In late 1940, the Australians realized that with the war in Europe and
the potential threat from Japan, there was a good chance that military
shipments to the country could be disrupted. One of the projects that came
out of this concern was designing a tank that could be produced locally.
Test models were ready by early 1942 and the first production Sentinels
appeared in August 1942.
Originally, the tank was to be based on the American M-3 medium (p.W:MP68),
but several problems prevented this, one being that the facilities to created
rolled steel armor were occupied. This led to one of the more unusual features
of the tank in that the hull and turret were each cast in one piece. The
tank also used three V8 automobile engines; it could be run on one, two
or all three of them. The turret was intentionally made larger than required
for the 2-pounder to allow the easy upgrading of the armament.
Only around 65 AC1’s were built and none saw combat. The first 13 tanks didn’t have their armor properly treated and were designated for training only. With sufficient numbers of American and British tanks coming in, all Sentinel designs were terminated in 1943. In an alternate WWII (where, for example Japan won the Battle of the Coral Sea), the Sentinel could be built in larger numbers and see combat in the Pacific. While the 2-pounder was obsolete by 1942 against Germany tanks, it would have still been perfectly adequate against the thin armor of tanks fielded by Japan. Likewise, the relatively thick Sentinel armor would have given the Japanese a hard time.
The AC1 Sentinel has a crew of five. The commander sits in the turret and operates the AA Bren gun. The gunner fires the cannon and coaxial MG, while the loader also operates the radio. The driver sits in the body with the co-driver, who also operates the hull MG. The turret electrically traverses at 20 degrees per second. The AC1 uses 11 gallons per hour at routine usage.
AC1 Sentinel Mk I
Subassemblies: Medium Tank chassis +3, full rotation Small AFV
turret [Body:T] +2, full rotation Mini open mount [Tur:T] +0, tracks +3.
Powertrain: 3¥82-kW HP gas engines w/ 246-kW tracked drive
train and 156 gallons fuel in standard fuel tank [body]; 12,000-kWs batteries.
Occupancy: 2 CS Body, 1 CS Turret, 2 CS both Cargo:
2.5 Body, 4.4 Turret.
| Armor | F | R/L | B | T | U |
| Body | 4/250 | 4/175 | 4/175 | 4/90 | 4/60 |
| Tracks | 4/40 | 4/40 | 4/40 | 4/40 | 4/40 |
| Turret | 4/250 | 4/250 | 4/250 | 4/95 | 0/0 |
Weaponry
40mm Medium TG/2-pounder QF [Tur:F] (130).
Ground LMG/Vickers [Tur:F] (*).
Ground LMG/Vickers [Body:F] (*).
Ground LMG/Bren [OM:F] (900).
* 4,250 rounds total
Equipment
Body: Small radio receiver and transmitter, 4.5-kW traversing
gear. Open Mount: Universal mount.
Statistics
| Size: 20’¥9’¥9’ | Payload: 1.45 tons | Lwt: 30.25 tons |
| Volume: 104 | Maint.: 39 hours | Price: $25,800 |
HT: 10.
HPs: 1500 Body, 150 Turret, 540 each Track, 30 Open Mount.
| gSpeed: 30 | gAccel: 2 | gDecel: 20 | gMR: 0.25 | aSR: 6 |
Design Notes
The design purchased 5,000 rounds of LMG ammo and 150 rounds of tank
gun ammo. The historical values have been used instead. The design weight
was increased 9.5% to match historical weight and gSpeed was increased
from the 29 mph design speed. Turret armor is Expensive.
The Sentinel could be fitted with a 53 gallon external fuel tank if required.
Variants
The Mark IA upgraded the main gun to a 6-pounder (57mm Medium TG).
None were produced.
The Mark IB replaced the 2-pounder with a 25-pounder howitzer (87.6mm Short Howitzer). None produced.
In 1941, work began on the Mark II (AC2), which was similar to the Mark IA, but replaced the three petrol engines with two diesel engines. It was lighter and simpler than the AC1 but was hated by the military. None built and design dropped in September.
The Mark III (AC3) Scorpion was similar to the Mark I, but with a single 298-kW engine. None built.
The Mark III (AC3) Thunderbolt was similar to the Mark IB, with 120 rounds for the 25-pounder and 2,500 rounds for the coaxial Vickers MG. The co-driver and the hull MG were removed. Weight was 31.9 tons but, with a 296-kW engine, speed was unchanged. Fuel tankage was increased to 222 gallons. Although turrets and hulls for 81 vehicles had been produced, only one was completed.
The Mark IV (AC4) would have mounted a larger turret and a 17-pounder gun (75mm Very Long TG) with 50-60 rounds. None built.