Strap-on boosters are extra rocket motors fixed to the sides of the core stage that add thrust at lift-off and are dropped once their propellant is spent.
The hardest moment for a rocket is lift-off. It is at its heaviest and moving slowest. Strap-on boosters solve this by bolting extra motors around the core. They add thrust when it is needed most, then fall away once empty so the rocket does not carry their weight to orbit.
PSLV is a good example because the number of strap-ons changes from flight to flight. Its strap-ons are S12 solid motors, each carrying 12 tonnes of propellant and giving up to 719 kN of thrust.
The core stays the same. ISRO picks the version that suits the mass of the satellite and the orbit it needs.
GSLV uses four liquid strap-ons, called L40, each powered by one Vikas engine. They surround its S139 solid first stage.
LVM3 takes the idea further. Its two S200 solid boosters are 25 m tall and 3.2 m wide, and each holds about 205 tonnes of propellant. They are so large that, at lift-off, they are the only engines firing. The L110 liquid core lights about 113 seconds into flight, at about 41.5 km. The boosters burn out after about 134 seconds and separate at 137 seconds.
Strap-ons let one design serve many missions: add more boosters for a heavier load. They also drop away early, which is the same benefit as staging. Their empty casings are left behind within a few minutes, long before the rocket reaches orbit.
The numbers show how much work they do. PSLV-XL, with six extended strap-ons, lifts off at about 320 tonnes and can carry 1,750 kg to a 600 km sun-synchronous polar orbit. LVM3's two S200 boosters together carry about 410 tonnes of propellant — about two-thirds of the rocket's 640-tonne lift-off mass.
Spot the variant letters — CA, DL, QL, XL — which tell you how many strap-on boosters each flight used. Explore PSLV's history
LVM3 shows how much of a rocket's lift-off mass can sit in its boosters.
Mass share: two S200s × about 205 t = about 410 t of propellant, out of a 640 t lift-off mass. That is about 64% of the whole rocket, as booster propellant alone.
Burn rate: 205,000 kg ÷ 134 s ≈ 1,530 kg/s per booster, about 3,060 kg/s for the pair.
Mass at booster separation (137 s), a rough estimate:
So within the first two and a quarter minutes LVM3 has lost about two-thirds of its lift-off mass. That is exactly what boosters are for: spend a lot of propellant early, when the rocket is heaviest, then shed the hardware.
The air-lit core is a design choice worth noticing. On GSLV and PSLV the core stage fires from the pad. On LVM3 the L110 waits until the rocket is above 40 km. A liquid engine started higher up, in thinner air, faces a lower outside pressure p0, so in the thrust equation F = ṁ·Ve + Ae(pe − p0) its pressure term is larger.
Six extended S12 solid strap-on motors.
About 410 t ÷ 640 t ≈ 64%.
The Vikas engine — one in each of the four L40 strap-ons.