Only three engines have plumbing for TEA/TEB reignition, and all three receive a reignition pulse regardless of which ones are lit.
Quote from: sevenperforce on 05/11/2016 07:17 pmOnly three engines have plumbing for TEA/TEB reignition, and all three receive a reignition pulse regardless of which ones are lit. Was this ever confirmed? I remember some speculation on it, but I don't recall if there was ever a source for it. In the Orbcomm 2 landing video there is only one green flash during the landing burn startup. If the igniter was firing in three engines, I would think the green flash would be visible in the other 2 engines as well as the center one. https://www.youtube.com/watch?v=ANv5UfZsvZQ#t=2m35s
That was stated as fact on reddit by someone who seemed to know (u/EchoLogic). And I haven't looked again at the OG2 landing video, but maybe all 3 engines were on line-of-sight to the camera, so the viewer would see only one flash.As it happens, the OG2 stage is the one where we noticed the white TEA/TEB residue on the 2 outer engines and made the conjecture about how the TEA/TEB was plumbed. So it's virtually impossible that there *weren't* three flashes on OG2. Later I noticed what seemed to be authoritative confirmation of the plumbing configuration on reddit by EchoLogic. Hopefully he wasn't just repeating what he read here.
That all sounds great in theory, but does anyone know precisely what they'd be basing those mass figures on? To know how fast it's burning propellant, does it use mass flowmeters - or something else? If they use mighty-accurate mass flowmeters, can anyone hazard a guess at a make/model??
It's likely a carefully crafted sequence developed thru many simulations then just played back and run thru...There is likely code to adjust some things in real time... but mostly just to adjust the timeline playing out...Example... compares timeline expected radar altitude to actual reading...Jumps back or forward in timeline to make equal and continues to monitor...Example... I'm falling faster then I should be at this point on timeline...Applies small plus offset to thrust commands in timeline to attempt to compensate...Point is with a good timeline sequence laid out and then played out... it works...Just has enough wiggle room built in to adjust for the actual deck height before it gets there... The above is very simplified and just my opinion...
Can someone with insight comment on how SpaceX is actually measuring or calculating thrust in flight?
Quote from: envy887 on 05/13/2016 05:20 pmCan someone with insight comment on how SpaceX is actually measuring or calculating thrust in flight?Not claiming insight into SpaceX specifically, but other LV's I have worked on (Pegasus, Taurus, which admittedly are different beasts as solids) don't measure or calculate thrust per se. The IMU just measures vehicle acceleration, etc.For liquid vehicles, the only difference is throttleability, and the thrust characteristics of the engines at given throttle settings are determined in static test firings. Once you have the engine thrust vs. throttle setting curves established through testing, you can program the autopilot accordingly. It can then command a given throttle setting expecting a certain thrust level, and the result is a given acceleration sensed by the IMU. If fine control of acceleration (and therefore thrust) is required, eg in the case of F9 landing burn, the IMU can adjust throttle up or down as needed to obtain the desired acceleration, because of course the vehicle mass is always decreasing. So the IMU ends up "closing the loop" on desired acceleration by adjusting throttle accordingly. But it doesn't need to measure (or calculate) thrust per se, only acceleration.
My question about thrust was more about the data sent home than anything used real-time for control.Isn't engine thrust a metric examined in post mission analysis?
Quote from: envy887 on 05/14/2016 02:34 amMy question about thrust was more about the data sent home than anything used real-time for control.Isn't engine thrust a metric examined in post mission analysis?As I posted earlier, "thrust" can be determined by measuring acceleration of the vehicle in response to control inputs. During post-mission analysis, the mass of the stage at any given time can be determined to a reasonable degree of certainty by integrating along the flight profile. SpaceX knows the empty mass of the vehicle; they know the mass of the payload; the know the mass of propellants loaded; they know the mass flow-rate curves for the engine along the throttle points for the M1D engine. From those starting values, they know all they need to. EDIT: Yeah, what the_other_Doug said.
But the flight computer probably just flies the mission assuming it has enough propellant to land safely. And you wouldn't want it to know if it didn't.
(But not sure if single engine landings will ever be planned again.)
Indeed. There is some value to knowing those things to some extent, but a control loop can be pretty dumb and still work just fine getting you to where you need to go.
Quote from: Robotbeat on 05/14/2016 01:41 amIndeed. There is some value to knowing those things to some extent, but a control loop can be pretty dumb and still work just fine getting you to where you need to go.I think the question wasn't so much about staying on track, but in cases where fuel reserves are at a bare minimum, knowing whether or not there will be enough fuel remaining to complete the landing.