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The Scale
In order to provide the performance stated above, the thrust needs to be, in SI units, ~1.2 MN. As a starting point there are some common "rules of thumb", to do with achievable "electromagnetic shear stress".
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Timothy Miller* gives the following table:- Stating that the values quoted here are for continuous rating, and that peak ratings may exceed these values by 2 - 3 times. |
| Motor Type and Size | (psi.) |
|---|---|
| Fractional TEFC Industrial Motors | 0.1-0.3 |
| Integral TEFC Industrial Motors | 0.5-2 |
| High-Performance Industrial Servos | 1.5-3 |
| Aerospace Machines | 3-5 |
| Very Large Liquid-Cooled Machines (e.g. Turbine Generators) | 10-15 |
*T J E Miller Brushless, Permanent-Magnet and Reluctance Motor Drives, OUP 1989.
As a very rough guide, taking the intermittent duty into account, we begin with 10 psi for an induction machine, 20 psi for a permanent magnet machine, and 30 psi for a wound field, possibly super conducting coils, providing very high flux density. In SI units these stresses are ~ 70, 140 and 210 kN/m2. Thus to provide a maximum thrust of 1.2 MN, the shuttle needs an active surface area of ~18, 9, and 6 m2 respectively.
While the above is a good starting point, scaling issues are very much determined, in large machines, by thermal management issues. If heat sinking is good (as it would be for an iron stator fixed to a steel ship structure), These numbers can go higher. Current research is identifying some likely sizes for detailed modeling.
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