SpaceX Tuesday...

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vescaegg

25,489 posts

166 months

Wednesday 28th January 2015
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This may be a totally stupid question but is there a reason why the rocket couldnt just guide itself to the proposed recovery location and then open some huge parachutes and come down gently rather than trying to 'land' properly? boxedin

MartG

20,622 posts

203 months

Wednesday 28th January 2015
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vescaegg said:
This may be a totally stupid question but is there a reason why the rocket couldnt just guide itself to the proposed recovery location and then open some huge parachutes and come down gently rather than trying to 'land' properly? boxedin
Wind - very hard to make a precision landing onto a specific pad when under a parachute

MartG

20,622 posts

203 months

Wednesday 28th January 2015
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Pic of the pad-abort test Dragon


SpeedyDave

417 posts

225 months

Wednesday 28th January 2015
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vescaegg said:
This may be a totally stupid question but is there a reason why the rocket couldnt just guide itself to the proposed recovery location and then open some huge parachutes and come down gently rather than trying to 'land' properly? boxedin
Parachute landings aren't particularly gentle in the context of a craft you're hoping to rapidly refuel & refly

They also aren't anything like as precise, they're aiming for helicopter precision.

Once again it also comes back to the SpaceX end-game - Mars.

Parachutes & a very light crash near home, perhaps assisted by whatever you can set up on the ground (lake?) would probably be a useful step forward from the current disposable approach but since accurate propulsive landing would provide by far the best result might as well shoot for that.

At least while it seems feasible, and it does.

SpeedyDave

417 posts

225 months

Wednesday 28th January 2015
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I'm a bit surprised parachutes aren't used to augment the current solution.

I would have thought the effect of chutes would substantially reduce the fuel required for the landing phase and more than pay for itself in weight terms. Even just a drogue seems useful.

Obviously the maths doesn't play out the way I imagine but it still seems a bit counter intuitive to me.

Eric Mc

121,779 posts

264 months

Wednesday 28th January 2015
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I mentioned parachutes at the very start of this thread months ago. It seemed an obvious solution to a lot of this but they obviously have their reasons not to use them.

It does seem rather strange NOT to make use of a free braking facility i.e. the atmosphere, rather than relying 100% on rocket thrust.

As others have said, accuracy is probably the driver rather than gentleness.

Here's one idea NASA looked at 50 years ago when they thought that the recovery of the 1st stage of the smaller (relatively speaking) Saturn 1 and Saturn 1Bs might be feasible.


SpeedyDave

417 posts

225 months

Wednesday 28th January 2015
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Eric Mc said:
It does seem rather strange NOT to make use of a free braking facility i.e. the atmosphere, rather than relying 100% on rocket thrust.
Exactly as I was thinking, it seems like free 'work'. I guess the terminal velocity when chute-less with the legs deployed is still close enough to the chute version that a moderate amount of fuel accounts for the difference.

Floating much slower all the way down through the atmosphere isn't useful. As long as you slow to a manageable speed by the time you are at the business end approaching the ground you're ok.

I wonder what the throttle profile looks like all the way down. Do they even run the engines during the upper part of the flight (apart from the initial deorbit* kick)

Eric Mc said:
As others have said, accuracy is probably the driver rather than gentleness.
I'm sure it's both. Canopy landings look very gentle right until the last few seconds near the ground when you see how fast they're really going. A damage free landing on solid ground would require an enormous chute system and more complicated shock absorbing legs.

In addition to the landing with location accuracy you also have the issue of settling in a controlled fashion to avoid tipping over etc.


  • I realize the first stages never actually get to orbit.

Eric Mc

121,779 posts

264 months

Wednesday 28th January 2015
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The S1B plan was to have the stage slither to a halt on its side using a Rogallo Wing. This would have resulted in some damage and I don't know how accurate you could "fly" such a chute to a reasonable spot - especially with the technology available in the 1960s.

However, in the modern age they could use a Rogallo or a Parawing - remotely controlled or autonomously controlled using GPS for accuracy - to a glide landing on the now disused Shuttle Runway at Cape Canveral. Maybe that could work?

scubadude

2,618 posts

196 months

Wednesday 28th January 2015
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You'd need to run the math but I wonder if a (some) chutes large enough to make a meaningful speed reduction to a returning first stage isn't in fact heavier than "a bit" more fuel?

Also chutes require separate control systems, pyrotechnics and I wonder if large cargo chutes aren't very reusable themselves?

Since the rocket already carries the engines and controls systems SpaceX's approach is in many ways simple a software upgrade! You still need legs and the hypersonic fins to get the reentry accuracy for the parachute landing so the difference is one would be floaty and empty and their method is a bit firey :-)

From an engineering perspective using something you already have without adding yet another thing to go wrong is the right way to do it.

London424

12,826 posts

174 months

Wednesday 28th January 2015
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My guess is a function of weight and less complexity for the solution they are going for.

