The Grumman X5F Skyrocket; Naval Hot-rod

September 20, 2022

In 1935 the US Navy had a bit of a crisis. They were just about to bring into service their newest carrier fighter, the Grumman F2F and were already flying the prototype of the next one, the F3F, which would remedy issues with the -2F and offer improved performance.

 

But though both aircraft were solid examples of their type, that type was biplanes and the writing appeared to be firmly on the wall as to where fighter aircraft designs were headed, and that was in the monoplane. So, in 1935 the US Navy also issued a request for a new fighter of that type as well, and this quickly led to the development and adoption of the Brewster Buffalo and the Grumman F4F Wildcat in the next few years.

But while this was going on, the Navy had another concern, one I suspect was created in fact by the actions of their sister service.

In 1934 the United States Army Air Corp (USAAC) issued a specification for a large, long-range multi engine bomber that could operate at high altitude and carry – for the day – a heavy bombload that it should be able to drop with great precision. And in 1935 an aircraft flew that aimed to meet this requirement; Boeing’s Model 299, which went on to become the famed B-17 Flying Fortress.

This was, by the standards of the time, a huge aircraft with unapparelled firepower and potential capability. But think about it from the US Navy’s point of view. If their sister service could get hold of such a formidable aircraft, so potentially could the United States’ rivals.

And this was a real issue. The Navy’s fighter were biplanes armed with two machine guns. In performance terms they would struggle to catch an aircraft like the proposed B-17, and even if they did, they might not even be able to shoot it down.

So, the Navy started to think about what sort of aircraft might be needed to counter such a threat. This process got a real boost in 1938 when the USAAC, in a blatant publicity stunt, “intercepted” an Italian ocean liner more than 600 miles out at sea and photographed it, using the event to trumpet that they could do a better job of defending the coast of the Continental United States than the Navy.

Putting aside the row that ensued from a tactical point of view the US Navy realized that the problem was serious. This was in the period before radar was fitted on ships, and so locating an enemy air attack was done by eye.

The US Navy realized that this raised a whole new issue. They were now facing the possibility of having to deal with big, fast, powerful, high-flying aircraft that the first warning of an attack might be when the lookout saw them. Even the new Buffalo and Wildcat didn’t have performance, especially in rate of climb, that would allow them to intercept such aircraft before they launched their attack.

What the Navy needed was a fast climbing, heavily armed bomber killing interceptor.

So, they put out a request for such an aircraft. This should be as fast as possible and with as high a rate-of-climb as could be achieved, along with an armament of two 20mm cannon and two machine guns.

Several companies submitted proposals, but most struggled to come up with a viable idea. In 1937, the problem was that the most powerful engines available for service fighters only produced around 1,000hp, and that just wasn’t enough to fulfil the requirement.

So, Grumman came up with what seems on the face of it the logical solution. If one engine doesn’t provide enough power, have two.

Meet the Grumman XF5F Skyrocket.

 

Now, I’m sure that you’ll see that Grumman’s solution was somewhat unconventional. The Skyrocket turned out as twin-engine, low-wing monoplane with a twin-fin tail assembly and the undercarriage retracting into the engine nacelles. But most notable was the nose, or lack of one really, with the wing’s leading-edge protruding at the front of the aircraft.

 

Odd looks aside, the Skyrocket did broadly meet the criteria.

The aircraft as initially proposed was intended to use two Pratt and Whitney Twin Wasp Junior engines which produced over 800hp each. But issues with that engine’s development meant that the prototype would actually be fitted with Wright 1820s.These were substantially more powerful, producing 1,200hp each, but were heavier and had a larger frontal section. This increased drag and negatively impacted the pilot’s view.

Neither Grumman nor the US Navy were particularly happy about that factor but accepted it as a fact as they wanted to see what the aircraft could do. This was the first two-engine carrier fighter the US Navy had properly considered and so it raised some interest, and an order was placed for a single prototype.

Grumman did at least apply some useful thinking by having the two engines rotate in opposite directions to one another, which eradicated engine torque and was to prove a very popular decision with the test pilots.

In terms of armament, the proposed layout would have been truly devastating by pre-war standards. Instead of the specified 20mm cannon, Grumman gave thought to fitting two Madsen 23mm ones instead. This is one of those pre-war guns that got a lot of attention before the Second World War broke out, but never really delivered on its perceived capability.

Because of its intended use as a bomber destroyer, apparently in 1939 the US Navy asked Grumman about adding racks on the wings for small bombs. This was an idea a lot of nations played around with, which involved dropping explosives onto a bomber formation.

In addition, the new interceptor concept proved interesting enough that the US Army decided to get in on the action and ordered a version for their own requirements for testing, the XP-50. But that is a story for another time.

