Trunnions on the Move, Part II Creating the "Gun-Over-Hull" Tank

Creating the ’Gun-Over-Hull‘ Tank The previous article proposed that the gun trunnions should be removed further rearward and centered over the vehicle’s hull. A variety of different gun mountings moved further rearward and centralized over the vehicle’s hull. A variety of different gun mountings can be interposed between the tank gun and the hull of the vehicle, but those that hold the gun permanently raised above the level of crew direct-view surveillance vision devices have been judged unsuitable for the MBT and have been rejected. Acceptable gun mounting configurations will be further reduced in number if all-around traverse is judged to be essential.

This concluding article examines developments which will have to be undertaken if these ‘gun-over-hull’ configurations are to be created. Finally, two different methods of installing the gun on the Main Battle Tank in ‘gun-over-hull’ mountings are sugg A New Gi and Automaric Loaaing The reasons for abandoning the conventional tank turret and moving to gun-over-hull mountings are to reduce the forward projection of the gun muzzle, which has now become excessive, to improve the mounting’s balance, to reduce its rotational inertia, and to prevent the gun from intruding downwards below the ring and dividing the fighting compartment. Such gun-over-hull mountings will place the gun centrally over the hull - actually within the hull in the case of the Swedish S Tank, in a shallow cleft in the hull roof in the case of a UDES-17 type vehicle, in the central cleft of a cleft turret, or finally, above the level of crew surveillance vision devices when raised up on a raisable gun mounting.

With the exception of the first of these methods of gun mounting, the breech end of the gun will extend to the rear outside armor protection. raising problems of the re-by Robin Fletcher More “gun-in-hull“ than “gun-over-hull,” the Swedish S-Tank design was revolu-liability of its remote operation, of its automatic loading and of its protection from attack from at least small arms fire and shell splinters. This may require the development of anew tank gun and ammunition system specifically designed for this new situation, with a breech which is power-operated and self-protecting and with its center of gravity much further forward. It has also been suggested that the gun might be closed at its rear end and divided at the front of the chamber so that parallel-sided, telescoped combustible-cased rounds might be loaded rearward into it.

If this new gun system is to differ so fundamentally from current conventional tank guns, a decision will soon have to be taken as to whether our next new tank gun should be configured along conventional lines, to be installed within a turret, or whether it should be designed to agree with the requirements of the suggested new gun system because it will be carried in gun-over-hull mountings. The same projectiles might be used and similar internal ballistics employed in both cases, but the configuration of the gun, and probably that of the ammunition, will need to differ substantially. There is thus a parting of the ways in tank gun design, with a decision having to be taken prior to any design work being started, as to whether the MBT will continue to be turreted or whether it will adopt some form of gun-over-hull mounting. tionary in its time. nnoaei iiiusrrares me sweaisn uuca configuration with gun in raised position. If a wrong choice is made and the gun is wrongly configured, it could

- in its conventional form - be enclosed and counterweighted by an armored box and placed in a cleft turret above a hull roof that runs horizontally. This is far from being an ideal arrangement, although height can be reduced by inclining the suspension, but it does allow the continued employment of conventional tank guns and may allow the extended use of existing hulls which are rear engined. And it is certainly possible for a conventionally configured tank gun to be installed in a raisable gun mounting, although the result might not be as satisfactory as could be expected from the use of a “purpose built” gun system. A gun configured to the new gun system could be forced into a conventional turret, although the forward location of its center of gravity would make this very difficult. The possibility does exist of using a divisible gun within an enclosed turret, allowing rounds to be loaded

- -. --rearward directly into the chamber. This could even be looked upon as being an advance on the present method of loading conventionally turreted tank guns.

These articles maintain that the conventional tank turret has reached the end of its development and that a change to gun-over-hull mountings is now needed. The gun and ammunition system is the key to this changeover and it can either delay or assist this revolution in gun mounting.

