Use of Hypergolic Thrusters
Hypergolic propellants are a fuel and oxydizer which ignite
spontaneously on contact with each other. The most commonly used
hypergolics in spacecraft today are monomethyl hydrazine and nitrogen
Hypergolic propellants avoid the need for an ignition system for the
thrusters. If something is going to fail, it probably will be an ignition
system; and you
don't want a thruster failure when you're maneuvering to dock with another
vehicle. There's also the issue of responsiveness of the system to command
inputs -- an ignition sequence could require several seconds -- and the amount
of fuel used when you're repeatedly relighting an LH2-LOX thruster. The
ignition system would either be a spark plug that will burn out the first time
you ignite the thrusters, a spark injector that's prone to failure and slow to
start, or a hypergolic mixture injected onto the combustion plate just as the
primary fuel starts to arrive. You can see that for jets we'll use hundreds of
times during the mission, we wind up with a hypergolic system anyhow.
We don't use cold-gas thrusters because they're so wimpy compared to the
of fuel used to maneuver the vehicle. A heated gas system would be new
technology development, with the additional complexity of maintaining the
temperature and pressure of the system. I don't know if heated gas would work;
but my gut feeling is that it would be a dead-end research project.
So, we ought to stick with what all the rocket scientists who have come
us have learned in 40 years of spacecraft development. With hypergolics, you
just need to tie your controls into a system that opens two valves (one fuel,
one oxydizer), and poof, you get a thrust vector you can predict to three
decimal places controlled to a hundredth of a second. Remotely operated valves
are very reliable and available off the shelf. Spaceship parts are pricey, but
lots cheaper than new development.
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