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ARMOURED VEHICLE THEORY.
Recently coming across a book which was written notes by the Driving and Servicing Wing at Puckapunyal, on the Churchill tank, Centurion Mk3 and M3A1 White Scout Car. As a modern day Military Vehicle Mechanic on A vehicles (Armoured Vehicles), I learnt from this old technology, many of the principles and reasons for many of the things which we complain about. There is a good and logical reason for most of it although these principles have somewhat become lost over time.
So I have reproduced a section of the D&S instructions on the description of the engine, many principles being common with all armoured vehicles to date.
The increasing weight of modern tanks and the higher standards of performance now expected, make great demands on those responsible for the design of the power units, the conflict between gun and armour following the lines familiar in Naval architecture.
In the case of the Battleship it is always possible to increase the dimensions, within the limits set by Dockyard and Harbour facilities: in the tank a limit of width has for long been imposed by the desirability of providing for transport by rail, the figure for English loading being about 9ft. 2 in. (Doug's comment: this is probably the single leading cause of the gunnery inferiority of English tanks in WW2 and thus the deaths of so many crew. The Germans built tanks to do the job and sorted out the transport issues afterwards eg Tiger with Combat tracks/Transport tracks.)
With the increasing weight of the machine, it is highly desirable to increase the width of the tracks to reduce ground pressures to a reasonable figure with the result that in the heavier tanks there is less relative space available for the engine than in the lighter models.
With the object of reducing to a minimum the superficial area of armour plate it is important to keep down the overall height of the power unit, while it's length is a serious matter with the increase in the size of the guns fitted, demanding larger turrets and more space for the storage of larger and heavier ammunition.
350 h.p. must be required as the minimum power requirement and conditions frequently arise in which 450 or 500 horse-power could be used to advantage.
The rolling resistance of a tank can at times reach enormous figures and this is momentarily increased when turning, though some improvement in this respect is afforded by modern methods of steering based in part on a differential action as distinct from early designs in which clutches and brakes alone were used for this purpose.
Assuming the employment of a gearbox with 4 or 5 speeds, it will be seen that a high torque over a wide range of speeds is very desirable if frequent gear changes on account of momentary increases of resistance are to be avoided.
In the matter of cooling the requirements of an armoured fighting vehicle impose severe conditions.
For obvious reasons, it is desirable to place all fuel tanks in the machinery compartment away from the crew space, while if the machine is to operate satisfactorily in places like Libya it is essential that the fuel tanks be not allowed to reach the high temperature prevalent in a steel box containing a 350 h.p. engine.
Again, every scrap of air which enters or leaves the machinery space and the radiators must pass through louvres of more or less tortuous shape capable of intercepting machine-gun bullets and shell splinters and offering considerable resistance to air flow.
Fan horse-power will therefore be high and the layout should be such that the entering air passes over the fuel tanks before being warmed by the radiators or the mass of the engine itself.
The problem of vapour lock must be dealt with and the petrol pumps must be located at the coolest possible part of the engine, even if this results in some in accessibility.
The average working life of a tank in active service is probably not a high one, quite apart from the rapid wear of the tracks, sprockets and idlers which can be replaced in the field; damage due to enemy action may quite obviously necessitate workshop attention at frequent intervals.
It would seem logical, therefore, to be satisfied with a lower standard of durability in the engine than would be demanded in a vehicle covering long mileages under normal operating conditions.
But those with experience of any class of machinery, particularly engines, know that the curve of life of a wearing part, plotted against it's bearing surface, is a very steep one. No appreciable saving in dimensions can be made on account of a reduction of the desired useful life - a part which will run for 2,000 hours without serious wear may well fail disastrously if 50 of it's dimensions (eg 50 components) are reduced by, say, 20 percent.
When in addition it is remembered that any suspicion of unreliability may involve the lives of the crew besides the loss of a very expensive piece of military equipment, it becomes clear that though the life of an engine of a tank may be short, it must be a thoroughly sound job throughout.
Hence when we look at any armoured vehicle, the above principles can be seen. Such as the Ferret Scout Car where the fuel tank is mounted directly behind the radios and the crew commander, the fuel pump low down tucked away in the dark recesses under and behind the fuel tank where servicing access requires near on a degree in yoga to remove. Hence also the air that eventually reaches the radiator at the rear most portion of the vehicle is already hot before it enters the radiator to cool it. Thus large and powerful fans are required to keep the air flowing and thus drag a great deal of horse power from the engine. A normal car engine fan takes 1 h.p. to operate and be driven thus reducing the engines output by 1 h.p. This loss is greatly reduced in AFV's due to the requirement of a larger fan or fans plural.
Weight being another huge factor in the reduction of engine and driveline component life. A Centurion Tank engine has an expected life of approximately 650 hours running time, although this is proven during peace time training, in Vietnam they were getting 2,500 to 3,000 hours out of them due to the fact that the engines were constantly running and maintaining an operating temperature rather than being started and stopped thus cold and hot running often. By comparison, the Ferret engine and drive line will last considerably longer as it's weight borders on the 4 ton rather than the 52 ton of the Centurion thus the load on the mechanical components is far less although the Ferret mechanical components will not last as long as those of a Rolls Royce sedan fitted with the same engine.
AFV design has increased greatly. Fans can now be electrically or hydraulically driven, thus greatly reducing the need for power from the engine to near on no power loss at all. Diesel fuel is now a preferred fuel and thus the requirement for vapour lock has been well dealt with. High maintenance requirements and low engine life has created developments for fast removal and change overs of power packs which allow the engine, transmission and most mechanical components to be removed and changed as a whole other than the tracks and final drives which also have easy and faster methods of removal and replacement. As such, the AFV can be kept working with very limited down time, the power pack and other highly serviced componentry can be shipped to well equiped workshops rapidly for repair.
Looking outwardly, the tank has increased it's speed and cross country performance with engines now producing in excess of 1,000 h.p. but as a result, the need to frequently remove the mechanical components for rebuild has never been and probably never will be rectified, but rather made far simpler to do in respect of keeping vehicles on the ground where they should be and thus reduced the required down time to remove engines etc by having more components than tanks, ready to just change over rapidly.
My thanks yet again to Alex McPherson.
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