[image credit enginehistory.org]
Starting in the mid nineteen forties there was a move to this newfangled jet turbine technology. Still it was mostly untested and the good old internal combustion engine was better known. Into this small fragment of time was an interesting innovation of Pratt & Whitney’s.
[image credit – enginehistory.org]
By 1944 the end of the line for piston engine technology was in sight. On the flip side jet engines were still to new and used too much fuel for long range aircraft. So an intermediate option was explored by engine manufacturer Pratt and Whitney. Their plan was to utilize the exhaust from their two largest engines, the R-3350 and R-4360, to be the first fitted with this technology.
[image credit – Wikipedia.org]
The idea of using the exhaust to provide additional forward trust wasn’t new to the industry. As you can see clearly here the Japanese laid out the exhaust for their relatively small and low powered radial motor to aid in forward momentum. This was also the case with the P-51, the P-40 and most aircraft using the Allison V-1710 inline engines.
To accomplish this Pratt and Whitney made several changes to their big radial. First they removed the mechanical supercharger. Next was a move from a carburetor to direct fuel injection and finally the addition of the variable discharge turbine to the rear of the engine. This final piece was to allow the exhaust gasses to be passed through a turbo with the exhaust exiting out as thrust.
This setup was expected to give an additional eight hundred horsepower. With this in mind several aircraft were designed for several bomber upgrades to utilize this power. Of these the final version of the Boeing B-29 Superfortress was to be called the B-54 Ultrafortress using these engines. As well as a very unusual variant of the Convair B-36 to be discussed later.
During testing issues were found with fuel distribution due to the removal of the mechanical supercharger. A larger issue was that due to this setup they had very limited control over the throttle. During testing they controlled the throttle by opening and closing of the turbine exhaust. A setup that would not work in combat or even in production. These issues could have been addressed but the writing was on the wall. The next generation of jet bombers were already on the drawing boards they found no need to put this into production.
[data from wikipedia.org]
- Type: 28-cylinder supercharged air-cooled four-row radial engine
- Bore: 5.75 in. (146.05 mm)
- Stroke: 6.00 in. (152.4 mm)
- Displacement: 4,362.5 in³ (71.49 L)
- Length: 96.5 in. (2 451 mm)
- Diameter: 55 in (1397 mm)
- Dry weight: 3,870 lb (1,755 kg)
- Valvetrain: Poppet, two valves per cylinder
- Supercharger: Gear-driven single stage variable speed centrifugal type supercharger
- Turbocharger: General Electric CHM-2
- Fuel system: Bendix-Stromberg PR-100E2 pressure carburetor
- Fuel type: 115/145 Aviation gasoline
- Cooling system: Air-cooled
- Power output: 4,300 hp (3,210 kW)
- Specific power: 0.99 hp/in³ (44.9 kW/L)
- Compression ratio: 6.7 : 1
- Power-to-weight ratio: 1.11 hp/lb (1.83 kW/kg)
This is but a footnote in the history of the large aircraft radial engine but it does lead to some interesting questions and discussion. The issues they had during testing that could have been ironed out but there really was no point. The early turboprops were rated at 5000hp and were far less complex then this monster. The maintenance needed to keep these running would have been very intense. So now we just step back and do the classic what if’s.
[image credit – retromechanix.com]