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#260
by
Avron
on 30 Sep, 2013 00:34
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Now this is the video I want to see... Jeff Foust @jeff_foust 4h
Musk: did recover video of the first stage reentry, hope to post it online later this week.
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#261
by
ChrisWilson68
on 30 Sep, 2013 00:44
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The legs are going to be aerosurfaces.
Yes, but I'm not sure we know whether they'll have any roll control capability. I think we only know they'll be used to slow the stage down. Or is there some information from SpaceX that says more than that?
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#262
by
QuantumG
on 30 Sep, 2013 00:46
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Basic physics says more than that.
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#263
by
meekGee
on 30 Sep, 2013 00:58
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The legs can be used as a steam-engine style rotary governor.
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#264
by
ChrisWilson68
on 30 Sep, 2013 01:07
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The legs can be used as a steam-engine style rotary governor. :)
Sure, but that won't entirely eliminate the roll. The angular momentum will be the same, it's just the angular speed will be smaller with the legs deployed. How much depends on the weight distribution of the legs versus the weight distribution of the rest of the stage relative to the axis of the stage. The legs supposedly mass less than a Tesla Model S, so I'm not sure if that alone is enough to keep enough prop at the bottom of the tanks.
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#265
by
Nilof
on 30 Sep, 2013 01:25
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I guess you could fix this by having legs that are bilaterally symmetric but not rotationally symmetric. Then you could modulate roll with them by extending/retracting them.
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#266
by
ChrisWilson68
on 30 Sep, 2013 01:31
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I guess you could fix this by having legs that are bilaterally symmetric but not rotationally symmetric. Then you could modulate roll with them by extending/retracting them.
It's not clear that SpaceX is designing in an extension/retraction mechanism. There's speculation that the system is one-shot -- open a valve and helium pressure pushes the legs into landing position. That would be a lot simpler, which would both save mass and reduce the odds of something going wrong.
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#267
by
Avron
on 30 Sep, 2013 01:32
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I guess you could fix this by having legs that are bilaterally symmetric but not rotationally symmetric. Then you could modulate roll with them by extending/retracting them.
Have not seen anything on retract yet from Musk et al
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#268
by
Nilof
on 30 Sep, 2013 01:42
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Well, what about using legs which generate lift when spinning? Worst case scenario, the rocket still spins, but you get positive Gs which prevent the fuel from sticking to the sides.
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#269
by
ChrisWilson68
on 30 Sep, 2013 01:47
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Well, what about using legs which generate lift when spinning? Worst case scenario, the rocket still spins, but you get positive Gs which prevent the fuel from sticking to the sides.
Positive Gs don't directly cancel out centrifugal Gs. The vectors add. To get the prop down, you might need a lot of Gs. Remember, the stage was at terminal velocity, so you already had 1 G down and that wasn't enough.
To get significant Gs from spinning airfoils, you would need huge airfoils spinning very fast. Think how fast the rotar of a helicopter spins, and how big it is compared to the helicopter.
So, interesting idea, but, unfortunately it's not practical.
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#270
by
meekGee
on 30 Sep, 2013 02:04
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The idea of using the legs as a governor was facetious, for the record.
It's not clear from Elon's comment what caused the spin.
My money is on aerodynamic forces, but those would be present from earlier on. Perhaps they indeed ran out of ACS gas. Maybe the forces became larger in lower altitudes.
(Contrary to previous posts, a symmetrical body would easily spin under a flow, especially if there's a small angle of attack, wind, etc. Since the rocket was far from a clean cylinder, then even more so)
I find the engine-related explanation a bit less likely. Since the braking burn is brief, it seems to me that the stage will have very little time to spin up. but maybe.
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#271
by
Lars_J
on 30 Sep, 2013 02:26
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Maybe I'm just "seeing things", but I got the impression that the v1.1 flame exhaust is significantly tighter (& longer) than the v1.0 flame exhaust - see attached comparison image. This would indicate the improved efficiency, right?
