The M56 is an amazing-looking little machine that was created to give US airborne forces a self-propelled anti-tank gun that could be parachute dropped out of an aircraft. It was a radical design with extreme attributes: the size of a car, the gun of a medium tank, and absolutely no armor.
The gun was so powerful compared to the chassis that the front portions of the track actually lifted off the ground when it was fired.
Its goal was to be dropped behind enemy lines with airborne units, but instead it was put into action in the jungles of Vietnam.
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Background
During the Second World War, the United States had fielded a number of turreted, tracked AFVs that were specifically designed to knock out enemy tanks. They were not classed as tanks, but tank destroyers, and were operated by a tank destroyer branch.
They were to be the anvil that any enemy armored thrust would slam into.
American tank destroyers were thinly armored, light, mobile, and packed a punch. However in practice the concept is regarded as a failure, and the branch rarely operated in its intended doctrine. In fact, many tank destroyers were simply used alongside tanks.
In the end, the tank destroyer concept was proven to be flawed, and failed to accommodate the actual tactics and conditions of the Second World War. In 1945 the branch was disbanded, and the job of killing enemy tanks was now to be carried out by tanks.
But as soon as the war ended, the Soviet Union became the United States’ primary foe. And what did the Soviets have lots, and lots, and lots of? Tanks.
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This wasn’t just T-34s either. The Soviets were building impressive, powerfully armed, and extremely well armored heavy and medium tanks.
New tanks like the Pershing, and later developments like the M103 were intended to counter the USSR’s enormous quantity of tanks, but there was one branch that sorely lacked any ability to defend itself against them: the airborne.
Airborne units didn’t have the option of relying on tanks to deal with enemy armor.
This line of thought wasn’t new; the British also faced this issue during the war, which resulted in tanks like the M22 Locust. However these vehicles were very lightly armed and incapable of fighting the tanks of their day. To fight post-war tanks, much, much more firepower would be needed.
Development of the M56
In October 1948 this problem was officially addressed at a meeting at Fort Monroe, Virginia. At this meeting, a requirement was established for an air-droppable vehicle that had the gun of a medium tank and could use its ammunition.
In many ways it was a revival of the old tank destroyer concept; being light, mobile and packing a punch, but was now airdroppable.
The following year it was settled that its gun was to be the 90 mm T119, the type used in the T42 medium tank that was expected to replace the M26 and M46.
In 1950 the vehicle was given the designation of Gun, 90 mm, Self-Propelled, T101. Weight was to be 8 tons (16,000 lbs), top speed was to be 25 mph, and it needed to have a range of 100 miles.
The T101’s 90 mm gun was essentially the same as found on tanks like the M46 and M47, which weighed almost 50 tons. Naturally, fitting it inside a vehicle that could achieve those requirements without exceeding 8 tons was a tall order. Some sacrifices had to be made.
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And by sacrifices, we mean zero armor. Seriously, this thing had absolutely no armor. The only thing that could be considered “protection” was the gun shield, which was probably more useful at stopping bugs from hitting the crew while driving.
Some slight modifications were made to the gun, namely to reduce its height. This involved changing the recoil cylinders from four to two, and moving them to the top of the gun. Thus, this 90 mm was designated the T125 (standardised as M54).
Two pilot vehicles were ordered, and these were completed in 1952. The T101 actually came in underweight, and was parachuted out of C-119s a number of times during testing.
In 1955 the T101 was accepted into service, under the designation Gun, Self-Propelled, Full Tracked, 90mm, M56. Later, it was officially named the Scorpion.
Production began in late 1957, with all M56s being built by General Motors at their tank plant in Cleveland, Ohio.
Design
As a vehicle that had to be dropped out of an aircraft, the M56 is extremely small, perhaps more so than you already think. Its tiny stature kept its weight down, and made it possible to fit inside the relatively small size of aircraft cargo holds.
The M56 is a genuinely tiny vehicle, perhaps more so than you already think. Its 4.5 meter (179 inch) long hull is shorter than many modern cars. It was slightly longer when including the gun, reaching 5.8 meters (230 inches), but it was still much shorter than the 8.6 meter (335 inch) long M47, yet it carried the same gun.
Width was 2.56 meters (101 inches), and height was 2 meters – a little taller than a person.
The M56 had a simple, very low profile, open top hull that was constructed from riveted aluminium. The engine was located at the front, with the nose forming the engine compartment, and the gun was situated in the center. Two tracks sandwiched the hull.
The driver sat in a tub in front and to the left of the gun while the gunner sat to the right of the gun.
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The commander and loader were given rather crude positions on the left and right track fenders respectively.
The rear of the hull dropped down between the tracks, creating a small ledge that the loader could stand on to access the gun’s breech. This area also served as the M56’s ammunition rack, with capacity for 29 rounds of ammunition, stored inside horizontal tubes.
