Why do liquid fuel rockets vent gas?
In rocketry, you want to pack as high an amount of energy into as little space as possible. Oxygen and hydrogen as fuel, in their liquid form is respectable in this regard. However those gases are only liquid at either very high pressure, or very low temperature (or a combination), so in order to get a tank full of those fuels in liquid form, you need to either a) Compress your gas until it’s liquid and inject it into a tank, able to withstand the pressure from the compressed gas, or b) cool your gas until it’s liquid and then pump it into a tank, able to hold the weight but at much lesser pressure than option a. Actually i think they use a combination of those methods. However, if you just let the tank sit there, the fuel is going to take on energy (heat) from the surroundings and will heat up. This will increase the pressure within, and if you only designed your tank to take a certain pressure you don’t want that. Thus the boil of. The phase change from a liquid to a gas is an endoth
Space is not the issue, mass is the issue. If they were concerned about space, they wouldn’t be using hydrogen and oxygen, because liquid hydrogen is ridiculously undense. Mass is the most important thing, because you have to lift it. The tanks holding the liquid hydrogen and liquid oxygen are reasonably well insulated, but too much insulation would add dead weight to the rocket, and they’ve decided that a better tradeoff is to accept a certain amount of boil-off in exchange for reducing the weight of the tanks.
