M1 Abrams Tank VariantsThe Abrams series started with the XM1 prototype that became the M1. Based on feedback from the troops in the field, certain modifications were added to those initial M1s in the field and to M1s during production. These became known as M1IP or M1 Improved Production. This variant was equipped with the M68 105mm main gun and had an asymmetrical 3-blast panel configuration. M1IPs can be identified from an M1 by the rear turret bustle rack and the redesigned turret ammo blast panels (went from 3 asymmetrical to two identical panels). All fielded XM1 and M1 tanks were brought up to M1IP standards (less test/evaluation tanks and museum pieces). The engine access cover does not have the two rear inspection hatches. The front fenders do not have the “X” stamping and the head light guards are straight across as opposed to the later stepped type. Also, there are sponson boxes on both sides of the hull.
The M1A1 was up gunned to the 120mm smooth bore cannon. It also added an NBC over pressurization system in place of the left hull sponson (storage) box. A "man hole cover" was added forward of the loader's hatch in anticipation of the development of Commander's Independent Thermal Viewer (CITV). The CITV was not installed on any M1A1 though. Other changes include the engine access cover having two rear inspection hatches added. The front fenders had “X” stamping for strength and the head light guards are the later stepped type. About the same time as the M1A1HC changes were being made (still during ODS), the Abrams changed track from the angled-block T-156 track to the square-block, “Bigfoot”, T-158 track.
The next variant included changes the USMC required making the Abrams suitable for amphibious operations. It also had the depleted uranium mesh added to the forward slopes of the turret and hull. Because these changes incorporated a heavier armor and was now a common USA/USMC tank, it was referred to as the M1A1 Heavy Common. The modeling world refers to this as the M1A1-HA for "heavy armor", but this nomenclature is not used in the real armor world.
The Marines made some cosmetic changes to their Abrams as well. They added a Missile Countermeasures Device (MCD) on top of the manhole cover. This is the square device that some people confuse for a version of the CITV. It serves a totally different purpose by directing anti-tank guided missiles away from the tank. The Marines do not use the CITV. They also swapped the Army 6-shot smoke grenade launcher for a double stack 4-tube version for 8 smoke launchers on each side of the turret. They also added a Tank Infantry (TI) phone to the right rear hull to allow ground troops to communicate with the tank crew for mutual support. Lastly, the USMC has started putting the track retaining ring or "Training Wheels" back onto their M1A1s. Originally developed for the M1IP the USMC started putting these on M1A1s that were stationed stateside in the early-mid nineties. It was an easy fix to counter the track losing tension during operation and reduced the numbers of track being thrown.
The M1A2 was the next step. It added a new digitized electronics package and the CITV. The CITV allows the commander to search and designate targets while the gunner engages the current target. This tank also incorporated an auxiliary power unit (APU or basically a generator) in the turret bustle rack to power the optics and electronics without the need to run the turbine engine. The APU has been retrofitted to many M1A1s as well. The M1A2 also required a new commander's cupola because the CITV data screens took up too much room in the commander's hatch.
The final variant (so far) is the M1A2SEP or Systems Enhancement Program. This upgraded several internal systems and allowed the APU to be moved from the turret bustle to the inside of the left rear hull (Under Armor APU or UAAPU). The engine was changed as well and the CITV now uses a 2nd generation FLIR. The biggest difference in the SEP is the new engine, smaller, more fuel-efficient (hence the removal of one fuel tank for the APU) and the same power. It also has a new Thermal Management System (Air Conditioner, mainly for all the electronic components) and part of it, the Vapor Compression System Unit (VPSU), is now in the turret bustle rack where the EAPU used to be in an M1A1/A2. There are also some new electronics installed in the SEP. All SEPs are rebuilt tanks that may have been M1IP or M1A1. Very few units in the Army use the M1A2SEP.
There are several additional rebuilt variants of the M1A1 as well. Since the original M1A1 tanks are over 15 years old, they were sent back to the factory and depot to be completely rebuilt. These tanks went through what is called the Abrams Integrated Management program (AIM). One is called the M1A1-D and incorporates the digitized electronics package that is in the M1A2 (FBCB2). Another is called the M1A1 AIM or Abrams Integrated Management Program which rebuilds older M1A1s to a zero time/zero hours "like new" condition. Older tanks are disassembled in a factory, then assembled again with all used up parts being replaced with new or rebuilt ones - effectively bringing these tanks to "new" condition. While AIM tanks are brought up to like new standards, components that check out are reinstalled and reused, most likely not on the same tank it came off of though. Also new electronics suites similar to an M1A2 are installed in these tanks, although they still lack some devices typical for M1A2, like CITV.
If you're asking why versions of the M1A1 are made after the M1A2, the answer is money. The Army can't afford any more M1A2 or M1A2SEP. Most units that did not get the M1A2 SEP (non III Corps units) are receiving M1A1 AIM tanks to equip their units.
The current US inventory consists of M1IP, M1A1 and M1A1HC in the National Guard. The Regular Army has M1A1HC, M1A1 AIM, M1A1D, and M1A2 SEP depending on the unit.
M1 Abrams APUsHull mounted APUs were used up until about 1993, after that the turret mounted APU was introduced in the M1A1 Heavy Common. Original M1A2s still had turret bustle APUs as well. The M1A2 SEP (Systems Enhancement Program) has an internal APU mounted in the left side of the rear hull.
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