problematic PIATs

[flamingknives]

Mr. flamingpicky would note that a link to that site appeared earlier in the thread.

[Scheer]

Originally posted by flamingknives:

90kg => 9.81*90=883N

F=ma => 883=1.35*a

a=654m/s^2

Or

Assuming that the spring is linear, is drawn back about 0.2m and that 883N is the final force, the energy stored is 0.5*0.2*883=88.3J

Kinetic energy is 1/2*m*v^2 where m is 1.35

=> (88.3*2/1.35)^0.5 = 5.7m/s initial velocity

Man, I just love this board ...

You guys are a cornucopia of unexpected facts ...

[GoofyStance]

After examining the photos and diagrams of the PIAT on the 6th Airborne web site (thanks to the folks who posted the link), I wondered about two things:

1) When the bomb was placed in the "tray," did the piston rest in contact with the propellant cartridge in the bomb's tail? Or was there a space between the tip of the piston and the cartridge? I ask because I wondered if the piston launched the bomb roughly in the manner of a "linear" catapult, or if it crossed a gap, slammed into the base of the bomb, and caused the bomb to fly out, much like kicking a soccer ball. This could affect the physics and accuracy of firing the PIAT, I believe.

2) The open-topped design of the "tray" in the photos suggests that the bomb's base was not inside the tube prior to firing. Is this true? In that case, it seems to me that whatever blast was issued from the propellant charge would emanate radially from the top and sides of the tray, at the latter's junction with the tube.

[Conscript Bagger]

Originally posted by Siege:

Of course you all have missed the other fun factor of the PIAT. Aim it down and the round falls out! From what I have read, the actual projectile was such a loose fit in the trough that they issued them with thin straps that went across the muzzle to hold the thing in place until firing.

I've seen this assertion a lot too, but there's contradictory info out there... consider the loading instructions from 6th Airborne (emphasis added):

Ascertain that the projector is cocked. With the right hand grip the projectile lightly by the tail drum and tail tube, with bomb head pointing forward and downward. Place the bomb head in the centre of the projectile support and push the nose of the bomb under the front ring of the support. Slide the bomb forward until the tail can be lowered. Keeping the loading clip against the front end cap of the projector, engage the loading clip behind the projectile guide plates. Release the grip on the bomb and, with the palm of the hand, press down until the tail drum rests in the support, thus bringing the tail tube in line with the spigot guide tube.

and this bit (from here):

With the rod retracted, you set the bomb in the flanges on either side of the hole. The base of the bomb is flared to fit these flanges and hold tight. The flanges served to hold it in place and align the tail for the rod. Holding in place was important when you were above your target, such as during the city fighting for Arnhem Bridge during Operation Market-Garden.

Edit to add: Maybe there was an early version without the guide plates?

[Mr. Tittles]

Originally posted by GoofyStance:

1) When the bomb was placed in the "tray," did the piston rest in contact with the propellant cartridge in the bomb's tail? Or was there a space between the tip of the piston and the cartridge? I ask because I wondered if the piston launched the bomb roughly in the manner of a "linear" catapult, or if it crossed a gap, slammed into the base of the bomb, and caused the bomb to fly out, much like kicking a soccer ball. This could affect the physics and accuracy of firing the PIAT, I believe.

2) The open-topped design of the "tray" in the photos suggests that the bomb's base was not inside the tube prior to firing. Is this true? In that case, it seems to me that whatever blast was issued from the propellant charge would emanate radially from the top and sides of the tray, at the latter's junction with the tube.

The cutaway drawings of the bomb show that the propellent is inside a cylindrical area inside the bomb. When the spring is cocked, the rod is back. In other words, the rod must come forward into the bomb to get to the charge.

I made a point earlier that the energy of the rod actuated spring may not be transfred to the bomb. That is, the charge launches the bomb.

When the bombs cylindrical area moves off the rod, the backblast would basically be pointed in a cone rearward. Part of that cone would be headed towards the firers face.

[ April 29, 2004, 05:06 PM: Message edited by: Mr. Tittles ]

[flamingknives]

If that were the case, there'd be little point to having a 90kg spring.

The designers came up with a bad design, but that doesn't mean that they were stupid.

[Mr. Tittles]

No. Theres a big point in having the mass of the rod and the large spring also. F=Ma

The propellent acts on both the bomb and the rod. Each is sent shooting in opposite directions. The rod is buffered into the spring. If not, the poor firer would be busted up horribly.

The main point of the spring is to stop the rod, not 'push' the rod so that it heaves the bomb. The bomb is heaved by the propellent.

[flamingknives]

Ah, good point. Having said that, some energy of the spring will almost certainly be transferred to the bomb (see my earlier calculations).

If the spring/rod system is moving forward before the propellant is detonated, then this momentum needs to be reversed, giving an extra push and also spreading the recoil force over time.

[Mr. Tittles]

The propellent reverses the rod. It would normally propel the bomb but must propel both the bomb and the rod (in opposite directions). So there is no free lunch. Getting the rod back takes away energy from the bomb.

Any way you slice it, the spring is not contributing much to the forward velocity of the bomb. Its bad intuition at work here. People see a spring and must understand it as a launching device. The fact is that the bomb is contacted at the end of the throw of the spring/rod system. The springs energy has already been put into the rod before it strikes the bomb.

[Mr. Tittles]

From images in this and other websites I would say that the throw of the rod compared to the depth of the bombs inner chamber does not allow the spring to even act as a pushing mechanism for more than a few inches at best. And this at the end of its throw where the spring is weakest usually.

[GoofyStance]

So the mechanism of the spring/rod combo sliding backwards as the bomb leaves the launcher is akin to diving off the bow of a small, unladen rowboat - the boat shoots backwards, and the diver doesn't go very far forward before doing a belly flop into the water? Hence the reason for the propellant charge. But that would mean the charge has to be significantly large, in order to propel the bomb forward 300+ meters. I understand the charge is actually a blank .303 cartridge, which means it'd be rather puny considering the weight of the bomb? Thus, the spring/rod combo HAS to lend some momentum to the bomb ...

[flamingknives]

Mr. Tittles, what is the distance between the spring at full back and full forward?

GoofyStance, If someone had the mass and MV of a .303 bullet, we could work out roughly how far it would project the PIAT bomb

[MikeyD]

"Getting the rod back takes away energy from the bomb."

That might be an overstatement. After all, a rifle propels the round forward while pushing back against the firer's shoulder; a normal artillery shell both pushes the round forward and recoils the gun tube backward; and a recoilless rifle is open ended. The PIAT rod/spring combo sounds more like a soft recoil system for the firer than a mechanical ejection system for the round. Not the most 'intuitive' weapon system to understand!

It woud be interesting to compare the PIAT firing sequence to the Churchill's big spigot mortar bomb.

[Mr. Tittles]

Originally posted by flamingknives:

Mr. Tittles, what is the distance between the spring at full back and full forward?

What 'distance'? Are you asking the throw or travel of the rod perhaps?

[flamingknives]

Yep, pretty much - essentially what distance does the spring apply force to the rod over.

Given that, you can plug the actual distance into the formula I used earlier to get energy stored in the spring

[Mr. Tittles]

This pic shows the cutaway. The rod has to travel up the hollow to reach the propellent. If this is a 90mm round, that internal propellent cavity is over an inch in diameter. It appears the propellent is as long as it is tall.

I would say that is more propellent than a typical rifle round has. perhaps twice as much?

[ April 30, 2004, 01:07 PM: Message edited by: Mr. Tittles ]

[Mr. Tittles]

Originally posted by MikeyD:

"Getting the rod back takes away energy from the bomb."

That might be an overstatement. After all, a rifle propels the round forward while pushing back against the firer's shoulder; a normal artillery shell both pushes the round forward and recoils the gun tube backward; and a recoilless rifle is open ended. The PIAT rod/spring combo sounds more like a soft recoil system for the firer than a mechanical ejection system for the round. Not the most 'intuitive' weapon system to understand!

Its showing that theres no free lunch. The basic thought is that ANY forward velocity component FROM the spring/rod may be negligible compared to the cost (it DOES take energy to get the spring ALL the way back) from the propellent energy loss.

I think that flaming knives sees that the spring is not acting for more than a fraction of its travel.

I dont know if comparisons to 303 bullets can be made due to the different length of barrel, etc.

[Mr. Tittles]

Originally posted by GoofyStance:

So the mechanism of the spring/rod combo sliding backwards as the bomb leaves the launcher is akin to diving off the bow of a small, unladen rowboat - the boat shoots backwards, and the diver doesn't go very far forward before doing a belly flop into the water? Hence the reason for the propellant charge. But that would mean the charge has to be significantly large, in order to propel the bomb forward 300+ meters. I understand the charge is actually a blank .303 cartridge, which means it'd be rather puny considering the weight of the bomb? Thus, the spring/rod combo HAS to lend some momentum to the bomb ...

