Hydrostatic Pressure...

[brady]

Originaly Posted in the WiTP Forum at the Matrix games Forum sight by mdiehl: (http://www.matrixgames.com/default.asp?URL=http%3A//www.matrixgames.com/forums/)

"It turns out that there is more to it than a hot piece of metal and lots of fuel. That was ONE of the Japanese a/c's weakness. The Zero had another one. In order to maximize fuel (and thusly, range) the metal fuel tanks in the wings went right up against the wing spars. Hydrostatic pressure from an incoming round hits this relatively incompressible fluid, sending a shock wave against the wing spars. The Japanese figured that would not be too great a problem against .30cal, but did not reckon with the use of .50cal. One guesses that they just wrote off 20mm+ as being too powerful to defend against. Anyhow, one of the reasons why many F4F pilots report the wing tearing off after a hit near the wing root is because the .50cal hydrostatic shock wave was too much for the relatively flimsy spar construction to handle, so the wing would basically shatter under the force of the impact. The subsequent fireball was usually just the leftover effect of relasing all that fuel into the air around a hot bullet. In other words, it sometimes wasn't the firball that killed the plane, the fireball was just the aftereffect of having your wing torn off.

The US figured out this problem pre-war, and so made their fuel tanks of a fabric/rubber/fabric combination and gave the fuel tank enough room in the wing that it could expand a little without hitting anything structural."


 Coment's:

[HoHun]

Hi Brady,

>In other words, it sometimes wasn't the firball that killed the plane, the fireball was just the aftereffect of having your wing torn off.

The problem with this interpretation is that the hydrostatic shock wave killed the wing spar directly. It couldn't, the spar was much too strong.

However, the hydrostatic shock was enough to tear holes into the aircraft skin, which - as the A6M was a stressed-skin aircraft - harmed its structural integrity. The wing subsequently might fail  - but not because the spar itself was damaged, but because the load-bearing system of spar and skin didn't work anymore.

That's the same effect you get from the gas blast of mine shells exploding inside the wing structure even when there's no unprotected fuel tank.

(A good illustration why mine shells were so effective: They made every plane look like a Zero to your guns.)

>The US figured out this problem pre-war and so made their fuel tanks of a fabric/rubber/fabric combination and gave the fuel tank enough room in the wing that it could expand a little without hitting anything structural.

Well, to be accurate, the US had recognized the need for it, but plenty of otherwise modern service aircraft weren't thus equipped and accordingly not considered combat-worthy.

Regards,

Henning (HoHun)

[VO101_Isegrim]

Interesting, though I tend to agree with HoHun that this hydrostatic force alone was just too less to damage the main spar to any real extent, those are usually forged steel pieces, very very strong material. Though I can think of an alternate method, if the fuel cells are located just next to the fusalge, and their sudden bulging under hydrostatic shock towards the fusalge may just rip them down... sounds a bit theoretical to me, though.
Wing root hits were generally preferred by pilot as aiming points, partly due to the fact that it was a place where many vital parts could be hit, but also because there was the place for the attachment points of the main spar to the fusalage. In general, that meant just a few, 2-4 bolts. If those were hit, the wings just come off. Not sure though about the Japanese Zero`s design, I recall there was something special about it (not just strenght), but I have no possibility to check it out now.

[brady]

I to found some of the statements to be somewhat suspect, non the least of which was nevver hearing of the hydrostatic effect causing the spar to fail, in fact I remember reading that the Zero's spar was increadbly strong.

 One would tend to think that the effect of this would vary considerably depending on the amount of fuel in the tanks as well.

[HoHun]

Hi Isegrim,

>In general, that meant just a few, 2-4 bolts. If those were hit, the wings just come off.

I'd guess that the real reason to attack the wing roots was that the bending moment on the wing was at its maximum there so that any structural damage there would have the greatest effect.

Regards,

Henning (HoHun)

[brady]

The Zero's spar was also one continious pice, the only bolts were whear the fuslage joined the wing.

[ra]

If a fuel tank ruptured because of hydrostatic shock, it would rupture at its weakest point, not through the spar, which would be its strongest point.  Also, self-sealing fuel tanks could also rupture from hydrostatic shock.  50 caliber could do it at close range, and anything over 50 caliber would probably do it from farther away.  

In "Report of the Joint Fighter Conference" they discuss a 60 caliber MG under development.  IIRC, they said it would rupture a tank so long as the tank was more than half full.

ra

[hogenbor]

Interesting. The physics for what they are (I wouldn't dare to comment) I once saw a video of firing trails on fuel tanks of the Su-27 jet:

Unprotected fuel tank on a test rig, shot at with 1 (one) round of 30mm (I presume the same gun as the Su-27 itself carries) > tank go boom.

