----- Original Message ----- From: "Henry Spencer" <[EMAIL PROTECTED]> To: "ERPS" <[EMAIL PROTECTED]> Sent: Monday, May 26, 2003 2:47 PM Subject: Re: [ERPS] Interesting analysis of X-Prize competitors launch modes
> On Sun, 25 May 2003, Ken Doyle wrote: > > ...Always operating only from incredibly > > long runways where that isn't a potential scenario is quite an operational > > restriction. > > I think you've missed a point Randall made: with a rocket-aircraft T/W, > you don't *need* an "incredibly long runway" to have abort-straight-ahead > capability throughout takeoff. Quite an ordinary runway length suffices, > because in a normal takeoff, you come off the runway so much more quickly. > (Yes, even compared to Concorde.) > 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. To get a feel for the reality of the situation, here's an exercise: Go download the latest (excellent) X-Planes software here: http://www.x-plane.com/ It's the full version, and it's free to try; it will just stop working after 6 minutes. Select the Mojave airport, with it's nice 9600 ft runway. 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. 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. All this will happen pretty quickly. Attempt to get back to the runway without crashing before you're past the end of it. Enjoy, and welcome to reality. For fun, rather than attempting to land straight ahead, see if you can circle to land. > As Jeff Greason pointed out at Space Access last month, HTHL rockets are > *not* just like jet aircraft. They have rather different characteristics, > and operate in a different part of the trade space. This is often bad, > because there is little experience and little COTS hardware, but sometimes > it works in your favor, giving you options that conventional aircraft lack. > 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. You're a smart guy, I'd encourage you to think for yourself along with considering the opinion of your favorite expert. > > My point was intended to express that in either HT or VT, there are a number > > of seconds where an abort caused by complete loss of thrust will be deadly. > > You haven't actually made that case very well for HT. Sorry you disagree. I maintain that both methods have dead zones during takeoff. 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. Both methods will do fine with redundant engines that don't all fail at once. > > While I prefer VTVL myself, complete loss of power is one case that HTHL > systems do handle rather better. > I'm partial to VTVL designs as well, and agree that Marti gave the rocket-powered VL case short shrift in his paper. 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. Some people had concerns with Rotary's hypersonic helicopter landing method. What people generally do have experience and trust in are winged landings on runways. Ken Doyle > > At the mentioned 100m abort altitude, either the 1g HT craft or the 2g VT > > craft can lose half the thrust and continue upward while proceeding with the > > Abort procedures. > > The risk is that a problem which affects half the propulsion will affect > the other half as well. This is more likely in the development phase than > in operation, but it can occur in either. (Witness the TriStar that lost > all three engines because the same maintenance mistake was made on all > three.) > > > ...And, while > > dumping propellant, the hybrid vehicle is only dumping oxidizer rather than > > a propellant combination designed to normally burn furiously in combination; > > a preferred option if one is already on fire. > > Any combustion from propellant dumping normally takes place aft of the > vehicle, so it's fairly irrelevant to an on-board fire. Aircraft seem to > have no great difficulty in this area, dumping flammable fuel into > oxygen-containing air. > > Henry Spencer > [EMAIL PROTECTED] > > _______________________________________________ > ERPS-list mailing list > [EMAIL PROTECTED] > http://lists.erps.org/mailman/listinfo/erps-list _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
