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Mid-air retrieval is a technique used in atmospheric reentry when the reentering vehicle is incapable of a satisfactory unassisted landing. The vehicle is slowed by means of parachutes, and then a specially-equipped aircraft matches the vehicle's trajectory and catches it in mid-air.
This is a risky technique, and so is only used when other forms of landing are infeasible. Successful mid-air retrieval requires correct operation of the retrieving aircraft, favourable atmospheric conditions, and successful execution of a tricky manoeuvre, in addition to correct operation of the vehicle itself. These risks can be mitigated somewhat: for example, multiple recovery aircraft can be used. The need for human aviators to perform a manoeuvre which would normally be classed as a stunt may in the future be avoided by advances in unmanned aerial vehicles.
The first successful mission use of mid-air recovery was on 19 August 1960, when a C-119 recovered film from the Corona mission code-named Discoverer 14. This was the first successful recovery of film from an orbiting satellite and the first aerial recovery of an object returning from Earth orbit.
Notable uses of the technique:
- The early-1960s era Corona reconnaissance satellite returned delicate film capsules to Earth that required mid-air retrieval by a JC-130 Hercules & HC-130 airlifter. These aircraft were manned by a crew of 10 personnel. The crew consisted of two pilots, one flight engineer, two telemetry operators, one winch operator, and four riggers. The telemetry operators would acquire the location of the satellite and relay the info to the pilots. Once visually acquired the pilots would head on course to the satellite descending towards the Pacific Ocean. One could visually acquire the satellite and its parachute at an altitude of approximately 50,000 ft. The winch operator and the riggers would deploy the retrieving apparatus called the "Loop", which consisted of high quality nylon rope with a series of brass hooks spliced into the apparatus. The whole snatching operation by the pilots was done visually. The winch operator and the four riggers would deploy the loop. Once contact was made between the parachute and the loop the winch line would pay out and stop. The winch then was put into gear and the retrieval process commenced. Once on board, the aircraft flew back to Hickam Air Force Base, where they were stationed, where the satellite or canister was offloaded onto a truck and then loaded immediately onto a running C-141 airlifter and then transported to a location in Maryland for analysis. The crews acquired these skills by practicing almost daily on practice missions, carried out with other aircraft dropping dummy bombs with chutes attached. The weights were 200 lb. in the early 70s and later to the conical parachute system which weighed in at 1,100 lb.
- The Genesis mission returned a sample of solar wind that was so delicate that it would have been damaged by a parachute landing, so a mid-air retrieval using helicopters flown by Hollywood stunt pilots contracted by NASA was planned. Its parachutes failed to deploy, leading to a disastrous high speed impact with the desert floor which shattered the delicate wafers holding the solar wind samples.
- An early design for SpaceShipOne called for a shuttlecock-like shape that would have made it incapable of landing independently, necessitating mid-air retrieval. This was deemed too risky, and the final design made the spacecraft capable of independent horizontal landing while cleverly retaining the desired aerodynamic qualities for the early part of reentry.
- During the Cold War, Lockheed HC-130 Hercules airlifters were used to recover instrument packages from unmanned spy balloons and UAVs.
- The United Launch Alliance's proposed Vulcan rocket is intended to have the main engines on its first stage recovered in midair by helicopter so that it can be reused for further launches.
Key Hole film recovery by JC-130