Awe is the sentiment that dominates  when you study  the development and features of the XF-91. The ingenuity and engineering skills of the Republic team were truly amazing, creating such an advanced and formidable piece of machinery in the late ´40s.

 The US military command had, already late in the war, envisioned a future bomber-threatfrom Soviet Russia(that was inevitably going to develop nuclear-bomb capacity before long). (The truth in this matter was, with hindsight, that the Russians didn´t have a long-range bomber-fleet until 1949-50, equipped only with the painstakingly copied B-29, the Tu-4 Bull.)

In response to this possible threath Secret Project No. MX-809 was created, proposing a high speed, high altitude daytime interceptor aircraft capable of  ”meeting and defeating any strategic bomber aircraft that any potential adversary might produce”. The Special Operational Requirement(SOR) included that the airplane was to be capable of 25.5 min. combat operation, divided into: 2.5 min. to climb to 47.500feet(19.000feet/min); 15 min. cruise time at 486knots indicated air speed; 3 min. of combat duration at an average of 688 kias; 5 min. of descent time from 47.500 feet to landing. These were very tough requirements indeed, given the technical level of the day. 688 knots average combat speed meant supersonic performance, climb-rate demands indicated rocket-power!

 Alexander Kartveli himself led the Republic design team that on May 29, 1946 got the contract to produce two prototypes of the company´s proposed AP-31 model(Convair simultaneously recieved a contract for their model 7, that was to become the XF-92, fore-father of the F-102/106 family).

The AP-31, now designated XP-91, was a tour-de-force of aircraft engineering:  Twin powerplants, turbojet+rockets, afterburner, swept wings with inverse taper and variable incidence and  different tail & nose configurations along the way.

The Republic team studied reports of the German Me 163 Komet rocket interceptor and the Me 262C Heimatschutzer, the latter combining turbojet/rocket power, to gain information on the planned propulsion system. The General Electric J47-GE-3(substitution for the planned GE-7, not yet available) was to be combined with the projected Curtiss Wright XLR-27-CW 1 four-chamber rocket motor, theoretically giving the Thunderceptor a top-speed of Mach 2 with everything burning.

The CurtissWright, however, was never to become reality and the rocket unit was replaced by the tried-and-true XLR-11-RM-9 from Reaction Motors. This was identical to the unit that pushed Chuch Yeager and the Bell X-1 through the sound-barrier on Oct.14, 1947.

 The XF-91(P changed to F when the prefix system was revised in June 1948) began flight tests at Muroc AFB on 9 May 1949, first flying with Carl Bellinger at the controls. Throughout flight testing the XF-91 performed brilliantly and most of the advanced ideas from the drawing board also worked well in the air. The inverse-taper wing, that was adopted to eliminate the risk of wing-tip stall, a serious problem for first-generation swept-wing aircraft, worked out  just as planned. The thin, narrow wing-root inevitably stalled first, with a fully controllable nose-down pitching as a result.

The thin inner wing proposed another problem for the engineers: where to put the undercarriage? The solution was a highly original arrangement with  bicycle-style twin main-wheels(looks like a Viggen!) retracting outwards into the wingtips. The variable incidence of the wing( -2 to +6 degrees) was also tested to good advantage, designed to enhance behaviour at takeoff and landing. These features combined gave the Thunderceptor a very broad speed-range and safe handling in all registers. During the first three years of flight testing the XF-91 was powered by the J47 alone and the first rocket motor wasn´t installed until Nov. 1952.

Bellinger was the first to take the XF-91 through the sound-barrier, in a 30 degree dive and on turbojet power only and on the 9^th December 1952,  Russel ”Rusty” Roth hit Mach 1.07 in level flight with two of the four rocket-chambers burning. This date marks the first soun-barrier-breaking flight of an American combat aircraft in level flight. With all five powerplants in action the XF-91 achieved a maximum speed of Mach 1.71! That´s fast for the early ´50s!

Originally proposed armament for the Thunderceptor was four nose-mounted 20mm cannon and other suggested configurations were different rocket-launchers in retractable bellymounts and a later variant proposal with a rocket-nose. During the later stages of  development different tail- and nose configurations were tested. A Vee-butterfly tail and a radar nose were tried in different combinations on the two prototypes that were built.

However well the Thunderceptor performed, this was a time when things changed rapidly. Technical advances, turbojet development  and new concepts of the typical interceptor mission combined with a lengthy test-period and cumbersome ground-handling due to the dual-propulsion system(refuelling and handling of the rocket-units alone required a specially designed truck, manned by ”space-men” in asbestos suits) made the XF-91 obsolete even before it´s first flight.

With the advent of a new proposal from the Air Force, the ”1954 All Weather Interceptor” (1954 optimistically indicating the year when such an aircraft would be in service) and new concepts and requirements given for such an aircraft, Republic entered with three designs of which one was a Thunderceptor derivative designed XF-91B. This project, however, never became a serious contenter for an Air Force contract.

On the 6^th anniversary of it´s first flight 6680, the first prototype, was shipped to the Air Force Museum in Dayton, where it can be seen today, in a radar-nosed configuration. The second prototype was used for firefigther crash-rescue training and was eventually destroyed.

