At orbit insertion the last stage shuts down at the planned height, speed and direction, then releases the satellite into its intended orbit.
The end of a launch is called injection or orbit-insertion. The upper stage cuts off its engine at the moment the rocket has the right height, speed and direction. Then the satellite separates. From then on it follows its orbit with no engine running.
ISRO publishes the orbit each launch is meant to reach:
These are long, stretched orbits. The spacecraft then fires its own engine to reach its final orbit.
Small errors at injection become big errors in the orbit. SSLV's first flight, SSLV-D1, in August 2022, aimed for a circular orbit of 356.2 km. After a sensor problem the rocket injected its satellites about 56 m/s slower than the required 7,693 m/s. That is less than 1% short. The result was an orbit of 360.56 × 75.66 km — so low at one end that it could not last.
This is why SSLV carries a Velocity Trimming Module at the top: 16 small 50 N thrusters that fine-tune the final speed before release.
Once separated, the satellite is on its own, and its team on the ground takes over. Chandrayaan-3, for example, relied on its own propulsion module to carry the lander from its parking orbit to a 100 km orbit around the Moon. If the satellite's engine fails, a perfect launch is not enough: NVS-02 was placed correctly by its rocket in 2025 but could not leave its transfer orbit. For satellites going to high orbits, the long job of orbit-raising begins — see why GEO satellites go via a transfer orbit.
Read the SSLV-D1 entry: a shortfall of about 56 m/s at injection left its satellites in an orbit that could not last. See SSLV's flights
The vis-viva equation links speed v at distance r from Earth's centre to the orbit's semi-major axis a:
v² = GM × (2/r − 1/a), with GM = 398,600 km³/s²
Check SSLV-D1's target. A circular orbit at 356.2 km has r = 6,378 + 356 = 6,734 km. Circular speed v = √(GM/r) = √(398,600 / 6,734) ≈ 7.694 km/s. This matches ISRO's 'required 7,693 m/s'.
What 56 m/s less does. Take v = 7.637 km/s at the same r, fired horizontally:
1/a = 2/r − v²/GM = 2/6,734 − 58.32/398,600 = 2.970 × 10⁻⁴ − 1.463 × 10⁻⁴ = 1.507 × 10⁻⁴ per km
so a ≈ 6,637 km. The injection point becomes the highest point (r = 6,734 km), and the lowest point is 2a − r ≈ 6,540 km, about 160 km up. A 0.7% speed error drops one side of the orbit by about 200 km. The simple calculation assumes a perfectly horizontal burn; the orbit ISRO reported was 360.56 × 75.66 km.
Long transfer orbits. For LVM3's 180 × 36,000 km orbit, r_perigee = 6,558 km and r_apogee = 42,378 km, so a = 24,468 km. Speed at injection (perigee): v = √(398,600 × (2/6,558 − 1/24,468)) ≈ √(398,600 × 2.641 × 10⁻⁴) ≈ 10.26 km/s.
ISRO's corrective actions after SSLV-D1 included changing the stage-separation system, making the sensor fault logic tolerate brief disturbances, and using NavIC data for guidance if the inertial sensors fail.
Orbit shape is very sensitive to speed at injection. Being slightly slow lowered the opposite side of the orbit so much that the satellites were in an unstable orbit that would decay.
r = 6,734 km; v = √(398,600 / 6,734) ≈ 7.69 km/s.
It is a liquid-fuelled top module with small thrusters that adjusts the final velocity so the satellite is released into the correct orbit.