McDonnell XF-85 Goblin

The McDonnell XF-85 Goblin is an American prototype fighter aircraft conceived during World War II by McDonnell Aircraft. It was intended to deploy from the bomb bay of the Convair B-36 bomber as a parasite fighter. The XF-85's intended role was to defend bombers from hostile interceptor aircraft, a need demonstrated during World War II. McDonnell built two prototypes before the Air Force (USAF) terminated the program.

The XF-85 was a response to an Army Air Forces (USAAF) requirement for a fighter to be carried within the Northrop XB-35 and B-36, then under development. This was to address the limited range of existing interceptor aircraft compared to the greater range of new bomber designs. The XF-85 was a diminutive jet aircraft featuring a distinctive potato-shaped fuselage and a forked-tail stabilizer design. The prototypes were built and underwent testing and evaluation in 1948. Flight tests showed promise in the design, but the aircraft's performance was inferior to the jet fighters it would have faced in combat, and there were difficulties in docking. The XF-85 was swiftly canceled, and the prototypes were thereafter relegated to museum exhibits. The 1947 successor to the USAAF, the United States Air Force (USAF), continued to examine the concept of parasite aircraft under three related projects following the cancellation: MX-106 "Tip Tow", FICON, and "Tom-Tom."

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 their 23-class airships. In the 1930s, the U.S. Navy had a short-lived operational parasite fighter, the Curtiss F9C Sparrowhawk, aboard the airships and . 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.

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. submitted a revised proposal, the extensively redesigned Model 27D. The smaller aircraft had a potato-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 long; the folding wings spanned . Only a limited fuel supply of was deemed necessary for the specified 30-minute combat endurance. To save weight, the fighter had no landing gear.|group=N 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, the pilot was provided with a cordite ejection seat, bail-out oxygen bottle, and high-speed ribbon parachute.

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. The anticipated production shift would see a mixed B-36 fleet with both "fighter carriers" and bombers 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.|group=N

thumb|upright|Hangar test of recovery trapeze, with wings folded in stowed configuration|alt=Black-and-white photograph of small potato-shaped fighter aircraft hoisted by a lifting mechanism underneath a large bomber.

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,|group=N 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. The second prototype had to be substituted for the remainder of the wind tunnel tests and the initial flight tests.

As a production series B-36 was unavailable, all XF-85 flight tests were carried out using a converted EB-29B Superfortress mother ship that had a modified, "cutaway" bomb bay complete with trapeze, front airflow deflector, and an array of camera equipment and instrumentation.

left|thumb|XF-85 suspended from an EB-29 via a trapeze|alt=Black-and-white photograph shows diminutive jet fighter suspended from a large aircraft in flight, through a trapeze.

McDonnell test pilot Edwin Foresman Schoch was assigned to the project, riding in the XF-85 while it was stowed aboard the EB-29B, before attempting a "free" flight on 23 August 1948. After Schoch was released from the bomber at a height of 20,000 ft (6,000 m), he completed a 10-minute proving flight at speeds between 180 and 250 mph (290–400 km/h), testing controls and maneuverability. When he attempted a hook-up, it became obvious the Goblin was extremely sensitive to the bomber's turbulence, as well as being affected by the air cushion created by the two aircraft operating in close proximity. He saved the prototype by making a belly landing on the reinforced skid at the dry lake bed at Muroc. All flight testing was suspended for seven weeks while the XF-85 was repaired and modified. Schoch used the down period to undertake a series of problem-free dummy dockings with a Lockheed P-80 Shooting Star fighter. During the fifth free flight on 22 October 1948, Schoch again found it difficult to hook the Goblin to the bomber's trapeze, aborting four attempts before hitting the trapeze bar and breaking the hook on the XF-85's nose. Another forced landing was carried out at Muroc. When testing resumed, on the 18 March 1949 test flight, Schoch continued to have difficulty in hooking up, striking and damaging the trapeze's nose-stabilizing section, before resorting to another emergency belly landing.

Two main reasons contributed to the cancellation. The XF-85's deficiencies revealed in flight testing included a lackluster performance in relation to contemporary jet fighters, and the high demands on pilot skill experienced during docking revealed a critical shortcoming that was never fully corrected.

Further developments

Despite cancellation of the XF-85, the USAF continued to examine the concept of parasite aircraft as defensive fighters through a series of projects. These included Project MX-106 "Tip Tow", Project FICON, and Project "Tom-Tom" – which involved fighter aircraft attached to bomber aircraft by their wingtips. Project FICON ("fighter conveyor") emerged as an effective Convair GRB-36D and Republic RF-84K Thunderflash combined bomber-reconnaissance-fighter, although the role was changed to that of strategic reconnaissance. A total of 10 converted B-36s and 25 reconnaissance fighters saw limited service with the Strategic Air Command in 1955–1956, before they were supplemented by more effective aircraft and satellite systems.

Aircraft on display

alt=|thumb|McDonnell XF-85 Goblin on display at the National Museum of the United States Air Force at Wright-Patterson AFB in Dayton, Ohio.

After the program's termination, the two XF-85 prototypes were stored, before being surplussed and relegated to museum display in 1950.

Specifications

thumb|Three-view image of XF-85 Goblin