**Previous message:**Ted Molczan: "Fobos-Grunt: decay estimate"**In reply to:**Paul Salanitri: "Re: Fobos-Grunt: decay estimate"**Next in thread:**Ted Molczan: "Fobos-Grunt: decay estimate"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

Paul Salanitri wrote: > I think I've worked out what the drag profile for Phobos Grunt means. > > http://twitpic.com/7vut8o/full > > The ballistic drag curve correlates with a spacecraft moving through 90 > degrees (see image). > > The clipping of the peak drag (when aligned with the trajectory) is where > the solar panels eclipse the base and therefore reduce drag). > > Absolute maximum is slightly side on with maximum solar panel, but all the > base incident to the atmosphere. > > Minimum is edge on when the solar panels present only the edge, and the > spacecraft sides (including hollow parts) is presented to the atmosphere. At launch, the spacecraft was almost totally covered with a thermal blanket, including the lattice of the cruise stage. > If this is correct, and nothing else changes to alter the spacecraft > orientation, it should return to the first "maximum" around 26 December. > > Thoughts? Based on drawings and other data, I estimate that the effective area for drag is ~16 m^2 when oriented sideways into the velocity vector, and ~17 m^2 end-on. I have not considered other orientations. The last time I searched the web for the s/c mass, the most commonly quoted values were 13,200 kg and 13,500 kg. The orbit manoeuvres consumed ~100 kg of fuel (I am nearing completion of report on this); attitude control consumed an unknown quantity, but not more than a few hundred kg, limited by supply. Here is a plot of estimated A/m since the orbit manoeuvres ceased, at typically ~2 day intervals: http://satobs.org/seesat_ref/phsrm/Fobos-Grunt_area_to_mass_ratio_evolution_v1.pdf The mean value, 0.001263 m^2/kg is in reasonable agreement with the above values of area and mass. If we assume mass is 13,000 kg, then the mean area is ~16.4 m^2. For the same mass, the plotted range, 0.001125 m^2/kg - 0.001408 m^2/kg, implies a range of effective cross-section for drag of 14.6 m^2 - 18.3 m^2. Although the A/m plot shows a gradual variation over time, it is probably not entirely due to precession of the s/c, if at all. At least some of the variation probably arises from variation of Cd and errors in the modelled atmospheric density. Ted Molczan _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-l

**Next message:**Patrick Schmeer: "Imminent launch of Soyuz TMA-03M"**Previous message:**Ted Molczan: "Fobos-Grunt: decay estimate"**In reply to:**Paul Salanitri: "Re: Fobos-Grunt: decay estimate"**Next in thread:**Ted Molczan: "Fobos-Grunt: decay estimate"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

*
This archive was generated by hypermail 2b29
: Wed Dec 21 2011 - 06:35:39 UTC
*