m18 hellcat

Picture 1:The Gun Motor Carriage M18, also known as the Hell Cat, was perhaps the finest US tank destroyer in WWII. It was nimble, fast and mounted apowerful gun, all necessary for the US tank destroyer doctrine of the time: "hit and run". In fact, the Hell Cat utilized the same Continental R-975engine of the medium tanks M3 and M4, yet was less than one half of the loaded combat weight. This drawing will give you a feel for the generallayout and main interior components. In particular, notice how the transfer case just in front of the radial engine lowers the drive shaft under theturret, allowing a lower silhouette. In order to maintain the extremely favorable power to weight ratio armor protection was sacrificed, and an AP30cal MG bullet could penetrate the thin turret. Never the less, Hell Cat crews loved their hot rods, and accounted for many destroyed enemyAFVs in the last two years of the war.

A general examination of the interior is provided in this Part 1. Part 2 contains some photographs taken by Martin Dogger of two Hell Cats thathave been recently advertised for sale at the web site Staman International Trading .

http://panzerfaust.ca/AFV interiors/m18.html

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Picture 2:Most of these scans are from the operator's manual (TM) for the M18. This oneshows the left side of the open topped turret and the gunner's position. In theseearly turrets you will find on the right the M76 direct sight telescope. In thecenter right you can see the hydraulic traverse control handle and to the left themanual traverse handle with palm grip sticking up. The turret lock lever is to thelower left in the picture and just to the right of it, at the bottom of the handtraverse gear housing, is another small bar handle, this being the shift (low/high)for the traverse speed). Just below these controls can be barely seen the top ofthe first row of 76mm ammo, stored above the sponsons on each side of thevehicle. The storage bin at the top of the picture is labeled "Hand Grenades" andthe edge of the main gun and shield are seen on the right. There was only apartial floor to the turret at the rear, the gunner had a small floor attached belowand in front of him, with a firing pedal for the main gun.

Picture 3:The 45 76mm rounds carried in the M18 were stored in racks on both side sponsons (18 each side)and in a smaller ready rack (9 shells) to the right of the gun (see Picture 6). This TM photoillustrates the center part of the left sponson rack. Each rack stored 10 rounds on the lower shelfand 8 on the upper. The base of the shell slid into guide races and was held in place by small thumbclips. The tip of the round slid into tube holders that were hinged to swing out when the shell wasreleased at its base and pulled from the rack. Above this left sponson rack is seen the turret lockagain to the left (locked this time) and the hand traversing mechanism and gear to the right, withthe speed shift bar also visible. Maximum traverse speed was 15 seconds for the full 360 degrees,fairly fast for those days. A good crew could get off 20 rounds in a minute and there was nostabilization of the main weapon. Each of the fixed rounds weighed between 15 and 20 pounds,depending on the type.


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Picture 4:This TM photo illustrates the 76mm M1A1 gun. The M1A1C and theM1A2 were also used, the latter two both threaded for muzzle brakes,but that was the only noticeable difference in any of these 76mmguns. At the bottom left of the gun is the elevation wheel and thegun's recoil cylinders flank either side of the barrel. The breech blockhandle is on the top of the breech and there is a long protectivebreech shield on the left and right of the gun due to the close quartersof the cramped turret. There was no coaxial MG in these main gunmounts; auxiliary firepower was provided by a ring mounted .50calMG at the commander's position.

Picture 5:Here is the right side of the main gun. Notice how the entire packageis rotated 45 degrees off horizontal. The 76mm gun fired eitherAPC-T M62 or HVAP-T M93 projectiles at 2600 or 3400 feet persecond. At 1,000 yards the APC-T could penetrate 3.5 inches (88mm)of homogeneous steel armor at 30 degrees obliquity and the HVAP-Tcould bore through 6.2 inches (157mm) at the same distance andangle. The gunner could also sight with a M4A1 periscope sight,mounted in the top front of the turret that we'll see later. Muzzlebrakes were handy in this AFV, as they would reduce the recoil of thegun by almost half and improved the view forward after dischargingthe 76mm gun. Still, the recoil rocked the little vehicle back on its

haunches after each shot.

Picture 6:There is a lot of equipment mounted on the left side of the gun breech and M1 mount and this TMimage helps identify most of it for us. Starting from the top of the picture, the telescope adjustingknobs are the primary methods of aligning the telescope with the gun during boresighting-- one knobadjusts vertical alignment and the other horizontal. The hand firing lever and rod is the simplemechanical hand method for firing the weapon when the electric solenoid is not working, typicallyused only as a backup firing mechanism. If you look closely, you will see that the firing rod is


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attached to the center pin of the electric firing solenoid, so using either one will actuate the firinglever and set off the percussion cap inside the shell when the gun is loaded and fired.

