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90mm Gun Motor Carriage M18


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The_Chieftain #1 Posted Nov 03 2014 - 22:11

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There is a somewhat tantalizing amount of information on the conversion of an M18 Gun Motor Carriage in mid-1945 to mate with the turret of the 90mm Gun Motor Carriage M36 in Hunnicutt’s Stuart, but it doesn’t really go very much into the background. I’m going to fill in a few gaps.

The Tank Destroyer Board gave the matter some fairly serious consideration at the beginning of the year. Army Ground Forces had asked Tank Destroyer Branch to look into the effects of having TDs with bigger guns and heavier armor. The response was dated 18th January 45. Although the study actually looked into more depth in general configuration (Responding that if a TD was created to 12th Army’s requirements of guns, machineguns, armor, you’d effectively have a tank), the following was pertinent to the upgunning proposal. 

To convert the present considerable stock of unassigned M18s to 90mm vehicles would require, after development, experiment and test has been completed, an unpredictable number of months. This would tie up this stock of an admittedly efficient weapon. The weapon resulting from the change would have, with present ammunition, about 1” greater armor penetration in the zone at which dependable sighting against tank targets is possible – out to about 3,400 yards. This additional penetration will not include the front plate of enemy heavy tanks. These front plates, now penetrable by the 76mm gun Hi-Velocity, T4 round out to 1,200 yards, would be penetrable by the 90mm with the same type round at ranges above 2,000 yards, possibly to the limit of the hitting ranges mentioned.

No change in the M18 is necessary to make it destructive to the enemy’s heavy tanks through hits on any part other than the front plate – Combat reports indicate its full capabilities in this respect demonstrated by actual combat.

The advisability of tying up completed, capable vehicles actually ready for combat assignment appears questionable. Further increase in the enemy’s frontal armor in new models might readily defeat the 90mm gun. New construction to effect a caliber increase should be carefully considered before a change is decided upon; also, the full possibilities of penetration of the frontal armor of the Panther and Tiger by the 76mm gun using still higher velocity projectiles, either half-weight or sabot, should be exhausted before the present M18 is radically modified.

However, in anticipation of exigencies as the war continues and of possible resumption of manufacture of the M18, which is a proven vehicle with no counterpart as a combination of fire-power and mobility, it is believed advisable that steps be taken now to explore from the design viewpoint the possibilities of replacing the 76mm gun on the M18 with the 90mm caliber, especially any improved versions of that caliber.

So, in other words, TD Branch didn’t think converting the M18s was a good idea at the time, unless the Germans came out with new vehicles they hadn’t encountered yet.

As a result of this submission, by the end of February 1945, Army Ground Forces sent a request to Army Service Forces a study to mount a 90mm gun onto M18. (Interestingly, with a coaxial. .50 MG machinegun)

By June 1945, the lads at Aberdeen had, for the sake of the experiment, managed to put the M36 turret onto the M18’s hull. The turret floor had to be lifted 2” by cutting and shortening the supports, the slip ring had to suffer a similar modification. The raising of the hull floor seems to have affected the range of elevation, as the mounting allowed only 17 degrees as opposed to the 20 as we believe would be found on an M36. In order to get any of the driver’s hatches to open or close at all in all conditions, a couple of inches needed to be shaved off the small doors, the larger doors remained unable to be opened if the turret happened to be in certain positions. The converter cooler blower’s discharge air vent needed to be cut back a bit to prevent interference. The vehicle weighed 43,075lbs without stowage, about 3,000lbs more than a standard M18.

The first thing they did, after the obligatory photographs and measuring, was to drive the vehicle around for a bit. Aberdeen reported “[T]he performance of the vehicle was not noticeably affected by this small increase in weight; however, due to the shift in the center of gravity, the rear of the vehicle rode lower than the front.”

Then it came time for live fire.

This firing indicated very definitely that the muzzle brake should be used. Two shot groups consisting of five shots each at 1,000 yards range were fired with the muzzle brake installed. One group fired to the front of the vehicle was 12” x 7”, the other, fired sideways, was 10” x 10” [Chieftain’s note: M36 GMC would expect about half that at that range]. In firing to the front with the brakes off, it was noted that the vehicle only moved backward ¾” or less. In firing without the muzzle brake the vehicle rolled backwards 22” when fired to the front, and rocked excessively when fired to the side.

The testing was not over yet, however, it was time for the endurance tests. They scraped up a travel lock from an M26 to hold the gun in place and put it onto the rear of the vehicle, and given the “considerable” increase in ground pressure caused by the heavier turret, they decided to put on the 21” wide T82 tracks which had been, unfortunately, rather well used in earlier tests. This required new sprockets and idlers be mounted.

