Revisiting the ACRA

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Finally, a visitor magnet again: Hardware-centric milporn.

Maybe you (the reader / visitor) remember the almost one-of-a-kind Shillelagh missile. The designation was probably meant to drive Soviet spies crazy, but it kind of works on me, too.

The concept was to use a very large calibre stub gun in a tank turret which could both launch a rather low velocity large calibre shell and a long-ranged anti-tank guided missile with a large calibre shaped charge.

The concept had some support in the United States which experimented with it on main battle tanks (M-60A2) and on light tanks (M551 Sheridan).

The missile was a kind of white elephant, since it wasn't used on any other platforms unlike the rather versatile TOW missile. Its range was disappointing and its penetration apparently very much below potential.

The M81E1 gun proved to be a corrosion nightmare (at least in Vietnam) and there were issues with the cartridge design. Finally, the recoil impulse was troublesome not the least for the fire control electronics. The gun was very lightweight and short itself, though.

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The key problem of this concept was the same as with all anti-tank missiles of the era; operations research showed a distinct advantage of tank guns over missiles up to about 2,000 metres distance. The missile accuracy was an advantage beyond 2,000 metres, but the shorter time of flight and higher rate of fire was a key advantage of the gun below that range.

Most terrains in Central Europe justified the expectation that most combat would happen at the same ranges as known from WW2; usually below 2,000 metres, possibly a majority at less than 500 metres. Experiences from Israel's conflicts pointed at relatively short effective lines of sight even in rather open terrain because of intended and unintended dust and smoke effects.

The title here is "Revisiting the ACRA", not "Revisiting the M81" for a reason. Unknown to many who know about the M81 and the Shilellagh, the French pursued a vastly superior concept at about the same time: ACRA.

French AMX-30 test vehicle with 142 mm ACRA gun

ACRA used a much superior muzzle velocity and its missile had a much shorter time of flight, extending the superiority of the missile approach over the gun approach to well below 2,000 metres. This 142 mm gun was not without troubles either (reliability), and the missile approach was still very expensive.

Good French source on ACRA

 

APFSDS projectiles gave conventional guns an improvement in time of flight, ease of aiming and armour penetration which made gun/missile launcher combos unnecessary. The Russians introduced some missiles for long-range fire with normal long-barrelled guns, and the Israelis did so as well, but overall the ammunition of choice against heavy armour is APFSDS.

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So why revisit this seeming dead-end? 

Simple: Stuff happened, and it did so for three decades.

The high/hyper velocity missile approach - mating a APFSDS-like long rod penetrator with a missile flying as fast as a APFSDS projectile and guided by an inertial navigation autopilot - could be applied to this kind of combo gun.

Combustible cartridges have long since proved their concept in the German 120 mm tank gun design, so one problem of M81 is solved now. A Mach 6 missile with long rod penetrator is about 1.2 to 1.5 metres long and happens to have a similar calibre as ACRA and M81. The length is a huge problem, of course: Current APFSDS cartridges are 'only' about 0.8 metres long. The use of CKEM-like missiles in a tank turrent would require a dedicated internal layout and ammunition storage in the turret bustle. Reloading would likely be restricted to a certain gun elevation as it was in old large calibre naval guns. ACRA's missile ammunition was 1.22 metres long as well, though.

Still, the combination of a APFSDS-like missile and a versatile squash head shell in a 140-155 mm rifled gun would be interesting. One advantage over MBTs such as Leopard 2A6 would be the negligible restrictions for turret movements in restrictive terrain with the same long rod penetrator power. You could easily traverse the turret even on woodland roads and on not very wide streets. The extremely long 120 mm L/55 guns (1.3 metres longer than the original Leo2 gun) are probably well past the optimum compromise.

A squash head (HESH, HEP) shell of such a large calibre would demolish almost all obstacles and many buildings in one shot and be terminally effective against armoured fighting vehicles within its effective range (which would be less than 1,000 metres against moving targets because of the modest muzzle velocity of both stub guns and the HESH concept each). A HESH / HEP warhead of such a calibre would possess such a quantity of plastics explosives that penetration isn't required to mess up a MBT: The shock itself would all but guarantee a mission kill even in hit locations which would be harmless with APFSDS.

The British army liked demolition guns of 165 mm calibre on dedicated combat engineer vehicles.  A combo gun would give about the same 'blow up' capability to regular MBTs.

This blog post should be seen in context to my other pet idea, the medium calibre (~76 mm) rapid fire gun option. A tank battalion with many such 76 mm / 30+ rpm guns would mostly lack raw power of 120 mm guns. A mix of 76 mm / 30+ rpm guns with 140+ mm stub guns on the company level would as a whole possess both specialisation advantages and much greater versatility than a 120 mm-only  MBT force.

S O

defence_and_freedom@gmx.de

edit: Obviously, a disadvantage of the concept would be the inability of ripple fire, which is otherwise a possibility with HVM. That's a trade-off. To launch a HVM with a gun adds initial velocity and allows for a delayed ignition of the missile ('cold launch'), potentially avoiding that it blocks the line of sight with hot gasses (if launch elevation is high). A stub gun is also much less prone to being damaged than exposed missile launchers and a cold launch may be less hazardous to exposed sensors and possibly crew heads or dismounted personnel than a launch from a normal missile launcher. It's still not good for a higher rate of fire than to be expected from comparable APFSDS gun ammunition. HVMs are powerful, so a 135-152 mm tank gun's APFSDS would be the equivalent.

edit again: I forgot to link to a related earlier blog post, Low signature propellants

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