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To: Valueman who wrote (2160)	3/11/1998 12:58:00 AM
From: Mr. Adrenaline	Respond to of 10852

I seem to have generated more questions than I had hoped to answer. I am aware that the flywheel-battery technology has hopes to also be a on-board momentum wheel. So, the hope goes, that you have one device do the work of two, and thus you save the mass/cost/power consumption of one device. But combining parts actually creates a whole bunch of questions. For example, you are "replacing" a battery with no moving parts with one that has moving parts. Engineers move mountains to reduce moving parts to increase reliability. Introducing a new moving part is generally met with opposition. One could argue that since momentum wheels are already have moving parts, you are really not introducing any new moving parts. I would argue, however, that this is too simple minded. Momentum wheels have back-ups. If one fails, you bring the redundant one on line. And even if you lose every momentum wheel, it doesn't necessarily mean you've lost a satellite. (For the curious, set your search engines on "Anik" on see what you come up with.) Batteries also have redundancy (note I didn't say they have a "back-up" - think of a bunch of Pink Energizer Bunnies you could lose one or two and not impact the entire system too much) so complicating the issue is if the battery/momentum wheel combo fails, you need to back it up with two items - a battery and a momentum wheel. Yes, you could back it up with another battery/momentum wheel combo but the argument would go something like this: "What if it had an inherent design flaw that wasn't caught in ground testing? Then you have a useless satellite." Everyone wants to use something that has worked in the past - it's a definite catch-22. There are legions of ways to solve this problem, and there is no fundamental reason why it wouldn't work on a satellite. But in my opinion it is years before it will fly on any satellite. I can think of no obvious reason why a flywheel-battery implementation would be (fundamentally) different on a LEO than a GEO. I stated earlier that I thought it wouldn't go on a commercial satellite in my lifetime. Maybe that was extreme, but I would be extremely surprised if it flies on a commercial satellite within 20 years. On the other hand, California has a mandate that requires zero emissions cars before sometime in the not too distant future. I think the flywheel-battery is going to be one of the enabling technologies that will allow that to happen. This is my opinion only. I have zero research on the subject, and my opinion on the subject is based on casual reading in magazine like Scientific American and Popular Science. Now having said that, it is certain that a flywheel-battery will fly on a commercial satellite within 5 years, and never be put on a car. (Above all else, I'm a cynic - even of my own predications.) As for fuel consumption for avoidance maneuvers. Remember that "avoidance maneuvers" are more to avoid having two satellites in the same neighborhood that have like frequencies so that there would be no RF interference, not to avoid a physical impact. Even for the example Valueman cited. In-plane maneuvers (east or west) are much less costly than out-of-plane (north or south). But the real variable in this problem is not so much "how far" a satellite was moved, as "how fast" Valueman cited an example of moving two degrees in-plane. If the satellite was moved that far in a months time, then the propellant would be nil (reducing a satellites lifetime on the order of days out of 10 plus years). But if it was moved that far in a couple of hours, then it would be significant (on the order of months lifetime). Because out of plane maneuvers are so costly, "avoidance maneuvers" would almost certainly be in-plane. Again, I can think of no example of a avoidance maneuver that was performed to prevent two objects from physically colliding. (Before someone pops up with an example from the Apollo era, I want to state that my experience is strictly from commercial satellites. There may well be an example of such a avoidance maneuver from the shuttle program, or Skylab, or Mir, etc.) Mr. A