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

[Lars-J]

So it looks like a bigger barge would have solve the problem?

Not necessarily. It depends on why it tipped over. If it was because too much lateral motion, it would have tipped over no matter what. But if it was due to the legs extending over the edge, then yes.

[SpaceBert]

Hi guys! I normally use to lurk around and enjoy NSF launch coverage but on this topic I have to add my five cents:

For me it clearly looks like the stage came down too much off center. If you look at the engine plume, you can clearly see it hits the barge on the withe circle, maybe even slightly outside of it. If we compare that with aerial views of the drone ship and keep leg span in mind it could really missed the landing surface with one leg - tipping over in consequence.

[Fr4nK]

So it looks like a bigger barge would have solve the problem?

Not necessarily. It depends on why it tipped over. If it was because too much lateral motion, it would have tipped over no matter what. But if it was due to the legs extending over the edge, then yes.

But with a bigger barge, it would have bigger margin to calculate required lateral correction and still hit the barge with less lateral motion. I'm sure it would help. I can't wait to find out if one leg extended over the edge!

[mvpel]

It looked so easy in CRS-3. 

[AJA]

Alright... my own first impressions seek to torpedo what I'm about to say, on the grounds that it'd be overly complicated... but nonetheless..


Would the lateral velocity problem be ameliorated at all, if the barge's engines were hooked up to a control program which, could be connected to a camera/transceiver? In that case, the landing system I'm envisioning would work somewhat like this.
1. Falcon nulls out most velocities - but primarily the vertical
2. Hovers at something like 10-15 metres above the barge
3. Barge quickly matches the remaining delta in the horizontal velocity.
4. When both guidance systems confirm that the relative horizontal velocity is zero, the stage proceeds to lower itself down onto the barge surface.


The advantages would be the saving of precious propellant, but perhaps - more importantly - keeping your altitude while still performing horizontal adjustments. Currently, the engine is required to be gimballed, and if you gimbal an engine that's perfectly offsetting the (rapidly diminshing) weight of the stage, then you're almost definitely going to be throwing off the vertical equilibrium as well.


This potentially allows independent control of X-Y and Z axes, although I myself am not convinced that this will lead to "easier" control.

[GWH]

Not necessarily. It depends on why it tipped over. If it was because too much lateral motion, it would have tipped over no matter what. But if it was due to the legs extending over the edge, then yes.

Not if the footpads are able to slide   

Koenigsmann on twitter just quoted that he is pretty sure there is something to recover from first stage landing. 

[MostlyHarmless]

Looking at the flag in the first frame, it seems as if there was a lot of wind.  If there was any sort of wind shear with altitude, it's very possible that the booster could have had to make a sudden adjustment for lateral drift in the final seconds.  Such a maneuver might have had insufficient vertical distance to complete before touching down. 

Of course, it's all speculation until we here from SpaceX. 

Regardless, they're getting closer.

[Bob Shaw]

Seriously, guys, you're *all* missing the point!

Elon Musk foolishly dissed the notion of superhuman AI development, and then...

...he trusted his toys to an autonomous vessel named after a Culture Mind, hyper-intelligent beings known for their devious behavior and wicked sense of humour!

And you're surprised that every time those first stages line up to land, the barge does a little jig out of the way? Ha!

I, for one, wish to be among the first to welcome our new overlords.

Just sayin'...

[AJA]

I was writing something related to first stage landing technical hurdles that SpaceX has already retired, but clearly something is missing.

What technical hurdles are left to solve?

They've never done a landing on a solid surface from high altitude free-fall at terminal velocity.  All grasshopper and F9R-Dev tests had the engine firing continuously and a slow controlled descent.   A F9R-Dev test involving engine cutoff and a separate last moment landing burn has never been attempted, let alone succeeded.   

But what difference does it make? Once the F9R-Dev is under power on the landing burn, it's analogous to the slowly decelerating Grasshopper, isn't it (aside from the differences between a test article and flight hardware)? The conditions at sea are perhaps quite different to McGregor though. Not to mention the vertically unstable (despite all the mention of the stablising ability of the) barge.

[Nilof]

So, is the lateral position control system underdamped?

[Jim]

So this tells use that the barge does not have the bandwidth to broadcast full live video at the moment. (something they will probably fix in the future)

Why do you think it needs to be "fixed"?

[meekGee]

So it looks like a bigger barge would have solve the problem?

Not necessarily. It depends on why it tipped over. If it was because too much lateral motion, it would have tipped over no matter what. But if it was due to the legs extending over the edge, then yes.

Not quite, IMO.

With a larger barge the algorithm could have chosen to land further off-center, but with lower lateral velocity.  Here, there's almost no margin of error.

[Lars-J]

So this tells use that the barge does not have the bandwidth to broadcast full live video at the moment. (something they will probably fix in the future)

Why do you think it needs to be "fixed"?

Why not? If the barge is in use years down the line, you find it such an implausible statement that it would have better bandwidth?  Let's wait and see who is right.

[Jim]

what if the lasers locked on the top of the rocket? Lasers mounted on the 4 corners of the barge... the farthest away possible from the bullseyes and thus less blocked by smoke, locking on the top part of the rocket?

The barge does not interact with the rocket except as a radar return.  There is no information that is broadcasted to the rocket, it is autonomous.

[Jim]

But with a bigger barge, it would have bigger margin to calculate required lateral correction .....

Based on what sensor data?

[Jim]

3. Barge quickly matches the remaining delta in the horizontal velocity.

No, there is no interaction between barge and rocket.  The barge is at a specific location and the rocket is told to fly to the specific location.  That is how it will work on land.

[Fr4nK]

But with a bigger barge, it would have bigger margin to calculate required lateral correction .....

Based on what sensor data?

Based on common sense. You aim for the X or you aim for no lateral motion while landing somewhere on the barge. Bigger barge means more margin

[Jim]

Why not? If the barge is in use years down the line, you find it such an implausible statement that it would have better bandwidth?  Let's wait and see who is right.

Because it is not needed.  It wasn't needed today, there is no engineering need..  The barge is going to be used less in the future.

[sanman]

But on land the ground can't shift beneath you. Sounds like they need to treat barge landing as a special case, that's different from a ground landing.

[Jim]

Based on common sense. You aim for the X or you aim for no lateral motion while landing somewhere on the barge. Bigger barge means more margin

Wrong and neither is it common sense .  The rocket is not aiming for the barge, it is aiming for a spot on the ocean.  The barge happens to be at the spot.  There are no sensors that say the rocket is to the right or left of the barge. The only sensors are the radars for height.