V.T. Fuses in Rockets?

[tankgeezer]

Just curious, were proximity fuses ever utilized in the 4.5" rocket artillery?  Given the Americans' penchant for '155MM Recon', it seems like a swarm of airbursts from a Xylophone battery would just about sterilize the strike zone.

[Xorg_Xalargsky]

Nope, the only batteries equipped with VT-fused shells are the 105mm, 155mm, and 203mm canons.

[Michael Emrys]

Which is slightly odd considering that they were already putting them in bombs. Were there perhaps not enough to go around and cover rockets too? That might be plausible given that the British had to do a bit of arm twisting before they got them for their anti-aircraft—specifically anti-buzzbomb—artillery.

Michael

[Bulletpoint]

I believe the reason is because rockets have the element of surprise - extremely high rate of fire means they can blanket an area quickly before anyone has the time to go to ground, let along get into a foxhole. But artillery has slower rate of fire, so without timed or VT fuses, only the first couple of rounds would have maximal effect.

With airplane bombs, it's a bit of the same thing. Often troops will spot or hear the planes approaching, so there will be time to get into shelter. Again, VT fuses are useful.

Edited July 29, 2017 by Bulletpoint

[Apocal]

15 hours ago, Bulletpoint said:

I believe the reason is because rockets have the element of surprise - extremely high rate of fire means they can blanket an area quickly before anyone has the time to go to ground, let along get into a foxhole. But artillery has slower rate of fire, so without timed or VT fuses, only the first couple of rounds would have maximal effect.

With airplane bombs, it's a bit of the same thing. Often troops will spot or hear the planes approaching, so there will be time to get into shelter. Again, VT fuses are useful.

VT fuzes don't work that good against cover either. It is hard to get the shells to detonate with such a trajectory that shrapnel is sent down into holes and it is useless against serious overhead cover. What they are good at is reliably generating a near surface burst (i.e. shell detonates a few feet off the ground at a steep trajectory, sending shrapnel sideways for a great distance) that cuts down anyone standing like grass and has a shotgun-like effect against those lying prone. But shells flying overhead at a steep trajectory then bursting just flings most of that shrapnel across the sky. It is a still better than a point-detonating fuze, but not so much as the game depicts.

[JonS]

Barrel-artillery rounds rely mainly on splinters for their effect. It takes distance for the splinter pattern t reach its maximum effectiveness - having the rounds burst consistently at 40m above ground level makes the splinter pattern more effective, and the splinter pattern is generally oriented downwards, further increasing it's effectiveness c.f. ground burst.

Rocket artillery rounds have a comparatively very high HE payload, in part because the stress of firing is far lower than for barrel artillery and so the projectile wall can be far far thinner. That means that the effectiveness of the round at the target end is due far more to blast effects than to splinter effects. However the effects of blast fall off in an inverse-square-law with increasing distance from detonation. Having the rounds detonate in the air would be very scary, but drastically reduce their ability to inflict physical casualties.

[Michael Emrys]

1 hour ago, JonS said:

Having the rounds detonate in the air would be very scary, but drastically reduce their ability to inflict physical casualties.

That would depend partly on how high they go off. I am content to agree with you if it goes off 40 meters above the ground, but if it goes off five or ten meters AGL, that would be a different matter. So the question in my mind is what would be the minimum distance AGL that the fuses could be reliably set to go off?

Michael

[JonS]

My bad - its an inverse cube law, not a square.

Mike, can you explain the mechanism by which blast increases with distance from the origin for the first 5 or 10 metres, then tails off?

Edited July 30, 2017 by JonS

[Michael Emrys]

1 hour ago, JonS said:

Mike, can you explain the mechanism by which blast increases with distance from the origin for the first 5 or 10 metres...

Eh? I never claimed that it did. I thought it was clear that I mean it still retained deadly blast effect for the first ten meters. More or less; it might be five, it might be twenty. but in any case it should be more than at the 40 that you postulated. (In fact, due to the inverse cube law that you mention, it should be far greater than than at the farther distance.)

Michael

[JonS]

Ah. I misinterpreted this exchange:

Quote

Having the rounds detonate in the air would be very scary, but drastically reduce their ability to inflict physical casualties.

Quote

That would depend partly on how high they go off. I am content to agree with you if it goes off 40 meters above the ground, but if it goes off five or ten meters AGL, that would be a different matter.

I took you to mean that bursting 5-10m up would NOT reduce the blast effectiveness.

For any sized detonation*, the largest blast footprint is created by a ground burst. Any burst above ground level reduces the size of that footprint, and lowers the peak blast overpressure experienced on the ground.

 

* for conventional weapons. Nuclear weapons are a bit different

Edited July 30, 2017 by JonS

[BletchleyGeek]

Very good exposition @JonS - that's not to say that rocket artillery is useless..  It just serves a different purpose. 