These tubes were spring loaded, and featured a small pressure-activated indicator on their covers that showed if they were empty or not.
The engine was a six cylinder Continental AOI-402-5 that made 200 hp. Looking at its name, we can deduce that it is an Aircooled, Opposed, fuel Injected engine that displaced 402 cubic inches (6.6 litres).
Power was sent through an automatic Cross-drive CD-150-4 transmission with two forward gears and one reverse gear.
The M56 rolled on a set of 20 inch-wide metal-reinforced rubber tracks. These were wide for the M56’s weight, and gave it a ground pressure of just 4 psi.
Suspension was the torsion bar type, and there were four air-filled rubber road wheels per side. The rubber tracks and wheels were a weight-saving measure.
The M56’s gun was the 90 mm M54, modified from the T119 gun intended for use in the failed T42 medium. The T119 and subsequent M54 were later developments of the M3, which dates all the way back to the M36 Jackson from the Second World War. The M3 was also used in the M26 Pershing and M46 Patton.
This meant that the M54 could fire the same ammunition as the M3-series of guns. However, because the M54’s maximum chamber pressure was much higher than these earlier guns (47,000 vs 38,000) they couldn’t fire its newer ammunition.
The casings of the high pressure rounds were shaped slightly differently so they couldn’t be accidentally loaded into the lower pressure guns.
While this power was great for knocking out enemy tanks, it was quite the nightmare for the M56. Its great recoil would rock the vehicle back so hard, that the front portions of the track would actually come off the ground!
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The gun could traverse 30 degrees left and right, elevate to +15 degrees, and depress -10 degrees.
When empty, the M56 weighed just 6.2 tons (12,500 lbs), increasing to 7.9 tons (15,750 lbs) when combat loaded. Its 200 hp engine got it up to a respectable top speed of 28 mph.
M56 in Service
When designed, the M56 successfully fulfilled the requirements laid out for it at the start of the project. It could be delivered by aircraft or parachute dropped, and had a very potent gun.
Functionally, the M56 worked very well. It had excellent mobility, thanks to its light weight and low ground pressure, and it was very easy to operate. It was so sparsely equipped that crews could be trained quickly.
It had plenty of firepower, and its lack of armor wasn’t an issue as it airborne units had no armor anyway. In addition, its open-top gave crews unbeatable visibility and battlefield awareness.
But on the battlefield they didn’t provide much value. The M56 was passed around a number of divisions, including both the airborne and infantry, but only the 173rd Separate Airborne Brigade used them in combat.
M56s were deployed with the 173rd to Vietnam in 1965. Here, the vehicle was tested in one of the most difficult environments for tanks to operate in.
Thick vegetation, mud, unforgiving terrain, poor visibility and dirt all work against heavy vehicles that lack visibility at the best of times. However, the M56’s excellent mobility came through, and allowed it to go places most other vehicles – especially tanks – couldn’t go.
But on the other hand, the jungle exposed the M56’s weaknesses. In particular, the crew were completely exposed in an environment that could hide threats from all angles – not to mention the elements.
This wasn’t helped by the 173rd regularly using the M56 outside of the role it was originally designed for.
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Tanks were not a threat encountered often, so the M56s’ 90 mm guns were put to work in the fire support role, assisting troop movements and clearing sections of jungle. Canister rounds were especially useful in the jungle.
M56s also performed base security, reconnaissance missions, and joined patrols. This was certainly not what it was designed for, and these roles would have ideally been performed by something with armor.
Its users would give special attention to the positioning of M56s prior to engaging the enemy. Usually, they would be concealed out of small arms fire range to more comfortably put the 90 mm to work.
The enormous recoil of the gun caused the entire vehicle to violently jump back, so it was advised that the commander and load get off the vehicle before firing. The commander would often stand next to the vehicle to observe the fall of shot on the target and give corrections to the gunner.
Due to the large cloud of dust thrown up, and the movement of the entire vehicle, follow up shots took longer.
Crews also learnt to establish other advantageous positions further back, should they need to fall back.
M56s would be pulled out of Vietnam toward the end of the 1960s, mostly due to their lack of armor.
Some success for these unneeded vehicles was found on the export market in the ’60s, with Spain buying a handful, and Morocco buying 87. Spain ran out of supplies and parts for the M56s by 1970, but it seems Morocco kept theirs for much, much longer – as recent as 2010, albeit in storage.
So in overview, the M56 was a radical design built to satisfy a radical set of requirements. It did this, and even exceeded them in some areas. It was mobile, had a powerful gun, and was easy to use. However its air-transportable nature wasn’t utilised, and its lack of armor proved to be its ultimate undoing.
Had the M56 been dropped behind enemy lines alongside paratroopers and engaged Soviet armor until the US’ medium tanks arrived, it may have been regarded as a great success.
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But instead the M56’s legacy was carved in the jungles of Vietnam, doing a job it simply wasn’t meant to do.