On the other hand the difference could be an optical illusion caused different light conditions, camera exposures, and atmospheric conditions.
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#272
by
sdsds
on 30 Sep, 2013 02:29
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Maybe I'm just "seeing things", but I got the impression that the v1.1 flame exhaust is significantly tighter (& longer) than the v1.0 flame exhaust - see attached comparison image. This would indicate the improved efficiency, right?
On the other hand the difference could be an optical illusion caused different light conditions, camera exposures, and atmospheric conditions.
Have SpaceX released Oxidizer/Fuel ratios for 1C and 1D? Maybe there's more unburnt fuel in the 1D exhaust plume, so it burns outside the engine for more time and thus appears longer in the sky?
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#273
by
llanitedave
on 30 Sep, 2013 02:41
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I wonder if they're taken at different altitudes, so the plume expansion is at different stages.
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#274
by
Hooperball
on 30 Sep, 2013 02:45
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I'm pretty sure the 1d's have a higher mass flow rate hence the new turbo pumps and higher chamber pressures of 1410 psi up from the 1c's 982 psi.
Higher mass flow rate through a smaller area (core without cowlings) and higher exhaust velocity (increased Isp) = longer flames?
Altitude and barometric pressure would also play a big role in plume shape.
Nice comparison pic!
S
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#275
by
Confusador
on 30 Sep, 2013 03:11
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By my count, 4 of the 5 attempted relights on this flight were successful. Who wants to speculate, with the limited information we have, what the difference is with the the M1Dvac that may have caused the issue?
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#276
by
sdsds
on 30 Sep, 2013 03:19
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For DANDE the planned insertion orbit was 300 x 1500:
semi-major axis: 7271 km
period: 6,171 sec
Ed Kyle derived the actual orbit is 328 x 1493 (from the DANDE-provided TLE):
semi-major axis: 7281.5 km
period: 6,184 sec
ratio of orbital periods: 1.002
So they put almost exactly the right amount of oomph into that payload, giving them a "better" orbit by 0.2%. For most payloads, though, the measure of better they want might be expected time it will remain in orbit before decay. I speculate the 28 km of added height at perigee buys them a lot. The attached atmoshpere info (from Vallado) seems to imply that....
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#277
by
Prober
on 30 Sep, 2013 03:38
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well, the Kerbel solution is fins. Aerodynamic control surfaces.
Too inefficient. Grid fins would be better. Better control, less mass.
Would also contribute to deceleration.
Soyuz uses these for the abort system
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#278
by
mlindner
on 30 Sep, 2013 03:42
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Some insider info:
The attitude control system was railed, wanted more thrust, but engines couldn't provide anymore. Doubts that they would solve it by stockpiling more RCS fuel.
Me:
Apparently there was so much aero roll that the RCS couldn't dampen it away. They probably just need to make some changes to the outside aero surface to fix things. I doubt they'll add pop out fins though.
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#279
by
edkyle99
on 30 Sep, 2013 03:59
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For DANDE the planned insertion orbit was 300 x 1500:
semi-major axis: 7271 km
period: 6,171 sec
Ed Kyle derived the actual orbit is 328 x 1493 (from the DANDE-provided TLE):
semi-major axis: 7281.5 km
period: 6,184 sec
ratio of orbital periods: 1.002
So they put almost exactly the right amount of oomph into that payload, giving them a "better" orbit by 0.2%. For most payloads, though, the measure of better they want might be expected time it will remain in orbit before decay. I speculate the 28 km of added height at perigee buys them a lot. The attached atmoshpere info (from Vallado) seems to imply that....
I found a planned DANDE orbit somewhere in the DANDE site that listed a planned 324 km perigee, so achieved is close. I'm still wondering about the inclination. It seems to have been aimed at 81 deg though prelaunch information suggested 80 deg.
- Ed Kyle
