Purpose.—Many of the younger officers of the service are unfamiliar with either theory or practice at the Naval Proving Grounds, and this paper is presented with the intention of reminding them of the vital necessity of the station's existence. It is also desired to bring up to date existing publications on the subject. The larger portion of navy ordnance must pass through the proving grounds before issue to the service, and upon their efficiency depends directly the efficiency of most of the material supplied by the Bureau of Ordnance.
To visit the proving grounds for at least a casual inspection should be part of the education of every officer; but a natural confusion as to what is going on is almost inevitable unless a clear conception is borne in mind of the purpose of the station. It may be likened to the testing laboratory of a large manufacturing plant, in which is tested, in accordance with specifications, all the material which goes into the finished product.
Similarly, all guns, mounts, projectiles, powder, fuses, ammution details and armor are tested, in accordance with specifications, at the proving grounds. Such specifications are laid down from time to time by the Bureau of Ordnance, and are compiled to include the best and latest ideas of the service and of ordnance engineers of this and foreign countries. If the material passes the prescribed tests satisfactorily, it is accepted; if it fails on any of the prescribed tests, it is rejected.
The desirability is thus indicated of having officers, both afloat and ashore, communicate with the bureau in regard to ordnance material which does not function perfectly, in order that such defects may be corrected in future types. This statement may seem too axiomatic to older officers of the service, but it cannot be too strongly impressed upon the younger members, whose tendency is to "make it work" and then forget that others may be experiencing similar troubles.
New devices are also tested at the proving grounds, and the increasing growth of the experimental department shows clearly the opportunity and the urgent need for research work in ordnance and gunnery. New ideas and developments of material in the service should, in a similar manner to reports of defects, be submitted to the bureau for consideration.
Location and Size.—The United States Naval Proving Grounds consist of the upper station, situated at Indian Head, comprising a tract of land of 409 acres on the left bank of the Potomac River, about 25½ miles below Washington, and the lower station, of 1365 acres on the right bank of the Potomac, 41 miles below Indian Head. The former has a water range of 18,000 yards, and the latter 100,000 yards. The entire reservation at Indian Head includes about 882 acres, of which the larger portion is occupied by the United States Naval Powder Factory. The proving grounds are entirely separate in administration from the Naval Gun Factory at Washington, D. C., coming directly under the Bureau of Ordnance.
Data Observed.—The bread and meat of proof work are velocity and pressure. Other data are recorded, but a clear understanding of the methods of obtaining these two vital necessities is essential.
Measurements of Velocities and Pressures.—Velocities are obtained usually from the Boulengé chronographs, which measure the time interval between the successive cuttings, by the projectile, of screens composed of copper wire. The first screen is placed about 80 feet in front of the muzzle, and the second 50 meters further. Separate electric circuits pass through the two screens, and the interval between breaking the circuits of the first and second screens supplies a measure of time for the chronographs. This, taken in connection with the space between screens, gives the projectile's velocity at a point midway between. The velocity is reduced to muzzle velocity at the physical laboratory, located about 400 yards from the firing battery, and is reported to the officer conducting the proof.
Shells recovered sometimes have the marks of screen wires graven on the nose by their rapid passage through. Two marks are usually cut, forming an X, of which one angle is the measure of rotation between screens.
Pressures.—Pressures are obtained by the crusher gage (see Fig. 1) from the comparison of copper discs calipered before and after firing. At least three gages are used, but where special accuracy is desired, six or nine are used, making due allowance for consequent reduction of chamber volume. The gages are placed loose before firing in the rear of the chamber of a B. L. R. or the base of the cartridge case of a R. F. gun. The copper discs are made of uniform metal from which sample discs have been calibrated. For anticipated pressures above nine tons, discs having had an initial pressure of nine tons are used. For pressures below nine tons, unpressed discs are used, except when, in experimental work, extremely low pressures are expected, when lead discs are used.
