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Raiz Aerospace raised an interesting question during one of his KSP Realism Overhaul play throughs.

If the RCS propellant Cavea-B is better than hydrazine and less toxic why does no one use Cavea-B, while hydrazine is a defacto industry standard for RCS and orbital maneuvering systems.

Russell Borogove
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DJ319
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    What's the basis for the claim that it's "better than hydrazine"? – Organic Marble Aug 20 '19 at 16:27
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    @OrganicMarble Specific impulse ~280 s, compared to monoprop hydrazine ~220 s. – Russell Borogove Aug 20 '19 at 22:38
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    Another consideration is that Monoprop is often used as attitude adjustment, rather than reaction mass. Thus squeezing out every m/s of delta V isn't as important as having a simple and reliable system that points you to where you want. Since you mentioned KSP, I have never depleted the monopropellant tanks of my spacecraft in what I'd call 'normal operation' (i.e. outside of doing dumb stuff on-purpose) and KSP has a monoprop Isp way lower than real life (100 s I believe). – Ingolifs Aug 21 '19 at 06:08
  • The RCS fuels used in the Realism Overhaul mod suit are meant to have realistic attributes, density specific impulse etc. – DJ319 Aug 21 '19 at 11:40

3 Answers3

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Monopropellant systems such as catalyzed hydrazine thrusters are attractive at very small sizes, where the simplicity of a single propellant tank outweighs their relatively low performance.

According to Wikipedia, Cavea-B requires a small amount of UDMH or a similar hypergolic to begin ignition -- every time you want to fire it, which can be a frequent occurrence for RCS thrusters.

This means you need a UDMH tank and a set of valves, so you lose the simplicity advantage of a monopropellant, and the requirement to store and handle UDMH negates the non-toxicity advantage.

At that point you might as well switch to a NTO/UDMH or NTO/MMH bipropellant thruster, which are very mature technologies and outperform Cavea-B.

Russell Borogove
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    This answer seems so much more appropriate than the other one; which seems to end with the conclusion that monopropellants are potentially dangerous; not that hydrazine would be preferred over Cavea-B. – JMac Aug 20 '19 at 18:49
  • Ah I see. When I read the article on Wikipedia I understood it to mean that it needed a constant supply of UDMH. Now that you point it out I see it's only for ignition making it harder to decide how much UDMH you need to carry and going for a true Bi Propellant makes more sense. Especially if that Bi Propellant is more efficient. Although you do end up with a more toxic fuel. – DJ319 Aug 21 '19 at 11:47
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    “More/less toxic” is moot. Once you plan to have any amount of hydrazine/UDMH/similar at the launch site, you need safety protocols, you need hazmat suits, you need decontamination facilities, and so on; to a first approximation it doesn’t matter if you’re using a liter of it or a kiloliter. – Russell Borogove Aug 21 '19 at 16:11
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John D Clark, Ignition!, p.165, on the testing of a Cavea B motor:

“Well, through a combination of this and that, the motor blew on startup. We never discovered whether or not the [detonation] traps worked —we couldn't find enough fragments to find out. The fragments from the injector just short-circuited the traps, smashed into the tank, and set off the 200 pounds of propellant in that. (Each pound of propellant had more available energy than two pounds of TNT.) I never saw such a mess. The walls of the test cell—two feet of concrete—went out, and the roof came in. The motor itself—a heavy, workhorse job of solid copper— went about 600 feet down range. And a six-foot square of armor plate sailed into the woods, cutting off a few trees at the root, smashing a granite boulder, bouncing into the air and slicing off a few treetops, and finally coming to rest some 1400 feet from where it started. The woods looked as though a stampeding herd of wild elephants had been through.

“As may be imagined, this incident tended to give monopropellants something of a bad name.”

Martin Kochanski
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    WOW! That's quite a spectacular failure. – DJ319 Aug 20 '19 at 13:26
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    So was that the only attempt at testing this fuel. I mean that's a spectacular failure but the benefits of Cavea B would seem to warrant further development surely? – DJ319 Aug 20 '19 at 13:28
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    Thus proving “Cavea-B is better than hydrazine” … at cutting down trees! – Jacob Krall Aug 20 '19 at 16:26
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    @DJ319: From Ignition, p 158: "Cavea B was the winner, and seemed to be the ideal monopropel- lant. And by the end of the year [1959?] it had been fired successfully by NARTS, GE, Wyandotte, and Hughes Tool, with JPL soon to follow. It performed very well in a motor, yielding 94 or so percent of the theoretical impulse with a comparatively small chamber." – Dennis Williamson Aug 20 '19 at 18:10
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    The conclusion this reaches seems to just be "incident tended to give monopropellants something of a bad name"; but why wouldn't that include hydrazine? – JMac Aug 20 '19 at 18:42
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    @JMac, hydrazine is a catalyzed monopropellant: in the absence of the catalyst, it won't decompose. Cavea-B, on the other hand, is a classic monopropellant: once you ignite it, the reaction is self-sustaining. This means that a Cavea-B engine needs some mechanism to keep the reaction from burning up the fuel pipe into the tank, while hydrazine just requires you to not build the fuel pipe out of iridium. – Mark Aug 20 '19 at 21:39
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    This sounds worthy of an entry in Things I Won't Work With, alongside FOOF. – Graham Aug 20 '19 at 22:41
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    @Graham The whole of Ignition! Is basically a rocketry 'things | would be very reluctant to work with'. At least real rocket scientists never went as far down the Florine chemistry for energetic rockets route as Clarke had thought they would. – Dan Mills Aug 21 '19 at 11:30
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    @DanMills I've just hit Google for it. Turns out it's out of print, but a PDF is available at https://library.sciencemadness.org/library/books/ignition.pdf so that's my reading material for the next couple of evenings! – Graham Aug 21 '19 at 13:09
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    @Graham - it is not out of print! Brought back in a very pretty little paperback - https://www.amazon.com/Ignition-Informal-Propellants-University-Classics/dp/0813595835/ref=sr_1_1?keywords=ignition&qid=1566415361&s=gateway&sr=8-1 - I quite enjoyed it (learned about it in the first place in a Q&A elsewhere on this stack - or maybe it was the Chemistry stack ...) – davidbak Aug 21 '19 at 19:23
  • Would have been good for a military missile as you might not need a warhead! – ikrase Mar 01 '20 at 10:59
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I think it comes down to this: you need both attitude control and thrust for a spacecraft to be able to do its job, and Cavea-B is only good at one of those jobs. Hydrazine, though less efficient, can power both your attitude control system and your primary thrusters, and so you only need one big pressurized tank. I do think it could be used for orbital maneouvers in things like space tugs since it's storable, dense and almost as good as bipropellant systems, but since there's no space tugs the point is, as of now, moot.

Tomas Boncompte
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