RogerBW's Blog

American Parasite Fighters 14 March 2014

One of the great scars on the American military-aviation psyche was the unescorted bomber. As the men who'd been on the front lines during the Second World War became the leaders of the Air Force, they tried to do something about it.

Since early jet engines were noticeably more fuel-hungry than piston engines, one approach that seemed potentially fruitful during the B-36's heyday was to carry a fighter along with the bomber in some sort of quiescent mode. When it was needed, it could be fired up and fly separately to defend the bomber; afterwards it would dock back on and everyone could head home together.

XF-85 Goblin

The first attempt along these lines used a specially-designed fighter, built by McDonnell in 1948 to a specification first issued in 1942 though heavily modified since. The Goblin had a single crewman with four .50-cal machine guns, no landing gear, and half an hour's supply of fuel for its Westinghouse J34 engine.

The plan was to carry it inside the bomb bay of a B-36 Peacemaker, but those weren't yet available in 1948, so flight tests were done with a B-29 Superfortress: the Goblin wouldn't fit entirely inside its bomb bay, but this seemed a reasonable start for testing.

The major unexpected problem, compared with earlier American experience flying fighters off airships, was buffetting and turbulence. Launching wasn't too bad, as the Goblin could simply drop away from its carrier, but connecting hook and trapeze in flight was challenging even for the experienced test pilot, and was never done reliably even in perfect test conditions. This very high demand on pilot skill was one of the two things that killed the project by mid-1949; the other was that, even if it could be got to work, the Goblin's performance was unimpressive compared with contemporary fighter aircraft.

Tip Tow

Once the B-36 had entered service, demand for an escort fighter rose again. Cue Dr-Ing. Richard Vogt, one of the German scientists taken to the USA by PAPERCLIP.

As an aircraft designer, one of Vogt's obsessions was wing aspect ratio: the ratio of wingspan to chord (fore-aft distance). The higher that number, the lower the wing's drag, other things being equal. This was first tried with "floating panels", non-structural wing elements that would extend the effective wingspan, but Vogt felt that wingtip-to-wingtip connection of two separate aircraft was more practical.

The aircraft in question were a B-29 mothership and a pair of F-84 Thunderjet fighters, and flight tests started in 1950. Attachment and detachment were less of a problem than with the Goblin, and the Thunderjets' engines could be safely shut down once they were hooked on. However, the Thunderjets' pilots retained control at all times, and this was a problem: the B-29 pilot had to coordinate manoeuvres with them, and there were concerns about the structural strength of the wings. Some sort of automatic system to control the Thunderjets was needed: when the B-29 banked left, the right Thunderjet would put on some up elevator, and so the strain on the connector would be minimised.

In 1953, this was finally built. The first F-84 hooked on, and the switch was thrown. The F-84 immediately made a hard roll, colliding with the wing of the B-29, and both aircraft crashed with the loss of all crew.

FICON

FICON (Fighter In CONvair) was a return to the bomb bay principle, using a trapeze lowered some distance from the parent aircraft to try to cut down on turbulence problems, and using a modified Thunderjet rather than a custom-designed fighter. The role of the parasite fighter had shifted slightly: now, the plan was for the Thunderjet (plus nuclear bomb) to make a fast run over the target, while the Peacemaker stayed a few miles back outside major air defences.

First trials were in 1952, and this actually worked rather better than previous attempts. The Thunderjet was too big to fit into even the Peacemaker's bomb bay, so its fuselage and wings protruded and cut the mission range, but the pilot was able to leave his aircraft while docked, making a ten-hour flight to or from the target rather more bearable. The Thunderjet was replaced with the faster Thunderstreak variant, then with the Thunderflash reconnaissance aircraft, its design role now being to gather intelligence over heavily-defended targets.

This combination actually saw service in 1955-1956, but hooking on was still a challenge even for experienced pilots in ideal conditions. As the B-36 started to look increasingly obsolete, and the U-2 approached its entry into service, the project was discontinued in 1956.

Tom-Tom

The other strand of Vogt's high-aspect-ratio project hooked Thunderjets onto the wingtips of a Peacemaker. This essentially had the same problems as Tip Tow, enlarged; the wingtip vortices, caused by air spilling round the end of the wing, were even larger from a Peacemaker than they had been from the Superfortress. One Thunderjet was even torn off its mount point by turbulence, though this time everyone landed safely.

In the end, the adoption of in-flight refuelling allowed aircraft ranges to be extended by safer methods.


  1. Posted by John Dallman at 11:25pm on 15 March 2014

    The greater turbulence problem with these projects, as compared to airships, is presumably due to the greater airspeeds at the time of hook-on. There are square and cube laws in here somewhere, and the D-21 disaster.

  2. Posted by RogerBW at 08:05am on 16 March 2014

    I agree that greater air speed is one of the problems, but in the case of the wing attachments I think that vortex formation will also be a factor. The closer the docking aircraft gets, the more the carrier's vortex will be trying to tilt it away; and the tip of a B-36 is much broader than the tip of an F-84, so even once they're docked there will be a partial vortex formation. (This would probably undo much of the good work of the higher aspect ratio too.)

    Hooking onto a trapeze is probably not much harder than mid-air refuelling, but while that may now be a routine operation it's still a source of problems.

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