So, the Pentagon Wars is a pretty hilarious little HBO movie about the design of the M2 Bradley and probably worth your time if you enjoy a painfully honest look at the cluster fuck of government bureaucracy, but the M2 isn't the only mess the Army has churned out. Now, generally, the US either produces some pretty quality equipment or just cuts programs that aren't meeting goals (While wasting millions, obviously), but it's really fascinating to look at the times something bafflingly ridiculous makes it all the way through the process. Now, I actually love the Sheridan, mostly because it IS so ridiculous, but man does this tank have some problems.
So, this story gets off to great start when the XM551 program gets rushed pretty heavily to keep the congressional budget committee from looking to closely at it, but hell if I can figure out why. This was in the middle of the cold war, so there wasn't much reason to believe congress wouldn't approve a viable program or that this program wouldn't be viable so early. It might have been that the Army was undertaking the M60 program at the time and didn't want congress seeing 2 different tank programs in the budget...and incidentally, the Sheridan is totally not a tank, at least officially. It's an Armored Reconnaissance/Airborne Assault Vehicle (AR/AAV) because I guess the military thinks of congress like a dog and if they spell out W A L K instead of saying walk they won't suspect anything. The goal of the program, at least officially, was to create an air droppable and amphibious armored vehicle to provide fire support to cavalry and airborne units. The amphibious requirement was added after the Americans saw the Soviets had an airborne tank that was also amphibious and they didn't want to be 1 uped. They added the amphibious requirement, scrapped the current project and started over. Incidentally, this also happened with the Bradley because of the BMP-1 and is the reason for some of the dumber additions to that vehicle as well.
Let's start with the big problem, and it is, in fact, quite big; the gun. You see, in order to be dropped out of C-130, they had to keep the weight in the 16 ton range, which seems like a lot, but really isn't for a tank. The standard 105mm tank gun would've had far too much recoil for the mass of the light tank to absorb. The 90 and 76mm options simply didn't meet requirements for lethality, both for delivering high explosive or for delivering anti-armor rounds. This is where they had the light bulb moment for the Sheridan. What if we put a 152mm short barreled howitzer in the Sheridan? The 152mm had a great HE round, but was so low velocity that it was useless against tanks, but they solved that as well by adding the ability to fire the Shillelagh anti-tank missile from the barrel. Brilliant! If you're asking yourself, "Wait, if a 105mm had way too much recoil for this tank, then how does a 152mm not?" then you're thinking about this way more than anyone involved in the project did. Firing this gun would generally lift the first wheel or two off the ground.
The early electronics for the tank weren't adequately hardened against shock, so firing the 152mm HE rounds would basically destroy all the electronics you needed to fire the missiles (and the electronic firing mechanism). Partly for this reason, Sheridans deployed to Vietnam initially just weren't issued with missiles. Also, it turns out the barrel wasn't terrifically well thought out either because it needed a groove to fire the missile, but this created a structural weakness that would cause non-trivial barrel cracking issues when firing HE rounds repeatedly. Fortunately, there was little danger of barrel overheat because the gun had to use a special bore evacuator to keep the crew compartment from being flooded with fumes after every round that took a while to work giving the tank the impressive rate of fire of 2 rounds per minute. And because this was a function of how slow the evacuator was, no amount of loader skill could make this go faster. But wait, there's more. The gun used caseless ammo with a projectile and propellant charge in a nylon bag that the loader was supposed to unsheathe before each shot, but there were also strict directions that if the projectile and charge became separated in the loading process, they couldn't fire the round for safety reasons. This, ironically, lead to the very unsafe situation where a Sheridan in combat may have a number of shells and propellant charges rolling around on the floor of the tank. Unfortunately, this is where this writeup takes a dark turn. The ammo was so poorly stored and so easy to detonate that every Sheridan lost in combat was a total loss of vehicle and crew.
Also, about the whole "amphibious" thing...yeah that didn't really happen. It had a trim vane built into the front hull and if you really wanted to, you could build this utterly ridiculous flotation screen for the tank that had a convenient piece of clear plastic in the front for the driver to see from, but that never worked. In practice this feature was basically never used. It took too long to put together and it didn't have a drive mechanism for water, so it just dog paddled with it's tracks which was painfully slow and just too vulnerable to use in combat. In practice, if you have enough time to use this feature, you could just call the engineers to build a bridge. Incidentally, the Bradley totally had the same feature and if you can find pictures of the early models you can even see the trim vane and that feature was just as useless.
In order to keep the tank super lightweight for the airborne drop, they had to use high tech aluminum armor. Even with the high strength to weight ration of aluminum, however, the Sheridan was still highly vulnerable to mines and RPG-7s. There was also an unpleasant side effect that burning aluminum tended to produce toxic gas (another flaw of the early Bradley), but it turns out when you're ammo is going to detonate and turn your tank into confetti anyways, this drawback never really came up.
Well, we made a lot of compromises, but at least we have an airborne tank...wait seriously? The only time this tank was air dropped in combat was in Panama. 8 tanks were dropped, 2 were destroyed or rendered non-operational on landing (fortunately the crews jump in separate from the tank). Literally, the one thing this whole project is for and it only has a 75% success rate the only time it was ever used. I guess this completes the circle of failure where this tank basically achieved none of it's goals. Most of them were pulled from service after about 10 years, but because there was no replacement, the 82nd held on to theirs all the way into the mid 90's and they even fired a few rounds in Desert Shield.
You'd probably be unsurprised to hear that soldiers hated this tank, but be prepared to have your mind blown because it was actually well liked by it's crews. I guess the guys who would object to the terrible armor and poor ammunition storage weren't around for the AAR's. Features cited include the low ground pressure, relative ease of maintenance and repair and the ability to provide responsive fire support to light units that normally wouldn't have access to it. But this was almost exclusively at the low levels. The higher up you go, the worse the opinions you get.
It's interesting that this tank is so little known, but, despite being an 1.8 Billion dollar program, there weren't that many tanks put into service and with most of it's service taking place in Vietnam, it probably slipped under the radar with the myriad of other problems in that conflict. This tank is like trainwreck in that it looks pretty bad, but every piece of rubble you move out of the way, it somehow manages to get even worse. I love the Sheridan in the same way I love other terrible things too stupid to exist, but that somehow do anyways.