Monday, 1 February 2021

Light and Long Lasting

The light tank as a cheaper and more numerous version of a medium tank was obsolete by the end of WWII. The Germans were one of the first to stop building classical light tanks, the USSR followed soon after in the fall of 1943. The USA was the only major tank building nation to continue light tank development. The result of this development was the Light Tank M24, the best light tank of WWII. Its success is underlined by its longevity: some nations kept theirs in service into the 21st century!

Less armour, more gun

Military vehicles in multiple nations were developed along the same path. This applies to light tanks as well. It's hard to say that the USA was special in this regard. A successful design was used as a starting point and development went from there. The Light Tank T2E1 concept was refined over the course of eight years, leading to the development of the Light Tank M5A1. This tank was produced up until the summer of 1944 and became the most numerous American light tank variant. This was a true American light tank: fast, not that well armoured, and with a low caliber gun, albeit the most powerful in its class. The Americans didn't escape the temptation of making a light tank with the armour and armament of a medium one. That is how the Light Tank T7 was created and later evolved into the Medium Tank T7. The Americans were the only ones who took this idea to its logical conclusion (the T-50 doesn't count, as it still had the armament of a light tank).

The story of the Medium Tank M7 was a sad one, but it warned the American military away from such experiments in the future. However, there was another light tank that never moved past the design stage. This was the Light Tank T21 (a Medium Tank T20 lightened to 21,319 kg). This idea was born in February of 1943, but did not live for long. The Ordnance Committee saw that the Light Tank T21 was following down the road of the Medium Tank M7, and so the program was closed. 

Trials of the 75 mm M3 gun in the HMC M8 SPG. Trials showed that the powerful gun could be installed, but with some caveats.

The developmental dead ends did not change the fact that American light tanks were becoming obsolete. This became clear in late 1942 when American tank units first faced off against German and Italian tanks. The Americans knew that the 37 mm M5/M6 gun was too weak, as back in July of 1941 the Ordnance Department ordered the installation of a more powerful gun into the Light Tank T7: the 57 mm gun descended from a British design. This was one of the causes for the tank's rapid weight gain.

The armament of the T7E2 changed again in early August of 1942. Instead of the 57 mm gun, the 75 mm M3 would be used. Practice showed that the more powerful gun took up about the same amount of space, but had a much more potent HE shell. This redesign inspired the idea of doing the same thing with a lighter vehicle. The same gun was installed in an HMC M8 in late January of 1943. Trials showed that the vehicle became too cramped, but then again the HMC M8 was not particularly roomy. The main result was proof that such a powerful gun could be installed even in a light tank.

The 75 mm M5 aircraft gun in the M9 gun mount. This system became the basis for creating a 75 mm gun for a light tank.

Several conclusions were made that were then used to create requirements for a light tank. First of all, the HMC M8's 1380 mm wide turret ring was deemed insufficient. This was already clear during trials of the Light Tank T7E2. Even with a 1600 mm turret ring, the gun was still too big. The solution came from above. Work began in 1942 to create a ground attack variant of the B-25 medium bomber armed with a 75 mm gun. This resulted in the B-25G armed with the 75 mm M4 gun in the M6 mount.

Further development resulted in the 75 mm T13E1 gun, later standardized as the M5. This was the aircraft variant of the M3 gun with the same barrel length, but slightly smaller overall length (2956 mm vs 3007). The gun ended up weighing 2.5 less than the M3 and had a completely different set of recoil mechanisms. If the M4 had two buffers above and below the gun, the M5 received a more compact cylindrical buffer that wrapped around the barrel. The result was a compact gun with a smaller recoil distance (just 305 mm). This gun was used on the B-25H bomber.

75 mm M6 gun on the M64 mount, the final version of the tank gun based on the aircraft gun.

The concentric recoil system was not that new for tank guns. The same Light Tank T7E2 used the T19 recoil mechanism that was very similar to that used in the aircraft gun. The issue was that the trials were brief and the M7 went into production with a regular M3 gun. The success of the M5 gun led to the development of a tank variant, which was done at the Rock Island Arsenal. The M9 mount didn't work as it, since it was designed for an aircraft. A less bulky recoil mechanism indexed T33 was developed for use in a tank. It was installed in the T90 gun mount.

