Bottle Rocket B

[UnknownyMous]

What if I just cut off the top and taped a ping-pong ball on top? You know, so that the nose cone would not be pointed?

[chalker]

What if I just cut off the top and taped a ping-pong ball on top? You know, so that the nose cone would not be pointed?

That would probably be a good start to changes to the design

[UnknownyMous]

Thanks for the help Mr. Chalker

[UnknownyMous]

okay, so I brought my 1L bottle rocket to my school so that me and my partner could finish building it and one of my friends commented, "the fins should be bigger so that the rocket would go straight up" . I was just wondering, is this true?

[andrewwski]

Without knowing their size right now, it's impossible to even make a haphazard guess.

Fins can add to the stability of the rocket my moving the center of pressure toward the aft, but once the rocket is stable, increasing the fin size is just adding drag.

Considering the "backslider" design is only marginally stable in the first place, and only requires the CG to be ahead of the CP but behind the center of lateral area, making the fins too large could be detrimental to an unstable recovery ("backsliding").

[davidkim2106]

So, 1 liter bottles this year. I wonder if a design in the plastic of the bottle would give an advantage over a bottle with no design at all.

[davidkim2106]

Did anybody start launching yet? What kind of times are you getting?

I honestly don't think anyone has tested yet, and if they have it's most likely a crude prototype (i.e. not something they would bring to a competition).

We actually tried to launch a couple rockets my son and I made with the Aquaport launcher we recently got (thanks to the advice of this forum). As expected, it was our first attempt to make and launch rockets so we didn't have great success - but it was fun and got my son and his friend / partner excited about it. He's already made a better one that we'll try to launch this weekend.

Our rockets didn't go high and our times were only about 5 seconds, but they did at least backslide!

My bottle rocket got 5 seconds too

[Lady Epidemic]

If you're looking for a time that is competetive at the state or national level, my guess would be about 19-22 seconds. 23 seconds a rocket got us first at Solon, so anything above 15 is probably good. However, that was my sister's and another teammates rocket. Mine went up, tipped nicely at the apex, and continued tipping right into the ground. Apperently, 2 grams is too much weight at the top of a cone. =( Doing testing right now to determine what weight is right, because weight defenitely helps if you have a longer cone like ours.
Referencing an above post, a cut-off ping-pong ball works. We used it all last year. It's light and relatively aerodynamic. And, if you have a not-very-sturdy cone, cutting a small hole in the top can strengthen the cone by allowing the air to rush in, almost inflating it. Despite how it sounds, it doesn't actually subtract that much time through drag.

[jma]

Without knowing their size right now, it's impossible to even make a haphazard guess.

Fins can add to the stability of the rocket my moving the center of pressure toward the aft, but once the rocket is stable, increasing the fin size is just adding drag.

Considering the "backslider" design is only marginally stable in the first place, and only requires the CG to be ahead of the CP but behind the center of lateral area, making the fins too large could be detrimental to an unstable recovery ("backsliding").

Does moving the fins toward the nose make the rocket unstable? Or will the rocket fly okay as long as the CP is ahead of the center of gravity?

[A-Ro]

Does moving the fins toward the nose make the rocket unstable? Or will the rocket fly okay as long as the CP is ahead of the center of gravity?

First of all, the CP should definitely stay BEHIND the center of gravity. Otherwise, the rocket will not be stable at all and flip over almost immediately after launch. What you want ahead of the center of gravity is the CLA (center of lateral area) to make the rocket backslide without nosediving. So as long as your fins are in a position that keep the CP below the CG and the CG below the CLA, it should work, but only one position will work best for your rocket.