JP Aerospace is an interesting company - in the city of Rancho Cordova, CA., California, JP Aerospace, America's OTHER Space Program. Their aim is to develop ways to send airships up into the stratosphere - and more controversially, all the way to orbit with their "Orbital Airships" vision. The airships would accelerate very slowly, at a rate of perhaps a few centimeters per second increase in speed every second, over several days, until they reach orbital velocity. Can they, or can't they, or how far can they go?

If you do it the other way, build airships to re-enter Earth's atmosphere from orbit, then this is much more widely accepted to be possible, with many scientists exploring plans such as the VAMP proposal for atmospheric re-entry for Venus, Titan and Earth by an airship that inflates as it re-enters, and slows down because it presents such a large surface area to the atmosphere. Also there was a 1964 test of a small inflated paraglider that achieved hypersonic speeds of over Mach 3 at a height of 400,000 feet, though only for minutes. So hypersonic flight by airships seems to be feasible in principle at those heights, but how fast and for how long? It is also definitely possible to have an inflatable in orbit, from the Echo project, which used inflated balloons as satellites for communications purposes.

Here is John Powell outlining his idea

Throughout this article, page numbers refer to John Powell's book "Floating to Space, the Airship to Orbit Program". The ideas here are from his book and talks unless I clearly label them otherwise.

AIRSHIPS TO ORBIT CONTROVERSY

The most controversial part is their idea of slowly accelerating to orbit. Their airships would be constructed and inflated at an altitude of 140,000 feet on a high altitude platform, his "Dark Sky Station" so called because it is so high in the atmosphere that the sky there is dark, just as it is when seen from space, even at midday. They would be far too flimsy to survive at ground level, but would be fine for flight at that height above all the weather in our atmosphere. Passengers would travel up to this platform in a more conventional stratospheric airship, then transfer to the orbital airship for the final stage of their journey. They then slowly accelerate to orbit over a time period of several days.

Artist's impression of JP Aerospace's orbital airship in orbit. It would get here by slowly accelerating over a period of several days, with the speed increasing at a rate of centimeters per second every second. Do that for several days - and if you can also have enough lift to stay high in the atmosphere above the denser layers of the atmosphere to avoid having impossibly high levels of drag - and you will eventually reach orbital velocity. That is, if you can find enough power, make the airship light enough, if an airship can survive flying at hypersonic speeds in a near vacuum, if it can generate lift, enough to stay above the denser parts of the atmosphere as it gets faster and faster, etc., etc.

The idea has been used in science fiction, for instance for Geoffrey Landis' "Sultan of the Clouds" about traveling to and from the clouds of Venus using a similar orbital airship.

He says that the V shape of their airship is optimized for hypersonic flight at Mach 3 and above in a medium to hard vacuum. It, never lands. It is too fragile, and would blow apart in a gentle breeze. It leaves and returns to the Dark Sky Station at 140,000 feet. He says that their 1.8 kilometer long model would take 20 tons to orbit (page 124) which is not far off as the payload of the Falcon 9 full thrust. If it works, he thinks that they can do it at far lower cost per ton than conventional rockets, and of course the whole airship is fully reusable with nothing discarded except the fuel.

This gigantic orbital airship is the very end point of the program. They would build up to it over many stages, of smaller airships and other designs, able to fly at 140,000 feet and a bit higher, then higher still, and eventually a small orbital airship, and then finally this one. Right now they are flying smaller airships of the same basic design but with a tougher skin and designed to fly from Earth up to 140,000 feet eventually (so far they have reached 15,000 feet). They also test high altitude balloons and structures and one of their main activities is taking "pongsats" up to high altitudes for schools.

Most physicists I've talked to say these orbital airships are impossible. Amongst other things, they say that:

• He can't get enough power to accelerate all the way to orbital velocity.
• He won't be able to generate aerodynamic lift above the conventional "Karman line"
• The Dark Sky Station would be too heavy to be buoyant at 240,000 feet, or else would be too fragile to last long

His answers on those three main points are

• Solar power using vast kilometer square solar panels using thin film solar cells only nanometers in thickness integrated into the material that makes the airship skin. These would be used to accelerate the exhaust of conventional rocket fuel to get much more thrust from the same amount of fuel as a "dirty ion thruster" with performance in between a rocket and an ion thruster, both in terms of the amount of thrust and the amount of fuel needed.
• The large wing size and low density construction lets his airships fly aerodynamically above the Karman line for smaller denser conventional air planes. - I'll discuss this briefly right away
• The Dark Sky Station would be constructed from numerous large balloons that would be swapped out in a maintenance cycle every few months. See "Dark Sky Station" section below.

On that second point, the idea of a "dirty ion thruster" is that it has charged ions mixed with the rocket fuel. The ions are created by combustion as in a flame then accelerated electrically as in a conventional ion thruster, which helps to accelerate the unionized fuel too, rather like the operation of a rail gun. So it's got more thrust than an ion thruster but has less thrust than a rocket. It doesn't generate enough thrust to hold up its weight against gravity so it needs a combination of aerodynamic lift and thrust once it gets above the buoyancy limit. It also needs more fuel than an ion engine but a lot less fuel than a rocket.

There are numerous other detailed challenges and responses. Of course the physicists who criticize him then challenge the practicality of these solutions.

If the physicists who say it can't be done are right - how far can he get? Can he achieve airships that fly several times faster than the speed of sound in the high upper atmosphere like the 1964 inflated paraglider? What about his "Atmospheric escape airship?" Can these airships launch from the Dark Sky Station and take passengers up to heights of 200,000 or 300,000 feet? What about his Mach gliders? Those at least should work, based on the 1964 experiment, and could be of scientific interest in themselves, hypersonic inflated gliders traveling in the upper atmosphere until they get too low to fly.

He is not suggesting any new physics like the EM drive, so it's not controversial in that sense. Rather, the reason it's controversial is because we have almost no practical experience of huge airships traveling at hypersonic velocity in a medium to hard vacuum to draw on. He is doing a lot of extrapolation on the basis of his own wind tunnel experiments and rather limited data. So, what makes it so controversial is that this is a different regime from any current experience of ordinary flight. He is talking about kilometer scale airships in a near vacuum that stay aloft through a combination of buoyancy and aerodynamics, and eventually also through ballistic motion as they approach orbital velocity.

This also means that in the course of trying to get his airships to fly, he will get this practical experience and that will help refine our understanding of what happens in practice to airships that attempt to fly in these extreme conditions. We do have some examples of hypersonic flight of rather small airships, or balloons - including that 1964 test, and some other similar examples (ballutes, ICBM mylar inflated decoys), but none of that is powered flight. We don't have any examples yet of airships that can reach speeds anywhere near to breaking the sound barrier under their own power. If he doesn't achieve his orbital airships, I think we will still learn a lot from that experience of trying, and who knows where it might lead in terms of new applications.

It is possible for an airship to have aerodynamic lift. Some hybrid airships are heavier than air, and depend on aerodynamics combined with buoyancy to stay afloat. The Lockheed LH-1 is a heavier than air hybrid airship which is going to be used to move freight and personnel to remote areas of Alaska and Canada. 

Still, though these hybrid airships still stay up mainly on the basis of buoyancy, with a little aerodynamic lift to assist. Also they could not travel anywhere close to the speed of sound without breaking up.

Meanwhile, conventional planes, can fly at supersonic and even hypersonic speeds (some of them). However, they are heavier than air, and are unable to fly higher than the Karman line. This is at 330,000 feet, and above this height a heavier than air airship has to travel faster than orbital velocity to generate enough aerodynamic lift to support its weight.

ESA invests in 4,000mph hybrid rocket jet engine firm, a Mach 5 airplane, the Skylon. Any heavier than air plane like this will be restricted to flying below the Karman line at 330,000 feet, the height at which you need to go faster than orbital velocity to achieve enough lift to hold up the mass of your plane against gravity. The Skylon would fly to orbit but it does that by converting to a rocket for the final section into orbit.

However, that's for heavier than air vehicles. What happens when you combine buoyancy with aerodynamic flight, at hypersonic speeds, and what's more, do it in conditions that would count as a hard vacuum in a laboratory, at a height of 300,000 feet or more?

First, could you do it just with buoyancy? Well in theory, perhaps, if we had the materials to do it, then we could float a sufficiently huge balloon above the denser part of our atmosphere even at the height of the ISS. So long as there is hydrogen inside, and higher molecular weight gases outside such as nitrogen and oxygen, it doesn't matter how thin the vacuum is. The principles are the same, and a balloon will still float, so long as there is some atmosphere, even if it counts as a medium to hard vacuum, for it to float in.

However, we don't have the capability to do that. Even the thinnest materials would be far too heavy for a balloon at the height of the ISS. But what is the highest point at which a balloon can float?

Well the world record so far is the JAXA BU60-1 experiment which was 74.5 meters in length, 53.7 meters in diameter and weighed a total of 39.77 kg including instruments. It reached an altitude of 53 km, or around 174,000 feet. This is just short of the height where JP Aerospace eventually plan to build their Dark Sky Station, where passengers will disembark to transfer to their orbital airships.

