Help with building a school rocket

[StellarisVoid]

Hello,
I attend a physics club at my school and now, we are building rockets until winter. I need some help because I really want to beat the GCSE students. We are in self chosen teams and the winner is who gets their rocket the highest(hopefully it will go high enough so we can't see it)
I have already gathered some information:
I should have a rounded nose for the rocket(decreased drag)
Centre of mass should be towards the tip (increase stability)
Placing fins at the bottom increases stability.

My current thoughts for fuel are:
Water
Vinegar and Baking soda.
Is there anything else I could use as a more powerful fuel? *I am in the UK so therefore, I cannot use solid fuel*

Should I 3d print the rocket, or at least attempt to? (most people will be using bottles so I hope I can do something different)
How do water rockets work? I can't wrap my head around how to push the water quickly.
What is the optimal fin shape?

Any other advice and things I should know? Help is greatly appreciated!

 

[anorlunda]

How do water rockets work? I can't wrap my head around how to push the water quickly.
What is the optimal fin shape?

Water rocket toys are inexpensive. Buy one to experiment with and to learn from. What is the optimum ratio of air volume to water volume?

 

[berkeman]

We are in self chosen teams and the winner is who gets their rocket the highest(hopefully it will go high enough so we can't see it)
I have already gathered some information:

Sounds like a fun project. Hopefully the judges have a quantitative way to measure the height reached by each rocket.

Can you post the full text of the project/assignment/competition? It will help us to offer suggestions if we know all of the rules you are working under. Also, it sounds like you are not allowed to use combustion for propelling the rocket?

Can you post links or pictures to previous competitions? Thanks.

 

[StellarisVoid]

Sounds like a fun project. Hopefully the judges have a quantitative way to measure the height reached by each rocket.

Can you post the full text of the project/assignment/competition? It will help us to offer suggestions if we know all of the rules you are working under. Also, it sounds like you are not allowed to use combustion for propelling the rocket?

Can you post links or pictures to previous competitions? Thanks.

This is just a competition the physics department organised and not an official competition.
Aim is to make our rockets travel as far up as possible.
Solid fuel is illegal in the UK which is sad otherwise I would've used "Rocket Candy".

 

[StellarisVoid]

What is the optimum ratio of air volume to water volume?

I am not sure.
I am in Year 8 so I do not know much about rockets and how the aerodynamics work(sadly) so I was hoping to find out some stuff here.

 

[berkeman]

This is just a competition the physics department organised and not an official competition.

It's still important to understand the rules of the competition. For example, there are other ways to launch a rocket without propellant...

https://www.spinlaunch.com/#p1

 

[StellarisVoid]

It's still important to understand the rules of the competition. For example, there are other ways to launch a rocket without propellant...

https://www.spinlaunch.com/#p1

View attachment 316598

I don't think that there are any particular rules apart from not using solid fuel and not to make anything too complicated as well as making sure we won't set the field on fire. Basic safety rules only seem to apply but I will check tomorrow.
Thanks for the link! I'll have a look at it.

 

[berkeman]

Other things to check:

** Does the rocket need to survive the landing? (may require a parachute system if your rocket gets high enough)

** Can part of the rocket/launch system stay on the ground after the launch, or does everything need to go up with the rocket system?

** How tall and how heavy can it be?

** Any bonus points for carrying a small WiFi camera along to send back video?

 

[StellarisVoid]

** Does the rocket need to survive the landing? (may require a parachute system if your rocket gets high enough)

I don't think so because most people attending the club aren't too good at physics so the teachers aren't expecting much from us.

** Can part of the rocket/launch system stay on the ground after the launch, or does everything need to go up with the rocket system?

** How tall and how heavy can it be?

I will check tomorrow, I too was wandering what size rocket am I allowed to make.

** Any bonus points for carrying a small WiFi camera along to send back video?

That's a good idea! I will look into it.
Do you have any recommendations for a camera?

 

[anorlunda]

I am in Year 8 so I do not know much about rockets and how the aerodynamics work(sadly) so I was hoping to find out some stuff here.

The way to find that out is to try it. In other words experment using the toy rocket.

There are also many Youtube videos of larger water rockets. You can learn from them.

It is hard for us to believe that there are not written instructions and rules for this competition.

 

[FactChecker]

Hello,
I attend a physics club at my school and now, we are building rockets until winter. I need some help because I really want to beat the GCSE students. We are in self chosen teams and the winner is who gets their rocket the highest(hopefully it will go high enough so we can't see it)
I have already gathered some information:
I should have a rounded nose for the rocket(decreased drag)

A pointed nose might be better.

Centre of mass should be towards the tip (increase stability)

This sounds wrong. CORRECTION: His statement is not wrong. Aerodynamically, it is correct. Furthermore, the fixed nozzle makes this correct even for the thrust force.

Placing fins at the bottom increases stability.

My current thoughts for fuel are:
Water

Standard, easy to load, and safe.