Types of Proofs.—Tests may be divided into the following:
1. Guns and breech mechanisms.
2. Mounts.
3. Powder.
4. Projectiles.
5. Fuses and tracers.
6. Armor.
7. Explosives.
8. Cartridge cases.
9. Primers.
10. Bombs.
11. Special work.
12. Experimental work.
Proof of Guns.—In accordance with specifications, every gun and breech mechanism must be tested with an overload pressure one and one-quarter times that which it will ever thereafter experience in service. Assurance is thus given that the gun will not "blow up" on board ship. If a gun is weak, it is better to have it blow up at the proving grounds, where everyone is under shelter. As pressure is a function of the weight of charge, proof pressure is obtained by firing a charge heavier than the service charge.
When guns are completed, they are sent to the proving grounds, having been prepared in all respects for firing. By means of locomotive cranes for medium and minor calibers (see Fig. 2), and gantry cranes for major calibers (see Fig. 3), they are transported to the firing battery, where the mounts, or girders, are clamped to slotted gun circles. The gun, breech mechanism, firing mechanism and mount are then carefully inspected. Data as shown on firing record are collected and recorded in advance of firing, as regards the characteristics of the gun, mount, projectile, powder, ignition, primer, case, etc.
The officer conducting the proof refers to records, specifications and curves or graphs of previous firings of guns of the same type (see Fig. 4). These curves are plotted with the velocity and pressure as functions of the weight of charge. Having determined the charge to give three-quarters service pressure, he weighs, checks and, after seeing that all personnel is under shelter, fires the charge.
Plotting, as a function of the weight of charge, the velocity and pressures actually obtained, the proof officer predicts the charge which will give service pressure and velocity. He then successively fires, plots and predicts the charges which will give just below proof pressure, and then proof pressure. In cases where doubt exists, a second proof round is fired. One more service round is then fired to relieve the strains set up by the proof round.
After firing each round, a thorough inspection make's certain that any defects which have developed are discovered. Records are taken, noting particularly: (1) Cracks in screw-box liner, plug or at muzzle; (2) condition of rifling; (3) crawling of tube or liner, at breech or muzzle; (4) length of recoil and counter recoil; (5) condition of case or gas check; (6) condition of primer; (7) blow-backs; (8) signs of failure of mount; (9) special notes, as directed by the bureau.
When predicting charges above service from the velocity and pressure curves, the proof officer must exercise great care when passing the critical point in density of loading, as pressures will jump excessively when this is reached. Cases have been known where the addition of one-tenth of a pound over the "near proof" charge in a medium caliber gun has caused a 10-ton increase in pressure; i. e., from 18 to 28 tons. As the highest specified proof pressure is 20 tons, the gun was thus subjected to a pressure three-quarters in excess of service pressure. To the credit of the gun, it is noted that no signs of strain appeared, except that the firing lock was wrecked. Attention is especially called, however, to the fact that such an occurrence could not happen by the addition of a small amount to the service charge, as the critical point for powders issued to service is always safely higher than around the point of service charge and pressure.
Indicator cards showing the work done by the recoil liquid during recoil and counter recoil are taken with Tabor indicators, and are of much assistance to the gun factory in connection with design and determination of proper functioning. The action of the indicator is similar to one on the cylinder of a reciprocating engine, as the recoil is analogous to the piston stroke, and the recoil liquid pressure to steam pressure.
In special cases, the velocity of recoil may be taken by the chronoscope.
Various guns are given special proofs, in accordance with the specifications, which have been developed from the needs of the service. For example, all S. A. guns are fired 10 rounds rapid fire.
After proof, the gun is returned to the Naval Gun Factory, where it is bore-searched and star-gaged, and stamped with "P" and the initials of the inspector of ordnance in charge of the proving grounds.
Proof of Breech Mechanisms.—Breech mechanisms on proved guns are proved with three rounds—service, proof, service. Rapid-fire test of new S. A. breech mechanisms is in addition.
