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#380 by Antares on 03 Oct, 2013 06:24
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Attached video is a cutout of the coverage and show the leak after SECO.
Yeah, I've been wondering about that too. The fluid coming out looks like it's coming off the engine, and from several different azimuths. -
#381 by ChrisWilson68 on 03 Oct, 2013 08:03
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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.
Me:
What about if they put RCS thrusters at the end of the legs. Wouldn't that provide a lot more roll control authority for the same amount of prop?
cheers, Martin
Yes, but with downsides that would likely far outweigh the benefits.
One big downside is having to pipe propellant through the legs to the thrusters. The legs fold out, so now you need some kind of flexible piping of propellant. That's a lot of additional weight and complexity, and more things that can go wrong.
Another downside is that the ends of the legs smash into concrete at the end of the flight. They're legs, after all. The thruster probably won't be very happy being pushed against the ground.
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#382 by Jakusb on 03 Oct, 2013 08:40
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What about some simple 'flaps' or wing tips on the legs. If roll is consistent, countering should be relatively easy, right? Probably not even required to make them adjustable.
Ps: not a rocket scientist, not even close, so using educated guessing... Feel free to explain how this would not work.
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#383 by guckyfan on 03 Oct, 2013 08:52
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What about some simple 'flaps' or wing tips on the legs. If roll is consistent, countering should be relatively easy, right? Probably not even required to make them adjustable.
Ps: not a rocket scientist, not even close, so using educated guessing... Feel free to explain how this would not work.
I think not even that is necessary. Just a slight unsymmetry of the legs could cause some roll moment countering the unwanted roll. -
#384 by ChrisWilson68 on 03 Oct, 2013 08:53
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What about some simple 'flaps' or wing tips on the legs. If roll is consistent, countering should be relatively easy, right? Probably not even required to make them adjustable.
Ps: not a rocket scientist, not even close, so using educated guessing... Feel free to explain how this would not work. :)
Sure, control surfaces on the legs are possible, and more practical than putting thrusters there. But it still adds complication (because you need to move those control surfaces, and they're out on the end of a moving leg), and it adds drag on ascent.
Sometimes, the simplest solution is best, even if it's boring. Adding more propellant for the existing thrusters is a simple solution that requires little work and is very likely to be successful.
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#385 by ChrisWilson68 on 03 Oct, 2013 08:55
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What about some simple 'flaps' or wing tips on the legs. If roll is consistent, countering should be relatively easy, right? Probably not even required to make them adjustable.
Ps: not a rocket scientist, not even close, so using educated guessing... Feel free to explain how this would not work. :)
I think not even that is necessary. Just a slight unsymmetry of the legs could cause some roll moment countering the unwanted roll.
Are you talking about a permanent asymmetry or something you can dynamically modify? A permanent asymmetry is unlikely to be helpful because the roll forces are likely variable. And dynamically modifying the asymmetry means it's a control surface.
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#386 by guckyfan on 03 Oct, 2013 09:07
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Are you talking about a permanent asymmetry or something you can dynamically modify? A permanent asymmetry is unlikely to be helpful because the roll forces are likely variable. And dynamically modifying the asymmetry means it's a control surface.
I am talking permanent asymmetry just like flaps, nothing variable. And while it is true that the roll forces are variable they go with the wind speed and so will be the roll induced by asymmetry of the legs. Any residual roll will be low enough to be countered by the RCS hopefully. -
#387 by ChrisWilson68 on 03 Oct, 2013 09:09
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Are you talking about a permanent asymmetry or something you can dynamically modify? A permanent asymmetry is unlikely to be helpful because the roll forces are likely variable. And dynamically modifying the asymmetry means it's a control surface.
I am talking permanent asymmetry just like flaps, nothing variable. And while it is true that the roll forces are variable they go with the wind speed and so will be the roll induced by asymmetry of the legs. Any residual roll will be low enough to be countered by the RCS hopefully.
What you're describing is more like a rudder trim tab than flaps. Anyway, it depends on how predictable the roll forces are. If they are very predictable, there are lots of easy ways to compensate for them.
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#388 by gospacex on 03 Oct, 2013 09:22
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What about some simple 'flaps' or wing tips on the legs. If roll is consistent, countering should be relatively easy, right? Probably not even required to make them adjustable.
