GT 101
Müller then turned to designs that removed the separate power turbine and instead required some sort of torque-maintaining transmission. The best solution to the problem would have been to drive an electrical generator and use the power to drive motors for traction (a system Porsche had tried to introduce several times), but a serious shortage of copper by this point in the war ruled out this solution. Instead some sort of hydraulic transmission was to be used, although not initially specified. Additionally, the new design included the rotating fuel injectors in the combustion chamber that the Heereswaffenamt was interested in. Müller presented the new design on 14 September, and the Heereswaffenamt proved considerably more interested – the deteriorating fuel supply situation at this point may have been a factor as well.
Oddly, they then suggested that any engine core developed for this role should also be suitable for aviation use, which led to the abandonment of the rotating injectors after all, and eventually to the use of a modified BMW 003 core. The basic layout had to be modified with the addition of a third bearing near the middle of the engine to help absorb shock loads, and a third turbine stage was added to the end of the engine to take off more power. Unlike the earlier design, the power-take off could be placed anywhere (not just off the free turbine stage) and was in fact moved to the front of the engine in order to make the design as compatible as possible with existing engine compartments. The basic design was completed in mid-November, and assigned the name GT 101.
Originally they had intended to mount the new engine in the Tiger tank, but although the engine was smaller than the engine it replaced, it was too long to fit in the Tiger's engine bay. Attention then turned to the Panther, which by this point in the war was to be the basis of all future tank production anyway (see the Entwicklung series for details). For experimental fitting, Porsche provided one of the prototype Jagdtiger hulls.
Fitting of the GT 101 in the Panther hull took some design effort, but eventually a suitable arrangement was found. The engine exhaust was fitted with a large divergent diffuser to lower the exhaust velocity and temperature, which also allowed for a larger third turbine stage. The entire exhaust area extended out of the rear of the engine compartment into "free air", which made it extremely vulnerable to enemy fire, and it was realized this was not practical for a production system.
A new automatic transmission from Zahnradfabrik of Friedrichshafen (ZF) was built for the fitting, it had three clutching levels in the torque converter and twelve speeds. The transmission also included an electrical clutch that cut off the engine completely at 5,000 rpm, below which the engine produced no torque on the output. At full speed, 14,000 rpm, the engine itself also acted a huge flywheel, which greatly improved cross-country performance by allowing some of the engine's speed to be dumped into the transmission to pull the tank over bumps.
In terms of performance the GT 101 would have been surprisingly effective. It would have produced a total of 3,750 hp, using 2,600 hp to operate the compressor and thus leaving 1,150 hp to power the transmission. The entire engine assembly weighted 450 kg (922 lb), not including the transmission. In comparison, the existing Maybach HL230 P30 it replaced provided 620 hp yet weighted a comparatively huge 1,200 kg. With the Maybach the Panther had a specific power of about 13.5 hp/ton, with the GT 101 this would improve to 27 hp/ton, outperforming any tank of WWII by a wide margin (for instance, the T-34 was 16.2 hp/tonne). For other reasons, essentially wear and tear, speeds would be deliberately limited to those of the gasoline-powered Panthers. The only downsides were poor torque at low power settings, and a fuel consumption about double that of the Maybach, which presented problems in finding enough room for fuel tankage.
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