CBSE Class 10 Physics Space Physics Notes Set 16

Rockets and Satellites

A rocket works on Newton's third law. Hot gases rush out of the rear nozzle (action) and the rocket is pushed forward with an equal force (reaction), like a released inflated balloon. A jet engine takes oxygen from the air to burn its fuel, but a rocket works in airless space, so it carries its own oxidiser. The fuel and oxidiser can be liquids stored separately and mixed in the combustion chamber, or solid propellant containing an oxidiser.

The final speed depends on the speed of the exhaust gases and on the mass ratio: mass at take-off divided by mass after the fuel is used up. A larger ratio gives a higher speed.

Multistage Rockets

A multistage rocket is two or three rockets stacked one above another. The lowest stage fires first and is dropped when its fuel is used up, then the next fires, and so on. Dropping the used stages reduces the mass, so the mass ratio and the acceleration increase. This is how satellites are placed in orbit and spacecraft sent to the moon, as with the three-stage Saturn V. Sounding rockets for the upper atmosphere use two stages.

Orbital and Escape Velocity

A satellite stays in a circular orbit when the gravitational pull supplies the centripetal force: mv²/r = mg, so v = √(gr), where r is the distance from the centre of the earth and g is the acceleration due to gravity at that height. For a height of about 150 km this gives roughly 8 km/s, about 28 000 km/h, and the speed is less for higher orbits. The escape velocity is √2 times the orbital velocity at the same height, that is about 11 km/s, and a body launched at this speed does not return.