How Can Fins Enhance the Stability of a CO2-Powered Rocket?

[themanonthemo]

Hi everyone,

My high school class is doing a competition where we have to build a rocket out of a 2L pop bottle powered by compressed CO2 and launch it (not directly up but at an angle)

we need to...
-achieve maximum distance
-have minimimal horizontal deviation
-protect an egg that the rocket must contain

Does anyone have any experience with model rockets?

I have some ideas but I'm not sure which would be best

-crumple zone
-wings
-parachute for the egg
-jelly to hold the edgy


I'm looking for a way to really maximize the distance for the rocket but I'm stuck.

Any ideas?

 

[haruspex]

What angle do you think would maximise range?

 

[K^2]

1) Biggest factor is your gas to water ratio. This depends on how much pressure the bottles are subjected to. They can hold up to about 100psi. If they pump them that full, you want about 60% water. If it's closer to 30psi, it needs to be about half and half.

2) There is going to be enough mass from egg etc for stable flight, so you'll need to try to make everything as light as possible. Balsa is a fantastic material for these sort of builds. It's fairly "soft" as far as woods go, so it will be good for protecting egg from impacts as well. It's also pretty good material for fins, if you can find some very flat sheets of balsa. Some hobby/arts stores have them.

3) Don't try to make it glide. Focus on ballistic flight. Yes, glide can give you extra distance, but odds are, you'll mess up stability and the thing will end up flying a lot worse. Even I wouldn't try to make it into a glider.

4) With that in mind, you need very good fins. You want 4, because 4 fins is easiest to align straight. And this is definitely a measure twice cut once thing. These need to be as straight as you can possibly make them. For the geometry, no need to reinvent the wheel. Have you ever seen a V-2 rocket? That's how you want your fins to be. Very simple, kind of large, sticking out back beyond the nozzle. That last bit's important. That will give you extra stability early in the flight. (That works even better with actual rocket engines, but it'll help anyways.)

That should give you maximum range with minimum deviation. (Oh, and if you have a choice of the launch angle, go for a bit bellow 45°.)

You're on your own with the egg, because I have no idea.

P.S. With balsa, this thing will only fly once. If you need multiple flights, you better use plastic for fins.

 

[themanonthemo]

So my biggest concern at the moment is the egg, in terms of resisting impact force, I am adding a giant crumple zone as the nose of the rocket. Does anyone have any experience with protecting eggs from impact. I considered perhaps putting the egg in some sort of light weight jelly. Is that a good idea?

Another idea I had was a sort of cotton spring

the picture should be attached.

is this flawed? I'm sort of thinking that it wouldn't be effective because the egg's weight in the back would cause the rocket to dip backwards, and if it lands on its back then the egg will have no protection.

 

[A.T.]

Here the same question (And they won):
https://www.physicsforums.com/showthread.php?t=517010

You can cut off one half of a second bottle for the egg, fill with paper/styrofoam, and duct tape it to the front of your rocket:

As for the airfoils, two different glider configs are shown at the bottom. But K^2 might be right that pure ballistic flight with a small fin behind the nozzle is better.

 

[themanonthemo]

I am a little concerned about putting the egg on the front of the rocket to be honest.

My current questions are

1. Is it not a good idea to put the egg in a tube above the rocket that will use the whole rocket's length as a crumple zone? (My issue is that the added weight on the back will cause the rocket to dip backwards)

2. Should I scrap the idea of wings and focus on ballistic flight?

3. Will the 4 fins behind the nozzle actually stabilize the rocket completely? Or will the rocket still roll?

 

[themanonthemo]

Thankyou for all your help everyone

I am now interested on knowing the actual physics behind the tailfins. How to they actually stabilize the rocket?

 

[mfb]

If you put the egg above the rocket like in your image, your center of mass is not aligned with the thrust, and your rocket will fly some uncontrollable curve.
A second structure below could balance that, but it would increase the mass a lot and it would be hard to do it properly.

 

[themanonthemo]

What would be the optimal center of mass?

 

[mfb]

Close to the center of the bottle (in 2 dimensions) and therefore in front of the nozzle. Can be a bit different with stuff like wings, but probably not much.

 

[themanonthemo]

Great, now I am considering a parachute, it will be pretty easy to make and I was thinking to attach a string to the rocket and to the ground so when the rocket reaches a certain distance, the string will get taught, which would pull a hatch that will release the parachute.

Does it sound do complicated to time correctly?

 

[themanonthemo]

And also, what will prevent the rocket from rolling, I intend to put a little compartment on the body of the rocket and I would like it to stay on top during flight. Is that realistic?

 

[Skeetss]

Are you able to have the egg leave the rocket and still have it count?

 

[drakken1985]

To your question are there any other rules we have to know about...like does the egg have to remain with the ship or can you have a sperate modual?

Still you have two options. Option A: follow the KISS strategy (keep it simple stupid) or Option B: Internship at NASA

For my part I would pick option B. First having working knowledge of military rockets your first concern is to maintain a stable ballistic flight. Keeping fins light but strong enough to absorb the shock of lift off will be the key. If one where to take a coat hanger and bend it into a DELT shape...will need 3 fins...with Mylar foil or other light wieght material one can cover the open area of the DELTA.

If the Rules allow, I would opt for a separate modual that detacts at the height of its arc. Depending on how much access one has to material, I'd make a cage of plastic straws that can "telescope" inside one another. The idea is to absorb the shock of landing. Using a collapsable cage and a parrachute the egg hopefully will survive

 

[GT1]

4) With that in mind, you need very good fins. You want 4, because 4 fins is easiest to align straight. And this is definitely a measure twice cut once thing. These need to be as straight as you can possibly make them. For the geometry, no need to reinvent the wheel. Have you ever seen a V-2 rocket? That's how you want your fins to be. Very simple, kind of large, sticking out back beyond the nozzle. That last bit's important. That will give you extra stability early in the flight. (That works even better with actual rocket engines, but it'll help anyways.)
.

How do fins stabilize the rocket's flight?