Author Topic: SpaceX Falcon 9 v1.1 - SpX-6/CRS-6 DRAGON - Discussion Thread (Read 516201 times)

GalacticIntruder · « **Reply #400 on:** 04/15/2015 01:19 am »

Not sure about this hover-slam idea on a very small target. I don't see how they will ever get the time to correct for their late (or unexpected) lateral movements if they are falling/landing that fast that close to the target. I would have assumed almost zero velocity well above the lander. Might need a larger landing area, and better landing gear.

Elmar Moelzer · « **Reply #401 on:** 04/15/2015 01:19 am »

Just look at how tiny the barge is compared to the stage! It is no wonder that they are having such a hard time getting right on top of it (especially with strong cross winds, etc). To me it seems very obvious that landing on a (MUCH larger) landing pad on land, would be trivial compared to this and would have probably succeeded.

abaddon · « **Reply #402 on:** 04/15/2015 01:20 am »

Oh man that was so close. Much closer than I had thought from the barge pics.

sanman · « **Reply #403 on:** 04/15/2015 01:21 am »

Gee, I would hate to be a human passenger in that thing. Even if it hadn't toppled over, it would still feel like a "brown pants" landing. They should slow the vertical descent rate near the end.

jabe · « **Reply #404 on:** 04/15/2015 01:23 am »

Quote from: Elmar Moelzer on 04/15/2015 01:19 am

Just look at how tiny the barge is compared to the stage! It is no wonder that they are having such a hard time getting right on top of it (especially with strong cross winds, etc). To me it seems very obvious that landing on a (MUCH larger) landing pad on land, would be trivial compared to this and would have probably succeeded.

my thoughts too...if off target 10 m on land just land it..not so for barge...have to adjust.. Amazing how close they got!! higher resolution vid is going to be awesome!!
jb

butters · « **Reply #405 on:** 04/15/2015 01:23 am »

It looks like the leg deploy upset the attitude control. The mark 2 legs with early deploy might help quite a bit.

CameronD · « **Reply #406 on:** 04/15/2015 01:24 am »

So.. I suppose the ASDS Landing Bingo jackpots again?

sanman · « **Reply #407 on:** 04/15/2015 01:24 am »

Infrared would have been awesomer, to penetrate that exhaust cloud at the end.

Space Ghost 1962 · « **Reply #408 on:** 04/15/2015 01:24 am »

Quote from: sanman on 04/15/2015 01:18 am

Also, somehow it looks like this stage is descending at a much faster rate at the end, compared to Grasshopper. Can't they spare a bit more fuel to slow the vertical descent just before the end?

It's like the equivalent of a guy screeching into his parking spot and slamming on the brakes.

Could be easier to control coming in fast - not as susceptible to gusts.

The lateral/pitching is more worry some.

The video is impressive, especially when you know the size/weight of the vehicle.

MarekCyzio · « **Reply #409 on:** 04/15/2015 01:29 am »

Regarding Vine movie:
https://vine.co/v/euEpIVegiIx

This is a book example of an uncontrolable, unstable system with long delay between measurements and controls.

Lars-J · « **Reply #410 on:** 04/15/2015 01:33 am »

Quote from: Rebel44 on 04/15/2015 01:13 am

To me, it looks like part of the problem is overcorrection.

Yes, it does appear so.

But one cool thing you can see in the Vine is how the RCS thrusters fire just before touchdown. It wasn't enough, obviously. But this data will be immensely helpful it improving the landing algorithm.

NWade · « **Reply #411 on:** 04/15/2015 01:33 am »

Anyone else notice the top of the stage as it nears the barge? I'm assuming we're seeing the N2 thrusters going full-bore trying to adjust the vehicle and either assist (or compensate for) the gimballed-dance that the main engine is doing...

[Answer: Yes, and Lars is just a bit faster on the "Post" button than I am, apparently!]

--Noel

Lars-J · « **Reply #412 on:** 04/15/2015 01:34 am »

Quote from: GalacticIntruder on 04/15/2015 01:19 am

Not sure about this hover-slam idea on a very small target. I don't see how they will ever get the time to correct for their late (or unexpected) lateral movements if they are falling/landing that fast that close to the target. I would have assumed almost zero velocity well above the lander. Might need a larger landing area, and better landing gear.