The M5 gun was also changed. The M3's breech opened to the right, the T13E1/M5's breech opened downwards, the new gun's breech opened to the left. This was due to the position of the loader in American light tanks. The result of this development was a compact gun mount standardized as the Combination Gun Mount M64. As for the tank gun, it was standardized under the index M6. In practice, the M5 and M6 barrels were so close in design that early tanks used T13E1 and M5 "aircraft" barrels.

A model of the Light Tank T24. The chassis was being developed at the time, so the turret was merely a placeholder.

Work on the armament took place in parallel with work on the tank. The tank was designed from the start to use as many components from the Light Tank M5A1 as possible. However, it was clear that an M5A1 with a bigger turret and new gun was not enough. The suspension developed by Harry Knox in the early 30s reached its weight limit. This was proven by trials of the experimental GMC T16 and HMC T64. There was also a new design developed by the Tank-Automotive Center in Detroit (modern day Tank-automotive and Armaments Command, TACOM) for the GMC T70 SPG. A torsion bar suspension was used instead of volute springs. The number of road wheels increased to 5 per side. Experience with the German Pz.Kpfw.III and Soviet KV-1, a sample of which was sent in late 1942, was used during development. Reports of transmission trouble in Light Tanks M5 and M5A1 were also starting to arrive.

The first experimental Light Tank T24, Aberdeen Proving Grounds, October 1943.

The first work on the new tank began in March of 1943. Officially, the Ordnance Committee approved the work on April 1st. This is when the tank received its index: Light Tank T24. The leap in the index is explained by the existence of the T20, T22, and T23 medium tanks, as well as the aforementioned Light Tank T21. Development of the Light Tank T24 was approved by the Army Service Forces on April 29th, 1943.

The first experimental GMC T70 entered trials in April of 1943 and served only to show that the new torsion bar suspension was very promising. This didn't mean that the new light tank would be built on its chassis. Of course, experience of the designers headed by Joseph Colby was used to its full extent, but the Tank-automotive Center worked with a different division of General Motors on this project: Cadillac. While the GMC T70 was Buick's first foray into tank building, Cadillac was already the main producer and developer of light tanks by the spring of 1942. This introduced a number of restrictions. The GMC T70 inherited a lot of components from the Medium Tank M7. Cadillac had no such shortcut. One of the requirements was the maximum use of components from the Light Tank M5A1 to speed up time to production.

A change to the proportions of the hull makes this tank look smaller than the Light Tank M5A1, but that is not the case.

The look of the Light Tank T24 came together by the fall of 1943. The chassis developed jointly by Cadillac and the Tank-automotive Center seemed lower to the ground. Its looks were deceiving: the only part of the M5A1 hull that was taller was the engine deck. The top of the turret was more or less on the same level. The T24's hull was wider and longer, creating the illusion of lower height. After the story with the Light Tank T7/Medium Tank M7 the armour was left as is, especially since even the armour of the Medium Tank M4 was considered inadequate by the spring of 1943. The thickness of the armour was the same (25 mm). The rear portion of the sides and the rear was thinned out to 19 mm. In part, protection was improved by sloping the armour (both the front and the sides). 

The suspension was similar to that of the GMC T70, but only in concept. The diameter and width of track links on the T24 and T70 were different. The T72 metallic track link developed for the T24 was 406 mm wide, 102 mm wider than on the T70. The Light Tank T24 used tilted telescoping shock absorbers rather than horizontal bidirectional shock absorbers used on the GMC T70. The tank also had a system of levers linking the last road wheel and the idler. This system ensured that the track was always taut, but forced the move of the last torsion bar backwards.

A change to the layout of the engine compartment and cooling system allowed the designers to get rid of the hump above the engine deck that was present on the Light Tank M5A1.

The engine and transmission group was also different from the M5A1. Even the Cadillac Series 44T24 engines were slightly different from those used on its predecessor. The maximum power output was still 296 hp. The tank became a bit heavier, but this was still enough power. Due to complaints about the GM Hydra-Matic 250-T gearbox GM developed a new model, the 255-T. It had no rear drive planetary bank. A two-stage reduction drive with a mechanical linkage was used to drive backwards. The ability to disable one engine in case of damage was also introduced. Some transmission elements (such as the double differential) were taken from the GMC T70. The Light Tank T24 also had dual steering.