The BU60-1 balloon, when fully inflated, reached a height of 174,000 feet, launched by the Japanese space agency JAXA.

 It uses a polyethylene film only 3.4 microns thick,with a breaking strength of 400 kilograms per square centimeter which they developed in 1998.

However, it is one thing to send a balloon that high. Is it possible to design airships and a "Dark Sky Station" balanced to float at 140,000 feet or so as he suggests? And then - can you send them even further? Well for a while you are assisted by buoyancy, but not for long. When you get high enough, the air rather quickly gets so thin there is almost no lift.

However, the equation that defines the Karman line has a variable for the wing area, and another for the lift.

There, ρ is the density of the atmosphere V₀ is the velocity for a circular orbit at that altitude in a vacuum, S is the wing area, C_L is the lift, m is its mass and g is gravity. So, you can fly in a lower density atmosphere, and so higher in our atmosphere, if you have either more wing area, more lift, or less mass. Above around 330,000 feet (depending on the design) then a conventional plane needs to fly at orbital velocity to get enough lift to counteract its mass.

But if you can increase both wing area and lift, while not increasing the mass by the same amount you can decrease the density of the air needed for aerodynamic flight and so increase the height of the Karman line. The Karman line will be somewhat higher if the wing area of the vehicle is greater. Also, if you can design a vehicle with more lift for the same wing area, again, then the line will be higher.So where is the Karman line, if your airship has a wing area of square kilometers, and its wings are extraordinarily low mass for their wing area and amount of lift, with a skin made of thin film polymers that couldn't survive even a slight breeze at ground level? That's the big question here.

He is not going to get to the height of the ISS with aerodynamic lift, but that's not his aim. His aim is to use aerodynamic lift to get high enough in the atmosphere to reduce the atmospheric pressure, and so the drag significantly. At whatever height it reaches, it then accelerates, kept aloft by aerodynamic lift, and perhaps rising a bit more as it gets faster, until it reaches orbital velocity. Then once it reaches orbital velocity, it can slowly accelerate into a higher orbit like a conventional spacecraft. The drag depends in a complex way on the atmospheric density and the speed, with variations in the drag coefficient. .I think the physics and modeling of this is likely to be rather complex and also to need wind tunnel and practical tests to check it out, if there are any gaps in the data measured so far.

His idea only works if they get aerodynamic lift while traveling at hypersonic speeds as they accelerate to orbital velocity. He can't do it on the thrust alone. But the acceleration can all happen in a thin layer at the top of the atmosphere, in a sweet spot which he says exists with enough lift to hold up its weight, and yet the air is still so thin so that there is hardly any drag. If he can find a layer like that for his airship, then there is no need to rise any higher until they reach orbital velocity.

Many physicists seem certain that these airships are impossible. But after taking into account everything they say, and also taking account what John Powell says in his book and talks, I haven't yet been persuaded by their arguments, and feel it remains a possibility that these airships just possibly might be able to get us to orbit. I don't think even the most enthusiastic supporter of his ideas would go much further than that, to wonder if they just possibly just might be possible.

David Livingston asks if he still thinks they can do it, 22 minutes into this interview. He answers that it's really hard stuff, but that nothing suggests they can't keep going and keep doing the research and figuring it out. It's clear also to anyone, John Powell not excepted, that the final goal of airships to orbit is a major engineering challenge. We don't yet have airships able to operate even up to the site of his proposed "Dark Sky Station" at 140,000 feet, so we are a long way at present from orbital airships,

Please have a read and see what you think, and comment,. Do explain what I am missing here, if you are one of those physicists who are sure that his idea is totally impossible. Also, there are lots of really neat physics ideas involved, which are fun to think about, whether you think it will work or not. If you are a physicist or engineer who hasn't come across it before, you may find that it's fun just to think over whether it might work or not, and how far they can go.

Also the discussion is so often a "yes / no". But they have so many stages on the way into orbit. How far do you think they can go, if they can't go all the way?

Also, if JP can't do it the way they plan to, is there any other way that it could be done?

Anyway let's start with an introduction to the company, and with the much less controversial idea of airships re-entering our atmosphere from orbit. It's easy to see how an airship can achieve re-entry, as you will see, and I think just about everyone is agreed on this point, including scientists who plan to use such airships to explore the Venus and Titan atmospheres (do of course comment if you think they can't). The tough thing is to see how they can accelerate to orbit.

INTRODUCTION TO JP AEROSPACE

Their idea is that they don’t do any big expensive “succeed or go bankrupt” type tests like SpaceX did in their early years. Instead every stage along the way pays for itself. At present they pay ways to lift material to the edge of space (and occasionally crowdfunding for the pongsats) - though they never charge for their pongsts. Their tests involve high altitude balloons, and V shaped airships rated for the lower atmosphere. They have also tested a high altitude balloon based airship design.

JP Aerospace hold the altitude record for an airship, propeller driven, remotely controlled from the ground, and flying at a height of 95,000 feet above sea level.

Later on they plan a “dark sky” station at the edge of space which will be of a lot of interest for itself both scientifically and for tourists. It gets the name because at that height the sky will be dark even in daytime, as for the Moon. Next, they plan small airships doing test hypersonic glides back to Earth. Finally they do test flights to orbit with smaller airships, then the first human pilots to orbit, and then huge orbital airships with passengers and cargo.

The idea started off as a US Airforce contract for a near space reconnaissance airship. But the US canceled the contract in 2004 or 2005 after first persuading them to attempt to launch one of their prototypes for a lower atmosphere airship in a 50 mph wind (which would count as a “strong gale”). It was only rated as sturdy enough for launch in a 2 mph wind at the time (an airship is particularly vulnerable in the short time it takes to launch it from the ground). They did this with some reluctance - and it blew apart in the strong winds, causing some minor injuries. The inventor himself sustained three broken ribs. That was enough for the US Airforce to cancel the contract.

JP Aerospace have now solved the problem and can launch their lower atmosphere V shaped airships in any wind conditions. You can read their account of this story here. It’s now a civilian company entirely self financed, and they are not interested in any more such contracts, naturally enough.

It’s probably going to take them a fair while, with this approach, maybe decades but it’s interesting: “watch this spot”.

ORBITAL AIRSHIP CONSTRUCTION

It has no internal girders. Its outer shell covers an interior of many large bags of hydrogen to give it rigidity and to stop the gas bunching up at its nose. It also has inflatable trusses, with nitrogen filling the gaps in between these components. The nitrogen is vented if necessary and then replaced from liquid nitrogen tanks.

Note the use of airbeams - at this altitude they would consist of very thin materials, that nevertheless give it some strength because of the cylindrical shape.

It is balanced to float at anywhere between 140,000 and 180,000 feet altitude in the atmosphere. But since it is aerodynamic, it also behaves like a glider when descending. It doesn't look much like a glider to our eyes perhaps, but that big voluminous V shape makes a great glider in the very tenuous upper atmosphere during re-entry.

So what keeps it up is partly aerodynamic lift and partly buoyancy. As it descends from orbit, then to start with it is kept up mainly through aerodynamic lift, as it slows down on its long glide through the upper atmosphere. The aerodynamic effects keep it higher in the atmosphere for longer, and so keep it cooler on the way down. As it slows down to a halt in the atmosphere, it’s finally kept up by buoyancy.

Many details of the design are given in their Floating to Space: The Airship to Orbit Program. They don’t actually give expected skin temperatures. But the design uses nylon rip-stop polyethylene (page 111) which suggests that they expect external skin temperatures well below 100 °C (212 °F) for continuous use.

(Most commercial grade Polyethylene starts to soften at 60 °C (140 °F) and has a maximum continuous use temperature of 65 °C (149 °F), High Temperature Polyethylene can retain its properties up to 100 °C (212 °F) )

On page 109 they say

"By losing velocity before it reaches the lower thicker atmosphere, the reentry temperatures are radically lower.... This makes reentry as safe as the climb to orbit"

DAMAGE BY METEORITES

You might wonder what happens if the airship is hit by a meteorite or orbital debris. Well, it wouldn't burst, because the inner cells are "zero pressure balloons". He says:

"One of the most common questions asked about ATO is about meteorites. "What happens if a meteor popped the airship?" The answer is very little would happen. A balloon pops because the inside is at a higher pressure than the air on the outside. The inner cells of the airship are "zero pressure balloons". ... There is no difference in pressure to create a bursting force. All a meteorite would do is to make a hole. The gas would leak out staggeringly slowly... "

(Page 112)

That seems reasonable to me, I don't see meteorite damage as being an issue. What do you think?

FLOATING TO SPACE - THE AIRSHIP TO ORBIT PROGRAM BOOK

Much of this is based on their book Floating to Space: The Airship to Orbit Program. 

When I give page numbers, they refer to that book.

RE-ENTRY CHALLENGE

The JP Aerospace orbital airships are so lightweight they could never survive at ground level. The slightest wind would tear them apart. So in their plan, they have conventional airships that take passengers up to a docking station in the upper atmosphere, the “Dark Sky Station”, where they then transfer to the orbital airships.

You might wonder how they could ever manage re-entry from orbit, such thin flimsy seeming things. After all the Space Shuttle heats up to high temperatures at around the same altitude that these airships would dock with the high altitude space station. Well the answer is that it slows down much much higher in the atmosphere.

