The XF-12 Rainbow Crushed Every Requirement Except Timing
This is the story of an aircraft unique in form and function. No other four-engine aircraft driven by reciprocating engines could touch its performance. Boeing’s prototype XB-39, a one-off experimental B-29 driven by Allison V-3420-11 liquid-cooled W24 (double-V) engines putting out 2,100 horsepower each, barely topped 400 miles per hour.
Still, it was only meant to prove that other power plants could power the B-29 should the R-3350 engines standard on the B-29 encounter problems. They did, but that’s another story. Even the RB-50, powered by the same engines as the object of this story, could only reach 385 miles per hour. In fact, not until the Lockheed P-3C Orion came along was an American four-engine propeller-driven aircraft capable of (barely) superior performance.
The President’s Ear
The year is 1943. America is in its third year of war. In the Pacific, seemingly endless expanses of deep blue water between island fortresses were the order of the day. Long-range aircraft were desperately needed and in short supply. Consolidated B-24s were doing the best they could with what they had. The Boeing B-29 was nearing service entry, but it was desperately needed as a bomber first.
However, a need was identified by Colonel Elliott Roosevelt (son of President Franklin Delano Roosevelt), commander of the 12th Air Force’s 90th Photographic Wing in the MTO, for a high-altitude long-range reconnaissance aircraft. The original proposal for the aircraft was made at the end of 1943 by the Air Technical Service Command of the United States Army Air Corps (USAAC).
The Impossible Dream Machine
This new aircraft would be required to fly at an altitude of 40,000 feet and at a speed of 400 miles per hour for 4,000 miles- a tall order indeed. The aircraft was intended primarily for high-altitude photographic reconnaissance of the Japanese homeland and those island fortresses- most of which would have to be amphibiously assaulted.
Other existing aircraft had been adapted to the role of photographic reconnaissance. USAAF reconnaissance aircraft designations began with F. P-38 Lightnings with cameras were designated F-4 or F-5. P-51 Mustangs with cameras mounted were F-6s. The photo recon versions of the B-24 were designated F-7A and F-7B. B-17 recon ships were F-9As or F-9Cs. B-25s adapted for camera work were designated F-10. The photo recon version of the B-29 would be designated F-13. Later reconnaissance aircraft designations switched to an R prefix when the US Air Force was born in 1947.
The (Lack of) Competition
Republic Aviation envisioned a large but aerodynamically smooth airframe powered by four of the most powerful radial piston engines available- the 28-cylinder 3,250-horsepower Pratt & Whitney R-4360 Wasp Major. Legendary Republic designer Alexander Kartveli and his team began drawing what is still considered one of the most aesthetically pleasing aircraft ever built. Still, the no-compromise design was more than just shapely.
Hughes Aircraft came up with the XF-11- essentially a larger version of the P-38 Lightning and also powered by R-4360s, but two of them. Howard Hughes crashed the first prototype on its first flight, and the Hughes entry went downhill from there. No other companies were designing to the requirements, though the B-29/XB-39 was considered a potential option. Foreshadowing.
First Fast Flight
Though ordered by the Army Air Force in March of 1944, when the war ended in 1945, Republic’s design, designated XF-12 and named Rainbow, was not yet complete. Work continued anyway on the two prototypes, assigned Air Force serial numbers 44-91002 and 44-91003. Rolled out in December of 1945, XF-12 002 flew for the first time on 4 February 1946 with Lowery L. Brabham, who had taken the P-47 up on its first flight five years earlier, at the controls.
The aircraft was said to be as pleasing to fly as it was to behold. But the proof was in the performance, and the Rainbow did not disappoint. The aircraft exceeded every design requirement by a healthy margin- flying at an altitude of 45,000 feet, at a speed of 470 miles per hour, for 4,500 miles. Republic had pulled it off- exceedingly well.
But, as with so many late-war designs, timing (and the advent of the jet engine) doomed the hottest prop job on the planet to notoriety only as a footnote or curiosity.
Film at 11
Republic continued to fly the Rainbows, and it seemed there was some interest from the Air Force, but it was short-lived. The XR-12’s performance and capability were showcased during Operation Birds Eye on 1 September 1948. XR-12 003, first flown on 12 August 1947, departed Muroc AFB in California, climbed to 40,000 feet over the Pacific, and then turned eastward.
The aircraft shot one continuous roll of 10-inch film as it passed over the country, photographing a 490-mile-wide swath of the earth below. Three hundred twenty-five feet of film and six hours and 55 minutes later, 003 landed at Mitchel AFB on Long Island in New York.
When later fitted with more powerful engines and additional sensors providing improved all-weather capabilities, the XR-12 was still seen as a potentially important intelligence platform. Day or night, good visibility or bad, the Rainbow could get the job done. This was the ultimate high-speed low-drag Foto-mat.
Flying Foto Mat
Aerial reconnaissance pioneer Brigadier General George W. Goddard was involved in the design of the mission equipment for the Rainbow. The XR-12 was equipped with three six-inch Fairchild K-17 aerial cameras oriented for vertical, split vertical, and trimetrogon (simultaneous vertical and side-looking) photography located in compartments aft of the wing.
The Republic engineers designed heaters for the camera lenses and aerodynamically efficient inward-retracting doors for the cameras. Also carried aboard the aircraft were high-intensity photo-flash “bombs” dropped to provide target lighting at night. The aircraft even had a fully-equipped darkroom for film development and printing of the “take” from the cameras while in flight.
There was additional capacity for additional photo or other reconnaissance equipment as well. The combination of performance and capability was unprecedented.
High-Tech and Looking Every Bit of It
The Rainbow displayed advanced engineering everywhere one looked. The wings were high-aspect laminar-flow shapes for efficiency without added drag. The elliptical vertical stabilizer and straight horizontal stabilizers were also efficient and low-drag. Later, the aircraft received revised, rounded wingtips and stabilizer tips.
The engines were tightly cowled and equipped with two-stage impeller fans located behind the 16-foot four-blade Curtiss propellers and large bullet spinners for increased engine cooling. High-pressure engine intake, intercooler, and oil cooler air were provided by leading-edge intakes, which were then routed to the rear of the engine nacelles, providing additional thrust. Each engine was equipped with twin General Electric turbochargers located at the trailing end of each nacelle.
