Options for light/medium AFV gun armament

Transcription

1 Options for light/medium AFV gun armament Anthony G Williams Based on presentations given to the DCMS Mounted and Dismounted Close Combat Symposium, July 2015, and the NDIA Armament Systems Forum, April 2016 This is a huge field to cover, so this can be no more than an overview of the issues involved in armament selection. I will not be addressing the armament of selfpropelled anti-aircraft systems or of vehicles armed with mortars or large calibre guns of 60+ mm. There are three elements in armament selection: - the gun and its ammunition - the type of mounting - the context: the purpose of the vehicle on which the weapon is mounted; the purpose of the weapon; the expected target set; and the expected mode of employment. This is of course of critical importance but involves various factors which are outside the scope of this presentation. Guns and ammunition Pictures of guns tend to be uninformative so I'll start by showing comparative photos of the ammunition instead, which tell you more about their relative performance, starting with the least powerful group. Rifle Calibre Machine Guns (RCMGs) are primarily for self-defence, while AGLs and HMGs enable a useful measure of fire support to be provided for dismounts. However, while an HMG delivers a high velocity bullet to long range its destructive effects are limited; and while the explosive charge in a 40 mm grenade provides significant destructive power, the velocity is so low that the steep trajectory and long flight time make hitting moving or fleeting targets increasingly challenging as the range lengthens. One attempt to replace both HMGs and AGLs with one lightweight weapon was the Orbital ATK LW25 firing 25 mm calibre grenade ammunition, but this achieved no sales and is no longer advertised. In a similar size and weight class comes the smallest of the cannon, the GA-1 from South Africa, better known as the Mauser MG 151/20, the Luftwaffe's principal aircraft cannon in World War 2. 1

2 Another, more recent, development from South Africa is the 20mm Inkunzi Strike which fires the same projectiles as the GA-1 but from a much smaller 20 x 42B cartridge case, providing a subsonic muzzle velocity of 310 m/s. Whether you regard it as a large machine gun or a small grenade launcher is a matter of opinion! Cannon ammunition To obtain both a high hit probability through high velocity and rapid fire, combined with significant destructive effects, it is necessary to select a cannon. These come in various calibres between 20 and 57 mm, with muzzle energies ranging from 30 to 1,400 kilojoules. The photo on the next page shows rounds in the 20 to 30 mm group. Over the past few decades there has been a gradual increase in the calibre of the gun and the power of the ammunition used by IFVs. The first generation of cannonarmed troop-carrying armoured vehicles had 20 mm guns but these have mostly been supplanted, although 20 mm guns are still offered in light vehicles, mainly of French or South African origin. The German Marder, now finally being replaced, is one of the last IFVs in a major army still using a 20 mm gun (the Rh 202 in 20 x 139 mm calibre). By the 1980s the baseline gun calibre had become 25 x 137 mm, 2

3 epitomised by the US Bradley IFV, with the British and Russians stepping up to 30 mm, a calibre which in recent years has become the new baseline. The standard Russian 30 x 165 is slightly less powerful than what has become the NATO standard 30 x 173 round (available in the Mauser MK 30 cannon and the ATK Bushmaster II Chain Gun, now designated the MK44 and further modified for AFV use as the XM813). The Rarden's 30 x 170 ammunition is similar in size and power to the 30 x 173, but is not interchangeable with it and there is a far more restricted range of ammunition natures available. Some countries have gone further: in the 1990s the Japanese Type 89 IFV entered service with a 35 mm Oerlikon gun and the Swedes adopted a 40 mm Bofors for the CV90. More recently, the Danes and the Dutch both chose the 35 mm Bushmaster III for their CV9035, while within the next few years the British Army will begin to receive vehicles armed with the unique 40 mm cased-telescoped weapon system (CTWS): the Ajax reconnaissance vehicle and the upgraded Warrior IFV. The 40 mm Super 40, based on the 30 x 173 case, is currently offered by ATK as an upgrade to the XM813. It is not yet in service, although reportedly an export order has been placed. 3