USING THE UPPER ATMOSPHERE AS A BRAKE - SPACE SHUTTLE

This is the way it is done today, to use the upper atmosphere as a brake, then slowly parachute to the surface or glide down in the lower atmosphere. How easy that is to do depends on the spacecraft.

If it is a heavy one like the Space Shuttle (now retired of course) then it can only slow down deep in the upper atmosphere, where it is dense. So it gets very hot. That’s why the Space Shuttle had to have ceramic tiles able to withstand temperatures up to 3,000 °F (1,650 °C)

Space Shuttle Enterprise banking on its second approach and landing test, during early flight tests.

NASA artwork for Space Shuttle re-entry - it’s high density, so can only slow down deep in the upper atmosphere, and gets very hot during that stage of its flight

LOWER TEMPERATURE REENTRY - SKYLON

Skylon is a plane being developed by the British company Reaction Engines with funding from the UK government and ESA. It will be able to fly to orbit from a conventional runway (though reinforced to carry the extra weight of all the fuel), return back to Earth, and then take off again within a couple of days with a crew of 200 to assist.

Its design is much lower in density than the space shuttle, once it has used up its fuel to get into orbit. So it slows down in the atmosphere at higher altitudes on the way down.

What really matters is the mass per cross sectional area it presents to the atmosphere or more exactly, its ballistic coefficient. Skylon could slow down even higher in the atmosphere if it presented a large blunt face like an aeroshell, but it has to be streamlined for the other stages of its flight. However it is also able to compensate for that to some extent by steering during the early part of the flight to slow down more quickly.

Skylon (future design being developed by UK / ESA). It flies to orbit from a normal length runway, reinforced to take the weight of fuel on lift off and may fly in the 2020s. It is heavy when it takes off, but during the landing, having used up most of its fuel, it is low density and so slows down much higher in the atmosphere than the Space Shuttle

As a result, it will reach lower temperatures than the Space Shuttle on re-entry though higher than a supersonic jet at Mach 3. Here are a few figures for skin temperatures for comparison, hottest first. These are the figures for the hottest parts of the spacecraft or plane:

• Space Shuttle re-entry: 1,650 °C (3000 °F)
• Skylon re-entry: 830 °C (1520 °F) - see section 4.3 of this paper
• US SR-71 Blackbird supersonic spy plane which flew at Mach 3, - external temperatures for the titanium around its cockpit windows of 232 °C (450 °F) - see (page 18 of this report)
• Concorde 153°C (307°F) when flying at Mach 2.2

SKYLON’S ZEPPELIN-LIKE TRUSS CONSTRUCTION, WITH A REINFORCED GLASS CERAMIC AEROSHELL

Modern planes have “stressed skin” structures, where the skin of the plane itself takes up all, or most of the external load from the wings, tail, other stabilizing structures and heavy components such as the engine (See fuselage for details). But the Skylon uses a structure much more like a zeppelin or a small plane. It’s girder-like with a thin glass ceramic outermost shell, which is just a heat resistant covering and doesn’t take any stress at all.

Structure of the Skylon - internal truss framework made from carbon fibre reinforced plastic composite held together with Kevlar ties. It has aluminium propellant tanks suspended inside it. Covering that, it has a thin outer aeroshell of a high temperature silicon carbide fibre reinforced glass ceramic material. For details see page 2 of this report

This ceramic outer skin is black, which is why Skylon is shown that colour in most of the artist renderings. This is an animation to show the concept for a mission to orbit, and back, by Reaction Engines who developed the idea. Re-entry starts about seven minutes into the video

LOW TEMPERATURE RE-ENTRY - ORBITAL AIRSHIPS

So with that background, now let's get back to JP Aerospace. Since their kilometer scale orbital airship is filled mainly with hydrogen, it’s not only lower density than a plane, and the Space Shuttle, and lower density than the Skylon. It’s also much lower density even than a normal airship. It also has a huge cross section which it presents to the atmosphere.

This spaceship design consists of a near vacuum of hydrogen floating in a near vacuum of normal air. If they succeed in building it, then it will be able to slow down just through friction in the very tenuous upper atmosphere. By the time it gets to levels dense enough to heat the skin significantly it’s already slowed down hugely, so the temperature of the skin during re-entry is much less of a problem.

JP Aerospace orbital airship - kilometer scale, extremely low density - this has the least temperature of any spaceship during re-entry, because it is so lightweight and large, has a low ballistic coefficient. This one is 2 km in length (actually this artist's impression is for ascent to orbit).

If it works out, the cost per kilogram to get cargo and passengers to orbit would be far less even than the space elevator ($500 per kilogram to GEO for the space elevator, and JP Aerospace’s estimate for orbital airships is $310 per ton to GEO, so 31 cents per kilogram), and it has much less development cost. It would be a leisurely journey as you would get there slowly over several days.

Although it may not look it, its huge V shape is designed to be aerodynamic at hypersonic speeds in the near vacuum upper atmosphere. They have done modeling, calculations and wind tunnel tests with scale models to test this.

ATMOSPHERIC RE-ENTRY FOR VENUS AND TITAN

There are several other places in our solar system with thick atmospheres like Earth, including Venus, and Saturn’s moon Titan. Mars also has a very thin atmosphere. The gas giants have thick atmospheres too (with no solid surface).

JP Aerospace hope the same idea can be used for Venus, with a high altitude staging post again, this time of course in the Venus atmosphere. The aim wouldn’t be to land on the surface, which is incredibly hot and high pressure, but to go down to the Venusian cloud tops to study them and perhaps build habitats there.

Perhaps they could use it for Mars too. The atmosphere of Mars is so thin that you could land an orbital airship like this on the surface. The strongest winds on Mars would only barely move an autumn leaf, fast though they are.

Though this prospect of JP Aerospace landing on Mars or descending into the Venus or Titan upper atmosphere is likely to be decades away, if they do achieve their dream of orbital airships - it could happen much sooner than that with VAMP.

VAMP ORBITAL AIRSHIP RE-ENTRY - FOR VENUS AND TITAN, ALSO EARTH

If you want to fly all the way down to ground level on Earth in one go, then you need a more massive airship. Northrop group’s “VAMP” project to study the Venus atmosphere uses an airship design like JP Aerospace, and they would inflate it outside of the atmosphere, so again that’s very like the JP Aerospace idea. It enters the Venus atmosphere already inflated, and because it is so large (55 meters in diameter) and low density, it doesn’t need an aeroshell.

However, unlike the JP Aerospace design, it’s able to fly in an Earth pressure atmosphere, so it’s not nearly as low density as an orbital airship. It still gets quite hot during the descent.

It would only descend as far as the Venus upper atmosphere, at the cloud tops, where temperatures and pressures are the same as for Earth. The cloud tops also have natural protection from cosmic radiation, and nearly all the ingredients for life. Indeed there are suggestions that it could be a good place for humans to settle outside of Earth. See my Will We Build Colonies That Float Over Venus Like Buckminster Fuller's "Cloud Nine?". Some astrobiologists think there may be life in the upper Venus atmosphere, already, which could have migrated there long ago when Venus was more habitable. The Russians are interested to search for this life, and may include an unmanned aerial vehicle, possibly VAMP in their Venera D mission to Venus in the mid 2020s.It inflates before it enters the atmosphere (see patent for details), and rather similarly to the JP Aerospace idea it decelerates slowly in the upper atmosphere, so generating much less heat, because of its low ballistic coefficient. So it doesn’t need an aeroshell, though because its designed to operate right down to the equivalent of ground level on Earth, its denser and its outer envelope is reinforced to withstand up to 1,200 °C (2,192 °F) along leading edges

They hope it can be used for Venus, and also Titan, and possibly Mars.

The first tests of VAMP would use the Earth’s atmosphere. So it could also be used for Earth re-entry. It might be useful for surveillance, photographing the Earth from above, and also for scientific studies of the upper atmosphere.

The same ideas could also be used for Titan - a moon of Jupiter with an extremely cold atmosphere at -180 °C, but it’s also dense, with the same pressure as Earth’s. This means that humans could go out of doors there, without needing a pressurized spacesuit. Of course they would need protection from the extreme cold and they would need air to breathe, so you are talking about warm clothing, as for Antarctica, perhaps heated clothes, and an oxygen mask.

However, they could get the oxygen to breathe by splitting water ice from Titan, and then burn the methane from Titan in that oxygen, a process that, rather neatly, creates an excess of energy which could then split more water, generating more oxygen for heat, and also for the colonists to breathe. The habitats could be built like many modern Antarctic bases, on legs to hold them above the cold surface. Since the air pressure is the same inside and out, the air could be kept in using double doors in a building of normal construction, again like an Antarctic base. See Let's Colonize Titan, and there are more details in their Beyond Earth: Our Path to a New Home in the Planets.

VAMP flying over Titan to sample and explore the upper atmosphere - Titan’s atmosphere is similar in pressure to Earth’s at ground level, though much colder, so you have similar methods for re-entry for Titan and for Earth. Though its gravity is much less - indeed a human falling from a plane or aerostat on Titan would easily survive the landing without a parachute.

