Saturday, January 24, 2009

Weekend Wings #32: Aerial Bombardment: The Beginning

Almost as soon as the first aircraft were developed, their proponents and advocates began to examine ways in which they could contribute to the art and science of warfare. The development of aerial bombardment began almost by accident, an afterthought on the part of an adventurous pilot, but was soon to become an all-consuming interest to many air arms.

This series of Weekend Wings articles will focus on bombing in the strategic sense - attacking an enemy’s infrastructure and means of supporting its war effort - rather than in the tactical sense of operating in support of ground forces. The latter is a fascinating study in itself, but involves a different set of challenges, which must wait for their own series of articles. Nevertheless, the principles involved in aiming bombs and hitting the target are very similar.

I'm afraid we have to begin by getting just a little bit technical. The problem of how to aim bombs must be analyzed and explained. The history of aerial bombardment is largely the story of how methods of bomb-aiming improved over the years. From World War I through World War II and the Korean War, 'dumb' bombs - i.e. unguided weapons - were the norm. Later, from the Vietnam War onwards, 'smart' bombs - i.e. bombs that are either guided, or guide themselves, to the target - would become more prevalent, until today the majority of weapons dropped in Iraq or Afghanistan are 'smart'. However, in this initial study, we'll examine how to get 'dumb' bombs on target.

The first factor is the aircraft's speed. An aircraft has to fly towards a target to drop bombs on it. Its actual forward speed must be known, otherwise the bombs can't be dropped accurately in level flight. Determining that speed is the single most important component in level bombing - and before the advent of the Global Positioning System (GPS), it was a very involved process.

An aircraft's speed is worked out as follows:

  1. First, the air pressure caused by the aircraft's forward motion is calculated. This is done by sampling the pressure of air flowing into a pitot tube. This is known as the pitot pressure (also known as 'ram air pressure'), caused by air being 'rammed' into the tube as the aircraft moves forward. The pitot tube is usually mounted out on a wing or clear of the fuselage to ensure an uninterrupted air flow, so that the pitot pressure can be accurately determined.
  2. The pitot pressure is then compared with the static pressure. This is the 'raw' air pressure at the aircraft's altitude, without any allowance for wind or aircraft velocity. It's measured through a static port, situated at a place on the aircraft's fuselage where the effects of air flow are minimized. (Static pressure is also used by barometric altimeters in a different way to determine the aircraft's altitude.)
  3. The difference between the pitot pressure and the static pressure, derived through the pitot-static system, is used to derive indicated airspeed. This shows only the aircraft's speed through the air - not necessarily the same as speed over the ground. It is displayed in the cockpit on the airspeed indicator (although that instrument may also - or alternatively - display corrected airspeed, as described in the following points).
  4. Indicated airspeed must next be converted to calibrated airspeed. This allows for instrument error on the aircraft (e.g. if a pitot tube is known to record a pressure reading of 10% below the actual pressure, the resulting error in displayed airspeed can be compensated for, either manually or electronically, to determine the true air pressure, and hence give a more accurate airspeed reading). It also allows for any known error in the static pressure system, whereby the air pressure surrounding the aircraft may be measured as being higher or lower than it is in reality. Such errors are inevitable, given the position of the pressure sensing systems on the aircraft, and are in fact called position error. Factoring position error into indicated airspeed gives calibrated airspeed.
  5. In aircraft flying at high speeds and/or altitudes, calibrated airspeed must be converted into equivalent airspeed. This is because compressibility error creeps in at altitudes of over 10,000 feet or airspeeds greater than 200 knots (230 mph). Since the magnitude of this error can be calculated, based on speed and/or altitude, calibrated airspeed can be corrected to equivalent airspeed - the airspeed at sea level which corresponds to the same dynamic pressure as true airspeed at altitude. Obviously, this step wasn't necessary in the slow, low-flying aircraft of World War I, but by the end of World War II it had become a significant factor.
  6. Calibrated airspeed (for slower, lower-flying aircraft) or equivalent airspeed (for faster, higher-flying aircraft) must now be converted into true airspeed. The latter is defined as the speed of the aircraft relative to the mass of air within which it is flying. To derive it, the direction and strength of the wind must be known, as this can slow down the aircraft (when coming from ahead), or speed it up (when coming from behind). This is complicated by the fact that winds within a weather system may be variable in strength, may come from different directions depending on the aircraft's location within the weather system, and may vary with altitude. At very high altitudes, the jet stream becomes a factor of great importance. (It was to make possible the Japanese balloon bomb campaign in World War II, and would significantly hamper US bombing of Japan, as we shall see). Correcting for such air currents, we derive the true speed through the air of an aircraft.
  7. True air speed is important for control of an aircraft, as its stalling speed, handling, etc. all depend on its velocity relative to the air through which it's moving. However, airspeed is not the same as speed over the ground - which is vital for navigation. If an aircraft is flying with an airspeed of 200 mph, into a headwind of 50 mph, its forward progress over the ground will be approximately 150 mph. It may be carrying enough fuel to fly for four hours at that speed, but instead of covering 800 miles in four hours, it will cover only 600. This might mean that it runs out of fuel before reaching its destination. Therefore, speed over the ground must be calculated by physical observation (i.e. the time it takes to get from one landmark to another), or (in modern times) by electronic means (radar vectors from an air traffic controller, or readouts from a GPS system, or other technical means).

