Assume that total propellant mass is a constant (limited by the launch vehicle) and temperature is a constant (because melting the rocket would be bad.) With c

Assume that total propellant mass is a constant (limited by the launch vehicle) and temperature is a constant (because melting the rocket would be bad.) With constant mass, you increase the total momentum by increasing the exhaust velocity. With constant temperature, you increase the exhaust velocity by making the molecules smaller.