A fleeting rainbow

In August 1943, Gen. “Hap” Arnold, head of the U.S. Army Air Force, issued the “Air Plan for the Defeat of Japan,” which called for B-29s to pound the Japanese home islands prior to an invasion, planned for early 1946. To effectively execute the plan, the AAF needed a very long range reconnaissance aircraft for bomb damage assessment and photographic intelligence collection.

To meet the need, the USAAF Air Technical Service Command issued a requirement for a “Fours” reconnaissance aircraft – a top speed of 400 mph, service ceiling of 40,000 feet and a range of 4,000 miles – for the missions over Japan. Such performance would make it invulnerable to interception by piston engine fighters.
Republic Aviation, which had previously built only fighters, saw an opportunity to move out of the fighter business and into the lucrative large aircraft business and began work with an eye toward building an aircraft that could double as a post war civilian transport.

The Republic design, the XF-12 Rainbow, was the brain child of Republic designer Alex Kartveli. It was, by any measure, an inspired design, so much so one was hard pressed to know where to start.

The XF-12’s main goal was high performance, so it had a long, cylindrical fuselage with a fineness ratio of 10-to-1 and topped by a huge, but necessary, vertical stabilizer and a mid-mounted horizontal stabilizer with slight dihedral for further stability. It had a smooth glazed nose, similar to the B-29, and a high aspect, 130-foot long wing with a Republic designed laminar flow airfoil section.

The wing was mounted at a right angle to the fuselage to further reduce drag.
The Rainbow had four 3,000 horsepower Wasp Major radial engines to ensure it could carry a heavy load of reconnaissance equipment in the fuselage.
The engines, unlike most radials, were long and slender and fitted into nacelles with the complicated turbo-supercharging ducting in the rear, making each nacelle about the length of a P-47 Thunderbolt fighter.

To keep the nacelles streamlined, they had a sliding cowls arrangement rather than drag producing cowl flaps, and the cooling air for the oil came from very efficient slot intakes that covered 25 percent of the wing leading edges.

By mid-1945, the prototype Rainbow was taking shape and was expected to be ready for the final assault on Japan in 1946. But the atomic bomb attacks on Hiroshima and Nagasaki ended the war before the Rainbow entered production.
The program was allowed to continue and the XF-12 made its first flight on Feb. 4, 1946. During the XF-12’s flight tests it exceeded all the requirements with a service ceiling of 45,000 feet, a top speed of 470 mph and a range of 4,500 miles.
For reconnaissance, the XF-12 had three separate photographic compartments usually fitted with six-inch focal length K-17 cameras, the forward one for vertical shots, the center station with three windows for a “trimetrogon” system of three cameras giving horizon to horizon coverage, and the rear station two split vertical cameras that produced a 3-D image.

Aft of the wing was a bay in the belly for 18 high-intensity photo-flash cartridges, and, most remarkably, the Rainbows carried complete darkroom facilities to develop and print film in flight.

The XF-12 demonstrated its capabilities on “Operation Birds Eye” on Sept. 1, 1948. The second prototype departed the Air Force Flight Test Center at Muroc, Calif., climbed to 40,000 feet cruising altitude and headed eastward toward New York, photographing its entire flight path over the U.S. The result was  a continuous 325 foot-long strip of film composed of 390 individual photos covering a 490-mile-wide field of view. The aircraft landed in Long Island, N.Y., completing a six hour, 55 minute flight and averaging 361 mph. However, just a month after “Bird’s Eye,” the second XF-12 prototype crashed because of an engine fire.

The first prototype continued the flight testing and development phase but the Air Force declined to order any additional aircraft because of the cost of a new aircraft program post-world War II and fact that the Air Force had the B-29s and its follow on, the B-50, available for the reconnaissance role. 

Republic had intended to build an airline version of the Rainbow, the RC-2, for the
commercial airlines but the airlines, like the Air Force, opted for less expensive but available aircraft like the C-54 (DC-4), ending the Rainbow program.

As a sad final note to the program, the sole surviving Rainbow, rather than being preserved, ended its life as a target on the Aberdeen Proving Ground in Maryland.

(For questions or comments, contact Dr. Michel at marshall.michel@ramstein.af.mil.)