120mm Smoothbore. Why?


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Coronach
March 30, 2003, 12:54 AM
OK, here is my question. Why is the main gun on the M1 Abrams smoothbore? I know that someone here is qualified to answer this.

Thanks,
Mike

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Blackhawk
March 30, 2003, 12:56 AM
The round it fires is not spin stabilized. Got fins...? :D

Tamara
March 30, 2003, 01:02 AM
Rifling limits the velocity of the projectile.

To get up to the speeds required to punch through modern armor, there's a good chance you'd rip the driving bands right off the projo.

A side effect is that a smoothbore gun is vastly better for firing shaped-charge projectiles. Spin from rifling degrades the plasma jet from the charge.

WilderBill
March 30, 2003, 01:46 AM
As usual Tamara nailed it. Who needs an encyclopedia when she's around? :)
Another reason is that when they started looking around for a main gun with more punch than the 105, they found one already developed and in production in Germany and that's the way they were made there.

dude
March 30, 2003, 01:46 AM
you can't get the same tank-killing power & loooong range from a rifled barrel............the APFSDS round has fins (the 'FS' part) on the AP depleted uranium sub-munition to stabilize the flight after the sabots have fallen away

don't know what was done to make the 'other' rounds work in the smoothbore

mons meg
March 30, 2003, 01:49 AM
So, what exactly is the velocity of the APFSDS projectile? Someone know off hand? It would be interesting to compare, say, to the 155mm M198 howitzer, which maxes out at about 2200-2400fps if I remember my TFT conversions correctly.


Edit: never had any complaints about the tank-killing power of a 95lb HE round....problem was hitting the little buggers without LOS. :D

Jim March
March 30, 2003, 02:06 AM
My understanding is that the round comes out of a smoothbore for the velocity reasons stated, then sabots fall away revealing stabilizing fins on the main projectile that get it spinning plenty fast enough.

dude
March 30, 2003, 02:09 AM
no spin at all.......the round 'glides'

Jim March
March 30, 2003, 02:49 AM
Wait...they don't spin?

http://www.globalsecurity.org/military/systems/munitions/m829a1.htm

They sure got fins:

http://www.globalsecurity.org/military/systems/munitions/images/m829-3.jpg

Oh...dang, you're right! I assumed the fins were set up to spin, but they ain't!

:uhoh:

See also:

http://www.cfis.org/ubb/Forum10/HTML/000730.html

Justin
March 30, 2003, 03:16 AM
Isn't there also a problem with large-diameter rifled barrels wearing out faster than smoothbores as well?

BTW, I was reading an article at msnbc.com that made reference to the use of the M1's main gun in a strange way.
They would fire a round down a street, and the pressure difference created by the speed of the round would basically pull Iraqi soldiers out from behind cover.

Any truth to that?

Feanaro
March 30, 2003, 03:17 AM
I somehow doubt the round would "pull" an Iraqi out of cover, unless he ran out of his own will.

Justin
March 30, 2003, 03:24 AM
Link to article (http://www.msnbc.com/news/892606.asp?0cv=CA00) In the opening battle, the tank unit fired two 120 mm high velocity depleted uranium rounds straight down the main road, creating a powerful vacuum that literally sucked guerrillas out from their hideaways into the street, where they were shot down by small arms fire or run over by the tanks. This is, of course, coming from the media, and they're about as competent at understanding weapons as I am at understanding Quantum Physics. But, I was reading some military training manuals that warned soldiers to keep a pretty far distance from the main gun when it went off because the pressure would be enough to kill you at short distances.:confused:

Feanaro
March 30, 2003, 03:28 AM
I'm slow to trust the media, but it might be true. Truth is, after all, stranger than fiction.

