m18 Hellcat

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Picture 1:

The Gun Motor Carriage M18, also known as the Hell Cat, was perhaps the finest US US tank destroyer in WWII. It was nimble, fast and mounted a powerful gun, gun, all necessary for the US tank destroyer doctrine doc trine of the time: "hit "hit and run". In fact, fac t, the Hell Cat utilized utilized the same Continental R-975 engine of the medium tanks M3 and M4, yet was less than one half of the loaded combat weight. This drawing drawing wil willl give give you a feel for the general genera l layout and main interior components. In particular, notice how the tra nsfer case just in front of the ra dial engine engine lowers the drive shaft under unde r the turret, allowing a lower silhouette silhouette.. In order to maintain the extremely favorable power to t o weight ratio armor protection was sacrificed, sacrificed, and a nd an AP 30cal MG bullet could penetrate the thin turret. Never the less, Hell Cat crews loved their hot rods, and accounted for many destroyed enemy AFVss in the last two AFV t wo years of the t he war.

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Picture 2:

Most of these scans are from the operator's manual (TM) for the M18. This one shows the left side of the open topped turret and the gunner's position. In these early turrets you will find on the right the M76 direct sight telescope. In the center right you can see the hydraulic traverse control handle and to the left the manual traverse handle with palm grip sticking up. The turret lock lever is to the lower left in the picture and just to the right of it, at the bottom of the hand traverse 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 of the first row of 76mm ammo, stored above the sponsons on each side of the vehicle. The storage bin at the top of the picture is labeled "Hand Grenades" and the edge of the main gun and shield are seen on the right. There was only a partial floor to the turret at the rear, the gunner had a small floor attached below and 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 P icture 6). This TM photo illustrates the center part of the left sponson rack. Each rack stored 10 rounds on the lower shelf and 8 on the upper. The base of the shell slid into guide races and was held in place by small thumb clips. The tip of the round slid into tube holders that were hinged to swing out when the shell was released at its base and pulled from the rack. Above this left sponson rack is seen the turret lock again to the left (locked this time) and the hand traversing mechanism and gear to the right, with the 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 th ere was no stabilization of the main weapon. Each of the fixed rounds weighed between 15 and 20 pounds, depending on the type.


Picture 4:

This TM photo illustrates the 76mm M1A1 gun. The M1A1C and the M1A2 were also used, the latter two both threaded for muzzle brakes, but that was the only noticeable difference in any of these 76mm guns. At the bottom left of the gun is the eleva tion wheel and the gun's recoil cylinders flank either side of the barrel. The breech block handle is on the top of the breech and there is a long protective breech shield on the left and right of the gun due to the close quarte rs of the cramped turret. There was no coaxial MG in these main gun mounts; auxiliary firepower was provided by a ring mounted .50cal MG at the commander's position.

Picture 5:

Here is the right side of the main gun. Notice how the entire package is rotated 45 degrees off horizontal. The 76mm gun fired either APC-T M62 or HVAP-T M93 projectiles at 2600 or 3400 feet per second. At 1,000 yards the APC-T could penetrate 3.5 inches (88mm) of homogeneous steel armor at 30 degrees obliquity and the HVAP-T could bore through 6.2 inches (157mm) at the same distance and angle. The gunner could also sight with a M4A1 periscope sight, mounted in the top front of the turret that we'll see later. Muzzle brakes were handy in this AFV, as they would reduce the recoil of the gun by almost half and improved the view forward after discharging the 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 TM image helps identify most of it for us. Starting from the top of the picture, the telescope adjusting knobs are the primary methods of aligning the telescope with the gun during boresighting-- one knob


attached to the center pin of the electric firing solenoid, so using either one will actuate the firing lever 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 breec h ring referring to the entire mass of steel that the barrel screws into and surrounds the sliding breech block. Part of the breech ring includes two attachment lugs for the recoil cylinders, and the one on this side of the ring is clearly shown to the right. The breech ring and block withstand incredible forces when the gun is fired, pressure inside may build to 40,000 pounds per square inch with some ammo rounds. That's roughly 20 tons of pressure on an area the size of your thumbnail. The firing lever at the bottom of the image is the actual mechanism that releases the spring loaded firing pin inside the breech block. If you follow the lever back up the breech ring, you can see where the solenoid comes into contact with it at the other end. As we have seen, the recoil guard completely surrounds the breech for the full extension of its recoil in the M1 mount in order to protect the crew from the recoil movement of the gun. The inset picture at the upper right illustrates how the firing shaft release lever can be set in the safe or ready position. This is simply a spring-loaded pin that can be used to immobilize the firing lever 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 which contained both MG and 76mm ammo. The upper right hand compartment is for the first aid kit and the drawers contained gun sight items. If you look closely you might see the small clips holding the 76mm rounds in the case. The bin top edge is protected with a black rubber bumper and the whole bin is obvious from a top view of the turret. The interior turret of the M18 arrived to the troops painted olive drab, while the hull was painted white. The mechanical equipment and gun were white or black. Notice how a white label with black border indicates the position for each stowed gear item. The labels didn't last long, but on occasion you will still see the remains in preserved vehicles that have not been "restored" by sand blasting everything in sight.


