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 tetroxide.

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 amount 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 before 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.