The Naval Bombing Experiments: An Account of the Bombing

Published in the U.S. Air Service

October, 1921

Captain A.W. Johnson, USN

Commanding Atlantic Fleet Air Force During All Bombing Tests

Note for Naval Historical Foundation

In this account of the bombing, I left out all psychological
factors, controversies, and references to Billy Mitchell

Published in U.S. Air Service
October, 1921


By Captain A.W. Johnson, USN Commanding Atlantic Fleet Air Force --
In Command of the Fleet Air Force During All Bombing Tests

The bombing exercises last June and July, which ended with the sinking of the Ostfriesland, furnished lessons of great value, if properly applied. It has been suggested that I tell my ideas regarding the bombing experiments as my duties placed me in a position to know all the details concerning them. I feel that I can discuss them without prejudice and perhaps throw some light upon what seems to be a much misunderstood subject.

Whether the exercise may be considered to have been entirely successful or not depends on the point of view. Many persons believed that a battleship could not be sunk by aircraft, others declared that a $20,000 airplane could sink a $40,000,000 battleship and that battleships were therefore useless. There were enthusiasts who believed any large bomb dropped on the deck of a ship would completely obliterate her, and there were skeptics who believed that bombing by aircraft was so erratic that they never could hit a ship at sea.

As a matter of fact these things were known before the tests took place. The Navy had conducted extensive bombing experiments on the Indiana in Chesapeake Bay. The various Naval Air Stations have regularly carried out target practice with live bombs against towed targets at sea, and the Atlantic and Pacific Fleet Air Forces have frequently conducted search problems against the Fleet.

The wide publicity given the exercises, and the partisan character of the discussions preceding them, in a measure obscured their real purpose. The general impression seems to have been that the sole object of the tests was to determine whether or not surface craft could be sunk by aircraft. As a matter of fact the exercises were planned to determine the effect of the explosion of various types of aerial bombs on the hull and structure of modern war vessels of different classes, to observe the effects of gun fire upon them, and to ascertain the accuracy with which aircraft could locate and bomb a vessel underway out of sight of land and other navigational aids.

On the whole, the experiments furnished a great deal of information to settle the disputed points, and to demonstrate the value of aircraft in operations against naval vessels. Much desired information was not obtained because some of the tests were omitted or not carried out with the necessary care to obtain all data required for a complete analysis of the experiments; but valuable experience was gained by those who took part in them, and enough information was obtained to enable the observers to draw conclusions from them.

All vessels attacked by aircraft were sunk by aircraft alone. It is perhaps just as well that this was so for it is doubtful whether, in any other way, some persons could have been convinced that battleships actually can be sunk by bombs dropped from aircraft. The fact that they can be so sunk was impressed upon those who witnessed the sinking of the Ostfriesland in 50 fathoms of water, 70 miles off shore in a way that will not soon be forgotten.

The Report of the Board of Governors expresses it well when it says:

The general result of those tests has been to convince the board that the airplane is a powerful weapon of offense. No matter what reasoning to the contrary may be advanced, such as the fact that the ships bombed were at anchor (except ex-Iowa), that there were no crews aboard to rectify hull damage and keep ships afloat, that the planes were guided to their targets by a line of destroyers placed there for that purpose (except ex-Iowa), that the ships were without anti-aircraft gunfire, or protection of their own planes, that ideal summer weather conditions prevailed except when it became necessary to postpone the operations on account of fog, wind, or sea, and that the planes bombed from very low altitudes, the fact remains that in every case of attack by airplanes with bombs, the ships so attacked, whether submarine, destroyer, light cruiser, or battleship, were eventually sunk, and by airplanes with bombs alone.

The facts have been carefully considered by the Joint Army and Navy Board, whose reasoned conclusions are given in a report that has been made public. This report should be studied not only by Army and Navy officers, but by those who formulate our national policies and who are responsible for providing the means of safeguarding them. It should clear up many erroneous ideas regarding the value of aviation in it application to naval warfare.

Before the experiments took place we knew that an airplane could sink any vessel by persistently bombing her, but we were not certain what types of bombs would give the best results, nor had we any information as to the number of bombs that would be necessary to put different classes of vessels out of action. We now have a better idea, but there is still much to be desired in the way of information on these points. For instance: Is it more advisable to carry four 500-lb bombs or one 2000-lb bomb in one plane? What is the smallest bomb useful against surface craft? Should bombs be standardized? Has the armor piercing bomb sufficient advantages to justify its use?

We knew that the effect of a bomb dropped alongside a vessel with delayed action fuse was the same as that of a mine of the same size exploding in the same place, but we had no information as to how close to vessels of different types bombs of different sizes should be dropped to insure crushing in the hull. We now have a record of the number of bombs of different types that were dropped for each vessel that was sunk, and the number of hits made. We learned something as to the action of the direct hits on the structure of the vessel, but nothing as to the effect upon personnel, communications, and machinery.

It is probable that the vessels would have been put out of action before they were finally sunk, but we are not certain as to these facts. Actually the effect of the direct hits on the material was not as damaging as generally was believed they would be. None of the bombs scoring direct hits were capable of penetrating the armored decks, or armored compartments. The tests with the specially prepared armor piercing projectiles to be dropped on the armored deck of the battleship, and the long range (high angle) gunfire with armor piercing 14-inch shell were not carried out. As a result, the effect of high explosive shell and bombs within the confined spaces of a ship was not ascertained. The aerial bombs could not penetrate sufficiently to determine this, and the explosion of direct hits of bombs up to 1,000 pounds was confined to the upper works or unarmored spaces. The 2000-pound bombs which finally sank the Ostfriesland registered no direct hits so that possible damage to personnel, communication systems, etc., by them could not be ascertained.

Regarding the uses of poison gas by airplanes, we have known that poison gas would knock out personnel exposed to it, but we do not yet know how far it will work into a vessel steaming into the wind at high speed, or whether the ventilation system of a ship will take in sufficient quantities of it to distribute it throughout the ship and disable the entire crew. It is probable that the ventilating systems as now installed on board ship will assist to distribute the gas in a very dangerous manner. If so the blowers and air ducts must be fitted with suitable appliances to secure immunity for the crew. The use of poison gas will not be confined to aircraft but will be used by naval vessels and shore artillery as well until a means is found successfully to combat this new menace.

The torpedo plane was not used in the tests and there is little reliable information available concerning torpedoes fired by aircraft. The mining effect of bombs dropped close to the side of a vessel brings up the question as to whether or not the torpedo is as desirable a weapon for aircraft as bombs. The torpedo has certain advantages over bombs and certain disadvantages. It can be fired with accuracy from great[er] distances than bombs, but is much more expensive to manufacture and is more complicated to handle. For a given weight of projectile the bomb carries a much heavier explosive charge than the torpedo.

As one of the objects of the tests was to see how different types of bombs would stand up on impact with ship's decks, and what penetration might be expected from them, different size bombs, and bombs of entirely different construction and fitted with various types of fuses, were used. Light case bombs with relatively heavy charges; and heavy case, thick walled bombs with relatively small bursting charges were tried out.

Explosions occurring on the exposed surfaces, such as on the upper deck of a ship or close to the surface of the water, are largely dissipated in the air since the explosive forces follow the line of least resistance; whereas, in confined spaces, as between decks, or underwater below the hull of the ship, they are extremely effective, depending on the quantity of explosive used and its distance away. This means delayed action fuses are necessary; but unless the walls of the bombs are thick and tough enough to penetrate, they will break up and scatter the explosive charge before the delayed action fuse has had time to act. With instantaneous fuses, explosion would take place before perforation, and much of the effectiveness be lost.

The bomb should also have sufficient velocity to penetrate. There is a maximum velocity for bombs dropped from aircraft that cannot be exceeded no matter from what height dropped. This maximum velocity, known as "terminal velocity," is reached when the retardation due to air resistance is just equal to the acceleration due to gravity and consequently all acceleration ceases. The form, weight, and cross-sectional area of the bombs, of course, affect this. With 1000-lb bombs, having good stream lines, we can expect a terminal velocity of about 1,100 feet per second, but to attain such velocity they would have to be dropped from an altitude of about thirty or forty thousand feet. From an altitude of 4,000 feet one of these bombs would attain a velocity of about 500 feet per second. Armor piercing bombs would, therefore, be useful only when the ceiling available was high enough to insure their attaining sufficient velocity to penetrate the armor.