They got about 25 miles into the 1,000 mile test before the track fell apart. They reinstalled the original running gear, and continued on. Incidentally, in a later attempt to create a more reliable 21” track than T82, they developed the T86 and tested that on an M18 as well, see image below. That test, undertaken in early 1947, indicated that the wider tracks showed more rolling resistance and less hill-climbing ability than the standard T69 track, but the whole project was cancelled by Army Ground Forces in April 1947 before full testing was completed as there didn’t seem to be any point in expending the effort.

The vehicle passed the endurance test fairly handily, the only significant problems being that the volute springs kept being knocked off or damaged, presumably the heavier weight of the vehicle causing more significant impacts resulting from higher motion of the roadwheel arms. Final damage tally in 1,000 miles was three springs, one thrown track, and one link somehow bent 90 degrees about 3" in from the end.

Once the vehicle was considered generally mechanically capable, it was to be sent off to Armored Board (Not TD Board, curiously) for further tests. If it ever made it that far, I have not yet encountered the records.

It was considered that in the event that the modified vehicle ever made it into production, or more likely, conversion in the field, the ammunition stowage would be provided for some 50 rounds. This would have involved the removal of the assistant driver and installation of ammunition racks. Overall, it was concluded that the conversion could be done in the field if required.

Of course, in the final analysis, none of this ever happened.

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ThunderHead47 #2 Posted Nov 08 2014 - 16:08

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Chieftain, thank you once again for your posts. Your posts and videos are one of the primary draws for me to WoT.

Regarding your note that the M36 GMC could put 5 round groups into 5" at 1000yds: this seems like amazing accuracy. That said, my studies have always concentrated on Infantry and Aerial combat, so I need to recalibrate. Is that kind of accuracy representative for a WW2 era main gun, or is that more accurate than most?  [Edit:  considering 75mm to 105mm rifled main guns]

Thanks,

Daigensui #3 Posted Nov 08 2014 - 16:12

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Interesting interesting.

Silvers_ #4 Posted Nov 08 2014 - 16:53

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hmm go figure....where are all those whiners who complained the Kitty never had a 90mm? seems like it did in testing so.... :D

Legiondude #5 Posted Nov 08 2014 - 17:02

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View PostSilveriferno08, on Nov 08 2014 - 09:53, said:

hmm go figure....where are all those whiners who complained the Kitty never had a 90mm? seems like it did in testing so.... :D

They're grumbling over on the War Thunder forums



CaptianNemo_VA_ #6 Posted Nov 08 2014 - 17:41

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View PostSilveriferno08, on Nov 08 2014 - 07:53, said:

hmm go figure....where are all those whiners who complained the Kitty never had a 90mm? seems like it did in testing so.... :D

 

The argument for the M18 having the 90mm is about as good as the argument for it having the 105mm. And it still does not have the 105mm. Partly because of that, it can be argued that the M18 does not need, or should not have, the 90mm on it in game.

 

 



1SLUGGO1 #7 Posted Nov 08 2014 - 17:42

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How did it affect the traverse of the turret with the increased weight?

Spacecomber #8 Posted Nov 08 2014 - 18:22

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I was a little confused by the context for this discussion of mounting a 90mm on the Hellcat, mostly in regard to the M36 Slugger, which already was being produced with the 90mm.  I gathered that the main issue was whether the existing Hellcats should be upgraded to the 90 mm.  (I wouldn't think that there would be a need to produce new Hellcats with this gun, if the Slugger was already in production with it.)  I was a bit surprised that there didn't seem to be any comparisons between the existing Hellcats and Sluggers in the discussion by the Army developers/testers of the desirability of putting a 90 mm on the Hellcat.  If the Slugger was proving itself as the more effective vehicle, I'd think that you'd have an argument for upgrading the Hellcat.  Anyway, it just seems that with two tank destroyers deployed in the field at the same time, there would have been very useful information available about what the relative strengths and weaknesses of each model were.

RiClassHeavyCruiser #9 Posted Nov 08 2014 - 18:43

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Fantastic article. Makes me want to pet my virtual Hellcat~.

Captain_Dorja #10 Posted Nov 08 2014 - 19:28

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Great read Mr. Chieftain. Linked to my friend John, a strong supporter of the M18 Hellcat.

The only thing better than this sort of thing is the Inside the Tank videos.

Life_In_Black #11 Posted Nov 08 2014 - 19:34

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View PostSpacecomber, on Nov 08 2014 - 12:22, said:

I was a little confused by the context for this discussion of mounting a 90mm on the Hellcat, mostly in regard to the M36 Slugger, which already was being produced with the 90mm.  I gathered that the main issue was whether the existing Hellcats should be upgraded to the 90 mm.  (I wouldn't think that there would be a need to produce new Hellcats with this gun, if the Slugger was already in production with it.)  I was a bit surprised that there didn't seem to be any comparisons between the existing Hellcats and Sluggers in the discussion by the Army developers/testers of the desirability of putting a 90 mm on the Hellcat.  If the Slugger was proving itself as the more effective vehicle, I'd think that you'd have an argument for upgrading the Hellcat.  Anyway, it just seems that with two tank destroyers deployed in the field at the same time, there would have been very useful information available about what the relative strengths and weaknesses of each model were.

