Report: Tank mounted guns in relation to clearing anti-tank minefields

TNA Catalogue Reference: DSIR 27/41

Context: Department of Scientific and Industrial Research: Road Research Laboratory Reports, Reports on War Researchers Authorised at the Road Research Laboratory

Scope and content: ID 1-30

Covering dates: 1941 May - 1944 Oct

Courtesy of Drew

 

Note No. ID/11/GG.
July, 1943.

DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Road Research Laboratory

SUGGESTIONS FOR THE USE OF GUNS TO CLEAR A PATH THROUGH AN ANTI-TANK MINEFIELD.




SUMMARY

The possibility of using guns mounted on tanks to clear a path through an anti-tank minefield is discussed.

A number of suggestions are made for increasing the blast pressure from guns used for this purpose, and schemes are proposed for fitting a tank for the clearance of mines.


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From time to time suggestions have been made for clearing a path through a minefield by means of a tank carrying explosive charges, which are detonated on or in front of the vehicle. Methods have been proposed for mounting the charge on the tank and directing the blast in the required direction, but the high explosive nature and consequent destructive power on the charges used makes it very doubtful whether directional charges, if developed, could be used in this way; in this Note it is suggested that suitably mounted guns are more likely to provide directed blast over the area required. It has to be remembered however that normal propellant charges in guns of reasonable size are small compared with the charges of explosive which could be employed. It is therefore necessary to make the best use of the charge of propellant in the gun; methods of increasing the blast from guns are therefore discussed in this Note.

 

Blast required to detonate anti-tank mines
Tests with British copies of German Tellermines [*1] have shown that when fitted with 230-lb. shear pins and buried 4 in. deep in earth at SHOEBURYNESS the mines were detonated by a blast pressure of about 12 lb./sq.in. accompanied by a positive impulse of about 25 lb. millisec./sq.in. Further tests on a sandy site near WESTON-SUPER-MARE [*2] showed that the British copies were not detonated at the same distance (within 1/2 yd. in 7 yd.) as German Tellermines. With 350-lb. shear wires tests showed that 15 lb./sq.in. maximum pressure and a positive impulse of 30 to 40 lb. millisec./sq. in. would detonate the mine.
[*1] Road Research Laboratory, Note No. MOS/180/HS.LHP.
[*2] Admiralty Report D.M.W.D./T.65/5.


Blast from guns
Few records of the pressure-time curves from guns in the region forward of the muzzle and close to the line of fire are available. A recent survey of a 4-in. Naval gun, made by the Road Research Laboratory but not yet reported, indicates that with Service rounds, a gun of at least this calibre would be required to clear an area extending 20 ft. in front of the muzzle and 8 to 10 ft. on either side of it, when mines with 350-lb. shear wires are being attacked; the corresponding figures for 250-lb. shear wires would be about 30 and 12 ft. respectively

A 4-in. gun is too large to be easily fitted on any normal tank chassis, and the success of the method therefore depends on increasing the blast efficiency of smaller guns. There are two ways of doing this,

(1) by increasing the blast produced by the gun, and

(2) by improving the distribution of blast over the area to be cleared.


(1) Methods of increasing the blast from a gun

(a) Greatest blast pressure will obviously be generated when, at the instant the shot is ejected, the whole gun is filled with gas at the maximum pressure that the gun will withstand. The gun used should therefore be one designed for a high muzzle pressure. To improve the blast performance the muzzle pressure must be increased with a corresponding reduction in the safety factor (generally about 2). Slight damage in the shape of permanent deformation of the muzzle end of the gun is not important so that it is probable that the muzzle pressure could be safely be doubled. At the same time it would be desirable to lighten the shot, thus reducing both the energy of the shot and the recoil of the gun. Blank sot would no doubt have the highest efficiency, but it is doubtful whether any Service propellant would be fast enough without at least, a light shot, but the choice of explosive for producing the blast need not be limited to normal propellants. Furthermore, if filled with shrapnel, a light paper shot might assist in destroying some mines by mechanical impact. The rate of burning of the propellant should be adjusted to give a continuous increase of pressure as the shot travels along the barrel.

