Getting to orbit

Rockets, stages, engines, launch windows and why India launches from Sriharikota.

1. Why do we need rockets to reach space? — 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.
2. How does Newton's third law make a rocket fly? — A rocket throws hot exhaust gas backwards very fast; the gas pushes back on the rocket with an equal and opposite force, which drives it forwards.
3. What are thrust and specific impulse? — Thrust is how hard a rocket engine pushes; specific impulse measures how much push it gets from each kilogram of propellant — its fuel efficiency.
4. Why do rockets have stages? — Stages let a rocket throw away empty tanks and engines during flight, so the part that remains is lighter and can reach orbital speed.
5. What is the difference between solid, liquid and cryogenic rocket stages? — Solid motors burn a fuel-and-oxidiser mix cast inside the casing; liquid engines pump separate propellants into a chamber; cryogenic engines burn super-cold liquid hydrogen and liquid oxygen.
6. What are strap-on boosters? — 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.
7. What is a payload fairing? — 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.
8. Rocket Launch Stations in India — India launches rockets mainly from the Satish Dhawan Space Centre (SDSC SHAR) on Sriharikota Island, Andhra Pradesh — ISRO's only operational orbital spaceport, with two active launch pads.
9. What is a launch window? — A launch window is the stretch of time in which a rocket can lift off and still reach its target orbit, planet or spacecraft safely.
10. Why do rockets curve over after launch? — After a short vertical climb, a rocket tips slightly and lets gravity bend its path towards horizontal — a gravity turn — so it gains sideways speed without large side loads.
11. What happens when a rocket puts a satellite into orbit? — At orbit insertion the last stage shuts down at the planned height, speed and direction, then releases the satellite into its intended orbit.
12. Why do geostationary satellites go via a transfer orbit? — Rockets drop geostationary satellites into a long, egg-shaped transfer orbit because it is cheaper; the satellite's own engine then rounds off the orbit at about 36,000 km.
13. What happens on launch day? — Launch day ends a long countdown: review boards clear the rocket, propellants are loaded, computers run the final minutes, and the rocket lifts off at the planned second.
14. Why do rocket launches fail? — Launches fail when one of thousands of parts or decisions goes wrong at high speed — a motor losing pressure, a leaking valve, a misread sensor or a shield that stays shut.
15. What is a reusable rocket, and what is ISRO's RLV? — A reusable rocket brings its costly parts back to fly again; ISRO is testing this idea with a winged vehicle, the RLV, designed to land on a runway.
16. What are small satellite launchers? — Small launchers are compact rockets built to put satellites of a few hundred kilograms into low orbit quickly, instead of waiting for a ride on a big rocket.
17. What does it mean to human-rate a rocket? — Human-rating means redesigning and testing a rocket so it is safe enough to carry people, including a way for the crew to escape if anything goes wrong.
18. How does a capsule come back to Earth safely? — A returning capsule uses a heat shield and the thick air to shed its orbital speed, then opens parachutes in stages to settle gently onto the sea.