Grumman Fast Twins Part 1 – The XF5F “Skyrocket”

Grumman made an undoubted name for themselves throughout the Second World War with their famous “Cat” series of aircraft, culminating with the Bearcat and Tigercat, arguably the pinnacle of piston powered flight. In the late 30s however, Grumman almost created another twin engined legend.

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An early mock-up of the Skyrocket design.

Grumman presented a design for a new twin engined carrier based aircraft in 1938. This was in response to the requirement issued by the Bureau of Aeronautics (BuAer) – SD112-14. The design proposal was simple; a lightweight airframe powered by two 1,200hp R-1820 engines. This new aircraft would also make use of contra rotating propellors to give better ground handling and a better rate of climb, much like types such as the P-38 and Hornet of later years.

The final design that Grumman brought out looks more like something out of Star Wars (Pod Racer anyone?) than anything we associate with 1930s aviation. Onlookers wouldn’t have been hard pressed to think the new aeroplane was an April Fools trick as the Skyrocket took its first flight on the 1st April 1940.

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The Skyrocket’s futuristic lines in flight.

Working to the BuAer specification, Grumman’s new machine featured twin engines, good visibility and a high concentration of armament in the nose. Early figures suggested that the new fighter would have a top speed of 335mph and a stalling speed as low as 67! As development went on the sole prototype expanded a little from the initial dimensions that Grumman proposed, with an interest in improving stability.

Obviously as a proposed naval aircraft, space would be at a premium when on an aircraft carrier. While the initial appeal of the safety of two engines seemed important there were concerns that this would make for a very large aeroplane. Luckily Grumman found the solution was to make the fuselage as thin as they possibly could, which allowed the engines to be placed closer together. This not only reduced the space needed to store the aircraft but also improved single engine performance.

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A look in the cockpit of the Skyrocket with the two large radial engines clearly in the peripheral. 

During the development phase a number of engine ideas were suggested, with Pratt and Whitney power plants eventually being dismissed in favour of the more powerful Wright Cyclones.

After much debate in the Grumman works about the centre of gravity of the aircraft and a number of variations in design, a full scale wind tunnel model was created and deemed ready for testing by October 1938. These wind tunnel tests found a number of stability issues earlier on which led to further modifications to the initial design.

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The prototype under construction.

In February 1939, a group of US Navy officers examined a full scale mock up of the fighter complete with folding wings. Those who saw the new machine at the time commented that it offered the “finest gun installation ever offered on a naval fighter.”

Alongside the pair of 50 cal and 30 cal machine guns, the US Navy hoped that the XF5F would also have a 23mm Madsen cannon. Grumman had concerns about constructing a suitable housing for the weapon. As it turned out they soon received confirmation from BuAer that the weapon would no longer be available for this project.

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A look at the Skyrocket’s simple wing fold design – essential for a naval aircraft. 

By the time the prototype had been completed, taking into account the Navy’s requested changes, the aircraft was 700 pounds over the specified weight. Grumman were initially in line to pay considerable fines for this oversight, though they justified this by pointing out the weight increase was at the request of the Navy. This resulted in the BuAer only viewing 196 pounds as overweight and as such no charge was raised.

In March 1940, the completed prototype was finally rolled out as BuNo 1442, the aircraft was all over silver except for the top of the wings which were finished in yellow. This roll out was soon followed by the design’s first flight on the 1st April in the hands of R.A. Gillies. Early flight tests were plagued by high engine temperatures, though 20 flights averaging 45 minutes were made during the first month of testing.

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Another inflight look at this distinctive design.

As a result of these temperature problems modifications were made to the oil-cooling system before further flight testing took place. In January 1941 dive tests began in the hands of “Connie” Converse, who managed to get the XF5F up to 505mph on the 1st February. Converse commented that the XF5F’s flying qualities were generally good, commenting that the counter rotating propellors made take off handling much better. Single engine performance was also considered good and aerobatics were found to be easily performed.

