# Thread: Some help for my water rocket

1. I am going to compete with a small water rocket competition.
Technically,its for the second graders and beyond,but i am a first grader and do not get aerodynamics and those things yet.
The rules are:
1.max length:76 cm
2.the radius of the nose has to be minimum 1 cm.
3.Water+pressurised air are the only allowed "fuells".
4. Pressure may not be greater than 80 psi

i have a few questions.
1. Where is the best center of gravity?
Right in the middle?
Below the middle?
Or a bit higher than the middle?

2.i have read somewere that the "holes" in a golfball reduce air friction. If i put these 'holes' in the nose,would i see any positive result?

3. I plan to use a 2 liter botle, and the thing wich is used to close wine botles ,and a basketball pump.

Like

. . . .\. _____. ./
. . . . .||. .| ...||
. . . . .||__|__||
. . . . . . . |

my idea is that if it got enough pressure,the th
ing wich you use to close wine bottles will shoot out the bottle,squirting the water out to shoot the rocket.

Is this a good way?

And what would be the right amount of water in it?

Later i will draw the plan on paper and post it here to make my plan clear.
Btw,my english is not that exelent...

Thanx for any help,crazy g

2. Originally Posted by Crazy_g
I am going to compete with a small water rocket competition.
And good luck to you. Really, you should not trust me for anything trust related but:

1: below the middle. Keep in mind that the water emptying out the rocket will contently change the center of gravity (toward the bottom).

2: aerodynamics is somewhat unpredictable. So turbulences will form, even on lean surface. So adding a few 'light' gutter/hole on the body will help, especially to reduce drag on escaping edges, but will likely be dependent on the speed of the rocket (you could test different setting by dropping it from a virtual Pisa tower)

3: A 2 litter bottle will be to wide in my perspective. The rocket will have to retain some water in it to store enough inertia to go as far as possible. I suggest puncturing holes in the pipe exhaust before all water has exited ... and not put too much water inside.

Have fun !

3. Originally Posted by Crazy_g
I am going to compete with a small water rocket competition.
Technically,its for the second graders and beyond,but i am a first grader and do not get aerodynamics and those things yet.
The rules are:
1.max length:76 cm
2.the radius of the nose has to be minimum 1 cm.
3.Water+pressurised air are the only allowed "fuells".
4. Pressure may not be greater than 80 psi

i have a few questions.
1. Where is the best center of gravity?
Right in the middle?
Below the middle?
Or a bit higher than the middle?

2.i have read somewere that the "holes" in a golfball reduce air friction. If i put these 'holes' in the nose,would i see any positive result?

3. I plan to use a 2 liter botle, and the thing wich is used to close wine botles ,and a basketball pump.

Like

. . . .\. _____. ./
. . . . .||. .| ...||
. . . . .||__|__||
. . . . . . . |

my idea is that if it got enough pressure,the th
ing wich you use to close wine bottles will shoot out the bottle,squirting the water out to shoot the rocket.

Is this a good way?

And what would be the right amount of water in it?

Later i will draw the plan on paper and post it here to make my plan clear.
Btw,my english is not that exelent...

Thanx for any help,crazy g
You need to have the center of gravity ahead of the center of pressure to maintain aerodynamic stability. This is most easily done by having large fins to assure that the center of pressure is well aft. Once that is done the location of the center of gravity is immaterial, but to make the task easiest it is best to start with the center of gravity well forward, as it will move aft as water is expelled. A geometry that tapers from the nose toward the tail will help (this is ot what you get with a 2-liter bottle.)

For a blunt object (and you do have a blunt object) at moderate velocity the addition of dimples separates the flow from the skin of the body a bit to allow the flow to converge a bit more quickly downstream and results in somewhat lower drag than would a smooth skin. You would need to put the dinples nose that the ogive (the curved area in back of the nose) to be useful. It will also helpif the body tapers from the nose toward the tail -- and again 2-liter bottles do not do that.

You will find that there is a trade-off between water volume and air volume that will govern performance. You will have to experiment a bit to find the best mix for your design. The more air that you have the greater will be the average velocity of the expelled water, and hence the more efficient will be your rocket. On the other hand you will get more thrust from the expulsion of water than of air, and so you cannot get by with only air or with only a little bit of water.

You did not the basis for judging the contest. Is it to be maximum altitude ? Something else ?

You will need to consider the size of the exit hole. That will affect the rate at which water is expelled and hence the maximum attainable altitude. Again you will have to do some experimenting. A large hole will allow the water to be expelled very quickly. If it were not for air resistance that would result in the maximum altitude, since you get all of the thrust very quickly resulting in high initial velocity and you do not have to carry much water upward. But that also results in the maximum degradation due to air resistance. On the other hand a very small hole results in thrust being extended over a long time, and lower air resistance, but also much greater velocity loss due to the effect of gravity on the water that you carry to higher altitude. In the extreme case a tiny hole will result in thrust that is not sufficient to overcome gravity at all and the rocket will fall rather than rise.

Finally, if you are allowed to use air pressure as high as 80 psi you might want to consider some sort of converging-diverging nozzle to squeeze the last little bit of thrust out of the compressed air after all of the water has been expelled.

