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To travel once in space, not only using gravity as a method of propelling, could pressurised gases work? The ubiquity of gases such as helium or hydrogen being brought into the craft, once compressed could they in turn be used to release pressure in a manner to generate thrust? The limited friction could enable gradual inclines of release to build momentum each time for speed. Reserving solid fuel purely for launch and recovery.

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  • $\begingroup$ I'm not quite sure what you're proposing here. Are you suggesting an interstellar ramjet variant? $\endgroup$ – Nathan Tuggy Dec 6 '17 at 0:32
  • $\begingroup$ Not quite. Similar to how earlier submarines experimented with compressed air for propulsion (before returning to conventional diesel, electric or nuclear) would the release of pressure for abundant gases in space provide sufficient thrust to realistically propel? For example would hydrogen under immense pressure from compression once released generate momentum to propel, whether in short bursts or just once? $\endgroup$ – Iwheaty Dec 6 '17 at 0:43
  • $\begingroup$ What do you mean by "abundant"? Do you mean "abundant on earth, easy to put in a spaceship and launch"? Or do you mean "abundant in space, easy to collect in-situ"? The latter is almost an interstellar ramjet; the former is just a cold-gas thruster. $\endgroup$ – Nathan Tuggy Dec 6 '17 at 0:45
  • $\begingroup$ Definitely the latter. So it's a ram jet thanks. The thought was to harness what would be in greater supply whilst in far remote regions enabling greater freedom without reliance on more precious fuel. $\endgroup$ – Iwheaty Dec 6 '17 at 0:50
  • $\begingroup$ Similar to how earlier submarines experimented with compressed air for propulsion A reference link would help. Don't make us go do your research for you. You should also edit add this additional information to your question text (comments can disappear). $\endgroup$ – Jan Doggen Dec 6 '17 at 9:24
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Collecting interstellar gas, compressing it and then releasing it (via decompression) takes more power than it yields. You have drag losses in the ram scoop, and compression losses.

Burning the interstellar gas would extract more energy, but oxygen seems to be far less abundant than hydrogen.

One variant is at least theoretically possible, and scientists have examined this method. This method is known as a Bussard ramjet, after the physicist who proposed it in the 1960s. This takes the hydrogen and runs it through a fusion reactor.

Since the time of Bussard's original proposal, it has been discovered that the region surrounding the Solar System has a much lower density of hydrogen than was believed at that time (see Local Interstellar Cloud). John Ford Fishback made an important contribution to the details for the Bussard ramjet in 1969,[2] T. A. Heppenheimer analyzed Bussard's original suggestion of fusing protons, but found the bremsstrahlung losses from compressing protons to fusion densities was greater than the power that could be produced by a factor of about 1 billion, thus indicating that the proposed version of the Bussard ramjet was infeasible.[3] However Daniel P. Whitmire's 1975 analysis[4] indicates that a ramjet may achieve net power via the CNO cycle, which produces fusion at a much higher rate (~1016 times higher) than the proton-proton chain.

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