Particle decay, Oberth effect and a relativistic rocket in the Schwarzschild background

TL;DR

This paper explores particle decay, the Oberth effect, and relativistic rockets near Schwarzschild black holes, deriving efficiency conditions and comparing ejection scenarios to optimize energy use and direction reversal.

Contribution

It extends the analysis of the Oberth effect and rocket efficiency into relativistic black hole backgrounds, providing new insights into optimal fuel ejection angles and energy recovery methods.

Findings

Maximum efficiency of 100% at the horizon for photon rockets.

Ejection along the trajectory yields the highest efficiency.

Two-step reversal can extract more mass near the horizon for non-photonic rockets.

Abstract

We relate the known Oberth effect and the nonrelativistic analogue of the Penrose process. When a particle decays to two fragments, we derive the conditions on the angles under which debris can come out for such a process to occur. We also consider the decay and the Oberth effect in the relativistic case, when a particle moves in the background of the Schwarzschild black hole. This models the process when a rocket ejects fuel. Different scenarios are analyzed depending on what data are fixed. The efficiency of the process is found, in particular, near the horizon and for a photon rocket (when the ejected particle is massless). We prove directly that the most efficient process occurs when fuel is ejected along the rocket trajectory. When this occurs on the horizon, the efficiency reaches 100% for a photon rocket. We compare in two ways how a rocket can reverse its direction of motion to a black hole near the event horizon by restoring the initial energy-to-mass ratio: (i) by a single ejection or (ii) in the two-step process when it stops and moves back afterwards. For a nonphotonic rocket, in case (ii) a larger mass can be taken out from the vicinity of a horizon. For a photonic one, there is no difference between (i) and (ii) in this respect. We also consider briefly the scenario when a rocket hangs over a black hole due to continuous ejection of fuel. Then, the fuel mass decays exponentially with the proper time.