Front Engined Hull If the gun is to be mounted centrally over the vehicle’s hull without front or rear projection, its center of gravity and trunnion position will lie to the rear of the center of the vehicle, which suggests the adoption of a hull which is front-engined. And the rear of the gun -especially if the new gun system is adopted -must be able to swing down to below ring level when the gun is elevated, which again indicates a front-engined vehicle. This might even include the provision of trailing rear idlers to lower the top run of the tracks at the rear of the vehicle. This approach, in fact, was used in the indirect-fire Swedish 155-mm SP Bandkanon IA1 which was built with a gun-over-hull layout.2 These idlers could be locked while firing to increase the vehicle’s stability.

Allied to theee reasons, which stem directly from the adoption of gun-over-hull mountings, is a growing conviction - now realized in the case of the Israeli Merkava MBT -that a front-engined layout should be adopted so as to use the engine compartment to provide ad: ditional protection, to allow rear stowage of ammunition, and to provide an entrance door at the rear of the vehicle. A final argument in favor of a front-engined layout is that obsolescent MBT hulls could be easily converted into heavy APCs or self-propelled mountings once their life as gun tanks had been exhausted.

With a front-engine design, the increased heat at the front of the vehicle might cause it to be spotted by thermal detectors, and might even interrupt its gun sighting by convection. This could be avoided by circulating coolant to the rear of The Swedish Bandkanon /A self-propelled artillery piece has movable rear idlers which drop to stabilize the vehicle in firing position. Engine is in the front. the vehicle (e.g. Marder and the Swiss NKPz) and possibly by su-perimposing a masking skin, kept at ambient temperature, over the front of the hull.

Unfortunately, the protection which will be afforded to the crewmen in the fighting compartment by having the engine compartment at one side of the front of the vehicle will be compromised by the presence of the driver’s station at the other side. This will take up part of the width of the vehicle and interrupt the armored bulkhead separating the two compartments. The driver’s station in its present position thus presents a problem to the tank designer. If it could be removed, the engine compartment -particularly if the engine were to be mounted transversely - could extend from one hull sideplate to the other and the bulkhead could remain intact so as to provide full protection.

The experimental MBT 70 relocated the driver in a counter-rotating station in the turret of the vehicle, but failed to produce a satisfactory answer.‘ An alternative might be to provide driving facilities to both turret crewmen, who would have an excellent view forward over the front of the turret and could see directly rearward to reverse the vehicle. Gun-over-hull mounting will have eliminated forward projection of the muzzle so the turret can remain locked forward for driving. To improve endurance in continuous operations,5 a third crew member will be needed to rotate to the two principal turret crew stations and relieve the other two crewmen. If this third man were able to lie and sleep at full length in the lower part of the fighting compartment, or with his feet against the rear entrance door, he would then be available to adopt a prone position and take over the driving function, using remote vision, should fire on the move become necessary.6 In this way, increased survivability and extended endurance in ‘round-the-clock combat’ would be gained while removing the driver from the front of the vehicle.

The changeover from rear to front engine will not be welcomed by the engine designer. Over the years, he’s been able to install the engine within comparatively thin armor which can easily be modified. A front engine would have to be confined within fixed hull dimensions and placed under heavy armor where conditions will be far more demanding. But 50-ton front engined test bed vehicles have been constructed already - as witness the VTF which started trials two years ago in Germany.7 Suspension System Earlier, suspension control was discussed as one means of overcoming the height penalty of first moving the gun rearward over the ring and then using contact between the gun and the top run of the tracks to establish the position of the gun trunnions. As already mentioned, this could be pursued to the ultimate; at least with the vehicle stationary, it would be possible -though not necessarily practical -to transfer all depression and elevation to a controllable long-travel suspension system with a consequent simplification of the gun mounting.