So, the aircraft's speed has to be determined accurately. If it maintains a constant velocity, the speed of the bombs as they drop, in the horizontal plane, can be fairly accurately calculated. They’ll begin their fall at the same speed as the aircraft. Wind resistance will slow them down (because, unlike the aircraft, they have no engines to maintain their speed), so their forward motion will gradually decrease. However, if the aircraft’s velocity is not constant - if it’s continually speeding up or slowing down - then the bombs’ horizontal speed can’t be accurately predicted at the start of their fall, and hence their speed during the drop becomes a matter of guesswork. In the primitive, low-powered aircraft of early days, which could be significantly slowed or speeded up by even a gust of wind from ahead or behind, this was a significant factor.

(Indeed, I’ve been in a Tiger Moth biplane, flying into a stiff headwind, where the aircraft was showing an airspeed of about 80 knots - but we were actually flying backwards! Looking over the side of the cockpit, and comparing our position to fixed objects on the ground, showed that the aircraft was slowly but steadily being pushed backwards, despite being perfectly controllable. The pilot eventually turned around and headed back to the airfield, unable to make any progress that day.)

The wind speed and direction are also subject to change. They may be known at the point of departure, but be completely different over the target. The crew will have to measure them as best they can while on the inward flight, and make allowance for them in dropping their bombs. In the primitive aircraft of World War I, this wasn’t always easy, or even feasible, particularly at night, when the drift of the aircraft could not be readily measured against a fixed position on the ground.

Second, there’s the aircraft’s course and heading. These are not necessarily the same. If an aircraft wants to steer a course of, say, due East, or 90°, in windless conditions, that’s fairly straightforward. However, if there’s a crosswind (let’s say from the South, at 180°), that wind will push the aircraft to the North of its course. It’ll have to steer slightly South of its intended course, so that its actual track over the ground is on the correct line. Thus, the aircraft may steer a heading of 100° in order to make good an actual course of 90°. The aircraft will give the appearance of ‘crabbing’, flying at an angle to its actual course.

This has implications for the fall of bombs dropped from that aircraft. For a start, they won’t fall along the line of the aircraft’s heading, but along its course. Its approach to the target will have to take this into account. The bombs will also be affected by the crosswind from the South. They don’t have rudders or engines or wings, to enable them to fly a different heading so as to maintain the desired course: so allowance must be made in the aircraft’s heading to compensate for their drift as they fall. This may mean, for example, that the aircraft must fly a heading of 110° - double the ‘offset’ it needs to compensate for its own drift - in order to have the bombs fall on target. However, that in turn means that the aircraft must adjust the starting point for its bomb run, approaching the target along a different line, so that it can release the bombs at a point in its flight where their fall will intersect the location of the target. This requires careful planning and preparation - and, of course, an ability to read the direction and speed of the wind. Techniques and technological aids to do this were not available in the early years of flight.