4 There are now three choices in 40 mm AFV ammunition on offer: the original Bofors 40 x 365 available in their L/70 gun; the 40 x 255 CTWS which delivers similar performance but is far more compact; and the smaller and less powerful 40 x 180 "Super 40" from the USA. This is based on the 30 x 173 case and can be fired from suitably modified variants of the 30 mm MK44 (Bushmaster II) Chain Gun - the MK44 "Stretch" or the XM813 - although it has so far not achieved any sales. Another option for further development is the 50 x 330 Supershot, which is based on the 35 mm case. Originally developed by Rheinmetall in the 1980s the project was stopped after the end of the Cold War but has recently been advertised by ATK as a Bushmaster III upgrade and has also been mentioned in presentations concerning the US Army's long-term thinking, although if that ever emerges it might take a different form from that shown here, with more conventional proportions. To top them all, Russian firms have recently been marketing an updated version of their old 57 mm AA gun installed in an unmanned turret, the AU-220, for fitting to medium armoured vehicles. To date it is only available with old-fashioned ammunition types, but new loadings are said to be on the way. Obviously, the more powerful the ammunition, the heavier the gun and mounting, and the smaller the quantity of ammunition that can be carried in any given vehicle. 4

5 The chart below gives another form of comparison, based on the approximate muzzle energies of the rounds. I am not quoting penetration figures because of the problems in finding exactly comparable data, but other things being equal (especially penetrator design) muzzle energy is a good indicator of relative performance in respect of armour penetration potential. You can see that not all 40 mm rounds are equal and that the 35 mm is unusually powerful for the calibre. The Russian 57mm is omitted as it would be way off the scale. One of the factors driving calibre growth has been the increasing protection levels of this class of vehicle, requiring more powerful AP ammunition to achieve reliable penetration. Medium IFVs are now being designed or upgraded to resist 30 mm AP projectiles over the frontal arc. The standard type of AP projectile is now tungsten alloy armour-piercing fin-stabilised discarding sabot (APFSDS). The US Army uses depleted uranium cores which boost penetration by perhaps 10%, while the British and Russian armies still use the older spin-stabilised APDS which is less effective, especially at longer ranges. There is clearly a relationship between the calibre of gun selected and the decision about whether or not to mount anti-tank guided weapons on the turret. With ATGWs 5

6 to deal with the heavy opposition, the gun could arguably be smaller. But that is a decision with lots of pros and cons which each army needs to determine. Another more recent factor in the growth in calibre may be the introduction of timed airburst ammunition (known as HEAB high explosive air burst or PABM Programmable Air Burst Munition) to engage troops in defilade, in which a very precise time fuze in the shell is set electronically on firing. These fuzes are relatively expensive, so it is logical to maximise "bang for the buck" and use them in bigger projectiles. The chart below shows typical weights of HE ammunition. There is a sales battle going on at the moment between HEAB and Kinetic Energy Time Fuzed (KETF - better known as AHEAD). The 30mm HEAB shell is primarily filled with HE but lined with small pellets, designed to form a spherical pattern of high-velocity fragments (although this arguably works less well with high-velocity projectiles, as the fragments start with a considerable forward velocity). KETF rounds, in both 30 mm and 35 mm, are filled with larger tungsten pellets, expelled mainly in a forwards direction by a small base charge. HEAB is better at catching personnel hiding behind walls or in trenches, KETF delivers a much denser pattern of heavy pellets onto the target and could be effective against even MBTs by damaging their optical sensors. 6

7 The cannon described so far have all fired high-velocity ammunition relying on kinetic energy to penetrate armour. There is one interesting alternative for use in lighter mountings: the M230LF (Link Fed), a version of the M230 Chain Gun used in the Apache helicopter with a longer barrel and a few other modifications. The smaller and less powerful ammunition relies on the shaped charge element in High-Explosive Dual-Purpose ammunition for armour penetration. HEDP is not as effective against armour as high-velocity APFSDS, so the gun is more suited to a general fire support role. In a nutshell, the medium-velocity M230LF delivers the destructive effects of a 40 mm HV grenade with a trajectory and hit probability closer to that of the.50 cal MG. It will obviously be of particular interest to those armies which already operate the Apache helicopter, since the gun is almost identical and the ammunition is already in the supply chain. I should add that the round shown in the photo below is actually for the French Nexter 30M550 series aircraft gun; the M230LF is able to use this ammunition, plus that for the British ADEN aircraft gun, as well as the usual Apache HEDP. Nexter make a competitor to the M230 using the same ammunition, the 30M781, which is fitted to the French Tigre combat helicopter and available in various land and naval mountings (though so far these have not been adopted). 7

