Quote from: jaufgang on 11/19/2014 09:32 pmSeems strange to me that they're first using these grid fins on the same launch where they've got everyone watching to see if they can accurately target a landing barge. The only flight data they have on the performance of those fins are from low altitude and low velocity manoeuvring on the F9R-Dev. They have very little data on how they will perform during a high velocity re-entry. Just playing devil's advocate here, but what makes you think they'll be used at all during re-entry? I suspect they'll stay safely stowed flat until terminal velocity, or at most used with very, very, small deflections until very late in the landing sequence.
Seems strange to me that they're first using these grid fins on the same launch where they've got everyone watching to see if they can accurately target a landing barge. The only flight data they have on the performance of those fins are from low altitude and low velocity manoeuvring on the F9R-Dev. They have very little data on how they will perform during a high velocity re-entry.
Quote from: Herb Schaltegger on 11/19/2014 10:43 pmJust playing devil's advocate here, but what makes you think they'll be used at all during re-entry? I suspect they'll stay safely stowed flat until terminal velocity, or at most used with very, very, small deflections until very late in the landing sequence.What makes me think that is that they're grid fins. IANAAE, but grid fins seem to be a special-use device for steady supersonic and high subsonic control in a compact fold-out profile. If they were targetting the low subsonic, I would expect them to use planar fins.
Just playing devil's advocate here, but what makes you think they'll be used at all during re-entry? I suspect they'll stay safely stowed flat until terminal velocity, or at most used with very, very, small deflections until very late in the landing sequence.
Grid fins perform very well at subsonic and supersonic speeds, but poorly at transonic speeds
Grid fins will save on RCS propellent, and provide better control at lower speed before powered landing.
Seems strange to me that they're first using these grid fins on the same launch where they've got everyone watching to see if they can accurately target a landing barge. The only flight data they have on the performance of those fins are from low altitude and low velocity manoeuvring on the F9R-Dev. They have very little data on how they will perform during a high velocity re-entry. I would have expected them to first try them out and collect characterization data during a high altitude/velocity test of the F9R-Dev, or perhaps on one of their earlier hail-Mary soft splashdowns in the big wide ocean. They could have performed a series of manoeuvres on the fins during various phases of the descent without caring if it sent the trajectory just a little bit off course, and used the collected data to refine the control system.I suppose they're just really confident in their computer modelling.
"You can just do an easy calculation of what's the terminal velocity and then how long you have to fire the engine, at what g-level, to get to zero velocity. If you then do some interesting things, like look at our landing gear, they're essentially like giant body flaps, so the drag - when we deploy the landing gear, the drag massively increases, so we have dual use of the landing gear as giant body flaps and as landing gear. That actually cuts the terminal velocity in half and therefore the fuel - the propellant we need to stop the vehicle in half, and actually it's quite an efficient method of landing precisely." - Elon at the MIT Aero/Astro Con 2014 via Sh!tElonSaysMaybe with the grid fins they'll be able to pop out the landing gear much earlier, not just before touch down like they've done with the previous water landings.
Quote from: Aerospace Dilettante on 11/20/2014 06:26 pm"You can just do an easy calculation of what's the terminal velocity and then how long you have to fire the engine, at what g-level, to get to zero velocity. If you then do some interesting things, like look at our landing gear, they're essentially like giant body flaps, so the drag - when we deploy the landing gear, the drag massively increases, so we have dual use of the landing gear as giant body flaps and as landing gear. That actually cuts the terminal velocity in half and therefore the fuel - the propellant we need to stop the vehicle in half, and actually it's quite an efficient method of landing precisely." - Elon at the MIT Aero/Astro Con 2014 via Sh!tElonSaysMaybe with the grid fins they'll be able to pop out the landing gear much earlier, not just before touch down like they've done with the previous water landings.Actually cutting velocity in half reduces kinectic energy by 75%. So it might cut fuel for the last burn by around 75% too.
Quote from: Herb Schaltegger on 11/19/2014 10:43 pmQuote from: jaufgang on 11/19/2014 09:32 pmSeems strange to me that they're first using these grid fins on the same launch where they've got everyone watching to see if they can accurately target a landing barge. The only flight data they have on the performance of those fins are from low altitude and low velocity manoeuvring on the F9R-Dev. They have very little data on how they will perform during a high velocity re-entry. Just playing devil's advocate here, but what makes you think they'll be used at all during re-entry? I suspect they'll stay safely stowed flat until terminal velocity, or at most used with very, very, small deflections until very late in the landing sequence.What makes me think that is that they're grid fins. IANAAE, but grid fins seem to be a special-use device for steady supersonic and high subsonic control in a compact fold-out profile. If they were targetting the low subsonic, I would expect them to use planar fins.Can we get an aerospace engineer to chime in?
I would be concerned about the deployment of grid fins happening at high speed in lower atmosphere, as the aerodynamic forces would tend to make them snap open violently and possibly break off. This leaves the opening either after slowed to terminal velocity or very high up where the air is very thin. I think they will choose to open very high up, about the time of the initial braking burn. This way they will have the added drag to aid slowing down as well as control during the entire decent.
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