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Retro-fire Attitude
Why is it that spacecraft tend to adopt nose-down attitudes for
de-orbit burns? If the acceleration vector were instead precisely anti-parallel to the velocity vector, then 1) a smaller impulse would suffice to reduce the pericenter by a given amount, and 2) the re-entry trajectory would be shallower, which should give lower g-loads and perhaps lower heating loads as well. If one wanted to de-orbit quickly, an inwardly directed acceleration vector would make sense. But for a typical civilian mission, there's no big hurry. A slightly inwardly directed acceleration might make sense just to be sure that control errors do not accidentally result in an outwardly oriented acceleration, but it seems to me that this does not explain 30 or 40 degrees of nose-down pitch. |
#2
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Retro-fire Attitude
"Proponent" wrote in message
m... Why is it that spacecraft tend to adopt nose-down attitudes for de-orbit burns? If the acceleration vector were instead precisely anti-parallel to the velocity vector, then 1) a smaller impulse would suffice to reduce the pericenter by a given amount, and 2) the re-entry trajectory would be shallower, which should give lower g-loads and perhaps lower heating loads as well. I assume you're talking about the Shuttle. This attitude puts the thrust vector anti-parallel. Remember, the thrust goes through the center of mass, not along the axis of the Orbiter. The trajectory won't be shallower. If one wanted to de-orbit quickly, an inwardly directed acceleration vector would make sense. But for a typical civilian mission, there's no big hurry. Nope. Any thrust away from the velocity vector is mostly wasted. -- James Summers IBM-ret, "old space guy". Apollo 201, 202, 203, 204, 1, & 9 RTCC Support. Apollo 13 "back room". email to: UseNet1 ayt mcsummation dotting com |
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Retro-fire Attitude
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#4
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Retro-fire Attitude
Where are you getting this 30-40 degrees of pitch? The shuttle pitches 10
degrees nose-down, but that's because the OMS engines are canted to point through the CG of the orbiter, so the orbiter must pitch to put the thrust line anti-parallel to the velocity vector. I think he's referring to earlier generations of ships (especially Gemini and Mercury). They pitched down about 40 degrees from retrograde for their deorbit burns. |
#5
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Retro-fire Attitude
"James Summers" wrote in message om...
"Proponent" wrote in message m... Why is it that spacecraft tend to adopt nose-down attitudes for de-orbit burns? If the acceleration vector were instead precisely anti-parallel to the velocity vector, then 1) a smaller impulse would suffice to reduce the pericenter by a given amount, and 2) the re-entry trajectory would be shallower, which should give lower g-loads and perhaps lower heating loads as well. I assume you're talking about the Shuttle. This attitude puts the thrust vector anti-parallel. Remember, the thrust goes through the center of mass, not along the axis of the Orbiter. The trajectory won't be shallower. Thanks to you and JRF for you replies. I had in mind Mercury and Gemini. I didn't realize that the Shuttle did a nearly anti-parallel burn, though I suppose it should have occurred to me that techniques might have changed in 20 years! If one wanted to de-orbit quickly, an inwardly directed acceleration vector would make sense. But for a typical civilian mission, there's no big hurry. Nope. Any thrust away from the velocity vector is mostly wasted. I beg to differ. Suppose we start in a circular orbit. If we then make a brief, completely anti-parallel retro burn, we'll now be in an elliptical orbit with pericenter 180 degrees around the orbit from the point of retro fire. If, on the other hand, we start in the same circular orbit and make a brief inwardly directed retro burn (I guess you couldn't really call it a "retro" burn, though), then the pericenter of the new orbit will be only about 90 degrees along the orbit from where the point of retro fire. |
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Retro-fire Attitude
"Bob Martin" wrote in message
... Where are you getting this 30-40 degrees of pitch? The shuttle pitches 10 degrees nose-down, but that's because the OMS engines are canted to point through the CG of the orbiter, so the orbiter must pitch to put the thrust line anti-parallel to the velocity vector. I think he's referring to earlier generations of ships (especially Gemini and Mercury). They pitched down about 40 degrees from retrograde for their deorbit burns. In that case, it was, if memory serves me correctly, so the "pilot" could line up the horizon with marks on the window. -- James Summers IBM-ret, "old space guy". Apollo 201, 202, 203, 204, 1, & 9 RTCC Support. Apollo 13 "back room". email to: UseNet1 ayt mcsummation dotting com |
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Retro-fire Attitude
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#8
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Retro-fire Attitude
"Proponent" wrote in message
m... Why is it that spacecraft tend to adopt nose-down attitudes for de-orbit burns? In the course of some other reading, I happened to run into some info on this... Mercury (possibly Gemini et al as well) used a 34deg nose-down pitch angle because this angle minimizes the effects of pitch-angle error. (Remember that attitude control was briefly a bit iffy during retrofire, the solid retros being rather more powerful than the little RCS thrusters.) Although as it turned out, the one Mercury retrofire that had a large attitude error -- Carpenter's -- was off mostly in yaw. -- MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer pointing, 10 Sept; first science, early Oct; all well. | |
#9
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Retro-fire Attitude
"Proponent" wrote in message
m... Why is it that spacecraft tend to adopt nose-down attitudes for de-orbit burns? In the course of some other reading, I happened to run into some info on this... Mercury (possibly Gemini et al as well) used a 34deg nose-down pitch angle because this angle minimizes the effects of pitch-angle error. (Remember that attitude control was briefly a bit iffy during retrofire, the solid retros being rather more powerful than the little RCS thrusters.) Although as it turned out, the one Mercury retrofire that had a large attitude error -- Carpenter's -- was off mostly in yaw. -- MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer pointing, 10 Sept; first science, early Oct; all well. | |
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