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#21
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U.S. astronauts are climbing back into space capsules. Here's how they've improved over the past 50 years
Jeff Findley wrote:
In article , says... JF Mezei wrote: On 2017-09-23 13:33, Fred J. McCall wrote: down. I think it's funny that NASA lets astronauts ride on Soyuz, which relies on power at landing on dirt for them to survive, yet is being obstructionist to safety certify Dragon V2 for fully powered landings on dirt. A rare instance where I agree with you. But being devil's advocate: At the time NASA started to purchase seats on Soyuz, it had already proven itself and it ability to land. Also, they had no choice since Soyuz was also the escape pod in case of emergency. And during post Columbia stand down and after permanent shutdown of Shuttle, NASA had no choice either. True, but doesn't address the issue. Why not let SpaceX prove powered landing? Higher (perceived) risk for returning payloads. But most don't return payloads. Why obstruct those? -- "The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man." --George Bernard Shaw |
#22
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U.S. astronauts are climbing back into space capsules. Here's how they've improved over the past 50 years
JF Mezei wrote:
On 2017-09-24 08:43, Jeff Findley wrote: itself during descent, even when the Super Dracos aren't firing. This is quite similar to Falcon 9 first stage landings where the Merlin engines only fire briefly for boost back, reentry, and landing. And how many stage 1s were lost before SpaceX got it right? Irrelevant. The problem with Falcon core is the engines aren't deeply throttleable. Dragon V2 with Super Draco doesn't have that problem. In terms of landing accuracy, can landing from a full orbit altitude be as accurate as landing after a 3 minute flight where speed during re-entry is much lower so minute deviations in angles etc have far lesser impact? Of course it can. Dragon isn't just a rock coming back. -- "Some people get lost in thought because it's such unfamiliar territory." --G. Behn |
#23
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U.S. astronauts are climbing back into space capsules. Here's how they've improved over the past 50 years
JF Mezei wrote:
On 2017-09-24 16:34, Jeff Findley wrote: From above article: "The reason we decided not to pursue (powered landings) heavily is it would have taken a tremendous amount of effort to qualify that for safety, particularly for crew transport," Musk said. Ok, so was Dragon gonna do powered re-entry like Stage 1, or be like Soyuz with lifting re-entry, parachutes and powered "cushion" very near ground? More like the former. I was told it was to be a powered one like stage I, where fuel available becomes critical. (and validating software a bigger challenge). No. The only reason there were 'issues' with the Falcon Core is that the engines can't be deeply throttled. That's not the case with Dragon V2. was a management/engineering trade-off and appears to be SpaceX's call, not NASA's. Suspect origimally NASA, then SpaceX realised water landings not as bad as thought. Why is it so hard to safety certify? Because NASA doesn't want to certify it. Yes. Dragon flies a lifting reentry and therefore has some cross-range capability. This was also true of both Gemini and Apollo (and Russian Soyuz and Chinese Shenzhou). Does "cross range" refer to only left/right, or also forward/backward? (aka, a narrow elipse or a full circle as possible landing locations? Capsule is round. Why would it be limited to only 'left/right' control? -- "Some people get lost in thought because it's such unfamiliar territory." --G. Behn |
#24
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U.S. astronauts are climbing back into space capsules. Here's how they've improved over the past 50 years
JF Mezei wrote:
On 2017-09-24 19:27, Fred J. McCall wrote: No. The only reason there were 'issues' with the Falcon Core is that the engines can't be deeply throttled. That's not the case with Dragon V2. Whatever the reason, it took a number of attempts before they fine tuned the software to make it work. And one should expect Dragon to have similar failures at first. And why is that? Why is it so hard to safety certify? Because NASA doesn't want to certify it. Not hard, But it would take a few flights to prove it, diuring which you can't return anything, at a time NASA was craving to return stuff. Of course you can. Nothing requires ALL reentries to use propulsive landing. This is why I suggested SpaceX should have insisted the text flight land on land to do that testing/certification/debugging. That's a bad plan, because you've just done the very thing you said was bad and prohibited returning anything until propulsive landing is safety certified. Considering it is likely they could do much of the testing/debiugging by dropping it from 30,000 feet or lower, they could have done this at faster rate than the space flight rate. Nope. Things that are different aren't the same. Capsule is round. Why would it be limited to only 'left/right' control? When you have 0 left/right speed, it is easy to use a bit of aerodynamics at right time to creare left/right cross range. Poppycock! You have to 'bend' your velocity vector and bending it one way is just as difficult as bending it some other way. But when you're coming in at 25,000km/h in forward speed, I would think that any miscalculation could get capsule to land very far before or after landing point. You shouldn't think. It doesn't appear to be one of your core competencies. The velocity vector isn't 'forward'; it's at a slant downward. Hence 'reentry'. But it doesn't matter. To change the landing point you need to 'bend' the velocity vector by applying lift. Bending it left or right is no harder (or easier) than bending it up or down. It's just a matter of how much 'lift' you apply and in which direction. [Actually you have slightly less authority 'up' and slightly more authority 'down' because of this stuff called gravity, so landing 'shorter' is a little easier than landing 'longer', but it's not THAT much harder.] -- "Some people get lost in thought because it's such unfamiliar territory." --G. Behn |