But the suspension has as its primary function the support of the vehicle during cross-country movement. Following the success of the hydrogas suspension of the British Challenger MBT, it is doubtful whether future tanks can afford to forego the use of similar systems. With more design effort being devoted to the suspension to raise cross-country speeds, automatic variation of suspension porting might be introduced to effect some degree of hull stabilization which would ease the task of stabilization at the gun mounting.8 This could be extended as an ‘active’ and possibly ‘forward-looking’ suspensions which might, again in the ultimate, assume full responsibility for gun stabilization. But it would also stabilize the crewmen and their vision devices and can be compared with the possible stabilization of the upper part of an oscillating turret, which has already been mentioned. With ride and attitude control being two of the functions of the suspension system, height control would be the third, allowing the vehicle to crouch down for best concealment or to rise up to move or to engage. But this function, as already noted, may not be able to be achieved with sufficient rapidity to be used during an engagement. The target which the vehicle would present when raised would remain large when compared to that of a raisable gun, which would reveal only its gun mounting. Although future control of the suspension will probably be exerted through a hydrogas system, both at the halt and also while moving, it can be applied immediately to existing torsion bar or hybrid system for use only at the halt or while moving slowly. O It would be a mistake if work on suspension development were to KS. ELKE prototype drawing shows the vehicle’s raisable 75-mm automatic cannon in raised and lowered positions. Prototype was built on a Sheridan chassis. done in a series of test bed vehicles with increasing degrees of sophistication until a fully stabilized system is finally created. slow down once hydrogas suspension has been adopted. There is a, the original reason for its intro-clear need now to move on to in- duction. clude at least control of the ve-There does not appear to be any hicle’s attitude. This should be particular difficulty in designing a Gun Raising The raisable gun has-so far only been used in the American ELKE test bed vehicle, carrying an ARES 75-mm automatic cannon above a Sheridan hull. The Improved TOW Vehicle (ITV) raises a TOW launcher to fire from the protection of defilade, and ELKE has been designed to do the same, but with a high-velocity kinetic energy gun. The variable geometry raisable gun mounting was originally introduced as a means of lowering the height of the turret and reducing the overall height of the vehicle following the adoption of the gun-over-hull configuration. It is only after this step has been taken that the full tactical value of such a system becomes apparent: the gun being raised temporarily above crew vision devices to display only a minimum target for the shortest time possible. This is the same small target once assumed to be the sole prerogative of the overhead external gun. In contrast, the raisable mounting exposes it without any vehicle movement and with only a momentary sacrifice of top vision. The tactical advantages of the raisable gun are unique and most valuable. They soon overshadow height reduction. which was raisable gun mounting and applying it to a cleft turret, to a traversable mounting of smaller diameter, or even to configurations based on vehicles of the casemate type. High-pressure hydraulics would be outside the armor and the whole gun mounting should be able to be lifted off as a unit for repair or replacement. It may be found beneficial to arrange the three separate functions of elevation and depression, gun recoil, and gun raising in an unorthodox fashion, and some integration may be possible between them. It must be remembered that the gun should be able to be fired without being raised from its fully lowered position and should also be able to remain fully raised when required to fire while moving. A development program for a raisable gun mounting might start by raising and firing a tank gun from a fixed test stand and then transferring the complete assembly to the turret ring of an existing tank hull to be fired tactically. With traveling, raising, and firing loads transmitted directly to the hull through the raisable mounting, the shell of the turret could then be constructed in canvas or in wood. Only then would the design of a plate turret and automatic loading system be undertaken. The same raisable gun mounting should be able to be installed in a variety of different turrets, which may be either lightly or heavily armored. If suspension inclination and gun raising are rivals in the reduction of the height of gun-over-hull con- - -figurations - employing different FIGURE 1. One of the author's two conceptual MBTs, this one has a conventional tank gun carried in a cleft turret with raisable mounting giving all-around traverse of both gun and crewmen. types of variable geometry to achieve somewhat similar effects

- which should be preferred and should any combination of the two be attempted? Clearly the raisable gun mounting is superior because of the small size and the small time of exposure of its target, but suspension control -particularly that which can be used only at the halt

- could be comparatively simple and could act at least as a low-cost standby system. For instance, if a raisable gun mounting in a cleft turret were to fail in action, the gun would remain in the lowered position but might then be laid onto targets in any direction by inclining the suspension. Should amounting of the UDES-17 type fail and remain lowered, the vehicle could engage by means of its suspension control but in one direction only. Suspension control and gun raising are both able to reduce the height of the vehicle; they are mentary. Both techniques should be further developed and both streams of technology should then be made available for the creation of gun-over-hull mountings.

So far, the gun has been considered as being on the center line of the turret or the hull with crewmen located on either side of it. This can be criticized for tending to present a large target to enemy return fire. But on the other hand, the rear of the gun is given good protection against fire directed at it from a considerable arc over the front of the vehicle.

