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SpaceX Dragon CRS-6 Launch Webcast

nasa.gov

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Re: SpaceX Dragon CRS-6 Launch Webcast

#141

Post by fit2rule »

david-given wrote: post #107
fit2rule wrote: post #84

I have an idea for how to improve the landing of the rocket stage: instead of fold-out landing-legs, why not fold-out wings ? Or, a combination of both - legs with wing-like features - they function as wings until close to touch-down and then feather to support the rocket laterally upon landing?

They'd be on the wrong end of the rocket. The centre of mass is close to the engines, but not that close; the landing legs have to go at the bottom for obvious reasons, but any aerodynamic surfaces have to go near the vehicle's stern, as it moves through the air. Since it's descending vertically, this means the top. This is what the pop-out grid fins way up are for. Fun fact: if the F9 first stage had traditional fin…

I believe you and I understand the problem (I make planes for fun, so know a little bit about aerodynamics and CG) .. but what I think is that the wings could indeed be mounted to the rocket higher up the frame, over the legs - so when the legs deploy at the bottom, the wings deploy as well over the top - they'd be part of the same leg structure, only extended all the way to the top of the rocket body. Actually they'd need to be rotors, a kind of mini-wing, to spin the rocket body on descent, for drag .. like maple seed wings that flip out in the last stage of the flight envelope.

Well of course all this sounds easy, but KISS principles apply I suppose. Still, I could see this happening at some point in the future ..

Re: SpaceX Dragon CRS-6 Launch Webcast

#142

Post by InclinedPlane »

tsotha wrote: post #136
InclinedPlane wrote: post #113

Earlier quoted context omitted.

That's why it's a probability. Do you not understand how estimate work?

Yes, I do, and that's why I'm laughing at what you wrote. Just how do you arrive at a numeric probability for an event for which you have no way of assessing the likelihood of occurring?

If one had complete clairvoyance and absolute understanding of every element in a system then there would be no estimates, no probabilities, merely outcomes foretold in advance.

For an estimate one needs to gather up what evidence and information is available; make a judgment on how well understood the system is, and its components; determine how predictable the behavior of the system has been in the past; figure out how close what is attempting to be done is to any previous testing or operations; and so on. Then take all of that and make an informed estimate of the likelihood of something occurring.

That's what an estimate is, a statement about how well a system seems to be understood and the probability of some event occurring. An estimate can build in uncertainty in the understanding of the system quite easily, by simply making a more cautious or pessimistic assumption.

SpaceX designed and built these rockets. They've flown them numerous times. They've done test flights exploring landing operations. They've done re-entry flights and landing attempts multiple times. All of these things increase confidence in understanding how the rocket operates under different conditions during landing, making it possible to come up with a reasonably informed estimate of the probability of a successful landing over a given number of attempts.

Perhaps they are wrong, perhaps they have made a key error in their modeling, who knows. That's why these are estimates, because there's always the possibility for unknown variables to affect outcomes. But to say that there is no basis for SpaceX's estimates is patently ridiculous.

Re: SpaceX Dragon CRS-6 Launch Webcast

#143

Post by InclinedPlane »

mikeash wrote: post #88
jerf wrote: post #86

Earlier quoted context omitted.

Thank you, that makes sense. All things considered I'd still call what happened today a good sign.

They're getting ever closer. And they have a great setup to experiment with. They have paying customers funding the whole thing, and they can take their time to get it right. If this was some pure R&D project, today's events would probably be a major setback. But SpaceX still made a healthy profit on this launch even with the bang at the end. That's the genius of their approach, and it's why I think they'll succeed w…

So far SpaceX has used approximately a third of a billion dollars in launch hardware in reusability testing, almost entirely subsidized through commercial launches. It's a very savvy way to do R&D. Note that it takes a lot of foresight to design a rocket which can serve as both a good expendable booster and also be sensibly made reusable, and most of the optimizations for expendability would push the design away from being good for reuse.

Re: SpaceX Dragon CRS-6 Launch Webcast

#144

Post by andygates »

ChuckMcM wrote: post #110
michaelmcmillan wrote: post #35

T+17m: Not a successful landing today. That sucks. Source: http://www.reddit.com/r/spacex/comments/32jnyd/rspacex_crs6_... Edit: "Looks like Falcon landed fine, but excess lateral velocity caused it to tip over post landing" @elonmusk

I'm a big fan of arrestor lines around the sides. Basically as the stage lands, stand up four outrigger arms with a cable going through them from the corners of the barge. If it falls over then catch it and lower it gently. But easy to armchair, hard to do! That they have arrived exactly where the barge is (or would have been) 3 times now is pretty awesome in itself.

Very easy to armchair -- but it's a fifteen story beer can, so you need a huge arrestor system, and it's pretty fragile except up its axis.

The ultimate goal is to land on a flat pad, so adding stuff now moves away from that goal.

Re: SpaceX Dragon CRS-6 Launch Webcast

#145

Post by clebio »

david-given wrote: post #65
ggonweb wrote: post #60

Looks like Falcon landed fine, but excess lateral velocity caused it to tip over post landing -@elonmusk https://twitter.com/elonmusk/status/588082574183903232

Scott Manley did precisely this in KSP the other day, trying to replicate the barge landing: https://www.youtube.com/watch?v=C9I55o8hQgs Admittedly, he also replicated all the other possible failure modes too. Turns out landing on barges is hard.

That's really fun to watch. Thanks for the link!

