Tank Scratchbuild: The Caterpillar Crawl

With a philosophical flourish Cato throws himself upon his sword; Ishmael quietly take to the ship; I go to the hobby store and buy a stack of scale model kits.

Good Intentions

The original plan was to build a dieselpunk tank kitbash, something like Miyazaki's Akuyaku #1. As I sketched out a few ideas, I realized that the project would be a great way to put actual tank design into practice. Instead of a model that was merely aesthetic, I could make a tank that was as functional as possible. Not good by any means, but functional. The sort of thing that could, in the '30s and '40s, get funding for a prototype.

The Caterpillar Crawl

 I started with the venerable 1/25 Caterpillar DH8 kit from AMT.

Since I'm converting it to a 1/35 tank, this means the final result will be ~30' long, which is long but not "you have made a terrible mistake" long. Caterpillar created the D9 by scaling up the D8, so scaling it up again should be fine. 

Bolting armour to a bulldozer, sticking some machine guns through a few holes, and calling it a tank is a time-honoured tradition. The most famous (or infamous) is the Bob Semple tank out of New Zealand, but there are other examples.

Bob Semple Tank (left), Disston Tractor Tank (right).

A bulldozer is not a great platform for a tank. It has a simple linear powertrain. The radiator, engine, torque converter, transmission, and differential are all in a front-to-back straight line. There's no room for a driver or turret near the front of the vehicle. The Bob Semple Tank had a front machine gunner lying on a mattress on top of the engine, which is obviously a bad plan.

The engine of a bulldozer also sticks slightly forward of the treads, meaning the tank cannot climb steep obstacles without smashing the radiator into them first. The suspension creates a very rough ride and the engine isn't designed for speed.

How can a bulldozer be converted into a "proper" tank? 

Screenshot from this video. The front of the bulldozer is to the left.

D8K brochure (pdf)

Reverse The Bulldozer?

An easy option, but this type of suspension doesn't work in reverse. When moving forward, each arm of the track can rise over a rock, pivoting vertically around the final drive and horizontally around the sway bar. In reverse, if you crunch into a rock, and all the force is transmitted to the drive sprocket, final drive, and differential.

Reverse the bulldozer and add more track in front of the drive sprocket?

The drive sprocket has to be at the end of a loop of track. It can't be in the middle. Otherwise, any slackness in the track, or going over a ditch, would disconnect the drive sprocket from the track. A high-drive system, like the modern D9s, has a sort of triangular loop of track, but dramatically increases the height of the vehicle.

Move the engine and rearrange the drivetrain?

This is the solution I chose. Instead of mounting the engine at the front of the vehicle, I moved it to the back. It's connected by a universal joint to the transmission, which then loops backwards to the differential and final drives.

Front is still to the left.

The transmission is under the radiator fan. The engine connects to it by a universal joint (currently disconnected). Then the transmission connects to the differential (the black and white plastic bit, from an old truck kit) and to the final drives (the yellow bulges between the chassis and the chrome drive sprockets).

Doctrine and Design

Why would anyone pick a caterpillar chassis as the basis of a tank?

The three traditional factors in tank design are Mobility, Armour, and Firepower. You need to balance all three to get a successful tank.

There are other major factors that the traditional triangle leaves out.

  • Feasibility. Can you actually manufacture this tank in sufficient numbers to make a difference? How complicated is it?
  • Reliability. Once in the field, can the tank actually perform its function without breaking down? Can an inexperienced crew maintain it? Is it over-engineered?

Caterpillar-type bulldozers are cheap, commonly available, and legendarily reliable. You can park one in a muddy field for twenty years and start it up again with some fresh oil, fuel, and a new battery. You can repair major structural components in the field without disassembling the entire vehicle. New parts can be manufactured from stock or scrap. A scaled-up tractor would retain these features.

In doctrine, an infantry tank was a relatively slow but heavily armoured tank designed to support the advance of massed infantry. 

I'm not really interested in the alternative history aspect of this project, but some sort of justification seems necessary. What if a nation's doctrine stressed the idea of a continuous attack? That, after a breakthrough was achieved, the attacking force must continue a methodical attack, without giving the enemy time to respond. An attack designed to overwhelm the decision-making powers of its leaders. Amphetamines for everyone.

A highly reliable tank might form part of that doctrine. A cruising speed of 6 miles per hour isn't much, but 6 miles per hour, day and night, rain or shine, might be appealing.

Original instructions. Model kits have come a long way since 1972, but the loss of functional labels (what each part does) is a bit sad.

Engine Alterations and Detailing

Ideally, this tank would use a more compact engine. The six-cylinder turbocharged engine in the DH8, scaled up, is basically a marine diesel engine. A V6 engine would be half the length. A radial engine, even a hideously complex radial engine, would be even shorter.

Still, the point is to use as much of the original kit as possible, so the huge inline six stays, with a few additions and changes. It's a very old kit, so the level of detail isn't great.

The right side of the engine (the left side of the vehicle) features new fuel injector lines and cables.

The left side (right side of the vehicle) features a re-positioned alternator, the start of the radiator fan drive, and a rearranged air filtration system. 

Originally, the air system was mounted above the engine, but I moved it to the side to reduce the height. It should still function; there are some later-model caterpillar engines with similar adjustments. 

I replaced the original paper canister air filter with a pair of scratchbuilt combined air-oil separators and cyclone dust collectors

Paper air filters are very efficient, but when they clog, they prevent air from entering the engine. This isn't a problem for a construction bulldozer. You can always open the filter, bang out the dust, and replace it. In the middle of a battle, leaving the tank to clean the air filter is not wise.

Oil-air separators continue to let air pass even after the oil becomes saturated with dust. This reduces the lifespan of the engine, but increases the lifespan of the crew.

The air intake and exhaust are not attached yet.

Finding a position for the radiator was difficult.

Back of the tank? It's a huge vulnerability. Armoured louvres are still weaker than actual armour, even if you angle them.

Front of the tank? As David Fletcher says, "If you're designing a tank at home, don't for heaven's sake put the radiator at a different place than the engine. It's just not meant to go there."

On either side of the engine? Could work, but since I'm aiming for simplicity, one large fan and one large radiator seemed best.

Above the engine? Could work, but increases the vehicle's height, and you'd need to remove and drain the radiator before working on the engine. 

Above the torque converter (further back from its current position?) Could work, but then you can't lift the engine and torque converter combination straight up to repair or replace it.

In the end, I put it at an angle above the transmission. This wastes a bit of space, but the fan can be driven by belt from the engine, and pulls air through the entire engine bay, which is nice. 

To Do

  • Add steering clutch and brake assemblies to the differential.
  • Add a trench-crossing rail below the engine, probably with some sort of shock absorber.
  • Convert the drive sprockets to include modular sprocket rim segments. Instead of removing the entire wheel, you can just replace a worn segment. Handy!
  • Build a turret.
  • Build a fighting compartment.
  • Convert 1/35 figurines for the inside of the compartment, because if you're fully detailing a tank, it makes sense to add crew.
  • Add weld lines where steel plates are welded, and rivets/bolts where they are joined. Most of the construction so far is welded. 
If any readers are tank experts (internet or real life), or have experience with heavy construction vehicles, feel free to leave notes in the comments.