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This may be a silly question, but I don't know how to approach an answer myself and I can't seem to find the right search terms to input.

A simple rocket might be a compressed air bottle, where a reaction does not take place and the exhaust gas is the same form as the exhaust itself. Thrust is produced because the air wishes to neutralize its pressure with the ambient, but this could be supplemented by a gas container that dramatically increases the pressure on the exhaust gas (or whatever form of matter), thus increasing exhaust velocity. There are many ways to do this, like increasing the temperature of the exhaust gas while maintaining volume, but I'm thinking of a complicated mechanical vessel or maybe a "sac" that can forcefully constrict and shrink.

Is there any case for hypothetical rocket engines that "squeeze" material in this way, rather than combustion or nuclear engines? Are there any sources I can read that might discuss it?

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  • $\begingroup$ Are thinking of a water bottle rocket? There are more complicated versions where you add something like dry ice or liquid nitrogen, that increases the air pressure a lot when it contacts water. But they are not as straightforward to make. $\endgroup$
    – mmesser314
    Aug 7 '20 at 20:00
  • $\begingroup$ @mmesser314 I'm thinking about a rocket design where we have some material for exhaust and the container-vessel itself does "squeezing," forcing the exhaust material through some small aperture producing thrust. The idea is vaguely familiar to me probably through some bit of science fiction I think I read. I think it was a mercury-containing vessel, but the vessel itself might've been some kind of active material or nanotechnology material that could constrict in on itself. $\endgroup$
    – BMF
    Aug 7 '20 at 20:23
  • $\begingroup$ So something like a spring ejecting a mass out the back.Or maybe a spring pushing a piston, which pushes water out the back. $\endgroup$
    – mmesser314
    Aug 7 '20 at 21:57
  • $\begingroup$ @mmesser314 maybe similar to, in a mathematical respect? But, no, not at all. Think of a balloon or some kind of sac filled with exhaust material being compressed--pressed upon by all sides--forcing exhaust through some aperture in an exhaust cone and yielding thrust. $\endgroup$
    – BMF
    Aug 7 '20 at 22:00
  • $\begingroup$ Instead of the exhaust material being forced out the back by some combustion or nuclear or other reaction, it's forced out by pure mechanical pressure. I suppose this could take many forms, like a piston pushing into a compressed cylinder with a tiny fluid aperture. I think the idea of squeezing material/fluid from a sac is a good example of the general idea. $\endgroup$
    – BMF
    Aug 7 '20 at 22:06
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the best example is a little CO2 cartridge, as used in pellet guns and to charge soda water bottles. These have pressurized gas inside them and a constricted nozzle. They make great rocket motors.

Amateur rocket people use this type of motor to propel motorcyclists like Evel Kneivel into the air; in this case the pressure vessel is filled with extremely hot water. opening the nozzle allows the water to flash into steam and a powerful rocket engine is the result, with the high-speed steam developing the thrust.

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Like a can of compressed Air or CO2? I've read warnings on spray cans to that effect. To my understanding Diet Coke & Mentos is purely physical as well, though of course it would be a ticklish matter to power a water rocket in this manner, what with overturning things fast enough.

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