Chair Force Engineer

Friday, September 21, 2007

Hybrid Theory

Over at Transterrestrial Musings, Rand Simberg speculates that Scaled Composites might switch to a LOX-Solid hybrid, or a purely-liquid engine, for SpaceShipTwo. While I believe that both are good ideas, I don't expect either to come to fruition.

I studied hybrid rockets extensively as a senior in college. As a result, I came to question the benefits of hybrid rockets. True, the motors are simple, the propellants are cheap, and the ground handling of the fuel is much safer than a traditional solid rocket. As far as I can see, the benefits end there. Specific Impulse is low, and does not exceed solid rockets unless cryogenic liquid oxygen is used as an oxidizer.

Why choose a hybrid over another propulsion system such as a simple, pressure-fed liquid rocket? I tend to think that most hybrid proponents are people who are afraid of handling cryogenic liquids. These hybrid proponents insist on using non-cryogenic oxidizers like nitrous oxide; this may make the rocket marginally safer than a LOX-solid hybrid, but it dramatically reduces the specific impulse of the rocket motor.

I disagree with these people (probably including the likes of Burt Rutan) who feel that cryogenic propellants are too unsafe to use for space tourism vehicles that will fly on a weekly basis. In the Air Force, it's routine to see 19-year-old Airmen who are fresh out of tech school and loading liquid oxygen into military aircraft. I see no major obstacles towards using LOX-Kerosene for space tourism vehicles.

If Scaled Composites cannot pinpoint the cause of the tragic nitrous oxide explosion that claimed the lives of three employees, I still don't foresee Burt Rutan & co. giving up on nitrous hybrid systems. Instead, they'll be talking to SpaceDev or Environmental Aerosciences Corporation (the two vendors who competed for the SS1 motor, with SpaceDev developing the successful flight motor,) to join in on the SS2 motor development.