The disadvantage is that it takes much power to generate a small amount of thrust this way, so acceleration is very low.
However, considering the mass of the source of the photons, e.g., atoms undergoing nuclear fission, brings the specific impulse down to 300 km/s (c/1000) or less; considering the infrastructure for a reactor (some of which also scales with the amount of fuel) reduces the value further.
Upon escaping the Earth's gravitational field the rocket will have a heliocentric velocity of 30 km/s in interplanetary space.
Eighty years of steady photonic thrusting would be then required to obtain a final velocity of 240 km/s in this hypothetical case.
It is possible to obtain even higher specific impulse; that of some other photonic propulsion devices (e.g., solar sails) is effectively infinite because no carried fuel is required.
, whereas for slow particles (that is, nonrelativistic; even the output from typical ion thrusters counts) the ratio is
(This is in a sense an unfair comparison, since the photons must be created and other particles are merely accelerated, but nonetheless the impulses per carried mass and per applied energy—the practical quantities—are as given.)
The limitations posed by the rocket equation can be overcome, as long as the reaction mass is not carried by the spacecraft.
However, BLP is limited because of the extremely low thrust generation efficiency of photon reflection.
[citation needed] A design proposed in the 1950s by Eugen Sänger used positron-electron annihilation to produce gamma rays.