Third, there’s the arc of the bombs’ fall. This is partly dictated by the aircraft’s speed. They begin their fall at the same speed as the aircraft that drops them. As wind resistance slows them, their forward speed decreases: but at the same time, gravity is accelerating their downward speed, until they reach ‘terminal velocity’, the speed at which their fall can no longer accelerate due to wind resistance. The shape of the arc can be affected by the shape of the bombs, of course. A fat, wind-resisting bomb will slow down more rapidly in the horizontal plane, and attain a lower vertical terminal velocity, than will a slim, streamlined bomb, optimized to cut through the air as cleanly as possible.

This can readily be seen in the photograph below. The upper bomb is a World War II-vintage US AN-M34 2,000 pound bomb, length 7' 6.2", diameter 23.3". The lower bomb is a modern US Mk-84 2,000 pound bomb, length 9' 10", diameter 14". The photograph is not exactly to scale, but is close enough to show the differences in shape and wind resistance. The modern Mk-84 bomb is much more streamlined (31% longer and 40% slimmer), optimized for carriage beneath the wings of a strike aircraft at near-supersonic speeds. As can be imagined, the older AN-M34 design, if carried in the same way by the same aircraft, would cause so much drag as to significantly reduce its speed.

The AN-M34 would offer much more wind resistance when dropped, so its horizontal speed would slow more rapidly and its vertical ‘terminal velocity’ would be lower than the modern Mk-84 bomb. Depending on which bomb was carried, allowance would have to be made for these factors in determining the release point.

Air resistance on the bomb would also be affected by minor variations in manufacturing tolerances, the presence or absence of mud, dirt and other excrescences, and even the uniformity of the paint on the bomb’s casing. A lump of thick, heavy mud on the bomb might make it fall unevenly, by making one side more wind-resistant than the other. Tail fins set at the wrong angle, even slightly, might affect its fall. Irregularities in the bomb casings, or uneven loading of the explosive charge, could also unbalance the bomb. Two bombs from the same factory, dropped at the same moment, might fall several hundred feet - or even yards - apart, due to factors like these.

The fourth factor is the altitude from which the bombs are dropped. The lower the aircraft, the less time the bombs will take to fall, and the less affected they will be by wind drift and other factors. Therefore, a low bombing altitude means they can be aimed more accurately. However, if they’re dropped from too low a height, the aircraft may be caught in the blast of their explosion, and be damaged or destroyed. In due course, delayed-action fuses would be developed to permit the aircraft to escape, but this would take years before reaching operational squadrons.

Another altitude-related factor is the presence or absence of air defenses. A low-flying aircraft is very vulnerable to ground fire, even from rifles and machine-guns. If it flies low enough to ensure great accuracy, it’s highly likely to be damaged or destroyed by such fire. On the other hand, if it flies high enough to avoid such defenses, its bombing accuracy will suffer.

Even in World War I, ground-based anti-aircraft defenses became a very important factor. The pictures below illustrate anti-aircraft weapons from this era.

World War I US short-range anti-aircraft gun

World War I German anti-aircraft machine-gun

World War I French vehicle-mounted anti-aircraft gun

As aircraft speeds increased and they became better protected against ground fire, so anti-aircraft defenses also improved. Guns were augmented (in some cases supplanted) by guided missiles, and today the first generation of beam weapons is under development. Prototypes have already shot down target aircraft, and even artillery shells and rockets, in mid-air, and operational weapons are expected to be deployed within the next decade. They may ultimately render low-altitude, close-range air support so dangerous as to be impractical.

Returning to our bombing problem, there’s also the difference between altitude and height above ground. Altitude can be referred to as height above a given norm (usually sea level). An aircraft flying at, say, 8,000 feet altitude is maintaining that height against the ‘reference mark’ of sea level. However, the ground beneath it is normally not level. There are flat portions; mountains and valleys; gentle rises and dips; and so on. In other words, the aircraft’s height above the ground may vary wildly, from a few hundred to several thousand feet, even as it’s maintaining a constant altitude above sea level.