Re: SpaceX Dragon CRS-6 Launch Webcast

#146

Post by ChuckMcM »

andygates wrote: post #144
ChuckMcM wrote: post #110

Earlier quoted context omitted.

I'm a big fan of arrestor lines around the sides. Basically as the stage lands, stand up four outrigger arms with a cable going through them from the corners of the barge. If it falls over then catch it and lower it gently. But easy to armchair, hard to do! That they have arrived exactly where the barge is (or would have been) 3 times now is pretty awesome in itself.

Very easy to armchair -- but it's a fifteen story beer can, so you need a huge arrestor system, and it's pretty fragile except up its axis. The ultimate goal is to land on a flat pad, so adding stuff now moves away from that goal.

I love the beer can analogy. Agreed it is a really interesting problem. Which I don't doubt SpaceX will solve. I was not impressed with the ULA "lets just catch the engines with a helicopter" idea.

Can't wait to see the recovered barge video.

Re: SpaceX Dragon CRS-6 Launch Webcast

#147

Post by mikeash »

InclinedPlane wrote: post #143
mikeash wrote: post #88

Earlier quoted context omitted.

They're getting ever closer. And they have a great setup to experiment with. They have paying customers funding the whole thing, and they can take their time to get it right. If this was some pure R&D project, today's events would probably be a major setback. But SpaceX still made a healthy profit on this launch even with the bang at the end. That's the genius of their approach, and it's why I think they'll succeed w…

So far SpaceX has used approximately a third of a billion dollars in launch hardware in reusability testing, almost entirely subsidized through commercial launches. It's a very savvy way to do R&D. Note that it takes a lot of foresight to design a rocket which can serve as both a good expendable booster and also be sensibly made reusable, and most of the optimizations for expendability would push the design away from…

It takes some smarts and will to realize that fully optimizing for reusability isn't the right move, too. It's a constant refrain in these comment threads to ask, why don't they use parachutes? Why don't they use wings? Why not do all sorts of things that would make it easier to reuse, instead of trying a crazy vertical landing suicide burn onto a boat?

The paradox is that making reusability easier would make it harder, because you'd have to get it right from the start, and spend a lot more money developing it, and ensure you can reuse each vehicle many times to make up for the increased cost.

So instead of inventing something new, they started with something known to work, and then adapted it. Because it's cheap, you can keep tweaking it in live tests and not worry too much when you break the stuff.

People often compare this stuff to the Space Shuttle, since that's the one example of a reusable rocket that actually flew, and it was never cheaper than expendable launchers. Imagine what would have happened if the Shuttle had crashed on its first two landing attempts. The program would have been dead before it got anywhere. Now imagine what would have happened if the Shuttle had been so cheap that they could build and launch one for $60 million, instead of $2 billion per orbiter and $500 million per launch (and imagine that somehow they could be lost without killing anybody). It would have been an enormous success even if a bunch had crashed.

Re: SpaceX Dragon CRS-6 Launch Webcast

#148

Post by mikeash »

ChuckMcM wrote: post #110
michaelmcmillan wrote: post #35

T+17m: Not a successful landing today. That sucks. Source: http://www.reddit.com/r/spacex/comments/32jnyd/rspacex_crs6_... Edit: "Looks like Falcon landed fine, but excess lateral velocity caused it to tip over post landing" @elonmusk

I'm a big fan of arrestor lines around the sides. Basically as the stage lands, stand up four outrigger arms with a cable going through them from the corners of the barge. If it falls over then catch it and lower it gently. But easy to armchair, hard to do! That they have arrived exactly where the barge is (or would have been) 3 times now is pretty awesome in itself.

The question is always whether you remove more risk than you add. That's a big part of why it's easy to armchair but hard to do. In this case, for example, I'd bet that the odds of the rocket getting tangled in the lines and ruining a landing are better than the odds of the rocket being saved by the lines.

From my armchair observations, it looks like they just need to tweak some constants in their control algorithm to avoid control-induced oscillation. This is the first time that the rocket has reached the ship while still under control (the first landing attempt ran out of hydraulic fluid and went out of control, and of course the second not-quite-attempt didn't have the ship there so it probably wasn't as obvious what was going on) and I imagine there's a fair amount of "oh, duh, how did I overlook that?" going around SpaceX headquarters now that they've finally seen it in action all the way to the end.

Re: SpaceX Dragon CRS-6 Launch Webcast

#149

Post by smackfu »

InclinedPlane wrote: post #142
tsotha wrote: post #136

Earlier quoted context omitted.

Yes, I do, and that's why I'm laughing at what you wrote. Just how do you arrive at a numeric probability for an event for which you have no way of assessing the likelihood of occurring?

If one had complete clairvoyance and absolute understanding of every element in a system then there would be no estimates, no probabilities, merely outcomes foretold in advance. For an estimate one needs to gather up what evidence and information is available; make a judgment on how well understood the system is, and its components; determine how predictable the behavior of the system has been in the past; figure out…

I feel like a truer estimate would be "100% chance of success unless some unknown factor causes failure." It's saying that the unknown factor subtracts 20% that makes me go hmmm.

Re: SpaceX Dragon CRS-6 Launch Webcast

#150

Post by krallja »

fit2rule wrote: post #84

I have an idea for how to improve the landing of the rocket stage: instead of fold-out landing-legs, why not fold-out wings ? Or, a combination of both - legs with wing-like features - they function as wings until close to touch-down and then feather to support the rocket laterally upon landing?

There are already grid fins on the F9, which provide aerodynamic control.
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