Picture 8:

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

Picture 9:

Now we're under the gun, looking at the mechanicals there. To the lower left is the large black electric motor that cha rged up the hydraulics for the turret traverse. Attached to the top of the motor is the white hydraulic "motor", or pump, for the system and is seen in a couple of the other photos in this page. To the right of the motor is an electrical switch box to activate the system and slightly to its right is the elevation handwheel. The shaft from the wheel passes under the gun through a reduction gearbox to a pinion gear and then to the main elevation race gear attached to the gun mount. Notice the gun cradle at the upper right is at a 45-degree angle. Not seen here is a hydraulic fluid reservoir located below our field of view in a white box-like tank.


Picture 10:

This is the view looking down into the turret. Above and to the left is the commander's position with the ring mount for the 50cal MG. Across the center of the t urret, from front to back, is a support rod for the c anvas cover. Also covered in canvas in this shot is the 50cal MG and 76mm main gun. Notice how the ammo bin on the right protrudes into the fighting compartment, the partial roof at the front of the turret, and the gunner's periscope 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 an interphone RC99 with five stations, one for e ach crew member. Two 12-volt batteries were located under the turret area of the hull floor with an access hatch, and there was also an emergency floor hatch on the right side of the turret floor area. Also seen in this photo are grab handles welded to the pa rtial turret roof for the loader and gunner to hang on to as the vehicle bounced across fields. The Hell Cat was first committed to battle in Italy 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. The hydraulic traverse control handle is seen to the center of the photo, and the telescopic sight is missing, but its mounting ring is seen forward of the control handle. Further back along the turret ring is the hand traverse mechanism with a handle underneath (shift lever) and just behind that is one of the turret locks (open he re). Up above is an angled storage box for the gunner's quadrant and the thin box below is for headphones, I believe. The larger box with the hasp latch is for hand grenades (12 were carried in the AFV). To the lower right of the photo is the loader's ready round storage box seen elsewhere in this tour. There is no ammo stored in this bin in this particular vehicle nor is any in the shelves over the sponson. You can 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.


Communication was the lynch pin in tank destroyer battalions and the radio installed in the bustle of the M18 had to be tough and reliable. Unfortunately, the technology at that time required tubes (or "valves") to operate the radio sets, and as our older readers will remember these were delicate glass cylinders and there was no place safe in a tank for a tube radio. Most US AFV radios had protective cover plates over the radio face and were shock mounted on trays with rubber bushings to help dampen jolts and bounces. But when a tube went bad, it could be a number of minutes until all of them could 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 includes the SCR-610 set in its two boxes with the protective covers over the control faces. The antenna is mounted nearby on the bustle roof while the battery case and terminal box are located to the left and below in the picture. Just the side of the interphone radio box BC-667 can be seen at the right. The SCR-610 could be man pack or vehicular and was the 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 steering lever design, and this TM illustration shows the major features of the setup. US tanks at this time still maintained the idea of seating both a qualified driver and assistant driver in the front hull area. This allowed either of them to drive the AFV from the left driver's seat during the extremely exhausting "pleasure" of controlling their AFVs over long periods of time. In the M18 the idea was taken a step further by designing a unique dual steering lever which was suspended from the front wall of the compartment, instead of the typical floor mounting only for the left seat. This overhead du al control lever system produced a very different "feel" to the driving experience. Also, the assistant driver's set of control levers could be unlocked from the steering linkage of the controlled differential and swung up and


Picture 14:

And what were the unique steering levers attached to? A Torqmatic, 3 speeds forward, 1 reverse gear case attached to a Detroit Transmission 900T Torque Converter. The steering was by controlled differential. This left side view of the entire unit shows the mounting legs at the bottom which bolted to the floor and the location of the entry of the drive shaft from the rear engine through the second transfer case which allowed the lowered drive shaft under the turret basket. Also in the picture you can see the left drive shaft mount, which would attach to the final drive and left drive sprocket and just above this mounting is the steering lever rising up and back from the unit. The gearshift knob (black) is at the top of the transmission and between the two steering lever brackets. The entire unit would be painted the same color as the interior of the driver's compartment, white in most cases.

Picture 15:

The engine of the M18 was the same radial Continental R-975 found on the heavier M3 Grant/Lee and many M4 Sherman medium tanks. One lesson learned from the high silhouettes of the earlier vehicles was to use two transfer c ases to lower the drive shaft passing under the turret. This photo taken from the old TM is taken from above, and shows the mufflers at the top (rear of the hull) radial engine cylinders in the center and air intake manifolds at the bottom. As you can tell from the cramped engine compartment, this motor was so tight that the entire engine had to be pulled to replace spark plugs. On the other hand, the engine developed over 400hp and pushed the Hell Cat along at 50mph on good roads.


Picture 16:

This last picture of the M18 illustrates one of the more interesting mechanical design features of the vehicle. This TD was built to ease maintenance problems. US tank designs had already built a reputation for simplicity and reliability and now the draftsmen were simplifying repair procedures. In this case, the R975 was built to slide out the rear of the vehicle on rails. Prior to this, when radials misbehaved badly, the entire engine had to be lifted out the rear engine deck to allow access to components at the sides or bottom of the engine. Now, after disconnecting the drive shaft and a few fittings the whole affair could be slid out, worked on, and slid back in. The same was true for the transmission in the front, which explains the large access hatch on the front plate of the M18. There too the designers had included a sliding rail system to ease examination and repair of the transmission. As far as the Tank Destroyer philosophy was concerned, the M18 was one of the best medium AFVs to come out of the war. Fast, light, easy to maintain, and packed with reasonable firepower, 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 th inking to come out of WWII.

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m18 Hellcat

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