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Space X CR6 First stage landing

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Re: Space X CR6 First stage landing

#101

Post by jccooper »

monk_e_boy wrote: post #71
grecy wrote: post #56

Elon says it's in slow motion too "High resolution, color corrected, slow motion rocket landing video https://youtu.be/BhMSzC1crr0" https://twitter.com/elonmusk/status/588463193070022656

How high did it get? Does it burn the whole way down (I assume it uses those little X-wings to 'fly down?) When does it start the descent burn? How fast is it going at this point? How much does it weigh, what sort of forces are we talking about? How hard is landing this puppy in terms of human achievement? Top 10% of tricky things humans have so far achieved? [edit] wouldn't it be easier to hover it a few meters abov…

Stage separation is somewhere around 100km and Mach 6 or more. The exact figures will depend on the mission profile.

There's a "boostback burn" at altitude to slow the horizontal velocity right after separation, an entry burn to put it into the atmosphere, and a landing burn just before it hits the deck. The guidance in between those last two burns is aerodynamic from the grid fins.

The first stage mass is ~18,000kg empty (which it nearly is on landing), and it's coming in at its terminal velocity, which is, as a WAG, maybe 70m/s.

What's more, it cannot hover, as even one engine has a greater thrust at minimum throttle than the rocket weighs. So it must do what SpaceX calls a "hover-slam", and reduce its velocity vectors to 0 at the same moment its altitude gets to 0 in a single burn. Burn too short, you crash, burn too long, you go back up (and then crash.) And the horizontal vectors also need to be very small, and you can't end up too tilted either, or you fall over. And explode, because you're a thin-shelled tube with plenty of fuel and oxidizer still left. Oh, and you have to hit a very small target within a couple meters.

So it's a pretty tricky task. Plenty of rockets can hover these days, but landing a full-sized launch stage coming in from the edge of space is pretty tricky. And they almost got it. Looks like it overreacted just a hair to a gust of wind or something.

Your solution has two problems: (1) it cannot hover and (2) you'd have to design and build and test big robot arms. And put them everywhere you want to land. It's probably easier to make your big robot spacecraft, which you already have, do the landing itself on a plain concrete pad.

Re: Space X CR6 First stage landing

#102

Post by robryan »

deeviant wrote: post #63
_Adam wrote: post #26

Earlier quoted context omitted.

I don't agree that failure is absolutely necessary. I think success is generally more informative, because it gives you data about something that works end to end. If your rocket blows up on the launch pad, you just learned that you shouldn't overpressure your fuel tank (or w/e). But if your rocket lifts off, goes to space, and lands, you learn thousands of things that work that you can use again.

I strongly disagree. I have seen a company be destroyed because they got things almost right, from the beginning. This "success" informed their every decision and created an enormous resistance to trying anything new and in the end it turned out almost simply wasn't good enough. The initial success destroyed them.

I agree, this video makes it look like there was a strong chance that it could have been a success if the random variables on the way down aligned better. If it had of been they may not have learned and iterated based on this possibility and may have had a failure down the track.

Re: Space X CR6 First stage landing

#103

Post by 31reasons »

waterlesscloud wrote: post #41
filvdg wrote: post #3

this footage give a real sense of how close they are to getting it right

Watching it made me realize I only had an abstract idea of how difficult it was to even attempt this. It's insanely hard, and they're so close!

Yes its a very hard problem but what makes it hard ? Is it engineering of the components, algorithms or expensive tests and economics of it ? Maybe people before Elon Musk were not interested in solving this problem not because its technically hard problem but because their existing business model is much more lucrative.

Re: Space X CR6 First stage landing

#104

Post by zaroth »

dperfect wrote: post #77

As if this weren't impressive enough already, according to a comment made by Scott Manley[1] it sounds like the final maneuver (where it appears the rocket is making a fairly strong lateral correction) is entirely intentional. Not sure if anyone here can confirm or deny the claim, but apparently the trajectory prior to landing is not supposed to be exactly above the barge as one would expect - I understand it may be…

I was wondering about that too! I guess it does make sense if the final burn fails to not have a fairly large bomb dropping at terminal velocity directly onto the barge! On land I guess less of a difference though...

