Quote@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
@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.
So the valves have been nowhere near as extensively tested in the regime of small, fast, and frequent changes.
Quote from: LouScheffer on 04/15/2015 11:34 amSo the valves have been nowhere near as extensively tested in the regime of small, fast, and frequent changes.It could have been extensively tested in McGregor, on an engine test stand.
Its not uncommon to see 30km/h wind shear from 1000ft to sea level. Given how fast the stage descents, the issue might be how quickly the stage can sense the wind. We routinely see 50km/h wind shear in my home town (southerly 10 kts @ 1000ft, northeast 20kts @ 50ft).A means to upload wind estimates for sea level, 500ft, 1000ft, 1500ft to the stage right before launch might be very helpful, if the rocket could incorporate wind estimates into its lateral profile.The rocket seemed to be compensating very aggressively towards dead center, perhaps its taking a wee bit too long to fully sense the wind.Windy conditions are common both at sea and at the coast. This would be an enhancement that might pay off in land and sea scenarios (although sea typically have stronger winds).Learn from aviation, every decent airport transmit fresh wind data to landing craft (ATIS, tower frequency, wind socks visually read).But it could be that this is just a natural fine tuning as a consequence of prioritizing landing hard as close as possible to bulleye vs landing softly at the edge of the barge, and now that they accomplished it, they can balance the two goals.Coming down a little slower would help if possible, but with T/W > 1 they might already be at the edge of how early they can slowdown.Wind shear/gusts probably a much bigger problem than total wind strength @ sea level.
While carriers do have systems that transmit landing data to aircraft for better precision landing, it's pretty much positional information for the aircraft to get on the right glide slope. I think a returning F9 stage would be in a similar position -- the wind data you'd send up is not terribly useful until the point where it's almost too late. Better to tune the system response including throttle response until it's quick enough to stick the landing based on information it has onboard the rocket.
Quote from: Ohsin on 04/15/2015 02:30 amQuote@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[...]I really appreciate this transparent failure analysis.
Testing on actual flights is the best way to go. There was considerable cross wind, aside from sticky valve weather variation is an issue that will need to be addressed.This landing attempt was close enough to good that if they modify the surface with some kind of numerous rapidly deployed grappling mechanisms to secure legs and pull rocket into deck securely they could have less precise landings become successes.I envision a landing pad at sea or on land with numerous such mechanisms to secure the base of the rocket as its landing. Otherwise the rocket is too tall, and the legs with to small a diameter to assure landing in a majority of landing weather conditions, even with the valve issue solved. I hope SpaceX starts working at securing the rocket more rapidly immediately to ensure both an initial successes and a higher rate of subsequent successes.
It would be nice if the Falcon9 had switchable power profiles for just the center engine. Maybe switch over to a different fuel plumbing circuit prior to landing that would produce less power and a better possibility of hovering.
We have a system, where the engine starts at e.g. 40m/s (downwards) and has to decelerate to e.g. 2m/s. Somewhere (e.g. 4m/s) is the limit of the system (landing legs able to compensate impact). The system must never come close to 0m/s as there is no way out (with a acceleration upwards the only thing to do is to switch of the engine). So effectively the final velocity setpoint is never 0 but something between 0 and the crash velocity. Taking away margin.
There are really a lot of constraints imposed on the system by not beeing able to hover. So many things have to work perfectly towards the final moments.
I beleive SX will get there but it's been interesting to see how something that is conceptually simple, and which should be pretty well simulated has taken 4 landings and still not resulted in a complete stage ready for reuse or analysis. Baysian statistics would have to penalize your chances of #5 being successful.
Quote from: Remes on 04/15/2015 12:24 pmThere are really a lot of constraints imposed on the system by not beeing able to hover. So many things have to work perfectly towards the final moments.True. And all due to a valve failing to open when commanded to?It seems the current Merlin 1d can't do deep enough throttling to cover (which surprised me).Perhaps a Merlin 1e will be needed?
finding the barge,
I'm still open to bet on this... 5 years from now, if a SpaceX barge is in service, it will be able to broadcast live video. Want to take that bet?
Looking at the video (https://vine.co/v/euEpIVegiIx) I'm struck by the dramatic attitude changes. [...]
Quote from: Lars-J on 04/14/2015 11:58 pmI'm still open to bet on this... 5 years from now, if a SpaceX barge is in service, it will be able to broadcast live video. Want to take that bet?I'll take that bet. The curvature of the earth isn't going to change, and if the barge is still in service landings will be taking place *further* from ground stations, not closer. So you're saying that in five years high-bandwidth video from the middle of the ocean will be stupid cheap. Presumably because of SpaceX's new constellation?It will happen eventually, but not within five years.We might eventually get access to the same very slow frame rate video which SpaceX is watching in the control room, but I'm assuming that's not what you're taking about.