Tracklink Article - The MBT-70 project

(This article was originally published in Issue 111 (Spring 2022) of Tracklink, the magazine of the Tank Museum.  The photos which originally accompanied it are owned by the Tank Museum, so I've replaced them with some photos from the public domain)

In the early 1960s, the US was very concerned that their current M-48 & M-60 tanks no longer had much technological advantage over the new Soviet tanks then appearing. Since Soviet forces could be expected to have a significant numerical advantage, this was obviously a problem, and lead to a project to develop a new vehicle.

This MBT-70 would incorporate very advanced features to restore its technological advantage, and would be developed jointly with the Federal Republic of Germany (as the Kampfpanzer 70) to share development costs. Germany was originally expected to buy 500 MBT-70, while the US would purchase 1,000 – 1,500. The two largest tank fleets in NATO sharing a common tank design would also have significant logistics benefits, simplifying spares and ammunition supply.

The basic agreement signed in 1963 created a project management board with one US and one German project manager. Technical trials were expected in 1967/8, and troop trials in 1968/9. Series production was expected to begin in 1971, with the vehicle in service by 1972.

As with any multinational project, there was considerable haggling over national preferences, which often reached a surprisingly high level before a memorandum of understanding was reached. For example, discussion of whether to use metric measurements for screws and fasteners or the SAE system used in the US reached the German Defence Minister and the US Secretary of Defence. Even then, the result was a rather unsatisfactory compromise which used SAE measurements for the US-made sections and metric sizes for German components and the common parts where the two joined, potentially requiring two sets of tools to work on the vehicle.

Work was split between the two countries, with the US responsible for the development of the main gun and its ammunition, Germany developing the autoloader and commander's night sight system, and both countries running parallel programmes to develop engines and suspension.

The MBT-70 included several novel technologies. It used a hydro-pneumatic suspension, similar to that used on the Swedish S-tank. In this system, the arm carrying each roadwheel acts against a cylinder filled with oil and inert gas. These offered steadily increasing travel resistance as the gas compressed, without needing dampers, bump stops or separate shock absorbers.

More interestingly, the gas pressure in each cylinder was controllable from the driver's position, so the suspension could be lowered to only 100mm (4in) ground clearance to minimise height and thus target size, or raised to a maximum of 710mm (28in) for cross-country travel, with track tensioning automatically adjusted by moving the front idler.

Indeed, the six roadwheels per side were divided into four suspension units (two each side) which could be adjusted independently, allowing the tank to “kneel” at the front, or level itself on uneven ground.

The US MBT-70 version was powered by a 1,475 horsepower Continental air-cooled V-12 diesel engine which used complex variable compression ratios (VCR) to maximise output for its size, while the German version originally used a longer and 500lb heavier conventional Daimler-Benz diesel unit before switching to a 1,500 horsepower MTU design. Both versions used the same German-designed transmission

Either of these engines gave the vehicle an excellent power-to-weight ratio, and combined with the hydro-pneumatic suspension to give a significantly better cross country performance than either of its predecessors.

The prototypes had a top speed of 43mph / 69 km/h, notably faster than even the very mobile German Leopard I, let alone the US M-60 (32 mph). Acceleration and cross country ride was also excellent, and the prototype completed a 10 km cross country course in 30% less time than an M-60. Indeed, the test vehicles tended to become briefly airborne crossing bumps at full speed.

However, problems with the complex US VCR engine meant that it did not develop full power in service, and any US prototypes after the first set were planned to use the German engine

The vehicle could ford water up to the turret top with all hatches closed, special seals (eg around the turret ring) inflated, and two air intake snorkels for the engine raised from their storage positions on the rear hull. A special deep wading kit allowed the vehicle to ford 18 feet of water. Characteristically, the two different engine designs took different routes to cope with deep wading – the German design sealed the engine compartment against water entry, while the US version allowed the engine compartment to flood.

In common with the German Leopard, the MBT-70 emphasised firepower and mobility over armour in order to keep within an acceptable weight, relying on its low silhouette (which could be lowered from 2.6m to less than 2m with the lowest suspension position) and the longer effective range of its weapons to survive.

It was protected by a type of spaced armour across the hull front and turret. This consisted of a 34mm outer layer of cold-rolled hardened steel, an air gap of 127mm then a 46 mm inner layer of softer steel. Kinetic projectiles penetrating the outer plate were deformed in the process, limiting their ability to penetrate when they hit the inner layer, which also served as spall liner, while the plasma jets from shaped charge warheads partially dissipated in the air gap. The fuel tanks also formed part of the protective system, since diesel fuel was hard to ignite and gave useful extra protection against HEAT rounds

Overall, the MBT-70s armour was specified to resist 100mm Soviet AP rounds or NATO 105mm APDS rounds at 800m across the frontal arc and turret, 20mm AP at close range over the sides and 14.5mm over the engine.

