Weapons Training: Bombing Basics

The text below on some of the theoretical aspects of horizontal and dive bombing - including bombing sights - is a (free) translation of a text by Dr. Müller, Fliegerstabsingenieur, "Der Bombenwurf". Published in: Das Buch von der Luftwaffe, Dr, Eichelbaum, Major im Reichsluftfahrtministerium (Hrsg.), Second Edition, Berlin 1940.

The purpose of the original publication is not clear. Most contributors are somewhere in the middle ranks of the RLM at the time of the publication. The book does certainly not disclose any restricted information. My guess for its purpose is to contribute to the all out "propaganda" flurry aimed at intimidating or "impressing" foreign decision makers with the might of the GAF and it's technical proficiency before the outbreak of WWII, which started together with the first official announcement of its inception shortly after 1935.

All additions or explanations will be in italics. Just keep in mind: The English language has not always been the chief aeronautical language.

Part 1: Horizontal Bombing

Let us assume we are in an aircraft moving at an altitude of 4000 meters (yards) with a speed of 100m/sec (or 360 km/h). This movement, caused by the engines, is the speed at which the AC is moving relative to the surrounding air, or from a different angle: The air surrounding the AC is moving as a kind of relative wind opposite to the direction of movement of this AC (Indicated Air Speed). This relative stream of air around the wings is what causes the lift opposing the weight of the AC to keep it air-borne at its current altitude. On the other hand: This stream of air is causing opposing forces (drag), which needs to be overcome by engine power (thrust) to maintain a given air-speed.

If we would drop an object (any object) out of this ac, the object would not only move downwards (caused by its own weight - or gravity), but at the same time - caused by the relative wind, or more precisely drag - move backwards in relation to the AC dropping the article.

Pretty much the same would occur, if we would drop an object out of a moving train or car.

This (relative) movement backwards is the more pronounced, the more pronounced the relative wind (or the speed of the ac) is and it is at the same time relative to the amount of drag the object (released from the AC) is facing, while moving through the air. At the same time, the movement down (and forward) is smaller, the larger the amount of drag acting on the object is. If it is deemed desirable for an object departing from the AC to move slowly through air, it is necessary to use a design with high drag characteristics.

Fig.1 Fall of an object as seen from the aircraft

This principle is used with parachutes. On the other hand, if it is found desirable to have an object with high velocity, well it is necessary to design it with very low drag characteristics. This is achieved by streamlining an object like a blimp (Zeppelin). The requirement of high velocity is certainly present with aerial bombs. Therefore streamlining techniques are employed in bomb designs.

An easy measurement to determine the drag characteristics of any aerial bomb is to determine the distance the bomb is lagging behind the dropping ac on its way down - given the AC will maintain speed, altitude and direction of flight. The difference in forward travel between the dropping AC and the relative (to the AC) horizontal path of the bomb is called "trail" ("Ruecktrittsstrecke" in German). This "retardation path" is very small with modern bombs. It does amount to 300 meters with a 250-kg bomb dropped from 4000 meters at a speed of 360 km/h.. The bombardier needs to know about these values on release. The other parameter the bombardier needs to know about, is the time it takes the bomb from say 4000 meters altitude  to actual impact. The time it takes for the aforementioned bombs from 4000 meters is about 30 seconds. This time to impact from any altitude is another measurement on the drag effectiveness of a (WWII) bomb. It goes without saying, that the drag effectiveness is the best, if the time to impact is very close to the time it takes to impact without any air interfering (vacuum). From an altitude of 4000 meters this "ideal" time to impact is 28.6 seconds. The fact, that the real times to impact are just slightly higher, shows the aerodynamic effectiveness of the bombs used.

If both the specific trail value (horizontal travel) and the time to impact (vertical travel) is known, it can be calculated, where a bomb released at 4000 meters altitude from an AC traveling at 100m/sec will actually hit. Since we know the bomb will be 300 meters behind the AC - given the AC travels at constant speed, course and altitude - we know the bomb will on impact be 300 meters behind the AC. Since we know how long it takes the bomb to reach the ground under these parameters (30 seconds), and since we know how fast the AC flies in 30 seconds (30 seconds x 100 meters = 3000 meters), we can assume the bomb will travel 2700 meters from release to impact horizontally. These characteristic parameters need to be known and understood to do any bombing with precision. This experiment was in still air. Now let us investigate the impact of wind on bombing technique.

