Yesterday ISRO operated the GSLV MK2 launch vehicle, which is a medium lift launch vehicle with a capability to lift 5 tonnes to LEO.

I just compared Falcon 9 with GSLV MK2. Falcon 9 capability to lift 23 tonnes to LEO is almost 5 times as much as the GSLV. So I expect a similar difference in thrust for each rocket.

GSLV First Stage : 760 kN * 4 (L40 Straps) + 4,700 kN (Solid core stage) = 7740kN

Falcon 9 FT First Stage : 825 kN * 9 (Merlin 1D+) = 7425kN

Both LVs first stages burn duration is around 160 seconds. Specific impulse also does not have much difference for both Vikas and Merlin engine. Also GSLV third stage uses a cryogenic engine with very high Specific impulse.

Still what are the main factors which allow Falcon 9 to perform 5 times better than GSLV Mk2?

Is the second stage of GSLV the main reason?


3 Answers 3


The biggest factor is probably the "inside-out" staging design, where the solid core burns out but is not jettisoned, while the liquid boosters continue to run.

Spaceflight101 gives the empty mass of the (steel-cased) solid stage as 28 tons (with an unremarkable 83% mass fraction); this means the otherwise decent specific impulse of the strap-on engines is horribly compromised by carrying 28 tons of dead weight for a full minute.

That unusual design is derived from the earlier PSLV launcher, which uses the same solid rocket first stage, but with smaller and shorter-burning solid strap-on boosters which drop away halfway through the first-stage burn. It was easier and cheaper to change those out for larger liquid-fueled boosters than to design an all-new high-thrust liquid first stage for GSLV.

GSLV MkIII will be a more efficient solids-on-the-outside design, launching 10 tons to LEO from a 630 ton launch weight.

In general, you can't expect to learn much about payload by comparing launch thrust. For orbital launchers, liftoff thrust is generally correlated with liftoff weight - typically you start with a 1.15-1.5 ratio -- but liftoff weight is not closely correlated with payload, as discussed in another Q/A: Why are rocket mass on the launch pad and payload mass to LEO not strongly correlated?


There are lots of factors that impact the mass a Launch Vehicle can bring to orbit. In this case, what we need to look at is not only the first stage's thrust, but also its efficiency, measured in rocketry as the Specific Impulse (Isp), which indicates the energy an engine can extract from a given mass of propellants.

According to wikipedia, the GSLV MkII's first stage has a very low Isp (237s) due to its use of a solid motor, while the boosters have a slightly higher Isp (262), still fairly low for liquid engines. For comparison, the Falcon 9 Full Thrust's Merlin-1D engines rate at 282s.

Additionally, it shows on the GSLV MkII's page that the first stage burns for 100 seconds, while the strap-on boosters burn for 160 seconds. This means that for the last 60 seconds of their burn, the four liquid boosters are carrying the first stage's casing which has become pure dead weight.

Lastly, looking at the launch mass of both vehicles gives us another reason: Falcon 9 Full thrust takes off at 549 tonnes, while GSLV MkII weighs 414 tonnes. We don't know the exact propellant mass fraction for either, but most rockets are around 90% fuel by mass at takeoff — GSLV just carries much less fuel than Falcon 9 FT


  • $\begingroup$ But is that enough to get 5times performance gain? $\endgroup$ Commented Sep 9, 2016 at 12:07
  • 1
    $\begingroup$ See the added paragraph on mass. If you combine less efficient engines, less efficient staging (which means carrying dead mass for a minute, everything but negligible), and nearly 25% lower liftoff mass, it wouldn't surprise me that much. Bear in mind, we don't have access to a lot of important numbers too: each vehicle's propellant mass fraction (very important when going to orbit), trajectory optimisations (I can't find a map, but I'm fairly certain launching to LEO from ISRO's launch complex requires a dogleg bend during the ascent). $\endgroup$
    – amyinorbit
    Commented Sep 9, 2016 at 12:11
  • $\begingroup$ @cesarparent another very important element is that the difference at thrust becomes very big.Because after 100s total thrust of GSLV MKII is only 3040 kN since the solid core stage has finished.Falcon 9FT thrust keeps rising,in vacuum it will be 8227 kN.This makes a big difference in payload.Another thing is that 2nd 3rd stage of GSLV are small. 2nd stage has a low Isp 295s and burns for 150s,compared to Falcon 9 348s burning for 397s. 3rd stage is very small and is used to put a little bit more payload but still can't compesate very big loses in 1st and 2nd stage.You can edit your answer. $\endgroup$
    – Mark777
    Commented Sep 9, 2016 at 12:35
  • $\begingroup$ @Mark777, But stage 3 specific impulse is much higher (454), And burns for 720seconds. Does that will not have enough impact? $\endgroup$ Commented Sep 9, 2016 at 12:39
  • $\begingroup$ We should combine all the elements together here,not taking only one,the Isp of 3rd stage.It has really the Isp 454s but has also a very low thrust 75 kN.And this kind of upper stages are helpful if you have put a big payload in a high orbit where you don't have to do much with gravity.In this case stages that fight gravity have a very low performance.At that moment that 3rd stage will be ignited the payload would be very small because it has lost to much performance in 1st stage mostly and 2nd.Combine all disadvantages of cesarparent and what I wrote,will give this performance for GSLV. $\endgroup$
    – Mark777
    Commented Sep 9, 2016 at 12:55

The additional velocity Del V is only matter in addition to total impulse. Velocity at the end GSLV is 2km/sec where as Falcon 2.5 km/sec. And at the end of Gs2 stage velocity is 3.4km/sec where as Falcon will go 4.km /sec in the same regime n further takes faster. That makes the payload difference. Also another factor is launch location site.


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