Balloon Killing

[Gurgast]

I've been trying to think through the thorny business of balloon killings. Much of the routine work of scout pilots seems to have been in the shooting down of static observation balloons, and I found myself wondering about the precise physical mechanism that must have been at work here.

Presumably these balloons were filled with hydrogen at a small increment above atmospheric pressure, and were only made just large enough to lift the necessary payload to get the job done, with perhaps some consideration given to acheiving a reasonable rate of ascent.

An ordinary machine gun round would enter a balloon through a small hole and leave again through a similar hole not much bigger than the round itself, and given that these holes wouldn't tear too far (a reasonable assumption given the low pressures and stresses at play on the fabric) then you could probably fire that a balloon all day and it would retain enough gas to stay up, unless the owners winched it down first.

Now try shooting incendiary ammunition -- rounds with a blob of burning (presumably white) phosphorus on the tip. What was the precise mechanism for the ignition of these rounds? Was it a matter of the phosphorus igniting by virtue of the friction they were exposed to by the round's passage through the oxygen in the air? How much bigger was the phosphorus blob on an incendiary round than on a normal tracer round? The latter certainly used to burn out in a matter of 3-400 yards. I would have assumed that anyone firing at a balloon with real hopes to destroy it would have had to do so from less than 100 yards range.

Does anyone know when these phosphorus rounds were first introduced? I've read it was 1916, is this right? And if so, when in 1916?

Such a round would make the same holes as the ordinary rounds, but would white phosphorus continue to burn inside the balloon where there would be no oxygen? I guess it was hot enough going in, and spent so little time inside, that it didn't have time to go out, but I'd also think that what the round did inside the balloon would be quite irrelevant.

It's not that hydrogen is "explosive" per se, because pure hydrogen is stable and doesn't ignite without an oxidant and an intense source of heat. A mixture of hydrogen and air burns, because air contains around 20% oxygen. Some air mixtures are hard to ignite because the relative strengths where burning happens is limited, but hydrogen is said to be "very flammable" because it will burn in as little as a 4% mixture or as much as a 75% mixture with air. A wide range.

Because the pressure inside the balloon is higher than atmospheric pressure, the only place where a flammable mixture can occur is outside the balloon, near a hole, where a plume of escaping hydrogen mixes with the outside air. Any large flammable mass of gas ignited will explode and not linger in the vicinity of the balloon surface, because hydrogen explosions have a strong tendency to blossom vertically upwards.

However, a flame created at the hole itself will burn invisibly (a feature of hydrogen flame is that it mainly emits ultraviolet light so it's hard to see with the human eye), and be continually fed by hydrogen fuel, like a little invisible blow-torch. This, I suppose is what would do the damage, setting fire to the fabric of the balloon and so enlarging the hole by that process. This would appear to an observer as a "delayed reaction" and I would suppose this to be borne out by contemporary reports.

Perhaps some incendiary rounds actually set fire to the fabric directly, but I would think there would be too brief a contact between bullet tip and fabric for this to work very often.

What was the fabric of balloons made from? Silk or linen, treated with nitro-cellulose dope? That ought to burn pretty well! On the other hand, if the upper surface of a balloon was liberally coated with dew or fine rain it would have a considerable protective effect. Is there any evidence that pilots like Coepens and Bourjade hunted in the afternoon?

Obviously hot, dry, sunny days would be best. The balloons would do their best work on clear moisture-free days, so I suppose I'd expect more kills then anyway, but the morning dew issue might be worth looking at. Does anyone know how many tries per kill the major "balloon busters" were obliged to make? It certainly looks like they weren't easy targets, even if we disregard the matter of ground protection and the perils of flying into the conflagration attending any success!

As for counter-measures, at first glance I should have thought hot-air balloons would have offered a much better chance, since there would be nothing inflammable there -- did they use them? I would guess that helium wasn't by that time stored in sufficient quantities to use, even on the Entente side since it's isolated as a by-product of natural gas and the helium industry wasn't really up and running. Still, I should have thought there must have been a non-inflammable kind of dope available ...