BALLUTE - INFLATABLE AEROSHELL

Though none of these airships have been tested doing a re-entry from orbit yet, we do have experience of something not that different with the Ballute. This is a cross between a balloon and a parachute. It works like an aeroshell but decelerates much higher in the atmosphere. It starts off small in the upper atmosphere and is used to replace all the stages of descent from the early aeroshell all the way through to the final parachute, down to the landing. This is from a 2000 ESA demo:

Ballute

In the first ever re-entry test of a ballute by ESA for instance, the maximum re-entry temperature on its skin was 200 °C (392 °F).

Artist’s impression of the ESA Ballute

Graph from this paper. Measured temperature reached a maximum of 200 °C (392 °F).

From that same paper, the ballute inflated the first cascade of its deceleration device as a heat shield at 100 km and with a velocity of 5.52 km / sec. That's 328,000 feet and Mach 16. 

PARAGLIDING FROM ORBIT

This is another idea originally developed for Gemini in the early 1960s. For a while, before they settled on the familiar parachutes, the engineers thought that after the fiery stage of re-entry, the capsules would glide down to Earth beneath a parasail or paraglider. Those tests were quite promising, though they ran into many issues, for instance getting the glider to unfold. Eventually this line of research ended in 1964, when they changed to the parachutes as used by Apollo. The Russians also used parachutes for the Soyuz flights. For details of the paraglider research, see: Coming Home

Anyway at around the same time, in 1960, the engineers came up with the idea of using the same paraglider approach to go all the way from orbit, right down to the surface, without an aeroshell. This was the idea of the inflatable paraglider (Rogallo wing), called “FIRST” (Fabrication of Inflatable Re-entry Structures for Test), another idea for a space self-rescue system for astronauts.

It could be folded up into a small cylindrical package which would be kept docked to a space station, much as our modern Soyuz TMA is. In an emergency, the crew enter this cylinder, and separate. The paraglider then inflates and deploys. It would re-enter at an angle of 1 degree, with the paraglider angle of attack of 70 degrees. They found that deceleration would not exceed two g’s, and that there would be minimal heating because of the way it glides down to the surface. It would approach the speed of sound at 43 km (141,000 feet) altitude, and from there it would be able to glide 345 km horizontally before eventually landing. Details here: FIRST Re-Entry Glider

BALLOON RETURN FROM ORBIT

This is an idea from 1966 to return a human being from orbit, in an emergency,  the seat for the astronaut dissipated most of the heat as an aeroshell, but the balloon dissipates additional heat. So, first, it's another example of an idea, though never flown, that the engineers of the 1960s thought could be used to return a human being from orbit inside what is basically a balloon.

SAVER

The space engineers in the early 1960s explored many other such ideas detailed here: Rescue.

JP AEROSPACE'S IDEAS AGAIN

So the descent from orbit of the airships doesn't seem so problematical. At least, VAMP is seen as plausible by most scientists.

It's really the ascent to orbit that's the most challenging part of their plans. On the way up it gradually accelerates to supersonic speeds, then to hypersonic speeds (by which time it is already in a near vacuum). It has solar panels over its vast upper surface to generate power. These use thin film, just nanometers in thickness perhaps, so they are not heavy like domestic solar panels. They have already done three flights with thin film panels (82 minutes into his spaceshow guest appearance).

It would use these to power ion thrusters. These then can let you accelerate with a very high exhaust velocity, and so, with a small total amount of fuel, so long as you have plenty of power. It would have no shortage of power with such a large area of solar panels. But storage of that power at night might be a major issue. So now let's look at this a bit more closely.

1964 MACH 3 INFLATED PARAGLIDER

This is the only data we have for hypersonic flight with a V shaped inflated paraglider. He mentions it in his book (see page 56). It was release in a suborbital flight and inflated, and at its fastest, on re-entry, flew at a speed of about Mach 3 at a height of over 100 km, so over 300,000 feet, launched in 1964.

This is what it looked like

It was expected to reach high temperatures during re-entry. The tests of the materials involved a 60-second temperature rise to 620' C, a 20-second exposure at this temperature, and a 60-second cool down, see page 14 of this article.

It was deployed at 96 seconds and collected data until 306 seconds, so for three minutes, thirty seconds.

This is a long way from sustained hypersonic flight of course.

Before his orbital airships, JP plans to test hypersonic inflated airship type gliders like that himself, flying Mach gliders downwards from 140,000 feet, at supersonic speeds (2 mbar according to the standard atmosphere calculator), then from a height of 400,000 feet (less than 0.003 millibars, as that's the pressure at 282152 feet), this time launched in rockets to reach that height like the original 1964 experiments (page 119).

If he does succeed in getting his Mach gliders to work, this would answer quite a few of the concerns that physicists have raised. After all in the 1964 experiment the Mach glider got very hot, projected to reach 620 C, for twenty seconds or so, though it stayed at hypersonic speeds for probably a minute or two. What happens to his, exposed to hypersonic flight?

This is a point where many physicists will already put their hands up and say "Whoa, that's a step too far". Still, it is possible to have hypersonic airships for at least a minute or so, from the 1964 experiment. That much we know we can do. And after all, if you can go high enough, then the VAMP airships also would be expected to do hypersonic re-entry until eventually they slow down enough to go supersonic and then subsonic. 

Also, as he mentions here (110 minutes in) then the USAF explored ICBM decoys - shiny inflatable mylar balloons released from a rocket at around 100,000 feet which can travel at speeds of up to Mach 12. So hypersonic flight by an airship at high altitudes doesn't seem totally impossible.

So the main question he has to answer here are, can we have sustained hypersonic and supersonic flight in an airship, for his gigantic kilometer scale orbital airships during re-entry, at enough altitudes? These would be so large and made of such light materials, that they would take several days to de-orbit, not just minutes or hours. How long can it remain at hypersonic and supersonic speeds, on the way down, at what altitudes? Can flight at such altitudes even at hypersonic speed generate the lift it needs to return to the upper atmosphere Dark Sky station as a glider?

And before that he needs to show that his airships can achieve supersonic and hypersonic flight in the upper atmosphere, and can achieve enough aerodynamic lift to not just glide down, but to rise up through the atmosphere, when combined with the buoyancy of his airships, to above the Karman line.

TRANSATMOSPHERIC AIRSHIP

Then after that he has a crewed version, the suborbital Transatmospheric Airship which will test the idea of using propulsion to actually leave the denser part of the Earth's atmosphere and return to it again (page 122). It's not zero g flight of course so the "sub orbital" may be a bit confusing. It's still relying on buoyancy together with aerodynamic lift to get to that height, first at 200,000 feet then in later experiments, to reach over 300,000 feet.

In this blog entry about the Trans Atmospheric Ascender he says that it would be capable of Mach 5 and reach an altitude of 350,000 feet

Eventually (from his book), he anticipates that it would be able to reach Mach 10, and to be useful for rapid transport of cargo around the world.

ACCELERATING TO ORBIT - FUEL EFFICIENCY AND SPECIFIC IMPULSE

Since VAMP should be able to achieve hypersonic re-entry of an airship into our atmosphere - why is it so problematic to do this in the opposite direction? Well it's mainly a matter of the power levels available, and whether it can generate enough lift, in the opposite direction from 200,000 feet back to orbit again. Also it's a matter or the amount of time it would need to spend at supersonic and hypersonic speed. In their plans it would take not just minutes, or hours, but days, though at a much higher altitude of course than for VAMP.

This in turn is mainly a matter of how much thrust the rockets produce and how fuel efficient they are. The fuel efficiency is measures using the "specific impulse". This is often abbreviated as I_sp or just written as ISP.

This is the ratio of the change in momentum to the amount of fuel used up to achieve that change. If it's a conventional rocket and you measure the fuel used in units of mass such as the kilogram, and it's flying in a vacuum, then the ISP is just the exhaust velocity. When flying in an atmosphere, it's the effective exhaust velocity. But often it's given in units of seconds. That's for convenience as you then have the same specific impulse no matter what units you use for the length measurements, meters or feet. The specific impulse in seconds gives the ratio of the change of momentum to the weight of the fuel changed as measured in pounds or newtons. When you divide a momentum by a force in newtons, you get a number measured in seconds.

Anyway the main thing you need to know is that you get from the specific impulse in seconds to the exhaust velocity in meters per second by multiplying by the standard gravity of 9.807 - if you use meters as your distance measure. That's because one kilogram exerts a force (weight) of 9.807 newtons in standard gravity (the Earth's gravity varies slightly depending where you are, reduced in equatorial regions because of the Earth's spin, counteracting it, also less as you get higher and varying depending on whether or not you are above a gravity anomaly, so they use a standardized gravity).

The Saturn V had a specific impulse of 260 seconds. So multiplying by 9.87, this means the exhaust velocity was about 2.57 km / sec.

If you could somehow arrange for the fuel to be expelled at a velocity of ten times that, then it would be as easy to get to orbit from Earth as it was to take off from the Moon. You would only need around half of the mass of your spaceship to be fuel. As an example, if we assume a rocket with ISP 2000, starting mass 2600 kg, then the final mass on reach orbit is 1609 kg for ideal rocket equation. with less than half its mass used to get it into orbit. You can try out other ideas with this online rocket equation calculation.

