Cold War, Experimental, WWII

The XF-12 Rainbow was WWII’s SR-71 Blackbird

It isn’t often that a manufacturer creates an aircraft so good that it smashes its own design requirements, but the XF-12 did exactly that, thanks to the ingenious work of Republic. This aircraft is the epitome of “If it looks right, it will fly right”.

Republic, the same company that produced the P-47 Thunderbolt, set out in 1943 to create a reconnaissance aircraft that flew so high and so fast that nothing could catch it – the same goal the SR-71 would set out to do much later on. Both Boeing and Hughes failed to achieve these goals, but Republic pulled it off.

They made a stunningly sleek aircraft that was powered by four of the most powerful American aircraft piston engines. It easily out-performed its requirements, but in the end, the XF-12 was simply beaten by bad timing. Today it is remembered as the fastest four-engined piston aircraft ever.

F-12 parked up.
Just look at it! It looks fast while sitting still!


Background and Development

In 1943, the United States listed requirements for a high altitude and high speed aerial photo reconnaissance aircraft. Up until that point, the Allies had gotten by using modified versions of aircraft built for different roles, like the Spitfire or P-38 Lightning.

But they had limited range and speed, and were unable to carry much camera equipment or reach high altitudes.

What the US wanted was an aircraft built specifically for the job, one that could fly so high and fast that no enemy interceptors could climb fast enough to catch it. These characteristics would also allow it to capture more of the earth below in a shorter period of time.

Lockheed F 5.
Lockheed F 5, the photoreconnaissance version of the P-38 lightning. Note the ports near the nose for the cameras. The P-38 platform gathered almost all of the film captured over Europe during the war.

Imagine a Second World War version of the SR-71, and you have essentially captured what the US was after in 1943.

They also wanted to speed up the process of gathering intelligence to having it on the desks of analysists and planners. Typically, a reconnaissance aircraft would fly over the target, snap photos and then return to base, where staff would remove the rolls of film and send them to be developed in a dark room.

The developed photos would then have to be dispatched to the appropriate office for analysis. This took time, and in war, time matters, a lot. So this new reconnaissance aircraft was to incorporate its own dark room, so that upon landing the valuable images were ready to go.

Recon cameras from a Mosquito.
Photoreconnaissance cameras were large and heavy. Converted fighters were limited in how much they could carry.

But other than this, the aircraft had to meet some incredibly difficult performance requirements for the day. It needed to reach 400 mph, operate at 40,000 ft, and have a range of 4,500 miles. For 1943 standards, this was a very, very high bar.

Such an aircraft was to be used in the Pacific, where it could travel over the vast expanses of the region and photograph large swaths of Japan without risk of interception.

When these requirements were announced in October of 1943, Boeing, Republic and Hughes aimed to earn the contract. Boeing proposed a modified version of its B-29, which was already far along in its development.

B-29 take off.
The B-29 had taken to the skies a year earlier and much of its development had been done.

Hughes proposed a very different approach, the famed, and doomed, XF-11.

Hughes had created the XF-11 specifically for the contract by adapting the earlier D-2 into a reconnaissance aircraft. It was of moderate size, powered by two Pratt & Whitney R-4360 28 cylinder radial engines.

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It was heavily opposed by many in the procurement process, but due to Howard Hughes’ connections and powerful influence over Colonel Elliott Roosevelt, President Franklin Roosevelt’s son, he was able to secure a contract for 100 examples.

Hughes XF-11.
Howard Hughes’ XF-11. The aircraft was eventually cancelled.

Meanwhile, Republic submitted the XF-12 – the only aircraft that had been designed specifically to satisfy the requirements.

Republic had made quite the name for themselves during the war as the creator of the famous P-47 Thunderbolt. This was a powerful, rugged machine that relied on brawn over brains to achieve its goals.

However, Republic’s XF-12 was about as far away from rugged as physically possible: they had designed a sleek, elegent and beautifully proportioned four-engined aircraft that was as futuristic-looking as it was capable.

Plans for the XF-12.
Plans for the XF-12.

Named Rainbow, Republic placed a major emphasis on making an aerodynamically flawless aircraft. Its wings were long, thin and tapered, while its fuselage was uninterrupted along its entire length. The cockpit was incorporated into a pointed nose, similar to the B-29, but with even greater aerodynamic efficiency.

Each section of the aircraft was meticulously tested in NACA wind tunnels to optimise airflow and reduce drag. Each of its four R-4360 engines were located in large nacelles that protruded far forward and behind the wing.

Republic had determined that the only way to achieve the difficult requirements was with four of the most powerful piston engines ever built in numbers by the US – unlike Hughes, who had half as many engines on their XF-11.

XF-12 model in a wind tunnel.
NACA wind tunnel testing a scale model of the XF-12, minus its tailplane.

