I’d written this as a brief essay in the ARCFT-115 class and figured it may be of value to the world:
Turbojet-based Unducted Fan
Turbojet-based engines — which continuously compress ingested air, combust the compressed air combined with injected fuel, and leverage the exhaust gases to drive the compressor assembly — provide a considerably higher power to weight ratio than reciprocating engines.
Presently, amongst fixed-wing, commercial airlines worldwide, there are two primary types of turbo-compound turbojet engines in use: the turbofan and the turboprop.
Subsequent to the oil crisis of 1973, NASA-guided research sought greater fuel efficiency gains in propulsion technologies for commercial aerospace than the ubiquitous turbofan and turboprop engines.
One of the proposed and tested implementations was of the “propfan” model.
The propfan combined the well-understood technologies of both the turbofan and turboprop puller-type designs but included a second, coaxial fan counter-rotating to negate the thrust torque imparted by a single-fan/single-propeller engine in a pusher configuration.
NASA installed a test engine, constructed by General Electric as the GE36 propfan, onto a Boeing 727-100 airframe replacing one of the aircraft’s three engines. Research on this test model found that the tested propfan realized 35% greater fuel efficiency than its turbo-compound counterparts. Additional tests were performed on a McDonnell Douglas MD-80 finding similar results.
The onboard perceived noise — described as “buzzsaw-like” by witnesses at the time — would be minimized at cruise speeds by mounting the engines aft of the passengers and crew. By modern standards, this requirement will present significant design challenges to the existing wing-nacelle preference.
Testing in the mid-1980s revealed that the ground-level noise pressure of 70 dBA from 30,000 ft overhead flights — as loud as a typical household vacuum cleaner. Testing revealed that the noise signature could be lessened, but with a complete loss of all efficiency-gains realized with the propfan model.
Combined development efforts on the planned Boeing 7J7 and McDonnell Douglas MD-94X were halted. The carbon-composite technologies realized in the GE36 Propfan have been implemented in the current generations of the GE90 and GEnx installed in the Boeing 747, 777, and 787 Dreamliner aircraft.
It would be a worthwhile model to investigate, learn, and explore further leveraging the combination of the additional 35 years of engine efficiency technology, noise abatement, and higher cruise altitudes.
And now, the addendum:
Celera 500L. Yes, it’s a V-12 reciprocating engine. But, what if…
What if engineers were to develop a suitably-sized propfan for it, might it be viable?