Driving the Rocket

by Jeanette Cain

Making, combining, and supplying propellant to the rockets has developed to a point of physicists and scientists having less fear of a rocket becoming a potential fireball or veering off course. Propellant is the container for fuel and an oxidant. An oxidant fuel needs to be able to burn while releasing energy. Today's shuttle allows for astronauts having a smoother ride when send into orbit, compared to yesterday's astronauts. Maximum acceleration is three times that of gravity, which is 3 g. It happens briefly, before the boosters fall, and occurs for 5 minutes before separation of the orbiter and external tank.

The Space Shuttle has an external fuel tank which is separated from the orbiter and begins falling back to the Earth after separation. The solid-fuel boosters will burn for a little over 2 minutes, which burn cannot be stopped once ignition has been made. Booster thrust may be reduced quickly by the venting of exhaust gases that are located through holes in the side. With special attachments, an orbiter holds to the boosters and external tank. It also has smaller liquid-fuel engines which help maneuver the orbiter into space and returns the orbiter to Earth. Scientists control the releasing of exhaust gases by the shape of the nozzle, which provides efficient action of the rocket. The Shuttle gives off an almost invisible trail of water vapor from the main engine. One of the most critical aspects for the orbiter is to survive the heavy vibrations given off by the booster.

About 90% of the weight of the Space Shuttle, at lift, off is propellant-solid and liquid. The liquid oxygen and liquid hydrogen needed for combustion are carried separately by the external tank. Each of the 3 main engines receive approximately 470 kg of propellant every second. Boosters on either side of the orbiter hold the solid fuel, weighing 91 tons and holding 556 tons of propellant.

Pellets containing an oxidant and a fuel are used as propellant in solid-fuel rockets. They also contain substances which help prevent decomposition when in storage. Energy is released according to the way in which the propellant is packed into the casing. An even thrust is provided when it is packed so that the surface will burn at a constant rate, or neutral burn. When the pellets have been packed so that the surface area, where burning occurs, increases gradually, then the thrust will increase gradually, or progressive burn. When the thrust decreases gradually, or regressive burn, the surface area has been decreased.

Liquid rocket fuel has a life of its own. Liquid oxygen reaches boiling point at -183 degrees Centigrade. This cold temperature will cause rubber to shatter and metal to crack. Liquid hydrogen reaches boiling point at -253 degrees Centigrade, and although both substances make excellent propellant, the low temperature makes them more difficult to handle. Liquid oxygen is needed to burn fuel; liquid hydrogen is stored separately, before both combine and burn in the combustion chamber.

Specific impulse refers to the propellant's efficiency. It is the time needed for 1 kg of propellant to deliver 1 kg of thrust. A propellant of 1 kg that has a specific impulse of 262 seconds may produce 1 kg of thrust for 262 seconds (like that within space shuttle's solid rocket boosters.). The mix will be more effective with a higher impulse. Solid fuels have lower specific impulses; liquid propellants have higher specific impulses.

Sources:
1. Editors. Secrets of the Universe. International Master Publishing: US. 1999

2. Editors. The World Book Encyclopedia. World Book-Childcraft International, Inc: Chicago. 1990

3. Couper, Heather and Nigel Henbest. Space Encyclopedia. DK Publishing Book: NY, 1999