Successful flight of GSLV-D5 was critical for ISRO, and they jumped the hurdle. It has been a week since then. Now that I got a few moments to spare I have few observations.
1. The CUSP is an uprated version of Russian Cryo stage. When Cryo deal was struck w/Russians Indian scientists had already seen the rocket engine has headroom to operate at higher thrust 8.5 tonne for some time, and they asked Russia to test and certify the higher thrust operating regime. The of course unkiil killed the Technology Transfer deal.
2. The temporarily available higher thrust allowed somewhat higher payload capability. For best mission performance the Cryo should be operated with higher thrust (to minimize ISP lost due to gravity) when the rocket has reach a velocity equal to the orbital velocity at current altitude, the thrust can be pared down significantly, and all impulse from that time onwards is gets used to raise orbit. So rated 7.5 tonne is OK as long as the engine is operating at maximum possible ISP.
3. On D5 mission the CUSP stage was operated in conservative thrust profile, to ensure it passes baseline benchmark with confidence. Hence the thrusting phase started with 7.5 tonne thust, and somewhere in the middle of the burn (by then teh engine was basically flight proven) the engine was put to its pace and operated in higher thrust mode.
4. The D6 due for middle of 2014 which is an identical configuration as D5 will however be operated in the preferred thrust profile, whereby when the Cryo stage is lighted it will operate in teh higher thrust mode for a good 1/3rd of the burn time, and then it will revert to slightly higher efficient (higher ISP) mode for GTO insertion. This will result in slightly higher payload.
1. The CUSP is an uprated version of Russian Cryo stage. When Cryo deal was struck w/Russians Indian scientists had already seen the rocket engine has headroom to operate at higher thrust 8.5 tonne for some time, and they asked Russia to test and certify the higher thrust operating regime. The of course unkiil killed the Technology Transfer deal.
2. The temporarily available higher thrust allowed somewhat higher payload capability. For best mission performance the Cryo should be operated with higher thrust (to minimize ISP lost due to gravity) when the rocket has reach a velocity equal to the orbital velocity at current altitude, the thrust can be pared down significantly, and all impulse from that time onwards is gets used to raise orbit. So rated 7.5 tonne is OK as long as the engine is operating at maximum possible ISP.
3. On D5 mission the CUSP stage was operated in conservative thrust profile, to ensure it passes baseline benchmark with confidence. Hence the thrusting phase started with 7.5 tonne thust, and somewhere in the middle of the burn (by then teh engine was basically flight proven) the engine was put to its pace and operated in higher thrust mode.
4. The D6 due for middle of 2014 which is an identical configuration as D5 will however be operated in the preferred thrust profile, whereby when the Cryo stage is lighted it will operate in teh higher thrust mode for a good 1/3rd of the burn time, and then it will revert to slightly higher efficient (higher ISP) mode for GTO insertion. This will result in slightly higher payload.