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GAU-7 : Gun for the F-15

RyanCrierie

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TomS said:
I came across two papers with interesting material on this.


AFATL-TR-84-03 Historical Development Summary of Automatic Cannon Caliber Ammunition: 20-30 Millimeter


This has a couple of sections specifically on caseless and telescoped ammunition, with some discussion of GAU-7/A (and a lot of other interesting topics).

"Page 87 hard Copy:


At the time this was written, we were working on the development of the 25 mm GAU-7 caseless (or more precisely, combustible case) gun system for the F-15 air superiority fighter. As it turned out, vulnerability to fire propagation in the ammunition bay caused a requirement to encase the round in a flame-retardant sheath which was stripped off prior to the round entering the gun, in effect, a "case" which only served part of the functions of a case. The funding and manpower required to develop this entirely new technology was taken from ammunition development; consequently, the problems of atmospheric humidity and inconsistent interior ballistics were not solved in time to get the GAU-7 gun on the F-15 aircraft and the program was cancelled"
 

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[font=verdana, arial, helvetica, sans-serif]The USAF's Armament Laboratory in 1967-68 proposed two armaments for the A-X:[/font]

[font=verdana, arial, helvetica, sans-serif]Option I: M61 scaled up to fire the US Army's 30x100B WECOM round at 2,000~ fps and 6,000 RPM[/font]

[font=verdana, arial, helvetica, sans-serif]Option II: 25mm weapon fired at 4,000 fps. This was eliminated because it would have delivered a fully developed gun by 1972; two years after the then-demanded 1970 IOC of A/X.[/font]
[font=verdana, arial, helvetica, sans-serif]
From the data I've found on GAU-7; the 25mm @ 4000 fps is it.[/font]
 

Grey Havoc

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PaulMM (Overscan) said:
Jeff Ethell Modern Combat Aircraft 12 : F-15 Eagle (Ian Allan 1981)


P19 has two photos of, apparently, the General Electric GAU-7 installation mockup. The text notes that Philco Ford and General Electric both submitted designs and Philco Ford won, so the mockup is of the losing contender. However it has 5 barrels which contradicts the below:


In an earlier life as an instrumentation techician for the US Gov't, I measured and recorded the behavior of internal ballistics energy on the outside of the barrel of the GAU-7 (caseless ammo, 25 mm gatling gun). It was abandoned due to the mechanical synchronization problem between feeder, loader and rotating chambers at high speeds.

Philco-Ford was competing against GE. In the ammo dept it was Hercules against Brunswick (I think). GE and Philco, I believe were also competing feeders. Each contractor supplied a gatling and a single shot breech loader (for ammo testing). GE's item had six barrels as I recall and Philco was five-barreled. I don't believe the GAU-7 was targeted for the F-15, but I could be wrong. I thought it was an A-10 project. The A-10 was a new project then too.PUT THE FIRE OUT? ARE YOU CRAZY? We ran like fugitives! Pretty much ruined the firing bay it was in, but as I recall, the test item (gun) was unharmed. 'Don't remember which one it was (GE vs Philco).

http://thefiringline.com/forums/showthread.php?t=13967&highlight=GAU-7

From a 1968 U.S Army report on the state of caseless ammunition R&D, a brief note:
25mm Caseless Ammunition/Gun Study

Two contracts, one to Philco-Ford/IITRI, the other to General
Electric Company/Hercules, Inc. were awarded by the Air Force in
January 1968 to conduct feasibility studies for 25mm caseless ammunition/
gun systems. No report is available on the progress of these contracts.

The Air Force has joined the Army in a joint effort with Aerojet
General Corporation to investigate their monolithic grain approach to
caseless ammunition.
 

Tony Williams

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Chinn's classic work five-volume work The Machine Gun has a chapter on each of the two GAU-7/A contenders: Philco-Ford and General Electric. The P-F had five barrels, the GE six barrels. Both were designed to fire at up to 6,000 rpm. Ammunition for the P-F was made by Brunswick, for the GE by the Hercules Corporation. Both were telescoped, with the projectile buried within them (on the outside, they were simple cylinders), and both had the same performance (1,200 m/s muzzle velocity). They were not interchangeable (the Brunswick was slightly fatter) and had different methods of construction: rather complicated for both, but the Brunswick round had a combustible case while the Hercules was effectively caseless.

P.S. In my experience, do not look for reliable information about aircraft guns in books by aircraft historians. With a few exceptions (notably Gunston and Francillon) they know very little about the subject.

