January 27, 2021

I want to build a mini rocket without a step-by-step tutorial.what content do you recommend me?

There are 3 important things to ensure when designing a mini-rocket

* Correct propulsion

* Stability

* Correct timing for parachute opening.

In my country (I don’t know in yours) people are not supposed to handle explosive. Especially young people (which are the ones more likely to play with minirockets). So my government forbid selling of minirocket engine… and in return, provide them for free themselves. But it requires then a validation by official before you fire the rocket (I once was in charge of giving the “go” stamps on such rockets).

So, for that reason, usually, you start by choosing the engine. And the rest of the design is done accordingly (those government issued engine are of 3 classes, for normal minirocket, bigger ones, and multiple stages ones).

(Note that I am not referring to micro-rockets here. Those whose dimension are around 20 cm, and with thumb-sized engines. Those you can buy from any modeling stores)

If you are American, I guess your situation is different and you can buy rocket engines at Walmart (at least, it would be strange, imho, to let people buy flamethrower and ar15, but impose a rocket engine control). Yet, it is still a good idea to start with choosing a popular, not to powerful at first, engine. And design from there. So that is just a matter of choosing on a catalog

One you know the engine, you need to design the shape and mass distribution of the rocket.

That is where design really starts.

For that you need to understand stability issues. The main driver for stability are the fins.

The rocket will have, like any object you throw, a tendency to spin around its center of gravity. Without any fin, that would be what occur.

If you have fins at the bottom, then the air flow would push on them, and put the rocket back in a straight position (imagine a rocket that you handle from its center of gravity. If it rotates a little bit, the push of air on the fins tend to compensate that rotation).

If those fins are too small, it won’t be enough to compensate the spinning and the rocket will be unstable, and will have the elegant trajectory of a deflating balloon.

If those fins are too big, then, it would compensate too much, and a spin on the opposite direction will be created, which, in turn will be compensated too much. So you will end up with amplified oscillations. And your rocket will have a sinuosidal trajectory (that could end up with a u-turn. I have already seen officials forced to get on the ground to avoid a rocket because of that! And it was harder to get launch authorizations after that…)

In between, each small variations is compensated by the push on the fins, and the rocket have a smooth, straight line, trajectory.

So, area of the fins, but also their shapes (it is a rotation momentum issue. So 1 square inch of fin near the axis is not the same than 1 square inch far from it), and positions (it has to be below the center of gravity, or else it works on the wrong direction, exagerating spin instead of damping it. How far below impact the coefficient).

So, that is probably what you need to read about.

“Rocket stability”

I don’t provide links, because yahoo bug half of the times I do. But just search that in google. There are at least two NASA page, “Conditions for rocket stability” and “rocket stability” and one wikihow page “how to calculate stability of model rocket” about just what you need to know.

All are quite simplistic. (They don’t really deal with all possible fin shape, which involve integral calculus. But for that you need to read courses on rotation momentum and things like that)

The 3rd point I’ve mentioned is the parachute.

You need to know when the rocket will reach its peak.

Because the parachute must be open at that moment (it is the moment when speed is slowest. And you don’t want to open the parachute at high speed. Besides, it is not like the flight would last minutes. So, a few second after the peak, even it the parachute resist being opened that fast, it is already too late to slow down the fall before impact. And obviously, before is not a good idea neither).

That is just quite easy ballistic computation. The engine you have chosen provides a certain amount of energy (kinetics energy that is then turn into potential energy). Your rocket has a certain weight. If it is fast, you should add a certain friction.

All together, you know at what height, and more importantly when, your rocket will start to go back to the ground.

And design what must be for the parachute to open at that moment.

For that, there is no reading. Just your imagination.

Usually you use a string, attached to the ground, that will be ripped at take of, and that ripping will start a countdown.

Some people use a small electric motor to slowly unscrew the chute door. Some other use chemicals.

In my case (but that was 25 years ago, and at that time, it was quite a new thing. I guess nowadays it is the easiest way) I used a counter (made of 74LS compenents and a small memory. But nowadays an arduino or microcontroller would do) to ignite pyrotechnic fasteners.

For a second rocket I used other electronics to measure acceleration, and fire the fastener at 0g (in a rocket, once the engine is stopped, you are in a parabolic flight. So it is almost 0 gravity anyway. But because of friction there is a small reverse apparent gravity first, then a small normal one, and in between real 0g. That was when I fired the chute)

That is the fun part. So use your imagination.

The tricky part, of course, is that everything is connected.

The system you will design to fire the chute impact the mass and the mass distribution.

Those impact both fin design, and the timings.

The fins have weight of their own.

And the timing impact the chute firing system.


Note: I assume you have already launched a microrocket. If not you should start with that!

You don’t want to discover the stability importance with a mini rocket. (micro rocket = arm sized rocket with a thumb sized engine. mini rocket = human sized rocket, with a campbell soup can sized engine)

The stability computation rules are the same

But the parachute opening is simpler. Because you don’t have the thrust to carry anything fancy anyway. And because toy engines usually provide all you need. Its an engine, when burning, provide first a downward outgoing ejection of gas (so thrust), then is silently burning for a few seconds, then explode at the top. So if your stuff your socket with a parachute, and the top cap is amovible, your rocket won’t explode when the engine does: instead the top cap will pop and the parachute will be ejected.

But that means that both timing and thrust are fixed by the engine.

All you have to do is to design a rocket with the correct mass fitting this timing (it is printed on the engine blister) and compute stability, with the documentation I pointed.

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