The design included fireproof armoured doors between the crew and the ammunition stowage in the turret bustle, which had blow-out sections in the roof to minimize crew injury if penetrated. The vehicle was fitted with an NBC system, allowing it to operate on the fallout- contaminated battlefields expected in a future European conflict as well as providing heating or cooling to maintain crew efficiency.

To minimise weight, the designers concentrated the entire crew in the large turret, significantly reducing the volume that needed to be heavily armoured and NBC protected. To achieve this, one crewman was replaced by an automatic loader, and the driver was placed in a rotating capsule to the left of the gun, with the commander and gunner in conventional seats to the right.

The driver's capsule rotated in the opposite direction to the turret, so that he always faced forward. Alternatively, his capsule could rotate 180 degrees to face the rear, allowing the vehicle to be driven backwards at full speed.

Moving the driver out of the hull also reduced vehicle height, so the MBT-70 was much lower – and thus a more difficult target – than the notably tall US M-60 it would replace. The driver's higher position in the turret also gave him a better view than in a conventional tank, and meant he didn't need to close down when the main gun was being fired. A TV monitor linked to a camera in the hull front gave him close-in visibility.

However, since the driver's capsule had to be mounted off-centre in the turret because of the gun breech, it travelled in a circle at the same time as rotating when the turret traversed, like the teacup ride at a funfair, and many drivers complained of disorientation and motion sickness.

The point of any tank, however, is its weapon system. The MBT-70 was armed with the 152mm XM-150 Gun / Launcher, a long barrelled version of the weapon fitted to the M-551 Sheridan and M60A2 “Starship”. This could fire the same combustible case HE, HEAT, cannister and smoke rounds as the Sheridan, but thanks to the longer barrel (43.5 calibres versus 17.5 calibres for the Sheridan, giving a muzzle velocity of 1,490 m/s versus 683 m/s) it could also fire a newly developed XM578E1 APFSDS (Armour Piercing Fin-Stabilized Discarding Sabot) round, with an improved tungsten alloy penetrator.

The MBT-70 gun was stabilised and the smooth ride from the hydropneumatic suspension was expected to allow firing on the move with minimum loss of accuracy. The gun was also one of the first to be fitted with a laser rangefinder to improve accuracy.

However, it was still considered unlikely that many hits would be scored beyond around 1,800m, then regarded as maximum tank gun range. To engage targets beyond this range, the MBT-70 could launch MGM-51 Shillelagh missiles through its gun tube. These had an effective range of around 3,000m, though they would take 8-10 seconds to travel out to maximum range.

Technically, Shillelagh was a “Semi-Automatic Command Line of Sight” (SACLOS) system, so the gunner simply had to keep the cross-hairs on target for the missile to hit, rather than having to “fly” it to the target with a joystick, as with first generation ATGMs such as the Soviet AT-3 Sagger. It also used a laser guidance link, which had significant advantages over the trailing command wires used by other contemporary ATGMs such as TOW, which could “hang up” on intervening obstacles. US modelling estimated a 90% first round hit probability against a 2.3x2.3m target at 3,000m, though this proved difficult to achieve in the real world.

The gun was fed by a belt type autoloader in the turret bustle, which could hold 26 rounds or missiles, and skip over rounds without firing them to get to the correct type. It was normally reloaded from outside the vehicle, though it could be reloaded – awkwardly – from within the vehicle

For anti-aircraft defence, the MBT-70 was fitted with a remote-controlled Rh202 20mm cannon which deployed from an armoured box on the turret roof behind the driver's cupola and was aimed by the commander using a periscope.

A 7.62mm coaxial machine gun was mounted alongside the main gun – an M-73 on the US prototypes, or an MG-3 on the German. It also mounted either 8 (US) or 16 (German) smoke dischargers on the turret sides.

The standard ammunition load was 42 main gun rounds, 6 Shillelagh missiles, 500rds 20mm and 6,000 7.62mm machine-gun rounds.

In practical use, however, the gun/launcher proved problematic.

The ammunition used combustible cases, which were lighter than conventional brass-case rounds and eliminated the need for extraction. However, they proved fragile and easily damaged by the autoloader. Worse, the cases were not always completely consumed, leaving smouldering fragments in the bore which could cause premature detonation of the next round. A compressed air purge system was thus installed to blow the toxic propellant residue and any burning case fragments out of the bore before the next round was loaded.

The large bore necessary for the missile required an interrupted-thread breech rather than the quicker-operating semi-automatic type generally used for tank guns, and the breech had to be powered as the recoil of the missile was insufficient to recock it for the next shot.

Along with the fact that the barrel had to return to the horizontal for the autoloader to feed the next round, this all reduced the rate of fire to a rather slow 7-8 RPM.

