SECOND DESIGN-REVOLVING PIN FUSE.

(Plates IV and V.)

The patterns include a centrifugal fuse simply and a centrifugal fuse for side impact of the several sizes required in service. They are illustrated by the accompanying drawings of the A size, viz:

Frankford Arsenal, November 6, 1901, centrifugal fuse, Plate IV.

Frankford Arsenal, November 4, 1901, centrifugal fuse, side impact, Plate V.

The fuses received at the proving ground comprised 25 C and 25 A, centrifugal, and 10 special A and 20 W, centrifugal, side impact. No change was made in the service form of fuse bodies, primers, etc., other than to decrease the diameter of the primer orifice to 0.1 inch. This was done in order to cut down the delay action of the service. percussion primer.

The fuse plungers are made by connecting two pieces of brass cut from a rod, practically almost semicircular in cross section and with lugs projecting from their flat faces to fit into corresponding recesses in the opposite half of the plunger. A special form of steel spring, stamped out originally and then bent into a U shape, with a hole through the reenforced center of the spring fitting over a circular projection from the bottom of the assembled plunger, holds the two faces of the plunger in position and exerts a constant force tending to keep them closed. Through the projecting lugs from the flat faces of the plunger pieces holes are drilled to furnish journals for two steel pins (m). The lower one of these forms the fixed axis about which the firing pin (4) revolves; the upper one, working in a slot in the firing pin, serves to guide the pin and steady it in its armed position. The opening of the plunger is limited by the contact of the firing pin with one of the sides in its armed position. It is necessary that the shell should have a more rapid revolution than that required to arm the fuse. An excess in rotation of from 50 to 100 revolutions per minute is sufficient for this purpose. The sides of the plunger are not permitted to bind against the interior surface of the fuse stock, as this would create frictional resistance to the forward motion of the plunger on impact. The drawings show a holdback spring and plunger () inserted as an experimental feature. It is intended to keep the plunger in contact with the rear surface of fuse cavity, giving freedom for the firing pin to rotate to its armed position; also to keep the firing pin away from the primer shield during the flight of the projectile, and secure the maximum path for travel of the plunger on impact of the shell. Inasmuch as the resistance of the spring in this holdback must be overcome, the device is considered of doubtful utility. At the time that the fuse arms, the acceleration of the projectile will insure the contact of the plunger with the rear of the cavity, and the pin will revolve into its armed position before any creeping forward of the plunger can take place due to atmospheric retardation.

The side impact feature, Plate V, consits in tapering the plunger to the rear and inserting a side impact wedge (7), fitting into a conical recess in the fuse body. On side impact of the shell this acts to force the plunger forward and explode the primer even if the shell falls with its base downward, provided the angle of inclination to the rear is considerably less than 45°.

This side impact device is the successor of that shown on Plate I, and is not liable to cause premature explosion during flight of the projectile. This improved device was also applied successfully to the McBride fuse, Plate II. The upper end of the armed plunger is of normal diameter and the lower end is centered by the wedge. Under these circumstances no eccentric rotation of the plunger can take place.

The following safety features of the design are thought to be of special value:

1. The natural tendency of the two faces of the plunger is to remain in the closed position, even without the assistance of the U-shaped spring. No jolting of the fuse can cause them to open.

2. While in the closed position there is no possibility for a premature explosion, since the firing pin is not directed toward the primer; no matter in what direction the fuse be dropped, there is plenty of solid metal to receive the blow.

3. Even if, in some unforeseen way, the fuse should be armed, it must, at the same instant, be dropped on its point or side to cause explosion, the natural tendency of the plunger being to close and return to its unarmed position.

During the construction of the fuse, it was suggested that possibly it might be advantageous to furnish some locking device which would cause the fuse when armed to remain armed. A simple device of this nature not shown in the drawing was furnished with some of the fuses sent to the proving ground. It also requires centrifugal force for its operation. The fuses supplied with it are marked on the base with the letter I. All of the fuses, excepting those of the C pattern, were supplied with the holdback spring so that in the test this spring could be removed or not, in order to determine its value.

The fuse is not armed by the shock of discharge in the gun, which, by creating pressure on the base of the plunger, effectually maintains the two faces in the closed position, and the arming is retarded until the force of acceleration in the direction of the longer axis has been overcome by the centrifugal force due to rotation, tending to open the sides. This will not occur until the projectile has passed out of the bore, and this removes all chances for a premature explosion in the bore due to the percussion fuse.

In tests made at Frankford Arsenal it was found that with the low rate of arming of 900 revolutions per minute it was impossible by rolling the A fuze down a 45° slope to reach a sufficient angular velocity under the action of gravity to arm the fuse when the perpendicular height through which it was rolled was 6 or 8 feet. In the 12-inch projectile this vertical height would have to be at least 72 feet, and it would then be necessary for the shell to roll without obstruction in a right line down this slope. The rotating band would cause it in a short time, if rolled on a concrete ramp, to turn base first, and would thus prevent the projectile from acquiring its maximum revolution. When the holdback spring was left out it was not found possible to arm the side impact fuse, even when rolled down a slope from a perpendicular height of 13 feet. This was due to the forward motion of the plunger, so that its front face rested against the fuse cap, and this offered a resistance to the rotation of the firing pin. The resistance to arming is higher in all nonside impact fuses.