So is there any way to do that? They used nozzles with the Saturn V but this idea seems to reach its limits at around the exhaust velocity they achieved. Even today we can't make chemical rockets with substantially faster exhaust velocities.

Now ion thrusters can achieve much higher exhaust velocities. But they also need huge amounts of power and to generate that power you need either solar panels or some onboard power generator. So the amount of fuel used is small - but the thrust is also small compared to the mass of the spacecraft. ESA is exploring a new type of ion thruster that in theory can achieve an ISP of 19,300 seconds - so an effective exhaust velocity of 190 km / sec, using xenon atoms. That's for "a single 20 cm diameter 4-grid ion thruster". See this paper. Also this page about their design. However it would need 250 kW power and would produce only 2.5 N of thrust. That's impressive for an ion thruster. But it comes nowhere near being able to hold its own weight against gravity. With 2.5 newtons of thrust, it is enough thrust for 253 grams (2.5/9.87) to hover against gravity. There is no way you could build a 250 kW power unit that weighs only 253 grams. So though it may well be very useful for navigation in our solar system, for next generation spacecraft, especially to the outer solar system, it's not going to be much use for taking off from Earth.

You can get much more thrust and also high specific impulse using either anti matter or nuclear fission or fusion. The Orion pulse rocket design from the late 1950s which worked by exploding nuclear bombs one after another to get into orbit had a theoretical specific impulse of 10,000 to a million seconds but also was powerful enough to carry its own weight into orbit. Details here.

Another way is to use beamed power from the ground. This was the subject of Kevin Parkin's thesis in 2006. He calculated that you can achieve an an Isp of 700–900 seconds and a thrust to weight ratio of 50–150 by beaming microwave power to your spacecraft, which gives plenty of margin to get a payload into orbit. (You need a thrust to weight ratio of 1 to hover against gravity). This was also the idea explored by Dmitriy Tseliakhovich in his thesis in 2010 who launched a company Escape Dynamics to try to get the idea to production. See also Jordin Kare's talks to the Space Show about power beaming.

JP Aerospace therefore have two main ways to overcome this problem of getting into orbit. They can beam power or collect vast amounts of solar power and somehow use ion thrusters, or they can build a rocket engine that somehow has a much higher ISP than a conventional rocket engine. It would need to have a higher thrust to weight ratio than a conventional ion thruster but it doesn't have to hover as that's done through buoyancy and then later on by lift. So it needs a better thrust to weight ratio than an ion thruster, but it doesn't have to have a thrust to weight ratio greater than 1. So that's a rather unusual specification. Such an engine would be useless for a conventional rocket to take off from Earth. And for an ion thruster the main requirement is to be as efficient as possible, to have as high a specific impulse as possible, so it wouldn't be optimal for an ion thruster either. But it's what they need for this application. So, is it practically possible?

THE SYMPHONY ENGINE - MIX OF CONVENTIONAL CHEMICAL ROCKET AND ION THRUSTER

An ion engine accelerates the ions to a high velocity, so can make velocity changes, eventually, for small amounts of fuel. But the problem is that the thrust is so small, it wouldn't be able to overcome drag. It could be operated up there but just would never be able to get the airship moving or keep it moving. While a chemical rocket would use far too much fuel. 

Well, one of the things that makes an ion thruster need so much power is that they have to ionize the atoms before they can accelerate them. So this is the first ingenious idea of their engine.

The engine they use is what they call a "mixed gas plasma". They call it the "symphony engine" but though they are actively developing it and doing many tests internally, they haven't published any papers on it, so I can't link to a detailed description of how it works. But he described the basic idea in his talks. They are accelerating the ions in the plasma, much like the way you accelerate materials in a rail gun - and this indirectly accelerates the remaining unionized gas along with them. The fuel he uses to create the plasma is a magnesium paraffin lox hybrid. So he is using chemical reactions to create the plasma, not relying on electricity to do that like a conventional ion thruster. He talks about it 74 minutes into his Spaceshow guest appearance.

Also 80 minutes in he explains their idea of a "dirty ion thruster" - that they need their engine to burn the fuel more slowly than a conventional rocket, but to have more thrust than an ion thruster and lower electrical power requirements enough so they can run on solar power and batteries. So that's their challenge, to do the plasma creation through a chemical process. The idea of creating a plasma in this way may seem exotic but all he is talking about here is combustion. Fire is a plasma and responds to an electric field. For instance a flame can be accelerated in this way, using an electric field, as this video from the Rino foundation in the Netherlands demonstrates. 

So the basic idea of the symphony engine is to burn a small amount of fuel to generate a plasma. The engine then uses electrical fields to accelerate the ions in the fire, and it mixes that with unburnt fuel and because of the charged particles mixed into the rocket fuel he can then accelerate the whole lot out of the back like a rail gun. It's like taking an ordinary chemical rocket, but then using the approach of an ion thruster to accelerate the exhaust as it comes out of the back to make it travel much faster, so getting more acceleration out of it, which is worth doing if it is burning slowly and you have lots of power available to accelerate the exhaust fuel. He also explains it 56 minutes into this talk.

It's a cool idea :).

The acceleration is very slow, the thrust to weight ratio isn't that great. He doesn't say how much but he says it would take several days to get to orbit (page 16 of "floating to space"). Not just minutes or hours.

He doesn't give details of the acceleration as far as I can see (may have missed it) but can work it out easily enough. With the ISS velocity of 7.66 km/s then if it took three days (say) to get to the orbit of the ISS at constant acceleration, the acceleration would be 1000*100*7.66/(24*60*60*3), or very roughly, 3 cms per second per second. Bearing in mind that you would need 9.87 meters per second per second to hold its weight up against gravity, this level of thrust is tiny compared to the likes of the Saturn V. It's 0.3% of the acceleration needed for the airship to hover under gravity.

At that rate of acceleration, which is just an example, then after one hour it would be traveling at 108 meters per second, or 389 kilometers per hour. It would take over three hours to accelerate to supersonic speeds and many more hours to reach hypersonic speeds.

All this time it is also ascending higher in the atmosphere.

Note, that the Space Shuttle was flying at far faster than hypersonic velocities when it burnt up, at a speed of Mach 22.8 and altitude of 230,200 feet, or about 28,000 kilometers per hour. His airships would reach that speed towards the end of the third day of acceleration if it's a three day flight to space.

Of course, he can't have it going at kilometers per second at the altitude where the Space Shuttle broke up. But he has buoyancy going for his airship, as well as aerodynamic lift. So it's not a priori impossible for him to get to high enough altitudes for hypersonic speed in the hours it will take to reach that speed. He doesn't give much detail about what height he'd be at which speed, but this blog entry about the Trans Atmospheric Ascender he says that it would be capable of Mach 5 and reach an altitude of 350,000 feet. The air gets thin rapidly, so that's both much higher than the Space Shuttle and at much slower speeds.

This calculation depends what he means by his rather vague "several days" - if he means six days it would be only 1.5 cms / sec / sec and it would take over six hours to reach supersonic speeds.

POWER FOR THE FLIGHT TO ORBIT

Potentially a large airship could have vast amounts of solar power available if it can use thin film solar cells - a film only a few nanometers in thickness. That is if he can succeed in integrating them into the skin of his airship. Such thin film solar cells have been integrated for instance into glass windows, where the layer is so thin that the window remains transparent.

He doesn't give a lot of figures, but you can work out some of them to fill the gaps. For instance the surface area of his giant airship would be over a square kilometer. If you have, say, 100 watts per square meter (probably would be more than that), that's a hundred megawatts per square kilometer.So if he can achieve thin film solar power as an airship cover material - then with he will have of the order of tens of megawatts of power at his disposal. Whether that's possible is then an engineering problem rather than fundamental science. But in those thin conditions with no winds, it will be much easier to have gigantic thin film solar arrays than on the Earth's surface.

He has already flown ultra thin film solar cells three times (82 minutes in here) on their balloon platforms. You get a lot more power out of them at 100,000 feet than at sea level. Their manufactures are now coming out with another lighter even higher powered version.

IDEA OF GETTING ELECTRICAL POWER FROM THE IONS IN THE FUEL ITSELF

However here, 64 minutes in, he says that they could alternatively use the electrical power from the exhaust of the fuel itself, using magnetohydrodynamics. As he explains it, they use the ions generated in the exhaust of the rocket through combustion, as a source for power, and use that to drive the acceleration of the fuel, so making the rocket more efficient, so it gets more thrust fro the same amount of fuel. He says they swing back and forth, depending on the current level of development of technology, between using this method and using solar cells that cover the airship.

I find this part puzzling. He claims that he can extract some of the ions without impacting on the exhaust. I don't really understand how this can be possible.

However, I may well be missing something here. The basic idea is that somehow he is able to increase the speed of his rocket exhaust up to about 19.74 kilometers per second instead of the usual 2-3 km / sec of a rocket exhaust. In the process he needs to achieve a thrust to weight ratio of at least 0.3% (from our calculation of the change of velocity needed to get into orbit). It would need to be more than that to overcome drag, the big unknown. That is more than most ion thrusters but of course much less than a chemical rocket.