8 Gun design issues The main issues concerning gun design are the ammunition feed and the rate of fire. The traditional belt feed is problematic in the cramped confines of a turret; the swept volume required by the movement of flexible feed chutes as the gun elevation alters is considerable, and their flexing is often a source of failures to feed. As a result, various alternatives have been developed, mainly involving magazines fixed to the gun breech and moving with it. Examples of this approach are the 30 mm Rarden, the 35 mm Oerlikon KDE shown below and the 40 mm Bofors, followed by the new Denel GI-30 and the XM813 (the latest version of the Bushmaster II/MK44 optimised for AFVs). Fixing the ammo supply to the gun removes the main source of feeding unreliability, but the supply of ammunition for immediate use is more limited especially so with the Rarden and the maximum elevation might also be constrained. A different solution is that of the 40 mm CTWS whose pivoting breech and cylindrical ammunition allow a fixed feed through the gun trunnions, completely unaffected by gun elevation. The downside in this case there is always a downside is that the gun is not balanced on the trunnions since most of the weight is ahead of them, so a compensating mechanism is required. 8

9 The rate of fire of AFV guns has been falling since the heyday of the 20 mm cannon which has cyclic rates in the 800-1,000 rounds per minute range. 25 mm cannon vary, with the original service weapon in this calibre the Oerlikon KBA firing at 600 rpm, the Nexter 25M811 originally at between rpm although now quoted at 100 or 400 rpm, and the Bushmaster M242 as used in the Bradley IFV originally given as rpm but now commonly used at just 200 rpm. In fact, 200 rpm seems to be the new standard for guns in 30+mm calibre, including the 30 mm MK44, the 35 mm Bushmaster III, and the 40 mm CTWS; the M230LF has its rate of fire reduced to 200 rpm compared with up to 625 rpm for the M230 helicopter version. Presumably this is felt to deliver the optimum compromise between effectiveness on the target on the one hand, and accuracy plus ammo conservation on the other. It also minimises the risk of propellant gases getting into the turret. The new South African GI-30 has a cyclic rate of only 110 rpm, almost as low as the Rarden in contrast, the Mauser MK 30-2 fitted to the German Puma and the Spanish and Austrian versions of the ASCOD IFV fires at around 700 rpm which is said to make the system more useful against helicopters. 9

10 Mountings There are various options in the mounting of MGs and cannon, basically divided into external or turret mounted. The simplest are the manned and unpowered external pintle mounts, which may be surrounded by shields to protect the gunner as a kind of half-way house to a manned turret. These are limited to lighter weapons and won't be considered here. There are several steps of increasing sophistication and cost for both external and turret mountings, starting with power assistance or operation; then adding stabilisation so the gun can fire on the move (although not all stabilisation systems are necessarily equal); and finally making them remotely controlled from within the vehicle by means of optronic sights on the mounting, incorporating a wide-range zoom lens, thermal imaging and/or image enhancement, a laser rangefinder and a ballistic computer. Such advanced fire control systems facilitate accurate fire from the first round, improving the chance of hitting the target before it gets out of the way, and reportedly reducing the consumption of ammunition in combat by up to 70%. Optronic sights are also very useful in surveillance and collecting intelligence. It is now common for the gun elevation to be separated from the sight elevation to avoid pointing the gun at people being observed; instead, the gun can be left at maximum elevation as shown with the OTO Melara ToT mounting above right, so that it doesn't 10

11 pose a threat and also makes it less obvious what the vehicle commander is looking at. External mountings operated from within the vehicle are generally known as remote weapon stations (RWS) and have the advantage of leaving the vehicle's interior clear of the turret basket, allowing more dismounts or equipment to be carried. They may be used with any calibre of gun, but are increasingly employed even with RCMGs, with which they typically weigh kilos including gun and ammunition. Larger models able to take.50 HMG or 40mm AGLs like the Kongsberg Protector shown above left weigh between 200 and 300 kilos. Installing both 12.7 mm and 40 mm AGL on the same mounting as shown by the Rheinmetall mounting below left (RLS 609M) puts up the weight to close to 400 kilos. The combination of a 20 mm cannon and an RCMG, a French example here below right, also weighs in at kilos. The M230LF is significantly lighter and generates much less recoil than other 30 mm or even 25 mm cannon, allowing the RWS to be much lighter also. The Viper VGS-30 from Nobles Worldwide is shown on the next page installed on a lightweight vehicle which weighs just 2 tons. This mounting is mechanically stabilised but manually aimed and weighs kilos complete with gun, but without ammunition or sights. 11