The different lots of these fuses were tested at the proving ground, as follows:

Lot 1. Ten centrifugal, side impact A special, arming at 900 revolutions per minute, intended to determine if this size of fuse could be used to explode 12-inch mortar

shell on water impact. After 3 had been tested in 12-inch mortar (October 29, 1901), and the first 2 on water failed to explode, the plungers of the remainder were returned to the arsenal to be fitted with the locking device which holds the pin in the armed position after it is once rotated to that position.

Lot 2. Twenty-five centrifugal A, arming at 1,500 revolutions per minute. Of these 11 were fitted with the pin-locking device, marked R. P. L., and 14 were without that device, marked R. P. In the tests in 7-inch mortar (November 20-26, 1901), 5-inch siege gun (November 27, 1901), and 5-inch R. F. gun (December 5, 1901), the R. P. and R. P. L. fuses were fired alternately with and without the holdback spring to determine the utility of the spring.

Lot 3. Twenty-five centrifugal C, arming at 3,000 revolutions per minute and without holdback spring. The primer shields were 0′′.004 thick. Of these 14 were fitted with the pin-locking device, marked R. P. L., and 11 were without that device, marked R. P. In the test in 3.2-inch field gun (December 6, 1901) 10 of the fuses were provided with the delay-action cap, which will be described later.

Lot 4. Twenty centrifugal, side impact W, arming at 900 revolutions per minute. Of these 10 were fitted with the pin-locking device, marked R. P. L., and 10 were without that device, marked R. P. In the tests in 3′′.2 field gun (December 18, 1901), and 12-inch mortar (December 23-24, 1901), primer shields 0.004 of an inch thick were substituted for those used previously (07.0015 thick) to diminish the liability to irregular delay action.

The following conclusions are drawn from the trials:

1. The design affords a remarkable degree of safety for handling and transportation, even with the lowest arming resistance (or number of revolutions per minute) required in service, as shown by the shop tests at Frankford Arsenal. In 79 fuses tested by firing under varied conditions at the proving ground no prematures have occurred, and with the different arrangements of parts provided for experiment, such as the removal of holdback spring, the introduction of the pin-locking device, etc., each pattern of fuse has shown generally satisfactory performance in the projectiles for which it is intended.

2. The exceptional failures will be noted as follows: In the 8 C fuses fired from 3.6-inch mortar, December 10 and 11, 1901, two shell at 10° elevation glanced from the ground without exploding, while three others fired under the same conditions exploded. The fall of the three remaining projectiles in this series was lost to observers and the explosion indeterminate. The exceptionally low angle of projection employed, coupled with the low velocity of this piece, is thought to account for the two failures observed.

One C fuse out of 17 fired through wood screens failed to explode.

The failures of the centrifugal A fuse in 7-inch mortar shell were overcome by using R. P. L. fuses without holdback springs (see rounds 5 and 9, records of November 20 and 26, 1901). With the centrifugal side impact A fuse, four out of five 7-inch mortar shell exploded on land and the fifth was recovered with the primer shield indented (see record of December 18, 1901). The irregularity in fall of 7-inch mortar shell at the shorter ranges can not but induce some failures in any fuse.

Some failures of the centrifugal side impact W fuse are noted in the 12-inch mortar firing records of December 6, 12, and 23, 1901. It is believed the record is largely in error in these cases and that explosions occurred so far beneath the surface of the water as to be unseen by the observers at the firing point. On December 24 three observers (officers) were stationed at the 3,000-yard target, and the shell were fired to strike water within about 400 yards of the observers. The first was a sand-loaded shell, and then two fused shell were fired, both of which exploded well under water. The explosion gave only a slight sound, but was clearly indicated by the continued disturbance of the water from below. The observers at the firing point saw no evidence of explosion and would have reported both of the fused shell as failures.

Three explosions of shell in 7-inch howitzer will be noted in the firings of December 9, 1901. These were undoubtedly due to the breaking up of the shell in the bore and not to the fuse. Cast-iron shell of 125 pounds weight for the 7-inch mortar were at that time being used in the 7-inch howitzer. The pressures reached about 27,000 pounds, and the shell proved too weak to withstand it.

3. The holdback spring should be omitted. It tends to cause failure of the fuse to explode when the projectile strikes with a side or glancing blow and is not required to prevent premature action in flight. It moreover detracts from the safety of the fuse in handling, since the firing pin will strike the blank face of the fuse cap and can not revolve to the armed position except when the plunger is in contact, or

nearly so, with the rear of its seat, and, in the absence of the spring to hold the plunger in that position, arming and ignition of the fuse under any circumstances of dropping or rolling, etc., in handling seems almost impossible.