[Machinbird]

Gurgast. That is a pretty workmanlike job outlining the problem of shooting down a hydrogen balloon. A tracer round is not on the tip of the round, but in the base of the round and is ignited by the propulsive charge.
If you consider the case where a round meets the balloon envelope at an acute angle and begins to depress the surface before finally tearing through, you then have the potential of making elongated holes. If the angle is acute enough, you can make some fairly long tears and thus cause substantial hydrogen leakage into the balloon boundary, but slight angular errors can cause a complete miss of the balloon. All this points up that a successful balloon attack probably involved sustained fire at the top (not the centroid) of the balloon with the fire continued and concentrated until necessary to break off the attack to avoid collision.
Observation balloons appear to have been constructed of a coated fabric based on some photo's I've found. What the actual fabric consisted of would be an interesting topic all of its own.
See this link from the Western Front Association for more detailed information. Observation Balloons On The Western Front
Sid

[Catfish]

Hello,
for what i have read the "killing" of balloons was not easy. Apart from common thinking Hydrogen gas itself will not burn. There are reports of shrapnels exploding within a hydrogen gas bag in a Zeppelin without causing the ignition of the hydrogen gas, and incendiary ammunition going right through it without igniting it. The sailmaker would instantly seal the small leaks, and overpaint it with cellon paint.

The shooting down of balloons was so dangerous for the attacker because he had to stay in the vicinity of the balloon for at least 2-3 minutes, after perforating it, until going in again and igniting the air-hydrogen mixture developing above the balloon. There was no quick going-in, shooting down the balloon and getting the hell out of there.
Ammunition used in the home defence squadrons (which may differ from the incendiary ammo used for ballons over the actual front?) consisted of three different types of bullets, being repeated in the ammunition belts. The use of different bullets additinally led to numerous gun jams.

Greetings,
Kai

[Romani]

Originally Posted by Gurgast

I've been trying to think through the thorny business of balloon killings. Much of the routine work of scout pilots seems to have been in the shooting down of static observation balloons,

Balloon busting was hardly "routine", rather it was mainly the domain of "balloonatics", specialized hunters. When it was a squadron assignment it was regarded as a dangerous, tough and generally disliked assignment. Many successful aces didn't want anything to do with balloons. Crediting a destroyed balloon as an aerial victory, to count for the score towards medals, was a way to encourage pilots to attack these targets.

As well, balloons themselves where used as a key element in major battles, both in the offensive and defensive so they were premium targets.

I used to think, due to the publicity given to Allied balloon busters, that balloon employ was more of a German thing to compensate for their overall inferiority in the air, and that they were used on quiet sectors of the front where there were scarce observation airplane resources.

I was thinking about top balloon buster Willy Coppens and the Belgian sector of the front in particular. I thought that if Coppens had flamed so many balloons is because he had no German airplanes to shoot at.

But then I read more, and the Germans were always short of balloons, as with many other things, the British and French deployed more balloons overall, even the Americans had their own sections.

Then I realized that the difference in numbers was made up because the Germans had held onto the high ground along the Western Front, so they had the commanding heights and thus owned the natural observatories for artillery fire, so the Allies had naturally to deploy more balloons all along their line to compensate the disadvantage.

In Flanders, specially near the coast, the lack of such heights forced the Germans to deploy more balloon sections here, giving Coppens something to shoot at.

Nevertheless, perhaps towards the end of the war due to the fuel shortages they had to rely more heavily on balloon observation, giving Frank Luke a "target rich environment" for his September rampage.

[Mogadeet]

The Germans had their fair share of balloon specialists, and one of the uses that the Pfalz D-III proved excellent at was this task. It's speedy and steady dive worked well for the swooping attack you needed for balloon attack, and they took a steady toll of Entente balloons.

Mogadeet

[Romani]

Originally Posted by Gurgast

Presumably these balloons were filled with hydrogen at a small increment above atmospheric pressure, and were only made just large enough to lift the necessary payload to get the job done, with perhaps some consideration given to acheiving a reasonable rate of ascent.

This is what I could find on German balloons, but since they were pretty much the same on both sides, it is also applicable to Allied balloons.

Following Hoeppner memories, early Drachen types had a volume of 600 cubic meters wich was insufficient,

"using good quality peacetime gas and with an observer on board its ceiling had been as great as 800 meters, but owing to wear and tear on the fabric and deterioration of the quality of the gas in mass production, it would rise on the average to only 500-600 meters wich was insufficiente for succesful observation"

I had wondered if balloons were filled with gas cells inside the envelope like in a zeppelin, but seems from the above passage that the envelope was simply filled.

Second, the gas is hydrogen, but I don't understand how the gas production method can result in anything other than pure H2, I thought it was simply electrolysis of water, unless some other method was used wich resulted in some percentage of nitrogen being mixed in the gas cylinders.

