Everyday we rely on information transmitted through satellites. Some are close, orbiting just above the earth’s atmosphere, while others circle at a distance of 30 or 50 times further into space.
They are placed into a given orbit based on need: what does the satellite need to see, and how does it need to travel with respect to the surface? These requirements must also come to terms with the essential physics for stable orbit, where the centripetal force equals the gravitational force. In short, satellites closer to the earth must travel faster, circle the earth with greater frequency, and observe less of the surface.
(Source: http://en.wikipedia.org/wiki/File:Orbitalaltitudes.jpg)
The Landsat 8 is one such satellite. At only 705 km, the
orbit is nearly twice that of the international space station, but far short of
GPS or communications satellites. Following a polar, north-south sun-synchronous
orbit, the satellite passes over the North Pole and records data while passing south
over the sun-illuminated earth. Intuitively, this makes sense. High-resolution
imaging and frequent satellite In an effort to achieve high resolution, and
constantly updating data for the world-over, circling the earth in only 99
minutes is advantageous.
Intuitively… But when you (or at least when I) think about
it more critically, there’s not much that seems intuitive about it. Maybe
ingenious is a better term. Awesome, really. (Any Eddie Izzard fans?)
It took only 80 seconds for a 2,800 kg satellite –- the mass of a Ford
F-150 –- perched atop a 330,000 kg rocket to accelerate to super sonic speeds,
fueled by 280,000 kg tanks of liquid oxygen and kerosene. 86 minutes lapsed between rocket liftoff to the
deployment of solar arrays for power generation of the satellite. That’s less
time than it takes to cook a sweet potato. Or less time than the daily commute
for the unlucky among us.
How does this remarkable feat occur? Well... Let's just see, shall we? It's all about the rocket...
1) At the base of the rocket is the Atlas V Booster paired with the RD-180 engine. The Booster contains the Liquid Oxygen (LO2) and RP-1 (Rocket Propellant, a refined kerosene) which are supplied in a 2.7:1 ratio to the RD-180 engine in a staged combustion process. Developed in Russia, Pratt&Whitney is now able to co-produce the engine critical to the Atlas V. First, the RP-1 and LO2 agent are mixed in a pre-burn cycle, powering turbines and a compressor that pumps the fuel and oxidizer into the combustion chamber. Developing and maintaining high pressure is critical to engine performance, and by venting the pre-burn byproducts into the combustion chamber, the engine achieves very high efficiency.
SpaceX, acting in the way typical of SpaceX (said with the utmost respect), is attempting to design an engine (Merlin) that produces a much higher thrust-to-weight ratio compared to competitors, and also focusing on re-usability. Although their engine also uses LO2 and RP-1, it is a different animal at heart. Unlike the staged combustion engine, the open cycle Merlin releases the preburn byproducts, decreasing the efficiency but simplifying the engine design.
There are several figures of merit used to evaluate rocket engines. Thrust-to-weight ratio is common, and one in which SpaceX aims to excel. Their Merlin 1D is being designed to achieve a thrust-to-weight ratio of over 150, compared to the RD-180's ~80. Given the nuances of what counts in the measure, this is a less hard and fast comparison. A more common metric is the specific impulse, Isp. This measures the efficiency of the engine by evaluating the amount of momentum imparted unto the rocket per mass of fuel. The RD-180 supplies ~310 s specific impulse at 100% thrust (sea level), compared to the Merlin's ~280 s. The decreased efficiency of the Merlin is due to the open cycle system employed. Lastly, simple thrust can be compared, and varies dramatically between the RD-180 and the Merlin. The two-chamber RD-180 produces upwards of 3,800 kN of thrust (100%, sea level), compared to a single Merlin engine producing only 620 kN (100%, sea level). I say only without being snide: the Falcon 9 is equipped with 9 of these babies. For comparisons sake: the Boeing 747 has two engines that produce about 200-300 kN of thrust at their peak.
(Source: http://www.spacex.com/falcon9.php)
The Atlas V Booster and RD-180 are fired at T-2.7 seconds, with a liftoff (where thrust > weight) at T+1.1 seconds. The Atlas runs steadily at ~95% through the max dynamic pressure (i.e., Max Q) at roughly T+87.3 seconds, and continues to run until the Booster Cutoff at T+242.2 seconds, and booster separation at T+248.2 seconds.
2) Going up the line, the
At the top sits the Landsat 8 (LDCM) satellite, enclosed in the payload fairing (PLF). This is used to protect the satellite from damage until it reaches an orbit of 113 km. A combination of thermal shielding on the external surface of the aluminum PLF -- consisting of cork composite tile ablators which reduce surface temperature by absorbing the energy by degradation, melting, and submlimation processes -- and an accoustic dampening system inside the PLF limits the payload temperature to below 50 C. Around 113 km the density of the atmosphere is reduced significantly, decreasing the density of gas particles that damage and heat the payload. The PLF can be jettisoned above this altitude, though the cargo still undergoes radiative and free molecular heating. An additional consideration is pressure: the payload bay must be continually vented during accent to prevent a buildup of the pressure within the payload bay.
Moving down the rocket, the Centaur contains a restartable engine powered by LH2 and LO2, paired with hydrazine (N2H4) thrusters to reposition the rocket during flight.
Lastly, and most dramatically, the he
And this isn’t even the heavy hitting rocket. No solid
rocket boosters, only one Centaur engine, and a smaller payload fairing (bay)
than the Atlas V 5xx series makes this the most petite offering from the United
Launch Alliance. (This is a video of the New Horizon launch, using 5 boosters…
it looks like a horse darting out of a gate.)
