Talk:Exoskeletal engine

A bit dubious

I know they claim that this would allow for ceramic blades, which would lower weight, but would that be enough to offset the weight of a metallic drum assembly stressed to spin at extremely high speeds without disintegrating? We're talking about taking the fastest moving part of a turbine engine, the lightweight blade tips, which are spinning at a very high rate, and replacing them with a solid metal drum assembly that has to be strong enough to spin at an equal rate while not tearing itself apart by centrifugal forces, as well as retaining the (lightweight) blades inside. There is also the question of bearings; a normal turbine has a small bearing area, located in the slowest moving part of the engine, the shaft. An engine of this type would require a larger bearing area (to support all around the diameter of the drum), and bearings that would be running at a far greater speed. I realize it mentions bearings in the text, but there are more challenges than it suggests. Unless of course, they intend to use bearings on a central shaft, supporting a rotating outer drum that retains the blade assembly...something like a "blisk", only extended fore and aft a great deal. I also don't understand how this is supposed to decrease the diameter of the engine...simply from this theoretical higher pressure ratio allowing for equivalent power from a smaller engine?

There is also the question of supporting a central structure; presumably, you would require a stationary central shaft of some sort, to mount the stator blades onto, and to keep the air flowing through the faster areas of the fan disc (basically just like a conventional engine, only with a fixed shaft and rotating casing). This would also prevent an area of stagnant flow in the center of the drum, where the fan blades would be moving very slowly and in a small circle. Since there is no way to run a support through the wall of a spinning drum, the central assembly would have the be fixed at each end, or at least at one end, with the drum assembly rotating around it (think of a toilet paper holder). This would not only require disruptive support members running through the intake and/or exhaust stream, but it would also add length to one or both ends of the engine, since the supporting assembly must logically run past the drum in order to hold the center shaft fixed. And most likely, some form of outer casing would be required, to hold the theoretical foil or magnetic bearing systems; the engine would have to be a unit which could be transported, installed and removed separately and easily from the parent vehicle, and unless you build the outer half of the bearing system into the aircraft itself, you'd require a outer casing to retain the inner and outer bearings.

This central shaft support would also have to be strong enough to absorb the thrust of the engine; normally, an engine has a number of mounting points connecting it to the parent vehicle, so when the engine thrusts itself forward, the vehicle is dragged along with it. Since you can't put mounting lugs onto a spinning drum, you'd have to have the mounting points connected to the center shaft support, unless your theoretical foil or magnetic bearings were capable of dealing with both radial and fore-and-aft forces. And of course, you would have to mount injectors, combusters, igniters, etc, on the central drum as well, and all the fuel piping and wiring would have to be run through the support members at the ends on the drum. And there is the minor matter of accessories; theoretically they could be mounted remotely on the vehicle, but there is the question of how to extract the rotational power to energize them. It's a simple matter to run gearing from the nose end of a turbine spool/shaft, to drive a generator and/or hydraulic pump, but it would be harder to do the same with a rotating drum assembly, unless you were to simply put a gear or wheel against the outside of the drum and allow the passing surface to spin it, like one of those little bicycle generators with the little wheel that rolls against the rim or tire as you pedal. Anyway, interesting notion, but I wouldn't be surprised if material advances make it not worth the effort to deal with the difficulties. Just a hunch..45Colt 01:33, 26 December 2015 (UTC)