Unlike its predecessors, the tank did not get a massive differential cover. A large hatch was installed in the front of the hull (this solution was first used on the GMC T16 and HMC T64). This hardly improved the front armour, but this decision doesn't seem so strange if you consider that the armour was only bulletproof. The engine compartment was also reorganized. The radiators were now in front of the engines rather than above them, which allowed the height of the engine compartment to be reduced.

Initial layout of the fighting compartment. The ammunition layout was typical for American tanks at first, but on production vehicles it was different.

The turret was developed from scratch, just like the hull. It had a complex shape. The sides were stamped from 25 mm thick steel, the front was 38 mm thick and cast. The gun mantlet was also cast. As required, the turret ring was widened to 1524 mm. The turret was designed to fit three men, but it sometimes only housed two. The loader was also supposed to double as the assistant driver. A 51 mm M3 bomb thrower was installed in the roof in addition to the 75 mm M5 gun and coaxial machine gun. The AA armament improved compared to the tank's predecessors: a .50 cal Browning M2HB was now installed on a pintle mount. The ammunition capacity decreased compared to the Medium Tank M7 to 48 rounds for the 75 mm gun (although the tank was also smaller). 

Both Light Tank T24 prototypes were subjected to intense trials while the design work continued.

The first Light Tank T24 was ready on October 15th, 1943. The Ordnance Committee decided to build 1000 of these tanks instead of Light Tanks M5A1 even before that, on September 2nd, 1943. AFS agreed with this proposal on September 3rd and contract T-11120 was signed with Cadillac. Trials revealed many issues with the suspension, engine, and transmission. Unlike with the T70, which was put into production before all design flaws were exposed, production of the T24 was postponed. Issues with the recoil mechanism confirmed this decision. They got so bad that the gun was removed and the tank continued trials without it. The tank drove for 4514 km by February 7th, 1944. Defective components were being improved as the trials went on.

Condition of the first prototype after 4500 km of driving. The gun was removed due to issues with the recoil mechanism.

The second prototype was built in December of 1943 and sent to the Aberdeen Proving Grounds. On February 10th, 1944, it was sent to Fort Knox for additional trials. A list of changes required before production could commence was composed. A decision was made to improve the second prototype (which would have been nearly identical to a production tank) and send the first 8 production tanks to trials to avoid issues like those experienced with the GMC T70. Nevertheless, the Light Tank T24 had a green light.

New generation

Even though the Light Tank M5A1 seemed obsolete back in 1943, its rate of production was quite high. This was because the tank was primarily considered a reconnaissance tank. Since they were better than the Light Tank M3A1, the objective of replacing the old radial engined tanks was the highest priority. Because of this, the Light Tank T24 was going to be produced in meagre numbers in the second half of 1944. No tanks were expected in April, then just 25 in May and 50 in June. Production of the Light Tank M5A1 at Cadillac would then stop. As for Massey-Harris Co., it would continue producing the Light Tank M5A1 through the second quarter of 1944.

The improved Light Tank T24 prototype #2, effectively the pilot production vehicle.

While plans for Light Tank T24 production were being made, Cadillac was working on their tank. New issues came up in addition to transmission defects. One of them was to do with the commander's station. Two periscopes (one in his hatch flap, one in the turret roof) turned out to not be enough. There were also issues with the AA gun pintle mount. As a result, the roof was thoroughly redone. The commander received a cupola similar to the one used on the Medium Tank T23, greatly improving his visibility. The loader's hatch was reduced in size and only given one flap. The AA machine gun pintle mount also changed. The ventilation fan was moved to the right and the antenna port was moved from the right side of the turret to the left. Finally, the loader got a pistol port and a spotlight was added to the left of the commander's cupola.

The commander's cupola, new loader's hatch, and new Browning M2HB machine gun pintle mount can be seen from above.

A number of changes were made to the chassis. The driver's compartment hatches changed slightly, the driver received a windshield, some changes were made to the toolkits. Like the Medium Tank M4, the Light Tank T24 received a wet ammo rack to reduce the likelihood of ammunition fires.