Its more like you are at the end of a pier waiting for this small boat driven by a midget (total weight of boat and midget equals your weight) to come at you (90 degrees to the pier end). As it arrives, you leap on and not only propel yourself forward considerably, you have to reverse the boat (reverse it so it goes back to its starting point). The thing is, when you jumped on the boat, you increased it mass and it did not maintain its speed. It also ran out of gas (end of spring travel). So you would have went much further if you had just jumped off the solid pier.

So, yeah the propellent charge has the energy.

The firer of this weapon had to hold the thing against any remaining kick or it would not cock. That means the total weight of firer and weapon is a very large mass compared to the bomb. But not the same ratios you see in a rifle or a artillery piece. Its orders of magnitude different.

If the spring/rod did not take up the energy back into the spring, the firer would be broken.

[ April 30, 2004, 11:48 AM: Message edited by: Mr. Tittles ]

[Mr. Tittles]

http://www.btinternet.com/~ian.a.paterson/equipinfantry.htm

This website claims a velocity of 250 feet per second. Even using flaming knives calcs above (which assume complete travel under spring effect), the spring would contribute 7 percent of the final velocity. In actuality, its probably less. The weight of the rod and bomb may be very close btw.

I am sure there were dummy practice rounds that could be used to practice loading (dummy meaning no HC or propellent. If one were to squeeze the trigger, I guess the dummy would shoot out at some small velocity.

[ April 30, 2004, 01:11 PM: Message edited by: Mr. Tittles ]

[Ant]

Interesting site, thanks.

One thing I did pick up on though:

but in 1941 rocket technology was still in it's infancy.

Yeah, we'd only been using rockets since the battle of Waterloo in 1815

[Mr. Tittles]

The site does mention a lag between unleashing the trigger and the bomb taking off. Pulling the trigger on this thing means unsnagging that spring. It also mentions that the firer has to grip that trigger guard stiffly and that there is a limited number of positions the firer can be in.

A bazooka firer can pop from behind a wall and firer with no recoil. The firing is electrical and there is no lag really. He can be in standing, kneeling, etc. He can fire with one hand actually.

The PIAT gunner really has to be prone on the ground or have the monopod resting on something.

[ April 30, 2004, 01:22 PM: Message edited by: Mr. Tittles ]

[flamingknives]

I would say that is more propellent than a typical rifle round has. perhaps twice as much?

Actually, it's about 1.4-1.5 times as much. The propellant for a rifle round is about 2" long (51-57mm) and roughly 0.5" wide.

A Bazooka may be better, but could it have been had in sufficient quantities to supply the Commonwealth forces as well as the Yanks?

[Siege]

--------------------------------------------------------------------------------

Originally posted by Siege:

[QB] Of course you all have missed the other fun factor of the PIAT. Aim it down and the round falls out!

--------------------------------------------------------------------------------

Outside of Arnhem, how often was this necessary?

I'd gather it would be a problem any time that the firer was a decent angle above the target. Even just steep hills or any building of a decent height could do it.

quote:

--------------------------------------------------------------------------------

But then again, they were derived from an oddball mortar design and it took a couple major revisions before they were considered ready for combat. I don't remember the name of the earlier version of the PIAT that was only issued to home guard units as it wasn't considered suitable for front-line use.

--------------------------------------------------------------------------------

Blacker Bombard IIRC.

Yeah, thats the one. IIRC they renamed the newer version as the PIAT when it was developed from the Bombard because of the horrible reputation it had developed.

-Hans

[flamingknives]

The Bombard and the PIAT were startlingly different weapons. For a start the Bombard was in a similar size range as the 2pdr ATG. IIRC, it was only used from fixed emplacements.

The design that they shared was called a spigot mortar, but the differenct between the two was similar to comparing a 60mm mortar with a 4.2" weapon.

In fact, the 290mm gun on the AVRE used a similar principle.

[Michael Emrys]

Originally posted by flamingknives:

A Bazooka may be better, but could it have been had in sufficient quantities to supply the Commonwealth forces as well as the Yanks?

We gave them in wholesale lots to the Soviets, who were not much impressed with them. The bazooka is actually a cheap, easy to produce weapon, especially compared to an AT gun.

Michael