The gained energy can be used for thrust amplification by increasing the propellant mass flow, or using the intensive neutron flux to produce a very high specific impulse amplification-even higher than the fission-fragment rocket, where in the pulsed rocket the final propellant temperature is only limited by the radiative cooling after the pulsation.
32.
However, air breathing engines are usually very much heavier than rocket engines and the empty weight of the oxidizer tank, and since, unlike oxidizer, this extra weight ( which is not expended to add kinetic energy to the vessel, as is propellant mass ) must be carried into space it may offset the overall system performance.
33.
Where M _ { \ text { ship } } is the non-relativistic ( rest ) mass of the rocket ship, and a is the fraction of the original ( on-board ) propellant mass ( non-relativistic ) remaining after annihilation ( i . e ., a = 0.22 for the charged pions ).