'Protected' fuel tank , shot at with 30mm > Nothing happens.

Of course not a word on how full the tank was, sort of round (AP, HE etc) used, how representative it all was and how the tank would hold when hit inside an aircraft's structure, onder G and at lower barometric pressure.

It still raises the question, how 'safe' are and were various types of fuel tank? Evidently even a self sealing tank of a certain design will explode if hit bad enough. Anybody any WWII documentation on that?

[VO101_Isegrim]

Hogenbor,

What do you mean under "protected" fuel tank ? There were many ways employed for that. One of the most common was using self sealing tanks, however, unlike the common opinion holds, these were not any less suspicable to fire/exploding as unprotected ones, they merely sealed the holes the bullets made (to a limited extent, they worked great vs. small rifle caliber hits, rather limited vs. HMG hits, and being practically useless above that caliber). Indirectly this also meant there was less fuel sprayed around, decreasing the chance of a secondary fire hazard, but again, the bullets that hit the tank could ignite it the same.

A fuel tank usually explodes if the the incendinary ammo hits it above the fuel level, where the fuel and air are mixed into each other. This oxygene-gas mixture is very prone to explosion, in fact it`s employed in some modern weapons such as vacuum bombs. If the hit occurs under the fuel level, there`s some chance of fire, which can also lead to explosion if it ignites the mixture above fuel level... To avoid this, some designs employed different protection methods, though the majority was unprotected, being made up a simple S-S tank.

The Soviet Lagg series used a smart preventive gas system, that allowed to lead the exhaust fumes of the engine into the fuel tank. Since the exhaust gas was low on oxigen, and high on CO2, N etc., it decreased the chance of the fuel tank exploding. AFAIK such "neutral gas" systems are used in today as well, with more advanced solutions. Another passive method was lightly  armoring the fuel tank, to stop the round or at least strip it of it`s incendiary head; such could be found on many bombers. A similiar protection can be found on the Bf 109 F/G series, being made up of a series of aluminium plates (about 20), that worked great vs. .303 and .50 caliber incendinary ammo, stripping them of their incendiary content as they passed through, also improving the pilot`s protection by slowing the rounds down and ruining their trajectory. The Zero itself in it`s later versions was using an active solution, the fuel tanks had fire extinguishers installed.

Also there`s the factor of the ammunition used, especially in larger calibers. Delayed action fuses can be effective vs. armored tanks, but if the tank is not armored, it may very well pass through the light structure w/o doing any serious harm. Similiarly, instant-action fuses may work very well on small aircraft with small dimensions (i.e vs. unarmored fighters), however vs. armored or large structures they could be ineffective. An example for that is the 3cm MK 108 ammunition, which employed instant-fused High Capacity HE shells, and as a result of experiance with US heavies, a special delay action Incendinary round with hydrostatic fuse to detonate only in liquid (fuel).

Quite a complex subject, it is.

[hogenbor]

Hello Isegrim,

Actually I don't know much about how a fuel tank is, or can be, protected, despite knowing a good amount about WWII aviation.

I do know enough about chemistry to understand the fuel-air mixture issue and I was aware of the system used on the Russian fighters, as well as the fire extinghuisers on the Zero. The effect of different calibers on fuel tanks and ways to protect them I was interested in. Is there any data on what happens when a mk108 30mm shell explodes in the fuel tank of let's say a B-17? I'll never forget the picture I once saw on this board of a Blenheim used for firing trails. Only one 30mm nearly blew the rear fuselage off, if this had happened in mid air the aircraft would have broken in two.

The Germans have estimated how much 30mm or 20mm it took on average to bring down a B-17, have hits on fuel tanks been taken into account? Or is is just weakening the structure, damaging engines etc.

[leitwolf]

The Germans have estimated how much 30mm or 20mm it took on average to bring down a B-17, have hits on fuel tanks been taken into account? Or is is just weakening the structure, damaging engines etc.

Afaik, only very few hits were necessary. Having seen the picture of the Blenheim tail mentioned and a modern 20mm HE in action (which sounds "small" but their effect is not a joke, really) everything from 1 to 5 30mm hits will take a B-17 apart or at least rip enough parts off to render it unflyable regardless whether they hit a fuel tank or something else.
Some sources also state that bomber crews were more afraid of the 20mm than the 30s because the 20mm would penetrate the thin skin and explode inside the bomber, killing or injuring the crew while the 30mm tends to explode on the surface and blows a huge part of the pane off but giving the bomber crews a better chance to leave the plane.