The engines will not be fired near the ground as nozzles dictate the performance of said engines. At lower altitudes that can lead to flow sep in the nozzle and cause more problems than it solves.
Quote from: Go4TLI on 06/01/2012 05:02 pmThe engines will not be fired near the ground as nozzles dictate the performance of said engines. At lower altitudes that can lead to flow sep in the nozzle and cause more problems than it solves. I agree that's the theory. But SNC's plan is to use the same motors for Pad abort and orbital maneuvering, so they are apparently willing to take whatever losses/nozzle design limitations this entails. The SpaceShipOne hybrid motor (from which DC's is derived) has already been flown from 50K ft to near-vacuum.
Placing the craft on top does remove the TPS debris problem, but you get new ones to manage. For example, with the tall skinny configuration the spacecraft adapter needs to be strong enough to deal with all the wind/turbulence the stack will face with a 10 t aerodynamically complex load on the far tip.The wind tunnel testing of the integrated stack that's going on now (or completed) should tell them what they need to know. But I wonder if the operational DC could have wind launch criteria limitations that will make it look less competitive than, say DragonRider. Remember, for an ISS crew launch you have to launch exactly on time; no waiting for favorable winds. It will probably have runway abort cross-wind launch criteria too, just like the Shuttle orbiter.
The hybrid propulsion system on the DreamChaser gives me the most pause. ...
it seems like SNC might eventually regret some of these early systems engineering decisions (i.e. going with the hybrids).
Quote from: Robotbeat on 06/01/2012 05:17 pmit seems like SNC might eventually regret some of these early systems engineering decisions (i.e. going with the hybrids).And what is your basis for such things? Because they are not SpaceX?
The hybrid propulsion system on the DreamChaser gives me the most pause. If it was a liquid bipropellant propulsion system, I'd be a lot more comfortable with the design. The fatal accident with SS2 should dispel this myth that hybrids are inherently safer than liquids. And the regression rate issues mean that you never really reach "steady state" combustion like you do with liquids. And having two separate motors on each side seems really difficult to me, especially because you have two hybrid rockets which both have to ignite at the same time and keep the same thrust. It seems like a system which will quickly become heavier than an equivalent liquid rocket system. And with probably a slower abort, too, since you have to wait for the whole hybrid rocket to come up to pressure and ignite properly... It seems like a really bad systems engineering choice for the abort phase.Capsules are still inherently more robust IMHO, but DreamChaser's case would be improved dramatically just by using a more conventional propulsion system (if you want non-toxic, there's always ethanol/LOx or ethanol/nitrous... but hypergols are still a great choice for abort because of the unbeatable ignition reliability and speed). Even some sort of solid rocket motors might be more appropriate for abort, kind of like MLAS.
And FWIW, I'm a little uncomfortable with Superdraco landing for SpaceX, which is why I'm glad they're staying with splashdown for initial crewed missions. But that's off-topic.Hybrids combine the worst of both solids and liquids, IMHO. And they are no safer.
Quote from: adrianwyard on 06/01/2012 04:33 pm+1.Placing the craft on top does remove the TPS debris problem, but you get new ones to manage. For example, with the tall skinny configuration the spacecraft adapter needs to be strong enough to deal with all wind/turbulence the stack will face with a 10 t aerodynamically complex load on the far tip.The wind tunnel testing of the integrated stack that's going on now (or completed) should tell them what they need to know. But I wonder if the operational DC could have wind launch criteria limitations that will make it look less competitive than, say DragonRider. Remember, for an ISS crew launch you have to launch exactly on time; no waiting for favorable winds. It will probably have runway abort cross-wind launch criteria too, just like the Shuttle orbiter.That "Main Propulsion System" can be used for abort and for cross range cruising. Will the craft normally glide to a landing unpowered? In a sudden and stiff crosswind, would it be possible to engage those engines and cross vector (crab) into the crosswind just like airplanes do all the time? To what degree could it crab unpowered?
+1.Placing the craft on top does remove the TPS debris problem, but you get new ones to manage. For example, with the tall skinny configuration the spacecraft adapter needs to be strong enough to deal with all wind/turbulence the stack will face with a 10 t aerodynamically complex load on the far tip.The wind tunnel testing of the integrated stack that's going on now (or completed) should tell them what they need to know. But I wonder if the operational DC could have wind launch criteria limitations that will make it look less competitive than, say DragonRider. Remember, for an ISS crew launch you have to launch exactly on time; no waiting for favorable winds. It will probably have runway abort cross-wind launch criteria too, just like the Shuttle orbiter.
Go4LTI - You were the one who brought up SpaceX here. Nobody else. Feeling cranky?
Quote from: Lars_J on 06/01/2012 08:30 pmGo4LTI - You were the one who brought up SpaceX here. Nobody else. Feeling cranky?Not at all, but I do appreciate you asking. I've lurked around here from time-to-time and know enough about certain posters who shoot down or downplay everyone else while essentially cheering others along as if they can do no wrong. Just my opinion of course.
BTW, I don't mean at all to discourage anyone at SNC. I hope and pray that my technical opinion of hybrids isn't accurate for their sake. I hope Dreamchaser does very well and that we see it fly soon!(I just hope they have a non-hybrid backup plan...)
I have technical reasons for my opinions. Do you?
Quote from: Robotbeat on 06/01/2012 09:15 pmBTW, I don't mean at all to discourage anyone at SNC. I hope and pray that my technical opinion of hybrids isn't accurate for their sake. I hope Dreamchaser does very well and that we see it fly soon!(I just hope they have a non-hybrid backup plan...)Really? I'm sure the phones are ringing off the hook in Denver and at various NASA centers right now with concerns that everyone has made an absolute poor choice over the design cycle, and too much CCDev money was stupidly given out, because someone in Minnesota is saying something on the internet.
Hey man, I'm not saying I would've foregone funding Dreamchaser, just that I think that Mercury, Gemini, Apollo, Shuttle, Soyuz, Vostok, Shenzou, HL-20 (on which Dream Chaser was based), Dragon, CST-100, and Blue Origin went with non-hybrid solutions for a reason. And I don't mind being wrong .
