The M551 Sheridan Tank – an Odd One

The M551 Sheridan Tank – an Odd One

by Bruce Lerner
February 2021

A model of the model showed up during the kit auction. Personally, I have a bit of a fondness for this
tank, mostly because it was an example of pushing the edge of technology. Roughly 2,000 were built, and
it was in service from 1966 until 2005 in a variety of front line and training roles.

The idea was to produce a light tank, a carry-over from WWII thinking. The first development was the M24 Walker Bulldog, which was too heavy and had a short deployed life. The Sheridan achieved a lower weight by using aluminum armor on the main hull, which proved easily penetrated by simple weapons like the Rocket Propelled Grenade (RPG).

Still, the tank was deployed in limited numbers to Vietnam, for
example, where it proved effective in certain combat conditions.

The truly interesting feature of the Sheridan, however, was its missile antitank round – the MGM-51
Shillelagh. A forerunner of the modern day TOWs (Tube-launched, Optically tracked, Wire Guided), it
was launched from the main gun as opposed to a separate launcher; certainly unique and not duplicated
in the U.S. military since.

In addition, rather than deploying a fine wire that allowed the gunner to send a
control signal, the Shillelagh used an infrared beam (basically the heat part of the light spectrum)
projected from a housing on the turret.

Although details on its operation are not readily available, the missile apparently did not use infrared light reflected from the target in the same way laser-guided munitions work these days – instead the beam literally steered the rocket after it left the barrel.

This is a missile on display at White Sands Proving ground. The missile had folding fins at the rear for
stability that would deploy once it left the gun barrel. The picture of the missile rear shows three rocket motors; I suspect the round part with the 4 circles housed the infrared receiver. I believe, based on my
work with optical systems that used real-time alignment, that there were separate detectors in each
circle. If the beam intensity seen by each detector is the same, then the missile is on its path. If the
intensity changes on one detector, then the missile is off the guidance beam and it steers to equalize the
signals again.

A major problem was the tank steering electronics being knocked out of alignment from the main gun
being fired. Although roughly 88,000 of the missiles were produced, only a few were ever fired in
combat. This was during Desert Storm where they were used against bunkers. The Shillelagh could not
be transitioned to a tube launcher unfortunately, and so they were eventually scrapped.

Anyway, this was a short review, so if you want to read more, may I recommend:

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