[InclinedPlane]

Isp is only one factor. The GSLV Mk. 3's upper stage is LOX/LH2 but it's also fairly small. The Falcon 9's upper stage is 4 tonnes empty 107.5 tonnes fueled, with an Isp of 348s. The GSLV Mk. 3's upper stage has an Isp of 443s (good) a dry mass of 5 tonnes (not as good) and a wet mass of 33 tonnes. The mass fraction on the F9 upper stage utterly dominates the GSLV's upper stage Isp.

Let's look at GTO payloads and upper stage delta V.

GSLV Mk. 3: 4 tonnes, which translates to a stage delta V of ln((4+33)/(4+5)) x 4.34km/s = 6.135 km/s

F9 FT: 8.3 tonnes -> ln((8+107.5)/(8+4)) x 3.41 = 7.72 km/s

So the Falcon 9's upper stage is able to push twice as heavy a payload through about 1.6 km/s more delta V despite having nearly a full km/s lower exhaust velocity, all because it has a better mass ratio.

[baq]

thanks for the writeup!