Detritus
March 30, 2003, 06:10 AM
note: this is a theory on my part, it makes sense to me. this does NOT mean it is in any way correct. and knowing this i demurr to those who understand physics better than me

that quote sounds more like a reporter watching the results of other shockwave effects and not understanding what's happening.

the speeding projectile DOES have a shockwave associated with it's flight. adn where there is a shockwave there is a void (the "trough of the "wave") imediately behind. but it wold take a heck of a shockwave to suck a guy out of a hole. (more on this in a sec)


Now if Said "troops in hiding" are behind a barricade that like many in an urban battel enviroment ends in, or spurs off of a wall, with an open end/side toward the street. and the round fromteh tank passes down the street at the right height. then the expanding shockwave, could be bouncing off of the walls of the buildings (interior and/or exterior depending on size of openings) the pressurewave bounces back just like a wave ont eh ocean from a seawall. if a troop is situated properly then the rebounding force could possibly propel him from behind his over.

or maybe the rounds ARE going down range fast enough to cause overpressure effects?? but if i remember right you'd need a pressure wave on the order of that put out by a nearmiss from a 105mm HE shell.


ok guys you;ll have to excuse my ramblings, it 5am and i am suffering a little insomnia. so the above is just that, Ramblings.

Feanaro
March 30, 2003, 07:02 AM
I too suffer insomnia. :(

Anyways, I don't think the round is quite big enough or fast enough to pull people, especially the big tall military types out of cover. It's possible, but anything is possible. I think you'd need a bigger round that traveled faster.

Could be wrong though. DU could be a human magnet or something. ;)

BTW, what about the shockwave hitting the soldier? Wouldn't that throw him deeper into cover, negating the throw of the rebouding shockwaves?

EMDII
March 30, 2003, 08:04 AM
MV of an M829A1 APFSFS-T round goes about 1640 m/s, or 5380 fps, over a mile per second.
http://www.army-technology.com/contractors/ammunition/apfsds02.jpg


The M829A3, nearing completion of fielding, is over 100 m/s faster.

The physics of 'shock waves' from hypersonic projectiles doesn't support all the conjecture here. Stick w/some reality folks. 'Lift people out of their cover' :neener: indeed. Air has mass. If a HV round passed close enough to me, the aerodynamic effect would be to push me away, as the bullet has displaced the air to its front and sides in a hypersonic shock wave traveling, oh, a bit faster than the speed of sound. Physics, people. Engage brain, leave mediatainment behind.

Troops in MOUT (military operations in urban terrain) will most likely load HEAT ammunition, as it is most effective against structures and LAVs which will be encountered inside the cities. The HEAT round of an M1 will kill a T72M1 quite nicely, at ANY range.

hksw
March 30, 2003, 08:32 AM
Concerning the round pulling a vacuum and sucking the combatants out of their hidding places, IMO I think, as another poster mentioned, is not caused by the round.

The muzzle blast of the gun is an explosion much like any explosion like a bomb (without the shrapnel). It would be so great that I have no doubt the concussive force would be enough to kill when positioned close to it in front of the muzzle. And, like an explosive, there will be an instant increase in air pressure. That ball of increased pressure would probably create a vacuum as it passed forward but it would be behind the blast, which would have seriously injured or killed anyone nearby.

A little farther down past the direct effects of the blast, the shockwave from the 120 mm would be enough to knock you around. If someone were next to a rigid surface behind cover but far enough away from the direct blast, the shockwave would probably knock him against the surface and he would bounce back out from the cover. The surface probably would also have some vibrational movement from the wave also to help propel the guy away from cover.

Even farther down where the shockwave loses force, you have the effects of the round. IMO, the crosssection of the round and the sonic wave it produces is too small to pull anyone out of their hiding place. As an illustration, suppose you stand next to the curb and a bus comes by. The crosssection of the bus is so great the the air it displaces first pushes you away then the slight vancuum as the bus passes pulls you back in. Now if a motorcyclist were to drive by (well behind the bus after the effects of the air disturbance from the bus have dissipated), the effects are much less.

All of this is, of course, IMO. I wounder how long it will take for the GSC to start saying the XXX caliber is so powerfull it will pull badguys out of there cover when the round goes by him by simple vacuum.

Feanaro
March 30, 2003, 08:36 AM
Engage brain, leave mediatainment behind.

