I think that the momentum achieved by a rocket that uses a pressurized ideal gas as it's only propellant, stored in a chamber of fixed volume before being rocketed away. will achieve a final momentum proportional to the Pressure of the gas times the fixed volume of the chamber. momentum ∝ (Pressure Initial) • (Fixed Volume). So if the gas pressure was twice as much or if the volume was twice as much you would get twice the momentum as before. Notice it is a proportional and not a = sign, I know the units are not right. Can some one tell why this proportionality is true or false as my Physics Book Haliday Resnick does not make it easy to figure out.