Proof of Mounts.—Mounts are ordinarily proved during proof of gun, but for proof, using a proved gun, three rounds are fired, working up to within a ton of proof pressure.
Proof of Powder.—In accordance with specifications, a sample of each index of smokeless powder is tested, before acceptance by the Bureau of Ordnance, to determine:
1. The weight of charge to give service velocity.
2. The weight of charge to give target practice velocity, in the case of powders for which a reduced velocity is used.
3. The service pressure per square inch.
4. The target practice pressure per square inch.
5. The regularity of a number of rounds, as shown by the mean variation.
Powder is always proved at a uniform temperature of 90° F. (32° C.). This temperature is reached by gentle heating, done in air-tight tanks to avoid loss of volatiles. If the power is for a R. F. gun, the cases used in the proof are also heated to the same temperature. When made up, the charges are exactly as issued to service, as regards ignition, bags, mouth cups, primers, etc. The projectiles used must be of the type to be used in service with the proof powder. Each is carefully measured and weighed before being brought to the firing battery.
The proof of a powder consists of the development of a velocity curve and a pressure curve, in the same manner described under "Proof of Guns." The officer conducting the powder proof examines the "characteristics" of the powder as reported on the accompanying description sheet, i. e.,
(a) Web thickness,
(b) Volatiles,
(c) Nitration,
and with these as a guide, consults the velocity and pressure curves of a similar powder. As hundreds of these curves are on file, and allowed variations in powder manufacture are small, approximately the same characteristics can always be found.
If the web of the proof powder is smaller than the reference powder, the former will be quicker; that is, will require a smaller charge for the same velocity. This may be remembered by reducing to absurdity, as, for example, a one-pounder powder is quicker than a 14-inch. If the percentage of volatiles of the proof powder is lower than that of the reference powder, the former will be quicker; reductio ad absurdum: a dry powder grain will burn quicker than one soaked in water. If the nitration of the proof powder is higher than that of the reference powder, the former will be quicker. Aid to memory: gun cotton, which has a higher nitration than smokeless powder, is much quicker.
The proof officer can thus make his allowances before firing the low round, which starts the curves at 150 to 200 foot-seconds below service velocity.
Charges are then fired and plotted to run the velocity curve up to 100 foot-seconds above service velocity, unless the accompanying pressures indicate that excessive pressures may be obtained. Four rounds are usually fired, two of which are at or near service velocity.
An examination of the velocity and pressure curves faired through the plotted points gives the information noted in 1, 2, 3 and 4 above; 5 is determined by dividing the total variation of the plotted points from the faired curve by the number of rounds fired.
A small sample enclosed in an air-tight jar is taken during the proof from the proof sample and sent to the chemical laboratory for chemical and physical analysis, as to stability, nitration, volatiles, solubility, web, etc., all of which must be in accordance with the specifications.
After proof a report is forwarded to the Bureau of Ordnance, recommending either acceptance or rejection.
If the powder is accepted, an index number is assigned. This number, together with the manufacturer's lot number and the weight of service charge, is stencilled on the boxes, and the index is forwarded by the manufacturer to a magazine.
Proof of Projectiles.—The types of projectiles purchased for the navy are:
I. Armor-piercing.
2. Common.
3. High capacity.
4. Flat-nose.
5. Shrapnel.
6. Target.
Armor-piercing projectiles are tested in accordance with the specifications under which the contract was let, to ensure their satisfactory operation in service. They are:
1. Fired against plate.
2. Fragmented.
3. Ranged.
For test against plate the inspector at the works selects four projectiles from each lot manufactured. The test simulates the extreme conditions as to angle of fall, striking velocity and thickness of plate which are anticipated in battle. The face-hardened armor plate used is one of a specified thickness which has previously passed test, and is set up on a heavy oak timber structure in front of a sand butt at a specified angle to normal. The posts of the structure are so set and the impacts so placed that the projectile will pass between the posts from the plate to the sand, without injury to the structure. A tube, shaped like an inverted U, (n), is placed in front of the plate, and this, with flat cover plates, stops the spread of fragments.
