Retro in the sense of 'oppose'; a retrorocket is a rocket intended to be fired in opposition to the direction of travel of a vehicle in order to reduce its velocity. Typically, this would be a
spacecraft, where rockets are the preferred mode of accelerating due to environmental limitations. However, retrorockets are used in other applications where quick, sharp changes in
velocity are required.
One of the earliest high-profile use of retrorockets was on manned spaceflight capsules. In order to deorbit in a controlled fashion, the capsules needed to reduce their orbital velocity in a quick and predictable manner. The U.S. Mercury capsules and the Soviet Vostok spacecraft both used retrorocket burns in order to re-enter the atmosphere. The U.S. craft had three (for redundancy) solid-fuel rocket motors attached to their base, while the Soviet capsule used a single liquid-fueled retro engine, but all of these can be considered 'retrorockets' as their only purpose was to thrust against the direction of travel in order to deorbit the spacecraft.
Retrorockets are not confined to space use. in 1968, the U.S. Army Materiel Command issued a report1 on the feasibility of using a ring of retrorockets fitted around the load attach point for cargo parachutes. These retrorockets would fire downwards just before the bottom of the load pallet reached the ground, decelerating it so as to significantly reduce the jolt of landing. The concept proved successful, and is useful for airdropping very large and/or relatively fragile loads such as vehicles.
Experimental high-speed vehicles such as rocket sleds occasionally make use of retrorockets for their high power and low complexity (especially in solid fuel versions) as 'strap on brake systems'.
One of the most impressive uses of retrorockets came as a possible hack designed to facilitate a commando raid on the U.S. Embassy in Teheran in 1980. The planners were unable to think of a way to evacuate the Special Forces team and the American hostages following an escape from the compound, and some mad genius or team thereof came up with the idea of flying a C-130 Hercules into a Teheran soccer stadium. Since the soccer stadium was a wee bit short for a short field landing, even in a C-130, the plan involved strapping rocketsall over the plane. It was to use a number of them as retrorockets in order to stop the aircraft within the stadium and then use the remainder to lift the aircraft off within 50 feet and achieve approximately 100 feet of altitude and flying speed on rocket thrust alone. 24 of the 30 motors were off the shelf units, taken from the ASROC. Unfortunately, during a demonstration of this aircraft (named project Credible Sport) the braking rockets fired early, and the airplane dropped 40 feet onto the runway. The impact tore the starboard wing from the airplane. The crew survived, but the mission was cancelled, as history records.
1 Chakoian, G. (U.S. Army Materiel Command, Natick Labs) and Michal, J.L. (Stencel Aero Engineering Corp) A parachute retrorocket system for low altitude airdrop of cargo andother special applications. American Institute of Aeronautics and Astronautics: Conference report AIAA-1968-956, El Centro CA, 1968.