Under the label for the firing shaft release lever is the breech ring label, the breech ring referring tothe entire mass of steel that the barrel screws into and surrounds the sliding breech block. Part of thebreech ring includes two attachment lugs for the recoil cylinders, and the one on this side of the ringis clearly shown to the right. The breech ring and block withstand incredible forces when the gun isfired, pressure inside may build to 40,000 pounds per square inch with some ammo rounds. That'sroughly 20 tons of pressure on an area the size of your thumbnail. The firing lever at the bottom ofthe image is the actual mechanism that releases the spring loaded firing pin inside the breech block. Ifyou follow the lever back up the breech ring, you can see where the solenoid comes into contact withit at the other end. As we have seen, the recoil guard completely surrounds the breech for the fullextension of its recoil in the M1 mount in order to protect the crew from the recoil movement of thegun. The inset picture at the upper right illustrates how the firing shaft release lever can be set in thesafe or ready position. This is simply a spring-loaded pin that can be used to immobilize the firinglever so the weapon can not be fired accidentally.

Picture 7:This is a good shot of the ammo bin on the right of the main gun whichcontained both MG and 76mm ammo. The upper right hand compartment is forthe first aid kit and the drawers contained gun sight items. If you look closelyyou might see the small clips holding the 76mm rounds in the case. The bin topedge is protected with a black rubber bumper and the whole bin is obvious froma top view of the turret. The interior turret of the M18 arrived to the troopspainted olive drab, while the hull was painted white. The mechanical equipmentand gun were white or black. Notice how a white label with black borderindicates the position for each stowed gear item. The labels didn't last long, buton occasion you will still see the remains in preserved vehicles that have notbeen "restored" by sand blasting everything in sight.


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Picture 8:Here is another view of the gunner's turret wall, this time the laterversion, showing the traversing controls and a few other goodies.Just to the left of the manual traverse handle is the hydraulic turrettraverse motor, and to the left of that is the black azimuthindicator. This piece of equipment simply tells the gunner what thedirection of the turret is compared to the hull, so the direction tonext target, indicated by the commander, can be located quickly bythe gunner. It is the azimuth indicator that differentiates the earlyfrom late turrets in this area. The gun quadrant lives in the bin onthe wall when not in use. The reason for the open top on the turretwas to provide the crew unobstructed vision for target acquisition.Unfortunately for the crews, this ment very cold days and nights ina poorly heated and open AFV. Since most crews were convertedfrom towed anti-armor soldiers, they maintained the typicalwheeled gun positions of gunner on the left and loader on the right,even though this was reversed from typical US tank positions at thetime.

Picture 9:Now we're under the gun, looking at the mechanicals there. To the lower left is the largeblack electric motor that charged up the hydraulics for the turret traverse. Attached to thetop of the motor is the white hydraulic "motor", or pump, for the system and is seen in acouple of the other photos in this page. To the right of the motor is an electrical switch boxto activate the system and slightly to its right is the elevation handwheel. The shaft from thewheel passes under the gun through a reduction gearbox to a pinion gear and then to themain elevation race gear attached to the gun mount. Notice the gun cradle at the upper rightis at a 45-degree angle. Not seen here is a hydraulic fluid reservoir located below our fieldof view in a white box-like tank.


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Picture 10:This is the view looking down into the turret. Above and to the left is the commander'sposition with the ring mount for the 50cal MG. Across the center of the turret, fromfront to back, is a support rod for the canvas cover. Also covered in canvas in this shot isthe 50cal MG and 76mm main gun. Notice how the ammo bin on the right protrudes intothe fighting compartment, the partial roof at the front of the turret, and the gunner'speriscope site on the left of the front turret roof section. Not seen here are the radios,carried in the rear turret bustle. Typically, this would include a SCR610 and aninterphone RC99 with five stations, one for each crew member. Two 12-volt batterieswere located under the turret area of the hull floor with an access hatch, and there wasalso an emergency floor hatch on the right side of the turret floor area. Also seen in thisphoto are grab handles welded to the partial turret roof for the loader and gunner to hangon to as the vehicle bounced across fields. The Hell Cat was first committed to battle inItaly in the Spring of 1944.

Picture 11:Another photo of the left of the turret shows a few of the details of an early vehicle. Thehydraulic traverse control handle is seen to the center of the photo, and the telescopic sightis missing, but its mounting ring is seen forward of the control handle. Further back alongthe turret ring is the hand traverse mechanism with a handle underneath (shift lever) and justbehind that is one of the turret locks (open here). Up above is an angled storage box for thegunner's quadrant and the thin box below is for headphones, I believe. The larger box withthe hasp latch is for hand grenades (12 were carried in the AFV). To the lower right of thephoto is the loader's ready round storage box seen elsewhere in this tour. There is no ammostored in this bin in this particular vehicle nor is any in the shelves over the sponson. Youcan make out clips for the M1 carbines stored along the hull sides, below the sponsons.Actually, there are a number of items missing in this vehicle.