4. The fuses for 7 and 12 inch mortar projectiles should be of the side-impact pattern, to meet possible conditions of fall of the projectiles at the shorter ranges. It appears, however, that for all ranges at which the 12-inch mortar will probably be employed-that is, from about 2,600 yards upward-the projectile will fall tangent to the trajectory and strike by direct impact. These two patterns should also be provided with the pin-locking device because of the relatively low velocity of rotation of the projectiles, which may, with the reduced charges, not afford such margin over the arming resistance of the fuse as to insure rigidity of the firing pin in its armed position during flight and on impact of the projectile.

The fuses for field and siege guns (including 3.6-inch mortar) and for direct impact generally, having a high velocity of rotation, do not require either the side-impact or locking features.

5. The revolving-pin percussion plunger can be applied in altering the service percussion fuses on hand, replacing the plunger only and using the same fuse bodies, caps, and primers. This will be particularly applicable for the preparation of any A, W, and M (or low C) fuses required for immediate issue, including the replacement of the A fuses recently recalled from service. The retention of the present fuse cap is permissible in this case, since it is fairly efficient as proved by experiment and in service use; its delay action is variable, however, and merits improvement

for future manufacture.

6. The revolving-pin percussion fuse of future manufacture should be provided with a delay-action fuse head or cap, having a primer orifice 0.1 inch in diameter, a primer shield 0.004-inch thick, and giving 0.02 second delay. This, however, does not involve the principle of the fuse, which resides essentially in the plunger and will be considered apart.

DELAY-ACTION PERCUSSION FUSE CAP.

Fuse heads of this description were furnished from Frankford Arsenal in response to the board's request of September 14, 1901. The device (drawing not furnished herewith) consists of a separate fuse head conforming generally in exterior dimensions to the ordinary head. The interior parts comprise a central, hollow metal stem containing compressed mealed powder, directly in front of the percussion primer and surrounded by a small magazine of rifle powder. The stem is pierced laterally with a hole communicating with the magazine to give a length of mealed powder train suited to the time of burning desired. The magazine communicates with the bursting charge of the shell by the interposition only of a crimped brass disk in the usual manner.

Several lots of these fuse caps, differing only in length of train as cut at the arsenal, were tested, and the firing records are inclosed herewith, as follows:

Lot 1. October 30, 1901, 8 rounds with service C fuse, 3.2-inch gun No. 113, 4 caps, marked No. 1, length of train 0.1 inch, and 4 marked No. 2, length of train 0.3 inch. Lot 2. December 6, 1901, 10 rounds with revolving pin C fuse, 3.2-inch gun No. 113, length of train 0.022 inch.

Lot 3. December 18, 1901, 4 rounds with service C fuse, 3.2-inch gun No. 35, length of train 0.022 inch, slightly reduced.

Lot 4. December 23, 1901, 12 rounds with service C fuse, 3.2-inch gun No. 113, 6 caps, marked No. 1, length of train 0.012 inch, and 6 marked No. 2, length of train 0.022 inch.

The projectiles were fired with full charges, giving a muzzle velocity about 1,685 feet per second, through pine-board screens to ignite the fuse. The screens used with lot 1 were in part 4 inches and in part 12 inches thick; the remaining screens were uniformly 6 inches thick.

WAR 1902-VOL 7—13

The distances of the points of burst in feet beyond the screen were observed as follows:

Taking the velocity of the projectile to be 1,650 feet per second after penetrating the screen, and considering only lot 2 for 0.022 length of train, these distances reduced to time become:

The results show a very consistent outcome of results for the device in the vicinity of the delay action desired-that is, 0.02 of a secondexcept for the irregular action of the 0.022 train in lot 4, wherein it was undertaken to duplicate the results previously obtained with the same nominal length of train in lot 2. It is known that lot 4 was prepared at the arsenal at a different time from lot 2, and it would appear that some variation was made in shop work. The results indicate that a train length of 0.01 inch will give the delay action required; but, because of the irregularity noted, the board deems further tests necessary, which should be made either with this device or a modification of it as suggested by Captain Dunn that can be made at Frankford Arsenal.

SHOP TESTS OF CENTRIFUGAL PERCUSSION FUSES.

The following shop tests are prescribed:

(1) Arming resistance:

An allowance of ±250 revolutions per minute will be permitted for all centrifugal acting plungers required to arm at 2,000 revolutions per minute or higher; 10 per cent to be tested.

For the 7M and 12M fuses arming at 1,200 revolutions per minute, the allowance will be 100 revolutions per minute; 100 per cent to be tested.

(2) Jumbling test:

In a wooden box 16 by 11 by 5 inches, inside diameter, revolving 30 times per minute about one of its diagonals for four hours, a loose fuse must not show any marks on primer shield; 1 per cent to be tested.