McDonnell XF-85 Goblin - Design and Development

Design and Development

During World War II, American bombers such as the Boeing B-17 Flying Fortress, Consolidated B-24 Liberator and Boeing B-29 Superfortress were protected by long-range escort fighters such as the Republic P-47 Thunderbolt and North American P-51 Mustang. These fighters could not match the range of the Northrop B-35 or Convair B-36, the next generation of bombers developed by the United States Army Air Forces (USAAF). The development cost for longer-ranged fighters was high, while aerial refueling was still considered risky and technologically difficult. Pilot fatigue had also been a problem during long fighter escort missions in Europe and the Pacific, giving further impetus to innovative approaches.

The USAAF considered a number of different options including the use of remotely piloted vehicles before choosing parasite fighters as the most viable B-36 defense. The concept of a parasite fighter had its origins in 1918, when the Royal Air Force examined the viability of Sopwith Camel parasite fighters operating from R23 airships. In the 1930s, the U.S. Navy had a short-lived operational parasite fighter, the Curtiss F9C Sparrowhawk, aboard the airships Akron and Macon. Starting in 1931, aircraft designer Vladimir Vakhmistrov conducted experiments in the Soviet Union as part of the Zveno project during which up to five fighters of various types were carried by Polikarpov TB-2 and Tupolev TB-3 bombers. In August 1941, these combinations flew the only combat missions ever undertaken by parasite fighters – TB-3s carrying Polikarpov I-16SPB dive bombers attacked the Cernavodă bridge and Constantsa docks, in Romania. After that attack, the squadron, based in the Crimea, carried out a tactical attack on a bridge over the river Dnieper at Zaporozhye, which had been captured by advancing German troops. Later in World War II, the Luftwaffe experimented with the Messerschmitt Me 328 as a parasite fighter, but problems with its pulsejet engines could not be overcome. Other late-war rocket-powered parasite fighter projects such as the Arado E.381 and Sombold So 344 were unrealized "paper projects".

On 3 December 1942, the USAAF sent out a Request for Proposals (RfP) for a diminutive piston-engined fighter. By January 1944, the Air Technical Service Command refined the RfP and in January 1945, the specifications were further revised in MX-472 to specify a jet-powered aircraft. Although a number of aerospace companies studied the feasibility of such aircraft, McDonnell was the only company to submit a proposal to the original 1942 request and later revised requirements. The company's Model 27 proposal was completely reworked to meet the new specifications.

The initial concept for the Model 27 was for the fighter to be carried half-exposed under the B-29, B-35 or B-36. The USAAF rejected this proposal, citing increased drag, and hence reduced range for the composite bomber-fighter configuration. On 19 March 1945, McDonnell's design team led by Herman D. Barkey, submitted a revised proposal, the extensively redesigned Model 27D. The smaller aircraft had an egg-shaped fuselage, three fork-shaped vertical stabilizers, horizontal stabilizers with a significant dihedral, and 37° swept-back folding wings to allow it to fit in the confines of a bomb bay. The diminutive aircraft measured 14 ft 10 in (4.52 m) long; the folding wings spanned 21 ft (6.4 m). Only a limited fuel supply of 112 US gal (93 imp gal; 420 l) was deemed necessary for the specified 30-minute combat endurance. A hook was installed along the aircraft's center of gravity; in flight, it retracted to lie flat in the upper part of the nose. The aircraft had an empty weight just short of 4,000 pounds (1.8 t). To save weight, the fighter had no landing gear. During the testing program, a fixed steel skid under the fuselage and spring-steel "runners" at the underside of the wingtips were installed in case of an emergency landing. Despite the cramped quarters, a pilot was provided with a cordite ejection seat, bail-out oxygen bottle and high-speed ribbon parachute. Four .50 in (13 mm) machine guns in the nose made up the aircraft's armament.

In service, the parasite fighter would be launched and retrieved by a trapeze. With the trapeze fully extended, the engine would be airstarted and the release from the mother ship was accomplished by the pilot pulling the nose back to disengage from the hook. In recovery, the aircraft would approach the mother ship from underneath and link up with the trapeze using the retractable hook in the aircraft's nose. The anticipated production shift would see a mixed B-36 fleet with both "fighter carriers" and bombers employed on missions. There were plans that, from the 24th B-36 onward, provisions would be made to accommodate one XF-85, with a maximum of four per bomber envisioned. Up to 10 percent of the B-36s on order were to be converted to fighter carriers with three or four F-85s instead of a bomb load.

On 9 October 1945, the USAAF signed a letter of intent covering the engineering development for two prototypes (US serial numbers 46-523/4), although the contract was not finalized until February 1947. After the successful conclusion of two reviews of a wooden mock-up in 1946 and 1947 by USAAF engineering staff, McDonnell constructed two prototypes in late 1947. The Model 27D was re-designated XP-85, but by June 1948, it was changed to XF-85 and given the name "Goblin". There were plans to acquire 30 production P-85s, but the USAAF took the cautious approach – if test results from the two prototypes were positive, production orders for more than 100 Goblins would be finalized later.