RobDickinson

31,343 posts

253 months

Wednesday 28th January 2015
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Eric Mc said:
As others have said, accuracy is probably the driver rather than gentleness.
Mars is the goal for Elon. Mars doesnt have enough atmosphere to make parachute landings feasable.

AFIK parachutes are also heavier than a little extra fuel for the rockets (& the rest of the landing system) to do this precice landing too.

It makes sense, but its not easy.

vescaegg

25,489 posts

166 months

Wednesday 28th January 2015
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This is a beautiful animation. Really hope one day this can actually happen.

http://youtu.be/4Ca6x4QbpoM

Barge attempt #2 is soon isn't it? About 2 weeks away?

Eric Mc

121,779 posts

264 months

Wednesday 28th January 2015
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RobDickinson said:
Eric Mc said:
As others have said, accuracy is probably the driver rather than gentleness.
Mars is the goal for Elon. Mars doesnt have enough atmosphere to make parachute landings feasable.

AFIK parachutes are also heavier than a little extra fuel for the rockets (& the rest of the landing system) to do this precice landing too.

It makes sense, but its not easy.
EVERY single probe that has landed on Mars has used parachutes - with rocket descent used for the last section of the landing.

And future landers on Mars will use even more sophisticated aerodynamic braking methods. There was a very good lecture on NASA TV the other night where two NASA engineers gave a great talk on the parachute development work that is currently under way to allow bigger items (such as habitats) and manned landers) to descend safely to the Martian surface.

As well as parachutes they are developing semi-rigid ballute type devices which can be deployed at over 1,000 mph without shredding.

RobDickinson

31,343 posts

253 months

Wednesday 28th January 2015
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Yep as you said with rockets.

Most stuff we have sent to Mars has been small and relativly light (apart from the last rover which had a complex landing system). Also no requirement to get back off Mars's surface.

Thats not going to work for a manned mission though, this is likely the best solution or at least part of.

Eric Mc

121,779 posts

264 months

Wednesday 28th January 2015
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RobDickinson said:
Yep as you said with rockets.

Most stuff we have sent to Mars has been small and relativly light (apart from the last rover which had a complex landing system). Also no requirement to get back off Mars's surface.

Thats not going to work for a manned mission though, this is likely the best solution or at least part of.
We shall see.

The latest rover, which weighed over 1 tonne, ALSO used a parachute.

And a big plus for chutes is that they are mature technology. They've been used to recover rocket segments, capsules, lander etc etc for over 70 years.
And in battlefield situations they've landed items heavier than many rocket stages.

The ONE reason against a parachute (as mentioned above) is lack of accuracy. This does not matter so much at the moment with Mars missions as there are no inhabited areas to worry about.
And it is the one reason I would see why SpaceX are trying their current system out.

RobDickinson

31,343 posts

253 months

Wednesday 28th January 2015
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It used a parachute but that wasnt enough to slow it down fully so then we had rocket crane to lower it..?

I assume larger heavier items will need more effective braking - chutes will still be part of the solution but couldnt ever be the whole one, not on Mars, not when you have to get back off Mars.

Caruso

7,422 posts

255 months

Wednesday 28th January 2015
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Any manned landing on Mars is going to have to be accurate as they'll need to land near to supplies and equipment previously landed. Parachutes have their place to reduce speed in the entry and descent, but for the actual landing they're not a good solution.

Eric Mc

121,779 posts

264 months

Wednesday 28th January 2015
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That's why I see a combination of both as being maybe the best solution - as already practiced on Mars.

On Mars the atmosphere is so thin parachutes alone would never slow the lander down sufficiently to avoid damage on impact.

On earth, up to certain sizes, parachutes can work. But a combination of parachutes and rockets can be used for earth landings too.

Just look at Soyuz.

SpeedyDave

417 posts

225 months

Thursday 29th January 2015
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vescaegg said:
This is a beautiful animation. Really hope one day this can actually happen.

http://youtu.be/4Ca6x4QbpoM

Barge attempt #2 is soon isn't it? About 2 weeks away?
Next launch Feb 9, I don't see any indication in the upcoming missions list of whether this includes a landing recovery attempt, I suppose they all will?

Kinda cool to be doing these landing tests largely for 'free'. Customer needs a satellite in orbit & pays for a launch so add some fuel and attempt a landing. As long as the customer payload isn't maxing out your lift capacity this is always an option.

Aside from landing attempts the next flight that looks really interesting is in March. They're going to test the Dragon2 LES at Max Q.

So half way through the first stage burn (about 1:20 after liftoff) at the point when the aero stress is at its highest they're going to fire the SuperDraco engines in the Dragon capsule and attempt to fly it away from the rocket.




Eric Mc

121,779 posts

264 months

Thursday 29th January 2015
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Here is a test of the Apollo escape system - which didn't quite go to plan -

from about 1.14 in -


https://www.youtube.com/watch?v=AqeJzItldSQ

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