The pic below of experimental ammo from the ammo photo gallery on my website shows the Brunswick case with the projectile next to it. By the way, my web address has changed which is why you don't see the earlier photos any more: www.quarryhs.co.uk



23x146SR (US Cal. .90 inch T1 experimental), 23x139RB (US Cal. .90 inch T2 experimental), 23x139RB (US Cal. .90 inch T3 & T4 experimental), 23x122 (HS.406/407: French aircraft gun 1940), 24x138B German WW2 (repA), 27x70B (for 1950s large-calibre Vulcan M61 variant), 25x116B (Oerlikon 251/25 RK aircraft gun), 26x130 (Colt 1960s AFV gun), 25x152 case and projectile (from US GAU-7 cannon 1960s - contained within combustible case), 25x145 (US plastic-cased)
 

Mark Nankivil

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Excellent Tony - thanks!

Enjoy the Day! Mark
 

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abstract : "The Air Force is sponsoring a program to seek out new concepts
for an air-to-air proj~ctile that will have superior performance in the air
combat mission. Air-to-air combat effectiveness can be improved by
increasing the probability of hitting the target and by greater warhead
lethality. A short time of flight at ranges out to 2500 feet is important in
achieving a satisfactory hit probability. Given a projectile caliber and
shape, the most effective means to reduce th.e time of flight at short ranges
is to increase the muzzle velocity. This can be accomplished by minimizing
the weight of the projectile; therefore, the objective of the program was,
working within a set of physical constraints, to develop a design for the
projectile that was as lightweight as possible consistent with the satisfaction
of the necessary performance parameters.
This pr0jectile will be used in a fully telescoped cartridge. This
removes some of the constraints associated with the cesign of conventionally
configured ammunition where the projectile and cartridge case are tandomly
arranged. This freedom to pursue new concepts was exercised in the
development of a plastic rotating band concept that restricts the propellant
gases to the region behind the projectile. This shielding effect minimizes
the lm.ds on the projectile walls and helps in achieving minimum projectile
weight.
The specified constraints were the caliber (25mm), the barrel length
and rifling, the ogivc shape, projectile length, fuze thread dimensions, and
the variation with time of the chamber pressure. Performance parameters
that influenced the design were the requirement for adequate projectile
stability and the need for satisfactory shell fragmentation properties. The
materials, the heat treat process, the interml projectile configuration and
the design of the rotating band were unconstrained areas. This provided the
freedom to achieve a lightweight projectile design that promises significant
improvements in short range projectile performance. For example,
compared to the GAU-7 / A, PJU-2/B HEI projectile the weight has been
reduced 21 percent, muzzle velocity increased 21 percent, time of flight to
2500 feet reduced 13 percent, the HE charge increased 30 percent, and the
charge-to-mass ratio increased 140 percent. Other improvements are expected
to be compatibility with the telescoped case design, good control of
balloting in the case and gun tubes, and improved reliability of a plastic
rotating band "
 

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TomS

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This would be a good addition to the GAU-7 thread here (I don't think it's there already):

http://www.secretprojects.co.uk/forum/index.php/topic,13941.msg242143.html#msg242143
 

NeilChapman

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Don't think I follow. This is dated 1977 and listed under "Army Projects".

Have there been advancements or a follow-on program?
 

TomS

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Have you read the paper? It was an Air Force project to develop an air-to-air projectile in a case telescoped configuration conforming to the GAU-7 chamber design.
 

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TomS said:
Have you read the paper? It was an Air Force project to develop an air-to-air projectile in a case telescoped configuration conforming to the GAU-7 chamber design.

took me a minute to catch up

thx t
 

Tony Williams

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I've noticed that some of the photos I posted have disappeared, or been replaced so that the captions are no longer accurate. My web address is now http://quarryhs.co.uk and my ammo photo gallery is here: http://quarryhs.co.uk/tankammo.html
Here's a pic from my photo gallery showing some unconventional auto cannon ammunition (all from the USA):



From left to right: 20x87 caseless, 20x171 Dardick Tround, 20x140 Cased-Telescoped, 25x145 neckless part-polymer case, 25x152 Cased-Telescoped Combustible case for GAU-7 aircraft gun (with projectile alongside), 30x166 AMCAWS tapered CT, 30x152 Hughes Lockless, 30x200 Folded
 

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Was the 20x140mm cased telescoped round what was intended for the ATF early in the program before they went back to the Vulcan?
 