Various modifications to the aircraft’s gun installations were discussed and proposed at this stage, but there is no evidence that any weapons systems were ever tested on’1442.  Modifications that were pursued involved extending the nose beyond the leading edge of the wing and the engine nacelles behind the trailing edge. This resulted in an increase in speed but caused a very abrupt stall compared to that of the first incarnation of the type. This aggressive stall led to the long nosed version having a faster landing speed.

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Bu. 1442 is looked over during the testing process. 

In 1941 as test flying continued it is noted in official Grumman correspondence that the XF5F picked up the name “Skyrocket” this also applied to the then proposed F7F, which would later become the Tigercat, though it was also known as the Skyrocket during early development.

The Skyrocket took part in a trial involving a section of the best fighter aircraft of the era in 1941, going up against the Spitfire, Hurricane, P-40, Wildcat and Corsair to name a few. A pilot by the name of Crommelin flew the XF5F in these trials and commented how easily he was able to pull away from the Corsair, to the extent that he thought the Vought machine was having engine troubles! He also felt that the lack of torque and excellent forward vision would have made the Skyrocket an excellent naval aircraft.

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Another look at the Skyrocket ready for carrier stow. 

As orders started pouring in for the Wildcat it became clear to Grumman that production orders for the XF5F were not going to be forthcoming, especially as the aircraft was getting heavier and less desirable with each development. These factors combined with the ongoing costs led to Grumman ceasing development of the navy fighter. Grumman made their stance clear to the BuAer in March 1942 and the Navy cancelled their contract in September.

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The long nosed Skyrocket waiting for flight testing. 

During its flight testing both pre and post cancellation the Skyrocket had its fair share of accidents on the ground, including an arrestor gear failure and the undercarriage collapsing whilst parked! The prototype was repaired from all of these minor instances. In December 1944 the aircraft was making an approach to NAS Floyd bennet field and the undercarriage would not come down, the subsequent damage from the belly landing was deemed beyond economical repair and the aircraft was struck off active service the next day. Though the wrecked machine was used as late as February 1945 as a crash simulation machine.

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A look at the long nosed, spinner equipped later modifications applied to the prototype.

At the end of what was, for a single prototype of a cancelled project, a long flying career, ‘1442 flew  211 flights and 155.7 hours a fine career for a unique and ultimately underused design. The Skyrocket was a wonderful machine to look at, looking like something straight out of science fiction and by all accounts, a great aircraft to fly. Ironically the Skyrocket’s longest lasting legacy was as the mount of the “Blackhawks” as part of the “Quality Comic” Series. This fictional squadron flew the Skyrocket in their stories throughout World War Two.

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Another look at the long nose modifications in flight. 

As can be seen from this article, government imposed changes made for an overweight and underpowered aircraft which at times became dangerous to fly, leading to a restricted test flying programme compared to others. This led to costs of development running out of control and ultimately this striking design being cancelled.  The Skyrocket name and design did not die with this machine however. Grumman would carry on and produce the XP-50 a tricycle gear variant of the XF5F, as well as the F7F Tigercat. Both of these latter designs will be covered in part 2 of this series.

All pictures in this article have been sourced from the San Diego Air and Space Museum Archives. 

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5 thoughts on “Grumman Fast Twins Part 1 – The XF5F “Skyrocket”

  1. Glen Judith Gillies says:

    Hi, great article. It was my grandfather that made the first flight, and he went by B. A. Gillies, his name is miss spelled in the article. Thanks for putting this information out there. I find it interesting that Grumman was competing against itself with the Wildcat! Cheers, Glen J Gillies

    Like

    • Warbird Tails says:

      Hi Glen,

      What an incredible story, I must apologise about the spelling, I will ensure that is corrected soon. I find the Skyrocket fascinating its such a shame that more hasn’t been written about it, a thoroughly unique machine. I’d love to speak more with you regarding this if you would be interested?

      Liked by 1 person

      • Glen Judith Gillies says:

        Hi Warbird – sure – I’ve learned a little more about it every time I get into a research frenzy. My Grandfather was involved in a lot of cool projects. I just found out about a Loening, folding wing, seaplane that was designed to ride in a tube on the deck of a submarine, and to take off when the submarine submerged enough to get its feet wet 🙂 Who knew !!!

        Like

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