Rate of ascent was not important, what was needed was ceiling, later balloons were larger at 800 and 1000 cubic meters and capable to achieve altitudes of 1000 and 1200 meters respectively.

Knowing the volume of gas, the density of hydrogen, the absolute ceiling, asume 100 kilos payload (70 kilo observer + 30 kilos in cameras and telephone cable) and I am sure you can find the weight of the wicker basket of a modern hot air balloon, and you can figure out rate of ascent, "buoyancy" and wether the balloons were "oversized" or just big enough for the job, plus the likely pressure of the gas.


What concerned them most was the rate of descent to lower the balloon quickly in case of danger, power winches were introduced in January 1915.

By the way, the Germans didn't at first appreciate the value of their balloons since there were only 9 balloons in the Western Front in February 1915.
By March 1917 there were 52 balloon sections, then it rose to 132 and 182 by the end of the war. According to British intelligence, each section had two balloons, one held in reserve.

What was the fabric of balloons made from? Silk or linen, treated with nitro-cellulose dope? That ought to burn pretty well!

I think we can safely discard silk, and with all certainty it was linen. To be precised it was rubberized fabric, that is fabric (linen) with a coating of rubber (natural latex, synthetic was not invented until the 1930s)

Now, the question is, why rubber ? Due to shortages the Germans had to stop making rubber gas masks and use sheepskin instead, while regarding balloons they used doped linen, just as in aircraft, wich shows linen could do the job, what was the rationale behind using rubber?

Perhaps because it's stretchy and will not leak so much gas in the seams as one made from doped linen or something like that?

Maybe because rubber is truly waterproof (I have my doubts about doped linen) and ensures that the balloon fabric will not get soaked adding weight and reducing the altitude it can ascend to?

On the other hand, if the upper surface of a balloon was liberally coated with dew or fine rain it would have a considerable protective effect.

This is a dumb question, I know, but how so? I know wet things are harder to burn, but I must have been checking out the girls in the classroom when that was explained in physics and missed that part

I know from reading accounts that indeed this was the case, a wet balloon could be very resistant to catching fire, but why?

Does anyone know how many tries per kill the major "balloon busters" were obliged to make?

I recommend the purchase and thorough reading of Osprey's Aircraft of the Aces 66 Balloon Busting Aces of World War I

Ok it depends, sometimes only a few bullets, les than a dozen would suffice, other times it would be neccessary several passes and a hundred rounds or more to achieve the desired result, you explained very well why it could be so, and the moisture would be another contributor.

I should have thought hot-air balloons would have offered a much better chance, since there would be nothing inflammable there -- did they use them?

From what I recall about the history of development of the aerostats, once they found out how to produce hydrogen in the early 19th century, they stopped messing around with Montgolfier hot bags since performance was beyond comparison.

Anyway, the technology for modern hot air balloons was simply not available back then. Starting with the envelope, no lightweight nylon back then. Second, the propane gas burner. Propane is a petroleum product, a liquefied gas obtained by the cracking of oil, if I recalling correctly, oil refining at this time was done by distillation.

Forget about helium, and about non flammable dope, the age of plastics had not yet arrived.

[Gurgast]

"A tracer round is not on the tip of the round, but in the base of the round and is ignited by the propulsive charge."

Ah! Suddenly it makes sense. I was told that the white phosphorus was in the tip and it didn't make any sense to me. How could they have stored ammunition? How could they have kept the Lewis drums, which couldn't have been airtight, from burning away ... none of it made sense. Of course the phosphorus was INSIDE the cartridge.


Thank you Machinbird also for that link -- a very interesting, information rich piece. I didn't know they used gutta percha. I would have thought rubberized materials too heavy -- think of those blow up mattresses made from rubberized cotton. And I hadn't appreciated the "drachen" had internal structure -- ballonets like airships -- nor that coal gas was sometimes used -- that of course would have given less lift, but also made them less inflammable.

It also makes sense that the kill would require an initial shooting up to create the perforations, followed by an igniting pass.

Romani -- I meant "routine" in the sense that balloons must have been on the army's agenda, since they would be suffering the artillery hits called down by them, and so there would have been constant requests to poke the enemy's eyes out. Certainly, I've just finished reading Rickenbacker's autobiography (which incidentally looked to me like nothing of the sort, but rather a ghosted text put together from scant diary entries, and was therefore a great let down to me,) however it does mention quite a lot of balloon busting flights.

