A payload fairing is the nose cone that shields a satellite from air pressure and heating during the climb, then splits open and falls away once the air is thin.
Satellites are delicate. They have folded solar panels, antennas and sensitive instruments. Rushing through the lower atmosphere at high speed would damage them. The payload-fairing, which ISRO often calls the heat shield, is a streamlined shell that covers the satellite on top of the rocket.
Once the rocket is high enough that the air is very thin, the fairing is no longer needed. It separates and drops away. Carrying it any longer would waste propellant.
On LVM3, ISRO's flight sequence shows the fairing jettisoned about 217 seconds after lift-off, at an altitude of about 115 km. The satellite then rides uncovered on the upper stage until it is released.
By then the rocket is far above the thick air. NASA gives the air density at Earth's surface as 1.217 kg/m³. A simple model puts the air at 115 km roughly a million times thinner — see the deeper section below.
The fairing's diameter limits how wide a satellite can be. ISRO's figures:
The cross-sectional area grows with the square of the diameter: π × (2.5 m)² ≈ 19.6 m² for LVM3's 5 m fairing, against π × (2 m)² ≈ 12.6 m² for GSLV's 4 m one. That is over 50% more room for wide satellites.
The fairing must separate at exactly the right moment. On 31 August 2017, PSLV-C39 lifted off normally with the navigation satellite IRNSS-1H. Every flight event went to plan except one: the heat shield did not separate. The satellite was released inside the closed fairing, and the mission was declared unsuccessful. ISRO said on the day that the cause was being analysed.
It shows that a part with no engine can still end a mission.
Find the PSLV-C39 entry — the flight where the heat shield stayed shut around the satellite. Explore PSLV's history
Why can the fairing be dropped at around 115 km? Because almost no air is left.
A simple model of the atmosphere is ρ(h) = ρ0 × e^(−h/H), with surface density ρ0 = 1.217 kg/m³ and scale height H = 8.5 km (NASA Earth fact sheet). At 115 km:
ρ ≈ 1.217 × e^(−115/8.5) = 1.217 × e^(−13.5) ≈ 1.6 × 10⁻⁶ kg/m³
This simple model is rough so high up, but the point holds: the air is about a million times thinner than at the surface.
The loads on the fairing depend on dynamic pressure, q = ½ρv². The NPTEL lecture notes that q peaks during the climb and then falls, because density drops faster than speed rises. That peak is what the fairing is designed for. By the time q is close to zero, the fairing is just extra mass.
Why not drop it earlier? Aerodynamic heating and pressure would hit the satellite. Why not later? Every kilogram of fairing carried longer is a kilogram the upper stage must accelerate. In the rocket equation, Δv = Veq × ln(m_full / m_empty), extra mass late in the flight reduces m_empty's advantage directly.
Size matters too. Fairing frontal area grows as d². Going from 4 m to 5 m adds about 56% more area: (5/4)² = 1.5625.
Once the rocket is above most of the atmosphere, the fairing no longer protects the satellite from anything, and carrying its mass further would waste propellant.
All flight events were normal except heat shield separation. The satellite separated inside the closed heat shield, so the mission was unsuccessful.
About 115 km, roughly 217 seconds after lift-off.