No, we aren’t going to drop a 70-ton M-1A1 tank using parachutes. But that doesn’t mean we haven’t had airborne tanks. I wouldn’t call any of them huge successes, but our main topic today, the M551 Sheridan wasn’t a complete flop, either.
Airborne forces got their start in WWII. We’ve all seen the movies of paratroopers jumping into Normandy. One problem they had was a shortage of ways to defeat German armor and take out targets like bunkers and pillboxes. The bazooka went a ways toward this, but a tank would help a lot. The US and the British developed a very light tank called the M22 Locust that could be landed by glider or transported by plane. Arriving in service too late to see combat in WWII, it was also badly undergunned.
After WWII, the Army still tried to come up with lightweight tank for the airborne forces, but had little success. To have any success defeating armor meant a bigger gun. A bigger gun meant a heavier vehicle, and heavier vehicles couldn’t be airdropped. That was pretty much the state of affairs until missile technology entered the picture.
Instead of using a solid shot to penetrate enemy armor, the plan was to use High Explosive Anti Tank (HEAT) rounds. These use a warhead that focuses the explosion to “burn through” enemy armor. The velocity of the round doesn’t matter to the penetration. The effectiveness of a HEAT round is directly related to the diamater of the warhead. The larger the better. But that takes us back to the problem of weight. The solution was to sacrifice muzzle velocity and accept a slow flying round, since the velocity on impact didn’t matter. This made the gun effective at short ranges. Unfortunately, the problem of long range defense against tanks was still there. The Army solved this by using the same gun as a launcher for a guided missle. After a protracted (and not terribly successful) development, the gun and missile combination was finalized. The gun was a 152mm bore (about 6″) that could fire HEAT rounds for short ranges, and the MGM-51 Shillelagh guided missile.
The gun/missile combination was mated to a lightweight, aluminum hull (or chassis, if you will) that was capable of both being airdropped from a C-130 and of swimming. Production started in 1966 and vehicles soon began to equip the 11th Armored Cavalry Regiment in Vietnam, and other units soon thereafter.
While the system worked to some extent, most of the users weren’t very happy with it. The aluminum armor was easily penetrated, and vulnerable to mines. The Sheridan was also prone to breakdowns. By the mid-70s, most Cavalry units had phased it out. The 82nd Airborne Division, however, had nothing to replace it and so held on to theirs until 1996. The 82nd actually airdropped eight of them during the invasion of Panama, and deployed 51 of them to Operation Desert Shield/Desert Storm.
By the late 1970s, the Army had several hundred relatively new, but obsolete Sheridans on its hands. It had also just opened the National Training Center at Fort Irwin, near Barstow, CA, and needed lots of armor to simulate a Soviet regiment attacking across Western Europe. Many Sheridans were modified with sheet metal and fiberglass to give them a distinctive, somewhat Soviet look to play this part. They served very honorably in this role until 2003 when they were replaced by highly modified M-113s.
Because there were few enemy tanks in Vietnam, and the recoil of HEAT rounds tended to damage electronics on board, Sheridans deployed to Vietnam had their missile guidance packages removed. In addition to the HEAT round, they carried a cannister anti personnel round. You’ve seen a shotgun shell before. Now imagine one six inches across and about 2 1/2 feet long. This was a fearsome weapon when the VC or NVA attacked Sheridan units.