No, hovering solves very little of the actual problem - it just wastes fuel. The targeting/landing algorithm will just need tweaking.

meekGee · « **Reply #413 on:** 04/15/2015 01:34 am »

Quote from: MarekCyzio on 04/15/2015 01:29 am

Regarding Vine movie:
https://vine.co/v/euEpIVegiIx

This is a book example of an uncontrolable, unstable system with long delay between measurements and controls.

It might just be. I can't tell if there's a sudden external forcing input (wind gust, failed leg deploy, etc) or just a rocket-induced-oscillation..

I think at the end it wasn't just lateral velocity - the tip rate at touch-down was not zero.

Lars-J · « **Reply #414 on:** 04/15/2015 01:35 am »

Quote from: MarekCyzio on 04/15/2015 01:29 am

Regarding Vine movie:
https://vine.co/v/euEpIVegiIx

This is a book example of an uncontrolable, unstable system with long delay between measurements and controls.

Unstable? Yes. Uncontrollable? Give me a break!

(you've never seen the grasshopper videos??)

If this was a piece of cake it would have been done before... But it certainly is possible.

sanman · « **Reply #415 on:** 04/15/2015 01:36 am »

Were these landing conditions a worst case example? Or would landings be expected under even worse wind/sea conditions?

matthewkantar · « **Reply #416 on:** 04/15/2015 01:36 am »

A few notes on the amazing Vine Spacex posted:

-One can see the thrusters at the top of the stage doing their thing at the last second.
-The camera seems to have been 'sidewind' to the barge not upwind or downwind, and the wind seems to have been a problem. The gimbal on the landing engine goes from hard left to hard right in the short video.
-I have no confidence at all that I am not seeing things, but it looks like the stage is bending in response to the maneuvering.
-A much larger landing surface would seem to be helpful, but the horizontal component of velocity is still going to have to be zeroed with rocket nearly plumb at the moment of touchdown.
-The video took away any notion that I had that there is a problem with the steadiness of the barge, the amplitude of the stage's movements are off the charts compared to the barge.

It would seem they have lots of work to do.

Matthew

Ohsin · « **Reply #417 on:** 04/15/2015 01:36 am »

Quote from: Lars-J on 04/15/2015 01:33 am

Quote from: Rebel44 on 04/15/2015 01:13 am
To me, it looks like part of the problem is overcorrection.

Yes, it does appear so.

But one cool thing you can see in the Vine is how the RCS thrusters fire just before touchdown. It wasn't enough, obviously. But this data will be immensely helpful it improving the landing algorithm.

Those could be grid fins catching the sunlight. Btw this all feels unreal amazing stuff we are witnessing.

EDIT: Nah too far up to be grid fins. You're right

Herb Schaltegger · « **Reply #418 on:** 04/15/2015 01:37 am »

Quote from: Space Ghost 1962 on 04/15/2015 01:24 am

Quote from: sanman on 04/15/2015 01:18 am
Also, somehow it looks like this stage is descending at a much faster rate at the end, compared to Grasshopper. Can't they spare a bit more fuel to slow the vertical descent just before the end?

It's like the equivalent of a guy screeching into his parking spot and slamming on the brakes.

Could be easier to control coming in fast - not as susceptible to gusts.

The lateral/pitching is more worry some.

The video is impressive, especially when you know the size/weight of the vehicle.

I need to watch the video a few more times to see if I can make out the moment of leg deploy, but I have to wonder if the late deploy, into a steady sea breeze, isn't what causes the pitch excursion due to the sudden increase in drag at the base of the vehicle relative to the wind. Those rapidly-telescoping legs and their bases have much greater area drag than the relatively featureless cyclinder of the rest of the stage.

meekGee · « **Reply #419 on:** 04/15/2015 01:37 am »

Quote from: Jim on 04/15/2015 01:11 am

2. No, they are not. Again, the rocket flies to a coordinate and not an object.

We haven't seen any evidence that there is or isn't a closed loop terminal guidance system.

Clearly most of the descent is towards an absolute coordinate, but we don't know about the last mile.

It is unarguable that active guidance (the rocket doesn't need to communicate with the barge for that) is more precise, since by using absolute coordinates you add the errors in barge and rocket station navigation.

The only question is whether SpaceX implemented a last mile terminal guidance system of some sort.

So far, all we have are opinions.