BTW you seem very thoughtful for a first grader. I am impressed. EDIT: In light of the subsequent conversation perhaps the reader should note that in the U.S. a first grader is usually six years old.

4. Thank you for your tips.
The contest is about which rocket gets the highest altitude when shot at 90 degrees.
I tried yesterday with different amounts of water,and i have found that 590~620 ml water gives the best altitude(i shot them with an angle of 45 degrees and watched wich one goes further in the sporthall)
today im going to test different shapes for the wings,and the size,and if adding holes to the nose would do anything.
I am also trying to make a new stopper,because the stopper shoots out around 38 psi,and thats only half of my goal.

I do not know the name of my school in english,but i am on the school thats before you go to the university.

5. @boring300
the suggestion of adding a pipe with small holes works...
I notice that i get less wet when it launches.
I forgot to test the difference,but i doubt it will be something negative

6. I made a guiding system of a 3/4 inch pvc pipe,and thickened the underside to function as a stopper with 2 component glue and had to sandpaper it to the right diameter.
This stabilizes the rocket at launch a bit and ensures less water is wasted in the first 50 cm of its path.
At 47~50 psi it starts to slide up(the rocket) so i have to hold it with my hands a bit.
I have to design a clamping system and make the thickened base a bit longer now.
I need to see what the correct amount of water is,because i tested the correct amount of water when there were no fins or nose. I added some sort of parabolic nose of packing foam and found that the best shape of the fins are

. . . .____
. . / . .. /
. /. . . /
/____/

dunno how the shape in english is called...

My rocket is becomming better and better now...

BTW, i read on a page that the center of gravity needs to be higher than the center of pressure
can someone here explain what the center of pressure is and how to calculate it for my rocket?

7. Originally Posted by Crazy_g
I made a guiding system of a 3/4 inch pvc pipe,and thickened the underside to function as a stopper with 2 component glue and had to sandpaper it to the right diameter.
This stabilizes the rocket at launch a bit and ensures less water is wasted in the first 50 cm of its path.
At 47~50 psi it starts to slide up(the rocket) so i have to hold it with my hands a bit.
I have to design a clamping system and make the thickened base a bit longer now.
I need to see what the correct amount of water is,because i tested the correct amount of water when there were no fins or nose. I added some sort of parabolic nose of packing foam and found that the best shape of the fins are

. . . .____
. . / . .. /
. /. . . /
/____/

dunno how the shape in english is called...

My rocket is becomming better and better now...

BTW, i read on a page that the center of gravity needs to be higher than the center of pressure
can someone here explain what the center of pressure is and how to calculate it for my rocket?
The center of pressure is the center of mass of a cross-section of your rocket with the assumption that the mass density is constant over the cross-section.

The whole purpose of the fins is to move the center of pressure aft to assure that the center of pressure is aft of the center of mass.

8. Is it assumable that with the fins atached,the center of pressure is already below the center of gravity?
Cuz with the fins atached the center of gravity is also lower than without fins..
And can you explain why the center of gravity needs to be higher than the center of pressure?
I always assumed that the lower the center of gravity,the more stable an object(and if im not wrong i learned that 2 years ago or last year)

9. Originally Posted by Crazy_g
Is it assumable that with the fins atached,the center of pressure is already below the center of gravity?
Cuz with the fins atached the center of gravity is also lower than without fins..
And can you explain why the center of gravity needs to be higher than the center of pressure?
I always assumed that the lower the center of gravity,the more stable an object(and if im not wrong i learned that 2 years ago or last year)
It is likely that the center of gravity is forward of the center of pressure with the fins attached, but it is not impossible for it to be otherwise. To check you can hang your rocket by a string so that it hangs horizontally and then use a fan to blow against the rocket. If it turns into the air flow (towards the fan) then your center of pressure is aft of the center of gravity.

The center of pressure needs to be aft of the center of gravity so that the rocket turns into the air flow rather than in the opposite direction (in which case it would be constantly spinning and aerodynamically unstable). If the center of pressure is aft of the center of gravity then the rocket will fly pointy-end-first.

A low center of gravity makes a wheeled vehicle less likely to tip over in a turn. That has absolutely nothing to do with the aerdynamic stability of a rocket.

Maybe, rather than just building a rocket based on suggestions from other people, you ought to study some physics.

10. Thanx for the info.
But im not gonna spend my days learning aerodynamics from the internet ,cuz i will get it next year and i entered the competition for fun not to break my head for something thats above my level...
I just search general knowledge about it if im a bit stuck.

11. Originally Posted by Crazy_g
Thanx for the info.
But im not gonna spend my days learning aerodynamics from the internet ,cuz i will get it next year and i entered the competition for fun not to break my head for something thats above my level...
I just search general knowledge about it if im a bit stuck.
I am happy to help young people who have a genuine interest in learning science. But I am not here to help someone who is too damn lazy to try to understand a subject, and to do the necessary work to do so. Breaking one's head is an important part of learning.

