> Date: Mon, 6 Oct 2003 15:11:02 -0700 > From: "ckpooley" <[EMAIL PROTECTED]> > Subject: [ERPS] CATS prize trajectory > To: <[EMAIL PROTECTED]> > > > 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. > > > **************************************** > 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 > > ------------------------------
We found exactly the same thing. An additional advantage of a very high initial acceleration is that we could use a launch tower/rail and not need avionics to do the "balance the yardstick" thing. The active guidence package only needed to remove the cumulative errors and keep the vehicle generally pointed towards "absolute straight up." We probably could have used a cheap rate gyro or angular rate sensor since zero rate is defined as the output value whilst sitting on the pad, so long as it did not drift in the last minute before launch or the 4 minutes of atmospheric flight. --- Doug Drummond. P.S. I *am* an avionics engineer. _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
