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SOCoach wrote:I am trying to help my kids prepare for bottle rocket in a few weeks - we have built and tested many rockets but our best time is right around 10 seconds.

We have used plastic flourecent light tubes for the nosecone . . approximately 3 feet . . 450 ml of water, 3 fins, 1 L bottle and the weight of the rocket ends up right around 120-130 grams. Our rockets tend to go fairly high, then fall backwards (straight down, the reverse of how they went up) for about half the height, then level out and backslide the rest of the way.

Are the teams that are getting 15+ seconds using paper nosecones? My kids are getting frustrated and I want to be able to help them but I am running out of ideas.

Maybe you're using too much water - we use about 200-250 ml of water for our 1-liter bottles. When you launch, look and see it the water comes almost glugging out, as opposed to in a spray - that can indicate if you're using too much water, which can weigh the rocket down on the way up. Center of balance and fin size might also be an issue, so I'd also check that. There should be some tips on that further down the posts.
Hope this helps, good luck!

Okay, I've been coaching 17 years and never seen this . . . .

Our team tested their rocket today. It weighed 119 grams, 105 cm long, we used 375 ml of water, 4 fins and I even had the kids determine the CLA, CG, and CP. According to their calculations there is about 20 cm between the CLA, CG, and CP (although the CP is nearly at the end of the bottle.)

1st flight, went high leveled out and backslid beautifully (we only tested with 250 ml of water on that flight) 11 seconds.

2nd flight (350 ml of water) it went up very high and then straight down, but not nose first . . . . bottle end first. Kind of like someone hit the rewind button . . . straight up, straight down.

3rd flight exact same thing as the second flight . . .

We tried adding clay to the front (very little) and it nose dived. Took the clay off . . . . same as before, straight up . . straight down.

Are our measurements off? (CP and CG) are they that close together that the smallest amount of clay overcorrects it?

SOCoach wrote:Our team tested their rocket today. It weighed 119 grams, 105 cm long, we used 375 ml of water, 4 fins and I even had the kids determine the CLA, CG, and CP. According to their calculations there is about 20 cm between the CLA, CG, and CP (although the CP is nearly at the end of the bottle.)

If the CP is at the end of the bottle, and if all 3 points are within 20 cm of each other, then that means all of the points are located near the nose of the rocket. If they are all located near the same area, the rocket will be unstable. I've heard that having the CG close to the tip of the rocket is the best, but IDK for sure. Another thing I'm not too sure about is that if the rocket is nosediving, then that means you're on your way to having a good backslider.

Play around with the locations of the CG, CP, and CLA, and use a simulator to predict the flight of your rocket.

Here are some things that will change the CP/G/LA:

1) Add mass to the front (moves CG forward)
2) Decrease mass in the back (moves CG forward)
3) Increase fin size (moves CP & CLA backward)
4) Increase number of fins (moves CP backward with little effect on CLA)
5) Move fins further back (moves CP and CLA backward)
6) Lengthen rocket (moves CP back in relationship to CLA)
7) Check Body Alignment (Cone, Extension, Pressure Vessel)

Thanks!

If their calculations are correct . . . the 3 (CLA, CG and CP) are fairly spread out which is why I can't figure out why adding so little clay to the front caused it to nosedive. We only added at most 2 grams of clay to the front . . . I guess we will go with 1 gram and see what happens.

I'd just never seen a rocket act so much like a pogo stick . . . straight up and down.

How are you doing the calculations. I have tried to download one and put in the numbers and it didn't work. I am using a cone shaped rocket. Do you need to figure in the shaping at the end near the nozzle. Can any one recommend a good simulation site? Any help would be appreciated.

ls20817 wrote:How are you doing the calculations. I have tried to download one and put in the numbers and it didn't work. I am using a cone shaped rocket. Do you need to figure in the shaping at the end near the nozzle. Can any one recommend a good simulation site? Any help would be appreciated.

We used programs from this website http://wrockets.trib-design.com/index.p ... e=programs

Hey guys do you know any general times that people are consistantly getting? States for New York is less than 12 school days away and I want to know about how bad I would fail if I took my current rockets to states

JKrafsur wrote:Shriek!! 1L bottles, guess the times will plummet this year, and im gonna have to cut the weight balance system in half. Still no parachutes?

notnecessarily. Times could go higher due to less mass, it could be effected by thermals or backslide better. The hight MAY drop, but not gauranteed.

rocketman55 wrote:Hey guys do you know any general times that people are consistantly getting? States for New York is less than 12 school days away and I want to know about how bad I would fail if I took my current rockets to states

we have been getting between 15-20 seconds several times 25 or even 30 seconds but that is just us.

rocketman42 wrote:

rocketman55 wrote:Hey guys do you know any general times that people are consistantly getting? States for New York is less than 12 school days away and I want to know about how bad I would fail if I took my current rockets to states

we have been getting between 15-20 seconds several times 25 or even 30 seconds but that is just us.

lol, rocketman42 is quoting rocketman55