For professional consulting I charge \$200/hour plus expenses -- 4 hour minimum. In the case of someone who is simply trying to avoid doing what he should be doing for himself, I have a special rate -- \$300/hour plus expenses. If you need more help on your project, prepare a retainer check.

12. For professional consulting I charge \$200/hour plus expenses -- 4 hour minimum.
Sweet Lord. I have the wrong job !!!!

13. Originally Posted by Markus Hanke
Sweet Lord. I have the wrong job !!!!
It helps a bit to be retired and not looking for work. So when consulting comes up it is because someone came to me. If they find the tab too high and want to go elsewhere that is OK.

This is not a great sales approach if you are trying to drum up business.

14. Wow
thats an average monthly salary in my country

its not that im lazy,its that im bussy...
I have to combine school,this project and my parttime job
if i was lazy,i wouldnt even take part in this contest(after all,no prize atached)
and the technical terms differ in language,making it even harder to learn.
anyway,thanx for the help. Im almost done with my rocket.i just need a way to put clamps to manually release my rocket.(no help needed for this,just materials)

btw,what country do you live in(Dr.rocket)?

15. Originally Posted by Crazy_g
Wow
thats an average monthly salary in my country

its not that im lazy,its that im bussy...
I have to combine school,this project and my parttime job
if i was lazy,i wouldnt even take part in this contest(after all,no prize atached)
Being too "busy" to do the study and research necessary for your task is just another expression for too lazy to do it.

16. Nope.
Too lazy is you go watch tv or play game while you got tons of work to do
too bussy is you dont even have free time to go do something nice like playing soccer or go watch a movie.

17. Originally Posted by Crazy_g
Nope.
Too lazy is you go watch tv or play game while you got tons of work to do
too bussy is you dont even have free time to go do something nice like playing soccer or go watch a movie.
You are merely rationalizing your intellectual laziness. If you don't do the intellectual work necessary to do your project and instead rely on free advice from others you are just plain lazy. And intellectually dishonest to boot. Period.

18. if i was to lazy to learn,dont you think i would go to the library to search a book about the topic,rather than to try to understand something in a language i dont even understand clearly?
The problem is,i search a page,find a new term. I search what the term means,find another new term,and goes on infinite.

19. Originally Posted by Crazy_g
if i was to lazy to learn,dont you think i would go to the library to search a book about the topic,rather than to try to understand something in a language i dont even understand clearly?
The problem is,i search a page,find a new term. I search what the term means,find another new term,and goes on infinite.
Quite the opposite.

In fact your words just demonstrate intellectual laziness. Go figure out what those words mean.

If you asked help with understanding the basics, including those terms then you would be trying to learn. But instead you come searching for someone else to help you design a rocket to win your contest, without really trying to understand the underlying physics.

20. How do you mean the opposite?
Like i said,learning in a strange language is HARDER than learning in your own language. So how is it considered lazy if you do the first one,rather than the latter one?

21. Originally Posted by Crazy_g
How do you mean the opposite?
Like i said,learning in a strange language is HARDER than learning in your own language. So how is it considered lazy if you do the first one,rather than the latter one?
Just like I said. You are trying to get others to do your work for you, and you have stated that you are not trying to learn anything about the fundamentals of aerodynamics, which the purpose of the contest. That is both lazy and intellectually dishonest.

Language has nothing to do with this. You have stated a lack of interest in learninng, but only an interest in producing a high performing rocket, but any means.

Lots of people have to learn in a language other than their native language. That is one reason why PhD programs typically demand that one demonstrate reading proficiency in at least one foreign language as a requirement for the degree -- and not just any language will do (In my program we could choose any two from French, German and Russian). Scientific and technical articles in a foreign language are usually a lot easier to read then would be general literature. There are a lot of cognates in scientific language.

I am not expecting the level of research associated with a PhD from you. But I do expect the same level of diligence is attempting to learn the basics of new material that was expected of me when I was about 10 years old.

The only purpose of this discussion, at this point, is for the benefit of lurkers. We have an entire thread on the subject of the lack of initiative in evidence by students in even attempting to learn a subject by dint of hard work rather than being handed answers on a silver platter. You just one more symptom of what appears to be a worldwide problem. http://www.thephysicsforum.com/gener...out-there.html

You will not even accept that you are being lazy. This conversation is over.

22. You are right on some points.
But i do not want people to give me things on a silver plate.
In my experience,if you do that,you wont ever understand it.
I see your point in that im too lazy to learn,but this debate started when i asked why the center of gravity should be higer than the center of pressure and i said i dont want to learn aerodynamics.
And that is a contradiction i just made.
My question indicates that i want to understand why it is that way.
I dont expect you to reply on this,cuz like you said,conversation over...
Im goin to read the topic you posted here now

23. Got second place in the competition

24. Originally Posted by Crazy_g
Got second place in the competition
Well done.

Next time, head for the moon !

25. Lol maybe....
Now i got another project. Showing the association/relevance of the fall height and the radius of a circle.
Just as how a rolercoaster works.
This is for a thesis/essay that needs to be submitted tomorow.easy compared to the rocket.

Have a nice day

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