SpaceX Falcon 9 v1.1 - SpX-6/CRS-6 DRAGON - Discussion Thread

[Jim]

Well, technically they *are* interacting - mechanically - which is why the "excess lateral velocity" probably caused the tipping in the end. In which case, it would make sense for the stage to directly measure its distance/velocity relative to the landing target. Some kind of landing beacon might be useful.

Again no.  There isn't going to be a beacon on land nor does there need to nor will beacon help.   The rocket has GPS and inertial guidance.  That is enough to tell distance/velocity relative to the landing target.  Again, the target isn't the barge but a spot in the ocean.  The barge is just placed over the spot. 

This system is to be landing site generic and not need outside support.

[Prober]

How about giant airbags? They inflate from the barge deck as soon as the stage touches down, in order to trap it upright.

You have making more problems than solving them.

the barge wasn't a cheap fix

kill the legs, just get the launcher close (horz), drop & recover what parts you can

[CameronD]

Well, technically they *are* interacting - mechanically - which is why the "excess lateral velocity" probably caused the tipping in the end. In which case, it would make sense for the stage to directly measure its distance/velocity relative to the landing target. Some kind of landing beacon might be useful.

Again no.  There isn't going to be a beacon on land nor does there need to nor will beacon help.   The rocket has GPS and inertial guidance.  That is enough to tell distance/velocity relative to the landing target.  Again, the target isn't the barge but a spot in the ocean.  The barge is just placed over the spot. 

This system is to be landing site generic and not need outside support.

Perhaps the key point is that in the last few feet before touchdown, you're already committed and, other than a full abort (go-around), you've not many control options available anyway.

eg. If the "excess lateral velocity" was caused by the wind (and it looks like there was plenty of it) pushing the stage bodily sideways across the deck, there's not much you can do about it other than stop engines and pray.  There's no point measuring anything because it's not like the ASDS can respond fast enough to help either.

EDIT:  I don't know what the sea/wind conditions were at the moment of landing, but if the wind was as strong as it looks in the pic I reckon all the video will show is that the landing was doomed from less than a dozen feet up..

[punder]

Well, technically they *are* interacting - mechanically - which is why the "excess lateral velocity" probably caused the tipping in the end. In which case, it would make sense for the stage to directly measure its distance/velocity relative to the landing target. Some kind of landing beacon might be useful.

Again no.  There isn't going to be a beacon on land nor does there need to nor will beacon help.   The rocket has GPS and inertial guidance.  That is enough to tell distance/velocity relative to the landing target.  Again, the target isn't the barge but a spot in the ocean.  The barge is just placed over the spot. 

This system is to be landing site generic and not need outside support.

Perhaps the key point is that in the last few feet before touchdown, you're already committed and, other than a full abort (go-around), you've not many control options available anyway.

Eg. If the "excess lateral velocity" was caused by the wind (it looks like there was planty of it) pushing the stage bodily sideways across the deck, there's not much you can do about it other than stop engines and pray.  There's no point measuring anything because the ASDS can't respond fast enough to help.

Which is actually an argument for some sort of active terminal guidance.  Null the wind by moving the ASDS, and let the stage track an active course to it.

But what am I saying!  Of course there is no need for active terminal guidance, and no one will ever implement such an outlandish notion.   

[llanitedave]

No, it isn't and you have nothing to back up your claim.  The barge is only temporary.  The fence is a kludge and not worth the effort

But Jim, what about for FH center core - is that supposed to always land back on land too?

That has infrequent flights.

"Infrequently used" is not the same as "temporary".

[CameronD]

Which is actually an argument for some sort of active terminal guidance.  Null the wind by moving the ASDS, and let the stage track an active course to it.

But what am I saying!  Of course there is no need for active terminal guidance, and no one will ever implement such an outlandish notion.   

As I said in my post, the ASDS can't move fast enough to "null the wind" - no matter how many "upgrades" you do or how big your engines are.  There are laws of physics working against you.

[Jim]

Which is actually an argument for some sort of active terminal guidance.  Null the wind by moving the ASDS, and let the stage track an active course to it.

But what am I saying!  Of course there is no need for active terminal guidance, and no one will ever implement such an outlandish notion.   

As I said in my post, the ASDS can't move fast enough to "null the wind" - no matter how many "upgrades" you do or how big your engines are.  There are laws of physics working against you.

That is not the reason.  How would they implement such a system on land?  Would they have a wheeled platform?  No.
Why can people understand that what is done on the barge will be the exact thing used on land? 

[craigcocca]

How about an electromagnet in the deck of the ASDS, with corresponding ferrous materials in each of the landing feet? The rocket would just need to hover near the center of the deck and then the system would pull it in and lock all of the feet down to the deck until the SpaceX red-shirts lock it down physically.

In light of this being an Elon Musk project, the magnetic landing system could be powered by SolarCity photovoltaics.

