SpaceX Tuesday...
Discussion
Watch the youtube video closely, in the first two secs as the stage comes into shot from above, you can see the rocket plume gimballing rapidly, and maintaining a vertical orientation.
Yet, as the engine goes to a higher power setting to decellerate the stage just prior to touch down, that gimballing seems to slow right down.
If you stop the video at 5 sec, it looks like the stage is pretty much vertically aligned with the centre of the pad, but the stage has a 15deg tilt to the right of frame.
At this point with limited altitude left, you have a big issue. Your control laws must get the rocket to vertical, but it also must get the rocket to the centre of the pad. ie:
1) In attempting to just get back to vertical, the thrust also pushes the stage to the right, and you miss the pad.
2) In attempting to just get to the centre of the pad, the rocket lands at an angle (ie not vertical)
In effect, you have simply run out of time to make corrections to be both centred and vertical at the moment of touchdown.
The question for me, is what caused the significant off vertical event in the final descent stages.
It could be a large number of things, such as cross winds, incorrect control system gains, and as mentioned, sticky control valves leading to phase lag. It certainly looks to me like between 4 and 5secs on that video, the system is slow to remove the thrust offset and this creates the off vertical alignment.
One good piece of news is that really, having to land on a small barge at sea makes all this a lot more complicated. A touch down on land, probably in the middle of a few square km of flat ground means the system can then bias it's control to maintaining vertical alignment at the expense of touchdown offset!
Yet, as the engine goes to a higher power setting to decellerate the stage just prior to touch down, that gimballing seems to slow right down.
If you stop the video at 5 sec, it looks like the stage is pretty much vertically aligned with the centre of the pad, but the stage has a 15deg tilt to the right of frame.
At this point with limited altitude left, you have a big issue. Your control laws must get the rocket to vertical, but it also must get the rocket to the centre of the pad. ie:
1) In attempting to just get back to vertical, the thrust also pushes the stage to the right, and you miss the pad.
2) In attempting to just get to the centre of the pad, the rocket lands at an angle (ie not vertical)
In effect, you have simply run out of time to make corrections to be both centred and vertical at the moment of touchdown.
The question for me, is what caused the significant off vertical event in the final descent stages.
It could be a large number of things, such as cross winds, incorrect control system gains, and as mentioned, sticky control valves leading to phase lag. It certainly looks to me like between 4 and 5secs on that video, the system is slow to remove the thrust offset and this creates the off vertical alignment.
One good piece of news is that really, having to land on a small barge at sea makes all this a lot more complicated. A touch down on land, probably in the middle of a few square km of flat ground means the system can then bias it's control to maintaining vertical alignment at the expense of touchdown offset!
They want to land the stages not far from the launching point. That limits the amount of landing space available - and causes a problem with proximity to human habitation.
With a launch from Cape Canaveral, you are limited to touch down points on the east coast of the USA - which means option for large expanses of flat, uninhabited areas such as Edwards Air Force Base or White Sands aren't available.
With a launch from Cape Canaveral, you are limited to touch down points on the east coast of the USA - which means option for large expanses of flat, uninhabited areas such as Edwards Air Force Base or White Sands aren't available.
Apparently SpaceX will be moving to a v1.2 version of the Falcon 9 later this year. The modifications include increasing second-stage engine thrust by 15 percent, increasing tank volume by 10 percent, and subcooling the cryogenic oxygen to obtain greater density.
I've read that some changes will also be made to allow recovery attempts from GTO missions which currently have insufficient fuel for a recovery attempt.
I've read that some changes will also be made to allow recovery attempts from GTO missions which currently have insufficient fuel for a recovery attempt.
Barge view of the landing - looks to me that when it started to topple one of the legs collapsed
https://vid.me/i6o5
https://vid.me/i6o5
It's already like trying to balance a broom stick on the tip of your finger. Quite remarkable that the computers can control it as closely as they do.
To my mind it needs some arms to fold upwards from the deck at the last second and form a ring around the upper rocket. That would stop it toppling over.
Sort of the opposite of the stabilising arms used on Russian rocket launches. As seen here about 0.39 seconds in:
https://www.youtube.com/watch?v=V8I7xJ0MBpg
To my mind it needs some arms to fold upwards from the deck at the last second and form a ring around the upper rocket. That would stop it toppling over.
Sort of the opposite of the stabilising arms used on Russian rocket launches. As seen here about 0.39 seconds in:
https://www.youtube.com/watch?v=V8I7xJ0MBpg
Beati Dogu said:
It's already like trying to balance a broom stick on the tip of your finger. Quite remarkable that the computers can control it as closely as they do.
To my mind it needs some arms to fold upwards from the deck at the last second and form a ring around the upper rocket. That would stop it toppling over.
