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 parachutes, and then a specially-equipped aircraft catches it in mid-air.
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It 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 and spacecraft reentry.
Notable uses of the technique:
- The first use of midair retrieval was in 1955, with Fairchild C-119 Flying Boxcar transports being used to recover Ryan AQM-34 Firebee target drones during test flights. On operational flights, the Firebee used Sikorsky SH-3 Sea King and Sikorsky CH-53 Sea Stallion helicopters as its recovery aircraft. [1]
- The early-1960s era Corona reconnaissance satellite returned delicate film capsules to Earth that required mid-air retrieval by a specially modified aircraft. Early in the program, modified C-119 Flying Boxcar airlifters were used, replaced in 1961 by modified JC-130B Hercules and supplemented in 1966 with JC-130H. [2] 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 (15,000 m). 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 Soviet Union also experimented with midair retrieval during satellite recovery trials at the Gromov Flight Research Institute, though they do not appear to have used this technique operationally to a great extent. While little information exists on these experiments, it is known that at least one Antonov An-12 airplane was modified for the midair recovery role. [3] A Mil Mi-8 transport helicopter was also modified for this task, possibly under the same program. [4]
- The Sikorsky CH-37 Mojave was used to recover the unmanned space capsules from suborbital Aerobee and Nike-Hydac rocket launches at the Wallops Flight Facility and the White Sands Missile Range during the 1960s. This became a routine operational procedure, and from the 1960s onward many sounding rocket payload capsules at Wallops Island and elsewhere were recovered in midair via helicopter. [5]
- The Hiller aircraft company at one point proposed to build an enormous helicopter with a rotor diameter of over 300 ft for the task of recovering the first stage of the Saturn V moon rocket in order to re-use it. This was never built, as the need to re-use the first stage of the Saturn V never arose due to the scaling-back of the Apollo Applications Program.
- The Genesis space probe returned a sample of solar wind particles in a "particle trap" device that was so delicate that it would have been damaged by a parachute landing. This task called for a plan involving a mid-air retrieval, using helicopters flown by Hollywood stunt pilots contracted by NASA. Its parachutes failed to deploy, leading to a disastrous high speed impact with the desert floor, which shattered the trap's delicate wafers holding the solar wind particle samples. Despite the return capsule being damaged by the impact, the mission was declared a success after intact solar wind particles were salvaged from the wreckage.
- 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 retaining the desired aerodynamic qualities for the early part of reentry.
- During the early 1950s, Lockheed HC-130 Hercules and Fairchild C-119 Flying Boxcar airlifters were used to recover film capsules ejected from unmanned high-altitude reconnaissance balloons under programs such as Project Genetrix and Project Moby Dick.
- The Lockheed D-21 high-speed reconnaissance drone was designed to be disposable, ejecting a camera capsule at the end of its mission and then self-destructing. This capsule would then be recovered in midair by a JC-130B Hercules. However, the D-21 met with very little success, and only a handful of missions were successfully recovered.
- The ALARR (Air Launched, Air Recoverable Rocket), an upper-atmosphere sounding rocket developed from the AIR-2 Genie air-to-air missile, was launched from an F-4 Phantom fighter, and then recovered in midair by a C-130 Hercules using the same technique used for the Corona satellites, spy balloons, and the D-21. [6]
- The United Launch Alliance's Vulcan rocket was at one point intended to have the main engine section of its first stage recovered in midair by helicopter so that it could be reused for further launches. [7]
- NASA's Wallops Island Flight Facility operated a Shorts Skyvan airplane nicknamed the “Ugly Hooker”, which was used between 1979 and 1995 to perform midair recovery of payloads ejected from high-altitude research balloons.
- The Dynetics X-61 reconnaissance drone is intended to be launched from a carrier aircraft and recovered in midair after their mission by a modified C-130 Hercules, using a device similar to that previously used to recover film capsules from spy satellites. [8] On the X-61's first flight, on January 17, 2020, the drone's main parachute failed to open, and the midair recovery failed, resulting in the loss of the drone. [9] Subsequent test flights have resulted in successful retrievals.
- US-New Zealand aerospace company Rocket Lab has announced plans to recover their Electron rocket for reuse by helicopter. [10] The first successful midair recovery of an Electron booster was made on 3 May 2022, using a Sikorsky S-92 as the recovery aircraft.