However, an alternative approach is to seat the two crewmen in tandem on the same side of the gun mounting (e.g., FMC Armored Gun System) in which case they can be better protected behind a reduced width of frontal protection. But at the same time, they will lose the advantage to be gained from the full duplication of their crew stations. This configuration might be modified by moving the gun off center, as has already been done in the case of a number of cannon turrets,'l which would move the two crewmen towards the turret center and give them more room. The process could be carried further bv moving the two crewmen rieht to the center line of the turret, offsetting the gun. While the gun would then be less well protected, there should be not loss of accuracy from such an arrangement. The shot would be out of the muzzle before buffering and rotation of both turret and vehicle had time to take place. The subsequent awkward motion of the vehicle might be uncomfortable for the crewmen, and a muzzle brake might be needed to prevent alight vehicle from being moved round by recoil.

If two crewmen were to be seated in tandem, one might be at the center of rotation of the turret where he could drive from a counter-rotated crew station without difficulty. A small thick block of armor would then give protection to both crewmen and to a ready round magazine to their rear. An armor skirt might even be extended downwards, within the turret ring, to provide increased protection, particularly when firing over the sides of the vehicle.

Considerable emphasis has been placed in these articles on engaging the enemy from behind a crestline, since most of the vehicle will then be protected and only the vision devices and the gun itself will be exposed. But in the forests and cities of Europe, the enemy is more likely to have to be engaged around rivals, but they may also be comple- - - -the comer of buildings and woods. This would suggest that guns might be offset or even ‘outrigged’ from the hull or the turret as they already have been in a number of German experimental designs.12 It would even be possible to envisage a vehicle which would follow the lead already given by certain German proposals for extendable missile launchers and to have the gun not only raised up to fire over cover but also moved over to one side or the other to fire round a left-handed or right-handed comer.

There are, however, problems in this approach. The first is that while a crestline provides cover both from view and also from fire, a wood or a building will not provide adequate protection from kinetic energy antitank projectiles. A second problem will be surveillance. Although sighting can be carried out remotely, whether the gun is raised or placed to one side, direct surveillance vision still requires the physical presence of a crewman. This can be provided at the top of the vehicle, but cannot be provided at one side or the other over its tracks.

Full implementation of firing round corners thus appears to be improbable, although a raisable gun might be swung down to one side of the turret or the other and might well be fired while it was lowered. While such a system would avoid having to make a cleft in the center of the turret, the gun would then remain virtually unprotected. And its lateral movement while being swung up to above the turret might be expected to attract been vertically raised. the hull.

The advent of the overhead external gun in the 1970s gave anew means of providing all around traverse for the gun. This allowed a choice to be made between the crew being “gun-oriented” and traversing in some form of turret and being “hull-oriented” and being seated down in the hull. Disenchantment with the overhead gun followed later as the disadvantages of the loss of direct top vision were more clearly realized.13 Perhaps a solution may be to accept the overhead gun for use in defensive positions while at the same time producing MBTs as fully traversing deigns.1 But the situation changes yet again when the potentials of raisable gun mountings are clearly understood. The use of a raisable gun in a cleft turret confers advantages a conventionally turreted tank has so far never possessed. The UDES-17 configuration, which provides the gun with all round traverse when raised to above the level of crew vision devices, could be considered in the MBT role. If all-around traverse for the MBT is demanded, then casemate designs, including those with limited traverse, must be passed over and only two designs for the MBT remain. One will have the crew seated in a cleft turret to face towards the target. The other will be based on the UDES-17 design, with the crewmen seated down in the hull.

In the UDES-17 case, the crew will have to raise its gun before traversing while the cleft turret can traverse discreetly and then raise the gun. The fact that the gun of a UDES-17 type vehicle cannot be lowered while it remains traversed detracts from the survivability of that design. But this is counterbalanced by the fact that the vehicle is not divided into two quite separate parts by the presence of a turret traverse race and so should be better able to withstand high levels of attack. In effect, the compactness and strength of the casemate type vehicle is carried forward into a design capable of all-around traverse.

