Stages let a rocket throw away empty tanks and engines during flight, so the part that remains is lighter and can reach orbital speed.
A rocket spends most of its flight carrying its own empty weight. Once a tank is empty, it is just dead mass that the engines still have to speed up. A stage is a section of the rocket with its own propellant and engine. When it burns out, it is dropped, and the next stage carries on with a much lighter load.
PSLV has four stages, and they take turns between solid and liquid propellant:
Notice how the thrust falls at each step. Each stage only has to push what is left above it.
ISRO's published flight sequence for LVM3 shows staging clearly:
For its last 11 minutes, the C25 stage is pushing only itself and the satellite. The two boosters and the core stage, which made up most of the lift-off mass, are long gone.
Every separation is a moment when something can go wrong. On SSLV's first flight in August 2022, a short burst of vibration during second-stage separation disturbed the rocket's navigation sensors. The flight software wrongly marked them as failed, and the satellites ended up in an unstable orbit. ISRO changed the separation system to a lower-shock design for the next flight.
Engineers accept this risk because, without staging, today's chemical rockets could not reach orbit with a useful payload.
Read how each LVM3 mission ends with the C25 cryogenic stage, the only part of the rocket still attached when the satellite is released. See LVM3's flights
The rocket equation shows why staging works: Δv = Veq × ln(m_full / m_empty). Speed grows only with the logarithm of the mass ratio, so carrying dead mass hurts a lot.
Compare two imaginary rockets, each with 100 t at lift-off: 89 t of propellant, 10 t of structure and engines, 1 t of payload. Take Veq = 3,000 m/s for every engine.
Single stage. m_full = 100 t; m_empty = 10 + 1 = 11 t. Δv = 3,000 × ln(100/11) = 3,000 × 2.21 ≈ 6,620 m/s. Not enough for orbit.
Two stages. Stage 1: 70 t propellant, 7.6 t structure. Stage 2: 19 t propellant, 2.4 t structure. Payload 1 t. Same totals.
The same propellant and the same structure give about 40% more speed, just because the empty first stage was thrown away. Real rockets add a third or fourth stage and use more efficient engines (higher Veq) on the upper stages, where every kilogram matters most.
The gain shrinks with each extra stage, while the extra engines, separation systems and failure points remain. That is why most orbital rockets use two to four stages.
The rocket equation depends on the ratio of full mass to empty mass. Dropping empty tanks and engines lowers the mass the next stage must accelerate, so the same propellant produces more change in speed.
PS1 (S139 solid), PS2 (liquid, Vikas engine), PS3 (solid) and PS4 (liquid, two small engines). They alternate solid, liquid, solid, liquid.
Vibration during second-stage separation briefly saturated the navigation accelerometers. The software declared them faulty and switched to a backup mode, and the satellites were placed in an unstable orbit.