A recent article on Slashdot got me thinking.
While the article is about using electric turbopumps for moving fuel from tanks to engines during launch phase and is just a small, incremental advancement on current design, it got me thinking about using electric pumps in space flight phase in zone where solar energy is abundant - zones where we normally use ion engines.
We can store extra energy in fuel by pressurizing it. The extra pressure converts to extra delta-V. Water-bottle rockets work that way trivially; ion propulsion uses highly pressurized xenon to give it initial boost as it enters the acceleration chamber. It's nothing new.
What puts a limit on pressure available is durability->weight of the container. In case of some fuels, like solid propellant, compression is pointless. In other cases like liquid hydrogen, the pressure isn't all that high because the volume and pressure if kept highly pressurized, would necessitate unduly heavy container.
But I can picture a tiny solar-powered pump capable of creating very high pressures with minuscule throughput - possibly multi-stage piston pump with very small cross-section pistons, although choice of the exact pump design is an engineering detail, a subject of discussion and brainstorming later on.
The pump can create enormous pressure in a tiny volume of a small "buffer tank". Keeping these things small means they don't weight all that much despite handling maybe gigapascals of pressure. Releasing that pressurized propellant through a nozzle would give it a very high initial velocity. It can then still be further accelerated by a ion drive or react with similarly pressurized oxidizer creating additional thrust.
Nevertheless, we're considerably increasing initial speed of the propellant, without changing the amount of propellant used. Unlike with ion engines we're not limited by available voltages and length of the acceleration chamber - by scaling the device down we can achieve staggering pressures in a small and lightweight package, and as result - exit energies, without increasing fuel usage. And still we can use "second-stage acceleration" be it chemical, ion, or whatever we can think of, on the pressure-accelerated propellant.
Would it be a viable avenue of research or am I out on a limb here?