This has obvious implications for bomb-aiming. The aircraft must know the target’s altitude as well as its own, and release its bombs so that their fall will intersect the target’s location, not only along a horizontal line, but also a vertical line - the difference in height between them, measured in terms of the time the bombs will take to fall that distance. If the target (for example, a road through a mountain pass) is on the slope of a hill, bombs aimed to hit the bottom part of the road will have to be released at a distance which takes account of its lower altitude, while those intended to hit a higher part of the road, further up the hill, will have to make allowance for a higher target altitude (and hence a shorter bomb drop duration). This can be a very complex and involved calculation, even with modern computer assistance.

All of this, of course, presumes that the aircraft is capable of determining its own altitude with accuracy. Before the days of radar altimeters or GPS, barometric altimeters, operated by air pressure, were the only way to determine altitude. However, air pressure at the the aircraft's departure point was seldom equal to the air pressure at its target. In addition, weather systems (storms, etc.) could bring with them wildly fluctuating air pressures, which could induce error in earlier, more primitive systems even before the aircraft reached the target.

Apart from altitude, the wind speed and direction over the target would have to be determined, as these would affect the path to be flown into the target area and the corrections needed to drop the bombs accurately. It would become increasingly important to obtain information about the weather, atmospheric pressure, etc. over the target before dispatching bombers to strike it, so as to be able to correct for such variations - but the very presence of a weather reconnaissance aircraft could signal a forthcoming raid, and alert the defenders. This dilemma has never been completely resolved to this day, even with satellite reconnaissance and other high-tech systems at the disposal of air planners.

Weather was, in fact, the single biggest obstacle to successful bombing throughout both World Wars. Bad weather could prevent bombers getting off the ground, or reaching their target, or even seeing the target to bomb it, or landing safely at their bases after completing their mission. Until reliable technological methods of blind flying and bombing were developed, bombing accuracy in anything but fine weather would prove an elusive goal. In addition, the primitive open-cockpit aircraft of the early years exposed their crews to the full fury of cold, bad weather, etc. This could so exhaust them, and slow down their responses at the controls, that they might be incapable of fulfilling their mission. Even in the enclosed crew spaces of World War II aircraft, this continued to be a problem.

This list of bombing problems is not fully comprehensive. As I said earlier, there were other factors which emerged as aircraft flew higher and faster. However, we’ll cover them in a future instalment in this series. For now, let’s take a look at the beginning of aerial bombardment, and see how the combatants fared in the face of such obstacles.

The first aircraft were woefully underpowered, hard to control, and had more than enough trouble simply lifting themselves and a pilot into the air - never mind the added weight of a passenger! However, improvements were rapid. By the end of the first decade of the twentieth century, aircraft capable of carrying the weight of two people were reasonably common. It didn’t take long for airmen to figure out that instead of a second person, equipment (such as cameras) or weapons (such as bombs) could be carried instead.

There was considerable scepticism as to whether the aircraft could ever be a useful instrument of aggressive warfare. An editorial in the Scientific American magazine of October 1910 pontificated:

Outside of scouting duties, we are inclined to think that the field of usefulness of the aeroplane will be rather limited. Because of its small carrying capacity, and the necessity for its operating at great altitude, if it is to escape hostile fire, the amount of damage it will do by dropping explosives upon cities, forts, hostile camps, or bodies of troops in the field to say nothing of battleships at sea, will be so limited as to have no material effects on the issues of a campaign . . .

However, events were soon to prove such scepticism unfounded.

The first aerial bombardment by powered aircraft recorded in history was conducted during the Italo-Turkish War of 1911-12. Italy sent nine aircraft of four different types to Libya as part of its invasion force, under the command of Captain Carlo Piazza, a well-known racing pilot. Piazza flew a Blériot XI (the same type used by Louis Blériot to fly the English Channel for the first time in 1909) on the first military reconnaissance flight by an airplane in history on October 23rd, 1911.