*engages brain, rolls over EMDII*

The whole thing seems to be a pot dream to me, but as a writer once said, only fiction has to make sense.

Bulldozer
March 30, 2003, 08:46 AM
Would someone please explain to me why the Brits are intent on using rifled 120mm main guns when the rest of the world is using smoothbore? What is there rationale? Does it have something to do withy their love affair with the HESH round?

Feanaro
March 30, 2003, 08:47 AM
It could be because they are.... well, British. :)

EMDII
March 30, 2003, 09:24 AM
.....What is there rationale? Does it have something to do withy their love affair with the HESH round?

That is how it was explained to me by a CR1 crew and boffin in FRG back in 1997. The HESH uses the spin to achieve some uniform 'spread', and bang! PIBD (point-initiated, base-detonated) round BTW.

SABOT rounds in rifled cannon require bands that essentially null, to a large extent, the spin. Also minimizes spin-drift.

Apple a Day
March 30, 2003, 09:55 AM
the round won't pull the guerillas out from behind cover but I bet it'll pull the brown stuff out into their BDUs. :what:

Tamara
March 30, 2003, 09:58 AM
Because from the 17-pounder to the L7, the Brits were the Official Tank Gun Designers of Western armies. Now, with the smootbore revolution of the Rheinmetall/M256, even the froggies new GIAT-designed 120mm is a smoothbore and will run NATO standard ammo. The Brits are throwing a snit fit. (This is a country that uses the SA80 instead of the M-16 or G36 and has the only MICV without an ATGW, rather than buying foreign gear.)

EOD Guy
March 30, 2003, 10:32 AM
That is how it was explained to me by a CR1 crew and boffin in FRG back in 1997. The HESH uses the spin to achieve some uniform 'spread', and bang! PIBD (point-initiated, base-detonated) round BTW.

The HESH (high explosive squash head) projectile does not use a PIBD fuze, it uses a straight base detonating fuze. PIBD fuzes are used in HEAT projectiles.

Rich

Hand_Rifle_Guy
March 30, 2003, 12:56 PM
As I understand it, Kinetic darts are too long to spin-stabilize. If a projectile's length exceeds something like five times it's diameter, spinning it will induce a destabilizing yaw.

Longer projectiles require faster spin, until a point of diminishing returns is reached: Kinetic darts would have to be spun faster than rifling and velocity can conveniently accomodate, and then of course the yaw factor would corkscrew the round off-target.

I THINK...

I can't remember where I read this, it was way back when the Abrams was being introduced. I hope I'm not confusing the issue, but I believe the length-to-diameter bit I got from an old ballistics text brochure produced by the gubmint in the fifties.

coonan357
March 30, 2003, 02:41 PM
and all this time I though it was because the fine folks at barsto said " you want us to rifle a WHAT???":D

EMDII
March 30, 2003, 03:07 PM
The HESH (high explosive squash head) projectile does not use a PIBD fuze, it uses a straight base detonating fuze. PIBD fuzes are used in HEAT projectiles.

Rich

!:o

I am corrected, and THANKS! Isn't HESH similar to our old HEP round? Can you tell I used to live on an M60? But I did graduate as far as the M1A1!
:)

Me, I'm waiting for X-Rod. As far as spin/length goes, the problem probably comes from initial effortsto get a clean release. A FS round separates nicely from its bourrelettes (petals), and so retains ballistic form/truth. A spin-stabilized round has not nasties in it's physics when using LRPs (Long Rod Penetrators). Suffice it to say FINS work!

Tamara
March 30, 2003, 04:54 PM
The problem may come from finding an optimal twist rate to stabilize both long/dense long-rod penetrators and short/less-dense HEAT rounds, no? Fins stabilize anything...

Thumper
March 30, 2003, 05:24 PM
Fins stabilize anything...

Pshaw...tell that to the Ruskies buried on the Karelian isthmus.

