Retrorocket

Retrorocket, or retro-rockets, is a rocket motor that produces thrust opposite to the direction of travel in order to slow a spacecraft. This retrograde impulse is used during descent toward a surface or during reentry from orbit, providing controlled deceleration in addition to whatever aerodynamic braking or main propulsion is available.

Most retrorockets are either solid-fuel motors that ignite quickly and deliver a short, high-magnitude burn or small liquid-fuel thrusters that can be pulsed or throttled. They are typically mounted at the base of the vehicle or on a dedicated retro module. The burn is timed to achieve the desired delta-v and is coordinated with the vehicle’s guidance system to control descent rate, attitude, and touchdown dynamics.

Applications of retrorockets include deorbit propulsion for returning spacecraft from orbit and soft-landing systems for planetary or lunar landers. In some configurations they provide a final braking impulse immediately before touchdown, while in others they assist braking during the descent phase until a primary landing engine or landing legs take over.

Historically, retrorockets have been used to improve safety and accuracy in spaceflight by providing reliable, rapid deceleration. Solid retro motors offered simplicity and reliability, while liquid retro thrusters allowed greater control and pulsed operation. Modern landing and reentry systems may rely on integrated descent engines or attitude-control thrusters, but the concept of retro thrust remains a fundamental option for braking in spaceflight.