Replica of Blériot XI

On November 1st, 1911, Lieutenant Giulio Gavotti took aloft with him four small 4½-pound grenades. Flying his Rumpler Taube monoplane at an altitude of 600 feet over Turkish positions at Ain Zara in Libya, he took the grenades from a leather pouch, screwed in the detonators, and dropped them over the side of the aircraft. No-one was killed or injured, and little damage done: but Lt. Gavotti had earned his place in history as the first person to drop bombs on an enemy in wartime from an aircraft.

Rumpler Taube

Italian airships were used for military reconnaissance West of Tripoli, behind Turkish lines, on March 5th, 1912, and in doing so became the first dirigibles ever used for a military purpose. They also bombarded Turkish positions near Tripoli later that same month - once again with little to show for it apart from some spectacular plumes of sand.

Of course, in both cases there were no means to aim the bombs. The pilots simply tried to determine ‘by eye’ when they were over their targets, then physically manhandled the tiny ‘bombs’ over the side of their cockpits, hoping for the best. They weren't even able to determine their true airspeed, in the absence of any reliable instruments. Needless to say, the results weren’t very satisfactory. Nevertheless, these first primitive aerial bombardments were a portent of the far more lethal and damaging techniques to come.

Other nations were swift to follow Italy’s example. A Bulgarian pilot dropped home-made bombs on a Turkish military base during the First Balkan War in 1912, without much success. The French Air Force began experimenting with bombing from aircraft in the same year, followed by Britain in 1913.

The USA was the first to demonstrate the use of firearms from aircraft. On August 20th, 1910, at Sheepshead Bay racetrack near New York City, Lieutenant Jacob Fickel, a passenger in a Curtiss Model D aircraft piloted by Glenn Curtiss himself, fired a rifle at a target from an altitude of approximately 100 feet. This proved that the recoil of a weapon would not damage the primitive, flimsy aircraft (a question which may seem laughable today, but was much debated at the time).

Glenn Curtiss and Lieutenant Fickel

In 1912, Captain C. D. Chandler of the US Army fired an air-cooled recoilless machine gun successfully from a Wright B Flyer over College Park, Maryland. The weapon was based on the patented design of Samuel Neal McClean, who sold it to the Automatic Arms Company, where Isaac N. Lewis developed it further into the world-famous Lewis Gun, which saw service in World Wars I and II.

Wright B Flyer

Lewis guns installed on Royal Aircraft Factory F.E.2d.

Another important development, not immediately bombardment-related but destined to become so, was the use of ships as aircraft carriers. On November 14th, 1910, Eugene Ely made the first takeoff from a warship, the cruiser USS Birmingham, anchored near Hampton Roads, Virginia, in a Curtiss Model D. On January 18th, 1911, he made the first carrier landing onto a 125-foot (38-meter) platform on the warship USS Pennsylvania, anchored in San Francisco Bay.

Ely lands aboard USS Pennsylvania, January 18th, 1911

The Mexican Revolution saw the first aerial bombardment of warships. Captain Gustavo Salinas Camiña, who had learned to fly at the Aero Club of America at Flushing Meadows, New York, in 1912, was one of five aviators serving General Venustiano Carranza in his struggle against General Victoriano Huerta, who had seized power in Mexico after President Francisco Madero was assassinated. Camiña was one of two pilots assigned by Carranza to assist General Àlvaro Obregón in the North-West campaign.

On April 14th, 1914, Camiña was ordered to engage two Huertista warships, the Morelos and the Guerrero, that were attacking one of Obregón’s gunboats, the Tampico. Flying a Curtiss Model D biplane (a later model, without forward canard wings, known as the ‘Headless Pusher’), that he’d named Sonora, and taking his mechanic, Teodoro Madariaga, with him, Camiña dropped home-made dynamite bombs on the Guerrero, not scoring any hits, but scaring her and her compatriot out to sea and saving the Tampico. The combined naval and air action became known as the Battle of Topolobampo, after a nearby port.

