Propforce - 10/10/2006 11:20 PMTo understand why, you must know the difference between a GG cycle and a SC cycle. In addition to the pressure compression limit of its turbopumps, the GG cycle is inherent less efficient with its C* efficiency than both the SC cycle and the expander cycle because the MR is inherently higher in the main combustion chamber (MCC) of a GG cycle engine. All these compound to a less efficient engine, but less expensive to build than a SC cycle engine. It's suitable for a throw-away (expendable) engine.
TyMoore - 12/2/2007 4:40 PMI think that what some of the others were getting at is that: sure, you could put a bigger, higher pressure pump on an RS-68. Then you'd have to redesign the injection manifold to handle the increased flow and pressure drop. And then you'd have to redesign the chamber to increase the (I think it's called the Critical Length?) the Volume of the combustion chamber slightly to handle the increased flow. And the nozzle throat would have to be changed (increased slightly.) Then you'd have to re-analyze the acoustics of the engine and redesign the injector baffles to suppress harmful resonances--build a few boilerplate engines and test them on the stand, maybe even blow a few up in the process. The short answer is that it would be a different engine--and I think that's what is trying to be avoided. What I understand about liquid propellant rocket engines, is that they are extremely fickle beasts. Once you get the design just so it works, it's better to not mess with it too much after that...Atleast, that's my naive, non-professional take on it!
publiusr - 18/2/2007 5:14 PMAccording to Space News, RS-68 upgrades with have both Air Force and NASA support. Very good.
publiusr - 18/2/2007 5:33 PMAnd which bit would that be? Space News has good people working for it.
TyMoore - 13/2/2007 6:08 AM...well, how else are you going to increase the chamber pressure? I suppose you can make the injector orfices slightly smaller, and squeeze the volume of the chamber slightly, and also decrease the area of the throat. It's still going to be a different engine--but I'll admit it ought to be closer to the original than going the other way...[I don't have my Rocket Propulsion Elements, 4th edition infront of me either!]