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In John D. Clarke's Ignition! (1972), the author spends the last chapter making predictions about the future of liquid rocket propellants. I thought these were very interesting, but I realize the book is decades old. So what's the status of these predictions now, based on contemporary works?

  • Chemical rockets will never have more than 600 seconds specific impulse.
  • Storing free radicals in propellant to defeat this limit is impractical.
  • ~500km range rockets will use chlorine pentafluoride and a hydrazine derivative.
  • Monopropellants, gels, and slurries will not be used in main propulsion.
  • Long-range missiles will use N2O4 and hydrazine for the foreseeable future.
  • First-stage space boosters will use liquid oxygen and RP-1, even with reusable boosters.
  • Upper-stage boosters will use J-2 hydrogen-oxygen. The final stage may use hydrogen-fluorine or hydrogen-lithium-fluorine.
  • Past this point, nuclear rockets will have to come into play.
  • Lunar landers and other extraterrestrial modules will use N2O4 and hydrazine.
  • Deep space probes will use methane, ethane, and diborane, possibly propane, with OF2, ONF3, or NO2F as oxidizers. Perchloryl fluoride (ClO3F) will be useful out to Jupiter.
  • Hydrogen peroxide will be used as a monopropellant, but not as an oxidizer for main propulsion.
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    $\begingroup$ I guess it's of interest that this book is from 1972. $\endgroup$ – DarkDust Dec 29 '16 at 21:50
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Chemical rockets will never have more than 600 seconds specific impulse. Storing free radicals in propellant to defeat this limit is impractical.

Validated. Chemical rockets in use top out at 450-460 seconds, with a demonstrated test-stand record of 542 seconds.

~500km range rockets will use chlorine pentafluoride and a hydrazine derivative.

As far as I know, most rockets in that range are using hydrazine derivatives with NTO (N2O4) or MON oxidizer, or are solid fueled. Chlorine and fluorine compounds are not in wide use.

I don't know why Clark thought anyone would use ClF5/hydrazine; its specific impulse is comparable to kerosene/LOX and its bulk density is quite good, but it's finicky and dangerous. He goes into detail about the hazards of the similar ClF3, and notes that chemists and engineers have very different attitudes towards these things.

Monopropellants, gels, and slurries will not be used in main propulsion.

Correct.

Long-range missiles will use N2O4 and hydrazine for the foreseeable future.

US likes solids in ICBMs for their relative safety. Russia used to use UDMH/N2O4 liquid fueled ICBMs but seem to also be preferring solids nowadays.

First-stage space boosters will use liquid oxygen and RP-1, even with reusable boosters.

Again overlooking solids, which are more cost effective.

Upper-stage boosters will use J-2 hydrogen-oxygen. The final stage may use hydrogen-fluorine or hydrogen-lithium-fluorine.

Fluorine is just too unsafe to be worth its small specific impulse advantage. Upper stages generally use hydrogen.

Past this point, nuclear rockets will have to come into play.

Politically infeasible currently.

Lunar landers and other extraterrestrial modules will use N2O4 and hydrazine.

Yes.

Deep space probes will use methane, ethane, and diborane, possibly propane, with OF2, ONF3, or NO2F as oxidizers. Perchloryl fluoride (ClO3F) will be useful out to Jupiter.

Again, Fluorine has been rejected. Deep space probes use hydrazine/NTO or electric propulsion such as ion engines (gaining an order of magnitude in specific impulse instead of the marginal increase possible from fluorine, at the price of very low thrust, which is acceptable for certain interplanetary missions).

Hydrogen peroxide will be used as a monopropellant, but not as an oxidizer for main propulsion.

True.

Overall it's a decent prediction record, mostly overestimating the industry's tolerance for very hazardous propellants. Solids partially displaced more efficient liquid hypergolics, and fluorine never supplanted LOX, for a combination of interrelated safety, environmental, and cost concerns.

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    $\begingroup$ Good point on the solids. The author admits he's a liquids specialist, and that probably swayed him bit. $\endgroup$ – bright-star Dec 29 '16 at 21:18
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    $\begingroup$ It seems the main "miss" was failure to predict electric propulsion and its notable place in the "final stage" territory. $\endgroup$ – SF. Dec 29 '16 at 23:38
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    $\begingroup$ Ion thrusters were demonstrated in space use as early as 1964, with thousands of run-hours demonstrated in 1970, but I think Clark overlooking solids was more surprising given the success of the Titan IIIC from 1966 on. $\endgroup$ – Russell Borogove Dec 30 '16 at 0:19
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    $\begingroup$ Having skimmed the text, I see he did mention ion propulsion, claiming he was unqualified to discuss it in detail. He also says his predictions are for the liquid propellant realm. $\endgroup$ – Russell Borogove Dec 30 '16 at 4:48
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    $\begingroup$ Yeah, I noted the other day that Cassini's main thrusters are in fact modernized versions of the attitude control thrusters used on Apollo. $\endgroup$ – Russell Borogove Sep 17 '17 at 22:26

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