Dragon 2 will have a ballast sled that lets them control the AOA during descent. So they do indeed have aerodynamic control features.
You forgot accuracy, which I suspect is one of the primary reasons Dragon v2 landing was abandoned. Chaotic aerodynamic forces during reentry alter the post-reentry trajectory and this will vary the landing point quite dramatically (for example, Soyuz landings are often kilometers away from their targeted landing site), and Dragon doesn't have any aerodynamic control features to direct the capsule to a precise landing point like the Falcon 9 does. They could use thruster firings, but my suspicion is that they realized they couldn't reserve enough fuel for both worst-case scenario trajectory corrections and the landing burn.
I'd like to speculate on why NASA rejected the powered land landing concept for Dragon, which is the reason (excuse) for delaying (canceling) Red Dragon.Performance: the weight of landing propellant, tanks, pipes, engines, and legs is bigger than that of parachutes and their deployment hardware. Any thruster failure has to be compensated by throttling back other thrusters (or cutting one al al N-1). So full thrust cannot be used in the nominal landing profile. All this extra weight is subtracted at 1 for 1 from the return payload.
Reliability: Each thruster and landing leg is an individual failure point, but you can survive failure of one drogue and one main parachute in the Apollo/Orion system.
Safety: You need to open doors in the heat shield for the thrusters and legs. The five doors on the belly of the Shuttle were a source of obsessive worry throughout that program. Recall that when COLUMBIA's wing started to heat up, the mission controllers assumed immediately that a main gear door had failed. Shuttle ops veterans now occupy high management positions in the manned space program; they probably regard any head shield penetration as unacceptable.
I've never understood the obsession with vertical powered landings on Earth. Since we have thick air and soft warm oceans here, it seems silly not to use them as free shock absorbers. Musk would probably argue that the time involved with bringing the spacecraft back to the launch site would prevent the high flight rates he fantasizes about, but obviously this is not a factor in ISS crew exchange operations.
Quote from: Arch Admiral on 08/21/2017 08:48 pmI'd like to speculate on why NASA rejected the powered land landing concept for Dragon, which is the reason (excuse) for delaying (canceling) Red Dragon.Performance: the weight of landing propellant, tanks, pipes, engines, and legs is bigger than that of parachutes and their deployment hardware. Any thruster failure has to be compensated by throttling back other thrusters (or cutting one al al N-1). So full thrust cannot be used in the nominal landing profile. All this extra weight is subtracted at 1 for 1 from the return payload.The design of Crew Dragon was baselined with powered landings in mind, including a pre-determined return payload capacity.NASA did not suddenly demand a higher return payload capacity. Return payload capacity is not the reason why SpaceX gave up on powered landings.
I've never understood the obsession with vertical powered landings on Earth.
And since the reason seems to be the cost of doing to NASA's specs in time for commercial crew, and a powered landing obviously has enough the utility to justify it, I believe SpaceX will go ahead and get it done on their own time to their own satisfaction-
My guess is that propulsive landing isn't worth doing for only 6 flights. SpaceX had already stated that it wouldn't do it for the first few flights. The question then becomes, is it worth doing for just 3 or 4 flights? The answer seems to be no.
Quote from: whitelancer64 on 08/21/2017 09:01 pmYou forgot accuracy, which I suspect is one of the primary reasons Dragon v2 landing was abandoned. Chaotic aerodynamic forces during reentry alter the post-reentry trajectory and this will vary the landing point quite dramatically (for example, Soyuz landings are often kilometers away from their targeted landing site), and Dragon doesn't have any aerodynamic control features to direct the capsule to a precise landing point like the Falcon 9 does. They could use thruster firings, but my suspicion is that they realized they couldn't reserve enough fuel for both worst-case scenario trajectory corrections and the landing burn. The reason that Soyuz sometimes lands many miles off has nothing to do with "chaotic aerodynamic forces during reentry", but everything with lack of precision timing of the reentry burn as well as lack of precision management of the reentry burn profile.On a system with proper systems, such as the current cargo Dragon, it's no big deal to do precision parachute landings. Cargo Dragon usually lands within a kilometer from the pre-positioned recovery ship.
The only reason it isn't any closer is for safety reasons: you don't want all that mass to land on your head. Precision parachuting into a radius of just a few hundred feat is no big deal actually as the both the US Amry and US Navy can tell you. And SpaceX has already accomplished that parachuting accuracy.
As an additional consideration, SpaceX now has a pretty good understanding of how much work it is taking them to refurbish and recertify Dragon 1 capsules after they get immersed in salt water. So, maybe it is turning out to be less problematical (read: costly) than they had originally expected? In which case, the cost driver for land landings maybe isn't such a big factor for the crewed capsules. Though this thought does maybe "over-downplay" the difference of having the SuperDraco system get dunked for D2s.
You people who cite earlier programs like MOL, Soyuz, and Shuttle as evidence for what NASA managers consider acceptable in 2017 need to get real. Safety standards have risen at NASA just like everywhere else. Also, the political base for manned space flight has shrunken so much since The Good Old Days that a single fatal accident might well end the government program for good. I don't say I agree with this, but it is the reality that NASA management has to consider.
Quote from: woods170 on 08/22/2017 06:16 amOn a system with proper systems, such as the current cargo Dragon, it's no big deal to do precision parachute landings. Cargo Dragon usually lands within a kilometer from the pre-positioned recovery ship. Citation needed.
On a system with proper systems, such as the current cargo Dragon, it's no big deal to do precision parachute landings. Cargo Dragon usually lands within a kilometer from the pre-positioned recovery ship.
Quote from: woods170 on 08/22/2017 06:16 amThe only reason it isn't any closer is for safety reasons: you don't want all that mass to land on your head. Precision parachuting into a radius of just a few hundred feat is no big deal actually as the both the US Amry and US Navy can tell you.Citation needed.
The only reason it isn't any closer is for safety reasons: you don't want all that mass to land on your head. Precision parachuting into a radius of just a few hundred feat is no big deal actually as the both the US Amry and US Navy can tell you.
Quote from: woods170 on 08/22/2017 06:16 amQuote from: whitelancer64 on 08/21/2017 09:01 pmYou forgot accuracy, which I suspect is one of the primary reasons Dragon v2 landing was abandoned. Chaotic aerodynamic forces during reentry alter the post-reentry trajectory and this will vary the landing point quite dramatically (for example, Soyuz landings are often kilometers away from their targeted landing site), and Dragon doesn't have any aerodynamic control features to direct the capsule to a precise landing point like the Falcon 9 does. They could use thruster firings, but my suspicion is that they realized they couldn't reserve enough fuel for both worst-case scenario trajectory corrections and the landing burn. The reason that Soyuz sometimes lands many miles off has nothing to do with "chaotic aerodynamic forces during reentry", but everything with lack of precision timing of the reentry burn as well as lack of precision management of the reentry burn profile.On a system with proper systems, such as the current cargo Dragon, it's no big deal to do precision parachute landings. Cargo Dragon usually lands within a kilometer from the pre-positioned recovery ship. Citation needed. QuoteThe only reason it isn't any closer is for safety reasons: you don't want all that mass to land on your head. Precision parachuting into a radius of just a few hundred feat is no big deal actually as the both the US Amry and US Navy can tell you. And SpaceX has already accomplished that parachuting accuracy.Citation needed.
Jerry, I've done a lot of joking about the spacecraft hitting the aircraft carrier, but the more I think about it the less I feel it is a joke. There are reports that the C Prime command module came down right over the aircraft carrier
Quote from: tdperk on 08/22/2017 01:20 pmAnd since the reason seems to be the cost of doing to NASA's specs in time for commercial crew, and a powered landing obviously has enough the utility to justify it, I believe SpaceX will go ahead and get it done on their own time to their own satisfaction-Wrong. Unsupported statement. How do you know it is "NASA's specs".
" “It was a tough decision,” he said when asked about propulsive landing capability during a question-and-answer session. “Technically it still is, although you’d have to land it on some pretty soft landing pad because we’ve deleted the little legs that pop out of the heat shield.”SpaceX planned to transition from splashdowns, which is how the current cargo version of the Dragon returns to Earth, to “propulsive” landings at a pad at some point after the vehicle’s introduction. Certification issues, he said, for propulsive landings led him to cancel those plans.“It would have taken a tremendous amount of effort to qualify that for safety, particularly for crew transport,” he said. " <-- http://spacenews.com/spacex-drops-plans-for-powered-dragon-landings/
How about it is just too expensive (maybe you don't understand the math) for anybody, since the only way to properly qualify it is to launch it?
Please note he did not say it would be hard to design or build it, but only hard to qualify it. It's not reading into it too much to think is it the specs vs. payment offered of the only paying customer which made the effort unreasonable, when plainly not dunking the ship in salt water is better if it can be avoided.
I would not be shocked--unless they have near term plans to replace the Crew Dragon with something else entirely--if in the next several years they went ahead and did propulsive landings with the Crew Dragon for all customers but NASA, which organization is welcome to pay for as much replaced and refurbished hardware as it likes
Quote from: tdperk on 08/23/2017 07:33 pmPlease note he did not say it would be hard to design or build it, but only hard to qualify it. It's not reading into it too much to think is it the specs vs. payment offered of the only paying customer which made the effort unreasonable, when plainly not dunking the ship in salt water is better if it can be avoided.Wrong interpretation. NASA "specs" have nothing to do with it. It can't be cheaply qualified to anybody's specs, short of having no qualification program. "hard to qualify it" means it is costly and not that specs are the issue. Quote from: tdperk on 08/23/2017 07:33 pmI would not be shocked--unless they have near term plans to replace the Crew Dragon with something else entirely--if in the next several years they went ahead and did propulsive landings with the Crew Dragon for all customers but NASA, which organization is welcome to pay for as much replaced and refurbished hardware as it likesThat is completely inane. Spacex or other customers are not going to pay for X number (about as many parachute drop tests for Dragon 1) of $180~200m flights to qualify the capability. It is dead and gone.Replacing and refurbishing hardware from water landings is cheaper than qualifying propulsive landings.
Just another case of unsupported NASA bashing.