To quote from Machinbird's link:

"The sausage shaped balloon of both sides on the Western Front was around 200 feet (60m) in length, 50 feet (15m) feet in diameter, and contained about 30,000 cubic feet of highly inflammable hydrogen gas. Some of the larger balloons were filled with coal gas, also then known as illumination gas."

So that's what they must mean by gas quality, because coal gas is a mixture of H2, carbon monoxide and methane, which is got out of coal when making coke. (There would have been varying degrees of CO2 and N2 in it as impurities, both heavier than air, so no lift there.)

Again from Machinbird's link:

"Inside the envelope so formed, were located cylindrical hydrogen gas-bag(s) and a balloonet, which was filled with a constant supply of air from the wind further tautening and stabilising the envelope. A valve attached to a fixed chain inside the balloon would be activated when the balloon expanded an was in danger over-inflating as it ascended, or warmed in the sun, releasing gas so the balloon would not burst under its own pressure."

If so, there would have been internal cavities that would develop flammable mixtures after an initial strafing.

Yes, your point about rubber is well taken. I was wondering the same thing, since there was a pretty serious naval blockade going on, and latex comes from the tropics!

Thank you, gentlemen!

[TonyWilliams]

This might help. Some of the various .303 explosive / incendiary bullets tried by the British, from this article on my website: RAFHS 08



.303 rounds sectioned: Brock incendiary, PSA Mk I HE, PSA Mk II HE, RTS Mk II HEI

[Catfish]

Hello,

very interesting thread !

Concerning the material of those observation balloons i think it was cotton, not linen ? But i could be wrong ..

The outer skin of a Zeppelin, or Schuette-Lanz airship was made of cotton, but it was not gas- or watertight at first. This was intended, so leaking hydrogen gas from inner gas cells would vent through the outer skin, and not cause problems while mixing with air, and becoming "Knallgas" (detonating gas, oxyhydrogen gas), a highly explosible matter then residing at the upper parts of the airship.

So early airships in damp conditions, like fog or rain would become heavier, this caused especially problems for the wooden Schuette-Lanz airships - one reason why Strasser (commander of the Naval Airship Force) preferred Zeppelin airships, made of a Duralumin framework rather than glued wood.

Later in the war the Zeppelins and Schuette-Lanz were indeed coated with a gas-tight paint (cellon, with metal particles), which prevented moisture, and made the ships faster due to the smooth skin.
When Zeppelins were built with special venting-hoses, thus obtaining the technique of the SL airships, the ventilation through the outer skin became obsolete. The coating then caused some other problems, one of it (implosion) is described in the Breguet crash challenge #386 with the naval airship L49.

However silk was only used towards the end of the war, and only for airship gas cells - it was lighter than the formerly used cell material gathered from animals.


Hydrogen, hmm - there are three methods to produce Hydrogen, one is to dissolve iron chippings in hydrochloric acid (e.g. Blanchard's balloon gas was produced this way, and also the gas for the observation balloons used in the US civil war). As already mentioned another one is to apply electrical current to water, however you nee a LOT of energy to get some hydrogen out of this. A third method is letting moist air stream along warmed (hot) iron, and as far as i know the latest method was foremost used for Zeppelins, balloons and to fill those gasometers.

If you speak of "low-quality" gas i take it the author meant hydrogen gas mixed with air, which would still lift, but certainly less than 100 percent hydrogen. Indeed airship cells were routinely filled to a 100 percent of hydrogen while waiting in its shed, since the hydrogen gas becoming polluted with air was thus minimized. I thought taht hydrogen (light) and air ("heavy") would build up "strata" in a gas cell, but not so ! It mixes, and that was always the problem with hydrogen gas.

Balloon gas was mostly transported to the front, the balloons were not filled at the gasometers at home. Instead it would be transported in steel bottles, which were not very tight, and the filling of those bottles itself did impure the hydrogen.

Greetings,
Kai

[TJJ]

There were also Le Prieur rockets - like giant fireworks fired electrically from tubes mounted on the interplane struts. These were used from 1916 onwards, but being inaccurate they had to be fired as close as 125 yards, which meant some nifty flying to avoid a collision.

They were successful against captive balloons, but never brought down a Zeppelin. The British Home defence squadrons certainly used them.

The various types of incendiary bullets were far more successful at balloon busting, and more practical too.

TJJ