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.
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, 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. However Daniel P. Whitmire's 1975 analysis 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.