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Even if you don't care about hypergolicity, there's the serious problem that most storeable chemical fuels are either horrifically toxic (like N2O4), dangerously unstable (like high-test peroxide), or both (like hydrazine).

LOX does not have this problem, but as a cryogen it is difficult to store, and it also has handling hazards due to its tendency to turn flammable or combustible materials into contact explosives.

On the other hand, cold gas thrusters have abysmal Isp and arcjets require nuclear-tier power sources.

But then there's hydrocarbon (possibly propane or butane, since these are H-heavy and can be stored in pressure tanks at reasonable mass ratio for small quantities) with N2O.

While this doesn't look like a high performance fuel, it seems like it would be pretty good from the standpoint of safety. Is this indeed the case, and does it have any deal breaker like unmanageable combustion?

(I'm specifically interested in cases like RCS thrusters and orbital maneuvering thrusters where liquid-fuel levels of controllability are necessary.)

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Substances equivalent in hazard to the hydrocarbon are available at any gas station. It's the oxidizer that's an issue.

Gaseous nitrous oxide is used as a propellant in some aerosol cans with rather low pressures. However, there's a variety of hazards that appear at higher pressures, or when using the liquid form. Under certain conditions, the gas can undergo explosive decomposition to hot nitrogen and oxygen. It can also form explosive mixtures with contaminating greases or oils, and when dealing with the liquid, the vapor pressure is unusually sensitive to temperature. One relevant incident would be an explosion at a test site for the RocketMotorTwo used in Virgin Galactic's SpaceShipTwo in 2006 which killed 3 and injured 3 others.

More info:

It's certainly more convenient than liquid oxygen, but it seems arguable whether it's actually safer, and the convenience may lead to a false sense of safety.

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  • $\begingroup$ I am mainly comparing it to nitrogen tetroxide, high-test peroxide, and RFNA. $\endgroup$
    – ikrase
    Mar 1, 2020 at 2:26
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Yes. It is the oxidizer of choice of more advanced hobbyist rocket builders, like the Copenhagen Suborbitals. It's not entirely safe, but it's way safer than most alternatives. Its performance is deal-breaker in professional rocketry - 1 oxygen per 2 inert nitrogens, vs 4 for N2O4, and it gives you most of the headaches of liquid propellant (proper mixing in combustion chamber, pogo effect, ullage...) while removing only the ones connected with handling extremely hazardous materials (vs only moderately hazardous...).

For the 'lower tier' rocketry hobbyists, there's KNO3, potassium nitrate. Even worse performance, but better stability and safety, and being a solid allows building much easier solid rocket motors.

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  • $\begingroup$ Has Copenhagen been using liquid bipropellants? I somewhat doubt if hybrid rockets are usable for things like RCS thrusters. A lot of maneuvering-type applications seem pretty inevitably married to fully liquid fuel. $\endgroup$
    – ikrase
    Feb 28, 2020 at 10:11
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    $\begingroup$ @ikrase: RCS usually runs on monopropellant. It has completely different priorities - efficiency is far less important than restartability and precision. Sometimes it's cold gas. It's also not entirely rare for RCS to be some puny ion thrusters way too weak to use as main propulsion - probably more frequent than bipropellant RCS. $\endgroup$
    – SF.
    Feb 28, 2020 at 11:29
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    $\begingroup$ Copenhagen suborbitals are using LOX - alchocol bipropellant. Virgin Galactic is using Nitrous oxide. $\endgroup$
    – WOW 6EQUJ5
    Feb 28, 2020 at 14:04
  • $\begingroup$ Nitrous oxide is N2O...1 oxygen per 2 nitrogens. It does also contribute some additional energy because its decomposition is exothermic, and can be used as a monopropellant. $\endgroup$
    – Christopher James Huff
    Feb 28, 2020 at 14:48
  • $\begingroup$ @SF. This is somewhat true, but unless my research has mislead me both the Apollo Program and the Space Shuttle used hypergolic bipropellants for their RCS systems, and for cases where cold gas is insufficient (since IIRC even peroxide gives much better ISP). $\endgroup$
    – ikrase
    Mar 1, 2020 at 2:26

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