Cutaway diagram of the Light Tank M24.

All aforementioned changes were introduced on the second prototype, which essentially became the first pilot tank. It was given serial number 2 and registration number USA 30112595. The first prototype also counted towards the 1000 tanks described by contract T-11120. Overall, the contract covered tanks with registration numbers 30112594-30113593. The second prototype arrived at the Aberdeen Proving Grounds for trials in June of 1944. Without waiting for trials to finish, the Ordnance Committee recommended standardizing the tank as the Light Tank M24 on June 22nd, 1944. On July 25th the chief of the Ordnance Department Major General Levin Campbell approved the standardization. The order increased to 5000 tanks.

The final version featured a wet ammo rack. This photo also shows the tank's dual controls.

Production of the Light Tank M24 was already being set up when it got its name. As of July 11th, 66 tanks of this type arrived at the Lima tank depot, and by August 4th 92 vehicles had been shipped. Cadillac delivered 100 tanks in July of 1944, and the Massey-Harris Co. factory also switched to M24 production, delivering 10 vehicles. Contract T-12367 was signed to produce 250 units with serial numbers 1001-1250 and registration numbers USA 30119020-30119269. Interestingly enough, artillery production couldn't keep up with the tanks. As mentioned above, the tank received a special version of the gun indexed M6. Meanwhile, the first and second prototypes still had the M5, the aircraft variant of the gun. It can be distinguished by rings on the barrel. Production tanks continued to receive M5 and even T13E1 guns.

Production of the Light Tank M24. It was quickly mastered, as it shared a number of components with the Light Tank M5A1.

Production of the Light Tank M24 slowly ramped up. Cadillac delivered 200 tanks in August of 1944, 212 in September, 277 in October, 377 in November, 499 in December, and 200 in January of 1945. Initial plans called for 408 tanks in November, 350 in December, and 250 in January. Massey-Harris was expected to deliver 40, 134, and 131 tanks respectively. Cadillac already filled up the allocated pool of serial numbers by November of 1944 and required signing of additional contracts. Contract T-12368 for 550 tanks with serial numbers 1251-1800 and registration numbers USA 30119270-30119819 was signed, but that lasted for only two months. Contract T-13780-1 was signed in December of 1944 for even more tanks. 1548 tanks with serial numbers 1801-3348 and registration numbers USA 30120329- 30121876 were built to satisfy this contract.

As for Massey-Harris, they produced Light Tanks M24 at a much slower rate. 16 tanks were delivered in August, 34 in September, 40 each in October and November, 50 in December, and 125 in January. The pool of serial numbers allocated by contract T-12367 was used up in January and contract T-13781-1 was signed. It requested 360 tanks with serial numbers 3349-3708 and registration numbers USA 30121877-30122236. The first of these tanks was delivered in January of 1945, the last of them in March, after which contract T-13781-2 for 415 more tanks with serial numbers 5452-5866 and registration numbers U.S.A. 30139011-30139425 came into effect.

Reliability trials of a production Light Tank M24, January 1945. The upper front plate has a large hatch for transmission maintenance, a characteristic feature of this tank.

Unlike the GMC M18, which changed visually during production, the Light Tank M24 had nearly no changes made to it. The biggest change was the gun. Use of M5/T13E1 barrels continued throughout 1944, for instance tanks produced in December 1944 that ended up in the USSR still have them. The majority of tanks that fought in WWII had these aircraft guns. The instruction manual states that these guns were later replaced with the M6. In reality, some tanks that survive to this day still have the earlier barrels. Mid and late production tanks were built with M6 guns, but they didn't make it in time for the war.

The tank carried a full set of friend or foe markings.

When production finally ramped up at Massey-Harris in early 1945 Light Tank M24 production stabilized. Cadillac delivered 300 tanks in February, 350 tanks in March. 205 in April, 350 in May. Contract T-13780-2 kicked in then. In this time Massey-Harris delivered 155, 192, 138, and 190 tanks respectively. The end of the war in Europe meant that production of the Light Tank M24 was coming to and end. Cadillac delivered 280 tanks in June of 1945, 86 in July, 81 in August. 494 tanks with serial numbers 3709-4202 and registration numbers U.S.A. 30137268-30137761 were delivered to satisfy contract T-13780-2. The last tank built to this contract was the last Light Tank M24 to be produced. As for Massey-Harris, they made 79 tanks in June of 1944, of them 44 for contract T-13781-3, another 40 in July, and the last 30 in August. Contract T-13781-3 covered 114 tanks with serial numbers 5867-5980 and registration numbers U.S.A. 30142464-30142577. In total production ran for 4731 units, 3592 built by Cadillac and 1139 by Massey-Harris.

