From Wikipedia, the free encyclopedia Photon rocket
Photon rocket
A photon rocket is a hypothetical rocket that uses thrust
from emitted photons for its propulsion. The standard
Derivation
textbook case of such a rocket is the ideal case where all We denote the four-momentum of the rocket at rest as Pi,
of the fuel is converted to photons which are radiated the rocket after it has burned its fuel as Pf, and the four-
in the same direction. In more realistic treatments, one momentum of the emitted photons as Pph. Conservation
takes into account that the beam of photons is not per- of four-momentum implies:
fectly collimated, that not all of the fuel is converted to Pph = Pi − Pf
photons etc., see e.g. Nuclear photonic rocket.
squaring both sides (i.e. taking the Lorentz inner product
of both sides with themselves) gives:
Speed
The speed an ideal photon rocket will reach in the ab-
sence of external forces, depends on the ratio of its initial
According to the energy-momentum relation, the square
and final mass:
of the four-momentum equals the square of the mass,
and because all the photons are moving in
the same direction. Therefore the above equation can be
written as:
where mi is the initial mass and mf is the final mass. The
gamma factor corresponding to this speed has the simple
Solving for the gamma factor gives:
expression:
References
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Categories:
• Spacecraft propulsion
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