Read the stage and booster configuration

PSLV’s four main stages alternate solid, liquid, solid and liquid propulsion. The second stage uses a Vikas engine; the fourth stage, PS4, uses two liquid engines. A main stage continues the ascent after earlier hardware has done its job. Strap-on boosters instead provide additional thrust alongside the first stage.

Separating spent hardware avoids carrying its empty structure through all later acceleration. That advantage comes with interfaces and events that must work: ignition, guidance, shutdown and separation are coordinated. Counting engines alone does not tell you how many sequential main stages the vehicle has.

ISRO lists CA, DL, QL and XL versions with zero, two, four and six strap-on boosters respectively. The main four-stage sequence remains. A designation therefore communicates a configuration choice; it should accompany any quoted payload capability. The photograph of PSLV-C54 shows one flight, not every possible PSLV arrangement.

In an original comparison, Flight A needs less launch performance than Flight B but both must satisfy their own orbit, mass and operational constraints. More boosters may supply capability that A does not require. “Choose the largest version” is not a complete design rule: suitability must be established for the actual mission, including loads, interfaces and qualification.

A launcher’s payload figure applies to stated conditions. Changing destination energy, orbital inclination or the flight configuration changes the problem. Do not transfer a mass advertised for one orbit directly to a harder destination. A liquid upper stage also needs the appropriate systems and qualification before a mission can rely on restarting it.

For a deliberately simplified budget, suppose the allowed delivered mass for one fictional configuration and orbit is 1,600 kilograms. Spacecraft total 1,350 kilograms and adapters add 120 kilograms within that same allowance. The unallocated amount is 130 kilograms: 1,600 minus 1,350 minus 120. This is an arithmetic exercise, not a PSLV performance promise or a complete flight approval.