SriKumar wrote:1 I know CUS is dialable, but it was not known to me if the variable thrust was _actually_ used in the previous GSLV flight or is this the first flight where it was actually used during flight (the commentators mention this one going 11% beyond nominal and then back to nominal). I am sure it would have been done on ground test.
2 About the last ruskie stage that flew (and failed), there was an interstage fairing..the shroud, that was weaker (thinner) than expected and that collapsed, breaking the connectors between the top stage and the rest of the rocket, and that was the end of the flight. I think it was traced to the weak shroud. This is when they went looking at the last remaining ruskie stage. I am not aware of a shake test but there was some note in the press that the ruskies did not give permission to ISRO to open up the reminaing stage for a post-mortem, so it makes sense that they might have atleast shaken it. (As for finding actual rocks, that's bizarre, this is the first I am hearing about it).
3 Cameras- I am sure they had a camera pointed to the shroud that had collapsed in the previous (ruskie CUS) flight.....if one recalls, the payload fairing was larger for the rusike CUS GSLV than the previous (desi CUS) GSLV.
The variable thrust of the CUS is known. It was used in previous launch as well. The nominal thrust is 75KN (wiki has got it wrong)., the lower-higher range is 73.5 - 82 KN. The higher thrust is used to give the last orbital kick to the sat.
Variable thrust is a standard feature for all liquid engines. CE 7.5 (ISRO should come up with better name like 'laghu urja' or something like that) is indeed a powerful engine., if you study the GSLV MKII., you would notice that the solid boosters provide some 70-75% of the thrust but the CUS provides 50% of the velocity. In layman terms, the solid booster takes the 2 ton satellite some 75% up - but this will fall back to earth (with a parabolic trajectory somewhere nearby in andamans) unless the CUS kicks it enough to make it fall the earth very very very far. In fact the CUS has kicked it so far now that the satellite is constantly falling to earth.
Coming to the failure analysis., the ruskies did not give the shear and other engineering data of the shroud. Basically they did not want to admit that their shroud was not within the tolerances. Lot of information that pinpoints on how the shroud would have behaved under various loads was withheld. On BRF itself we had concluded that the shroud did not perform to the spec and buckled - disconnecting the chords. However that is past.
PSLV when it first launched successfully, it launched *only* @800 Kg IRS sat., and everybody bemoaned it as a disaster for India. But nobody congratulated ISRO when RISAT was launched at @1800 Kgs. This is indicative of the progress being made. As more flight experience is gained (results in more data)., the tolerances are narrowed and also better material is added. I will not be surprised that if the GSLV Mk II is allowed to continue, it will reach some 3.5 tonnes to GTO by say 10th flight.
I still feel ISRO should continue research and development of make the CE 7.5 into CE 25 staged combustion into a monster like the SSME. Also a multi-restartable cryogenic engine is needed. Astute observers would have noted that US, Russia, Japan, ESA and India only have the staged combustion engine.