Randall, Wings don't work the way you describe. A 2g pull up is not available at rotation V. The 15 degree AOA I mentioned is 15 degrees Angle of Attack, meaning the pitch angle relative to the inertial direction of motion vector. Go much over 15 degrees and you're going to stall the wing. The lift generated changes that direction of motion vector pretty quickly in such a hot aircraft, but you cannot instantly pull straight up. The 30 degree deck angle I mentioned is about right for the 300ft / 100m AGL engine failure point suggested earlier by someone.
BTW, the F-22 is described as having "35,000 lb thrust class engines" (2 of 'em), and in the X-Plane simulation I referenced one can indeed climb straight up from takeoff even at the 64,000 lb gross weight. Engine thrust quickly drops with altitude, being a mere air-breathing powerplant, but at takeoff it has better than 1:1 thrust at gross weight. It's a rather hot airplane. :-) I think it makes a pretty dang good first-cut takeoff simulator for a HTHL rocket plane. It is also very much easier to accomplish an engines-out landing when it is has no fuel onboard vs. being fully loaded, of course. It would/will take some work to suitably prove my conjecture, "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'm not yet even convinced that a HTHL design not allowing for the few seconds of dead zone discussed is doable in reality. As Marti pointed out in his paper, an HTHL rocket plane is carrying along an oversized wing and overly robust landing gear and brakes that are only necessary to be so oversized (heavy) during takeoff. Make your wing large enough for the thing to be a decent glider at gross weight and the HTHL design is even less suited for the very high speed flight at the other end of the envelope. Given some dimensions and realistic weights and tankage and engine numbers, (and a little free time), I'd be happy to build an X-Plane vehicle of the Xerus or whatever HTHL design you like to simulate with. Better yet, you guys make the model and we'll see how it flies in the simulator. But hey, so long as you're already presuming better than 1:1 thrust, consider just taking off on your tail and only carrying the wing and gear mass necessary to land with after the propellant is gone. :-) At least you can rule out blowing tires on the takeoff roll then. What I've tried here to do here is dispel the notion that a HTHL rocket-plane gets a free pass on surviving the same all-engines-out scenario that is an obvious issue for the alternate/competing VTHL design. No free pass on safety, and much greater mass and wing area penalties. My mention of the rocket-powered Vertical landing mode was just that because of the necessary aggressiveness you mentioned, it would be a far more terrifying landing mode than gliding on wings; tough to market for a passenger-carrier. When not used in concert with some sort of high-drag terminal velocity limiter it does use a not-so-competitive amount of propellant mass, as Marti pointed out. I like the idea of using rocket-powered Vertical landing as the precision soft-landing endgame of a parachute landing. Ken Doyle ----- Original Message ----- From: "Randall Clague" <[EMAIL PROTECTED]> To: "Ken Doyle" <[EMAIL PROTECTED]> Cc: "Henry Spencer" <[EMAIL PROTECTED]>; "ERPS" <[EMAIL PROTECTED]> Sent: Wednesday, May 28, 2003 6:47 PM Subject: Re: [ERPS] Interesting analysis of X-Prize competitors launch modes > 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
