On Tue, 7 Jan 2003, Ian Woollard wrote:
> ...I figure the first stage
> could land back at the takeoff site with hardly much fiddling- DC-X
> style; so it looked about as good as a TSTO is ever going to be. Using
> HTP for this might be good, because then the engines restart easily.
In fact, HMX was building peroxide engines for the "launch assist platform"
of Kistler's "K-0" technology demonstrator, until Kistler got shaken up and
reorganized and its vehicle turned into a much more vanilla TSTO.
> It would be nice to airbreath, but that sounds rather too complex and
> lox and tanks are dirt cheap, and you need them above 10km or so anyway.
Quite so.
> Now John Carmack and Gary Hudson had tried a rocket powered rotor; but
> both had abandoned them. And I wasn't entirely clear why that would
> help, what's the theory?
Using rockets to run a propeller means that you no longer have to carry
all your reaction mass -- you can recruit some of it from the air. Your
tanks still have to supply all the energy, but not all the mass. That
raises your Isp; a given amount of energy yields greatest impulse
(thrust*time) at the lowest possible exhaust velocity. (Kinetic energy
is 0.5*m*v^2, momentum is m*v, so momentum/energy is m*v / 0.5*m*v^2 = 2/v,
highest at lowest velocity.) The price is higher dry mass.
The gain is modest: Gary later said that the gain from rotor aerodynamics
basically just carried the mass of the rotor into orbit, providing an
aerodynamic landing system with no payload penalty. Bear in mind that
propeller efficiency goes down the drain once the blades go supersonic.
The original Roton's rotor was primarily a high-pressure centrifugal pump,
secondarily a drag brake and landing system; using it as a propeller for
ascent was a minor bonus.
> optimal energy efficiency of a rocket engine is when the velocity of the
> exhaust is about the same speed as the vehicle moves at...
Correct, but for chemical rockets, in general you care overwhelmingly
about *mass* efficiency and not at all about energy efficiency. What
matters is the momentum imparted to the vehicle by a given amount of mass
from the tanks, and that is maximized by recruiting as much outside air as
possible, taking the exhaust velocity as low as possible.
> I think HMX abandoned this idea because of weight growth didn't they?
> Anyone?
Roughly correct; according to Gary, the *payloads* kept growing, and
scaling up the rotor systems to match took them into a region with some
uncomfortable technical challenges.
Henry Spencer
[EMAIL PROTECTED]
_______________________________________________
ERPS-list mailing list
[EMAIL PROTECTED]
http://lists.erps.org/mailman/listinfo/erps-list