He says he may be able to do that by extracting ions from the exhaust and using those as a source of electrical power that he would use to accelerate the remaining ions to high exhaust velocities and so accelerate the fuel along with it. The ions are ionized due to chemical reactions.

So anyway that's his idea. If anyone reading this knows more about it do say. Sadly it's not published. But that's normal in the is topic area.

The precooler for the innovative SABRE rocket engines for the Skylon space plane. It aims to fly as a high tech version of an airbreathing jet at up to more than five times the speed of sound by cooling the incoming air. It uses some of their liquid hydrogen fuel (at -238 C) to cool liquid helium which in turn cools the incoming air from 1000 C down to -150 C in a hundredth of a second. Even more impressively, it does that without causing any condensation or blockage from frozen water vapour. That seems almost impossible too, yet they have actually done it in tests. They keep the details secret for commercial reasons but had to disclose some of the details in 2015 for a patent, which reveals that they use 3D printing technology to create a part that couldn't be made using more conventional approaches.

So - is their symphony engine something like that - an almost impossible sounding thing that actually does work, but they aren't releasing the details for reasons of commercial secrecy? Also - has he explained how it works properly? Maybe he has left out some detail that's important to help us understand how it works?

I have to just "pass" on this, whether this can work. However all the rest of it, the idea of creating a plasma chemically, then accelerating the ions by solar power, that all makes sense. The idea that you can replace solar power by extracting electricity from the exhaust is the one part I'm not sure about. I just don't understand the concept yet.

ENERGY REQUIREMENT TO GET TO ORBIT - THEY HAVE PLENTY AVAILABLE FROM SOLAR POWER IF NEEDED

In this program at 97 minutes in, Charles Pooley rang in asking him to explain how it can achieve enough energy to get into orbit, when the requirement is a minimum of 33 megajoules per kilogram. (This is the sum of the potential energy of moving up in the Earth's gravitational well and the kinetic energy which is about 30 megajoules for LEO - there's a nice interactive graph here on Wired.com showing how both vary depending on the orbital radius -as the radius gets more, the potential energy increases, the kinetic energy decreases, and the sum increases)

So anyway it is hard to know how much power he'd get from his fuel, if he manages this harvesting of electricity power from the exhaust. He didn't give any figures there. 

But we can work out how much power he will have available from the solar panels, if it takes six days to get into orbit, and say, 100 megawatts of power for half that time, so for 72 hours? So that's 7,200 megawatt hours of power per square kilometer, or around 26 million megajoules. So the total power those solar panels accumulate over six days would be enough for about 788 tons into orbit, from one square kilometer of solar panels, assuming no drag at all.

His large Orbital Ascender (page 124) is 6,000 feet long, so about 1.83 km long, and he says, capable of carrying 40,000 pounds to orbit, or 18.14 metric tons. At that size, if covered in thin film solar panel tech, he doesn't have detailed dimensions in the book, but the wings seem to be about 300 feet in diameter, so that's about 0.3 square kilometers exposed to sunlight, maximum. So that's 1.8 times as much power, so enough for about 238 tons, or more if the thin film solar panels are more efficient.

Of course that's assuming no loss to drag which is not feasible at all. But it's also more than 10 times the amount of power needed to get the mass to orbit from just the potential and kinetic energy differences between the surface of Earth and orbit. That's a fair bit of leeway he has there. And he doesn't run the figures there - can you make a giant airship that weighs so little? And what about the lift? (Earlier version of this article said it was 60,000 feet and it's easier to make the numbers work if it is somewhat larger). 

So, anyway, am I missing anything here? It would seem that he could get enough energy from solar power alone even if it is fairly inefficient. And he is also carrying fuel, and using the power to accelerate the fuel. Some fuels have high energy density and may be worth the mass needed to carry them most of the way into orbit especially if combined with tens of megawatts or even over a hundred megawatts of electrical power.

That particular approach of using solar cells to supply all the power to get to orbit assumes a future where you can build a thin film hundred megawattsolar power station in space (or on the Moon or elsewhere) using ultralightweight balloon fabric. If we could do that it would have major implications for space exploration generally. But my main question is, does it need new physics, or just new engineering and materials? If this calculation is correct, and I haven't made a mistake somewhere, it would seem to be more the latter.

It seems it may be just within the margin of possible, or perhaps it is just outside it?

It would also be possible to beam power from the ground to orbit using microwaves or laser. It's a huge challenge, e.g. to integrate all those thin film solar cells, power distribution etc, but I'm not sure that it is totally impossible, as in, not possible according to the laws of physics and known materials properties. And if you can do it in principle with vast amounts of solar power for six days, you can also maybe do it with beamed power, or in some other way too, and may find an easier way to do it.

What do you think? As usual do say if I have missed anything in my calculations. I know how easy it is to forget an order of magnitude or some such :).

STORING THE POWER DURING PERIODS OF DARKNESS

Solar power is fine so long as you have sunlight. But as his airships travel to orbit, they will encounter periods of darkness. So what do they do at night?

He looks into many ideas for storing power in his book. All of them require advances. The aim of course is to have the least mass for the amount of stored power. So, one of the most promising methods is to use flywheels, because large diameter flywheels are amongst the densest forms of energy storage available, with far more power stored per unit mass than a battery.

The larger the diameter of the flywheel, the more energy you can store for the same amount of mass and the same strength of materials. With such large airships, potentially, they could be even tens or hundreds of meters in diameter. Normally flywheels are not built to such a large scale because they operate in a vacuum. You would need to evacuate a large volume in order to house such a huge flywheel. But in the orbital airships, the entire interior of that large envelope is a medium to hard vacuum already. He doesn't say where they would go, but a likely place seems to be in the nitrogen filled near vacuum gaps between the helium filled bags that produce the lifting power. Or perhaps they could be located within the lifting bags themselves, since they would be vast in scale?

Modern flywheels for energy storage normally operate at between 0.1 and 100 Pa (0.001 to 1 millibars). The top of the mesosphere at about 85 km or 280,000 feet has a pressure of about 0.3 Pa according to the ISA 1976 standard atmosphere model (online calculator). So it would seem that flywheels in the orbital airships would be able to operate without a vacuum housing, , and the higher it gets, the harder the vacuum, and so the less the loss to friction and the longer it will be able to store energy in flywheels.

The flywheels would come in counter rotating pairs. As a result they would have net angular momentum of zero and there is no torque induced precession. He doesn't say this, doesn't go into that level of detail, but it is how you would do something like this. Alternatively, you can mount the flywheels on a gymbal so that the torque induced precession has no effect on the airship itself, but when you have plenty of space, as for an airship, then counter rotating pairs seem the best solution to this problem. See this page: The Mechanical Battery for a history and discussion of use of flywheels for storage, including the "Gyrobus".

He suggests several other ideas on page 118, including conventional lithium ion batteries and nuclear micro batteries. But he says

"For raw power storage, nothing can beat a Maglev flywheel. The volume inside the airship allows for flywheels of truly tremendous size. With a large diameter of flywheel, the mass density can be lower. Experimental city buses are now being run on these flywheels. More study is needed to see if this is a practical power source."

IDEA - EARLY STAGES OF THE FLIGHT IN CONSTANT SUNLIGHT?

Whether he can get enough power density for his batteries or not I don't know. How about this idea though, would it work? He suggests putting his Dark Sky Station in the polar vortices, which are at a latitude of 60 degrees. 

That's close enough to the Arctic circle so that in summer they would have only five hours of darkness a day. Also the elevation of 140,000 feet will help make the nights even shorter. As the airships head off to orbit, if they fly west instead of east, they could even keep up with the sun more or less for the first day. After 24 hours they would have gone around the Earth 1.4 times and by then would be traveling at 1.3 km / sec. or about Mach 3.8. So, if you timed it carefully, so that when the airship reaches 60 degrees south in that first orbit, it is there just as that part of Earth comes into sunlight, perhaps you could arrange for the entire first 24 hours to be in sunlight?

By the end of the second day they have gone around the Earth five times. At this point, then the day / night cycles have decreased to a bit over four hours and they are traveling at 2.6 km / sec. They would of course eventually need storage for power for the night - but if you can do that at a later stage, then maybe you need power only for a couple of hours rather than twelve hours or more. Also his airship is after all a glider. He anticipates that the glide back to orbit would take several days.

So when there is no battery power available, it can glide back down again, a bit like solar powered airplanes circling the globe, but in this case an airship balanced to an equilibrium height of 200,000 feet, so there is no risk of it getting lower than that. It would rise under solar power in the day time and glide down at night. So long as he has a good head start, rising for maybe 24 hours or more in constant sunlight, maybe that can then work for the rest of the way into orbit.

When higher in the atmosphere, the vacuum is harder too, so flywheel storage of power would be more efficient without a need to pump out a vacuum around the rotors. It's just an idea and of course lots of details to flesh out. Surely, power storage for night time use is a significant issue.

Just a suggestion, I wonder, what about beamed power from the Earth's surface, in the form of laser or microwaves. Could these fill in useful gaps with strategically placed base stations for the night time flight? With such large airships, there would be plenty of space to add a rectenna to receive beamed microwaves from Earth.