:D

Frohickey
March 30, 2003, 05:50 PM
The fins are for stability, not for spinning the projectile. Its really there to add a little drag to the projectile so that the front stays at front, and the rear stays at the rear (no tumbling)

Now, when Rheinmetal making the 120mm smoothbore, will the US start making its own, since Rheinmetal is a German company?

Detritus
March 30, 2003, 09:25 PM
Isn't HESH similar to our old HEP

not a tanker, but the sources i can find say No.

HESH is still an Anti-armor round, HEP (if you;re referring to High Explosive Plastiqe, the one that is basicly a semi-aerodynamic chunk of C4??) is a soft target, or breaching round.

EMDII
March 31, 2003, 05:25 AM
Now, when Rheinmetal making the 120mm smoothbore, will the US start making its own, since Rheinmetal is a German company?

We've been making our own M256 cannon for over 15 years. the gun is a German design, but no longer an import.

HEP was used as an anti-armor round (just not the primary one) and yes it is a large flying blob of C4. The detonation caused large scabs and spalling to occur on the inside of the AFV. Not pretty.

Now I'm back on the PIBD issue:
High Explosive Squash-Head. Use plastic explosives and a ballistically capped round. The round strikes a hard surface and the cap flattens the round evenly onto the surface. A piezoelectric crystal in the base shatters on striking the armor surface, detonating the plastic explosive. Really effective against armor, but not so effective against soft-skinned targets.

And this from the Indian MBT Arjun technical data:
High Explosive Squash Head (HESH) is the secondary ammunition for Arjun and is effective against a variety of soft targets, tanks, fortifications, etc. The complete HESH round consists of a semi-combustible cartridge case, primer with steel obturating cup, and a single-base propellant. The optimised explosive composition of HESH defeats rolled homogenous armour plate detaching a scab of about 9 kg mass moving with a velocity of 100 to 120 m/s. Besides the scabbing effect, blast and shock imparts a tremendous jolt to the enemy tank stripping off explosive reactive armour and incapacitating the crew severely, thereby affecting their fighting capabilities. The accuracy of the HESH is of the order of 0.25 mil standard deviation.

And this from a monologue on Soviet (sic) Armor-defeating rounds:
'base-detonating fuze"

Ah well,I won't wonder at my confusion: it's been 15+ years since I had to fire a HEP round. Seems as if only HEAT used the piezo-electric PIBD fuze. Tada! And my last SABOT and HEAT were fired in 1997. I never got to crawl on the new Challenger 2. Interesting how the Brits apparently treat this as a common Anti-Armor round, where we Americans use the APFSDS-T.

EOD Guy
March 31, 2003, 08:58 AM
I am corrected, and THANKS! Isn't HESH similar to our old HEP round? Can you tell I used to live on an M60? But I did graduate as far as the M1A1!

not a tanker, but the sources i can find say No.

HEP (high explosive plastic) rounds and HESH round are exactly the same thing. HESH is British nomenclature and HEP is US nomenclature.

EOD Guy
March 31, 2003, 04:21 PM
High Explosive Squash-Head. Use plastic explosives and a ballistically capped round. The round strikes
a hard surface and the cap flattens the round evenly onto the surface. A piezoelectric crystal in the
base shatters on striking the armor surface, detonating the plastic explosive. Really effective against
armor, but not so effective against soft-skinned targets.

Unless something new was developed since I left the service, piezoelectric elements were not used in base detonating fuzes. The normal application was as the point initiating element for an electical base detonating element in PIBD fuzing. The piezoelectric crystal generated an electrical charge when stressed by impact. The electrical charge was transmitted to the base element via a wire.

PIBD fuzing is used in HEAT (shaped charges) rounds, not in HEP (HESH) projectiles. HEP Projectiles used conventional striker/detonator base fuzing.

Rich

EMDII
March 31, 2003, 07:26 PM
Gracias!

The several articles I read on HESH said only BD IIRC. I recall clearly the LIBD arrangement on HEAT from AOBC classes, and even earlier when I took 11E class in 1970.

They (and their surrogates AND the Russians) still use this as an anti-armor round. Weird. I guess we make interesting Silver Bullets, too.

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