ORBITS

A rocket can propel a vehicle to high speeds and high altitudes, both of which are required in order to achieve orbit. An orbit represents a delicate balance between a vehicle’s velocity and the Earth’s gravity. Imagine that you toss a ball away at low speed. You will see it follow a curved trajectory and hit the floor some meters away. If you want it to go further, you will need to throw the ball a bit faster. It will still fall and accelerate towards the ground at the same rate as before, because the force of gravity remains the same. However, because its initial horizontal speed is higher, it will cover a larger horizontal distance before landing. Now imagine that you shoot

your ball away with a cannon: watch it fly, all the way over the horizon! Because the Earth is round, its surface drops away under the ball while it is falling. The result is that it takes the ball longer to reach the ground, and as a consequence it will manage to fly farther away than if we were living on a flat world. If you get the ball up to a velocity of about 8 km per second (5 miles per second), the curvature of the ball’s trajectory under the pull of gravity is exactly the same as the curvature of the Earth. In effect, the ball is continuously falling around the world, without ever hitting the ground: it is in orbit! If you shoot the ball eastwards, it will circle around the planet and reappear from the west just under 1.5 hours later.

In reahty, the atmosphere would slow the ball down so much that it would never come back, it would either burn up or fall short. However, at altitudes over 100 km (60 miles) there is hardly any atmosphere left to slow down a moving object. If you can get your ball up to speed there, it will be able to circle the Earth and become a satellite.

To launch a satellite, a rocket is initially fired straight up in order to rapidly climb above the thickest layers of the atmosphere and minimize aerodynamic drag. It then starts to pitch over, so that as the rocket keeps accelerating it increases both altitude and horizontal velocity. On the way up a conventional launcher drop stages to shed the dead weight of the empty tanks and engines that are no longer required. Without this ‘staging’ it would be too heavy to reach orbit. Most launch vehicles consist of two or three rocket stages, one on top of the other, plus sometimes two or more rocket boosters attached to the side of the first stage. Once the last stage reaches the proper orbital altitude and velocity, the satellite is released. The last rocket stage usually stays in orbit as well, but the other stages splash into the ocean, crash on land, or burn up in the atmosphere when falhng at high velocity. In principle it would be possible to design these stages to be recovered and reused, but that would add an enormous amount of complexity and, most importantly, weight. A

recoverable upper stage would need a heat shield to protect during its re-entry into the atmosphere, parachutes, and probably also airbags to cushion the impact on land or more likely at sea. Such a stage could easily become so heavy that it would not be of any use in a launcher. In addition, the recovery and refurbishment would be very expensive. All currently used satellite launch vehicles (except for the Space Shuttle, until recently) are therefore expendable, meaning they can only used once because everything except the satellite payload is discarded during the short flight up. As we will see later, in the case of the Space Shuttle the two Solid Rocket Boosters and the Orbiter itself were reused, but the large External Tank which held most of the liquid propellant for the Orbiter on the way up was still discarded. In operational terms, it would be best to use a fully reusable vehicle requiring only a single stage to get into orbit, just like you would not want to have a commercial airliner dropping off tanks and engines on the way to its destination. Expendable rocket stages are expensive to build, but usually it is more cost effective than providing a soft-landing capability and then retrieving, refurbishing and reintegrating reusable rocket stages. A Single Stage To Orbit (SSTO) vehicle must carry everything all the way up, including the large rocket engines and propellant tanks which will be used only for the ascent. It must also carry into orbit all the propellant, heat shielding, parachutes, wings and so on, that it will need to return to Earth. All the extra weight that an SSTO has to take into space diminishes the amount of cargo, or payload, it can transport, which is the actual reason for the launch. Each 100 kg of empty tankage that an SSTO takes into orbit is at the cost of 100 kg of payload. Since it takes about 30 kg of propellant and rocket hardware to place 1 kg of payload into a low orbit, and the payload typically represents only 3.5% of the total weight that leaves the launch pad, the design of an SSTO can easily result in a vehicle with a zero payload capability if the tank mass is a bit

higher than expected or the rocket engine is slightly less performing. Such a launcher would only be able to put itself into orbit, if at all.