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Topic: Why the different Apollo RCS fuels? (Read 1,919 times)

ka9q
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Re: Why the different Apollo RCS fuels?
« Reply #30 on Nov 22, 2011, 7:39am »

Nov 21, 2011, 10:38am, Bob B. wrote:

Also note that one of the big advantages of hypergols that you didnï¿½t mention is their good temperature range — they are easily storable.

Although these fuels are liquids at STP, you did recently point out that N₂O₄ has an uncomfortably high freezing point. It can present a thermal control challenge especially if you expect to be in eclipse for a long time. I suppose if that's an insurmountable problem there's always IRFNA.

N₂H₄ also has a high freezing point, and while the methylated hydrazines have much lower freezing points they're not suitable for monopropellant engines.

What about N₂O (nitrous oxide)? I'm familiar with its use as an oxidizer in hybrid rockets, especially amateur high power rockets because it is far less toxic and corrosive than the other oxides of nitrogen. Its main drawbacks are lower performance than N₂O₄ and a much higher vapor pressure at 20°C: 58.5 bar. It can be kept liquid at room temperature if you don't care too much about the weight of the tanks but I suspect that's a much bigger deal in a spacecraft than in a low performance amateur rocket. Still, it was used successfully in Space Ship One.

I had forgotten that N₂O also has a positive enthalpy of formation, meaning it can be used in a monopropellant thruster with a catalyst though the performance isn't great. Has anyone actually done this?

« Last Edit: Nov 22, 2011, 7:42am by ka9q »

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Bob B.

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Re: Why the different Apollo RCS fuels?
« Reply #31 on Nov 22, 2011, 10:05am »

Nov 22, 2011, 7:39am, ka9q wrote:

What about N₂O (nitrous oxide)?

I've done some past ISP calculations on N₂O and found that is not a particularly good performer as an oxidizer. It depends on the fuel it's burned with, but generally it is about 3% lower than IRFNA and about 7% lower than N₂O₄. If you're looking for a less toxic alternative, H₂O₂ actually looks better than N₂O in regard to performance. Peroxide seems to be comparable to IRFNA at 85% concentration and comparable to N₂O₄ at 95%.

Nov 22, 2011, 7:39am, ka9q wrote:

I had forgotten that N₂O also has a positive enthalpy of formation, meaning it can be used in a monopropellant thruster with a catalyst though the performance isn't great. Has anyone actually done this?

I've read a paper that promoted the use of N₂O as a monopropellant. It's better than H₂O₂ in this role but not nearly as good as N₂H₄. The real gist of the paper was to call attention to N₂O as a low toxicity alternative in dual mode systems.

A dual mode system is one having a bipropellant main engine and monopropellant thrusters for attitude control, where one of the two main engine propellants is also the monopropellant. Typically, hydrazine is used as the monopropellant and fuel; however, N₂O can be used as the monopropellant and oxidizer. Likewise, H₂O₂ can be a monopropellant and oxidizer. N₂O would perform better in the thrusters, but H₂O₂ would perform better in the bipropellant engine. Neither is as good as N₂H₄

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ka9q
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Re: Why the different Apollo RCS fuels?
« Reply #32 on Nov 22, 2011, 1:29pm »

H₂O₂ scares me. The stuff is just too damn unstable. Lots of chem labs have had serious accidents by accidentally synthesizing organic peroxides. (Many terrorists do it intentionally.) I think even the Germans, Japanese and Russians who tried to propel submarines and torpedoes with it have largely given up.

While it's easy to dilute with water to a non-toxic concentration, as 90+% rocket fuel it's probably as deadly as any other oxidizer.

Interestingly the FEMA HAZMAT threat-to-life classification for both H₂O₂ and N₂O₄ is 3. I would have expected 4 for the latter. Peroxide has a 2 rating for reactivity while it's 0 for N₂O₄. Naturally both have the OX qualifier.

« Last Edit: Nov 22, 2011, 1:29pm by ka9q »

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theteacher
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Re: Why the different Apollo RCS fuels?
« Reply #33 on Nov 23, 2011, 5:53pm »

Nov 22, 2011, 1:29pm, ka9q wrote:

H₂O₂ scares me.

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ka9q
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Re: Why the different Apollo RCS fuels?
« Reply #34 on Nov 23, 2011, 7:19pm »

That cartoon really has been making the rounds lately. I don't know anyone who hasn't gotten it pretty quickly.

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raven
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Re: Why the different Apollo RCS fuels?
« Reply #35 on Nov 25, 2011, 9:39am »

While the concept of that joke is funny, my sense of comedic timing is enraged.

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ka9q
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Re: Why the different Apollo RCS fuels?
« Reply #36 on Nov 25, 2011, 9:59am »

How so?

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raven
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Re: Why the different Apollo RCS fuels?
« Reply #37 on Nov 25, 2011, 11:12am »

You could cut at least three panels and a line of dialogue and make it funnier in the process. Say, cut from the scene of them taking a sip to the chemist standing beside a grave marker. If you still want the "idiot" line, you could have the dead guys ghost looking morose over the grave with the chemist telling him off.
Enraged is probably too strong a word, but it could be punchier.

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ka9q
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Re: Why the different Apollo RCS fuels?
« Reply #38 on Nov 26, 2011, 12:38am »

According to Bob B's page, straight N₂O₄ has a freezing point of -9°C while MON-3 freezes at -15°C and MON-25 at -55°C. That last one is quite an improvement, but what is the effect of all that NO on performance as an oxidizer?

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