What would a robot tank look like?

Lately, I’ve read about some interesting experiments to jury-rig helicopters and tanks to be autonomously controlled:

• https://www.janes.com/article/84156/us-army-flies-automated-sara-helicopter
• https://www.janes.com/article/76439/usmc-onr-conduct-final-autonomously-operated-uh-1-demonstration
• https://www.telegraph.co.uk/news/2018/03/21/china-testing-unmanned-tank/

I’m unsure if any of these projects will get past the prototype stage, but they’re fascinating nonetheless since, if adopted, they could extend the useful lives of many old pieces of military hardware by enhancing them with machine intelligence and, maybe in some cases, with robot crews. As with all types of jobs, those in the military will inevitably be taken over by machines of some sort, and drop-in-kits installed into the cockpits of old helicopters could be the “bridge” in that transition.

However, in the longer run, planes, tanks, ships, and other pieces of military hardware will be redesigned around the needs of machines instead of humans. Returning to the helicopter example, a clean-sheet design meant to be flown by a computer wouldn’t have a cockpit at all: its shoebox-sized computer “pilot” would just need a small, armored compartment in the fuselage, which could be accessed through a little door. Deleting the chairs, controls, displays, and windows that a human pilot/co-pilot duo would have needed would make the autonomous helicopter lighter, sleeker, faster, and cheaper to make. In a fight between the old helicopter jury-rigged with a drop-in computer and the new helicopter designed specifically around a computer pilot, the latter would win.

This makes me wonder what a “robot tank” that was as good as the best modern tanks would look like. A tank’s quality is determined in aggregate by its 1) firepower, 2) speed, and 3) armor, so the theoretical robot tank will need to equal or surpass the U.S. M1 Abrams or Russian T-14 Armata. For this exercise, I think the best place to start is with the first criterion, “firepower.”

The Abrams tank has a 120mm diameter main cannon and the Armata has a 125mm cannon. Their capabilities are about the same. Our robot tank would thus need one or other. Now, a tank’s cannon sets a minimum diameter for the tank’s “turret ring,” which is the big hole in the top of the tank’s hull that the turret is dropped into in the factory. As a general rule, bigger cannons need wider turret rings. And for obvious reasons, the width of the turret ring in turn sets a minimum width for the tank’s hull.

In the early 1980s, the U.S. Army built an experimental version of the M1 Abrams tank that had an unmanned, 120mm turret. They dryly named this vehicle the “Tank Test Bed” (TTB). It was never put into mass production, but its weapon reportedly worked very well. I couldn’t find figures on the internet, but eyeballing the photo below, it looks like the diameter of the TTB’s turret ring is about 80% of the tank hull’s diameter. The M1 is known to be 12 feet wide, so I’ll estimate the turret ring is 9.6 feet in diameter. To add the necessary structural support for the turret ring and space for side armor, let’s make our robot tank’s hull 11 feet wide.

Take note that the Russian T-14 Armata has a 125mm, unmanned turret, and the vehicle’s overall width is 11.5 feet (3.5 meters), which suggests my estimate is credible, and that the Russians might have made a robotic tank gun that is as compact as physically allowable. Note that the T-14 Armata has a three-man crew, and they are seated in a row inside the hull, so it’s possible the tank’s width was determined by human considerations rather than mechanical ones. That said, I’ll stick with my assumption that a robot tank could be a slightly thinner 11 feet wide thanks to the deletion of space-consuming humans with their huge shoulder widths.

As this very long, very awesome RAND report says, the optimal ratio between a tank’s width and length is 1 : 1.5 (ignoring the length of the cannon). Our hypothetical robot tank’s length should thus be 16.5 feet (198 inches). Looking at modern tanks, however, I see the ratio ends up more like 1 : 2.16 (M1 Abrams), 1 : 2.49 (T-14 Armata), 1 : 2.08 (Leopard 2), and 1 : 1.8 (T-90), which suggests to me that there’s some other design constraint forcing tank engineers to make their vehicles longer than they should ideally be. And you guessed it, I think the extra length owes (mostly or wholly) to the need for interior crew compartment(s) for the humans. Look at this diagram of a T-90, to which I added a Little Green Man to indicate where the driver sits.

Sitting down, the driver adds about 3 feet to the tank’s overall length. Subtract that, and the T-90 ends up being 19 feet 6 inches long, giving it a width : length ratio of 1 : 1.56, which is very close to optimal. Additionally, shortening it by that amount might allow for the elimination of one set of road wheels (down from six to five), cutting weight and cost, though I think there’s also a risk that could make the tank unbalanced and back-heavy.