[Apocal]

21 hours ago, JonS said:

Barrel-artillery rounds rely mainly on splinters for their effect. It takes distance for the splinter pattern t reach its maximum effectiveness - having the rounds burst consistently at 40m above ground level makes the splinter pattern more effective, and the splinter pattern is generally oriented downwards, further increasing it's effectiveness c.f. ground burst.

Are you sure you mean forty meters and not forty feet? Because that is a higher height of burst setting than I've ever heard of IRL and I'm pretty sure the pattern on a 105 HE would be screwy at that distance. It certainly would be with a 5" shell.

Edited July 31, 2017 by Apocal

[John Kettler]

tankgeezer,

My research follows. Believe I have a partial answer for you, and what I found while getting it is remarkable. To begin, I can the US Navy's 5-inch spin-stabilized rocket had a VT fuze available for shore bombardment, as did air-launched versions of same. Here is the official skinny on the VT fuzes available for various Navy projectiles. To have a clean copy. of this rare document is practically a miracle, and there is much to learn from what's there, for those of a mind.

VT Fuzes For Projectiles and Spin-Stabilized Rockets.

OP 1480 (First Revision). BUORD. May 1946 

https://maritime.org/doc/vtfuze/index.htm

Also the top page of this UXO information site contains a bunch of additional VT material, together with all sorts of ordnance grog goodness. There was a VT fuze for Army 4.5 inch rockets for air use. 

http://uxoinfo.com/blogcfc/client/includes/uxopages/downframeset.cfm?X=4010&Y=4019

Ordnance - Historic Tech Data

TB 9X-94 Fuze Rocket, P.D., T4 and T5

1944 Technical Bulletin (TB) on the T4 and T5 variable time (VT) proximity fuzes for aircraft firing of 4.5-inch Army rockets. TB 9X-94 includes a detailed and technical description of the fuzes including sketches and deployment tactics. De-classified document found in the National Archives.

Rocket_Fuze_PD_T4_and_T5_Dec1944.pdf

1.21 MB

War Department

Finally, I can now show the Army redesignated the Navy's Mk 170 VT fuze for the 4.5 inch rocket, changing it to M402. This is taken from a section of OP1664 US Explosive Ordnance. It makes an excellent point about fuze reliability--50%. Only half of the rockets fired will detonate at all when they arrive in the target zone.

http://michaelhiske.de/Allierte/USA/OrdnancePamphlets/OP1664/Volume01/Part02/Chapter06/Section05/Section05_17.htm

Given the above, it seems to me the capability certainly existed, but were the fuzes shipped in time, and if they were, were the Xylophones (or any other surface-to-surface 4.5 inch rocket system) ever fired in battle using the VT M402 fuze? I don't have that answer, but I consider everything rock solid on the technical side and that the VT fuze for the 4.5 inch rocket was in service with the US Army for surface-to-surface attacks. 

Regards,

John Kettler

 

 

Edited July 31, 2017 by John Kettler

[JonS]

Back in the day calling in 105mm light guns I was taught to go for 40m HOB. Mind you,  this was in the 90s and 00s, not the 40s.

Also,  now I think about it ... that may have been for MTSQ (time fuzes) rather than VT. Now I think about it,  I have a vague recollection that Prox was set to burst at 20m ... I'd really have to dig out my aide memoire to be sure though.

Anyway,  assume that - whatever the actual distance is/was - the intent is to maximise the splinter pattern effectiveness.

[Michael Emrys]

On July 30, 2017 at 3:56 AM, JonS said:

My bad - its an inverse cube law, not a square.

Are you sure? The formula for finding the area of a sphere is A=4πr^{2_{. You may have been right the first time.}}

Michael

Edited July 31, 2017 by Michael Emrys

[TheForwardObserver]

I wonder if you're recalling 40 because UP 40 is the first adjustment made to every Graze HOB Spotting for VT.  We generally try to bring it back down to 20 after that.

[A Canadian Cat]

1 hour ago, Michael Emrys said:

On 2017-07-30 at 6:56 AM, JonS said:

My bad - its an inverse cube law, not a square.

Are you sure? The formula for finding the area of a sphere is A=4πr^{2_{. You may have been right the first time.}}

Its volume that matters.

V = 4/3(πr³)

So it is cubed.

[cool breeze]

frag density is squared but blast goes one power up since the pressure decreases as the volume increases. 

[JonS]

4 hours ago, TheForwardObserver said:

I wonder if you're recalling 40 because UP 40 is the first adjustment made to every Graze HOB Spotting for VT.  We generally try to bring it back down to 20 after that.

Thanks - yeah that's ringing bells. "Three on the ground: up 40. Two up, one on the ground: go to FFE. Three in the air: down 40"