What about small fins at/near the top of the stage? It can even *save* weight since actuated fins don't need to spend propellant to affect the roll, unlike thrusters. -
#389 by cambrianera on 03 Oct, 2013 09:56
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What about some simple 'flaps' or wing tips on the legs. If roll is consistent, countering should be relatively easy, right? Probably not even required to make them adjustable.
What about small fins at/near the top of the stage? It can even *save* weight since actuated fins don't need to spend propellant to affect the roll, unlike thrusters.
Small fins at the bottom of the stage.
http://forum.nasaspaceflight.com/index.php?topic=32859.msg1103830#msg1103830
http://forum.nasaspaceflight.com/index.php?topic=32859.msg1103859#msg1103859 -
#390 by Jakusb on 03 Oct, 2013 10:27
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What about some simple 'flaps' or wing tips on the legs. If roll is consistent, countering should be relatively easy, right? Probably not even required to make them adjustable.
What about small fins at/near the top of the stage? It can even *save* weight since actuated fins don't need to spend propellant to affect the roll, unlike thrusters.
The whole idea of something on the legs is: it only effects the decent and only after leg deployment. Anything permanent on stage will effect ascent too. -
#391 by Ben the Space Brit on 03 Oct, 2013 12:25
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Credit to WetMelon on the SES general discussion thread.
Quote"The second-stage engine initiated ignition, got up to about 400 psi, encountered a condition it didn't like and initiated an abort," SpaceX spokeswoman Emily Shanklin said Oct. 2. "We need more time to review the data before we come to any sort of definitive conclusion, but we are confident we will be able to address it before the next flight," she said, adding that the abort "wasn't anything fundamental" and that a planned late-October launch of the SES-8 satellite for Luxembourg fleet operator SES "is still on track to launch in about a month."
http://www.aviationweek.com/Blogs.aspx?plckBlogId=Blog:04ce340e-4b63-4d23-9695-d49ab661f385&plckPostId=Blog:04ce340e-4b63-4d23-9695-d49ab661f385Post:59d72274-2bb3-4d33-aa94-fd879fe649b2
I wonder what the 'condition' was that it didn't like? Impeller bearing temperature? Injector pressure?
Musk says that he thinks it was something to do with the engine being on an 'extended spin' start, which I presume means that a long wait with the impellers spun up but the fuel/oxidiser valves into the combustion chamber closed. -
#392 by Zed_Noir on 03 Oct, 2013 22:58
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What about some simple 'flaps' or wing tips on the legs. If roll is consistent, countering should be relatively easy, right? Probably not even required to make them adjustable.
What about small fins at/near the top of the stage? It can even *save* weight since actuated fins don't need to spend propellant to affect the roll, unlike thrusters.
Small fins at the bottom of the stage.
http://forum.nasaspaceflight.com/index.php?topic=32859.msg1103830#msg1103830
http://forum.nasaspaceflight.com/index.php?topic=32859.msg1103859#msg1103859
Think it's better to have pop-out vanes on the trailing part of the core. There is room to mount the vanes internally in the interstage. Fins position near the bottom or leading part of the core will probably be external structures that is able to withstanding the exterior environment during launch. There isn't much room internally to mounted the fins/pop-out vanes there I think.
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#393 by LouScheffer on 04 Oct, 2013 01:04
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1) Propellants well settled. Single engine starts. Heavy rotor spinning several thousand RPM in one direction.
This does not seem plausible from the physics. Spin is imparted to the rocket only during turbine spin up. When the rotor reaches constant speed, the acceleration ceases, as the rotor torques and viscous and other drag cancel - otherwise the rotor would speed up or slow down. At this point the entire rocket is rotating in the opposite sense at a constant angular rate.
2) All ACS gas gone to overcome rolling torque due to viscous and rolling friction forces inside the single-shaft turbopump transmitting roll torque to the housing and therefore the entire vehicle.
3) Roll moment imparted to the vehicle draws propellant out of the sumps, up the walls. Engine starved.
I'm not getting paid enough to explain this.