If the air around the AC is moving, the movement of the AC is no longer exclusively relative to the ground (speed over ground) but at the same time relative to the direction and speed of the surrounding air. Let us assume the air is moving in the same direction of the ACs heading at a speed of 15 m/sec. From the earth as a reference the speed of the air adds to the speed of the AC which in this case would be traveling at 100 m/sec + 15 m/sec = 115 m/sec. which in turn is the actual speed over ground, as opposed to the ACs airspeed which would still read as 100 m/sec on the instruments (if true airspeed would be indicated, which is not the case - as we will see).

During the time span from release to impact of the bomb, the AC will not only have traveled 3000 meters over ground but additional 450 meters (15 m/sec x 30 seconds) because of the wind. The total travel of the AC is 3450 meters. Since the speed of the air is the same for bomb and the AC - and since the characteristic trail value is not affected - the bomb will impact at 3150 meters after release. (2700 meters plus 450 meters)

A side note: Keep in mind, the speeds etc. are given as speed over ground - True Airspeed (TAS) if you will. However, the instruments in any AC will only show Indicated Airspeed (IAS), which is not only relative to the flow of the surrounding air (speed and direction) but to pressure altitude as well. More on this in the "Ground School". When using a bombsight, the bombardier needs to know whether his gear makes "automatic" adjustments for this or not.

While the bombardier sees the bomb fall in a pane slightly inclined backwards, an observer on the ground has a very different impression. (see image below)

An observer on the ground sees the bomb travel at first at the approximate speed of the AC forward and at the same time downward. The motion forward will decelerate (because of drag) and at the same time the motion downward will accelerate under the influence of the mass of the bomb. This constitutes a curved trajectory. (checked by a final weight (mass) to drag ratio)

Fig. 2: Fall of a bomb from the ground perspective

From the ground perspective: A bomb is released at point A, at constant rate of travel, the AC will be at point E on impact. The points B (identical with release) and C (point of impact)  mark the horizontal travel of the bomb (Wurfweite) while the points C-D indicate the difference in horizontal travel of the bomb and the  AC (Ruecktrittsstrecke - trail). Whereas the straight line of sight between the eye of the bombardier and the point of impact (Vorhaltewinkel) includes an angle of "deflection" at the time of release.  However, to come up with a predictable trajectory - and deflection angle - every time, it is necessary to assure a high level of manufacturing accuracy. The primary means by witch the trajectory and in flight stability of a bomb is established are its fins at the back end of a bomb. The old ways of stabilizing the "flight" path of a bomb, by inducing a circular motion around its center axis (spin) is no longer in use with modern bombs.

To determine the right time of release of a bomb to hit the target, special devices - "bomb sights" - are employed. These are mechanical or optical devices, the bombardier is operating. These are supposed to assist the bombardier to determine the proper release point at a given situation. The values needed to be taken into consideration are the speed of the AC over ground, the speed and direction of the surrounding air, the time to impact and the retardation of forward travel of the kind of bomb used and the altitude of the AC. The bombsight will assist the bombardier to determine the horizontal travel of the bomb from the point of release in a given situation - that is it will help determine the deflection or if you will "lead". If all calculations are done correctly, the bomb sight will give the bombardier a precise line of sight at which he has to release the bombs - taking all the parameters mentioned above into account. Another important parameter that determines the accuracy of bombing is the proper direction of the flight path of the AC relative to the target. (Sideways deviation) That means: The vector of velocity of the AC has to point precisely at the target. As far as I know only the US made Norden bombsight had any reliable adjustments for that parameter. Including cross winds.

Fig. 3: Mechanical German Bomsight

The picture above shows a (early - Battle of Britain and German)  bombsight which is operated in a simple way: The line of sight is determined by aligning a simple ring and bead aiming device. The aiming point can be modified by actuation of several scales and sliders. With the help of these scales the time to impact and the retardation of the bomb is dialed into the device. Using this information it takes a stop watch to determine the speed off the AC over ground - using known reference points. With this information the correct line of sight is determined. The bombs are supposed to be released, at the very instant when the  determined line of sight angle touches the target.

More sophisticated bomb sights use optical aids like telescopes to achieve better precision. See below for an example.