Finally, the rounds also absorbed moisture and swelled up, so they would no longer fit into the breech. In Vietnam, this problem lead to the 152mm rounds being issued in neoprene wrappers that had to be removed immediately before manual loading. This obviously slowed rate of fire and was not possible with an autoloader, suggesting problems had MBT-70s been deployed in certain areas.

The shillelagh missiles proved expensive ($3,000 each, as much as 20 APDS rounds) and unreliable, while the remote control 20mm anti-aircraft gun was over-complicated, awkward to deploy and difficult to use effectively.

Prototype construction began in 1965, with two mild steel prototypes, followed by six examples of each of the US and German versions, for a total of 14 vehicles. The lower hull and drivetrain were tested in 1966, the vehicles were shown publicly in October 1967 and full trials began in 1968.

Concern was already being expressed that the MBT-70's sophisticated systems would be too complex and time consuming for the crew to service, especially as both Germany and the US were operating conscript armies at that point, and the replacement of one crewmember by the autoloader meant maintenance tasks were spread over just three men per tank.

Like many tanks, the MBT-70 increased in both cost and weight during the design process. The original specification envisaged the MBT-70 weighing 46 metric tons (50 US tons), slightly less than the M-60 (48 metric tons) or about 10% more than the Leopard 1 (42 metric tons).

During the design process, however, the projected weight ballooned to 54 metric tons. before redesign work managed to bring it back to a final weight of 50 tons. Even so, this meant the vehicle was pushing the German 50 ton bridging limit, though within the higher US 60 ton limit.

The problem of cost inflation was even harder to deal with. The original estimate of $80m development costs ($40m per country) had been rather optimistic. Although the idea was to split costs and save money, in practice the need to balance conflicting national requirements resulted in costs ballooning, as duplicated programmes for things like engine development meant costs doubling rather than halving.

$303m had already been spent by 1969 to build and test the 14 prototypes, with development still to be completed. By comparison, the entire development of the Swedish S-tank – which was happening over the same period, and also included a hydropneumatic suspension – was only $24m, including 12 prototypes

Worse, the projected unit cost had risen to $600k per vehicle by 1967. By 1969 it was estimated as $750k - $850k, and pessimists worried it might reach $1m per vehicle.

US analytical modelling took the existing M-60 as a baseline, with a combat value of 1, and scored the British Chieftain at 1.5. The modelling scored the MBT-70 as having a relative efficiency of 3+, but even so, with the M-60 costing only $250k, the numbers no longer added up even if the optimistic modelling was correct.

The rising costs and diverging requirements lead to West Germany withdrawing from the project in late 1969 to develop their own Leopard 2 MBT instead. In early 1970 the US Congress also objected to the increasing price and cancelled further funding for the MBT-70 programme.

The US Army persevered with what was essentially a simplified version of the MBT-70 as the XM-803. This used a simpler single cylinder hydropneumatic suspension, stripped out features such as the NBC system and replaced the problematic 20mm remote control AA gun with a simple .50 calibre machine gun.

However, it quickly became obvious that even this simplified version only reduced unit cost to around $600k, still several times as much as the M-60 it was replacing, without a guaranteed equivalent gain in capability. The project was thus finally cancelled in November 1971, and funding was redirected to the XM-815 project, which ultimately became the successful M1 Abrams.

An XM803 was tested with a Lycoming AGT1500 gas turbine, but this was not completely successful, and never became the engine of choice.

None of the technologies planned for the MBT-70 were unreasonable in themselves, and almost all aside from the driver's position in the turret have been used subsequently in successful vehicles. However, combining so many new technological innovations – most of them still in the early stages of development – into a multi-national project without it becoming excessively complex and costly was always going to be ambitious.

Perhaps the US should have remembered that the French AMX-30 and the German Leopard I had emerged from an earlier joint programme which was intended to produce a joint Franco-German vehicle but ended up with each country producing its own vehicle.

As a side note, the Germans had concerns over Shillelagh, both because of its cost and because they believed that in a European war 50% of tank-versus-tank engagements would be at less than 1,000m range and 75% would be at less than 2,000m. They believed the best weapon would be something that delivered rapid first round hits at short and medium range, whereas the Shillelagh accepted a low rate of fire in return for a long range capability the Germans believed would rarely be used.

They therefore planned to use the MBT-70 in conjunction with another tank armed with a conventional 120mm smoothbore gun, provisionally termed the “Keiler” or “Boar”. The idea was that the Keiler would engage Soviet tanks at closer ranges, while the MBT-70 provided longer range support with its missiles before switching to the gun as the enemy closed.

Since there would be obvious benefits in commonality between the two designs, a version of the Keiler based on an MBT-70 chassis was produced as a wooden mock up and nicknamed “Eber”, but lost out to the Leopard II.

                    The MBT-70 - note the size of the turret, which contained all the crew

A photo showing how much the setting of the hydropneumatic suspension could change the height of the vehicle - apologies for the poor photo quality

The remote controlled 20mm AA cannon deployed from its armoured box on the turret