TRANSITION THROUGH THE SOUND BARRIER AND HYPERSONIC FLIGHT

He says in this recent SpaceShow guest appearance that it gets to 180,000 feet through buoyancy but doesn't reach Mach 1 until around 240,000 feet (56 minutes in to the talk). He thinks that the limit for neutral buoyancy may be 280,000 feet here (53 minutes in). But for practical purposes they think 250,000 feet is the end of buoyancy. By 320,000 feet then they are using aerodynamic lift alone, and can only get higher by having an exceptionally large low density vehicle with a large wing area.

You might think that crossing the sound barrier would be hard, but with such thin atmosphere, he says it wouldn't be.

"Visions of Chuck Yeager with the "right stuff" exploding through the sound barrier need to be left behind. At 200,000 feet, the air pressure is extremely low. The low pressure greatly diminishes the forces involved. The teeth are pulled from the tiger of the sound barrier"

The transition would happen at 200,000 feet (page 206). The next part of the flight up to Mach 3 is the toughest part according to him

HYPERSONIC WAVE RIDER - HIGH LIFT, LOW DRAG RIDING ON TOP OF THE SHOCK WAVE

The vehicle is tuned to fly at Mach 3 and above. It's a "hypersonic wave rider" designed to ride on the huge shock waves that form around it as it flies. As it goes through the sound barrier, the vehicle passes through a wave of air that forms in front of it, at which point the air instead of flowing round the plane, deflects off it at large angles. As it gets faster still, the air gets pushed back close to the wing, which helps with lift (page 109).

This is a point that often gets ignored in the "impossibility proofs". What is the drag and lift for a kilometer scale, light weight airship flying as a hypersonic wave rider, nose up, flying on top of its shock wave, in a near vacuum? I don't think there is much data available on this.

What are your thoughts on all this? Would it be able to go through the sound barrier and into hypersonic flight smoothly because of the hard vacuum conditions, or would it experience stresses that would tear it apart? What about the skin temperature, what would the effect of flying in a hard vacuum be on that? And what about the later stage as a hypersonic wave rider?

Any hard data on all this that we can apply to a large airship flying in a vacuum?

FIRST STAGE AIRSHIP

Before he can do any of this, apart from the Mach glider tests, he needs a way to get from the ground up to those altitudes of 140,000 feet. That's what the first stage airship does.

Later on his plan is that these airships will bring cargo and passengers to the Dark Sky Station. They will also bring the materials of the airships all ready to be inflated, and hydrogen to inflate them in the form of liquid hydrogen (easy to transport). His idea is to use these to build his orbital airships, designed to be neutrally buoyant at that height. These high altitude airships can't be built on the ground because their thin film skin would be so lightweight that they would break apart with the slightest breeze of wind.

So he needs to build a first stage airship. This already is a major challenge, an airship able to get from ground level, to a height of 140,000 feet. The German dirigibles could fly up to 24,000 feet. In 2005, Raven Industries demonstrated powered flight with a five pound payload for five hours at 74,000 feet.

The StratoBus project led by Thales Alenia Space in Europe plans to send a dirigible able to carry 200 kilogram to a height of 20 km, or about 66,000 feet. From this you can see that JP Aerospace's plan of an airship able to travel from ground level up to 140,000 feet is rather ambitious. But he has already achieved flights up to 104,000 feet with his prototype ascenders, as he mentions in this program from The SpaceShow.

Artist's impression of the Stratobus, which will be able to carry a payload of 200 kg to 66,000 feet, half way between a drone and a satellite in its capabilities.

As I mentioned above, JP Aerospace do hold the record for powered flight -just a propeller attached to balloons and a framework but they have flown their Tandem unmanned twin balloon airship to 95,085 feet.

They are also working on the actual V shaped airships that they hope eventually to send up to these heights from ground level.

Test flight of Ascender 26 (26 foot long) in 2015, first desert release nose-up flight.

Recent test flight of an Ascender 36 which reached a height of 13,512 feet.

They have flown them to several thousand feet so far in unmanned flights. This shows that their V shaped design works as an airship. It also gives them experience in launching them, which they can now do even in windy conditions. Here is one of the arms of their hundred foot long Ascender 6 which they plan to test later this year, in an inflation test

And from inside

These are low altitude vehicles so far. He says in this guest appearance to the SpaceShow (56 minutes in) that the ones they built for the USAF were designed to fly at 100,000 feet but were not actually tested because they didn't get that far before the USAF pulled out of the project. So, their only high altitude experience so far is for the tandem design at 95,000 feet, a propeller attached to a framework kept aloft by balloons. So far they are using much smaller ascenders, all "subscale", only 26 feet for Ascender 26 for instance. He explains in that Spaceshow guest appearance that his aim is not to try to get to the desired height but to work on integrating all the systems and refining the basic V shaped design airship, which later they scale to larger and larger vehicles to get to higher altitudes. It's a lot cheaper to do that on smaller scale vehicles, and they flew it to 6,000 feet. Next was the ascender 36, which they flew to 15,000 feet in 2016 on its first flight. 

Their Ascender 100 will fly to 45,000 feet some time later this year (that's 13.7 kilometers, or one and a half times the height of Mount Everest, above the highest level commercial jets fly at, and the same height that Concorde flew at).

Artist's impression of JP Aerospace's Ascender 100, a 100 foot airship they are building now, which they expect to fly to 45,000 feet.

Then probably early September they will do a 175 foot vehicle which will be able to fly up to 65,000 to 75,000 feet, so by then they will be flying much higher than a commercial jet, though within the range of military aircraft (the now venerable but still in service Lockheed U2 spy plane cruises at 70,000 feet).

The early USAF designs which were designed to fly to 100,000 feet look superficially similar, gigantic V shaped designs. Internally the new ones are very different with many improvements which is why they have gone back to smaller models again to work on the design for the new vehicles. So, they probably won't get ascenders crossing 100,000 feet until towards the end of 2018.

DARK SKY STATION

This is so called because the idea is that it would float at such a height that the sky would seem black in daytime just as it does from space or for the astronauts on the Moon. You might think it is impossible to have a structure of any size floating at a height of 140,000 feet. But this is easier to achieve the larger the structure, because of the way the mass per volume goes down as the structure gets larger, so long as the envelope for the structure contributes most of its mass.

For instance, Buckminster Fuller's "cloud nine" was a design for an entire city, one kilometer in diameter, floating in the atmosphere. A sphere as large as that would be so low in mass, if constructed as a tensegrity sphere, relative to the mass of the atmosphere that just a one degree increase in temperature would be enough for it to float. As a tensegrity sphere it would also be very robust in any weather conditions. There's never been any need to build such a structure, and maybe there never will be, nor did he expect there to be, but it seems generally agreed that the engineering for it is sound. He thought of them only as an "exercise to stimulate imaginative thinking".

So, going on from that, there was a serious proposal in 1980 by doctors Ernst Okress and Robert Brown of the Franklin Institute to build a large platform called STARS, half a mile to a mile in diameter, as an upper atmosphere research station. See Solar Thermal Aerostat Research Station (STARS) and the news story about it in the Washington Post:

"Solar Powered Balloon Station Proposed For the Edge of Space".

For an artist's impression of STARS, see Peter Elson's "Orion Shall Rise" painting, an illustration from Poul Anderson's novel of the same name, Orion Shall Rise, which features the STARS aerostat.

One of the covers of Poul Anderson's "Orion Shall Rise" featuring the STARS aerostat floating at 100,000 feet, which in his novel is an old pre-war relic from our time. Painting by Peter Elson.

This construction would have flown at a height of about 100,000 feet. And it would have been kept aloft only by the heat of the air inside it, like Buckminster Fuller's "cloud nine", not needing hydrogen or helium or any kind of lifting gas.In this paper he discusses temperatures between 27 C and 100 C for the interior, kept that warm basically by the greenhouse effect on a transparent balloon if I understand them right.

With that as background, perhaps JP's idea of a dark sky station, with use of hydrogen for lift,  is not so impossible?

The dark sky station would be starfish shaped and use lots of large gas bags along each of the five arms, and not one huge spherical one. This is a very stable configuration.

Artist's impression, shows the Dark Sky Station at 140,000 feet. It would be visited by airships from below and could be used as a launch complex to launch rockets to orbit, which then would not have to push their way through the dense lower atmosphere. This much could be achieved without orbital airships.

Again he goes into a fair bit of detail. The lifting bags would be damaged by radiation and would be replaced every 100 days (page 88) at least unless we can develop new materials to prevent that - and he sees that as part of a continual maintenance cycle, with old bags deflated, replaced and the new ones reinflated with the lifting gas. Instead of trying to get exotic materials, he uses larger (see 94 minutes into the interview). He's already done a balloon swapout in flight at 50,000 feet on his "Away 6" mission, which continued up to 100,000 feet.

He would bring hydrogen up to the dark sky station in cooled liquid form, something that has already been done in a high altitude helium balloon flight in 2002 by Julian Nott, He would use this to keep the bags inflated.

