Astronautics: The Space Shuttle Launch System

Understanding Astronautics: The Space Shuttle Launch System

Rocket engines work by throwing hot gas backward at very high speed. The gas pushes the vehicle forward through Newton's third law of motion. The shuttle main engines used liquid hydrogen because it gives a very high exhaust speed when burned.

Hydrogen takes up a great deal of space, however, because it is not dense. This is one reason the fuel tank had to be so large. Liquid oxygen was carried because a rocket cannot depend on oxygen from the atmosphere.

It must keep burning even after reaching the thin upper air and space. During ascent, engineers watched maximum dynamic pressure. This is the period when the fast moving vehicle experiences its strongest aerodynamic forces.

Getting to orbit is mainly a problem of gaining sideways speed, not simply reaching a great height. After the first part of flight, the shuttle gradually turned from a near vertical path toward a mostly horizontal one. This planned turn is called a gravity turn.

It reduces the energy wasted fighting gravity. The engines had to keep accelerating the vehicle while its fuel supply became smaller. Dropping empty hardware helps because every kilogram left behind is mass that no longer needs to be accelerated.

The large tank was released only after it had done its job. It was not built for recovery because adding wings, engines, or heat protection would have made it much heavier.

The shuttle had an unusual shape for a launch vehicle. The orbiter was attached to the side of the tank instead of sitting on top of it. This arrangement created difficult airflow, vibration, and balance problems.

Engineers had to make sure the exhaust plumes, vehicle structure, and guidance system remained stable throughout flight. The main engines could change their thrust during ascent.

Throttling helped limit loads when the air was thickest and helped place the orbiter on the required path. Small errors in speed or direction could lead to an orbit that was too low, too high, or tilted at the wrong angle.

Returning from orbit created a very different challenge. An orbiting spacecraft travels far faster than a passenger jet. It must lose most of that speed by pushing through the atmosphere.

Friction and compressed air heat the outside surfaces to extreme temperatures. The orbiter used many heat protection tiles, each designed for a particular location. The tiles were light and effective, yet they could be damaged easily.

This made inspection important before launch and during missions. The Challenger accident showed another lesson about complex systems.

A joint seal in a booster became less reliable in cold weather, and a small failure led to a catastrophic result. Students should notice that spacecraft safety depends on materials, weather, measurements, procedures, and clear decisions as much as on powerful engines.