Since I don’t know what all the different design constraints are, I’ll give a range of possible lengths rather than a hard number: The hypothetical robot tank could have a width : length ratio of anywhere from 1 : 1.5 to 1 : 1.7 (slightly less than the T-90), which means its length would be 16 feet 6 inches up to 18 feet 8 inches.

Estimating the robot tank’s weight is harder still, but I’ll give it a shot. The RAND report has weight figures for three types of tanks: the four-man M1 Abrams (66 short tons), a hypothetical three-man tank (61 short tons), and a hypothetical two-man tank (55 short tons). Graphing those on a simple line chart yields the following:

As you can see, the elimination of each human crewman allows a roughly five ton weight reduction, and extending the trend to zero crewmen, our robot tank should weigh 44 short tons.  It’s fair to scoff at this straight-line extrapolation as overly simplistic, but consider this: the T-90 is longer (22 feet 6 inches) and wider (12 feet 5 inches) than our hypothetical robot tank (16 feet 6 inches up to 18 feet 8 inches long and 11 feet wide). The T-90 weighs 51 short tons, and trimming length and width to make something the size of our robot tank could absolutely result in a new weight of 44 short tons. A real-life datapoint supports my back-of-the-envelope line graph extrapolation.

In terms of height, the robot tank couldn’t be much lower than the T-90, which is 7.3 feet tall (not counting the machine gun and any sensors mounted on the top of the turret). Returning to the T-90 diagram, this time with a Little Green Man drawn to represent the commander, we can see that the turret might be a little taller than is mechanically necessary because it must accommodate a human. However, the height of the hull can’t be decreased since it is constrained by the height of the engine (outlined in red). The T-90 also can’t have its suspension lowered without sacrificing ground clearance and damaging its cross-country performance.

Aside from flattening its roof, there doesn’t seem to be any good way to make the T-90’s turret lower. The diameter of the cannon and the height of its breech establish a hard limit on how low the turret can be. Additionally, the turret’s ammunition carousel (shown in the diagram below) stacks the shells and propellant horizontally, which already minimizes the overall height of the carousel. There doesn’t seem to be any way to shrink it further.

I estimate that getting rid of the bulge at the top of the turret would reduce the T-90’s height from 7.3 feet to 6.5 feet. The hypothetical robot tank would use the same type of autoloading turret and hence would be the same height.

So there we have it. I estimate that an autonomous tank equivalent to today’s best manned tanks in terms of firepower, speed and armor would have the following specifications:

Length	16 ft 6 in (min) to 18 ft 8 in (max)
Width	11 ft
Height	6 ft 6 in
Weight	44 short tons
Armament	120mm or 125mm cannon with autoloader
Armor and mobility	Comparable to T-90 or M1 Abrams

This is close to the Chinese Type 59G tank (a modified version of which is used by Pakistan):

Length	19 ft 10 in
Width	10 ft 8 in
Height	8 ft 6 in
Weight	40 short tons
Armament	125mm cannon
Armor and mobility	Inferior to T-90 or M1 Abrams

The fact that tank with similar dimensions and firepower to my hypothetical robot tank already exists shows that there aren’t any engineering or practicality barriers to building the robot tank per my specifications. The Type 59G proves that a 125mm cannon can be mounted in a relatively small, lightweight hull and fired without tearing the vehicle apart. I don’t have the software or artistic talent to make a computer rendering of the robot tank, but combining the hull of a Type 59G with the lower-profile turret of a T-90 or the narrow, “naked turret” of the M1 Abrams TTB would give a fair approximation of its appearance.

Since the robot tank would be much smaller and lighter than an equivalent tank built around a human crew, it would be cheaper to manufacture, harder to hit since it would be a smaller target, and more easily transportable. A computer would take the place of a human commander, gunner, and driver, allowing for a significant reduction in internal volume and overall tank size. A space-efficient autoloader already found in the T-90 would be incorporated into the robot tank. Additionally, to perform maintenance and repairs in the field, the robot tanks would need to internally carry two smaller, human-sized (but not necessarily humanoid) robots, but they could be stored much more compactly than human crewmen during transit. They might be able to curl up into fetal positions and fit into small lockers in the back or sides of the vehicle, or in a bustle at the back of the turret.

Finally, I think the robot tank would carry a small UAV that it could launch to provide aerial reconnaissance footage, vastly improving the tank’s situational awareness. Something as simple as a 25-pound quadcopter could do.

Links

1. https://www.rand.org/content/dam/rand/pubs/reports/2009/R3837.pdf
2. https://tankandafvnews.com/2016/03/15/from-the-vault-ammo-loading-systems-for-future-tanks/ 
3. https://youtu.be/cWw5SRa4vu8