Model rockets all work in essentially the same way. Occasionally things are done a little differently, but here I’ll describe how a typical model rocket works.
A model rocket is typically made from cardboard, wood and lightweight plastic: the idea being that if it hits something the rocket will take most of the damage, not the thing it hit. Usually this is a successful strategy.
A model rocket is powered by a commercially made, tested and certified motor that usually uses either black powder or ammonium perchlorate as the propellant. Not fuel: propellant. Like “real” rockets, model rocket motors contain both fuel (the stuff that burns) and oxidizer (provides oxygen or oxygen-like qualities to support combustion). Rocket motors don’t require air to work: you can (and people have) launch a model rocket from under water and it’ll ignite and lift off just fine.
Also like “real” rockets, model rocket motors work by propelling mass, in the form of gasses produced by combustion, through an expanding nozzle at very high velocity, producing force (remember from physics: force is mass times acceleration). At that point, Newton’s Third Law takes over: every action creates an equal and opposite reaction. The action of hurling hot gasses out the back of the rocket at roughly 4000-6000 mph produces the reaction of propelling the rocket forward.
Model rockets are guided in the first fraction of a second of flight by a rod or rail: something that ensures the rocket can fly straight until it reaches a velocity (typically around 30 mph) where its static stability can keep it on track. What’s static stability? I’ll go into more detail another day, but it’s essentially the ability of the fins to counteract forces like gusts of wind, weight imbalances, even insect impacts, and keep the rocket flying straight without requiring any of the rocket’s parts to actually move.
The motor burns briefly — the “whoosh” part of the flight: usually not for more than a few seconds and often much less — and most of the rocket’s upward flight is propelled by its own momentum. Eventually air resistance and gravity slow the rocket’s upward flight, it reaches apogee, tips and begins its return to earth. Around that time, a small charge in the motor explodes (pop!), blowing the nose cone off the rocket and deploying a parachute or streamer. The rocket descends slowly enough to avoid serious damage when it reaches the ground, is recovered and can be immediately flown again.
That’s the basic idea and that’s how the majority of model rocket flights go.
But as simple as it is, there are details and nuances. For big enough, fast enough, or high-flying enough rockets, things can get … involved.
There’s also the very real possibility that even the simple things won’t go quite right. Sometimes there’s nothing the rocketeer can do: a defective motor, for example, is likely to destroy a rocket not matter how well it’s built or how carefully the rocketeer has planned for contingencies. Other times, the rocketeer has more control and can make decisions while building the rocket or prepping it for flight that improve the odds of it surviving to fly again.
There’s always risk with a launch: every time you launch a rocket, you take the chance that you may not be able to recover it, or it may simply be destroyed in flight. For me, much of the challenge is in mitigating that risk.
A note about words: Even though the term “model rockets” rubs me the wrong way — they’re not just static models: they’re actually rockets — it’s the preferred term because, to be honest, it settles peoples’ nerves. “Amateur rockets” or just “rockets” stirs up cultural memories of the “basement bombers” of the 1950s and 60s: both kids and adults who would try to build their own rockets and engines in their basement or garage and end up badly wounded or worse when touchy propellants accidentally ignited or all-metal rockets exploded at launch. Model rocketry, thanks to its outstanding safety record, enjoys a considerable amount of forbearance from government at many levels, and most of its enthusiasts understand the long-term importance of sustaining both the reality and appearance of safety. So they’re model rockets. Even if they sometimes go faster than the speed of sound and higher than commercial airliners.
Also, some people say model rocket “motors” and some say “engines”. I prefer “motors” because they have no moving parts. I admit “engines” sounds sexier and don’t mind folks using that term as well.