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Space X CR6 First stage landing

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Re: Space X CR6 First stage landing

#191
post #61

I wonder what happened there. Looks at 4 seconds the guidance system figured it needed to pull an ultra manoeuvre to slow the the horizontal speed(was drifting left) but ended up with a solution putting the rocket upright again at nearly 0m. If it would have left some altitude for stabilizing I think they would have had it. Arm chair rocket science is fun.

As mentioned, it's control system lag, which led to over-correction oscillations. This is very straightforward to deal with.

I find this a bit surprising given the amount of testing they did with F9R.

Re: Space X CR6 First stage landing

#192
post #58
post #48

Question: Why don't they land into the water instead of a hard surface? (Wouldn't this give them more leeway in terms of impulse?) Edit: i.e. landing on a net stretched (under the lake/ocean-surface) by 3 or 4 ships - reel the net it in to raise stage out of the water, I'm pretty sure seawater is not significantly corrosive from 2-3 hours of exposure.

The eventual goal is to land it on land, they're only doing water landings right now due to the high chance of failure. In the future, the first stage will land back at the rocket pad, which massively reduces the logistics for re-using it. There's a cool mock-up video of what that might look like: https://www.youtube.com/watch?v=4Ca6x4QbpoM

Sorry, but I'm failing to see how sending a large barge out into the ocean constitutes "massive logistics".

Re: Space X CR6 First stage landing

#193

Earlier quoted context omitted.

Judging from the ensuing explosion there was still fuel left over.

Making it a little puzzling ask to why the engines didn't try harder to lower the vertical velocity on approach. Perhaps there are improvements to be made in the control systems.

Vertical was fine, it's the residual lateral velocity that tipped over and killed the rocket. Also, I can't find the tweet now (I think it was deleted) but initially Musk said they determined that 'static friction in bipropellant valves introduced a lag into system response that the control system wasn't prepared to handle'.

Re: Space X CR6 First stage landing

#194
post #141

Earlier quoted context omitted.

Making it a little puzzling ask to why the engines didn't try harder to lower the vertical velocity on approach. Perhaps there are improvements to be made in the control systems.

I have a feeling their engineers may have considered this. This is after all rocket science.

You'd be surprised what Bill lets Jeb fly at times... ( ;) ).

Re: Space X CR6 First stage landing

#195
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…

Maybe it needs to hover? There must be something that can be squeezed to throttle it down some more. Call the NASCAR guys with their restrictor plate designs.

IMO that landing clearly proved suicide burning is working perfectly fine - they just need to fix something to get rid of the remaining lateral velocity.

Re: Space X CR6 First stage landing

#196
post #121

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…

In general, burning while you descend is a terrible waste of fuel, because you have to generate 1G of acceleration just to keep your speed constant. (See http://en.wikipedia.org/wiki/Gravity_drag. ) That's fine if you're not fuel limited (like the grasshopper tests) but if you want to land with minimal use of propellants, you want to use as high acceleration as possible. So the idea is to let the rocket drop freely a…

[deleted]

Re: Space X CR6 First stage landing

#197

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…

just the fact they can hit a barge after falling back from space impresses me. I wonder if there are any islands they could use instead, surely they had to have looked for one. Would a stationary oil platform work better?

A volcano base if one is to judge by Elon's tweets.

Re: Space X CR6 First stage landing

#198
post #104

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...

Not terminal velocity, just fairly fast: there's a deceleration burn earlier in the descent plan. If that doesn't work, the big red abort button can be used to blow it up at high altitude.

Re: Space X CR6 First stage landing

#199
post #26

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…

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've +1'd because while I disagree, yours is a reasonable opinion that's provoking a useful discussion, and I think the downvotes you're getting are not appropriate.

The problem with aiming for success from the start is that it promotes a highly conservative, over-engineered solution. If it does work, it also acts as a brake on iterative improvement because your system already works, why risk failure by trying to make it more optimal? Also if you have a proven system, people start making bigger and bigger bets on it's success (more expensive payloads, for example), so the cost of any failure goes up.

If you aim for optimal design form the start, you're almost bound to experience failures. That's because say there are 100 parameters you need to tone and you aim for the estimated optimum for all of them. Statistically, half of those estimates are goign to turn out to be sub-optimal. As you iterate on the design, you fix those untill you end up with a truly optimal design. At each iteration, you have a positive incentive to make the necessary changes.

Depending on the cost of failure, this can be more expensive than aiming for a conservative, over-engineered design to start with, but not necessarily. Over-engineered systems, for a high chance of initial success, are by definition more costly than optimised designs and there's still a non-zero chance of initial failures anyway.

Re: Space X CR6 First stage landing

#200
post #171

If any of the engines has a TWR above one when landing, even at it's lowest setting, i can't help but wonder why don't they use a smaller engine which can be more effectively used to land. I'm sure theres a perfectly good reason for this, there's a reason SpaceX has its reputation, but i'm curious as to why.

For one, developing reliable rocket engines takes a long time, you can't just whip up a new one. The current Merlin has a very large throttling range already when compared to existing engines. The size of the engines they use now is defined by the need to get payload's to space, the landing has to be secondary.

Grasshopper was able to hover, or even perform a powered descent and was probably lighter than an F9 first stage, so I don't think this can't be it. They likely do have an engine configuration capable fo a lower minimum thrust, but they're choosing not to use it presumably because it's not optimal for the primary mission, and they believe it's not necessary for the landing.
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