> Doug Drummond here: ... > One big problem with short burn times for CATS or XPF category > rockets is the drag and resulting destructive forces due to high > mach numbers at low altitudes. [ Q = rho * Vee ^ 2 ] To keep > "Max Q" down to a low roar, 'Everybody' waits until rho is at least > one fourth of see level, meaning at least 30-40 kilofeet. It is NOT > a coincidence that jet aircraft also cruise in that altitude range. > I'm using a very simplified atmospherical model, in which the density > is down by half for every 20 kilofeet of altitude. This is a fairly > good model up to 50-60 Kfeet. > **************************************** When I was considering a CATS Prize entry, I found that designing the rocket to reach a high subsonic speed quickly, followed by a reduction of thrust to about 1.5 times the vehicle mass for a long burn time of 100-120 sec worked best. The velocity at the end of the quick acceleration was to be nearly constant for a while (thrust overcoming drag by a small margin), and that mach 1 would be reached at over 30,000 ft. Following that, the acceleration builds rapidly due to reduction of air density and mass. An overall MR of about 5 was needed to exceed 200 Km. The calculations suggested that the rocket could be as light as 100 lbs at end of quick acceleration period.
The initial acceleration was to be accomplished by a separate booster, or an "afterburner" arrangement which would inject additional propellant downstream of the throat. Charles Pooley _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