The lower end of the telescope does protrude from the fuselage, next to the telescope is a so called computing box (Rechenkasten), having the required controls for measuring and computing deflection (Vorhaltewinkel). On the upper side of the computing box are two dials, one for time to impact at a given flight altitude, on the other dial the deflection angle can be read off  as a result of some automated computations. Both, the mechanical and the telescopic bomb sight can be used at night, since the input scales and the aiming points are illuminated. ....

As mentioned above, all bombsights need two values: Time on impact and trail. The bombardier is provided with numerical tables, giving him the numbers for all usual or practical conditions regarding altitude, trail of a given munitions and aircraft speeds. The required ballistitical data are obtained by empirical tests.  .... The data are extracted by numerous bombing runs at night under controlled conditions using photographic techniques, with illumination of the bombs trajectory and long exposure rates in the cameras are used.

A large number of exposures and great care is taken to come up with precise numbers.

So far we have covered the implements of horizontal bombing techniques - which requires a number of very precise computations to achieve a desirable amount of accuracy.

Part 2: Dive Bombing (with purpose build AC)

By diving a flightpath attitude of close to vertical is referred to. If it is possible with  a dive bomber (an aircraft build to support this kind of attitude) to dive vertically at a target, any issues of bomb trajectory, especially of the bombs flight path and line of sight/deflection issues are eliminated. However, in practical terms a perfect vertical dive will not be executed, not the least to avoid the hazards of the bomb exploding in close proximity. By the same token: If the dive is not perfectly vertical the horizontal rate of travel of a bomb needs to be taken into consideration again. In the example above (horizontal bombing) the rate of travel of the bomb was 2700 meters. In a dive of 80 degrees (measured against the horizon) this horizontal travel is only 450 meters with the same kind of bomb, given a speed of 130 m/sec is maintained. Besides, the smaller horizontal travel is caused by the much smaller time to impact. For comparison: A bomb dropped form 4000 meters will take 30 seconds to impact. At a dive bombing attack from the same high it will take 20 seconds. At a release altitude of 2000 meters it will take only 12 seconds to impact. A moving target (ship, truck, tank) has got a lot less time and space to evade.

A dive bomber will approach a target at a given altitude and will commence a gradual decent with an ever increasing angle of dive up to an angle of 70 or 80 degrees. The speed of this AC will increase, up to a speed where the mass of the AC and the drag of the AC will meet an equilibrium.  The dive bomber will approach the target in a way, consonant with the defenses met, dangers to itself  by the blast of it's own ordnance and the air space required to recover from the diving attitude. At the very moment of release of the ordnance the recovery from diving takes place, On the other hand, the direction and momentum of movement at the moment of release is transferred to the ordnance. If gravity and drag had no impact on the way of travel of the bomb, the bomb would maintain the speed and direction until impact inherited by the releasing AC. The bomb would hit where pointed - as a kind of extension of the flight path of the AC at release.

Fig. 4: Dive Bomber Flight Path

However, this is not the case. Because of the results of gravity the trajectory of a bomb is not straight but curved. Instead of impacting at A the impact is at B. (see below).

On top of gravity (and it's accelerating force) drag has an influence on the trajectory of a bomb.  This means: trailing - because of drag - has an influence. In order to hit properly in a dive bombing run, the pilot has to reckon for that trail - the difference between aiming point and impact of his ordnance. Of course the impact of wind on accuracy has to be taken into account. If the wind is coming on in the direction of flight (and bombing), the pilot has to take this into account on top of the ballistic "lead"  required.

Fig. 5 Aiming from a Dive Bomber

It goes without saying, that a dive bomber pilot requires a lot of awareness of the specific situation, the capabilities of his equipment and the things that might go wrong. in order to be successful. Since he is attacking pin-point targets from relatively low altitude, the margin for error is extremely small. The issues to spoil his effort are: Late or unregular release of his ordnance, mechanical interference of the release gear, mutual interference of ordnance released at the same time, prop-wash deflecting the ordnance and so on. The pilot has to rely on the mechanical soundness of his bombing gear. From experience, it is most advantageous in terms of accuracy to use ordnance already exposed (externally mounted) to the airstream around the AC .

Fig. 6: Various German optical Bomsights

Fig. 7: German Bomb-Racks and "Reihenwurfgeraet" (Salvo Controller)