 

It was never called Slugger.

feanval #12 Posted Nov 08 2014 - 20:04

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Hmm... up gunned M18 still retained more less same driving characteristics which brings back the ingame Hellcat speed discussion back. Devs already gave us tanks capable of higher speeds than current top speed of M18 question comes back when we will see M18 moving as fast as it should.



Meplat #13 Posted Nov 08 2014 - 20:08

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View Post1SLUGGO1, on Nov 08 2014 - 09:42, said:

How did it affect the traverse of the turret with the increased weight?

 

Probably not much at all, both used similar traverse mechanism/systems.

The power traverse in the one I've dealt with is carnival-ride quick, I doubt it'd have an issue with the heavier turret and ordinance.



callmecrazy #14 Posted Nov 08 2014 - 21:59

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View PostLife_In_Black, on Nov 08 2014 - 10:34, said:

It was never called Slugger.

 

I'm not an expert, nor do I play one on tv, but I thought I'd google this as I've seen it referred to as the "Slugger" in other web sites.  Here's a few clips from the indicated web sites.  You decide about their validity.

 

http://www.100thww2..../776combat.html

In early September 1944, the Battalion was relieved from assignment to Fifth Army and ordered to embark from Naples for France, where it would be reassigned to the Seventh Army. Before the 776th left Naples, however, it was reequipped with the most powerful antitank weapon in the Allied inventory: the M36 “Slugger” tank destroyer, mounting a high velocity 90mm main gun.

 

http://www.100thww2....ort/776m36.html

By the time the battalion supported the 100th Infantry Division in January 1945, it was equipped with the M36 “Slugger” tank destroyer. Sluggers carried the most powerful US antitank weapon to see combat in WWII, the M3 90mm gun.

 

http://www.militaryf...sp?armor_id=251

Like other armored American vehicles lacking any sort of imaginative name, the British stepped in to nickname the M36 the "Jackson" after famed American Civil War General "Stonewall Jackson" (consistent with the M5 General Stuart, M3 General Lee/Grant and M4 General Sherman).  To others, it was simply known as the "Slugger".



ket101 #15 Posted Nov 08 2014 - 22:09

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I wonder why they mention volute springs when the Hellcat had torsion bars?

_M18_ #16 Posted Nov 08 2014 - 22:15

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:D my fav thread

callmecrazy #17 Posted Nov 08 2014 - 22:22

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View Post_M18_, on Nov 08 2014 - 13:15, said:

:D my fav thread

 

You're such an attention [edited].  :smile:

 

The best word was edited out, but it's the oldest profession. :)



ramp4ge #18 Posted Nov 08 2014 - 22:57

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View Postket101, on Nov 08 2014 - 13:09, said:

I wonder why they mention volute springs when the Hellcat had torsion bars?

 

It still had them, or some form of shock absorption, on the 1st, 2nd, 4th and 5th roadwheels.

 

T-bars don't necessarily absorb shock. They're just a method of bound and rebound. You still need something to dampen the movement. Even cars/trucks today with torsion bars still have shocks on the suspension component that's being sprung by the torsion bars.

 

I like how the vehicle, even modified and significantly over-weight, still managed to travel 1,000 miles with relatively few problems, and the most major problem being owed to tracks that were worn out before even being put on the vehicle.

 

That's what, nearly twice the distance you could expect to drive a Panther before the transmission took a crap?



Taiho #19 Posted Nov 08 2014 - 23:05

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View Postcallmecrazy, on Nov 08 2014 - 15:59, said:

I'm not an expert, nor do I play one on tv, but I thought I'd google this as I've seen it referred to as the "Slugger" in other web sites.  Here's a few clips from the indicated web sites.  You decide about their validity.

 

In this case, I'd point you to Chieftain's What's in a Name? article. It's shown that the official nickname for the M36 was decided as "General Jackson" by the Army. "Slugger" is not listed in any way. Given that other tanks and equipment have kept either their original nicknames provided by the British or the nicknames in wide use in the field, then it's fairly safe to accept that it was not in wide enough use in the field to matter. I would say "not in use in the field" in there as an alternative, but in the linked thread someone indicates that their grandfather's unit referred to it as "Slugger" (incidentally, it's the same unit that is listed in two of your examples as calling it a Slugger).



Dominatus #20 Posted Nov 09 2014 - 00:10

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View Postramp4ge, on Nov 08 2014 - 16:57, said:

That's what, nearly twice the distance you could expect to drive a Panther before the transmission took a crap?

Closer to 10x, actually.






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