Primary Propulsion

Solid Propellant Systems

A typical solid propellant main engine on a spacecraft looks like the system shown in Figure 9.3. In concept, it is a simple device, composed of a tank containing solid fuel, an engine nozzle, and some kind of pyrotechnic device to light it. Just like the large space shuttle solid propellant booster rockets (it maybe helpful to recall the material associated with Figure 5.2 in Chapter 5), once lit, it will burn until the propellant is exhausted, and then it becomes

Figure 9.3: Diagram of a typical solid propellant primary engine.

inert and plays no further role in the spacecraft's mission. The engine burn is usually of short duration (a few tens of seconds) and high thrust (tens of thousands of Newtons), producing high accelerations—typically a 0 to 60 mph in 1 second kind of performance! In our orbit transfer example from LEO to GEO in Figure 9.1, the engine burn at point 2 has been performed many times with this kind of rocket engine. To get the right A Vat this point, the mission analysis team has to do careful calculations, using the rocket equation, to determine how much propellant is needed. This precise amount is then loaded into the motor. Despite the simplicity of the system, one major disadvantage of using a solid propellant motor is the one-shot characteristic; multiple rocket engine burns are not possible.

Liquid Bi-Propellant Systems

Over time, spacecraft missions have become more sophisticated, requiring more than a single primary engine burn. To accommodate this requirement, a unified liquid bi-propellant system has become increasingly popular as an alternative to the solid propellant systems. The system is unified in the sense that the spacecraft's primary engine, and the small thrusters that deal with orbit and attitude control are on the same circuit. The big and small rockets onboard the spacecraft share the same fuel supply. The propellant used is in liquid form, and it is a bi-propellant because there are two liquids—a fuel and an oxidizer. The commonly used fuel and oxidizer are monomethylhydrazine and nitrogen tetroxide, respectively. These liquids are hypergolic, which basically means that if we mix them together, they spontaneously explode! So to fire the main rocket engine, for example, all we have to do is feed the fuel and oxidizer into the rocket's combustion chamber (see Figure 5.3), and a hot gas is produced explosively, which is then exhausted through the engine nozzle to produce thrust. The same principle is used to operate the small thrusters on board the vehicle. The liquid propellants are held under the pressure of a gas, such as helium, so that the feed system in this case operates by squeezing the liquids down the fuel lines under this gas pressure. Of course, the hypergolic character of the fuel/oxidizer mix means that the plumbing associated with the propulsion system is complex, in order to ensure that the two liquids do not mix before they get to the combustion chambers of the thrusters. The consequences for the spacecraft were this to happen would be catastrophic! Figure 9.4, which shows an example of the plumbing associated with such a system, gives a good idea of this complexity. There is also the issue of the safe handling of the fuel and oxidizer at the launch site, when the spacecraft's fuel tanks are being loaded. These workers have to be equipped with protective, pressurized suits to safeguard them from the effects of an accidental escape of these nasty substances.

Propellanttank

Propellanttank

gas tank

nozzle

Figure 9.4: A diagram of the unified liquid bi-propellant propulsion system used on the European Space Agency (ESA) Venus Express spacecraft. As well as the main elements that are labeled, there is also an array of valves, sensors, and regulators to manage the hypergolic fuel safely. (Image credits: left—courtesy of EADS Astrium; right—courtesy of ESA.)

High pressure gas tank

nozzle

Figure 9.4: A diagram of the unified liquid bi-propellant propulsion system used on the European Space Agency (ESA) Venus Express spacecraft. As well as the main elements that are labeled, there is also an array of valves, sensors, and regulators to manage the hypergolic fuel safely. (Image credits: left—courtesy of EADS Astrium; right—courtesy of ESA.)

The main engine used in unified liquid bi-propellant systems usually has a much lower thrust than solid propellant primary engines. A thrust level of around 400 Newtons is common, giving a 0 to 60 mph in 2 minutes type of performance for an average-sized spacecraft! Consequently, the duration of an engine firing is often much longer, up to 1/ hours, to give enough time for the required AV to be achieved.

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