Monday, June 20, 2016

M18 Hellcat Tank Destroyer

Powertrain: 2 L motors
Turret traverse: M motor
Gun elevation: M motor
Suspension: Torsion bars
Controlled by Sbrick
This model was rather interesting to build, as I wanted to achieve the functionality of the open turret, and include the details of the turret's interior.
Along with the turret, I wanted to replicate the drivetrain and suspension of the Hellcat. The Hellcat is known. For its high-speed, and powerful gun. So I first began by using the road wheels to scale the model. The scale is 1/22, making this a rather small vehicle with only 8 studs of space between the sides of the hull. I first began by replicating the offset torsion bars. The torsion bars are spaced 5.5 studs apart to allow for proper travel of the swing arms. It also features full-length torsion bars, this means the torsion bars run the full-length of the hull and are anchored on the opposite side. This provides a relatively soft suspension for the lightweight model.   The real vehicle was known for its higher kill to loss ratio amongst Allied tank destroyers, and the high on road speed of 55 mph (88kph). To provide power to the sprockets, I used 2 L motors drive the sprockets with a 1:1 ratio, so the model features a decent speed and torque. The position of the sprockets made it a challenge to connect the drive motor, and the sprocket, I attempted several different solutions with M motors to replicate the speed, but they lacked the proper torque to drive the model over an obstacle or resulted in an over complex amount of gearing that further hampered the torque or caused the vehicle to not track straight, because of the power difference.

The suspension, drive motors, and battery box take up a majority of the space available in the hull. With the remaining space available, the turret traverse and gun elevation motors are mounted directly below the turret in a compact configuration. Below you can see a couple renders of the turret traverse and gun elevation system. The turret traverse has a 11.2:1 reduction. I wish there could have been a greater reduction, but because of the limited space I was forced to use this solution. I attempted to reduce the turret traverse speed, by using the Sbrick profile designer and set the maximum value for the motor's output to different values, 75%, and 90%, but the m motor had issues with stalling, so I decided to leave the motor at 100% power.

The compact turret traverse, and gun elevation mechanism mounted in the hull allowed the turret to be internally detailed. The main components of the turret's interior is the gun breech guard, the side ammunition storage, and the radio in the turret's bustle. The external structure of the turret was entirely composed of SNOT techniques. I decided to use a similar technique that was utilized on my M4A2 Sherman Crab, this allowed for a compact structure that replicated a relatively round turret structure. I decided to omit some of the external features of the turret, such as the front storage boxes, and the stowage racks on the rear. As these features disrupted the smooth shape of the turret.

This model was an interesting challenge to build, it contains the more basic functions of a tank, and retains the detailed exterior and open turret.  I am pleased with the functionality and mobility of the model. There was some compromises made for the structure, such as the limited gun elevation. This is because of the breech guard limiting the upwards elevation, because of the turntable directly mounted below it. Maybe in the future I can better replicate an open topped turret, by completely removing the turntable and mounting it on the floor of the hull, this would allow for the "turret basket" to be connected to the turret, though the model would have to be at a much larger scale. Along with the turret, the hull was difficult to build in some places. One of the more difficult challenges that required several revisions is the "cheeks"that mesh with the upper glacis armor in the front of the model, I tried several solutions, using both bricks and SNOT configurations, though nothing seemed to look correct. Another was the lower glacis plates, as they are floating directly above the bevel gears for the drive motors. I wish more details could have been added to the turret's interior, but there isn't much else to add, besides the power traverse, ammunition storage, and more detail to the gun breech (controls, optics). Despite the minor flaws with the model I am pleased with the compactness and form of it.


More photos

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