(I am actually serious about the core engineering idea here)

[jimbowman]

Video from the chase plane coming soon according to Musk. Amazing photo on his last tweet from the plane.

[jabe]

look at this tweet!!
https://twitter.com/elonmusk/status/588142879245238273

Ok, looks like we got some killer footage from the chase plane. Big ocean, small ship. Posting vid shortly

jb
edit:got beat by a hair I guess..

[topsphere]

Incredible stuff!!!

I posted this in the ASDS thread as well, so mods remove one if necessary.

[jabe]

Vid shows some instability..the wind???
https://twitter.com/SpaceX/status/588143946800463874

[sanman]

Why does the methodology for barge landing have to be exactly the same thing as for land?
Barge and land are different in reality, so why shouldn't this be reflected by small differences in the landing techniques for each?

Is the wind at sea inherently greater than wind on land?
Why not use Doppler radar to measure the wind conditions?

Thrustmasters don't have to null the wind - the gimbaled Merlin engines should be able to do that, shouldn't they?

You should be homing in on whatever surface you're landing on - if that surface is a barge surface, then it seems strange to want to home in on something else (ie. GPS) which is not that surface itself.

[CameronD]

Why does the methodology for barge landing have to be exactly the same thing as for land?
Barge and land are different in reality, so why shouldn't this be reflected by small differences in the landing techniques for each?

Is the wind at sea inherently greater than wind on land?
Why not use Doppler radar to measure the wind conditions?

Thrustmasters don't have to null the wind - the gimbaled Merlin engines should be able to do that, shouldn't they?

You should be homing in on whatever surface you're landing on - if that surface is a barge surface, then it seems strange to want to home in on something else (ie. GPS) which is not that surface itself.

Something people take for granted with "land" landings is protection from the wind:  On a flat-topped barge in the middle of the Atlantic with miles of fetch in all directions, wind speed at ground level is generally a lot stronger than on any strip of land with even only a few trees around, due to friction.

To make this work I suppose they could ditch their itsy bitsy nitrogen thrusters and replace them, top and bottom of the stage with SuperDracos acting horizontally in all directions, even after touchdown.  IMHO that's about the *only* thing that would give you sufficient lateral control authority to keep an upright stage on an unprotected barge on the open ocean. 

What SpaceX are trying to do here makes landing on a nice concrete pad on solid ground surrounded by a few trees look like child's play.  Yes, it really is that much harder.

[sanman]

Vid shows some instability..the wind???
https://twitter.com/SpaceX/status/588143946800463874

Some kind of axial/rotational momentum there. The stage has to tilt in order to counter any lateral forces, but then it doesn't have enough time to vertically reorient itself - so it seems to still have rotational momentum when it's touching down.

So is this all due to low-altitude wind gusting near the surface?

[CameronD]

Vid shows some instability..the wind???
https://twitter.com/SpaceX/status/588143946800463874

Some kind of axial/rotational momentum there. The stage has to tilt in order to counter any lateral forces, but then it doesn't have enough time to vertically reorient itself - so it seems to still have rotational momentum when it's touching down.

So is this all due to low-altitude wind gusting near the surface?

IMHO, yep.    From the look of that ocean, wind wind is blowing at least 15kts.  I reckon they might have nailed it if it hadn't been for the wind.

[Space Ghost 1962]

It doesn't null out before landing.

It overcontrols after adjusting to prevent overshoot.

However it looks adequately under control at the end to land.

Might be tipping end frame.

add:
Judging from backwash, no wind gusts of significance on last frame.

My guess is either oscillation in flight control model, dynamic instability possibly from a mis deployed leg, or collapse of a leg on contact.

[Jim]

1.  Why does the methodology for barge landing have to be exactly the same thing as for land?
2.  Barge and land are different in reality, so why shouldn't this be reflected by small differences in the landing techniques for each?
3. Is the wind at sea inherently greater than wind on land?
4.  Why not use Doppler radar to measure the wind conditions?

5.  Thrustmasters don't have to null the wind - the gimbaled Merlin engines should be able to do that, shouldn't they?

6.  You should be homing in on whatever surface you're landing on - if that surface is a barge surface, then it seems strange to want to home in on something else (ie. GPS) which is not that surface itself.

1. Because they only want to have one configuration of vehicle.  one set of sensors, one version of software, etc.
And the if the barge uses a different version, it does no good for certifying for land landings.

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

3.  Not always. the land landing spots are close to the ocean

4.  not needed.  The vehicle senses the wind and corrects for it.  When you drive and there is a high cross wind, do you need outside measurement of the wind speed and direction?  No, you just slightly turn into it to stay on the road.

5.  No, the thrustmasters keep the barge over the predetermined spot.

6.  No, because rocket flies from multiple pads and will have multiple landing spots.  All that would need to change for each flight is just to input the coordinates of the landing spot (work will have already been done to make sure the site is within the capabilities of the vehicle).  That make everything simple and clean and easy to verify and validate.

[Rebel44]

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

[sanman]

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.