Now the rotor masses several kg, but the stage masses several 1000s of kg. Furthermore most of the weight of the tanks are far from the axis, several meters as opposed to several cm for the rotor. That's a factor of 100, more or less. Furthermore, the moment of inertia goes like the distance from the axis squared. So the rocket will indeed rotate, but the rate will be something more than a million times slower than the turbine. So if the turbine goes 25,000 rpm, the rocket will get something less than 0.025 rpm. So whatever caused this problem, it's not angular momentum transfer from the turbine. -
#394 by Jim on 04 Oct, 2013 01:44
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This does not seem plausible from the physics. Spin is imparted to the rocket only during turbine spin up>
So a helicopter only needs the tailrotor during engine start up? -
#395 by LouScheffer on 04 Oct, 2013 02:01
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In steady state operation, the helicopter rotor torque is balanced by the torque imparted by the external air. In the rocket turbopump case, the corresponding drag from the pump rotor and impeller are against the vehicle itself. That's why the helicopter keep accelerating, but the rocket only acquires a constant rate dictated by conservation of angular momentum. The helicopter, in a vacuum chamber, would pick up a constant angular rate just like the rocket does. But in air, it will first acquire the same rate, then keep accelerating, unless the tail rotor stops it.
This does not seem plausible from the physics. Spin is imparted to the rocket only during turbine spin up>
So a helicopter only needs the tailrotor during engine start up? -
#396 by jcm on 04 Oct, 2013 02:11
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Not sure if this belongs here or in update thread.
It is now fairly clear that 2013-055B, object 39266 is CASSIOPE.
Of the remaining 19 objects, 11 of them were last tracked around Sep 30; for the past three days only 8 of them
have had new TLEs. Perhaps this is an issue of visibility to sensors, but it will be interesting to see if it holds up.
Of the 8 still being tracked, D is a slow decayer and may be the rocket. H, U, L, R and T are decaying rapidly.
M and N are slower but perhaps because they started in higher orbits.
I looked at the orbits and tried to calculate the closest approach point of the first TLE set of each object to CASSIOPE.
For each object I give the time of closest approach, the close approach distance DR in km, the relative velocity DV in m/s,
the difference in perigee and apogee height (km) from CASSIOPE. The delta-V's are surprisingly high, and
I'd encourage others with more sophisticated orbital analysis software to check my results. The closest approach times are consistent with the approximate expected time of the second stage restart, and a little before the observed venting phase.
Objects with recent TLEs
Time DR DV DP, DA
D 1640 28 97 +79 -57
H 1623 27 61 -49 -70
U 1638 6 63 -1 -81
L 1638 4 79 -1 -110
M 1645 45 91 +120 +98
N 1645 10 85 +83 +106
R 1652 10 118 +9 +78
T 1646 3 74 +66 +83
'Lost' objects
Time DR DV DP, DA
A 1634 21 81 -63 -150
C 1654 17 3 -48 -18
E 1644 13 12 -60 -29
F 1634 32 61 -90 -29
G 1643 78 162 +86 -69
J 1638 11 93 -32 -100
K 1636 15 47 -48 -86
V 1638 29 70 +43 +108
P 1638 30 64 +45 +105
Q 1645 14 78 +47 +94
S 1634 6 40 +67 +100 -
#397 by Jim on 04 Oct, 2013 02:20
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In steady state operation, the helicopter rotor torque is balanced by the torque imparted by the external air.
Wrong. You got your physics all wrong. In steady state operation, a helicopter that loses its tail rotor starts spinning in the opposite direction of the main rotor. If left spinning, the helicopter will reach a RPM dictated by the torque caused by the rotor transmission friction and air drag on the spinning fuselage, which offsets the drag on the main rotor blades. -
#398 by Helodriver on 04 Oct, 2013 02:46
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Jim has his helicopter physics correct. Source: first hand knowledge. Torque is always present so long as the rotor is being powered. I would expect the impeller and turbines in a rocket engine would also impart torque so long as they were being driven by combustion gasses.
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#399 by LouScheffer on 04 Oct, 2013 02:50
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I agree the helicopter spins the opposite way. I was speaking of the torque as seen by the rotor. The helicopter is exerting torque on the rotor, trying to speed it up. The air is exerting torque trying to slow it down. So in steady state the rotor's torques are balanced. From the helicopter's point of view, it sees the torque it is exerting to spin the rotor, which tries to accelerate the body of the helicopter in the opposite direction.In steady state operation, the helicopter rotor torque is balanced by the torque imparted by the external air.
Wrong. In steady state operation, a helicopter that loses its tail rotor starts spinning in the opposite direction of the main rotor.