Tony Williams

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Was the 20x140mm cased telescoped round what was intended for the ATF early in the program before they went back to the Vulcan?
These are my notes on it:

  • 20 mm AGT (1982-1992): USAF Advanced Gun Technology: 20 x 140, 42 mm diameter; total weight 453 g; propellant 112 g, projectile 90.7 g; MV 1,500 m/s, ME 102,000 J.
 

Colonial-Marine

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These are my notes on it:
  • 20 mm AGT (1982-1992): USAF Advanced Gun Technology: 20 x 140, 42 mm diameter; total weight 453 g; propellant 112 g, projectile 90.7 g; MV 1,500 m/s, ME 102,000 J.
Thanks, I can't recall reading the source but I'm fairly certain that Advanced Gun Technology program was related to the ATF at some point. I wonder if it was a direct evolution of the 25mm ammo used in the GAU-7.
 
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Basil

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Tony,

I know it is off-topic but: in your book "Flying Guns - The Modern Era" you mention a US Air Force project of the early 1950s for a six-barrel rotary aircraft gun fed by liquid propellants, calibre 53 mm (or 52 mm?) with a high cadence, intended for arming interceptors. Is there any more information available regarding this development?
 

Tony Williams

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Incidentally, there was one conventional follow-up to the GAU-7/A, intended to replicate its performance while avoiding the technical problems. This was based on a necked-down and shortened 30 x 173 round from the GAU-8/A (25 x c.140 - there were various case lengths between 136-144 mm) with the gun based on the GAU-8/A with one less barrel, but speeded up. The ballistic performance matched the GAU-7/A, as did the rate of fire, but the gun was bulkier and heavier, and the ammo was heavier too. In the end it was dropped due to lack of time - by then, the F-15 needed a gun now!
 

marauder2048

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Thanks, I can't recall reading the source but I'm fairly certain that Advanced Gun Technology program was related to the ATF at some point. I wonder if it was a direct evolution of the 25mm ammo used in the GAU-7.
The AGT projectile was boom tailed; were any of the GAU-7 projectiles even boat tailed?
 

Tony Williams

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The AGT projectile was boom tailed; were any of the GAU-7 projectiles even boat tailed?
Not that I'm aware of. Given the relatively short range of an aircraft gun, it makes more sense (within any given projectile length) to make the nose long and finely-pointed rather than combining a boat tail with a shorter nose. This will deliver better ballistics at very high velocities.
 

Tony Williams

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There is one aspect of the GAU-7/A programme, compared with the GAU-8/A, which puzzles me. For the GAU-8/A, the USAF ran a parallel programme with an existing weapon (the Oerlikon KCA, designated GAU-9/A) as a back-up in case anything went wrong with the GAU-8/A. It wasn't needed - not surprising as the GAU-8/A was quite conventional.

However, the GAU-7/A, despite being much more technologically advanced with various untried features, had no conventional back-up. Big mistake! The 25 mm cut-down version of the GAU-8/A could easily and cheaply have been developed in parallel with the GAU-7/A, but by the time it was realised that the GAU-7/A's problems were never going to be resolved, it was too late to start work on a conventional replacement.
 

marauder2048

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The AGT projectile was boom tailed; were any of the GAU-7 projectiles even boat tailed?
Not that I'm aware of. Given the relatively short range of an aircraft gun, it makes more sense (within any given projectile length) to make the nose long and finely-pointed rather than combining a boat tail with a shorter nose. This will deliver better ballistics at very high velocities.

This doesn't make sense to me.

A. practically all medium caliber A2A projectiles are either boom tailed or boat tailed
B. The goal of both GAU-7 and AGT was range increase and the latter projectile was boom tailed

Boat tailing and boom tailing do make the round more difficult to manufacture and
subjects the round to greater stresses at points where you don't necessary want it.
 

sferrin

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The AGT projectile was boom tailed; were any of the GAU-7 projectiles even boat tailed?
Not that I'm aware of. Given the relatively short range of an aircraft gun, it makes more sense (within any given projectile length) to make the nose long and finely-pointed rather than combining a boat tail with a shorter nose. This will deliver better ballistics at very high velocities.

This doesn't make sense to me.

A. practically all medium caliber A2A projectiles are either boom tailed or boat tailed
B. The goal of both GAU-7 and AGT was range increase and the latter projectile was boom tailed

Boat tailing and boom tailing do make the round more difficult to manufacture and
subjects the round to greater stresses at points where you don't necessary want it.

