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It's become a sad and oft-demoralizing truism that it's quite difficult and expensive to design or build even low-performance liquid-fueled rocket engines, and that amateurs -- even those with skills in machining, technical plumbing, etc -- are doomed to be disappointed.

But I wonder if this applies to the use of small rocket engines -- when you have the engines, how hard is it to, say, assemble them into a workable RCS system or the like?

Is this a "tube cutter and Swagelok catalog" task?

(A motif that's been common in science fiction is the hypergol or monopropellant powered "scooter" with fractional-G acceleration and minimal instruments built out of spare parts by a teenager -- can this be dismissed as another thing that's flatly unrealistic?)

(Answers that are about the legality of owning this stuff are not on topic.)

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Hydrazine monopropellant engines are stunningly easy to use. They do emit ammonia (and some minor portion of hydrazine) - so not trivial to use terrestrially - you need to pump and dispose of that crud in some way. But otherwise, you pre-heat the catalyst bed, open the valve, and away you go. Same with AFM, LMP - just the cat bed temperature is higher. Realistically, all you need is a valve, a pressure vessel with hydrazine, a thruster, and some sort of controller with valve driver, telemetry, etc for a minimal system.

Real combustion systems - rather than hydrazine or other monopropellant catalyst-based decomposition systems - are much more complicated and have igniters, important valve timing and throttle tables, etc. This becomes somewhat obvious when you look at things like the flammability triangle. You need to keep your O/F ratio combustible the whole time as both propellant and oxidizer ramp.

When you throw in pumped systems, supercooled systems, autogenous systems - rather than pressure-feed - I'm sure you can imagine that the complexity again rises.

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    $\begingroup$ I'm very much thinking of exclusively pressure-fed engines. What are small hypergolic pressure-fed biprop engines like? Are valves often integrated with the engine -- sounds like more complicated than "turn two identical solenoid valves on simultaneously". $\endgroup$ – ikrase Dec 10 '20 at 4:39
  • $\begingroup$ Definitely the toxicity of hypergol is easier when you can just go outside (and already have a spacesuit.) $\endgroup$ – ikrase Dec 10 '20 at 4:39
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    $\begingroup$ Yes, valves are integrated into the engine (typically double seat), because of the typical number of valve requirements for dealing with hydrazine. But also latch valves are used upstream. For monoprop it really is "crack em all at once." For bi-prop, it's just an offset. (I say this only knowing anything about hydrazine biprop...other propellants, eh). $\endgroup$ – phyllis diller Dec 10 '20 at 5:30
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    $\begingroup$ Oh and...welds, hermeticity, & cleanliness are really important for this kind of thing. So maybe not off-the-shelf Swagelok level. Additionally, operation in vacuum can be important because of oxidation effects at high temps. $\endgroup$ – phyllis diller Dec 10 '20 at 5:42
  • $\begingroup$ I was under the impression that swagelok was pretty good at that. $\endgroup$ – ikrase Dec 11 '20 at 13:08

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