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In most of chemical propulsion engines the product of burning fuel and oxidizer is the propellant; in some cases the ratio between the two is non-ideal, in which case some unburnt fuel acts as propellant as well.

OTOH, in concepts like NTR, or with RTG-powered ion drives, the two concepts - fuel and propellant - are entirely distinct. The nuclear fuel provides energy; the inert propellant is the reaction mass. Another trivial example is from amateur rocketry - the compressed air rocket, where compressed air is the energy carrier, but water expelled by air pressure is the main propellant.

Are there any designs of chemical propulsion though, that would use some low (-to-none) energy density propellant of a very high thermal expansion ratio, together with fuel and oxidizer that when combined provide a lot of thermal energy but not nearly as much exhaust velocity and/or thrust?

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    $\begingroup$ Probably not what you mean, but technically any chemical bipropellant where the mixture ratio is not stoichiometric would qualify; hydrolox engines are often run at 4:1-6:1 instead of stoichiometric 8:1, so a lot of unburnt hydrogen passes through. yarchive.net/space/rocket/fuels/fuel_rich.html $\endgroup$ – Russell Borogove Mar 28 '16 at 17:18
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    $\begingroup$ Personally, I was thinking about amateur rocketry engine using acetylene and oxygen (easily obtainable welding gases) as energy source, plus water or other mostly inert propellant. Acetylene has excellent energy density but it doesn't produce much exhaust. $\endgroup$ – SF. Mar 28 '16 at 22:38
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    $\begingroup$ @Andy Nuclear Thermal Rocket, in short: inert propellant pushed through a nuclear reactor, evaporating/expanding due to extreme heat. ISp around 800s, decent thrust, never launched. And yes, the water in jets would be a perfect example - except not a rocket :) $\endgroup$ – SF. Mar 29 '16 at 14:39
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    $\begingroup$ Did some reading. Forget my comment about water injection, I was wrong; seems it's only used to cool the (jet) engine to allow higher takeoff power. Also for rockets, I suspect adding ballast mass would just mean the rocket exhaust giving some of its chemical energy to this mass, so there would be no overall gain. (At a guess.) $\endgroup$ – Andy Mar 29 '16 at 15:23
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    $\begingroup$ @Andy: The engine is limited by thermal durability of its components. With very high energy density propellants of lousy expansion rate it would melt. Imagine a thermite-powered rocket. One way to make it work: add water. $\endgroup$ – SF. Mar 29 '16 at 19:38
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The V2 rocket used 75% ethanol / 25% water for fuel. Purer ethanol was available but led to combustion temperatures that were too high; diluting the fuel limited the temperature so the engine could survive the burn but also changed the rocket's thrust properties.

Aerojet has demonstrated thrust augmentation of a (simulated) NTR by injecting liquid oxygen into the hot H2 stream in the expansion nozzle. ISP drops but thrust/weight increases; varying the LOX mass ratio allows throttling of the ISP vs thrust until the LOX is expended.

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John D. Clark's book Ignition! (PDF available here) has at least two examples, although in each case the fuel/oxidiser combination provides a good deal of thrust on its own.

  1. On pages 177-179 of the book (pages 193-195 of the PDF), he describes some work on adding considerable oomph to a rocket (at the cost of specific impulse) by injecting liquid mercury (!) into the combustion chamber (apparently, this grew out of an idea to use dimethylmercury (!!!!!) as fuel, which was rejected for a number of reasons, not least the inability to find anyone willing to supply them with sufficient quantities of the toxic death juice). This dropped the specific impulse way down, but provided a considerable power boost. (Apparently the author did the calculations for the mercury-injection rocket, and submitted the proposal for such, in the expectation that their superiors would reject it as insane; it was accepted, however, "and NARTS, horrified, was stuck with the job of firing a mercury-spewing motor in the middle of Morris County, New Jersey." Fortunately, it got reassigned to a lab somewhere in the middle of the desert Southwest before they got to the actually-firing-the-damn-thing stage. Which did, in fact, provide the expected power boost.) How one would launch anything with such a rocket without poisoning everyone in the micropolitan statistical area is left unstated.
  2. On pages 188-189 of the book (204-205 of the PDF), he describes a Rocketdyne hydrogen-lithium-fluorine rocket that, when running with a stoichiometric lithium-fluorine ratio, "and injected hydrogen to make up 30 percent of the mass flow" (presumably indicating that the hydrogen was intended mostly as propellant, rather than reactant), managed to achieve a specific impulse of 542 seconds, the highest specific impulse ever achieved by a chemical rocket (a record that still stands), presumably due to having all that very light (and, thus, very-fast-moving) hydrogen as propellant on top of having a very energetic reaction to start with.
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