sasbrb@sas.UUCP (Brendan Bailey) (10/26/88)
I remember watching the first shuttle launch on television and the
announcers kept talking about when the shuttle took off, it would
do something they called "walking" which took place when the
main engines ignited. I remember them showing a side view of the
launch close-up and showing the boosters actually move several feet
from their original position to look as if it was "walking". Maybe
it wasn't when the main engines ignited, but the boosters ignited.
Since that launch I don't think I've heard anything ever mentioned
about this. Has anyone heard of this?
Bren Bailey
rogers@orion.SRC.Honeywell.COM (Brynn Rogers) (10/27/88)
In Ben Baily's artical he commented on walking. I have heard it called Twang. It happens on every launch when the MAIN engines are ignited. since the main engines are not on the centerline of the spacecraft they produce offset thrust. When they are ignited the whole STS twists (in relation to the pad, I think the orbiter is pretty ridged) in its mounting and springs back to close to its original position before SRB ignition, which balances out the thrust so it is centerline and not offset. The top of the ET moves something like 1 or 2 feet (a guess) during this period. The Clamps holding the SRBs down apparently absorb this torque. (another guess) Brynn Rogers rogers@src.honeywell.com
dsmith@hplabsb.HP.COM (David Smith) (10/28/88)
In article <665@sas.UUCP> sasbrb@sas.UUCP (Brendan Bailey) writes: >do something they called "walking" which took place when the >main engines ignited... the boosters actually move several feet >from their original position to look as if it was "walking". Maybe >it wasn't when the main engines ignited, but the boosters ignited. When the boosters ignited, the shuttle lifted off and became free to "walk" under the side force of the liquid engines. If you review a tape of the STS-26 liftoff, you can see the lateral movement. I seem to recall talk after STS-1 on the order of "Now that we know how much it walks, we can compensate for it in the future." But I don't know why that aspect of the flight should have been an unknown to the engineers, nor do I know how it is compensated. It doesn't appear to tip back any. But STS-26 walked less than my fuzzy recall says STS-1 did. -- David Smith HP Labs dsmith@hplabs.hp.com
craig@think.COM (Craig Stanfill) (10/28/88)
In article <4964@hplabsb.HP.COM> dsmith@hplabsb.UUCP (David Smith) writes: >In article <665@sas.UUCP> sasbrb@sas.UUCP (Brendan Bailey) writes: >>do something they called "walking" which took place when the >>main engines ignited >When the boosters ignited, the shuttle lifted off and became free to >"walk" under the side force of the liquid engines. If you review a >tape of the STS-26 liftoff, you can see the lateral movement. >I seem to recall talk after STS-1 on the order of "Now that we know >how much it walks, we can compensate for it in the future." But I don't >know why that aspect of the flight should have been an unknown to >the engineers, nor do I know how it is compensated. I'm guessing on this, but the dynamics of the shuttle-EFT-SRB system when the solids ignite and the holddowns release are probably complex, and could not be exactly determined without a launch. Remember that the spacecraft is an elastic system, and at the moment of launch, between SRB ignition and holddown release, the forces acting on the system and the point at which they are acting suddenly change by several million pounds. The result will be short-lived system oscilation. So what you've got is a system with 3 SME'e and two SRB's flexing around. The result is that the axis of thrust is going to change, and the space craft is going to ``walk''. The crux of the problem: how much and how fast? If you know the dynamics of the system, you can use active control methods (gimboling the engines) to compensate. I'm sure the engineers had a pretty good idea of system dynamics before STS-1 based on computer simulation, but these simulations are never exactly right. If your simulation is off by, say, .1%, you're going to walk a little between release and the time you can gimbol the engines to stop the walk. The only way to be exactly right is to light the candle and find out. Then you know how much to compensate for the walk before it happens. An additional source of walk could be interference between the SRB plume and the launch pad, but I'm less sure of this; most of the plume is deflected sideways by the launch pad, and the space craft doesn't care what happens to the plume after it leaves the nozzle. However, if SRB ignition changes atmospheric pressure at the pad noticably, it would affect the thrust of the various engines. You might also get aerodynamic forces acting on the wings as air displaced by the SRB plume rushes forward. Again, you don't know accurately enough to compensate until you launch it. A final possible source of walk could be if you don't know the thrust profile of the SRB. I suppose it's possible that there is a differnce in the thrust profile between a horizontal SRB at a test facility and a vertical SRB on a launch pad, but this seems unlikely. By the way, to the extent that these effects vary from launch to launch, every launch will see some walk. If you change the distribution of weight in the cargo bay, you change the system dynamics, and you walk a little more or less. Each SRB probably has a subtly different burn rate at ignition, and again this changes the system dynamics and you walk a little. As I say, I'm guessing on this, but it seems reasonable based on first principles. My guess is that uncertainty as to system dynamics is the major cause of the large walk on STS-1, but I wouldn't completely rule out the other causes.
karn@ka9q.bellcore.com (Phil Karn) (10/28/88)
The phenomenon you call "walking" is easily seen on any shuttle launch when the westward-looking camera is used (the one that shows the wings edge-on). The northward (rightward, as seen from this camera angle) "walk" is caused by the shuttle's center of mass not being aligned directly above its thrust vector for the first few seconds of flight. This is a designed-in phenomenon, though I'm not sure why. You will note water "rainbirds" on the top of the launcher platform; these cool the platform when the shuttle's plumes hit it during its northward walk. Perhaps the walk was intentional, to spread out the impact of the plumes during launch, lessening damage to any one part of the pad. Phil
apratt@atari.UUCP (Allan Pratt) (10/28/88)
Somebody I know who had his master's thesis go up on the Shuttle said that it goes like this: At T-5, the main engines start, and their thrust rocks the whole assembly (orbiter + ET + SRBs) forward some distance. As the thrust settles down, the orbiter rocks backward again, and as it passes through the vertical (at T=0), the SRBs ignite and off you go. Is this what you mean? ============================================ Opinions expressed above do not necessarily -- Allan Pratt, Atari Corp. reflect those of Atari Corp. or anyone else. ...ames!atari!apratt
knudsen@ihlpl.ATT.COM (Knudsen) (10/29/88)
Don't forget that a certain amount of "walk" is deliberate. Because the SSMEs are off-center, and angled, it is necessary for the whole stack to clear the tower a little off-vertical, so the SSMEs are more nearly under the center of gravity of the whole thing. Otherwise the SSMEs would pitch the stack right over. So on liftoff the shuttle has to walk under the tank a little bit. I predict that if you check the tapes carefully you will see that the top of the stack (ET) moves sideways less than the bottom, so the whole thing goes up a little tipsy. I've heard that the SRBs are ignited just as the "twang" effect is swinging back to the neutral point. This rotational momentum is in the right direction to help the shuttle walk its way under the ET. Thanks to Phil Karn for explaining the above to me years ago; apologies to him if I screwed it up here. -- Mike Knudsen Bell Labs(AT&T) att!ihlpl!knudsen "Lawyers are like nuclear bombs and PClones. Nobody likes them, but the other guy's got one, so I better get one too."