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U.S. astronauts are climbing back into space capsules. Here's how they've improved over the past 50 years
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U.S. astronauts are climbing back into space capsules. Here's how they've improved over the past 50 years
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U.S. astronauts are climbing back into space capsules. Here's how they've improved over the past 50 years
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U.S. astronauts are climbing back into space capsules. Here's how they've improved over the past 50 years
Jeff Findley wrote:
In article . com, says... On 2017-09-24 16:34, Jeff Findley wrote: From above article: "The reason we decided not to pursue (powered landings) heavily is it would have taken a tremendous amount of effort to qualify that for safety, particularly for crew transport," Musk said. Ok, so was Dragon gonna do powered re-entry like Stage 1, or be like Soyuz with lifting re-entry, parachutes and powered "cushion" very near ground? No. Parachutes would not have been used for a nominal powered landing. Parachutes would have been necessary in case of an abort because the abort burn would have used too much fuel to allow for a powered landing. I was told it was to be a powered one like stage I, where fuel available becomes critical. (and validating software a bigger challenge). It's critical in the sense that you have to budget enough for landing, but the terminal velocity of the capsule limits the amount of fuel needed for landing, so it's not hard to make sure you have enough. Since Dragon V2 has internal fuel tanks and that fuel is pretty much used only for the Super Draco engines (I think they use cold gas for OMS now), you come down with virtually full tanks. So, since it's not as important to Mars as SpaceX once thought it was, I smell excuse here. Funny, I saw a clear engineering/management trade-off. That's because you know something about engineering and management and Mayfly knows something about taking the trash out for Mom and the living conditions in her basement. The Mars plans were announced years after Dragon was designed. I suspect this is more about SpaceX realising that the refurb costs for salt water landings aren't as high as thought, so it is more economical to land in water than to spend the extra money to develop/test land landings. I suspect Mars landing will be different enough to not be applicable to Earth landings. We'll see what has changed with the next update from SpaceX on its Mars plans. It could be that a (relatively simple) powered vertical landing has been abandoned in favor of something else a tad more complicated. I think they may have tripped over the same issue landing anything of size on Mars has. The air is to thin for aerobraking adequately and the gravity is too heavy for a pure propulsive landing. was a management/engineering trade-off and appears to be SpaceX's call, not NASA's. Suspect origimally NASA, then SpaceX realised water landings not as bad as thought. Possible. But there is the cost of the capsule recovery force. Yes. Dragon flies a lifting reentry and therefore has some cross- range capability. This was also true of both Gemini and Apollo (and Russian Soyuz and Chinese Shenzhou). Does "cross range" refer to only left/right, or also forward/backward? (aka, a narrow elipse or a full circle as possible landing locations? Roughly speaking, by rotating the capsule, you can direct the lift vector anywhere along the circle that's normal to the velocity vector. So in your terms, yes you can direct the lift vector left/right and/or forward/backward. Yep. -- "The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man." --George Bernard Shaw |
#29
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U.S. astronauts are climbing back into space capsules. Here's how they've improved over the past 50 years
"Jeff Findley" wrote in message
... In article , says... Jeff Findley wrote: In article , says... Anthony Frost wrote: In message Jeff Findley wrote: This has changed somewhat recently. Reportedly SpaceX is the one who shelved development of Dragon V2 vertical landing. The reasons for this aren't terribly clear, but there are hints from SpaceX that this is because they've decided to change the (Mars) landing mode of their (eventual) Mars vehicle. Also apparently NASA weren't happy about cargo flights being used for testing powered landings. Why would they care? They get their cargo on the way up, not the way down. Because Dragon is the only way that NASA can get things like EMUs back to earth for refurbishment. There have been articles on how few functioning EMUs are left. Sorry for the word-wrap on the cites: But most flights aren't carrying those (or anything else) back down. So why is NASA apparently being obstructionist about landing the 'empties' propulsively? I suspect that it's mostly the pointy haired bosses at NASA being "cautious", not the engineers raising concerns. They've had it hammered into their heads that "Failure is not an option", even though that's not at all what was meant by that phrase during Apollo 13. Like you said, if they lost a capsule or two (as long as it didn't carry something critical to be refurbished like an EMU), it's not like it would impact ISS safety in any way. I'm going to say even then it doesn't matter. Consider that they haven't been able to count on cargo return since Shuttle. So, in a sense, bringing back a failed EMU and having it crash is really no worse than not bringing it back in the first place. Jeff -- Greg D. Moore http://greenmountainsoftware.wordpress.com/ CEO QuiCR: Quick, Crowdsourced Responses. http://www.quicr.net IT Disaster Response - https://www.amazon.com/Disaster-Resp...dp/1484221834/ |
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