Rocket-powered target drones from Van Nuys

In my previous post about unmanned air vehicles that I saw at the Western Museum of Flight during my visit there in January 2020, I offered brief details on three Radioplane/Northrop-built UAVs preserved at this museum, the MQM-57 Falconer and MQM-36 Shelduck derivatives of the prolific MQM-33 Quail target drone family, and the NV-144 prototype jet-powered reconnaissance UAV. However, one Radioplane-built UAV on display at the Western Museum of Flight that I virtually overlooked happened to be one of just a handful of rocket-powered UAVs to be designed and built in southern California, the Radioplane AQM-38. Having recently gotten a copy of the book 50 Years of Target Drone Aircraft (published in 1985 by the very company that built many of southern California's most notable 20th century drones besides the Firebee) and done some brief yet painstaking research into the genesis and early development of the AQM-38, I now have the opportunity to dedicate this post to telling the story of rocket-powered unmanned aerial vehicles developed by the Radioplane Division of Northrop in the 1950s.

A trio of XKD4R-1s on their towing platforms at the Naval Air Missile Test Center (NAMTC) in Point Mugu, southern California, January 1957.

In March 1955, Radioplane proposed an air-launched rocket-powered target drone under the company designation RP-70, which used molded plastic in its construction and had a sharply pointed nose, with longitudinal stability provided by three forward control fins (one on top of and two on the sides of the forward fuselage) and a horizontal stabilizer mounted below the ventral vertical stabilizer. Power was provided by a single 37 lb (0.16 kN) thrust Aerojet 530NS35 solid-fuel rocket motor with a burn time of 530 seconds, and the RP-70 had a length of 9 feet 6 in (2.90 meters), a wingspan of 5 feet (1.52 meters), a diameter of 12 inches (30 cm), a weight of 305 lb (138 kg), a top speed of Mach 0.95 and a service ceiling of 60,000 feet (18,300 meters). After launch, the drone would rely on autopilot to remain on a constant heading and altitude for a flight endurance of 9 minutes, with a bright flashing light in the tail utilized to facilitate visual tracking. For recovery, the RP-70 was equipped with a parachute system. The Navy assigned the designation XKD4R-1 to the RP-70, and the first XKD4R-1 drones were built and first flown in late 1956, with deliveries to the Naval Air Missile Test Center (NAMTC) at NAS Point Mugu in January 1957. The main launch platforms for the XKD4R-1 wer the Douglas F3D Skyknight all-weather jet fighter and McDonnell F3H Demon jet fighter, both of which could fly at the subsonic speeds that the XKD4R-1 attained. A handful of XKD4R-1s were manufactured but the drone was not approved for series production despite exhibiting satisfactory performance.

Left: An AQM-38A (RP-76) on display at the Western Museum of Flight, photographed by me on April 17, 2021.
Right: An AQM-38A under the wing pylon of an F-89 Scorpion.

Months before the XKD4R-1 began flight testing, in early 1956 Radioplane proposed a variant of the RP-70 to be used by the US Army for surface-to-air missile training, the RP-76. Despite having the tail empennage, rocket motor, and flight duration of the RP-70, the RP-76 differed in having a blunt nose section to house the Luneberg lens, straight wing/rocket exhaust fairings, and the dorsally mounted forward control fin moved to the underside of the forward fuselage. The Luneberg lens was designed for radar reflectivity augmentation, and the RP-76 also utilized a Northrop RPTA-1 tracking aid system, necessitating elimination of the bright flashing light in the tail developed for the XKD4R. After being launched from an aircraft, the RP-76 was be controlled in flight by autopilot with an optional override by radio command, and recovery of it was done by a two-stage parachute system.  The Army awarded Radioplane a contract for full-scale development of the RP-76 in June 1957, and test flights of RP-76 began in early 1958, with deliveries of service test drones to US Army units commencing later that year, and the first successful hit against an RP-76 by a Nike Ajax surface-to-air missile taking place on September 18, 1958 at the Red Canyon Guided Missile Range near Carrizozo, New Mexico. Series production of the RP-76 began in late 1959, by which time the drone had been cleared for operational service, and launches were conducted from the Northrop F-89 Scorpion all-weather fighter but also the RP-77DL mothership variant of the RP-77D turboprop-powered target drone. 

Left: An AQM-38B (RP-78) under a wing pylon of an F-89 Scorpion, late 1962
Right: An RP-76-4 target drone under a wing pylon of an F-89 Scorpion, circa 1961

In parallel with design of the RP-76, in April 1956 Radioplane envisaged a supersonic variant of the RP-76 for the Air Defense Command of the US Air Force, the RP-78, which had the same airframe and guidance system as the RP-76 but was designed for a top speed of Mach 1.25. The RP-78 had a service ceiling of 78,700 feet (24,000 meters) and a range of 44 miles (70 km), and the top speed and altitude for which for the RP-78 was designed meant that this drone utilized a slightly more powerful solid-fuel rocket motor generating 100 lb (0.44 kN) of thrust. The RP-78 began flight tests in 1960, but by then the US Air Force had lost interest in the RP-78 in favor of the Ryan Firebee, so the US Navy took over the RP-78 program, and the operational deployment of the RP-78 with Navy units commenced in October 1962, with the F-89 being used as the launch platform. Radioplane in April 1960 proposed another supersonic derivative of the RP-76 to be used by the US armed forces and NATO member states in Europe, the RP-76-4, which began test flights in December 1961. The RP-76-4 was 11 feet (3.35 meters) long with a wingspan of 4 feet 4 in (1.32 meters), a top speed of Mach 2.25, a service ceiling of 80,000 feet (24,384 meters), and a 200 lb (0.89 kN) thrust solid-fuel rocket motor. It differed from the RP-76 and RP-78 in having delta wings and a conventional dorsal vertical stabilizer as well as anhedral horizontal stabilizers, and it had an endurance of four minutes over a range of 62 miles (100 km) under rocket power and 30 minutes in controlled glide mode. Over 20 test flights of the RP-76-4 were made by 1964, and despite its stellar flight performance, no production orders for this advanced drone followed. When the US Defense Department introduced a new designation system for missiles, unguided rockets, and drones on June 27, 1963, the RP-76 and RP-78 were designated AQM-38A and AQM-38B respectively (ironically, the RP-78 had not been assigned a Navy designation prior to 1963). Production of the AQM-38 ended in 1968 with more than 2,400 targets built, and the AQM-38 itself remained in operational service until the mid-1970s.

References:

Botzum, R.A., 1985. 50 Years of Target Drone Aircraft. Newbury Park, CA: Northrop Corporation (Ventura Division) Publishing Group.

Taylor, J.W.R., 1963. Jane's All the World's Aircraft 1963-1964. New York, NY: McGraw-Hill.

Taylor, J.W.R., 1965. Jane's All the World's Aircraft 1965-1966. New York, NY: McGraw-Hill.