Air intakes for the engines were added into the leading edges of the wings. This rammed air into the superchargers at high speed, improving cooling and power simultaneously.

It was critical that these openings were perfectly sized and shaped, and this was achieved via extensive wind tunnel tests.

The high velocity wastegate gasses from the engines were ducted out the rear of the nacelle for additional thrust, adding the equivalent of around 300 hp per engine.

F-12 mock-up.
Full-scale mock-up of the XF-12.

A number of cameras were positioned near the rear of the fuselage, and were arranged to take photos vertically and to either side at a 30-degree angle.

The images could be overlayed and combined together. Each camera lens was electronically heated to prevent them freezing over.

Also inside the fuselage was a dark room, equipped with the facilities necessary to develop photos, along with racking to store the film.

RC-2 cockpit mock-up.
This is a mock-up of the RC-2’s cockpit, but it gives an idea of the view from inside the XF-12, which the RC-2 was based on.

It was also capable of taking photos at night. This was achieved via the use of photoflash bombs, which the XF-12 carried in a dedicated bay on its belly. Following their minimal-drag approach, the bay doors retracted internally so they wouldn’t interfere with the airflow around the fuselage.

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Photoflash bombs were air-dropped canisters filled with brightly-burning materials, like magnesium. They were set to detonate above the ground, releasing an extremely bright flash millions of candlepower bright.

XF-12 assembly.
XF-12 prototype under construction.

The flash would briefly illuminate the ground below, and the reconnaissance aircraft above would snap a photo.

In March of 1944, Republic was awarded a contract to produce two XF-12s along with the required spare parts for testing.

A mock-up had been completed and inspected by June, and the first prototype was finished in December of 1945.

F-12 in flight.
The stunning XF-12.

The XF-12’s Beautiful Design

As mentioned, the XF-12’s design focused on minimizing drag. Republic’s work produced one of the sleekest, most futuristic-looking aircraft of the 1940s.

At the front in the nose was a “double windshield”, comprised of an inner glass screen that sealed the pressurised cockpit, and an outer glass cone that reduced drag around the nose.

This used similar principles to ballistic caps used on some types of ammunition.

XF-12 nose.
This close view of the nose shows the “double windshield”. You can see the inner windows, behind the clear nose cone.

From here, the cigar-shaped fuselage continued back uninterrupted, ending in a fine point at the tail.

The vertical stabiliser was thin for increased aerodynamic efficiency, and the horizontal tail surfaces were slightly angled up so they wouldn’t encounter turbulent air from the wings.

The engine nacelles were circular in section and mirrored the fuselage’s stunning simplicity.

Rainbow, P-47, RC-3.
The XF-12 Rainbow beside a P-47 Thunderbolt and RC-3 Seabee.

They extended in front and behind the wing, an arrangement used by Republic that emulated the layout of a smaller fighter aircraft on each engine, so they were effectively “mini fuselages”.

This is apt, as each nacelle was longer than the fuselage of Republic’s P-47.

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Their length also increased their fineness ratio in a similar way to an aircraft fuselage.

Rainbow F-12 nacelles.
XF-12 nacelles.

Inside each nacelle was a Pratt & Whitney R-4360 Wasp Major, a 54.9 litre (3,350 cu in), 28 cylinder, twin turbo-supercharged radial engine that produced 3,500 hp. These were the same engines used in the B-36 Peacemaker, and it is the most powerful mass-produced American aircraft piston engine.

It was a four row engine, and because of this the air-cooled R-4360 often suffered from cooling issues, caused by the last row of cylinders not receiving enough airflow.

One way of resolving this is to increase the size of the engine cowling to improve airflow, but this increased drag.

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Republic used tight cowls to reduce the nacelles’ width, and omitted the cowling flaps found on most US designs of the time as they also increased drag. Instead, the entire cowling slid backwards and forwards automatically to increase or decrease airflow.

This function was performed automatically via the use of thermostats.

To further aid cooling, two-stage fans were fitted at the front of the nacelle, behind the propeller, to draw in more air.

XF-12 engines nacelles.
Note the fans installed behind the propeller cones.

Large and carefully designed air intakes for the engines were incorporated into the leading edge of the wings, between the inner and outer engines.

These took up about 25 percent of the wing’s leading edge, and provided significant ram air pressure into the turbo-superchargers.

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Gasses from the turbo-supercharger waste gates were directed out the rear of the nacelles through jet nozzles, providing an appreciable amount of thrust to further increase the XF-12’s power.

Rainbow nacelle nozzles.
The nozzles at the rear of the engine nacelles.

There was some thought of fitting an afterburner system that sprayed fuel into these nozzles, but this wasn’t pursued.

The fuselage contained three bays for photoreconnaissance equipment, and these could be fitted with any type of reconnaissance cameras used by the US at the time.

