|Animation showing the operation of a free piston turbine engine|
Most airborne turbine development during the Second World War wasn't focused at first on jet engines but on superchargers that were driven mechanically by the engine to compress the thinner air of higher operating altitudes so that aircraft engines operated as if they were in the richer air of lower altitudes. Turbochargers operated on the same principle and tended to be more complex do the exhaust ducting used to drive the turbines that compressed the air for the engines. During the war, General Electric was one of the top firms in supercharger and turbocharger development. However, as early as the late 1930s, Pratt & Whitney had been sponsoring small research programs at the Massachusetts Institute of Technology (MIT) to forward its own efforts in supercharger development. In early 1941, one of the MIT engineers, Andrew Kalitinsky, along with his Pratt & Whitney liason, John Marchant, began discussing a new sort of propulsion system based on a free piston engine- basically two opposing pistons would compress air in a combustion chamber between them with the exhaust being ducted to drive a turbine which in turn drove a propeller. Turbochargers already were in use that collected exhaust gases and drove the turbine for the supercharger- aircraft like the P-47 Thunderbolt used this method. The difference in a free piston engine is that the pistons aren't connected to a crankshaft, hence being "free" but are used to compress air for combustion to drive a turbine which is what provides the rotational power to a propeller.
Free piston engines were already in use as air compressors. On German U-boats, for example, a four pairs of free pistons in series were used to generate compressed air for launching torpedoes. Kalitinsky and Marchant's idea for an aircraft free piston turbine engine went up their respective chains of command at both MIT and Pratt & Whitney with all involved interested in the concept. On 6 September 1941 a formal report based on MIT's studies was submitted to Pratt & Whitney titled "Free Piston Gas Turbine Power Plant for Aircraft". The proposed engine had eight pairs of free pistons (eight stages) as a gas generator that drove a turbine that in turn drove a propeller through reduction gear. The exhaust gases after spinning the turbine were discharged through a variable area nozzle for additional propulsive thrust. In addition, the duct work incorporated a burner for extra power like an internal afterburner. The report suggested that such an engine could drive a fighter aircraft to Mach 0.75 at 40,000 feet or power a four-engine bomber at Mach 0.6 at 40,000 feet.
The proposed fighter engine at low altitudes had nearly all the propulsive power coming from the propeller, but the proportion from the turbine exhaust increased as altitude increased. At the operating altitude of 40,000 feet. 2/3 of the propulsion would come from the prop and 1/3 of the propulsion could come from the turbine exhaust. In the bomber engine at operating altitudes 3/4 of the propulsion came from the prop and 1/4 of the propulsion came from the turbine exhaust. For comparison, consider the widely-used PT6 turboprop engine- about 85% of its propulsion comes from the prop and 15% comes from the exhaust gases. The specific fuel consumption of the proposed free piston turbine was 0.36 lbs/hr/HP for the bomber engine and 0.41 lbs/hr/HP for the fighter engine. This represented about a 30% reduction in fuel consumption over the piston engines of the day. Again, for comparison to a modern turboprop, the PT6 engine has an SFC that ranges between 0.64 to 0.59 depending upon the variant.
The report summarized the potential advantages of the free piston turbine over the piston engines of the day:
- 1. Improved fuel economy.
- 2. Reduced weight.
- 3. Reduced cooling requirements.
- 4. Flexibility in installation due to the smaller size.
- 5. Since the turbine exhaust contributed to propulsion, the propeller could be smaller.
- 6. Less fatigue stress since the engine torque would be minimal.
- 7. Use of alternate fuels than avgas.
|The PT1 test article at the Pratt & Whitney Museum|
A new engine designation system would be needed since the free piston turbine was a departure from Pratt & Whitney's established business. "P" would stand for propeller and "T" would be for turbine. The free piston turbine was launched as a company-funded program on 27 October 1941 as the PT1. The PT1's pistons were the same as that used on the R-1830 radial engine. The initial test engine was built out of cast iron since it wasn't going to be a flight-worthy engine. The main challenge for the small PT1 team was getting the two pistons to oscillate symmetrically at high frequency. First run was in August 1942, but again, the technical challenge was getting just two opposing pistons to synchronize. Imagine getting eight for the proposed engine! By March 1943 the PT1 test article was running as intended. In parallel to the free piston work were two other efforts- work on the turbine and work on the burner. Two types of burners were tested, one before the turbine that could boost power to the prop and a second one that was in the turbine exhaust as a rudimentary afterburner.
|Cross section model of the PT1, showing the opposing cylinders|
It's important to realize at this time, the PT1 was a low priority program despite the potential advantages. The US military didn't want resources to be diverted from established radial engine programs that were crucial to the war effort. In 1943 there were only 74 personnel assigned to the PT1 program but the engine had run for 400 hours and 280 hours were run with the turbine. Considering the modest budget, small staff and that the PT1 pushed Pratt & Whitney's metallurgical techniques and limited turbine experience, that run time is quite an accomplishment for the technology of the day. Based on the testing so far, the PT1 team concluded the bomber application was most promising and should be the focus of the team's efforts. A comparison was done with a B-29 compared with a B-29 powered by the PT1. For the same bomb load, the range was better with PT1 engines going from 5200 miles to 8900 miles. On a long distance 2900-mile mission, the bomb load was also heavier, going from 7600 lbs to 25,600 lbs! In March 1945 Pratt & Whitney proposed developing the PT1 further to a 4500 hp demonstrator engine with the military designation T32. By this point, though, jet engines were a better-known quantity and offered even more power for relative simplicity compared to a production PT1 engine.
At war's end the PT1 program was canceled but it wasn't for nothing. Many of the engineers who worked on the turbine section of the PT1 would go on to have great influence in the design of Pratt & Whitney's jet engines like the J57/JT3 turbojet and much of what was learned in the PT1 went to the the company's first true turboprop, the PT2 program which was started at the end of the war. The PT2 became the T34 turboprop engine that powered the Douglas C-133 Cargomaster and was flown prior to the Cargomaster on a Boeing C-97 Stratofreighter, a Lockheed Constellation, and a Douglas C-124 Globemaster. The PT2/T34 was Pratt & Whitney's first axial flow gas turbine engine that set the stage for later engine designs.
Source: The Engines of Pratt & Whitney: A Technical History by Jack Connors. American Institute of Aeronautics and Astronautics, Lockheed Martin Library of Flight, 2010, pp161-172. Photos: Wikipedia, www.enginehistory.org (Kimble D. McCutcheon)