Posted on 08/15/2003 6:36:49 AM PDT by bedolido
Should really be:
Government bureaneers beg the government to save their jobs.
The external forces (that cause a torque of the vehicle) our satellite “feels,” are mainly from atmospheric drag for the low Earth orbiters and the solar radiation pressure for the higher ones. Other torques may include magnetic fields, gravitational field gradients, solar wind particles, etc.
Also, once you start adding angular momentum to a wheel, isn't it hard to subtract exactly the same amount of momentum?
On some satellites, that is exactly what the torque rods are used for. To unload the wheel momentum.
(Hmmm... maybe that is why the wheels need to be normally still, so you can just apply a brake to stop a turn of the telescope
The wheels are used constantly to keep the satellite pointed properly. The onboard control laws perform the “dance” between the wheels, satellite, and torque rods. :-)
Here is a really nice write up on a reaction wheel strategy for the ESA’s Integral Gamma Ray Telescope:
The stable Lagrange Points are much further out. They fall within the orbit that the moon sweeps out. So those points are on the lunar orbit track both leading and laging the moon by 60 degrees.
:-)
Lets get even pickier. Add the effects of both Jupiter and the Sun. :-)
What Vade was pointing out was a glaring error in my statement about the center of mass for the Earth/Moon system. It is not exactly at the center of the Earth due to the large mass of the moon in Earth's orbit.
It so happens I worked on a satellite that did that very thing. No matter what the orientation, there was enough solar cell area to power the satellite. Often though due to weight, power, and stability reasons, panels are just sail areas that need to be orientated.
I assumed you guys would be way ahead of me. At least I'm not in the same league as the writer of a famous letter to the London Times, who seriously suggested that moon shots should be scheduled for dates when there is a full moon, so we'd have a bigger target to aim for.
ROFL! I had forgotten that. :-)
Thanks! :-)
Your welcome :-)
(You liked my "note to self") :-)
Let me tell you an anecdotal story. This is from a time not too long ago, when I worked at another place. They had recieved a Small Explorer Mission grant from NASA, worth about 3 million. They selected a company of some reknown to build the satellite bus and software (not one who's name I thankfully remember). It wasn't anyone too well-known, since it was a small satellite and a small university. The company had made prior satellites, but bad luck had pretty much followed them the entire way before this. Their first satellite had minor attitude issues, which were believed to have been caused by problems with coming out of the nosecone . They fixed them, I believe, and the satellite worked, sort of. The second satellite never made it out of the Pegasus rocket vehicle's nosecone, and ran out of batteries without ever seeing the sun. Finally, the last satellite was built for the group, and it passed all of the tests necessary for the SMEX. It was launched into space, and it was the first satellite made by that particular company to make it out of the nosecone of the rocket cleanly. Unfortunately, the satellite had one solar panel on one side of the spacecraft bus, instead of panel wings extending from the outside of it. The satellite came out of the cone, and as it turned to orient the solar panel, oriented itself so that the panel was directly away from the sun.
With the solar panel not recieving insolation, the spacecraft quickly ran out of batteries and died. The spacecraft began to tumble, and every time the panels recieved solar power, it would immediately turn away from the sun, and the batteries would die, and it would begin to tumble again. A team of engineers were turned loose on the problem, and after about 15 hours, they discovered that the attitude control code had a sign error in it, and the satellite would turn away from the sun while trying to align the panels to the sun. It turned out that that particular instance of code had been in all of the other satellites, but it was never discovered because of all of the other calamities. I felt bad for a graduate student who had helped with every stage of the process, but had to find a different topic because of a lack of satellite to take any data. How depressing is that?
Remember the Lewis satellite? There waq a flaw in the Attitude Control System. They did not take into accout the natural spin axis of the spacecraft body. It entered a flat spin that resulted in no solar power to the panels and a fatal battery discharge as well. It too was a total loss of mission.
The companion Clark satellite (yes they were named for the famous Lewis and Clark) was canceled and never launched.
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