He would use superpressure balloons like the NASA big pumpkin (page 91) so that they stay in the same altitude in the sky and don't rise and fall as the temperature changes. He would put it in the polar vortices, so that it travels around the Earth once every 14 days (page 90). His station would be a starfish shaped structure with five arms for stability (page 85). They have done experiments with balloons attached to struts for an early first try at the configuration of a dark sky station, with success (page 97).

The balloons clearly have to be huge. In this diagram a slightly under 40 million cubic foot balloon (the orange curve) can hold up around 1,500 pounds at a height of 140,000 feet using helium.

That's around 0.75 tons held up by a balloon about 424 feet in diameter or about 129 meters in diameter.  The diagram is from this page - from the Columbia Scientific Balloon Facility page.

That's for a helium balloon, but though helium is twice as dense as hydrogen, what matters is the difference in density between these gases and air, which is much denser. So a hydrogen balloon can carry only 8% more mass, in the Earth's atmosphere, in this case, 0.81 tons.

His DSS Block Three station, in the book, would have balloons with a total volume of 2 billion cubic feet. He doesn't say what the mass would be, but with these figures it would be able to support 40.5 tons. with a crew of 15, and with 20 passengers.

This may seem to be too little mass for a station with so many people - especially if you compare it with the ISS, which weighs ten times as much. However, this is still low enough in the Earth's atmosphere so that there some protection from micrometeorites. The shooting stars in meteor showers glow and vaporize at heights of between 100 km and 70 km, and this is a height of 42 km. It's above the ozone layer (at around 20-30 km so below 100,000 feet) so there wouldn't be protection from UV - but they would be inside habitats anyway and there is no difficulty protecting from UV. This would make a difference.

You can actually survive in space outside of the ISS in a thin plastic balloon, which was the idea for this rather hair raising

 Indeed he talks about inflated fabric tunnels which the crew could use for jogging in. Or at least, that's his idea (page 102). Do you agree, could he have a low mass station at the edge of space at 140,000 feet? Or would it have to be a massive construction like the ISS? Or a mix of the two?

Atmospheric pressure at that height is around 2 mbar according to the standard atmosphere calculator. So the outward pressure would be pretty much identical to the ISS, about ten tons per square meter if he maintains a sea level pressure atmosphere.

Both of these also show the idea of being able to survive at least for a while inside a not very heavy balloon in orbit. Which suggests, maybe you can also do lightweight construction at 140,000 feet. Does the ISS have to be as heavy as it is for life support? Of course all that mass helps with meteorite shielding. It also provides radiation shielding. But the Dark Sky Station would fly far too low in the atmosphere to have any problems from radiation. The ISS gets so much radiation because it regularly flies through the South Atlantic Anomaly, which happens because of its inclined orbit, which in turn was required because of the need to partner with Russia which has its launch facilities at high altitudes.

South Atlantic Anomaly - where Earth's inner Van Allen Belt dips down to an altitude of 200 km - the ISS flies through this regularly and this significantly increases the radiation dose for the ISS crew. The Dark Sky Station would be located in the polar vortices and anyway is far too low to be affected by the South Atlantic Anomaly.

Some ideas for future space stations would have air beams in between modules built more like the ISS - and those air beams could be very light. Joe Carroll suggests this approach in his "Design concepts for a manned artificial gravity research facility".

So that's not unlike this idea from JP Aerospace of air beams as tunnels the crew could jog in for the Dark Sky Station. On page 15 of his paper Joe Carroll, who is an expert on space tethers and has flown several tethers in space, works out that a air beam strong enough to be used in space, able to hold in 10 atmospheres of pressure (so with a big safety margin) would weigh 10 kilograms per meter at a diameter of 1.55 meters, at an areal density of 2 kg / m2. The mass of a cylinder scales up linearly with the diameter, so if we make it a more roomy 2.4 meters in diameter, then it would be 3 kg / m2. So you could then have, for instance, a 10 meter long habitat, 2.4 meters in diameter for a total mass of 30 kilograms. Or 4.8 meters in diameter, 20 meters long, mass 120 kilograms.

So - it seems that you could have rather lightweight structures for the habitats, if this sort of construction is safe. So perhaps the analogy of the ISS can lead us a bit astray here. Could you have really lightweight structures like this, and still achieve reasonably safe construction at 140,000 feet? It is of course just theory and hasn't ever been tested in reality.

What do you think?

WORK AROUND - IF HE CAN'T ACHIEVE ORBITAL AIRSHIPS, PERHAPS HE CAN RENDEZVOUS WITH A LIFT TETHER SYSTEM?

This is my own suggestion here. It's based on the lift tether system, which is an idea which has been investigated for an easier way to get into orbit, related to the space elevator. We don't have materials to make the space elevator, which is also a massive construction. So, until we have those materials, we need to use a “watered down version” which is not able to hover stationary over the Earth’s surface. However, it would still be enough to make a difference. Normally, you need to travel at about Mach 20-25 to go into low Earth orbit (depending on how high the orbit is). The launch assist tether reduces that to Mach 12 or less.

From: Hypersonic Airplane Space Tether Orbital Launch (HASTOL) System.

This video explains it well:

For more details, see Launch Assist Tethers. You could use the same process in reverse to de-orbit a spacecraft to Mach 12 in the upper atmosphere, and then it glides down from there.

So this would give you a way to slow down your orbiting spacecraft to Mach 12 instead of Mach 20 upwards, which would be quite a plus.

That’s still very fast. It’s four times the speed of the Lockheed SR-71 Blackbird supersonic spy plane, or Virgin Galactica’s SpaceShipOne (Mach 3.09), and more than twice the speed of the ESA’s projected Mach 5 aircraft:

However it's not far off the speed of JP Aerospace's Mach 10 transatmospheric airship, so it may be a way for them to get into orbit if they can get that far, and can't make the last stage of the orbital airship to get all the way to orbit just using airships.

SO WHAT DO YOU THINK? HOW FAR COULD THEY GET?

This debate can get so polarized, if you ask the question "Can they do it or can't they". But perhaps we can defuse this a bit by asking a more nuanced question - "How far could they get?"

To get the debate started, here are a few possibilities to think about. Could JP Aerospace

• Get all the way to orbit by gradually accelerating all the way?
• Achieve a Mach 10 transatmospheric airship, for instance to permit fast transit of cargo, and then perhaps also to manage transfer to a launch tether system to get into orbit?
• Achieve supersonic airships, but not hypersonic?
• Get above 320,000 feet with his airships, the point at which the airship has to be flying using pure aerodynamics with almost no assistance from buoyancy?
• Never manage supersonic flight, but achieve powered flight in airships neutrally buoyant from 140,000 to 200,000 feet and perhaps some distance above that through aerodynamic flight.
• Build a large Dark Sky station, also used as a launch pad to launch rockets to orbit ("rockoons" - i.e. rockets launched from balloons).
• Build a smaller Dark sky station at 140,000 feet, maybe their Dark Sky 3 with 15 crew and 20 passengers?
• Even the Dark Sky Station can never be built, perhaps a few balloons but never a permanent station at that altitude. However they could achieve powered airships that reach the upper stratosphere and maybe into the mesophere, not just robotic, but with humans on board.
• Will only ever have robotic balloons and airships at those heights of 140,000 feet upwards.
• Or some other view?

Also what are your thoughts about re-entry of really large kilometer scale airships? Or the smaller ones for VAMP in the Titan, Venus and Earth atmospheres? And what about accelerating to orbit? Is it a problem crossing the sound barrier for instance at such high elevations in a near vacuum?

It will be interesting to see what the range of views is of physicists on this matter. I will surprised if they all agree on how far he could get. So often it is presented as black or white "Will they do it" or "will they not". Do say what you think in the comments area below. Also, if I have missed out anything significant in my presentation of JP's ideas do say. 

ANALOGY OF THE WRIGHT BROTHERS

I see it as a bit like the Wright brothers, many doubted they could fly at all. They managed a short hop. Their machine was high maintenance and could only travel short distances, and that remained the situation for many years, with the early aviation pioneers. What they did was regarded as an amusement like hot air ballooning. Few could have guessed where that would lead, even themselves. Hot air ballooning has remained something of mainly amusement value, while the plane has become something of great economic value. Back then if anyone had tried to predict what would happen they could have easily predicted that it would be the other way around, or that neither would be of any economic value except for entertainment..

If he achieves the equivalent of their short hop - if he gets as far as his V shaped airships lifting from ground level to 140,000 feet, or he achieves his Mach 5 airship, or the Dark Sky station - any of those would be like the Wright brothers hop. After that - well it's probably looking too far ahead to see for sure where it would go after that.

What do you think? Do say in the comments. Be sure to say if you spot any mistakes in what I've said here, thanks!

BTW got a mention on JP Aerospace's blog:

"There’s a article about JPA at Science 2.0. It’s a good article.A few minor errors (Dark Sky Station will be at 140,000 feet not 180,000 feet and we never charge for PongSats). However overall it really captures what we’re up to."

Great! I've fixed those minor issues he mentions there. I've also fixed a mistake in the si\ze of his orbital airship. I thought it was 60,000 feet, don't know how that happened. On re-reading, it's 6,000 feet. Also fixed conversion of 40,000 pounds to tons. It's 20 US tons but only 18.14 metric tons. Anything else do say anyone!

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