Long-living scout

As mentioned above, the first Light Tanks M24 were test vehicles destined for trials. The first vehicles meant for army use were built in May of 1944 and were issued starting in June of 1944. Even though production of the Light Tank M24 gradually grew, deliveries to the army were lagging behind. For instance, as of September 4th 158 tanks had been delivered to Lima, but only 87 were issued. The tanks began to arrive in active units even later, towards the end of 1944. This kind of delay was typical for the American army.

Transporting Light Tanks M24. The tanks are being shipped with a full set of spare parts, just like how they were transported across the ocean.

The final Light Tank M24 was not a very light vehicle. Its mass reached 18.4 tons, although there was still enough engine power to reach a top speed of 56 kph and maintain an average speed of 35 kph on a highway. This was less than what the Light Tank M5A1 could develop, but with a caveat. The Light Tank M5A1 could keep up its top speed of 60 kph only for a short time, at regular power its speed capped out at 48 kph. The effective mobility of these two tanks was close, but the new tank had much better firepower. However, recall that the Light Tank M24 initially had a crew of 4, so the loader had to work double duty.

Inspecting a new arrival, late 1944.

The first Light Tanks M24 reached France in November of 1944. These tanks were supposed to replace Light Tanks M5A1 in tank battalions. The first units to get these new tanks were supposed to be the 744th and 759th Tank Battalions which were a part of the 2nd and 3rd Armored Divisions. The rearmament was supposed to take place in December of 1944, but the German counteroffensive in the Ardennes upset those plans. As a result the first two M24 tanks that saw battle were "borrowed" by the 740th Tank Battalion. The 744th and 759th battalions received their own tanks (for instance, the 744th got 18 tanks), but this happened later, on December 24th, 1944.

The silhouette of the new tank was slightly reminiscent of the Panther, so it was demonstrated to both tankers and infantry.

The combat debut of these reconnaissance tanks was a success, albeit with a caveat. The silhouette of the tank differed from the M3 and M5, more closely resembling the Panther. The comparison might seem silly, but even though the M24 was a light tank it was quite tall and the shape of the turret was quite similar to the Panther's, especially from the front. This kind of resemblance could become and issue, and so the tanks were demonstrated to tankers and infantrymen in January of 1945. After the war the Light Tank M24 played Panther tanks in movies.

The tank showed itself quite well in the winter of 1944-45.

Let us talk about the tank's reviews for a moment. Often people compare apples to oranges, without understanding that there are certain roles even within the light tank class. For instance, it is incorrect to compare the Valentine and Light Tank M24, not is it correct to compare it to the T-70 or Pz.Kpfw.38(t). These tanks are "classical" light tanks, designed to complement medium tanks, with equivalent mobility but weaker armament. The closest analogue of the Light Tank M24 is the Pz.Spw.Luchs, another reconnaissance tank. Here is where the tank shines. The German tank loses to the M24 in firepower and in amount produced. As for the Light Tank M24, it was perfectly suited for its role and could play the role of a support tank if necessary (of course, in this role the criticisms of insufficient armament and thin armour began). If the tank was used as intended, the reviews were very favourable.

Light Tanks M24 from the 8th Armored Division crossing the Ruhr, March 1945.

The Light Tank M24 became truly common towards the spring of 1945. The main recipients of these tanks were tank battalions used in armored divisions. Usage showed that the idea of having a four man crew was a bad one and a fifth crewman was reintroduced. The tank showed itself a mobile and reliable vehicle, although it suffered from issues with damage to road wheel tires. This was a problem common to American tanks with steel road wheels.

Weak armour was the cost for high mobility. However, most American tanks didn't fare any better against German anti-tank artillery and rockets.

