Q & A: Bottle rocket fins

what about the fin affect on bottle rockets? whats the best way to make your fins on a pop bottle rocket?
- mark (age 15)
crestline oh.

Adding fins to bottle rockets, or any rocket, for that matter, contributes to the stability of the rocket in flight. A rocket works by conserving momentum -- as water (in the case of the bottle rocket) or burning gases escape from the rocket's nozzle, the rocket will feel a push in the opposite direction. Unless the escaping material is exquisitely aimed, then the rocket may not get pushed exactly in the direction you want it to go. Even a small deviation from the right direction will make the rocket take a curved path, and maybe even go in circles or tumble randomly. A balloon with its lip untied, with air escaping, is an example of an uncontrolled rocket.

The fins counteract sideways motion of the rocket. Air flows smoothly past them if the rocket is traveling along its axis. If there is any sideways motion, then the air striking the fins pushes the rocket back towards straight motion. It's not perfect -- the rocket may still go around in circles, but the idea is to make the path straighter than it otherwise would be.

The fins can be made out of cardboard and taped to the bottle. Their shape probably isn't too important (the fins on real rockets take a wide variety of shapes), although their area is. It probably isn't good to make them too heavy. If you make them with pointed ends it will be easier for them to bend out of shape when you set the rocket down. The plane of the fins should contain the axis of the rocket -- don't bend them or they will cause drag on the rocket in flight and may cause it to spin. Spin is used to help stabilize rifle bullets, but in the case of a water bottle rocket it probably won't make much difference because the bottle may spin but the water inside would not. That would rob energy from the flight anyhow, so it sounds like not such a good idea.

Tom

(published on 10/22/2007)

I would like to know some information on pop bottle rockets! Mainly I would like to know if boiling hot water or ice cold water would make the pop bottle rocket go farther up into the air, or if they go the same distance?
- Hillary (age 15)
Lewiston, Idaho,United States

Hillary -

I don't know for sure if hot water or cold water would work better with a bottle rocket. I bet it wouldn't make much difference, since the water is just there to provide some mass for the exhaust. I think that it works best when the mass of the water is about the same as the mass of the rocket. The density of water only changes a little as the temperature changes, so the room left for compressed air would be nearly the same whether the water were hot or cold. Hot water is less viscous (it flows with less friction), so it wouldn't lose quite as much energy going out the nozzle, but that too probably isn't a big factor.

I would really suggest that you don't try this yourself. Putting boiling hot water in a bottle rocket is a very bad idea since the water can spray out and hit people on the ground. In situations like this, boiling water can cause very serious burns.

-Tamara

(published on 05/07/11)

where should i place the fins on the rocket or would it make a differnce at all. and does a smaller bottle work better than a big pop bottle.
- t dog (age 16)
wooster ohio usa

People  usually have the fins toward the back. I guess that's because the fins add drag, so if they were in front it would tend to destabilize the rocket.

I'd expect that the bigger rockets would go farther. Let's say you've picked some thickness of bottle. That sets the maximum pressure that can be used. Then the total stored energy is proportional to the volume. The mass of the bottle itself is only proportional to the area, for fixed thickness. So your energy/mass ratio gets better for bigger bottles. Once the mass of the water is a lot bigger than that of the bottle, that won't improve much. However, air friction is relatively more important for small bottles. So I bet the bigger bottle will go farther.

Mike W.

(published on 05/07/11)

What if i fixed the fins that it would make a spiral when it launches in the air?Would it go farther?since additional thrust can be used from the spin on the fins. or would the wind resistance slow it down?
- Hinh N.K (age 14)
Mississauga,Canada,Ontario

The fins help keep the rocket on a straight path. In principle, spinning would help that some but, as Tom pointed out earlier, since it's hard to get the water spinning that might not help much.

One thing that definitely won't happen is for the spin to add thrust. It takes energy to set things spinning, and that energy comes out of the same stored energy that provides the thrust. Spinning will reduce the thrust. As you suspected, the mechanism is that the same force from the air on the fins that sets them spinning also pushes them backwards.

Mike W.

(published on 10/03/11)

i am making a bottle rocket. im not sure how much pressure to put in it or water and i am using a 2 litter bottle
- danielle (age 14)
sidney ohio united states

We've marked this as a follow-up.
See also http://van.physics.illinois.edu/qa/listing.php?id=17080

Mike W.

(published on 05/04/12)

In science class, we are doing a project on 2L pop bottle rockets and we I need mine to go the highest. I have no idea where to start, but i would like to know how much water to put in it and some tips on how to get it to go the highest.
- Angie

We've marked this as a follow-up.
See also http://van.physics.illinois.edu/qa/listing.php?id=17080

Mike W.

(published on 05/04/12)

hi so in science we are doing bottle rockets and i have found the amout of water and compressed air to put in the bottle(1 LITER) but i was wondering what does this amout have to do with how high it will fly p.s this is a compition so i would really like to know so it could maybe help me win.
- Gabriella (age 13)

We've marked this as a follow-up.
See also http://van.physics.illinois.edu/qa/listing.php?id=17080

Here's why you need to use a good part of the bottle for compressed air and a good part for the water. The compressed air provides the energy for the rocket. However, by itself it would just rush out the back heating up the nearby air and hardly pushing on the rocket. You need something heavy to push backwards, and that's where the water comes in. The reason you need something heavy is that a basic law says that the total "momentum" doesn't change. Here that means that the mass of the stuff going forward times its forward speed equals the mass of the stuff going backwards times its backwards speed. The water supplies the mass for the backward-going part.

Mike W.

(published on 05/04/12)

Hello, im lookig for some for infromation about bottle rockets ? What could the fins be made out of? Where are you going to attach the fins to a bottlerocket.? How Many Bottles would you need ? Also, it is going t be a water and air bottle rocket.
- Whitely (age 13)
Sidney Ohio

We've marked this as a follow-up.
See also http://van.physics.illinois.edu/qa/listing.php?id=17080

One thing we should add to our previous remarks is that the fins should be toward the bottom of the rocket, for stability.

Mike W.

(published on 05/04/12)

Hi, which is better, a water rocket or air, also, what are good websites to find out how to build ones that go REALLY high? -Sally
- sally (age 11)
U.S.A

It turns out that you really need both water and air. As we've discussed in old answers (http://van.physics.illinois.edu/qa/listing.php?id=17080), about 1/3 of the bottle should be filled with water.

Mike W.

(published on 07/19/12)