Pressurizing the tanks. Consider Apollo - https://en.wikipedia.org/wiki/Apollo_command_and_service_module#Reaction_control_system https://en.wikipedia.org/wiki

Pressurizing the tanks. Consider Apollo - https://en.wikipedia.org/wiki/Apollo_command_and_service_module#Reaction_control_system https://en.wikipedia.org/wiki/Apollo_command_and_service_mod... > The command module attitude control system consisted of twelve 93-pound-force (410 N) attitude control thrusters, ten of which were located in the aft compartment, plus two in the forward compartment. These were supplied by four tanks storing 270 pounds (120 kg) of monomethylhydrazine fuel and nitrogen tetroxide oxidizer, and pressurized by 1.1 pounds (0.50 kg) of helium stored at 4,150 pounds per square inch (28.6 MPa) in two tanks.