Vinegar and Baking soda.

Not sure how you can load this in.

Is there anything else I could use as a more powerful fuel? *I am in the UK so therefore, I cannot use solid fuel*

Should I 3d print the rocket, or at least attempt to? (most people will be using bottles so I hope I can do something different)

3d print is probably not strong enough. Try to get an aerodynamic bottle.

How do water rockets work? I can't wrap my head around how to push the water quickly.

Compressed air.

What is the optimal fin shape?

Any other advice and things I should know? Help is greatly appreciated!

 

[anorlunda]

By the way, hobbyists have quite a bit of success making water rockets out of 2 liter soda bottles.

You may be able to finish a soda bottle project and launch it before sunset today. Another team may want a year or two to design and build something that performs better. Does this competition have a specific deadline date? Is there any measure of merit other than how high the rocket goes? Do the physics teachers have a reliable way to measure how high the rockets go?

Edit: I think what I'm hinting at is that there are amateur rocket societies almost everywhere. Contacting them for advice, or assistance in running the competition might be a very smart thing to do.

 

[berkeman]

** Can part of the rocket/launch system stay on the ground after the launch, or does everything need to go up with the rocket system?

https://i.pinimg.com/originals/01/a1/b1/01a1b14807c1df91732106e59d005f17.jpg

 

[Vanadium 50]

Hopefully the judges have a quantitative way to measure the height reached by each rocket.

Trig! It was practically invented for this!

 

[Vanadium 50]

This sounds wrong.

It's not. You want the center of gravity in front of the center of pressure.

That's why fins are on the bottom and not on the top.

Solid fuel is illegal in the UK which is sad

No it isn't. I can see several places that sell it.

It's entirely possible that your rules prohibit them, but that's why we asked you about them.

 

[berkeman]

It's not. You want the center of gravity in front of the center of pressure.

I've always wondered about that. Can you elaborate please? The helicopter is an obvious counterexample, but a helicopter is not a rocket...

 

[FactChecker]

It's not. You want the center of gravity in front of the center of pressure.
That's why fins are on the bottom and not on the top.

I stand corrected. Aerodynamically, that is clear. And for a fixed nozzle, I was (once again) misled by my intuition. (I forget what that error is called, but it is well known and I have made it here before.)

UPDATE: The intuition mistake that I am referring to is called the "pendulum rocket fallacy".

 

[Vanadium 50]

a helicopter is not a rocket...

Exactly.

The thing is we are used to thinking "stable means weight on the bottom". But stable actually means "a small perturbation yields a restoring force (and restoring torque) in the opposite direction".

For a vase, you have two forces on it - gravity and the normal force. You want the center of gravity low so that the perturbing torque is minimized. In a rocket you have gravity and no normal force - instead of the normal force you hgave the sum of all the aerodynamic forces on it, The two axes about which these two torques operat are called the "center of gravity" and "center of pressure". Torque is the more relevant concept - we know the forces don't balance because the rocket zooms up.

Long story short, if the COP is behind the COG, a perturbing torque about the COG has as aerodynamic forces turning the rocket in the opposite direction - providing a restoring torque. Put it ahead and they are in the same direction, cwusing instability.

 

[StellarisVoid]

I checked at school today and there are no rules which is weird because health and safety? Furthermore, the physics teacher encouraged us to launch rockets at the PE Department and the school on the opposite side of the road
Although I would love join a rocket club or something similar, I do not have one in my area and the nearest rocket club is about 50 miles away :(

 

[anorlunda]

OK, then why not try for the KISS prize? (KISS=Keep It Simple Stupid). You may be able to build and launch it tomorrow. The main things you need are two 2 liter bottles and a bicycle pump.

 

[dlgoff]

OK, then why not try for the KISS prize? (KISS=Keep It Simple Stupid). You may be able to build and launch it tomorrow. The main things you need are two 2 liter bottles and a bicycle pump.

I use to do this as a kid and have seen the results of the bottle rupturing.
If it were me in the Youtube video, I'd have on safety glasses. Just sayin'

 

[berkeman]

I use to do this as a kid and have seen the results of the bottle rupturing.
If it were me in the Youtube video, I'd have on safety glasses. Just sayin'

I was wondering about that. Good safety tip, Don!

 

[StellarisVoid]

OK, then why not try for the KISS prize? (KISS=Keep It Simple Stupid). You may be able to build and launch it tomorrow. The main things you need are two 2 liter bottles and a bicycle pump.

I might end up doing this. It would be quite funny bringing a whole 2m tall "launcher" into school.

 

[StellarisVoid]

Another question, in the video, the demonstrator removes a 'pin' to let the water out. Is this pin in the bottle or the pipe?

 

[anorlunda]

Another question, in the video, the demonstrator removes a 'pin' to let the water out. Is this pin in the bottle or the pipe?

There are many ways. Try this search.

https://www.youtube.com/results?search_query=water+rocket+release+mechanism