In the aftermath of the second world war tank designers around the globe were trying to solve one of the classic problems of armoured warfare.
How can you do damage to enemy tanks when their armour is getting thicker and thicker?
To put this in perspective, at the start of WW2, almost all tanks of the period had an armour thickness of between 15 and 30mm.
As a result, a standard tank gun was a 20mm autocannon, or even a .50cal machine gun,
with the largest guns being, at most, around 40mm.
By the end of the war, just 6 years later, the armour on tanks had ballooned
with typical medium tanks of the period having armour at least 60mm thick, usually with a significant slope.
This meant guns had been forced to get bigger and more powerful too, with medium tanks typically having a 75mm gun at minimum,
and heavy vehicles mounting massive guns, up to 122mm as seen on the IS-2.
So, as tank development continued after the war, designers were concerned that, if armour kept getting thicker, there would be a point where the guns needed to penetrate it would be just too big to be practical.
The British Conqueror and American M103 mounted 120mm main guns and were pushing the limits of how massive a tank and its ammunition could realistically be.
British engineers even tried to mount a 183mm anti-tank gun on a centurion chassis. Twice.
A different approach was badly needed, and some American designers thought they had the solution.
By the end of the second world war, anti-tank weapons were relying less and less on kinetic energy to destroy enemy vehicles.
Weapons like the Bazooka and Panzerfaust were hugely effective, and used shaped charges to penetrate the thick armour of late war tanks.
Shaped charges, like those used in HEAT shells, works by using explosives to shape and propel a jet of molten metal out the front of the warhead and into the target.
Unlike kinetic penetrators, this charge is in no way dependent on the velocity of the projectile,
and this meant that lightweight or low velocity weapons could, for the first time, have impressive anti-armour capabilities.
Was the age of the high velocity anti-tank gun over?
The US Army thought it might be, and decided to use a shaped charge warhead to create one of the first Anti-Tank Guided Missiles.
Shaped charge weapons at the time were often fired from relatively low velocity guns or shoulder mounted anti-tank rockets.
This meant that they could be incredibly hard to aim over long distances, and the thinking at the time was that mounting the charge on a powerful, guided missile solved this issue.
By 1958, the Americans had bet big on missiles being the future of armoured warfare
and wanted their new main battle tank to fire missiles from its main gun as the vehicles primary anti-tank weapon.
The new missile, known as XM13, was incredibly advanced for the time period.
A small charge at the base of the missile would propel it out of the gun tube,
before fins extended from the rear of the missile and the solid rocket motor ignited, powering the missile to over 700 mph.
All the gunner would have to do is keep his aiming reticle on target, and the 6.8kg shaped charge warhead would detonate on impact,
with XM13 being able to penetrate 600mm of armour on a target up to 2km away.
The first test shots were fired in 1960, enjoying success on the test range. Promptly adopted into service, the XM13 was redesignated as the MGM-51,
and named the Shillelagh, after a big stick from Ireland.
However this big stick wasn’t out of the woods yet.
It became clear that Ford, who were developing the Shillelagh, had underestimated the complexity of the task in front of them,
and there were major problems with the missile and it’s 152mm XM81 launcher.
At this stage testing was actually done using M60 hulls with modified T95 turrets, a fact which will become a lot more important later on.
While this was all going on, the army were in the middle of developing the Armoured Reconnaissance/ Airborne Assault Vehicle,
a new air-deployable light tank that was to replace the M41 Walker Bulldog after the cancellation of the T92 project.
This vehicle was going to be the first to use the Shillelagh, but a decision had to be made on its armament by April of 1962.
The missile was far from perfect, but by this stage some improvements had been made and Ford were confident they’d be able to make it work.
And so it was adopted for use on the new vehicle, which would enter service as the M551 Sheridan.
We’ll fast forward to June 1967.
The Sheridan had just entered service in the 63rd Armor Regiment, and was armed, as planned, with the M81 launcher that could fire the Shillelagh as well as conventional 152mm HE, HEAT or smoke rounds.
Testing of the missile during the development of the Sheridan had discovered even more issues.
Due to the layout of the tank, the missile wouldn’t actually be visible through the gunner’s sight until it had travelled around 730m from the vehicle.
This gave it a very long minimum range, and meant the gunner only had seconds to correct the path of the missile before it would reach its maximum range of 2km.
But there was more. Missile technology in the 60s was still in its infancy, and the equipment needed to track and guide the shillelagh was expensive, complex, and, importantly, quite fragile.
As it turns out, 152mm shells don’t really care about how fragile the equipment was, and firing the main gun was known to completely destroy the missile guidance equipment.
This isn’t very surprising, as firing would lift the first and second road wheels of the Sheridan clean off the ground, before slamming the 15-tonne tank back to earth.
Ok so maybe the Sheridan and the Shillelagh just weren’t a good match.
The missile had undeniable performance against armoured targets, with much higher penetration values than the 105mm guns currently in service with frontline NATO tanks.
Maybe a heavier vehicle was a much better candidate for the M81 launcher. Something like the M60?
Since the entire premise of the Shillelagh was that it would arm main battle tanks, the M60 hull had been used to test the missile in the early 60s.
These test vehicles had been designated M60A1E1.
A new, low-profile turret had since been designed for the M60, which reduced frontal turret area by as much as 40%.
The idea was that this turret would be paired with the M81 launcher, and would allow the M60 to use the Shillelagh to destroy enemy armour at distances of up to 4km,
providing a stop-gap solution until the next American MBT was ready.
These missile armed M60s were to be designated M60A2.
The vehicle looked so bizarre and futuristic that they were unofficially referred to as “Starships”.
Unfortunately for the Army, the Starship was also plagued with technical difficulties, despite the vehicle not entering service until 1974,
after some of the more glaring issues with the missile system had been addressed.
The M60A2 would end up succumbing to the same issues that had been seen on the Sheridan,
and was taken out of service in 1981, after just 7 years.
The MBT-70 project, the tank that, in essence, the entire shillelagh system was designed for, had actually been cancelled in 1971,
years before Starship would even enter service.
MBT-70 would’ve used the Shillelagh in a longer barrelled M150 launcher, an improved variant of the M81.
Some aspects of the hyper-advanced MBT-70 project would end up being carried over to the M1 Abrams, but the Shillelagh missile was not one of them.
The story doesn’t end there though - The M551 Sheridan was still kicking, even if it was only because the Army had nothing to replace them with.
Sheridan was still in service with the 82nd Airborne until 1996,
and 51 vehicles served in Operations Desert Shield and Desert Storm in 1991.
This was the first and only time a Shillelagh missile was fired in anger, with as many as 6 being fired at Bunkers in Iraq and Kuwait.
To put that into perspective, 88,000 missiles were produced.
In the end, the Shillelagh was considered a failure.
In the years following its development, kinetic penetrators would see huge technological leaps,
allowing regular tank guns to have incredible anti-armour capabilities without the drawbacks that guided missiles bring.
The issues with the Shillelagh meant Sheridan and Starship were likely both doomed from the very beginning.
Luckily the American designers were smart enough not to put all their eggs in one basket, and ended up with the M1,
arguably one of the most capable vehicles ever to see service.
Shillelagh was meant to be a world beater, but will go down in history as a lesson learnt,
a steppingstone to more capable technologies.
Missiles like TOW, Hellfire, and Javelin owe a lot to the shillelagh, and all are still in service with US Forces today.
Maybe missiles were a good idea after all.