It's Just Rocket Science - Part 2

A further examination of the basic principles of rocketry

Jul 10, 2006

Last week we looked at some of the basic principles of rocketry, now we are going to examine the more complex issues of rocket flight.

If the rocket is pointed at a 45 degree angle, it will travel the furthest distance horizontally, barring obstruction. This is because it balances vertical lift with horizontal distance. The height gives the rocket time to travel further than it would if it were aimed in a shallower trajectory. You can test this principle yourself by throwing a ball into the air in different directions at about the same velocity. Some rockets can be steered in flight by redirecting the flow of fuel exhaust.

In the simplest of cases, one only needs to know the distance to the targeted landing area to plan a trajectory. If the horizontal distance is known, then an efficient vertical distance can be found. This means that the initial energy needed can be calculated, and the required amount of fuel determined. This does get a bit complicated because there are other practical factors to be considered. Weather for instance, is a major factor. It takes more energy to counteract winds that might throw the rocket off course. Even if most of the rocket's path is above the atmosphere, it will still have to pass through a significant layer of weather. In many cases shuttle launches have been delayed to avoid severe Floridian weather.

Another factor that makes a rocket trajectory more complex is that as the rocket burns fuel, it looses mass. The rocket gets lighter and lighter as it travels, and the force required to push the rocket diminishes. If fuel is burned at a constant rate, the parabola shape of the trajectory will flatten out a bit near to the end of the trajectory. This can be compensated by varying the rate of fuel burn over time.

In the 1980s, the Reagan administration proposed a "Star Wars" program, that would shield the United States from missile attack by shooting down incoming missiles with other missiles. The greatest difficulty with this tactic is that missiles are fairly small targets, and even if the trajectory is known in advance, it is difficult to aim another missile to meet and hit it high in the atmosphere, while trying to compensate for highly variable factors such as wind. So far, the program has had very little success in testing, but it may have some value as a deterrent nonetheless.

The copyright of the article It's Just Rocket Science - Part 2 in Physics is owned by Katharine M. J. Osborne. Permission to republish It's Just Rocket Science - Part 2 in print or online must be granted by the author in writing.