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Oberg: "The real significance of the ISS thruster test failure"
John Doe wrote in :
"Jorge R. Frank" wrote: And ISS does exactly that - they call the mode "Night Glider". If the arrays are at an angle to direction of travel (with the sun in the back), do they provide any lift at all ? "Any", yes. "Non-negligible" (in terms of being useful for orbit maintenance), no. The station isn't just hypersonic; it's in a "free- molecular" flow regime. L/D is generally so poor ( 1) that ISS is better off following a strategy of minimizing drag (even though lift goes to zero) rather than maximizing lift. Once the truss os fully deployed, if they were to put one side at 45° and the other at -45°, would it create sufficient force to actually put the station into a spin ? It would generate a measurable aero torque, and could in theory generate a spin over time, though the angular acceleration would be quite low (the station's moments of inertia are very large) and would take a long time to build up a visible rate, even in the absence of control torques. Will the surfaces be large enough that they could use the arrays/truss to help desaturate the CMGs ? In theory, yes. The software accounts for this (rather than seeking gravity gradient attitudes, it seeks torque-equilibrium attitudes that balance the aero and gravity-gradient torques), but currently does not take advantage of it. -- JRF Reply-to address spam-proofed - to reply by E-mail, check "Organization" (I am not assimilated) and think one step ahead of IBM. |
#92
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Oberg: "The real significance of the ISS thruster test failure"
Chuck Stewart wrote: NASA PAO flack at a press confrerence: "No one has ever done this before! It will be a huge leap in space solar power applications that will undoubtedly have many spinoff effects for life on Earth... and also we're using RMS technology engineered for the shuttle/ISS program so costs will be reduced by a factor of 10!" I'm still shaking me head at Deep Space 1 and the ion engine of wonder that has never been done before and is like the one Scotty's going ecstatic about in Star Trek TOS. Of course it had been done long before, and powered by a RTG isotope generator to boot. What I can't figure out is why Scotty is so damn awed by an ion engine, considering he's got a pair of antimatter powered warp field generators to play with. Maybe it's like running into a steam engine made out of titanium or something. Pat |
#93
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Oberg: "The real significance of the ISS thruster test failure"
Chuck Stewart wrote:
"Feather"... (The zapkitty has a vision of ISS circling the globe with the solar arays windmilling freely in the breeze... Except the idea of feathering is to *prevent* windmilling. D. -- Touch-twice life. Eat. Drink. Laugh. -Resolved: To be more temperate in my postings. Oct 5th, 2004 JDL |
#94
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Oberg: "The real significance of the ISS thruster test failure"
On Mon, 22 May 2006 17:02:45 +0000, Derek Lyons wrote:
Chuck Stewart wrote: "Feather"... (The zapkitty has a vision of ISS circling the globe with the solar arays windmilling freely in the breeze... Except the idea of feathering is to *prevent* windmilling. *ouch* OK, I asked for that... .... but what about the ISS autorotation drills? D. -- Chuck Stewart "Anime-style catgirls: Threat? Menace? Or just studying algebra?" |
#95
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Oberg: "The real significance of the ISS thruster test failure"
JRS: In article , dated Thu, 18
May 2006 20:32:38 remote, seen in news:sci.space.station, Jorge R. Frank posted : Dr John Stockton wrote in news:$n9DF9AxAObEFwW9 : What does the drag amount to in actual Newtons and pounds-of-force, for a typical or mean attitude? It can hardly be 166 kgf/m^2. Right, the ballistic number is not pressure (force per unit area) but rather mass per unit area. So 166 kg/m^2 means the station has 166 kg of mass for every square meter of projected area. Drag force is F = 0.5*rho*v^2*Cd*A, drag deceleration is a = F/m. The ballistic number is BN = m/(Cd*A) so drag simplifies to a = 0.5*rho*v^2/BN or F = 0.5*rho*v^2*m/BN. m is around 185,000 kg, v is around 7700 m/s. I don't have my atmosphere models handy so plug in your favorite value for rho and chug away. That's all very well, no doubt; but neither do I. The Heavens-Above site indicates that, between boosts, ISS drops by about 3 kilometres/month or 1 millimetre/second. Now ISTM that dR/dt = A T / pi where T is the period (Does anyone know if that is right?) which puts the deceleration A at around 0.05 micro-gee and the force therefore as about one centi-newton (1 gram). Someone please check that! -- © John Stockton, Surrey, UK. Turnpike v4.00 MIME. © Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links; Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc. No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News. |
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