The Rare Norm 6 months ago Share Tweet Pin Share The same reason jet engines burn their fuel instead of just pumping it out the back. Could you imagine just using a water pump to spray jet fuel out the back of the plane? When you burn the fuel it combusts and expands outwards. Think of it as an explosion that pushes a lot of gas with a lot of force. Much more effective than just pumping the fuel backwards. Bonus: Want to stay up to date on our latest Rare Norm news ? Sometimes we actually do use non-combusting gas rockets in space. For a given amount of fuel, they produce about 2.5%(1/40th) as much kinetic energy as a combustion rocket. I think that number highlights the problem. The only reason we use them is because they’re extremely simple, reliable, and safe, and for something like spacesuit maneuvering they still have enough performance. Also, if you wanted to use a pump, you’d need to power the pump somehow. The best place to get that power would be by burning the fuel you have, which is in fact how most rockets run their fuel pumps. Interesting fact of the day, nuclear rocket engines don’t burn their fuel but are still much more fuel efficient than regular rockets. They do however superheat their propellant to several thousand degrees to make up for the lack of combustion.