retropropulsors

Retropropulsors are a class of spacecraft propulsion systems designed to produce thrust by expelling a propellant in the direction opposite to the desired motion. This fundamental principle, governed by Newton's third law of motion, allows a spacecraft to accelerate, decelerate, or change its trajectory in the vacuum of space.

There are several types of retropropulsors, broadly categorized by how they generate and expel propellant. Chemical rockets are the most common, utilizing the rapid combustion of fuel and oxidizer to produce hot gases that are expelled through a nozzle at high velocity. These systems offer high thrust but are generally limited by the mass of the propellant they carry. Electric propulsion systems, such as ion thrusters and Hall effect thrusters, use electrical energy to accelerate a propellant to much higher velocities than chemical rockets. While they produce much lower thrust, their high specific impulse (a measure of propellant efficiency) makes them suitable for long-duration missions and orbital maneuvering. Nuclear thermal rockets, a less common type, use a nuclear reactor to heat a propellant (typically hydrogen) to very high temperatures before expelling it.

The term "retropropulsor" specifically emphasizes the deceleration aspect, often used for critical maneuvers like orbital insertion, deorbiting, or landing on a celestial body. However, the underlying technology is essentially the same as for forward propulsion. The direction of thrust dictates whether it is used to slow down or speed up relative to a given frame of reference. Control systems manage the firing duration, thrust level, and orientation of retropropulsors to achieve precise maneuver execution.