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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Just as a crazy thought experiment ive been wondering if a cold metal turbine driven by an electric motor could exceed Mach 1 ?
Since combustion does not take place it would seem to me that slowing the incoming air to subsonic speed would be unnecessary. Diverter plates might not be needed either.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I googled this a little as it is mildly interesting,
- despite much effort, no one has achieved the speed of sound in horizontal flight using a propeller driven aircraft, due to the reason in the reply above, this is why it was called the sound barrier.
- I guess that a turbine designed for this purpose may work, as the increasing temperature (due to compression) at each stage increases the internal speed of sound, but the electrical power and weight involved probably means that a complete aircraft would not be feasible.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I am not an expert at supersonic flows, but it seems in principal (ie, with arbitrary amount of electrical power) one could use the scheme used in supersonic wind tunnels. Essentially, slow the incoming air (expand/cool it) to be subsonic, compress/heat it (while maintaining subsonic speeds in the compressor) and then speed/cool the exiting air using a nozzle to achieve supersonic exhaust velocity. Given the incredible amount of power required to overcome the effective viscosity of supersonic fluid flow you need an absurd amount of power density.
As a (perhaps unfair) comparison point, in amateur rocketry it is possible (but not completely trivial) to achieve transonic and even supersonic speeds using mid- and high- power APCP amateur rocket motors. For a 10lb (wet) rocket with Cd of 0.5 (at 0 velocity) you can just get to supersonic speeds using a 2000 N thrust motor with a 1s burn. At 1000mph that motor is producing a sizable fraction of a megawatt of *mechanical* power (and and order of magnitude more thermal). That includes accelerating the rocket body, but at 1000mph the lion's share of that thrust is just to overcome drag.
*calculations are all very rough and only order of magnitude accurate but are intended to show that it is not going to work using a lithium ion battery and electronic ducted fan to get to supersonic speeds on earth no matter how much you overdrive the motor
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Is it possible to exceed mach 1 with a electric turbine ?
