The whole pressure differential analog is only true for an ideal M/E thruster, with zero losses.
Okay... that kinda sorta makes sense, but you should realize an ideal M/E thruster (i.e. one that can generate thrust at arbitrarily high energy efficiency) is far more
valuable as an energy generator, as 93143 went into.
It would not be. Even an ideal thruster would comply with the law of conservation of energy: It may be able to supply a tremendous amount of static
thrust, but once you start things moving and the thruster is under load, it will draw just as much energy as the work being effected on the target object.
Energy in means energy out, no strange physics here.
It's the same fallacy made by some 'free energy' enthousiasts out there who confuse 'voltage' with 'power'. Just because an electric motor generates a certain amount of back EMF, does not mean you can extract power from that back EMF. For that, you need an element that can supply current as well.
Physically, force (thrust) is actually the analog of voltage, and if it has no path for current (momentum) to carry, or has no means of supplying that momentum as well, power output is zero. Practical example:
Suppose you have an ideal M/E thruster that gives you 500N of thrust at 12V of voltage across it. It's happily pushing against a wall, not doing any work, so the current (and therefore power
) through it is zero.
Now we remove that wall and let the M/E thruster push a 1kg cart forward. It will do so with the same 500N, accelerating the cart with 500m/s each second. In the first second, the power draw will be (on average) 250W, in the next, it will increase, up to the point where air resistance takes the cart to terminal velocity. At which point, the amount of power drawn will be equal to the amount of power spent in drag at the front.
The actual thrust on a moving object now only depends on the power source upstream. As long as it can source enough current through the thruster at the same voltage, thrust will sustain. Hence the term ideal
truster, with an energy conversion efficiency of 100%.
Remember the 'Newtons per Watt' figure is only a static
measure of the amount of losses when the thruster is not
doing any work. If it is doing any work, all of the energy required for that will be added to that. However, dynamic situations have not been tested.
You can calculate the same values for a wheel or other mover, just bolt some cart down to a track and apply power to the wheel while its target cannot move. A wheel with a lot of slip will have a lousy figure of motive force versus power, while an ideal wheel will never
slip and the motive force for 1W will reach infinite.
Release the brakes, and the amount of power required will just be dependent on the efficiency of your wheel. A non-slipping wheel will require just as much power as is needed to keep the vehicle going, where a lousy wheel will require a crap-load of power more to keep the vehicle going and
slip at the same time.
Because we can.