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ISRO's PSLV-XL has 6 solid strap on boosters.

In a typical flight profile 4 strap on boosters are lit on ground just before liftoff. The remaining two boosters are lift 25 seconds later in air.

Similarly 4 boosters are jettisioned 12 seconds before air lit boosters. My question is what are the advantages of igniting boosters mid air?

PSLV-XL Flight Profile

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This probably has to do with optimizing the launch efficiency of the rocket.

More specifically, a rocket has two goals:

  • Get out of the atmosphere
  • Reach orbital velocity

The second step can't really be accomplished before the rocket has "left" the atmosphere, because drag against the atmosphere doesn't increase linearly with speed: it increases with the square or even a higher power, once supersonic regimes are considered. This means that an "optimal" rocket launch is rather slow at the beginning, while the rocket is still in the thicker parts of the atmosphere, and then power is increased once the air is thinner and drag plays less of a role.

At the ~3km in altitude where the additional set of solid rocket boosters are lit, the atmosphere is already only about 70% as thick as it was at sea level, which means that the rocket is "allowed" to go faster without losing enormous amounts of energy to drag. If all boosters were lit directly on the launch pad, the rocket would presumably go too fast too quickly and thus waste a lot of energy. By spreading around these ignitions, a higher mass-efficiency is reached and more payload can be transported to orbit.

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    $\begingroup$ Yes indeed. In developmental PSLV flights D1,D2 and D3 only two strap-ons were ground-lit and about 30 seconds later remaining four were air-lit. This was changed for PSLV C1 operational flight of PSLV where four were ground-lit and two were air-lit. Per ISRO's official brief-history 'From Fishing Hamlet to Red Planet' for developmental flights this was done mainly to "reduce the dynamic pressure at the cost of payload reduction." and for operational flight the originally planned sequence was restored, based on flight experience. $\endgroup$
    – Ohsin
    Feb 12 at 20:22
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    $\begingroup$ Delta II's also could ignite some of their SRMs in flight. space.stackexchange.com/a/41747/6944 $\endgroup$ Feb 12 at 23:16

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