Sort of the opposite of the stabilising arms used on Russian rocket launches. As seen here about 0.39 seconds in:
https://www.youtube.com/watch?v=V8I7xJ0MBpg
They are already managing to hit a 10m bullseye in the middle of the atlantic and now you need them to get the landing ellipse down to the order of centimeters?To my mind it needs some arms to fold upwards from the deck at the last second and form a ring around the upper rocket. That would stop it toppling over.
Sort of the opposite of the stabilising arms used on Russian rocket launches. As seen here about 0.39 seconds in:
https://www.youtube.com/watch?v=V8I7xJ0MBpg
Eric Mc said:
They want to land the stages not far from the launching point.
I wasn't really talking about needing miles to land in, more say ~500m. They have demonstrated targeting accuracy of something like 25m by the looks of things, but opening that up to say even 100M means you can have a orientation system that at low altitudes (say <100m) can totally bias itself to attitude rather than absolute position. Hence, if the stage suddenly gets blow sideways by a wind gust, the system can stay upright and still land. (with the barge, anything more than about a 50m offset in position and you're going to have a rocket full of seawater ;-)Eric - they plan on landing at the old LC-13 http://spacenews.com/spacex-leases-cape-canaveral-...
I wonder how long before they start attempt to recover the 2nd stage too, as in this old promo video https://www.youtube.com/watch?v=sWFFiubtC3c&fe...
I wonder how long before they start attempt to recover the 2nd stage too, as in this old promo video https://www.youtube.com/watch?v=sWFFiubtC3c&fe...
Eric Mc said:
The point is where?
I do not see the relevant authorities allowing any of this to be carried out anywhere on the eastern seaboard of the US - which makes barge landings compulsory.
It will take someone a lot smarter than I to work out all the possible ballistic trajectories from the various plausible abort scenarios, and the probability of each those trajectories occurring, and mapping that out onto the ground around any given touchdown point. Assuming that the stage has some form of self destruct capability to avoid very large pieces coming down, as the altitude reduces, the possible landing circle decreases with it. I guess most likely (and also a possible "safe mode") is a complete failure of the descent control systems, leading to a pure ballistic flight?I do not see the relevant authorities allowing any of this to be carried out anywhere on the eastern seaboard of the US - which makes barge landings compulsory.
Better footage of the attempted barge landing:
https://www.youtube.com/watch?v=BhMSzC1crr0
BTW the barge is called "Just Read the Instructions", which Ian M Banks readers may recognise.
The one under construction for operation in the Pacific is called "Of Course I Still Love You".
https://www.youtube.com/watch?v=BhMSzC1crr0
BTW the barge is called "Just Read the Instructions", which Ian M Banks readers may recognise.
The one under construction for operation in the Pacific is called "Of Course I Still Love You".
Edited by Beati Dogu on Friday 17th April 22:59
Beati Dogu said:
Better footage of the attempted barge landing:
https://www.youtube.com/watch?v=o2NlwvXaFjo
BTW the barge is called "Just Read the Instructions", which Ian M Banks readers may recognise.
The one under construction for operation in the Pacific is called "Of Course I Still Love You".
Wrong link ?https://www.youtube.com/watch?v=o2NlwvXaFjo
BTW the barge is called "Just Read the Instructions", which Ian M Banks readers may recognise.
The one under construction for operation in the Pacific is called "Of Course I Still Love You".
MartG said:
Eric - they plan on landing at the old LC-13 http://spacenews.com/spacex-leases-cape-canaveral-...
I wonder how long before they start attempt to recover the 2nd stage too, as in this old promo video https://www.youtube.com/watch?v=sWFFiubtC3c&fe...
I think we are looking at a lot of wishful thinking. I still cannot see them getting permission to attempt land landing in and around the Cape for a long time. If I was a certification officer I would not allow any attempts to land back at Cape Canaveral until they had landed successfully on the barge at least ten times - demonstrating that at all stages in the descent they were fully in control of the proceedings.I wonder how long before they start attempt to recover the 2nd stage too, as in this old promo video https://www.youtube.com/watch?v=sWFFiubtC3c&fe...
In order to get something to work at a level where everybody is satisfied that it WILL work properly each time, you need to be critical and pose hard questions.
I hope very much that they succeed and I do recognise that they are doing this at their own expense - which means they can try as often and as long as they like.
But I don't want people to think that this is easy - it's not. And there is no harm in throwing into the discussion the various issues that they have to surmount to turn this technique into something workable.
I always knew landing Thunderbird 3 back into its underground launch facility was harder than it looked.
I hope very much that they succeed and I do recognise that they are doing this at their own expense - which means they can try as often and as long as they like.
But I don't want people to think that this is easy - it's not. And there is no harm in throwing into the discussion the various issues that they have to surmount to turn this technique into something workable.
I always knew landing Thunderbird 3 back into its underground launch facility was harder than it looked.
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