yee@trident.arc.nasa.gov (Peter E. Yee) (01/11/90)
1/10/90 12:20 p.m. CST The following information is the latest data on the time for LDEF grapple. This information takes into account the normal corrective burn that took place at 11:23 a.m. CST today. As the orbiter performs other correction burns the time of capture will change. Updated capture times will be provided by the PAO commentator who will check with flight controllers following each burn. LDEF CAPTURE Flight Day 4 - Friday, January 12, 1990 3 days 2 hours 13 minutes 29 seconds MET 8:48 a.m. CST Prepared by: Ed Campion Source: Jeff Carr
paulc@hp-lsd.COS.HP.COM (Paul Carroll) (01/13/90)
During the LDEF capture this morning, there were comments about how stable the LDEF was (i.e., not tumbling). Is there any reason why LDEF is not rotating or tumbling in orbit? My only thought as to why it might not is that the LDEF might be flying end-on in orbit so any tendency to rotate might be suppressed by atmospheric drag on that side (that is, if there is enough atmospher to affect it). Does anyone have a definitive answer as to why LDEF was behaving so nicely in orbit? ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + Paul Carroll "Dear Lord, please break the laws + + HP Logic Systems Division of the universe for my convenience."+ + hplabs!hp-lsd!paulc - Emo Phillips + ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
poirier@dg-rtp.dg.com (Charles Poirier) (01/13/90)
Excuse me if this has been covered before and I missed it. Something is puzzling me. The LDEF was more-or-less on the verge of falling out of the sky due to atmospheric friction. The shuttle caught up to it by orbiting at a lower altitude. My question is, how much lower? How much drag did the shuttle experience by orbiting lower than LDEF? And, were any special adjustments made to the shuttle's attitude or configuration to reduce drag in such a low orbit? Cheers, Charles Poirier
animal@eunomia.rice.edu (Carl Rosene) (01/14/90)
In article <2187@xyzzy.UUCP> poirier@dg-rtp.dg.com ( Charles Poirier) writes: > The LDEF was more-or-less on the verge of >falling out of the sky due to atmospheric friction. The shuttle >caught up to it by orbiting at a lower altitude. My question is, >how much lower? How much drag did the shuttle experience by orbiting >lower than LDEF? And, were any special adjustments made to the >shuttle's attitude or configuration to reduce drag in such a low orbit? > Lower and Higher are rather relative here. Fact is, the LDEF is(was) at an altitude higher than typical shuttle missions. The LDEF was about 200 hundred miles or so up. The shuttle was in an orbit maybe 20 miles lower (I'm sure people will be correcting this number). It is unnecessary for a shuttle to go more than about 150 miles high for a typical satellite launch. So, there were no special precautions necessary to reduce drag, since it was not any greater than an orbiter experiences normally. One moral is that the atmosphere does not end suddenly. At one hundred or two hundred miles there is little enough drag that you can get into an orbit. But, how long you stay there is another question. If you only plan to stay a week it doesn't matter much. A few months and it's time to get out your slide rule. Carl Rosene Rice University
tneff@bfmny0.UU.NET (Tom Neff) (01/15/90)
In article <9090002@hp-lsd.COS.HP.COM> paulc@hp-lsd.COS.HP.COM (Paul Carroll) writes: > Is there any reason why LDEF is not rotating or tumbling > in orbit? The reason it's not tumbling IN GENERAL is that long satellites tend over time (if they didn't start that way) to assume what is called 'gravity gradient' attitude, with the major axis pointing to the center of the earth. This is essentially a stable solution to the orbital dynamics problem of how to fly the two ENDS of a long satellite in 'forced tandem'. The satellite IS tumbling -- at 1/90th rpm! Now you know why the LDEF retrieval was such a fun job :-) Nevertheless as satellites re-enter the atmosphere they do start tumbling. The reason LDEF isn't tumbling YET is that Our Friend Mr. Sun was a good enough sport to quiet down the last couple of months, so the exoatmosphere (which gets fluffed up by an active sun) subsided and gave LDEF a time bonus. If the sun hadn't quieted, this delayed Columbia launch would probably have been too late. As it is, LDEF tumbling and re-entry was predicted for March. -- 'We have luck only with women -- not spacecraft!' \\ Tom Neff -- R. Kremnev, builder of failed Soviet FOBOS probes // tneff@bfmny0.UU.NET
pat@grebyn.com (Pat Bahn) (01/15/90)
The question was why is LDEF so stable? Answer Large permanent magnets installed to maintain orientation to earths B Field. -- ============================================================================= Pat @ grebyn.com | If the human mind was simple enough to understand, 301-948-8142 | We'd be too simple to understand it. =============================================================================
henry@utzoo.uucp (Henry Spencer) (01/15/90)
In article <9090002@hp-lsd.COS.HP.COM> paulc@hp-lsd.COS.HP.COM (Paul Carroll) writes: > During the LDEF capture this morning, there were comments > about how stable the LDEF was (i.e., not tumbling). > Is there any reason why LDEF is not rotating or tumbling > in orbit? ... LDEF was designed for passive stabilization. It was built as basically just a platform for experiments, with essentially no intelligence or subsystems of its own. However, stabilization was considered desirable, as quite a few of the experiments want a predictable orientation. By making the thing relatively long, you get two axes of stabilization for free: long satellites tend to line up with the long axis pointing toward Earth, as a result of the slight difference in Earth's gravitational pull between the Earthward end and the spaceward end. This is "gravity gradient" stabilization; it's not enormously effective but it works well enough when done carefully. As I recall, LDEF gets the third axis with a permanent magnet, which tends to align one of the short axes with Earth's magnetic field. There are also some provisions for damping, which helps keep small perturbations under control. When LDEF was deployed, it was lined up in the correct orientation, held there for a while, and then released very slowly and carefully to minimize disturbances. -- 1972: Saturn V #15 flight-ready| Henry Spencer at U of Toronto Zoology 1990: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu