New Supersonic BizJet - Radical New Turbofan

[allardjd 1,853]

New Supersonic BizJet - Radical New Turbofan

http://www.aviationweek.com/aw/blogs/aviation_week/on_space_and_technology/index.jsp?plckController=Blog&plckBlogPage=BlogViewPost&newspaperUserId=a68cb417-3364-4fbf-a9dd-4feda680ec9c&plckPostId=Blog:a68cb417-3364-4fbf-a9dd-4feda680ec9cPost:ed395b1c-7923-4f8b-95ed-30de4c7d050a&plckScript=blogScript&plckElementId=blogDest

I found this article linked in the Mutley's Hangar Review - not sure what to think about the link itself - glad I didn't have to type it.

Anyway, the article is about a company that is getting patents on a Mach 3.8, 85,000 ft. altitude bizjet. The really interesting thing for me is the description of the engine...

...the Magnetic Advanced Generation Jet Electric Turbine (MAGJET), described in another patent application. This essentially burns jet fuel to power a turbine generator that produces megawatts of electrical power to, in turn, drive the fan and compressor electrically, and not magnetically.

I think the last word of the quote is a typo - that should probably be "mechanically", since the usual way is to have the compressor/fan on the same shaft with the power turbine. Anyway, this is a pretty radical change in the concept for the turbofan engine. Driving the fan/compressor with an electric motor really relieves designers of the need for all the engine components to be co-axial. It will no longer be necessary for the front end to be shafted to the power turbine - obviously the air will have to be ducted between them but some current turbo-props have air-flow paths that are pretty convoluted already. Though the sketch of the bizjet in question shows everything looking co-axial, in other AC that might be designed later using this kind of engine, not having to have a shaft coupling the engine sections will allow them to be placed in non-conventional orientations, possibly allowing some new and interesting airframe configurations.

A question this raises with me is where the payback is for all the extra weight of a multi-megawatt generator and motor. Those devices require low-permeability iron laminations and lots of it - not lightweight stuff so this will be a heavy engine. Someone must think there's a benefit to be derived from doing that but it's not obvious to me where it comes from.

John

[stu7708 244]

That's an interesting concept, and the benefit I think will be fuel efficency even with that added weight.

I think I read about a concept hybrid car that ran on a similar concept, a Ford if I remember correctly. It had an electric motor for propulsion, and a small petrol driven engine with the sole purpose of powering a generator. The biggest benefit in that car was that the petrol enginge could be designed to run at an optimal RPM with regards to fuel efficency, and then hook it up to a generator that provides enough power to the electrical motor regardless of what speed you were traveling at.

This meant that it was incredibly fuel efficient. If I remeber correctly we have a slightly different approach on presenting fuel efficency in sweden compared to the US and the UK. We present it as liter per 10 kilometers instead of miles per gallon, so we look for as low a number as possible. A small car, say a Nissan Micra or Toyota Yaris, running on petrol will use about 0,5 liters per 10 kilometers. This concept car used less than 0,1 liters per 10 kilometers on a car much bigger than those, even with the added weight of the batteries for storing surplus electricity.

[Tim_A 997]

The main issue with a heavier engine will be in reduced load capacity, since tha MAUW of the aircraft will stay the same. Now, if the increased efficiency of the engine means it requires less fuel, that weight saving will offset some or all of the new engine weight.

[allardjd 1,853]

Thinking some more about this, there will have to be a pair of gearboxes, adding again to the weight. It's not practical to have a generator turning at rotor speed nor a compressor turning at motor speed. There will have to be a reduction gearbox between power turbine and generator and a speed increaser gearbox between motor and compressor/fan. Motors and generators typically arent' run at those speeds.

You guys may be right and the power turbine in particular could be run at a fixed speed. Some turboprops (e.g. C-130) already use that scheme. Electrical output would be controlled by varying excitation, probably. I just see this powerplant as being stupendously heavy, and there's almost always a lost power penalty to be paid for converting energy from one form to another. In this case it happens twice, once from mechanical to electrical and once back to mechanical (neglecting the conversions from chemical to heat to mechanical, which occur in any engine).

Anyway, it's an interesting concept and I hope to read more about it soon. Not sure this has gone very far beyond the patent stage so far.

John