Relativistic rocket means any spacecraft that travels close enough to light speed for relativistic effects to become significant. The meaning of "significant" is a matter of context, but often a threshold velocity of 30% to 50% of the speed of light (0.3c to 0.5c) is used. At 30% c, the difference between relativistic mass and rest mass is only about 5%, while at 50% it is 15%, (at 0.75c the difference is over 50%); so above such speeds special relativity is needed to accurately describe motion, while below this range Newtonian physics and the Tsiolkovsky rocket equation usually give sufficient accuracy.
In this context, a rocket is defined as an object carrying all of its reaction mass, energy, and engines with it.
No known technology can bring a rocket to relativistic speed. Relativistic rockets require huge advances in spacecraft propulsion, energy storage, and engine efficiency which may or may not ever be possible. Nuclear pulse propulsion could theoretically reach 0.1c using current known technology, but would still require many engineering advances to achieve this. The relativistic gamma factor at 10% of light velocity is 1.005. A 0.1c speed rocket is thus considered non-relativistic since its motion is still quite accurately described by Newtonian physics alone.
Relativistic rockets are usually seen discussed in the context of interstellar travel, since most would need a lot of space to reach such speed. They are also found in some thought experiments such as the twin paradox.
Relativistic rocket equation
As with the classical rocket equation, one wants to calculate the velocity change that a rocket can achieve depending on the exhaust speed and the mass ratio, i. e. the ratio of starting rest mass and rest mass at the end of the acceleration phase (dry mass) .
In order to make calculations simpler, we assume that the acceleration is constant (in the rocket's reference frame) during the acceleration phase; still, the result is nonetheless valid if the acceleration varies, as long as exhaust velocity is constant.
In the nonrelativistic case, one knows from the (classical) Tsiolkovsky rocket equation that
Assuming constant acceleration , the time span during which the acceleration takes place is
In the relativistic case, the equation is still valid if is the acceleration in the rocket's reference frame and is the rocket's proper time because at velocity 0 the relationship between force and acceleration is the same as in the classical case. Solving this equation for the ratio of initial mass to final mass gives
where "exp" is the exponential function. Another related equation gives the mass ratio in terms of the end velocity relative to the rest frame (i. e. the frame of the rocket before the acceleration phase):
For constant acceleration, (with a and t again measured on board the rocket), so substituting this equation into the previous one and using the hyperbolic function identity returns the earlier equation .
By applying the Lorentz transformation, one can calculate the end velocity as a function of the rocket frame acceleration and the rest frame time ; the result is
The time in the rest frame relates to the proper time by the hyperbolic motion equation:
Substituting the proper time from the Tsiolkovsky equation and substituting the resulting rest frame time in the expression for , one gets the desired formula:
The formula for the corresponding rapidity (the inverse hyperbolic tangent of the velocity divided by the speed of light) is simpler:
Since rapidities, contrary to velocities, are additive, they are useful for computing the total of a multistage rocket.