Picture 12:


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Communication was the lynch pin in tank destroyer battalions and the radio installed inthe bustle of the M18 had to be tough and reliable. Unfortunately, the technology atthat time required tubes (or "valves") to operate the radio sets, and as our older readerswill remember these were delicate glass cylinders and there was no place safe in a tankfor a tube radio. Most US AFV radios had protective cover plates over the radio faceand were shock mounted on trays with rubber bushings to help dampen jolts andbounces. But when a tube went bad, it could be a number of minutes until all of themcould be replaced and the radio put back in service.

The TM image shows the typical radio set up in the bustle of the Hell Cat and includesthe SCR-610 set in its two boxes with the protective covers over the control faces. Theantenna is mounted nearby on the bustle roof while the battery case and terminal boxare located to the left and below in the picture. Just the side of the interphone radio boxBC-667 can be seen at the right. The SCR-610 could be man pack or vehicular and wasthe field artillery version of SCR-510. It was a 20-28 Mhz FM radio and included a

BC-620 transceiver box, PE-97 power unit, PE-120 vibrapack mount and an AN-45 antenna.

Picture 13:The Hell Cat had a very interesting and unique steeringlever design, and this TM illustration shows the majorfeatures of the setup. US tanks at this time stillmaintained the idea of seating both a qualified driverand assistant driver in the front hull area. This allowedeither of them to drive the AFV from the left driver'sseat during the extremely exhausting "pleasure" ofcontrolling their AFVs over long periods of time. In theM18 the idea was taken a step further by designing aunique dual steering lever which was suspended fromthe front wall of the compartment, instead of the typicalfloor mounting only for the left seat. This overhead dualcontrol lever system produced a very different "feel" tothe driving experience. Also, the assistant driver's set ofcontrol levers could be unlocked from the steeringlinkage of the controlled differential and swung up and

out of the way from the seat. A similar set up can be found on the US Light Tank M5 series.


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Picture 14:And what were the unique steering levers attached to? A Torqmatic, 3 speedsforward, 1 reverse gear case attached to a Detroit Transmission 900T TorqueConverter. The steering was by controlled differential. This left side view of theentire unit shows the mounting legs at the bottom which bolted to the floor andthe location of the entry of the drive shaft from the rear engine through thesecond transfer case which allowed the lowered drive shaft under the turretbasket. Also in the picture you can see the left drive shaft mount, which wouldattach to the final drive and left drive sprocket and just above this mounting isthe steering lever rising up and back from the unit. The gearshift knob (black) isat the top of the transmission and between the two steering lever brackets. Theentire unit would be painted the same color as the interior of the driver'scompartment, white in most cases.

Picture 15:The engine of the M18 was the same radial Continental R-975 found on the heavierM3 Grant/Lee and many M4 Sherman medium tanks. One lesson learned from thehigh silhouettes of the earlier vehicles was to use two transfer cases to lower thedrive shaft passing under the turret. This photo taken from the old TM is taken fromabove, and shows the mufflers at the top (rear of the hull) radial engine cylinders inthe center and air intake manifolds at the bottom. As you can tell from the crampedengine compartment, this motor was so tight that the entire engine had to be pulledto replace spark plugs. On the other hand, the engine developed over 400hp andpushed the Hell Cat along at 50mph on good roads.


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Picture 16:This last picture of the M18 illustrates one of the more interesting mechanical design features ofthe vehicle. This TD was built to ease maintenance problems. US tank designs had already builta reputation for simplicity and reliability and now the draftsmen were simplifying repairprocedures. In this case, the R975 was built to slide out the rear of the vehicle on rails. Prior tothis, when radials misbehaved badly, the entire engine had to be lifted out the rear engine deckto allow access to components at the sides or bottom of the engine. Now, after disconnecting thedrive shaft and a few fittings the whole affair could be slid out, worked on, and slid back in. Thesame was true for the transmission in the front, which explains the large access hatch on thefront plate of the M18. There too the designers had included a sliding rail system to easeexamination and repair of the transmission.As far as the Tank Destroyer philosophy was concerned, the M18 was one of the best mediumAFVs to come out of the war. Fast, light, easy to maintain, and packed with reasonablefirepower, the M18's main drawbacks were its light armor and open turret, problems that would

eventually doom the vehicle. But, as a design experiment, it was one of the more interesting and successful attempts at new thinking to come out ofWWII.

TO M18 HELL CAT PART 2

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m18 hellcat

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