Originally posted by esecallum
but you are forgetting that high exhuast velocities means you have to carry less fuel ...which makes the ship smaller...lighter...which means you need less thrust...

in other words a downward spiral in overall energy requirements...

You won't get a high exhaust velocity without more fuel though. You need the fuel in order to get the exhaust obviously, but the faster you want the exhaust (and hence the aircraft) to go, the more fuel you need. Basically no matter how you look at it, you need more fuel. Unless you've perfected vacuum energy, then you can't get energy from nothing.

I'll second that (ye gods, a Rotherhamite backing up a Dee Dar - whatever next? ) . How do you propose higher exhaust velocities for less fuel?

This is readily achieved with HBPR airliner engines, which is what makes them fuel efficient in the first place, precisely because of the acceleration of atmospheric air, that is NOT compressed and burned, that we are all 'obsessed' with.

Or are you referring to this on board air that you have filled the fuselage with? Bearing in mind of course that the amount of air you can carry aboard a 747 will pass through its four engines in a matter of seconds, even if you could somehow close the fans off and pump it directly to the engine.

In the absence of this air, and with less fuel on board, how is this velocity achieved, fairy dust?





[edit on 28-1-2007 by waynos]

Higher exit velocity is indeed a way to increase thrust while holding massflow, no argument there. To increase the exhaust velocity of the engines on a 747 operating in the notional 'onboard fuel-oxy only' mode, the nozzle must be able to switch from the typical operating mode of conic to con-di, and be able create a nozzle Aexit/Ath ratio much higher than its typical area ratio (unity for sonic exhaust). Even if this mechanical design challenge can be overcome, then the combustor must be able to withstand the high chamber pressures/temperatures necessary to create a nozzle pressure ratio capable of accelerating the flow to supersonic velocities. Even if all the engineering challenges of converting a turbofan engine into a rocket engine (in mid-flight, mind you) can be overcome, then you have the problem of plummeting propulsive efficiency as you make thrust with exhaust velocity rather than massflow. Since you plan on flying the 747 at a roughly sonic velocity, every ft/s faster the exhaust velocity is than the aircraft, the propulsive efficiency goes down. This will only add to your fuel consumption problem.

Looks as if this topic has finally been put to bed. I don't know if I'm glad or dissapointed.