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There is a fundamental limit to the specific impulse of a chemical rocket because the propellant is also the fuel so you are limited by the fuels energy density. What if you burnt some fuel in a closed cycle, and then through the use of heat exchangers heated up the rocket exhaust? The exhaust temperature would be higher, so the exhaust velocity would be higher and you would get a higher specific impulse. You would then need to crack the spent fuel from the closed cycle back into fuel and oxidizer before the next thrust maneuver. This could be done with solar or RTG. Though this would be less efficient then hooking the power source directly to an ion drive, it seems like the bursts of thrust would allow more efficient orbital maneuvers. Has this idea ever been studied, or are the thermodynamic losses to to much to have any feasibility?

Lex
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  • Capturing exhaust back would slow down the rocket. The rocket is accelerated by the exhaust with very high speed. Stoping the exhaust to recapture it would decelerate the rocket. Studying this idea would only be a waste of time. – Uwe Nov 13 '17 at 19:19
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    This is akin to running a fan in a sailboat to blow on the sails. – Organic Marble Nov 13 '17 at 19:39
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    Also "through the use of heat exchangers, only expelled a heated up fraction of it" sounds like it violates the 2nd law of thermodynamics. You can't take a mixture at one temperature and use passive heat exchangers to make some of it hotter and the rest of it colder. – pericynthion Nov 13 '17 at 20:58
  • @Uwe Maybe "recapture" is poor wording a fraction of the fuel would be in a closed cycle so it never actually escapes to begin with. This would be used to heat the exhaust beyond what is possible in a standard open cycle. – Lex Nov 13 '17 at 21:45
  • @pericynthion I admit the thermodynamics hurt the efficiency a lot, but it could be done without violating any laws, the simplest way is by exchanging heat between a hot exhaust (in a closed cycle) and cold fuel/oxidizer. Now you have warm fuel/oxidizer which you then burn to end up with extra hot exhaust which is expelled for trust. – Lex Nov 13 '17 at 21:45
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    I also don't see why active heat exchange would necessarily be impossible in this situation. – Lex Nov 13 '17 at 21:51
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    The exhaust from the combustion chamber in real rocket engines is so hot that active cooling or ablative measures are needed to prevent burning through the nozzle. I don't think adding extra heat by carrying around a complicated chemical engineering plant is really going to do you any good at all. The opposite, in fact. How much extra propellant could you carry for the mass of that chemical engineering plant? – Organic Marble Nov 13 '17 at 22:50
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    Since you are using the same fuel and oxidizer the adiabatic fame temperature will be the same/similar and therefore the temperature gradient will be small. Keeping the pressure higher in the closed cycle might improve this, but I don't think a big improvement could be made this way. Maybe the fuel could be used in a fuel cell to generate electricity, which could be used to heat the exhaust gasses. – fibonatic Nov 13 '17 at 22:55
  • @OrganicMarble, Lockheed's Mars Base Camp already plans to crack water to fuel the lander, so the chemical engineering plant could be free. The mass-flow could be far lower than an earth to orbit engine which would reduce heating. As an example, the RL-10's nozzle extension is only radiatively cooled. The goal is to have an engine with enough thrust to exploit the Oberth Effect, not something to get to orbit. – Lex Nov 13 '17 at 23:50
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    Problem with additional heating of the exhaust gases using the same fuel in closed cycle is that while you could technically get better specific impulse for engine, you also should accelerate bigger mass, because fuel used for overheating is not exhausted the same way as overheated propellant. – ZuOverture Nov 14 '17 at 03:08

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