[CobaltBlue]

He may be right but he did not do a good job of explaining why he's right. In the context of the story that was being told, the example was almost a throwaway. It takes great skill to write a short example and make it coherent.

Most scientific concepts can't be boiled down into a sentence or two and do it total justice.

When you take away the long-winded explanations and the complex examples, and boil it all down to a couple of sentences, the original statement was probably as correct as anything you, yourself, could say in a sentence or two.

If you think you can explain it better in a sentence or two, please feel free. It doesn't appear that he can.

[gomaaa]

Yes! I've had to do that a lot when explaining stuff to friends and even on some posts I've made on this list. Doesn't mean he doesn't know what he's talking about, he's just trying to tailor it to his target audience.

On a side note, and I realize I am splitting hairs here just as others have been, but even if we could use a conventional propellor in space, we would still have to take all our fuel along with us. The whole example is somewhat unnecessary.

As for Zero Point Energy, cool stuff, and I am very curious to learn how they think they could use this as a energy source. These guys are totally legit and highly respected, but this sort of thing is easily twisted around by the "free energy" crowd. It has the potential to go right up there with cold fusion and the perpetual motion machine.

[XEHRpa]

He may be right but he did not do a good job of explaining why he's right

You are right, he did not.

the original statement was probably as correct as anything you, yourself, could say in a sentence or two.

As an engineer, and on a forum of nonengineers, I can live with approximate explanations. Were I a mathematician or talking to other techies, the distinction would be too great to blur over. The difference between applying an overt force vs. changing the momentum flux of a working medium are most definitely related, but not identical.

For example, a pure ramjet is an efficient air-breathing engine at high Mach. Ideally, it is an open cylinder (imagine a paper towel tube), that has burning fuel injected into the center of the cylinder. The problem with this design is it can't start from zero velocity, but if you could get the thing going fast to start with and then "turn on" the ramjet, it would work just fine. Clearly, the engine isn't "pushing" on the air flowing through this hollow tube. It is the burning fuel which raises the energy of the air in the tube, which makes it expand at higher velocity (momentum) out the back of the tube than it was entering at the front of the tube. It is, in essence a thermodynamic process, rather than a mechanical one, which is why the "pushing" analogy (a mechanical analogy) is not the best one to make.