The gun, about 100 yards from the butt, is bore-sighted on a chalk mark on the plate. Screens are placed in the line of fire to measure velocity, which in this case is calculated to give the striking velocity. A pasteboard screen is also used to record whether the flight of the projectile was true.
A charge of powder is chosen from curves of previous firings to give the specified velocity corresponding to the thickness of the plate at the point of impact. After all personnel is well under shelter, the projectile is fired.
Data as to the dimensions of the impact, flaking of the plate, penetration of the projectile, etc., are taken. Fragments of the projectile, if any are in front, are collected and weighed. Unless a second round at the same plate is to be fired at once, the projectile is then dug out of the sand by a bucket crane and after recovery is inspected, checked and photographed.
Two out of four projectiles so fired must be recovered in "effective bursting condition." A full report of the behavior of the test projectiles is forwarded to the bureau.
Four more armor-piercing projectiles are also selected from the first lot of each contract, of which three are ranged and one fragmented.
Projectiles are ranged by firing them at service velocity on a favorable day, from the highest powered gun for which they were designed, in a standard line of fire as laid by a theodolite, at an elevation of 8°. Under these conditions the mean dispersion of several projectiles should be within the limits of the normal dispersion, as indicated by many firings at the Naval Proving Ground. The range should also be as great as that shown by the range tables. Both range and dispersion should also compare favorably with those of station projectiles of uniform characteristics fired at the same time.
Observers at favorable stations on the Potomac River cut in the splashes with theodolites. Their observations are plotted on a plotting board on which are permanently marked the observing stations. The exact position of each splash is thus located and the mean range and dispersion both of test and station projectiles obtained.
During firing the projectiles must not strip their bands, break up or flight erratically.
Projectiles are fragmented in an explosion chamber by exploding electrically a service charge of explosive in the shell cavity. The resulting fragments must be satisfactory in number, size and shape, in order that the maximum damage may be done when bursting on board an enemy vessel.
Common projectiles are tested by ranging and fragmenting a specified number of projectiles per contract, as described for armor-piercing projectiles. Plate test is against special treatment steel plate of a thickness and at a velocity laid down in the specifications for the contract. Projectiles are recovered from the sand butt behind the plate by a bucket crane.
High capacity and flat-nose projectiles and shrapnel are fragmented and ranged, as laid down in the specifications. Shrapnel is also tested in accordance with specifications similar to those of the army.
Target projectiles are ranged, three per lot, at service velocity.
Proof of Fuses and Tracers.—Detonating fuses of various types are tested in accordance with the instructions of the Bureau of Ordnance. All detonating fuses in shell filled with high explosive are fired in a covered butt to prevent the spread of fragments.
All major, medium and minor caliber tracer and ignition fuses have samples from each lot submitted for test.
The proof consists of several tests which ensure their safety and satisfactory functioning in service. These tests consist of:
1. Drop test.
2. Fragmentation test.
3. Flight test.
4. Plate tests.
5. Tracer test.
The drop test consists of dropping a blind loaded projectile, in which the fuse has been inserted, a distance of 30 feet or more. The fuse must not arm or the tracer, if fitted, ignite.
The fragmentation test is made by causing a fuse to act in a service projectile filled with service explosive.
For the flight test, a projectile loaded with black powder, with the fuse inserted, is fired down the range. The fuse must not act prematurely or in flight.
For plate test, several projectiles, loaded and fused as for service, are fired through plates three-sixteenths of an inch, or above, at service velocity, or below, as specified. The fuses must function satisfactorily in all cases.
For tracer test of fuses so fitted, three are fired in service projectiles at night, down the range. Two out of three must function satisfactorily so as to be followed by the eye for 12 seconds. Two or more timekeepers are used and the gun is served as rapidly as possible.
