British MkV* Tank
R/C Tank Combat is the science, art, and mental disorder of equiping giant remote control tanks with paintball guns. Think about it for a moment.
At the moment of this writing, most of the tanks in the hobby are from WW II and later. Never having been one to follow the crowd, I've decided that - resources being available - I'll build a tank from WW I. The MkV (pronounced "Mark Five") is the quintessential WW I tank - big, awkward, clumsy, thin-skinned, lightly armored, slow, and way too cool for words.
1/6th scale British MkV* Paintball Tank
In R/C Tank Combat, the greatest enemy of the MkV is ... the rules. Designed around more powerful WW II tanks, the rules make a WW I tank, with it's thinner armor and smaller guns, almost worthless on the battlefield. But, considering that a WW I tank actually would have been worthless on a WW II battlefield, the rules are entirely realistic. I would argue, however, that realism is a less important goal than fun, and may in the future propose a modification that makes all tanks equal in offensive and defensive rating. I haven't decided if this is a good idea or not, though, since such a change has the potential to entirely change the nature of the game.
Section II(3)a, "Scale Size", specifies that tanks must be either 1/6th scale or " be at least 3 feet long." The MkV was 26'-5" long overall, a very big tank even by modern standards. If I scale it to be 36" long, it comes out to only 1:8.8 scale. This is small enough that I think it would look funny compared to the other 1/6th scale tanks. Furthermore, if I build it to the smaller scale to meet the minimum length, other problems develop. The hull of the prototype was only five feet wide between the tracks. At 1:8.8, the hull ends up only 6.8" wide, into which I have to cram two gear motors, a big battery, CO2 tank, etc. I don't believe that I could do it. At 1/6th scale, it comes out to be 4'-5" long, and 10" between the tracks. Much better.
Section III(4), "Defensive Rating", of the R/C Tank Combat rules specifies how many hits by paintballs the model can withsthand before being declared "dead" based on the armor thickness of the prototype. As the MkV had a paltry ½" (12.6mm) of armor, it would only take two hits to kill. This is half of what it would take to kill a more modern tank, such as the Königstiger with it's formidable 150mm of frontal armor (the bottom armor of the Königstiger was twice as thick as the frontal armor of the MkV!). More importantly, frontal hits would count against the MkV, but not against the Königstiger.
Section III(3), "Offensive Rating", specifies 30 paintballs for a tank with a gun of less than 70mm. In this case, I suspect the rules may actually be working in favor of the MkV, which only had 57mm (six-pounder) guns. 20 paintballs would be much more realistic. On the other hand, the MkV carried something like 380 rounds of ammunition for it's main guns, versus only 40 or so rounds for modern tanks.
The final, and perhaps the biggest, problem is found in Section III(2), "Asset Armament", which specifies that a tank is only allowed a "single, semi-automatic paintball cannon". Since the MkV has two main guns (or at least the "male" version does - the "female" version had only machine guns), does this mean that one gun must be a dummy? This would effectively limit the field of fire to 120° on one side of dead ahead, versus 360° for modern turreted tanks. In e-mail discussions with the R/C Tank Combat Gods (Frank Pitelli, et al), I proposed a solution to this dilemma.
Could it be possible to use two paintball guns, but somehow controlled so that only one would fire at a time? ... As the joystick was held over to the left, the right gun would swing left (forward) until it was pointed straight forward, and then disengage. At that moment, the left gun would engage and continue swinging left (rearward). Another way to say it would be that the left gun would hand off a target to the right gun when it's bearing passed 0°, and vice versa. So even though there were two guns, it would _act_ like a single gun.
Fortunately, a special exemption for this particular case was made.
Taking into account the "cool factor" of those tanks, along with the unique situation that they represent, I think that an exception is in order that would allow you to arm it with two paintball cannons, each with a magazine of 15 paintballs, giving you a 30 round total capacity. As far as firing them, I think your idea of using a separate half-channel for each gun is within the intent of the rules, which simply require a distinct remote control action for each shot.
- Frank Pittelli
And with that exemption, we're good to go!
The design is still under development, but here is what I've got so far.
The drive system is very similar to the drive system in the prototype. On each side a motor drives a sprocket inside the rear "horns". This sprocket drives a chain, which in turn drives the rear track sprocket. In the prototype, there was a second sprocket and gear between the track sprocket and chain, but I've omitted this. Hell, realism can only be carried so far, after all.
The inner hull is divided into three compartments - the radio compartment in front, the "fighting" compartment in the middle, and the motor compartment in the rear. In the real thing, there were no internal divisions, making life a living hell for the crews between the noise, heat, and fumes. The main reason I divided the hull up this way was to insure that it was rigid, but also make ventilation easier - you can't see it in the images, but there is a fan in the bottom of the motor compartment.