16 units received Light Tanks M24 in WWII, most of which fought in Europe. The tank showed itself quite well in Italy, as it was well suited for use in the mountains. However, the Marines were unsatisfied with this tank. 10 tanks were tested but never accepted into service with the Marine Corps.

Chaffee I, the first tank of this type to be sent to the UK for study. 289 tanks of this type were sent in total.

Unlike its predecessors, very few Light Tanks M24 were sent to other armies during the war. The British were the only ones to receive them in large amounts. American vehicles were allocated a pool of WD numbers from T.330410 to T.330809. The tanks were named Chaffee I. Fewer tanks arrived than expected, just 289. Trials showed that the mobility of this tank was high, but they didn't have time to test it in battle as the war came to an end. Only a small number of tanks were sent to the front lines in April of 1945, but they did not see combat. The tanks only served in the British Army of the Rhine (BAOR). 

T85E1 metallic track link introduced as a result of user experience.

Few examples were sent to other countries for study. One tank ended up in Canada and 31 more were obtained after the war. A pair of Light Tanks M24 were sent to the USSR in the spring of 1945 where one was used for trials. The Australians also tested the Light Tank M24, but did not accept it into service.

A modernized Light Tank M24. It has T85E1 tracks and an M6 gun.

The Light Tank M24 fully replaced older models in the US Army in the late 1940s. Some of the tanks were modernized. As mentioned above, some were given M6 guns. A study of the issues with road wheel tires found the culprit: the T72 metallic track. As a result, the T85E1 rubber-metallic rack was developed. It was narrower, just 356 mm wide. The design was close to that of the T48 track link used on the Medium Tank M4. If necessary, extended end connectors could be installed, widening it to 419 mm. The drive sprocket crown was changed to support the new track. The new sprocket had 13 teeth. 1600 tanks were modernized in total. T86 and T87 tracks were also developed, but not mass produced.

Not all tanks went through modernization. This tank from the Korean War still has old tracks and an aircraft gun.

WWII was the first war for the Light Tank M24, but not the last. The next time these tanks went into battle was in the summer of 1950 in Korea. 138 tanks of this type fought here. In part, they played the role of medium tanks, for which they made a poor substitute. The M24s could do little against North Korean T-34-85s. Despite their unsuccessful debut, these tanks fought for a long time in Korea and were used by the South Korean army after the war. In total, the South Koreans had about 30 of these tanks.

French Light Tanks M24 in Indochina. The French were the second most avid users of the Light Tank M24 after the Americans with 1200 tanks fielded.

The French were the most avid foreign users of American tanks. Various sources estimate that they received 1200 to 1245 tanks, or a third of the total production run. French tanks were used in battle many times (mostly in Indochina). Light and mobile tanks were perfect for the role of "colonial" tanks. Bulletproof armour was enough for police actions and real anti-tank weapons were about equally effective against light and medium tanks. It's no surprise that the Light Tank M24 stayed in Indochina for a very long time. After the French left, these tanks were used by Laos and South Vietnam (the latter used them until the 1970s). The Light Tank M24 was used by dozens of nations. The Italians bought 500 of these tanks, and plenty of other European nations used them as well, including Norway, who only wrote off their M24s in the 90s. These tanks were used in the Middle East, in Africa, in South America. Japan used M24 tanks for quite a while. Uruguay was the record holder and only disposed of their M24s a few years ago.

Italian M24 tanks on parade, 1961. The Italians received about 500 tanks of this type.

The tank's long career shows that it didn't earn the title of best light tank of WW2 for nothing. American designers rightfully abandoned the drive for more armour to focus on other parameters. The result was a successful light reconnaissance tank with good mobility and firepower for its class. It also turned out that the tank was quite useful in a certain theatre of war, which ensured its longevity.


  1. In terms of American light tanks- some people argued that in T-34 loader must stepping during turret rotation due lack of turret basket. In my opinion that's biased argumentation due seats located inside turret. But according my knowledge in early Stuart tank loader and commander must stepping during turret rotation due lack of seats and turret basket. Also- in my opinion late US WWII tanks show that turret basket make limited sense if tank use floor mounted main ammo rack (lack of turret basket in M24 and in M26).

  2. Yes, the Americans seemed to have no issues with dropping the turret basket. The M4(105) and later M4(76)es didn't have it.