"Boom" tail?
 

sferrin

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Interesting. I'm not sure I've ever seen ones like that before. Had never even heard the term.
 

Tony Williams

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In the examples I've looked at, the "boom tail" is there to house a tracer rather than for any aerodynamic reasons.

At high velocities it is the nose shape that is the critical element - the more sharply-pointed, the better the aerodynamics. The benefits of a boat-tail are mostly felt at lower velocities (and it becomes extremely important when the projectile drops to subsonic speed).

By comparison with the maximum ballistic range of cannon ammunition (typically around 8,000 m), air-combat gunfighting distances are very short at 1,000-2,000 m. The hit probability is directly linked to the time of flight, a product of the initial velocity and the velocity loss through aerodynamic drag. So it makes sense to use relatively light projectiles (maximum velocity) which are sharply pointed (minimum high-speed drag), which describes the GAU-7 projectiles very well. If you are primarily interested in attacking ground or naval targets, then time of flight is less critical and a heavier, boat-tailed projectile will give you a longer effective range. Aircraft guns tend to be used mostly for ground attack these days, so the projectiles are "general purpose" rather than optimised for aerial combat.
 

marauder2048

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In the examples I've looked at, the "boom tail" is there to house a tracer rather than for any aerodynamic reasons.
Or to hold a fumer for use as a base bleeder. Boat tails remain effective into the hypersonic but
at a loss of gyroscopic and dynamic stability (both vital to aerial gunnery) which boom tails help to address.

At high velocities it is the nose shape that is the critical element - the more sharply-pointed, the better the aerodynamics.
No argument there but the practical thermal and structural realities of sharply pointed front-ends
interacting with powder guns, breeches and rifled barrels limit what's doable on the front-end.

So you have to chase some other drag reduction methods.

By comparison with the maximum ballistic range of cannon ammunition (typically around 8,000 m), air-combat gunfighting distances are very short at 1,000-2,000 m. The hit probability is directly linked to the time of flight, a product of the initial velocity and the velocity loss through aerodynamic drag. So it makes sense to use relatively light projectiles (maximum velocity) which are sharply pointed (minimum high-speed drag), which describes the GAU-7 projectiles very well.
This is completely backwards; at the ranges you indicate you are beyond the crossover point where initial velocity due to
lighter weight is advantageous for time-and-flight and in the realm where the heavier projectile is desirable due to the better ballistic coefficient.

The baseline GAU-7 projectiles were ~200 grams which isn't relatively lightweight. There were lighter versions intended
for air-to-air but designed for shorter ranges that what's indicated above. And even there the boat tailed version
had the shortest time-of-flight but didn't mean the gyroscope or dynamic stability reqs.

And as it turns out that, according to Davis, the later GAU-7 round (the GE/Avco round) had a boat tail.
 
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Tony Williams

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A few observations:

Yes, boat-tails do provide some drag-reducing benefit at high velocities so most projectiles have them, but their contribution to drag reduction is nowhere near as much as at lower velocities. Base bleed/fumers also reduce drag, and tracers can have such an effect.

If your argument about the benefits of boom tails is correct, then logically projectiles will be fitted with them regardless of whether or not they contain tracers. I have just trawled through the auto cannon section of a copy of Jane's Ammunition Handbook and found multiple pictures of sectioned projectiles with "boom tails", but in every case except one the tail is occupied by a tracer (described as a tracer/base bleed in one case). The exception is a TP round with an optional tracer filling. The picture below shows a typical example:

614600

Perhaps the most significant example concerns Nammo's 35 mm HEI and HEIT (see below): the HEI-T has a "boom tail" with a tracer, the HEI does not:

614601

Incidentally, not all modern projectiles feature boat-tails or boom tails, even with a tracer - see below:

614605

I have just weighed the standard GAU-7 projectile in my collection and it is 192 g. I only know of one 25 mm service projectile which is lighter than this, and that is the 25 x 137 HEI-T which is 180-184 g. Other post-WW2 HE projectiles are heavier: Oerlikon KBB = 230 g; Chinese PG87 = 250 g; Soviet 25 mm naval = 280 g. So I think I am justified in describing the GAU-7's projectile as relatively light.