Internal Layout MkV
The hull will probably be 3/16" Luaun, one of my favorite building materials. The floor and bulkheads may be ½" plywood, though. The sponsons, however, would be made of sheet metal folded into shape and either soldered or pop-riveted. The cupolas on top will be cut from solid EPS foam. If the foam proves too vulnerable to damage from paintball strikes, it will be replaced with sheet metal. The driver's cupola (the one in front) would be the perfect place for a tiny remote TV camera, so it may well end up being built from metal anyway.
The tracks used here are somewhat different from the tracks I designed and built for my 1/5th scale Sherman. These tracks are more like the tracks used by bulldozers and tractors - which, in fact, were the basis for the tracks on all German and most French tanks of WW I, which used track designs taken almost intact from the Holt Tractor.
For tracks, I hope to use DV-325-820 linked conveyor chain, available from Dyna-Veyor, among other places. This track is 3.5" wide and has a 1.5" pitch. Unfortunately, it also has a significant price tag of $50 for a 10' section, or 63¢ per link. That wouldn't be bad if I didn't need almost 200 links! I haven't priced the matching P820-19 sprocket, since I'm confident that I can build my own. Once you understand how sprockets work, it's easy.
The motors will be the same AMD 265-1002 motors that I used in my 1/5th scale Sherman, though I may instead use the 12v versions. The road wheels will be plywood, or possibly plastic, running in roller-skate bearings.
The biggest design challenge will be the paintball guns, and not just because of the one-gun rule. I'm not sure I could fit a commercial paintball gun into the curved shield! I've got a design of my own that I'd like to try - a "blow-forward, open-chamber, semi-automatic" paintball gun with one moving part (unless you count the spring). Because no part of it has to hold pressure, it could be made from plastic. Experimentation is in order.
Update 2/21/2004 - Paintball Gun
I've started work on the paintball gun. Check it out.
Update 8/17/2004 - Construction Begins
After an unexpectedly long break between design and construction, I have finally started work. I've got the sides cut out, and the floor marked and ready to cut. Hopefully by the end of the week I'll have enough of it assembled that it looks like something so I can post some pictures.
I was originally planning on building a MkV* ("Mark Five Star"), which was identical to the MkV except that it was six feet longer so it could carry a platoon of infantrymen. Unfortunately, the conveyor-chain tracks that Joe Sommer, of R/C Tank Combat fame, sold me would not have been quite long enough, so I switched to the MkV. This track is not the DV-325-820 conveyor chain I had originally planned on, but the DV-400-820, which is 4" wide. I've also got a pair of 21-tooth sprockets to drive them, which are unfortunately too big.
Making the tank a foot shorter had little effect on anything (except to push the battery forward between the guns) so I haven't changed all of the drawings - some still show the MkV*. I hope this doesn't confuse anyone.
I bought a whole pile of 2-¼" wheels (rubber tread with ball bearing hubs) from The Surplus Center for 50¢ each. They'll make excellent wheels for the tank when mounted in pairs on eighteen ½" metal axles on a 5" spacing. I am making the axles out of ½ steel rod on my metal lathe, cutting to length and turning down and threading the ends. (It's going to take a while.)
Over the summer I picked up a pair of Ford remote mirror motors, and plan on using them to elevate the guns. To rotate the guns, I am considering mounting each gun on a 40B25 sprocket and connecting them together with a #25 chain. This would allow a single motor to aim them both. Still thinking about it.
I may have found a sponsor - a local hydraulics company may supply me with high-pressure (several thousand psi) hoses and solenoid valves for the paintball guns. (Can you imagine a tank covered in "Holly Carb." and "Crane Cam" stickers and with the "Tide" logo painted across the glacis? I can, and it hurts.)
Update 8/19/2004 - Photos
Left Side View
Update 6/13/2005 - Back at it
After a long break, I'm back at it. I can't believe it's been ten months.
I've purchased some Siemens AS02-15-150-F (at least, that is what I think they are) motors from a surplus dealer. These motors were originally two-speed cooling fan motors for trucks, and are quite similar to the legendary "EV Warrior" motors, both in size and performance. Here is some information on them.
Voltage was provided by a big ol' marine deep-cycle battery, and measured at the motor.
Still to be determined is the stall current (my meter doesn't go that high), the resistance (depending on how I measure it, I get different answers), and free rpm. According to Joe Sommer, the stall current is 60A high speed, and the free rpm is 2365 rpm, but some of his other measurements are significantly different from mine, so I suspect we may have different motors.
For my next trick, I plan on seeing what happens if I power both the low and speed coils at the same time. Stand by for the explosion.
Because they're not gear motors, I've had to redesign the drive train to include some reduction. I am hoping to hit 6 mph, so after fiddling with the motors a while I've settled on a total reduction ratio of 7.11 to 1, in two stages of 2.67 to 1 (8 to 3), using 15-tooth and 40-tooth sprockets. I'll be using the leftover #25 chain that wouldn't stay on the sprockets in my Sherman Tank - we'll see if they work this time. I've built one of the motor mounts and installed the motor and the reduction shaft bearings; once I get the 15-tooth sprockets I'll be able to install it in the tank.
"Tank? What Tank?"
© 2003 W. E. Johns