Two alternate designs for a future MBT are shown here (see Figures 1 and 2), based on drawings which originally appeared in December 1984 in Soldat und Technik.15 The turreted design (Fig. 1) adheres to the conventional as far as possible, employing a raisable gun in a cleft turret. It provides lateral protection for the breech of the gun by twin bustles, each containing ready rounds. Twin loaders transfer rounds from the bustles and swing over to ram the rounds forward into the gun. Driving is normally from the turret top, with the turret locked forward, although a third crewman, resting in the lower fighting compartment, can take over driving should this be required.

ROBIN FLETCHER was commissioned in the Westminster Dragoons in 1941 and later served in the Special Operations Executiveand 2d Special Air Service Regiment. After the war, he attended the technical staff officer’s course at Shrivenham, spent two years on tank design at Chobham, and returned to Shrivenham to lec-tureon tank armament. After leaving the service, he raised crops in Kenya and cattle in Ireland. Hisarticleson armor have been published inlnter-national Defense Review, Soldat und Technik, TANK. and other journals.

The lift-and-turn mounting of the UDES-17 type vehicle (Fig. 2) is considerably more radical. It carries the gun on a raisable mounting on a pedestal drum which does not itself move up and down. The drum would have both top and bottom bearings and would contain a number of ready rounds. This design uses a divisible gun system to accept rounds loaded directly upwards and rearwards to the chamber from the drum. Although full all-around traverse will be available, reloading may only be possible while the gun is within a 180-degree arc over the front of the hull.

These two vehicles are very different from the conventional main battle tank. They are front-engined, both for better protection and also to agree with the gun-over-hull theme. And they raise their guns to gain the tactical advantages that only such raisable gun mountings can confer. One tries to stay as close to the conventional as possible and uses a more or less conventional gun, while the second is deliberately more extreme in adopting a crew-in-hull layout and also a divisible gun.

These articles have been based first on the rearward relocation and then on the vertical adjustment of the gun trunnions of the main battle tank. Rearward relocation has been first with respect to the turret ring and then, in a wider context, with respect to the top run of the vehicle’s tracks. Height reduction will be effected either by inclina-Footnotes ‘Christopher Foss, “Swedish 155-mm Bandkanon l. A Self-propelled Gun,” Jane’s Armour and Artillery, 1984-85, p. 460. ZR. M. Ogorkiewicz, “Merkava Mark 2,” Znternational Defense Reuiew, March 1985, p. 311.

3Rudi Meller, “NKPz -The Swiss Tank for the OS?," ZDR, July 1979, Special Series -I 1 Armored Vehicles.

‘Walter J. Spielberger, “From Halftrack to Leopard 2,” A History of the Krauss-Maffei Ordnance Department, 1979. 5Emanski and Scharfen, “Continuous Land Combat Operations - Are We Only Half Effective?” RUSI Journal, June 1978. 6Robin Fletcher, “Crewing the Turreted Tank,” ZDR, February 1983, p. 199. 7 R l f Hilmes. “KAMPFPANZER - die *Patrick Mercillon, “Meesier Auto-Indus-trie,” Defence and Armament No. 33, September 1984. gPeter Massey, “Active Suspension,” ARMOR, September-October 1979, p. 35. loEric C. Ludvigsen, “South Korean Tank Tested,” ARMY, May 1984, p. 77. llChristopherFoss, “FMC 25mm Oneman Turret for AIFV,” Jane’s Armour and Artillery, 1984-85, p, 822.

12Hans-Rudolf Lembcke, “A Tank of the Future in the Context of German Defense Technology,” Krupp MaK Defense Journal No. 5, November 1980.

I3Brigadier Richard Simpkin, “Room at the Top,” ARMOR, January-February 1985, p. 18.

“Wolfgang Flume, “The Combat Vehicle Family of the Future,” Military Technology MZLTECH, April 1985, p. 76. 15Robin Fletcher, “Die Hublafette-ein neues Entwieklungen der Nachkriegszeit,” Soldat Konzept fur einen zukunftigen Panzer,” Soldat und Technik. December 1984. D. 622. I unri Trhnik. l9R4.