Twelve 163-pound bombs carrying 117 pounds of TNT were dropped on the submarine U-117 by three planes scoring two direct hits in two attacks, from 1,200 feet, both salvos straddled, resulting in sinking her in 12 minutes. It would seem unnecessary therefore to use heavier bombs than these against a submarine. The bombs exploded instantaneously on direct hits and with delayed action on striking the water.

The destroyer was attacked by eleven pursuit planes carrying four 25-pound bombs each. Forty-four of these bombs were dropped from an altitude of about 200 feet, of which twenty-one scored direct hits. This was followed by an attack by eleven Martin bombers which dropped forty-four 300-pound bombs, scoring three hits. The vessel sank as a result of their combined mining effect. No examination of the vessel was made by the Board of Observers and definite information as to the actual results to material is lacking.

The tests of the Frankfurt began with an attack by Navy planes carrying 250-pound bombs. Twenty-four of them were dropped, scoring five direct hits. This was followed by an attack by Army planes which dropped twenty-nine 500-pound bombs, registering two hits. Navy planes then dropped seven 520-pound bombs of which three were hits. The final attack, with fourteen 600-pound Army bombs, scoring two hits, resulted in sinking the vessel. It would be fair to conclude, then, that attacks upon light cruisers should be with bombs of 500 pounds or heavier. Of the Navy bombs scoring hits, none of the 250-pound bombs detonated, and one of the 550-pound bombs failed. All Army bombs detonated.

The Ostfriesland tests began with an attack by Navy planes dropping thirty-three 250-pound bombs, scoring eight hits, followed by eight 550-pound bombs, making four hits. The Army then dropped eleven 600-pound bombs registering one hit. An examination of the vessel after these attacks showed that she had sustained little damage from direct hits witht he exception of a hole in the starboard side of the forecastle made by a 600-pound bomb, which put out of commission the two 5-inch ammunition hoists directly under it; but the mining effect of the bombs that dropped close to her had damaged her considerably under water, and several compartments were leaking. She had gained water during the night and the following morning was three feet down by the stern, with a list of 5 degrees to port. Engine and fire rooms were partially flooded, when the attack with 100-pound bombs was launched.

Five of these were dropped and three direct hits were made on the main deck causing no vital damage to the ship or battery. However, her fighting efficiency might have been affected by a large hole on the starboard side of the forecastle, taking in water. By noon she was down five feet by the stern and one foot by the bow.

In the final attack, seven 2000-pound bombs were dropped, none of which hit the vessel. The possible effect of the explosion of this type of bomb on deck, or between decks, could not therefore be ascertained. Three of these bombs were close enough to do extensive damage to the hull. The most effective bomb detonated close under the port quarter, throwing water up under both sides of the hull. She immediately began to settle rapidly by the stern, listing heavily to port, water entering through injuries on deck, broken air ports and through gun ports. She turned completely over and went down by the stern at twenty-two minutes after the first 2000-LB bomb was dropped.

The Iowa experiments disappointed the spectators because there were no big explosions. Live bombs were not used because the Navy Department had decided to use this vessel for further experiments. Nevertheless, this problem was far more difficult for aircraft than any of the other bombing tests. The planes carried a full load of concrete bombs besides the standard war equipment of machine guns and ammunition, and as much fuel as possible to take off with.

The thirteen seaplanes which took part in the problem first had to locate the Iowa which was steaming toward the coast somewhere between Cape Hatteras and Cape Henlopen, 50 to 100 miles off shore. There were no marks of any kind to assist them in this search. An airplane locating the enemy would report by radio its position, course and speed and as each plane received the message she immediately altered course to intercept the "enemy."

Some of the planes had already been in the air two or three hours and were many miles from the enemy when they received the contact message. They not only had to find the Iowa and bomb her, but also had to have enough gas left to return to their bases after bombing her. The success or failure of this problem depended therefore not so much upon the skill of the bombers as upon the accuracy of navigation of pilots and the effectiveness of the radio chain. There was some adverse criticism because there were only two direct hits made out of 80 bombs dropped. As the Iowa test took place before the Frankfurt and Ostfriesland were sunk by bombs exploding alongside, and not by direct hits, the critics probably were ignorant as to the effects of the bombs dropping alongside.