Late input (April 2005) "Just read your review of the XF-91 kit.  Your information has an error.   The second prototype, 46-681 was not used for fire-fighting training but was destroyed in a crash in the summer of 1951 when the engine suffered a catastrophic failure on take-off from Edwards AFB dry lakebed in a test flight flown by Carl Bellinger.  In Chuck Yeager's autobiography, "Yeager" (Bantam, 1985) the incident is recounted on pages 189-190.  As the aircraft lifted off and Bellinger retracted the landing gear, Yeager, flying chase in an F-86, radioed that the engine was coming out the exhaust.  Bellinger put it down immediately and exited the aircraft as the tail melted off.  By the time the fire equipment arrived from seven miles away the airframe was ashes.  Time from lift-off to the pilot exiting the XF-91 was 90 seconds."    -----   Mark Sublett

The Thunderceptor was a remarkable feat of aeroplane design and engineering and provided much valuable information and experience for it´s contemporary F-84 family as well as the later Republic success-story, the F-105 Thunderchief. Many see the Thunderceptor as an ugly, oddball  beast, the ”flop-eared mule”, when it in reality was a sleek, well behaved racer, too advanced and complicated for regular Air Force service in a time when airplane technology raced ahead into the jet-era.

If Bill Koster is Mr. vacform-Tamiya,  then surely Gordon Stevens must be Mr. vac-Hasegawa. He´s the designer behind  those great Rareplanes kits and some of the Maintrack Project-X line that this kit comes from, the XF-91 being one of his works.

Kit includes parts to build all variants of the XF-91 with the different nose/tail/rocket configurations and two sets of different drop-tanks. It also includes, and this is most satisfying, beautifully cast metal undercarriage units. This part of the Thunderceptor is a very complex affair and a real challenge if you were to scratchbuild them. Also included are decals, thank you, as you´re not likely to find any aftermarket sheet for this one(to be honest, the only items exclusive to this decal sheet are the XF-91 logos and the tail numbers, stars ´n bars and USAF letters shouldn´t be too impossible to find). 

Panel lines are extremely delicate, to the point that you worry if it can even take the thin layer of natural metal finish required. The ”multi-faceted” paneling on the fuselage is especially well captured and is noticeable from certain angles only, just like on the real thing. Very impressive handicraft!( Please compare with the Lindberg 1/48 ”riveteers-wet-dream” of a kit! Talk about two ends of a spectrum!!) The only flaw I could find were a number of small pips from the moulding-process. These should, however,  be possible to remove with a new X-acto blade and a steady hand. Well, I hope so!

Also included, as metal items, are jet-pipe, alternative rocket-exhaust and different nose-probes. A metal seat is also included but seems to be thrown in for good measure only as it has no resemblance to the real seat of the airplane( it looks like an Aeroclub MB-seat of some sort and can always be a good addition to the spares-box).  One vacuformed canopy means no forgiveness for a slippery knife.

Instructions are the usual exploded view of the kit and directions on how to cut out and assemble. You also get the 1/72 scale drawing( as always with Rareplanes/Maintrack) that, in my own humble opinion, should be standard in all kits. Very helpful! Painting is very briefly described(and wrong: fuselage trim/walkways should be olive drab, not ”greyish black”) and there is also a short history/ref.listing.

Rareplanes and Maintrack are like Santa Claus to us who are obsessed with everything starting with X, XP- or XF. The Project-X line includes goodies like the XF-88, XF-90 and F-107, among  others, all in the one and only true scale of 1/72J Good quality vacuform-kits that have all the basic elements to make true models of unusual, not-so-middle-of-the-road, subjects.

With the main sub-assemblies more or less finished it was time for a true Rolls Royce paint-job(you know about this, don´t you? When all parts are painted before assembly. I had two friends that went to England with the perfect idea of a great business-deal, bought a vintage Bentley 1955, took it back to Sweden, put it through(what they thought) a super paint-job and put it out on the vintage-car market. Inspected by the true connaisseurs it was, of course, immediately declared worthless, not being painted with the Rolls Royce method. They had to ship it back to Britain and sell it for half the initial price( not to mention the travel costs). Even the best of ideas can boomerang right back in your face, just like some of my novelty modeling methods sometimes do!)

Allright, so my plan was to do the entire aircraft in Bare Metal Foil after having tried it out on the fin and found it looking super. After a good week of wrestling the somewhat snakey pieces of metal foil I had the entire plane covered. Looked like sh*t! I couldn´t get the overlaps between different panels right, no matter how careful I followed the instructions. So followed another good week of undressing the poor Thunderceptor and I  have some inside info. for you modelers out there: some Metal Foil come off easy, some don´t! Some of it leave a lot of hard-to-get-rid-of residue-glue, some of it just peels off.

Seems to be a difference between sheets of different shades of metal. Chrome is the worst(to get off, best to get to stick). I had to wash the entire plane with white spirit in order to get it cleaned up for another try at a NMF. I actually didn´t make it all the way with the undressing and chose to leave a band of Chrome Foil around the rear fuselage that I thought I could blend in to good advantage. Foolish me!

Anyway, I now chose SNJ for the next try and made a, let´s say, acceptable job of it. It was just that the area I had left in Chrome Foil stood out like a flashlight in contrast to the rest of the airframe. I hadn´t dared spray the SNJ over the metal foil as I didn´t know if it would adhere or just run off. I finally tried spraying the area with Testors Dullcote and a couple of layers brought the sheen down to a less blinding level.

Overall, I tried to copy the different shades of metal finish that, sort of, goes round the fuselage in bands, polishing the SNJ in different degrees. The wing/tailplane center sections are also in a markedly flatter and more grey tone than the rest of the wing and these areas were left in unpolished SNJ. Oh, and the fin was left in it´s ”original” metal foil finish.

I wasn´t entirely satisfied with parts of the spinal joint so I sanded/Mr.Surfacered this again, masked for the fuselage trim and sprayed  Olive Drab(for which I used Humbrol 98). This was also done on the wing walkways( All main parts still separate, remember? Nice/easy to mask/handle, the Rolls Royce folks know what they are doing!).

I also discovered, via the Pace book and correspondence on the Forum, that the wheel-wells weren´t in zinc-chromate yellow as you might have expected. They were in natural metal too and I had to repaint these, no big deal(just goes to show the importance of good  ref. material).