2013/10/05

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 lower 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.

 


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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13 comments:

  1. Didn't want to necropost on your ARES75 post, but I didn't see a mention there of the Rooikat 76 with the Denel GT4. That's the closest I could think of to a current version of that caliber of weapon in current use, although it doesn't seem to have met with any export success.

    Regarding this post, at its most basic level you add the problem of ammunition handling. A 142mm round/missile would not be lightweight and given most western Atvs continue to use human loaders, you'd be looking at a reduced rof compared to current 120mm systems.

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    1. It's a normal rate of fire gun, comparable to the one used in M41 light tanks or the French Panhard EBR throughout the Cold War.
      The closest thing is IMHO the 60 mm HVMS used on Chilean light tanks. 60 mm isn't versatile or powerful enough, though.


      The ACRA's missile weighed 26 kg complete and was 1250 mm long, the dumb round being 20 kg and 900 mm.

      German wartime cartridges for the 120 mm smoothbore tank gun weight 19-29 kg.
      The difference is really the length of the missile and the diameter, and not much else.
      The bulk and length requires a dedicated internal layout of the turret and its basket.

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    2. Oops, I forgot to make the comparison to the more relevant HVM instead of with the classic ATGM:

      CKEM: 1500 mm, "< 45 kg"
      HATM: 1270 mm, weight unknown.

      Rockets are less propulsion-efficient than cartridges, so they need more propellant. The high acceleration of a HCM and thermal effects also require a rocket wall of some significant weight.

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  2. If you are worried about munitions length, there's always the Soviet/Russian approach of using two-part ammunition. Loading might be marginally slower, but there would be fewer restrictions inside the turret.

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    1. This doesn't work well with long rod penetrators (Russians have this problem with their APFSDS) and is certainly impractical with a missile body.

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    2. . . . But the Russian missiles ARE in two pieces.

      I wasn't aware of Russian problems with APFSDS and would be curious to learn more, if you can point me to further reading.

      I get the desire to shorten tank barrels, but given the expense of current through-barrel missiles, do you really think that a hypervelocity missile would be affordable? Also, at what point do direct-fire systems draw the line between ammo bulk and quantity? I recall that being a problem with the 140mm gun (along with reduced elevation/depression, and a massive increase in weight).

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    3. Refleks isn't so much a missile as it is a guided round with a sustainer rocket. Its Mach 1 speed requires only 1/36th the kinetic energy as it would with Mach 6.
      The solid fuel rocket required for a Mach 6 missile with long rod penetrator is different by an order of magnitude. The propellant charge for pushing the missile out of the barrel at Mach 1 is negligible by comparison and the one used with Refleks has been designed that way because of the laser beam receivers and the autoloader dimensions.


      A HVM is basically a tungsten penetrator, a battery some casing, a simple autopilot and several kilograms of very high quality propellant. There's no expense for a Tantalum liner for the shaped charge and no expense for a non-cooled infrared sensor of 2+ km range as in modern ATGMs. I suppose it's affordable.
      Its problem isn't price, but its lesser versatility due to no explosive warhead. That's no issue in this case, though.


      The 140 mm tank guns would have been introduced if the Cold War hadn't ended. It's only the peace dividend which did put a premium on being able to use old 120 mm stocks. Western tank experts were still fixated on frontal MBT-on-MBT combat by the late 90's, but the budgets weren't.

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    4. About APFSDS:
      The long rod penetrator gets eroded when it penetrates armour - same as a shaped charge jet.
      The quest for more penetration wasn't only a quest for better material (tungsten and uranium are necessary because steels shatter at 1200+ m/s impacts). It was also a quest for more length, as more length means more length available for erosion = deeper penetration.
      Rule of thumb: Fin stabilisation works up to length:width of 6:1, fin stabilisation works at 7:1 and more. So at first APDS was fine, but as penetrators became longer fins became necessary for stabilisation, thus APFSDS. APFSDS became longer over time (DM13 to DM63 was ~463 to ~743 mm: http://www.kotsch88.de/m_120_mm.htm).
      By the 90's the length of Western APFSDS was greater than the length of the projectile in the Russian 125 mm two-piece ammunition and the Russians were disadvantaged by this restriction of their ammunition's potential.
      More here http://fofanov.armor.kiev.ua/Tanks/ARM/apfsds/ammo.html
      But AFAIK they preferred the cheaper HEAT munitions anyway.

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  3. Interesting idea. SO

    Amount of rocket fuel can be reduced by having a scramjet that uses oygen from the high volume of air that is being traversed within a short timeframe.

    Manual loading could be augmented with mechanical aids for the loader. These can help to solve handling problems of length.

    The two piece amunition process still might be considered. Is it possible to assemble a large pentrator in the barrel? You need to glue one penetrator section onto another. The sarissa is an ancient solution for the task. http://scottthong.files.wordpress.com/2007/05/sarissa1.jpg
    Something similar can even work today. Its energy transfer capability depends on the high precision of the two matching penetrator ends.

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  4. 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.

    This statement made me calculate a bit.
    Take the 100mm French naval gun. It has two thirds the diameter area of a 120mm gun and half that of a 140mm gun.
    Upping rapid fire for 120mm should be within the realm of the possible and a long rod pentrator round with two parts would provide your suggested solution that could be implemented into a 120mm. You have not really explained why a long rod rocket penetrator needs a 140mm tube.
    As an alternative one might consider the solution of altering the current system by introcuding a two part penetrator rod with matching precision ends and a scramjet, plus an automated rapid fire for the 120mm that can switch to manual loading for special munitions.

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    1. A 140 mm stub gun needs a HVM, for its AT capability would otherwise rest on a short range dumb round and a kind of ATGM which is very susceptible to active and reactive defences.

      Also, a greater diameter helps propulsion not only inside a barrel (pressure * area = force). It can also help rockets, as it provides enough volume for solid rocket fuel without excessive length. The successful HVM developments up to M6.6 used solid fuel rockets without breathing air, which is an additional source for failure in a concept that's still a technology frontier after 30 years of R&D.

      120 mm automatic guns exist, but you're not going to be able to give them a fine maximum elevation for indirect fire and ShorAD without a very high turret. That and the trouble of reloading 120 mm with elevated barrel are the real restrictions.

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  5. I suppose if you're going to go that route you might as well update the ballistic computers so that you can also fire the HE rounds indirect, giving you an expedient heavy mortar capability too.

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    1. In theory this resembles a Brummbär-like capability, yes. The maximum elevation of a 140 mm gun and the ammunition resupply (indirect fire would rather not make use of HESH) would be issues, though. It would probably be a niche capability.

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