How do rockets work?

This seems to be a dumb question, but it isn’t because rocket science is more complex than it appears. Let me begin with a little story.

When I was a young pup, I tinkered in rocketry, something that wasn’t that popular at the time. Back then I was able to acquire the chemicals I needed from the drug store or even by sending away for them (Today that would be impossible). I mostly used a modified gunpowder mixture that consisted of Saltpeter (Potassium Nitrate), Flowers of Sulfur, charcoal (carbon) and Agar (a gelatin-like binder). I ground these separately in a mortar and pestle, mixed them together and added water to make a paste, which I carefully packed into a wound paper tube, one or two inches wide and a foot long. The paper tube was placed onto a wooden base that had a tapered metal rod that ran up through the center of the wound paper tube. This rod provided an inner tunnel or channel in the rocket propellant. This is so that the fuel burned from the inside out and didn’t blow out through the rocket tube. The rod also had a flared bottom section to create a rocket nozzle. I started by adding a water mix of clay and agar to form the rocket nozzle. The propellant mix was added on top and a clay/agar plug was added at the top. Addition of balsawood fins and a nose cone completed the rocket. I usually allowed the newly loaded rockets a week to dry and then I used commercial fuse to ignite the rocket, which I placed inside a metal launch tube. I did this in an abandoned strip mine. Most of the time the rockets went up and out of sight. Occasionally, I would mount a parachute on the top of the rocket to recover them.

Gunpowder was invented in 9th century China, and it soon was used to make rockets that were used in war and in fireworks. Most fireworks use a form of black powder (gunpowder) as a propellant. However, there are other solid fuel propellants that are more effective. One is a Zinc and Sulfur mix. This mixture is more powerful than black powder, but it’s dangerous.

Another variant of the solid fuel is a Potassium Nitrate and sugar mix.

Another propellant that is used not only by NASA, as in its shuttle boosters, and also by commercial rockets is the Ammonium Perchlorate Composite, which is 70% finely ground Ammonium Perchlorate (a fertilizer) mixed with fine Aluminum powder in a base of Polybutadiene acrylonitrile (a form of artificial rubber).

The main advantage of solid rocket fuel is that it can be stored and used on demand, but it has one major flaw: once lit there is no stopping it. The shuttle used two solid boosters and simply disconnected them when the shuttle had achieved the proper velocity.

A liquid fueled rocket is the kind that most of NASA’s rockets use. The Apollo moon rocket used LOX or liquid oxygen mixed with kerosene, and it made a very powerful rocket propellant.

Other liquid fuels use a 90 percent Hydrogen peroxide and Hydrazine mix that needs no ignition. This usually results in unintended explosions, a problem that the early space rockets suffered.

So, how does a rocket work? The answer goes back to Isaac Newton’s third law of motion: when one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body. In simple terms, action causes a reaction.

A good example is if one jumps out of a wagon, the wagon moves opposite to the direction of the jump. This law is responsible for the kick of gun or why a moving pool ball knocks another pool ball into motion.

In the case of a rocket, the lift off is caused by the rapid flow of hot gases out of the rocket nozzle. The maximum velocity that a rocket can attain is based on the rocket equation where the final velocity is equal to the exhaust velocity times the natural log of the total mass of the rocket and fuel diveded by the mass of the fuel. A typical rocket has a mass ration of 9 to 1, which means that most of the rocket’s mass is fuel.

Specific Impulse is a measure of how efficient the fuel is. It’s essentially the amount of force that results with the amount of propellant consumed per second. A propellant with a higher specific impulse is more efficient, but it’s the velocity of the exhaust gas that determines how fast a rocket can go. Obviously, a conventional rocket--either liquid or solid--can’t get near the speed of light. That will require an ion-thruster rocket.

Thanks for reading.