Re: Space X CR6 First stage landing

#105

Post by 69_years_and »

sfeng wrote: post #87
monk_e_boy wrote: post #71

Earlier quoted context omitted.

How high did it get? Does it burn the whole way down (I assume it uses those little X-wings to 'fly down?) When does it start the descent burn? How fast is it going at this point? How much does it weigh, what sort of forces are we talking about? How hard is landing this puppy in terms of human achievement? Top 10% of tricky things humans have so far achieved? [edit] wouldn't it be easier to hover it a few meters abov…

It burns three times after the second stage detaches. The first is to get the landing zone closer to the takeoff, the second slows it down from 1300 m/s to about 250 m/s, the third slows it to (hopefully) 0 m/s on the landing pad. Here's a diagram: http://media2.s-nbcnews.com/i/newscms/2014_51/812491/141217-... If the 'x-wings' are what I think you're referring to, they're the landing legs. They are shaped like that…

What's the difference between the tests on earth (as seen on youtube etc) where the engine runs the whole time and seems to display good throttle control and this type of landing?

Re: Space X CR6 First stage landing

#106

Post by jccooper »

zaroth wrote: post #100

It was jaw dropping as-is, but when they finally do stick a landing I imagine the engineers will be running around like little kids screaming with the pure joy and exhilaration of it all. I hope they share the video at mission control as well when the day does come! The sheer drama of how fast it comes down, the awesome power of the Merlin engine just blasting away delta-v... I wonder how many g's it takes on that fi…

A drogue would seem unnecessary; a Merlin can scrub off plenty of m/s real fast, and if it can't do it fast enough, they have more of them. In fact, it might be easier if the thing is coming in faster, because you get to run the engine a bit longer and have more time to adjust your aim with the engine's greater control authority.

Anyway, they don't seem to have any problem managing the vertical velocity. It's been the horizontal each time. And this time, only a tiny bit off.

Re: Space X CR6 First stage landing

#107

Post by [deleted] »

monk_e_boy wrote: post #98
sfeng wrote: post #87

Earlier quoted context omitted.

It burns three times after the second stage detaches. The first is to get the landing zone closer to the takeoff, the second slows it down from 1300 m/s to about 250 m/s, the third slows it to (hopefully) 0 m/s on the landing pad. Here's a diagram: http://media2.s-nbcnews.com/i/newscms/2014_51/812491/141217-... If the 'x-wings' are what I think you're referring to, they're the landing legs. They are shaped like that…

Aah, grid fins were the things I was thinking of. That first burn must have some complex maths behind it. Do we know anything of their simulation software?

[deleted]

Re: Space X CR6 First stage landing

#108

Post by 31reasons »

tjradcliffe wrote: post #44
rebootthesystem wrote: post #22

Non-engineers see this as a failure. Engineers and scientists see this as an awe-inspiring success from which to learn, iterate and try again. I don't fault reporters for exagerated headlined focusing on failure. They don't know any better. That is not what their lives are about and most might be challenged to understand that nothing in that video constitutes failure in the context of evolving a design. In fact, fail…

This. This video is one of the most beautiful things I have ever seen. This is not a failure. This is an incredibly rich mine of data on how to do better next time. This is how we improve the world, and it's beautiful.

We are witnessing 0-1 technology of space exploration and its amazing. Its going to be an inflection point in the history of our species.

Re: Space X CR6 First stage landing

#110

Post by melling »

mikeash wrote: post #74
melling wrote: post #59

Earlier quoted context omitted.

Of course, their plan is to not have problems. That's everyone's plan. It's planning for when things don't go right that makes the difference.

They are planning for when things go wrong, which is why the landing ship is unmanned and in the middle of a big exclusion zone in the ocean.

I'm talking about the economics of failures. Insurance prices will go down, the cost to orbit will be cheaper. Could probably land at the Cape, decreasing turn around time. We'll have flights everyday!
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