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Rocket Propulsion Basics
Rocket motors and jet engines are two sides of the same coin; both combust fuel with oxygen to create a jet of high-velocity gas that pushes the vehicle forward. The primary difference is that jet engines bring oxygen in from outside and rocket engines carry their own oxygen supply. Pure liquid oxygen is generally too unstable to use in any kind of engine, which is why rocket engines typically use some sort of chemical oxidizer that carries a few loosely attached oxygen atoms that can easily break free to burn the fuel. These chemicals come in both liquid and solid or crystalline forms.
Basic Construction
Most model rocket engines use a solid fuel and solid propellant mixed together in a sort of dough and cooked or dehydrated to a chalk-like consistency. Simplicity is the rocket motor's primary virtue; it has no moving parts, only an outer case and fuel. The rocket motor starts as a cardboard outer case. Manufacturers set the case on end -- with the bottom facing up -- and drop in a small explosive charge. On top of the explosive charge they pour a slow-burning delay fuel. Then comes a thick layer of solid fuel-oxidizer mixture, and finally a hard plug with a hole in the middle.
Ignition and Burn
In terms of function, a rocket motor works like a bullet that carries its own charge. Launching a rocket is fairly simple; the user inserts a pair of electrical leads into the hole in the bottom plug and into the fuel-oxidizer charge and a plug holds the leads in place. An electrical charge causes a spark to jump the leads -- think of a taser -- to ignite the charge. The charge combusts and the hole in the plug turns into a nozzle to increase the gas velocity and then diffuse it to push the rocket upward. The rocket continues to accelerate until all of the fuel-oxidizer burns out.
Delay and Recovery
Just before the last of the fuel burns away, it ignites a slow-burning delay fuel that acts like a fuse. The longer the delay, the more time the rocket will coast before triggering the recovery charge. Once the fuse burns out, the rocket's explosive ejection charge triggers and blows the nose-cone off of the rocket. Once the nose-cone pops off, a parachute deploys and the rocket body floats gently earthward and into a pond.
Rocket Coding System -- Impulse Thrust
On the side of the rocket you'll find an alphanumeric code that reads something like "C6-3" or "A4-5." All rocket fuels aren't created equally; some burn very powerfully but burn out quickly, while others provide less maximum thrust but last longer. The first letter in the sequence indicates the rocket's impulse power, or its maximum output. The letter correspond to a rocket class, and tells you its maximum output in newton-seconds or foot-pounds-per-second. For instance, an "A" class rocket puts out 0.29 to 0.56 foot-pounds-per-second, a "C" class put out 1.13 to 2.24 foot-pounds-per-second and the largest "O" class rockets put out 4,602 to 9,204 foot-pounds-per-second.
Rocket Coding System -- Average Thrust and Delay
Absolute thrust isn't everything. If the motor burns out too quickly, your rocket will accelerate like a rocket, but will run out of juice too quickly to reach any real altitude. The second number -- the "6" in C6-3 or the "4" and A4-5 -- tell you the rocket's average thrust in newton seconds. An A10-5 and a D10-4 have the same average thrust, but the D-class offers more absolute thrust power.
Recommendations
Generally, a higher average thrust is better for heavier rockets that take a bit more time to get up to speed. Lower numbers are better for lighter rockets, which get up to speed quickly and then stop accelerating. The number after the dash indicates the delay in seconds between fuel burn-out and ejection charge ignition. This is the rocket's coasting time. You'll want a higher coasting time for heavier rockets that maintain a certain amount of inertia after the burn stops, and a lower coasting time for lighter ones that will come to a screeching halt once the fuel burns out. There's really no such thing as too much impulse power; that's mostly a matter of how much motor you can physically fit into the rocket body.