Andrew, I'm in agreement with most all you wrote. :-) I did want to comment on this part: > One of the possible VT risk mitigation strategies that S-K didn't > mention is assisted launch, like Brian Walker's vehicle. Coupled with > hold down clamps, assisted launch could increase the safety margin for > VTHL.
Some type of assisted launch/catapult would be helpful to either the HT or VT case. One big help for the VT case is that it can put the vehicle immediately at a velocity where the aerodynamic controls are useable. I think I once calculated that a 5G acceleration (experiencing 6Gs) up a 150 ft tower would put you at about 300 ft/sec. This could greatly reduce the requirement for gimballing of the engines. The ground support/facilities cost would be high, though. Brian Walker's vehicle is a bit complex for my taste. The shedding of all those fins and the multiple aerodynamic decellerators... Ken ----- Original Message ----- From: "Andrew Case" <[EMAIL PROTECTED]> To: "ERPS" <[EMAIL PROTECTED]> Sent: Sunday, May 25, 2003 1:14 PM Subject: Re: [ERPS] Interesting analysis of X-Prize competitors launch modes > > On Sunday, May 25, 2003, at 01:40 PM, Ken Doyle wrote: > > > This whole issue of aborts in the early takeoff/launch phase between > > HT and > > VT designs is being muddled by the variables of when and why the abort > > is > > necessary. Some seem only concerned with aborts during early flight > > testing, seeming to say that after you've done it once, you'll not > > have to > > worry about takeoff aborts again. I was considering the operational > > aspects, and concede that you can not do a high-speed taxi test with a > > VT > > design. > > I think the criticality of abort modes depends on the maturity of the > technology. You really want intact abort throughout the flight envelope > while you are testing, but once you've developed confidence it might be > OK to allow for small regions of the flight envelope where there is no > intact abort option (though IMO there should never be any portion of > the flight envelope where there is no option to save the crew). Ideally > you have abort throughout the envelope, but that's not always > operationally practical. > > There are probably ways to ensure intact abort for VTHL during flight > testing, though I think they might be incompatible with testing the > vehicle in operational configuration. I'll also readily concede that my > preferred mode (VTVL) has serious issues in the early phase of the > flight as far as intact abort goes. The way this can be handled is with > redundancy in the engines, which is also the way to handle it with > VTHL. The HL option still needs a pitch over in order to land (and some > horizontal velocity for lift). Not necessarily impossible to ensure, > but it makes for a sportier abort scenario than the VL early flight > abort, which is to dump fuel and plunk down in the nearest flat spot. > > > Operationally, you'll always have to be prepared for takeoff aborts at > > any > > point. An abort decision/event at the point where you're at rotation > > V on a > > HT ride will be dangerous. Recall the Concord takeoff crash a while > > back. > > They had warnings that they were on fire and had a blown tire while the > > aircraft was still on the runway but they were committed to flight > > because > > of the speed and runway remaining. Always operating only from > > incredibly > > long runways where that isn't a potential scenario is quite an > > operational > > restriction. > > Good point. At some point in the maturation of the vehicle you have to > be willing to accept that the trades may work out in favor of having > regions of the flight envelope where intact abort isn't an option. > Personally I think that's a situation that should be avoided wherever > possible, but it's not always possible (from an ops optimization > standpoint, that is). It is possible, however to design to minimize the > portion of the envelope where this is the case, and it's also possible > to design so as to avoid the *requirement* for dead zones in the flight > envelope - if they become operationally desirable later that's one > thing, but designing them in from the get-go seems to me like a poor > choice. > > > 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. > > I'm not convinced that this is the case for HT on a long runway, at > least not for a conservatively designed vehicle. For VT it's true that > early flight total loss of thrust means loss of vehicle. The solution > to this is redundancy, preferably redundancy that avoids the > possibility of common-mode failures. > > > The VT craft that Marti outlined as his preference has 2g takeoff > > thrust. > > 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. > > Yep. My objection to the paper is not to the choice of modes per se, > but to the presentation of VTHL as the only sensible path. I lean > towards VTVL myself, but I'm not theologically bound to disparage all > other modes. VT has some drawbacks that should be acknowledged by any > honest analysis. The Sarigul-Klijn paper has a lot of good stuff in it, > and it's well worth reading, but it has drawbacks in the author's > failure to critically examine their preferred vehicle. All possible > takeoff and landing modes have tradeoffs associated with them. > > One of the possible VT risk mitigation strategies that S-K didn't > mention is assisted launch, like Brian Walker's vehicle. Coupled with > hold down clamps, assisted launch could increase the safety margin for > VTHL. > > > You HT advocates appear to be accepting of remarkable otherwise > > unnecessary > > landing gear, brakes, and wing mass penalties all in the name of > > believing a > > Horizontal Takeoff mode to be intrinsically safer. > > That's one of the tradeoffs associated with HT. Whether it's more of a > drawback than the difficulty of safely aborting a VT launch, or the > impossibility of doing gradual envelope expansion in a mixed-mode > vehicle is something that has to be decided by the people who will > build and fly the ship. It's not a slam-dunk either way. > > > The CG control is actually less of an issue with the VTHL design than > > the > > HTHL design. HT craft must manage propellant mass location at high > > flow > > rates such that the CG stays within a relatively narrow range while > > the wing > > is being used for lift. This complicates tankage and plumbing layout > > for a > > HT craft relative to a VT craft. > > > > Indeed, the VT hybrid cannot abort and land completely empty of liquid > > oxidizer and solid fuel. Some oxidizer may need to stay onboard to > > balance > > the remaining solid fuel load. However, the remaining solid fuel load > > will > > normally be less than half of what was on board at takeoff. > > This need not be the case if the hybrid is a coaxial design, with fuel > down the center and oxidizer tanks around the outside. Another > possibility is to palletize the propulsion system so that in the even > of emergency you dump the whole thing - tanks, engine, everything. It > adds failure modes, but it gets rid of a lot of weight fast (assuming > it doesn't hang up partway out and make things a lot worse). A > palletized propulsion system is an advantage for hybrids anyway, since > refueling involves basically replacing the engine. > > > 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. > > Smart design says you put fuel dump ports on the opposite side of the > vehicle from the oxidizer dump ports, so the fire should be some > distance behind you. Not a great advantage if you're doing VL, but at > least the pilot has options. > > ......Andrew > > -- > Dr. Andrew Case, PhD. > Institute for Research in Electronics and Applied Physics, > University of Maryland, College Park > "It is seldom that liberty of any kind is lost all at once." > - David Hume > > _______________________________________________ > 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
