On Tue, 27 May 2003 15:49:20 -0700, "Ken Doyle" <[EMAIL PROTECTED]> wrote:
> I'm not so sure that I'm the one missing the point, Henry. The takeoff >isn't done when you've rotated. Consider Randall's takeoff; Vr at 1600 ft >and 200 mph/300 ft/sec. Great, but the engines just all shut down at the >aforementioned 100m altititude mark after you rotated. Being generous and >presuming that you've gone for the safety of altitude rather than kept the >deck angle shallow to go for speed first, you are now at 100m/330ft, heading >upward at about a 30 degree deck angle and barely over your 200 mph Vr, and >at least 3000 ft of your runway is now behind you. The runway ahead >required to get down and come to a stop from that predicament will indeed be >in the "incredibly long runway" class. If you make the deck angle 60 degrees, or 75, or 90, this changes. The dead zone gets smaller, and may go away. This becomes an operational restriction. It may become an onerous operational restriction, in which case you trade it against other failure modes until you make mission again. >Select the F22 Raptor as our rocket-aircraft takeoff simulator. Load it up >with full fuel, and add payload up to the Gross Wt. of 64,000 lbs. Set up >your joystick with buttons to shutdown the engines, deploy flaps, brakes, >speedbrakes, etc. >For takeoff, let the engines spool up into afterburner before releasing the >brakes. And what is their thrust at that point? If it isn't at least 50,000 lbs, I would characterize the vehicle as not operationally useful. >Rotate at 200kts, which will be right at the 2,000 ft. mark. Hold >about a 15 degree AOA, and hit the engine shutdown button at 300 ft. AGL. Again, too shallow. You're absolutely right that you'll have a big dead zone if you continue to fly down the runway. So don't do that. Go for altitude. Pull at least 2 g until you're at max climb. > HTHL rockets *are* just like jet aircraft in the case when the engines are >all off at very low altitude and you're at gross weight. Just like any high >wing-loading jet in such a predicament; you're flying a brick and the future >doesn't look pretty. I generally agree. I'm just not sure a vehicle -can't- be designed that doesn't have a dead zone on takeoff. It'd be hard, sure. If it was easy, someone else would have done it already. >Design your HTHL such that the wing loading is low enough to >reliably handle early takeoff aborts and you won't have an X-Prize capable >vehicle. I don't see why this is so. I'm not disagreeing, but I don't follow your reasoning. > Not everyone is comfortable with a landing method that happens so quickly >and is so critical that the only competent pilot will be a computer. My explanation of this mode for landing POGO in the simulator was, "If you're not certain you're going to die in the next two seconds, you're not doing it right." Only way to land without wasting too much fuel is to be really aggressive. Operationally, this requires a full up burn above abort altitude to make sure the engines work, and that pre-landing burn wastes fuel and introduces new failure modes. It's a tough problem. -R -- "We've all heard that a million monkeys banging on a million typewriters will eventually reproduce the entire works of Shakespeare. Now, thanks to the Internet, we know this is not true." -- Robert Wilensky, UC Berkeley _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
