Only a rocket carries its own oxidiser and can push a satellite up to the sideways speed — about 7.7 km/s at 400 km — needed to stay in orbit.
Getting into orbit is not mainly about going up. It is about going sideways, very fast. A satellite stays up because it moves sideways so quickly that, as it falls, the curved Earth keeps dropping away beneath it.
At 400 km — the height ISRO has chosen for the Gaganyaan crew orbit — that speed works out to about 7.7 km/s, nearly 28,000 km/h. Climbing 400 km is the easy part. Reaching that speed is the hard part.
Jet engines and propellers burn fuel with oxygen taken from the air around them. As they climb, the air thins out and they run short of oxygen. A balloon floats only while the air is dense enough to hold it up, and it gives no sideways speed at all.
A rocket is different. It carries both its fuel and its oxidiser, so it can make thrust in a vacuum where there is no oxygen. That is why it is the only machine that keeps working all the way to orbit.
Reaching orbital speed takes a huge amount of propellant. NASA's rocket-equation guide works through an example in which about 90% of a rocket's weight at lift-off is propellant, and only around 1% is payload.
India's PSLV shows the same pattern. In its XL form it weighs about 320 tonnes at lift-off. It can place 1,750 kg into a 600 km sun-synchronous polar orbit. That satellite is about half of one percent of the rocket's lift-off mass. The heavier LVM3 weighs 640 tonnes at lift-off and carries up to 8,000 kg to a 600 km low orbit — a little over 1%.
Every launch has the same three jobs. Climb out of the thick lower air quickly. Tip over and build sideways speed. Then release the satellite at the right height, speed and direction.
The rest of this part explains each step: how thrust works, why rockets have stages, the gravity turn and orbit insertion.
Look at how each PSLV variant adds strap-on boosters so the same rocket can lift heavier satellites. Explore PSLV's history
Two things must be paid for: height and speed. Compare them per kilogram of satellite, using Earth's GM = 398,600 km³/s² and equatorial radius R = 6,378 km.
Speed. For a circular orbit at 400 km, r = 6,778 km and v = √(GM/r) = √(398,600 / 6,778) ≈ 7.67 km/s. The kinetic energy is ½v² ≈ 29.4 MJ per kg.
Height. The gain in gravitational potential energy from the surface to 400 km is GM(1/R − 1/r) = 398,600 × (1/6,378 − 1/6,778) ≈ 3.7 km²/s², or about 3.7 MJ per kg.
So roughly 89% of the energy goes into speed and only 11% into height. That is why rockets turn sideways soon after lift-off.
The rocket equation shows the cost. NASA writes it as Δu = Isp × g0 × ln(MR), where MR is the ratio of full mass to empty mass and g0 = 9.80665 m/s². Turn it around: MR = e^(Δu / (Isp × g0)). With NASA's example values, Isp ≈ 350 s and Δu ≈ 25,000 ft/s (about 7.6 km/s), the exponent is 7,620 / (350 × 9.81) ≈ 2.2, so MR ≈ 9–10. About 90% of the lift-off mass must be propellant. Engines, tanks, structure and payload share the last 10%.
PSLV-XL's numbers fit this picture: 1,750 kg of payload out of about 320,000 kg at lift-off is a payload fraction of about 0.55%.
Its engines need oxygen from the surrounding air, which thins out with height. A rocket carries its own oxidiser, so it can keep producing thrust in a vacuum.
1,750 ÷ 320,000 ≈ 0.0055, or about 0.55% — around half of one percent.
Into speed. About 29 MJ per kilogram goes into the 7.7 km/s orbital speed, against about 3.7 MJ per kilogram for the climb to 400 km.