As a matter of fact, had the Ostfriesland's horizontal projection been superposed on the Iowa 15 hits would have been registered. Another fact to be considered is that the target was not anchored but was zig-zagging at slow speed; that the altitude of the bombing was 4,000 feet (twice that of the other tests) and that in the areas searched the wind velocity and direction was constantly changing. The Iowa search problem showed that properly organized air forces, trained in this kind of work, can search out given areas within the limits of their radius of action in the same manner as do other naval scouting forces.

So much has been made out of the very excellent work of the aircraft in bombing tests that there is real danger that enthusiasm may lead to overlooking the limitations of aircraft for work on the high seas. The enthusiastic claims made in some quarter as to the capabilities of aircraft are dangerous because they lead the public to believe that aircraft can do many things that they cannot possibly do, and they give one the idea that the art of aviation is more advanced than it really is.

The development of aviation depends far more on accurate knowledge of the present limitations and needs of aviation than upon the misleading information as to its present capabilities. Some persons have gone so far as to say that the development of aviation is being retarded by the Army and Navy, that the greatest need of aviation is to place it under an independent department of the government to control and administer its affairs, as if this would brush aside the difficulties. A little reflection should convince one that the development of aviation has been furthered more by the Army and the Navy than by any other agency. That its development has not been more rapid is not because of the Army or Navy, but because of limitations that can be overcome only by gradual development. One might as well say that the science of engineering has been held back because the Corps of Engineers is part of the Army, or that the development of radio telegraphy has been retarded by its association with the Navy.

Let us dissolve the mists surrounding aviation, particularly in its relation to warfare on the high seas, and recognize facts without prejudice. The advancement in aviation has been marvelous and is steadily going forward.

The exercise off the Virginia Capes have shown the limitations of aircraft as well as their capabilities. The wastage in material during the exercises and in preparation for them was considerable. Destroyers were spaced at intervals of seven miles from the flying field to the target to recover planes having forced landings, and to direct them to the target. None of the land planes were used in the search problem of the Iowa as it was decided that the distance from shore was too great to risk the planes and personnel.

Complaint was made in the other tests because some of the planes were delayed as much as 40 minutes in the air to permit the observers to inspect the target. There were protests because the experiments were conducted 70 miles off shore and "were made as difficult as possible for the aviators." Does this not point to the necessity of working along lines further to increase the reliability of motors and radius of action, as well as developing better means of navigation and all round efficiency of the planes?

We know that bombing planes carrying heavy bombs working from shore bases can sink surface craft operating 100 to 200 miles off shore. In a great measure, then, aircraft will supplant coast fortifications, or will be a most valuable auxiliary to them in making our coast immune from attack. But we are totally lacking in suitable flying fields along the coast from which to operate. These are as necessary to land planes as are good roads to automobiles and we must develop them immediately.

Seaplanes can operate now from any of the sheltered harbors along the coast if tenders or suitable air stations are available, but we have not yet developed any means of attaching heavy bombs to sea planes while in the water and they must be hauled out upon the shore for this purpose. Operations on the high seas by aircraft are impossible without carriers. We have none, and until we get them, there will be no real fleet aviation.

One of the difficulties is to get planes with slow landing speeds to fly on the carrier. Only the lightest type of planes are able at present to land on carriers. This means that the heavy bombing planes can not be used at this time from carriers. The light pursuit or combat planes cannot carry torpedoes nor bombs heavy enough to inflict serious damage to armored vessels. Until the heavy bombing planes are capable of flying on and off carriers, surface craft operating more than 200 miles from shore will not seriously be menaced by aircraft. The slow heavier planes cannot stand up in aerial combat against the fast light combat planes. This means that protection of surface craft from bombing attacks will be insured if they are simply provided with light combat planes.

For the above reasons it is probable that naval aviation on the high seas will be restricted to light combat, spotting, and scout planes which will be flown from surface craft accompanying the fleet, and that in operations over the water the use of bombing will be restricted to their limited radius of action from the coast line. It should be remembered that naval warfare does not consist always of operations against an enemy's coast line, but that more frequently the key to the naval situation is found in the control of a distant ocean.

We cannot expect that aviation will render battleships obsolete overnight. What the future may develop no one can foretell, and one man's guess is as good as another's. Let us look at the facts as they are and proceed in along same lines, neither casting aside the tried weapons of naval warfare nor neglecting the development of the new and untried. We should be wise and take warning, for naval powers which fail to develop aviation and make full use of it will be in serious danger when opposed to one which has in its possession a thoroughly trained and well-equiped naval air force.