Captain Camiña stands alongside his biplane Sonora

In 1943, as chief of the Mexican Army Air Forces, while on a visit to the USA, Camiña recalled:

The bombs I used were home-made, with a charge of 52 sticks of dynamite. Primitive as they were, they worked like a charm. At the time I was flying an old Wright pusher type. It occurred to me that the day would come when we would have planes of weight-carrying efficiency beyond one’s fondest hopes, and then the plane would come into its own as a military assault weapon of fabulous power. That day has come.

Until now all aircraft had been of light weight and very low power. It was widely assumed that a heavy aircraft with multiple engines would be unable to take off at all, and even if it did, would be impossible to control. The man who changed that view forever was Igor Sikorsky. On May 13th, 1913, his Russky Vityaz (‘Russian Knight’, also called ‘Le Grand’) aircraft took off from St. Petersburg, Russia, on its maiden flight.

Sikorsky's Russky Vityaz

This aircraft, which was indeed colossal for its day, had a wingspan of 92 feet (three-quarters of the distance flown by the Wright Brothers on their first flight a decade before, and similar to the wingspan of an early-model Boeing 737 jet airliner). It was initially designed to use two engines, but none of adequate power were to be had: so the design was modified to use four, each of 100 horsepower, mounted between its wings. This made it the first four-engined aircraft in the world. The plane weighed four and a half tons, and was amazingly luxurious, with two passenger cabins, a washroom, and even an exterior balcony in the nose on which passengers could promenade!

Only one example of the Russky Vityaz was built. It made 53 successful flights, and established a world endurance record of 1 hr. 54 m. with eight passengers aboard, before being destroyed in a freak accident. While it was parked on the ground, a Morane aircraft flew overhead - and its engine fell out! The engine fell onto the Russky Vityaz, damaging it beyond repair. Undaunted, Sikorsky used the knowledge gained from this aircraft to build another, even bigger model, of which more in a future article.

So, by the beginning of World War I, much of the foundation for aerial bombardment had been laid. Bombs (albeit small and crude) had been dropped from aircraft and airships; rifles and machine-guns had been fired from them; and larger aircraft, which could carry a heavier weight of munitions, had been demonstrated to be feasible. However, there were still no aircraft designed to carry armament from the outset, and no way to aim bombs from the air at ground or naval targets. All depended on the skill (and luck) of the pilot in judging his drop to hit the target. In every single case thus far, such skill (and luck) had proved lamentably lacking.

At the outbreak of World War I, the major combatants’ military air arms were approximately as follows, in descending order of their size:

France: 260 aircraft, 171 trained pilots.
Russia: 100 aircraft, 28 trained pilots.
Germany: 46 aircraft, 52 trained pilots.
Great Britain: 29 aircraft, 88 trained pilots.
Italy: 26 aircraft, 89 trained pilots.
Japan: 14 aircraft, 8 trained pilots.
USA: 8 aircraft, 14 trained pilots.

Remember that none of these aircraft were armed, or fitted to carry bombs. Indeed, the US Army’s doctrinal approach, as set out in a 1914 Field Service Regulation, stated explicitly that aircraft were only to be used for strategic and tactical reconnaissance - nothing more! There was also no such thing as a ‘standard’ aircraft type. The French total of 260 aircraft is said by one authority to have comprised no less than 37 different types of airplane, making standardization, training and maintenance a nightmare.

In the next episode of 'Weekend Wings' we'll examine aerial bombardment during World War I, and its development in the 1920's and 1930's.



JPG said...

As always, Peter, your aviation writings are intriguing. I note that they've been a bit less regular in recent months, and I've missed them.

I just recently completely re-read your fine article on defensive aerial gunnery. I got evewn more from it this time than on the first reading.


On a Wing and a Whim said...

I must say, I'm bookmarking this for the next time someone asks me "What's so difficult about a flour bombing competition? How come no one hit the target (outhouse) at the competition?"

As ever, incredibly thorough and quite delightful!

Anonymous said...

Bravo, Peter.