I'm not super well versed on literal rocket science, but my understanding is that the conventional hard requirements are that you need something that contains a lot of energy because you have to carry all your fuel with you, you need something that can release all that energy quickly because every second you take is another second you're being clawed back down by gravity and air resistance, and you need your craft to be able to contain and withstand both of those.
The Skaven solved this with warpstone-based rocket fuel, the downsides of which are fairly self-apparent. Thankfully humanity has evolved past these self-destructive methods, as the use of mercury-based rocket fuels has been banned by the UN since the primitive and dangerous era of *checks notes* two months ago.