# Q & A: Bottle Rockets

Q:
Ok, in my science class we are building rockets which are going to consist of a 2-liter bottle, water, and air pressure. Our goal is to get your bottle to fly the highest. Any suggestions???
- Jamie Keller (age 14)
Breg Middle School, Newton, Ia USA
A:
Jamie -

What fun! (I'm a big fan of bottle rockets!) The concept behind a bottle rocket like this is that you take a bottle and fill it part-way with water and part-way with compressed air. Then the compressed air pushes the water out of a hole at one end of the bottle. The force of the water going in one direction causes the bottle to go the other way. You can aim the bottle by attaching it to a very straight stick and setting the stick in some sort of holder on the ground. (Or you can attach a straw to the bottle and rest that on a stick in the ground.) As you can see, the basic idea is pretty simple. The hard part is actually getting it to work!

First you need some way to put lots of compressed air into the bottle. If you're doing this with stuff you have around, a bike-pump will work pretty well. Next, you need to figure out how to make the air stay inside the bottle until you're ready to launch it. This means that there can't be any holes for the air to get out of once you've put it in. If you're using a pop bottle, screwing the lid on really tightly should do the trick.

Ok, so you're probably wondering how you're supposed to be getting the air /in/ if the whole bottle's sealed up. This takes practice! You'll have to experiment with different ideas until you find one that works for you. Something that I've found works pretty well is if you use a needle attachment for the bike-pump (like the kind you'd use to pump up a basketball). You can stick the needle straight through the lid on the bottle and still keep a pretty good seal. (This is also nice because it makes a hole for the water to come out of later, and you don't have to mess around with unscrewing the cap when you're ready to launch.)

So now you've figured out how to get the air in. What about how much air you should use? Remember that part of the bottle has to be filled with water (put the water in first), so you can't fill the whole thing up with air. To make the bottle go the furthest (or the highest), you have to figure out what is the best amount of air and water to use. If you have too much water and too little air, then you'll run out of "push" before you run out of propellant (the propellant is the water - the stuff that gets pushed out.) But if you have too much air and too little water, then you'll run out of propellant when you still have more push left. Neither one will make your rocket go as far as it could.

Although you could spend a lot of time messing around with equations and stuff trying to figure out what the best amounts are, it's a lot easier (and a lot more fun) to just try it. Get some pop bottles, try different amounts of water and air, and see what works the best! While you're doing this, you can also try to see how much air you can fit in the bottle. (Remember if you fill it up too full, the bottle may explode, so be careful doing this.)

Most importantly, have fun! To make your bottle rocket work the best, you'll want to play around with it a lot. Try different types of bottles (lighter ones will go further, but they may not be as strong as thicker ones). Try different ways of getting the air in and launching it, and try different amounts of water and air. See what works the best. (Remember, worst case you lose a few pop bottles and have to drink more soda!) Good luck!

-Tamara

(published on 10/22/2007)

## Follow-Up #1: bottle rockets

Q:
well my class are making bottle rockets. It is kinda of a competion so i was wondering how much water you should put in it? since we really don't have time to experiment and will the shape of the cone affect how high it goes?
- colleen (age 13)
spokane valley,WA,United states
A:
There's a trade-off here. You need the water to provide exhaust momentum. You also need some space for the pressurized air, which provides the energy. I did a rough calculation, and got that the best combination would be to have the rocket about half filled.  If you can experiment a bit, you can optimize that fill level. For more discussion, search this site for more "bottle rocket" answers.

I assume that a more or less streamlined shape is best to reduce air drag.

Mike W.

(published on 06/04/2009)

Q:
How much water do you put in for NASA Project X-51? I know all about Newton's 3rd Law of Motion but I am unsure about how much I put in, taking into account the extra load. My bottle is a 2 Liter bottle, a Crush bottle if it helps. Please help?
- Micah (age 13)
Loma Linda, California, USA
A:

One of our old threads has a better discussion: https://van.physics.illinois.edu/qa/listing.php?id=17080.

Around 1/3 full seems to work well, for reasons we discuss in that thread. A little experimenting should help you fine-tune the amount.

Mike W.

(published on 02/20/2018)