Time fuses are tested in shrapnel to determine time of functioning and variation from the mean time of burst. They are also given percussion tests at plank, canister test at 0 seconds setting and graze test on a pile of sand. In all cases they must function satisfactorily.
Special fuses are given special proofs, as laid down in the specifications under which they were manufactured.
Proof of Armor.—Armor is proved by determining its ability to withstand the attack of an enemy's projectiles at battle ranges, and the test simulates actual conditions as far as possible. A sample plate from each lot is selected by the inspector at the works of the manufacturer, who chooses the plate expected to give the poorest ballistic results. The sample plate is shipped by rail to the proving ground. Class "A" (face-hardened) plates are set by a large locomotive crane against heavy oak timber structures in front of sand butts. The plate is backed or unbacked in accordance with specifications and is secured to simulate service conditions.
The test is carried out in a similar manner to an armor-piercing shell test. The gun is usually of approximately the same caliber as the thickness of the plate. Velocity and pasteboard screens are used. The projectiles are those known to be of uniform quality. Tubes and plates are placed in front of the plate to prevent the scattering of fragments.
Several rounds are fired. After each, the plate is inspected to be sure that it has defeated the projectile in every respect, as laid down by the specifications. Measurements are taken and recorded, as in the case of shell tests. After the completion of the test, or sometimes after each round, the plate is stencilled with the striking velocities and photographed. (See Fig. 5.)
Plates passing successful test become the government's propterty and are available for test of armor-piercing projectiles. Failed plates are the manufacturer's property and are returned at his expense.
Class "B" plates (special treatment steel, such as turret and conning-tower tops) and protective deck plates are set for test so that the projectile strikes at an angle equal to the anticipated angle of fall of an enemy's shell. As it is impractical to strike a plate in a horizontal position because of the ricochet of the projectile, the plate is set vertically and the projectile caught in a sand butt behind. The plate must not fracture under the impact.
Full reports are made to the bureau of the ballistic results.
Proof of Explosives.—Explosives are tested by using samples of each lot to fragment service projectiles in an explosion chamber. Each sample must function in a satisfactory manner.
Proof of Cartridge Cases.—Cartridge cases are proved by firing sample cases three rounds each, thus ensuring satisfactory functioning in service. The case must load and eject easily on each round and must be free of deformation, splits or other serious defects.
Proof of Primers.—Primers are proved in accordance with specifications laid down by the bureau. When possible they are tested during current work. Combination primers have several of the proof lot fired electrically and several by percussion. Primers must not misfire, split or rupture. Should they do so, the lot they represent is rejected, and the failed sample primers are returned to the manufacturer.
Proof of Bombs.—Bombs are tested in accordance with specifications, which, as in the case of other material, are to ensure their proper functioning in service. Service conditions are adhered to as closely as possible.
Special Work and Experimental Work.—A definite division between these two classes of work cannot be clearly made. However, such tests as are possible are carried out in connection with proof work. For example, data are collected relative to ranging, erosion, life of guns, new types of ordnance material and questions which have arisen in the fleet. Research and development work are done, using special types of apparatus designed to give accurate results. The proving ground is available for such tests as are desired by officers of the navy to determine definitely points in dispute or in development as to ordnance material or the effect of ordnance material on material under the cognizance of other bureaus. Such tests must receive the approval of the Bureau of Ordnance before being conducted.
Duty.—Duty at the proving grounds is one of the most satisfactory. shore details. The work is exacting, but is full of live interest and is in constant touch with modern developments in ordnance. Younger officers of the service considering their first shore details would do well to request this duty, either as a regular assignment or as a part of ordnance postgraduate instruction.
Recommendations.—Once again it is urged that officers at sea or on combatant shore duty remember that the bureau and the proving grounds are anxious to obtain data relative to unsatisfactory functioning of ordnance material. The only way the officers supplying material can know whether it is good or bad in actual service, with the exception of their own experiences, is to receive detailed reports from the officers using the material.