[1] Photon rockets have been discussed as a propulsion system that could make interstellar flight possible during a human lifetime, which requires the ability to propel spacecraft to speeds at least 10% of the speed of light, v ≈ 0.1c = 30,000 km/s.
A large amount of fuel would be required and the rocket would be a huge vessel.
[4][5] 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.
The speed an ideal photon rocket will reach (in the reference frame in which the rocket was at rest initially), in the absence of external forces, depends on the ratio of its initial and final mass: where
[6] For example, assuming a spaceship is equipped with a pure helium-3 fusion reactor and has an initial mass of 2300 kg, including 1000 kg of helium-3 – meaning, 2.3 kg will be converted to energy[a] – and assuming all this energy is emitted as photons in the direction opposing the direction of travel, and assuming the fusion products (helium-4 and hydrogen) are kept on board, the final mass will be (2300 − 2.3) kg = 2297.7 kg and the spaceship will reach a speed of 1/1000 of the speed of light.
If the fusion products are released into space, the speed will be higher, but the above equation cannot be used to compute it, because it assumes that all decrease in mass is converted into energy.
As we start in the rest frame (i.e. the zero-momentum frame) of the rocket, the initial four-momentum of the rocket is: while the final four-momentum is: Therefore, taking the Minkowski inner product (see four-vector), we get: We can now solve for the gamma factor, obtaining: Standard theory says that the theoretical speed limit of a photon rocket is below the speed of light.
However, his claims have been contested by Tommasini et al.,[6] because such velocity is formulated for the relativistic mass and is therefore frame-dependent.
Regardless of the photon generator characteristics, onboard photon rockets powered with nuclear fission and fusion have speed limits from the efficiency of these processes.
Assuming the fuel mass to propulsion-system energy conversion efficiency γ and the propulsion-system energy to photon energy conversion efficiency δ ≪ 1, the maximum total photon energy generated for propulsion, Ep, is given by If the total photon flux can be directed at 100% efficiency to generate thrust, the total photon thrust, Tp, is given by The maximum attainable spacecraft velocity, Vmax, of the photon propulsion system for Vmax ≪ c, is given by For example, the approximate maximum velocities achievable by onboard nuclear powered photon rockets with assumed parameters are given in Table 1.
Therefore, onboard nuclear photon rockets are unsuitable for interstellar missions.
The beamed laser propulsion, such as photonic laser thruster, however, in principle can provide the maximum spacecraft velocity approaching the speed of light, c.