Bog wrote:And, if you're running the engines - <snippage> You have linear thrust, shoving backwards, rolling the treadmill the other way.
Ummm. Nope.
Engines spooled up, causing a massflow through themselves, which I also assume to be pointing directly backwards and it's not a harrier or a WIG aircraft etc etc, resulting in an equal and opposite force acting on the motors themselves, and thus to the airframe to which they are attached. And not to the treadmill (except by the secondary and rather smaller action of air friction caused by air entrained by the main massflow through the engines and then impinging on the treadmill).
If the motors acted directly on the treadmill, then they would, in the normal case of things, be acting *directly on the ground*, at which point you would not be able to accelerate an aircraft as soon as it took off. One could, of course, achieve this situation "relatively simply" by attaching wings to the body of a car such that takeoff speed was lower than the maximum speed attainable by the car (with wings attached, obviously). Put the car on a runway. Floor the accelerator pedal. Super-flying-car rushes down the runway, reaches takeoff speed, lifts, and, suddenly, there is no power accelerating it any more. It's a glider. As it inevitably loses speed, it will touch down again.
Aircraft motors act on the medium through which the aircraft is moving to create a motion relative to that medium. Motion relative to the gound or any other point is irrelevant to the flight of an aircraft. It could not be otherwise.
Similarly, the "rocket with the big fan" thought experiment is pointless. Rocket motors don't "push" on anything, they create mass flow. They move mass (fuel) from the rocket and accelerating it backwards out of the motor, one way or another. Once thefuel or residue is out of the rocket, it ceases to have any effect on anything except the external environment. This is why rocket motors work in a vacuum; they don't have to "push" on anything except the fuel they are ejecting.
So, it doesn't matter what you do to the ejected fuel, how many big fans you put in the exhaust stream, the simple fact of having ejected the fuel will create thrust acting on the motors, and the rocket will lift off. If you don't want the rocket to accelerate, then don't start the motors.
Simon
