A few notes on the amazing Vine Spacex posted:...-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.
It would seem they have lots of work to do.
3) a) Discussion of wind here in this chat room of the internets is useless. Wind on that cylinder whether its a significant force or not is easy to model and has been thought through vastly more than our words and the wind from our mouths will accomplish.
b) Observations of a flag which is seen to be flying briskly in a direction radially away from an active rocket engine are not reliable indications of a meterological wind. A review of the Apollo 11 lunar surface flag during liftoff would under that logic lead one to believe that it happened on a windy day on the moon. I watched that moon walk and I can attest that there was no appreciable wind on the surface until they lit that thing.
Quote from: Jim on 04/15/2015 01:11 am2. 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.
2. No, they are not. Again, the rocket flies to a coordinate and not an object.
My two cents:Barring additional information, the way to fix the problem is to land with less lateral motion. It either had the lateral motion because it had not killed off enough horizontal speed or because it it overcorrected/corrected too late. The former is fixed by adjusting the arc that the stage "flies." The latter is fixed by refining the terminal landing algorithm.That's it. No changes to the barge, no need to anchor the barge, no new two-way communication required. We are well into the "fine tune" phase of the design/experiments. Wash, rinse, repeat.
Is it possible to actuate the legs to dampen or absorb some of the lateral landing momentum?
Quote from: meekGee on 04/15/2015 01:37 amWe 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.So far, all we have are opinions.Speak for yourself
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.So far, all we have are opinions.
Quote from: MarekCyzio on 04/15/2015 01:29 amRegarding Vine movie:https://vine.co/v/euEpIVegiIxThis 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.
Regarding Vine movie:https://vine.co/v/euEpIVegiIxThis is a book example of an uncontrolable, unstable system with long delay between measurements and controls.
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.
I have an aerospace engineering degree - the wind-induced drag on a large cylinder is significant but not as significant as the vertically-assymmetrical drag on the entire stage once those legs deploy.
To me, it looks like part of the problem is overcorrection.
@ID_AA_Carmack Looks like the issue was stiction in the biprop throttle valve, resulting in control system phase lag. Should be easy to fix.
Quote from: Space Ghost 1962 on 04/14/2015 11:53 pmOr on reentry. Possibly not enough control authority.How much scope is there to beef up the RCS / nitrogen thrusters to help keep the tipping stage balanced?Is it just a question of increasing the amount of available nitrogen gas? You would think that the RCS which is capable of flipping a stage for boostback would have enough control authority to balance against mere tipping.
Or on reentry. Possibly not enough control authority.
The RCS can do its job of flipping for boostback by thrusting in one direction for 10-30 seconds, coast for a minute (turning at a degree per second), then in the other direction for 10-30 seconds. In suborbital vacuum, it only has to modify orientation very, very slowly. In terminal landing, it has to work very, very rapidly, exerting lots and lots of thrust.A SuperDraco may or may not be overkill for this.
Quote from: Elon Musk@ID_AA_Carmack Looks like the issue was stiction in the biprop throttle valve, resulting in control system phase lag. Should be easy to fix.Source Tweet