You will see a reference in Davis to experimental 20 x 102 loadings with modified projectile characteristics which are relevant to this. These are my notes:

The original USAF ammunition was the M50 series which has relatively light projectiles of indifferent aerodynamic shape intended for high-altitude combat (projectile weights are c.100 g and muzzle velocities are 1,030-1,050 m/s); such projectiles lose velocity quickly in the denser air of lower altitudes. The Air Force Armament Laboratory investigated new loadings in the late 1960s; one was a ground-attack series using heavier as well as better-shaped projectiles to maximise velocity retention and armour penetration (136 g at 878 m/s, or 162 g at 808 m/s with a heavy-metal core). The other was optimised for low-altitude aerial combat and featured better-shaped but lighter projectiles to maximise hit probability (PGU-17/B HEI and PGU-18B TP); the target ballistics were a weight of 78-84 g at an MV of 1,130-1,160 m/s. The requirements for both of these variants were dropped.
 

marauder2048

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A few observations:

Yes, boat-tails do provide some drag-reducing benefit at high velocities so most projectiles have them, but their contribution to drag reduction is nowhere near as much as at lower velocities. Base bleed/fumers also reduce drag, and tracers can have such an effect.

If your argument about the benefits of boom tails is correct, then logically projectiles will be fitted with them regardless of whether or not they contain tracers.
Sure. They are.


614797




Last I checked, the Nammo 35mm and 30mm MPLD were all intended for ground-based auto-cannon where the dynamic and gyroscopic stability
reqs are far more relaxed than for aerial cannon.

I have just weighed the standard GAU-7 projectile in my collection and it is 192 g. I only know of one 25 mm service projectile which is lighter than this, and that is the 25 x 137 HEI-T which is 180-184 g. Other post-WW2 HE projectiles are heavier: Oerlikon KBB = 230 g; Chinese PG87 = 250 g; Soviet 25 mm naval = 280 g. So I think I am justified in describing the GAU-7's projectile as relatively light.
Again, aside from 25 x 137, were any of the other post-WW2 HE projectiles intended for aerial cannon?

You will see a reference in Davis to experimental 20 x 102 loadings with modified projectile characteristics which are relevant to this.
I'm not sure it's relevant; these rounds were intended for shorter ranges than what you indicated.
 

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The design of the PGU-46B is made clear by its title: "High explosive incendiary with inert tracer". The projectile was obviously designed to take a tracer if required, and that is what the "boom tail" is for.

I had never come across any reference to a "boom tail" other than your posts in this thread, and I've been involved in studying cannon ammunition for a long time. Could you please provide a reference to support your belief that "boom tails" are added to projectiles for aerodynamic reasons?
 

marauder2048

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The design of the PGU-46B is made clear by its title: "High explosive incendiary with inert tracer". The projectile was obviously designed to take a tracer if required, and that is what the "boom tail" is for.
The parent projectile is the mk266 HEI-T which has a tracer. Your challenge was to find a cartridge that didn't have the tracer
but retained the tail. What's the difference between them? The PGU-46B is intended for aerial cannon.

I had never come across any reference to a "boom tail" other than your posts in this thread, and I've been involved in studying cannon ammunition for a long time.
Could you please provide a reference to support your belief that "boom tails" are added to projectiles for aerodynamic reasons?

https://apps.dtic.mil/dtic/tr/fulltext/u2/b008388.pdf
https://apps.dtic.mil/dtic/tr/fulltext/u2/a160003.pdf (presumably AGT was interested in drag reduction)
 

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Thank you for the references, most interesting. Surprising that Davis said nothing about them in his book.

I am also surprised that (as far as I could see on a quick skim-through) nothing is said about the aerodynamic effect of tracers in the booms, since most of them have them. It also seems that most of the "tracer booms" have different proportions to the solid booms tested (see below - note that the examples shown are for AA uses as well as aircraft), being wider, shorter and tapered. That makes me wonder whether there really is any connection between them, or whether it's a coincidence. I am not convinced so far that there is evidence that specifically designed boom tails as tested ever got out of the lab and into production.

614872

614873

614874
614875
 

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A quick question, if I may?

How long before the tracer burns out in these "boom tail" rounds?
 

Tony Williams

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How long before the tracer burns out in these "boom tail" rounds?
A few examples of tracers in cannon HE-T rounds:

25 x 137 = 2,000 m (c.3.5 secs); Multipurpose heat relay SD 5.3 secs; MK210 6 secs

30 x 173 = >3,000 m for Nammo Multipurpose; 4 secs for HE-T

35 x 228: min 5 secs (3,100 m); Denel 6 secs.
 
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