The story of the HSTVL and its development

Out with the old

With an unreliable M81 gun and commonly failing atgm guiding system it was clear the current light tank of the US army, the M551 Sheridan was due for an replacement. Testing done during the Vietnam War showed that the tanks atgm system was not something that worked well enough which led to the Sheridan being mostly only used on Anti-Infantry missions.

In with the new

In 1976 the US army started the ARMVAL (Advanced Anti-Armor Vehicle Evaluation) program. The programs goal was to make a tank that would have the same firepower as a frontline tank while keeping the mobility and small size of a light tank. Out of the program came the HIMAG, one of the fastest tanks ever made, with it achieving speed up to 100km/h (62mph).

A gun that could fire telescopes?

For the ARMVAL project Ares Inc. developed the 75mm XM274. It was an fast-firing 75mm smoothbore gun firing cased telescopic ammunition, designed by none other than Eugene Stoner himself. This was the gun the HIMAG was equipped with and carried over to the other prototypes.

(The Xm885 telescopic round)

The HSTVL was born

With a proposal from AAI and Paccar inc. in 1977, armour was added onto the HIMAG along with a laser rangefinder and a hunter/killer sight. It had now all the upgrades that made the HIMAG into the HSTV(L) (High Survivability Test Vehicle (Lightweight). It was then accepted into testing to see this concepts feasability.

(The initial HSTVL mockup)

Overall design

The HSTVL weighed 22 tons and was 5.981m long, 2.794m wide and 2.414m tall. It had an 80 degree angled plate with special applique armour that was belived to be able to sustain fire from the 115mm cannon on the soviet T-62, hence the High Survivability in its name. The commander remained in the turret while the gunner was moved down into the hull sitting to the right side of the driver. A special feature on the HSTVL was that the driver and the gunner would both be able to fire the gun and drive the tank, while the commander could only fire the gun.

Its engine was an Avco-Lycoming gas turbine engine which was derived from an LTS101 helicopter engine and it could produce up to 650hp. With that amount of horsepower it achived a power to weight ratio of 32.6 hp/ton and an top speed of 83km/h (52mph).

Due to how the cannon was mounted in own compartment it could achive great elevation and depression angles. A maximum of -30 degees of depression and 45 degrees of elevation.

the HSTVL sported the previously mentioned 75mm XM274 cannon with its telescopic rounds. With its rotating breach block it could achieve a reload rate of 0.5s per round. This speed was however lowerd to 0.85s on the final design due to the limitations of the stabiliser system and the fire control system. It had 3 different shells that could be loaded, APFSDS, HE and HE-VT.

(An accuracy test of 40 round fired out of the XM274)

During testing the APFSDS round showed that it was as effective as an 105mm M774 shell that the Abrams used. However, it was decided that it was not enough so a shell called the Delta 3 was developed which led to an increase of lenght on the breach but also an higher muzzle velocity going from 1463m/s to 1,615m/s. It was further developed into the Delta 6 which had an even better muzzle velocity and could therefore penetrate up to 430mms of armour.

However

When testing was finally done in september of 1981 there was no issues found with the HSTVL. However, it was decided during testing that the 105mm Royal Ordnance L7 would be the bare minimum in terms of tank gun which ment that the 75mm XM274 was not enough. No tests were made to attempt to mount such a cannon on the HSTVL because of concers that the light hull could not handle the recoil. This was sadly the end of the HSTVL, with only 1 trial vehicle being made.

What now?

The HSTVL still exists and is being restored at Fort Benning, GA

Sources:

https://en.wikipedia.org/wiki/M551_Sheridan_replacement_process

https://en.wikipedia.org/wiki/High_Survivability_Test_Vehicle_(Lightweight)

tanks-encyclopedia.com/high-survivability-test-vehicle-lightweight-hstv-l

https://en.wikipedia.org/wiki/XM274

https://media.defense.gov/1996/Jun/14/2001715295/-1/-1/1/96-164.pdf