The fuselage was also pressurised, allowing the crew to operate normally without bulky flight suits or oxygen supplies, even when operating at 40,000 ft.

XF-12 camera compartment inside.
One of the XF-12’s camera compartments.

A crew of seven operated the XF-12. No armament was carried the aircraft, as its speed and altitude put it out of reach of enemy interceptors.

Overall, the XF-12 measured 94 ft (28.6 meters) in length, 28 ft (8.6 meters) in height, and had a wingspan of 129 ft (38.4 meters).

It had a maximum take off weight of 100,000 lbs (46,000 kg), around 15 tons lighter than a B-29.

XF-12 in flight.
Few aircraft can compete with the XF-12’s elegant lines.

With 14,000 hp on tap, the XF-12 actually over-performed on its difficult design requirements, with a top speed of 470 mph, a service ceiling of 44,000 ft (13,000 meters) and a range of 4,500 miles (7,200 km).

In fact, the aircraft had so much power that it was found it could climb to 25,000 ft with a full payload with only three engines running, and climb to 5,000 ft with only two engines running.

A Sad End to an Incredible Machine

The XF-12 first took to the air on February 4, 1946. The war had long ended, but the aircraft was so capable that many were still interested in it.

Republic tested the XF-12 for about a year before handing it over to the Army Air Force for their own evaluations. It had incredible performance, and its ability to photography huge areas quickly was unmatched at the time.

XF-12 first prototype after crashing.
The first XF-12 prototype after crash-landing with two out of three gear down. It was later repaired and put back into the air.

Unfortunately, after a test flight on July 10 1947, the prototype’s right main landing gear collapsed during landing.

The pilot gained altitude and began circling the airfield to burn off excess fuel to lighten the aircraft and reduce the chances of fire. On the second attempt they landed with only the nose and left landing gear down, resulting in the XF-12 prototype being heavily damaged.

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A second XF-12 prototype had been built, and this first flew on August 12, 1947.

F-12 during take off.
XF-12 taking off from Republic Field.

The following month, the aircraft was redesignated XR-12 when the Army Air Force became the Air Force.

In September 1948, the second prototype was able to show just how capable the XF-12 platform was at aerial reconnaissance when it crossed the entirety of North America at 40,000 ft, snapping 390 photos of the ground below as it went.

Travelling at an average of 360 mph, the XF-12 completed this flight in just under 7 hours. The photographs were developed, and presented as a complete strip by the US Air Force in New York.

Photographed portion of North America by F-12.
The portion of North American photographed by an XF-12 in just 7 hours. This was incredibly impressive for 1948 standards.

Sadly though, the second prototype would crash just a few months later after an engine failure. Two of the seven crew were killed, and the aircraft was lost to the ocean.

Meanwhile, the first prototype had been repaired and returned to the air. However, despite its incredible capabilities, the XF-12 received little interest from the Air Force.

Budgets had reduced with the end of the war, and developments with jet aircraft were soon going to make it obsolete.

XF-12 Rainbow from the side.
The XF-12 Rainbow’s sleek side profile.

For the photoreconnaissance role, Boeing’s RB-29 would win out. It was much less capable than the XF-12, and failed to meet all the 1943 design requirements, but it was ready and in production before Republic’s design.

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As they say, a good plan now is better than a perfect plan next week.

The lonely first prototype was retired in 1952. It would be transported to the Aberdeen Proving Ground in Maryland, and used as a target.

An RB-29, a similar configuration to the Lake Mead B-29
The RB-29, shown here, would become the US’ main high-altitude reconnaissance platform in the post-war years.

RC-2 Passenger Airliner

From its early development stages, Republic had designed the XF-12 with a passenger variant in mind. During the Second World War, aircraft manufacturers were well aware that a huge market for passenger airliners would open when the war ended.

The war reset the status quo, and now, anyone had the chance to dominate the age of the airliner when it arrived.

RC-2 cutaway drawing.
A cutaway diagram showing the internal layout of the proposed RC-2 passenger version of the XF-12.

Republic wanted in on this action, and planned an airliner version of the XF-12, named the RC-2.

The XF-12’s great speed and service ceiling leant itself well to the passenger airliner market, as it would reduce travel time, and could fly above the weather.

The RC-2 was to be slightly longer than the XF-12, and have room for 46 passengers in a luxurious air conditioned cabin.

RC-2 Rainbow Poster.
A Republic poster promoting the Rainbow passenger airliner. Note “Maker of the Mighty Thunderbolt” at the bottom right.

Large American airlines nearly ordered the RC-2, with one newspaper from the day claiming 38 had been ordered, with a price tag of $1.25 million each.

However all airlines pulled out when the XF-12 failed to enter production. Once again, it was beaten by designs from Boeing.

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No RC-2s were built.