Development of the XF5F proceeded gradually, and by April 1940 the aircraft was ready for its first flight. This found that the was a pretty good performer, proving to be agile and with reasonable recovery characteristics in a spin, and would also handle satisfactorily on a single engine.

Most impressively, the aircraft demonstrated a top speed of 383mph (616 km/h) at 20,000 feet (6,096 m) and a climb rate of around 4,000 feet per minute (c.1220m per minute). At this time, that was an astounding performance.

 

But the aircraft had some issues with engine overheating and this led to the need for further revisions to the design.

There were also problems now in acquiring the desired weaponry. Denmark had fallen to Germany and so the decision was made to alter the armament to two 0.5-calibre machine guns alongside the two .30-calibre ones and then after the test flights to four of the heavy machine guns.

This led to Grumman changing the design, and the XF5F now acquired a nose to house it, which also went someway to apparently alleviating the Grumman engineers concerns on the stability of the aircraft.

 

Because though the pilots might have liked it, the engineers worried about the novel arrangement and as such the XF5F was very much an experimental design.

The altered prototype was handed over to the US Navy in February 1941 and began fight testing with service pilots. And they too reported very favourably on the aircraft.

The Skyrocket had good take-off and landing characteristics, and the cockpit was very well laid out. Test flights were conducted against a range of available aircraft, including the Spitfire. apparently, the XF5F showed a clean pair of heels to all of them in the climb.

According to a later statement by Lt. Commander Crommelin, who was in charge of the test program, this included the Navy’s new wonder fighter under development, the XF4U. This had been developed to the same requirement as the Skyrocket, and at the time was equipped with the first versions of the Pratt and Whitney Double Wasp engine, which produced 1,850hp.

Crommelin would recall that in a climbing race to 10,000 feet against the XF4U he outpaced the single-engine aircraft so thoroughly he believed that it was suffering engine trouble.

But for all this, the Skyrocket never received a production order, and would end up being limited to a single prototype. This would be used for testing and development work until 1944 when it was scrapped.

So, with quite amazing performance and high praise from many of the pilots that flew it, why didn’t it get into service?

Well, like many such decisions, there were multiple factors.

As said, the XF5F was going up against the F4U. though it might show a clean pair of heels in the climb to the single-engine fighter, in level flight the Corsair was the rocket ship. It was also thought that it had more development potential, probably correct because the Skyrocket, for all its good qualities, was unlikely to be able to accommodate more powerful engines and therefore would only suffer if it got into service and was weighed down with all the associated equipment.

 

So, the Navy decided on the Corsair to meet the original requirement in July 1941.

There was also the factor that the Skyrocket’s novel two engine layout required more materials and would cut down on the production of other aircraft. After all, the engines in one XF5F could power two single-seaters. Even before the USA entered the War, by mid-1941 that was a critical factor, with every American builder getting into full speed production.

And Grumman was amongst them. They were now producing as many of the F4F Wildcat as they could, and they also now had an order for its successor, the F6F Hellcat. These aircraft were recognized as the future priority both by Grumman and the Navy.

And there is another factor that often gets overlooked. At the time of the XF5F proposal being formed in 1937, radar was still an interesting scientific experiment. But by 1941 the first air search radars were already deployed on US Navy warships, with twenty being in service aboard ship by the time of Pearl Harbor.

With that, rapid climb rate became far less important and existing aircraft were able to get up to altitude to counter attacks because of the earlier warning that radar afforded. The Skyrocket was a one-trick pony in a field – carrier operations – where diverse capabilities is a highly prized attribute, and that the existing and projected designs had.

Besides, the Skyrocket had pretty much been recognized as an experimental type at a fairly early stage, and though it did seem to have some potential, the problems it experienced in development and the lessons learned were more valuable in a successor aircraft that Grumman were planning.

This would ultimately evolve into the F7F Tigercat, a two-engine carrier aircraft that was, in the words of Fred Trapnell (one of the most experienced naval aviators in the world):

“…the best damn fighter I’ve ever flown.”

As the line of development of the Tigercat firmly starts at the Skyrocket, that’s not a bad epitaph.

But in fact, there is one more final aspect to the odd little fighter that bizarrely means it gets more recognition than you would expect.

In 1941 Military Comics began a new series about a mythical fighter squadron, the Blackhawks, who fought tyranny all across the world in rip roaring fashion, enthralling their young readers with their exploits. And their mount of choice, was the Skyrocket.

 

Not bad for a single aircraft that didn’t see any service.

Sources/ Related:

http://aviation-history.com/grumman/xf5f.html

https://planedave.net/2014/12/07/grumman-xf5f-1-skyrocket/

http://www.fiddlersgreen.net/models/Aircraft/Grumman-Skyrocket.html

https://en.wikipedia.org/wiki/Grumman_XF5F_Skyrocket

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