12 A couple of other mountings featuring the M230LF are the Australian EOS (Electro Optical Systems) R-400S (next page, lower left), which weighs 450 kilos with 250 rounds of ammunition and has been tested on vehicles down to 6 tonnes, and the BAE Systems Lemur, shown above and to the right of it, which is just 350 kg. 12

13 The Lemur is also available with the 25 mm M242 for a weight of only 450 kilos; this is remarkably light, since most such mountings for 25 mm guns weigh in at around 1,000 kilos. BAE's South African branch offers the TRT (Tactical Remote Turret) mounting with the M242 at 900 kilos with a version for the 30 mm MK44 at 1,400 kilos. In fact, most RWS for powerful 30 mm guns such as the MK44 weigh around 1,500-2,000 kilos; they range from minimalist mountings like the Rafael RWS above left to rather more substantial Kongsberg PROTECTOR Medium Calibre Remote Weapon Station (MC-RWS) above right (2,000 kilos), in which the distinction between RWS and unmanned turrets becomes rather blurred (unmanned turrets preserve a normal turret ring diameter, protected RWS have a smaller-diameter mounting with the gun accessible via a hatch in the vehicle roof). Stepping up in calibre to the powerful 35 mm has a significant impact with the KMW RWS shown below weighing 3,500 kilos (without ammo). 13

14 Turrets and a few of the more enclosed RWS protect the mounting under armour, with ready access to the gun and ammunition feed from within the vehicle, for reloading or clearing stoppages which in combat can be dangerous with most RWS. Turrets are almost always manned with the turret crew sitting in a basket which rotates with the turret, occupying a lot of the vehicle's interior volume, but a few are available which leave the crew in fixed positions within the hull, controlling the armament remotely. At least one turret provides the option of manned or unmanned installations. This is the Rheinmetall Lance (shown on the next page), which weighs between 2,000 and 3,300 kilos depending on the calibre (30, 35 or 40 mm) and whether it is manned or unmanned. The manned Nexter 40 mm CTWS weighs 4,200 kilos and the General Dynamics IFV-35 two-man turret with a 35 mm Bushmaster III weighs 4,900 kg. All of these weights can of course vary a lot depending on the armour and optional equipment fitted. Shown on the next page is the manned turret developed by Lockheed Martin for the Warrior upgrade, which weighs something like 6,000 kg because of the level of protection. 14

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16 The table below illustrates the wide variations in mounting weight, which might by itself be the determinant of what can be used, especially if existing vehicles are being upgunned. Conclusions Typically, IFVs are armed with cannon whose size and power is primarily determined by the need to punch holes in their enemy equivalents. This gives priority to armour penetration with the use of HE ammunition against other targets being a secondary, albeit very important, requirement. The high velocity 30 x 173 round must now be considered the minimum to deliver adequate penetration against medium AFVs, with 35 mm or one of the more powerful 40 mm rounds providing a margin for futureproofing. AFVs which are not intended to fight their opposite numbers might find RWS with lighter weapons, better suited to general fire support, more appropriate. In which case, the various mountings for the 30 mm M230LF might well be of interest, especially to those armies using the Apache helicopter. Broadly speaking, in IFVs in which delivering cannon fire is a primary role, a manned turret still tends to be preferred as the gunner and commander are always facing in the direction the gun is pointing and the commander can put his head out of the turret 16

17 to improve situational awareness, although given the risks of so doing and the development of 360 degree all-weather, day and night optronic coverage by cameras spread around the vehicle, the advantages of relying on the unaided eyeball may not be what they were. It is not difficult to foresee the adoption of technical developments from the computer gaming industry which will see crew helmets fitted with visors incorporating video screens, with the view slaved to the head and eye position so that the crew can effectively look through the vehicle in all directions as if it weren't there. Something like this is promised for the F-35 Joint Strike Fighter due to enter service over the next few years. No-one really knows how the pros and cons of the different gun and mounting options will work out until they are thoroughly tested in combat against sophisticated as well as irregular enemies, so lets hope we don't find out anytime soon. Finally, a reminder of where it all started in World War 1! ************************************************* 17