(b) In a normal gun, by the time the shot has travelled a short distance up the barrel the propellant is completely burnt. For the production of maximum blast, however, it would be desirable to prolong the burning period for a few milliseconds after the shot has been ejected, for until a wave of reduced pressure returns along the barrel from the open muzzle, the propellant will still be at a high pressure and will burn efficiently. The optimum time of burning after the shot has left the barrel will depend on the length of the barrel.

(c) It is known that the flash, or afterburning of the ejected gases, contributes to the blast from a gun. It might be well therefore to arrange conditions so as to produce maximum flash. Fortunately, high muzzle pressures and temperatures, which favour the afterburning of the gaseous products, are likely to result from the measures, suggested under (a) and (b); for the rest, it might be necessary to choose a propellant such as NH which is known to flash with some violence.


(2) Methods of improving the distribute of blast

(a) The gun or guns should be pointed downwards at a small angle so that the projectile strikes the ground near the outer edge of the area to be cleared.

(b) It is known that the flame and blast from a gun may be directed backwards by the deflecting plates of a muzzle brake, and improved distribution of the blast over the area to be cleared of mines could probably be achieved by deflecting plates mounted on the gun muzzle. The best distribution is likely to be produced by spreading out the flame as a horizontal sheet.

It is realised that the proposals outlined above may give rise to a number of practical difficulties; for instance, it may be found that a much larger propellant charge should be used in the gun than can be accommodate in the normal cartridge case; or it may prove that overhearing[*3] of the gun due to the use of larger charges is a serious problem. These matters which will have to be dealt with as they arise; that is possible however to increase the blast from a gun very appreciably even without drastic changes has been proved by a recent test with a 2-pr. gun at the Road Research Laboratory. The gun was arranged to fire horizontally, with muzzle 3 ft. 6 in. above the ground, and blast pressures were recorded at ground level 4 and 8ft. in front of the muzzle. Standard rounds, consisting of A.P. shot weighing 2.3 lb. with 9.4 oz. of Cordite WT 144/048 propellant, were compared with 17-oz. projectiles accelerated by 11/5 oz. of the same propellant, and it was found that with the lighter shot and increased charge, the blast pressures and positive impulses were increased by a factor of about 1.5.

[*3]A smooth-bore gun might be expected to absorb less heat than a rifled barrel.


Proposed arrangement of the gun or guns on a tank

Scheme 1.
As mentioned in an earlier section, a 4-in. gun irking Service rounds would probably clear a useful area, though with little margin to deal with mines less sensitive than those considered. The tests with the 2-pr. indicated that the same performance might be expected of a 3-in. gun with a shot of half the normal weight and a slightly increased propellant charge. Assuming that the blast might be still further increased by the measures outlined in the preceding section, the first proposal would be to mount at 3-in. gun on a tank chassis. The gun would be fired on a fixed mounting, arranged to fire forwards and downwards. The muzzle might be about 3 to 4 ft. above the ground and would be fitted with suitable deflector plates. The gun, which might be bored but smooth, would fire light paper shot filled with shrapnel, and would be expected to clear a path 20 ft. in front of the muzzle. Assuming an average rate of 10 rounds per minute, this would give a rate of advance of something over 2 m.p.h.

Scheme 2.
Measurements of the blast from a 6-pr. gun to the rear of the gun carriage, have shown it to be exceptionally severe for a gun of tis calibre. This is no doubt due to the high muzzle pressure.

It is suggested, therefore, that instead of one 3-in. gun, two 6-pr. guns might be mounted on a tank with their muzzles about 4 ft. apart. Other details would be as for the 3-in. gun, except that the deflector plates would be designed to spread the flame as a long narrow sheet. It would also be advantageous to fire the guns electrically at the same instant, as considerable reinforcement of the blast from one gun by that from the other would then occur.




Road Research Laboratory,
July, 1943,
F.