KarenRei wrote:Sockmonkey wrote:I think what some people are suggesting is essentially a scramjet/ramjet where the flame-holder is a rocket engine that provides the initial startup thrust to get it up to ramjet operational speeds then throttles back as the ramjet portion starts providing more thrust.
Not exactly. The concept presented doesn't inherently have to deal with any combustion with the air brought in - the air is a working fluid. It could just as well be martian air or venerian air or whatnot.
A rocket's efficiency is at a maximum when all of the heat of combustion and all of the pressure have been fully expanded to ambient temperature and pressure.
No. That's a finite length nozzle, except in a perfect vacuum. And even there, there's a optimum nozzle length where increasing the length adds weight and slows you down overall.
This would require an infinitely large nozzle, so one does as much as they can with the nozzle that they have. The more gas involved (even gas that doesn't take part in the reaction), the more fully they can approach ambient for a given-sized nozzle.
No, and actually the space shuttle main engines went quite a long way below ambient at take-off. The flow was about three psi. It's just that going below ambient reduces the thrust. That was fine for the space shuttle, they wanted to make sure that the engine was working properly before take off.
The point of air-augmentation in rockets - whether or not it can also attain some degree of ramjet functionality - is to make use of the air as additional working fluid to increase your rocket's efficiency.
The problem is that the extra weight of the duct and associated equipment usually cancels out the extra performance advantages.
SABRe is about the only one that looks like it's a major advantage.