Why Fill Bottle Rocket 1/3?

What is the reason for using 1/3 of the bottle rocket to fill up with water... What is the science behind it?
- Anon (age 12)
Canada

We've discussed this before. See the discussion here: https://van.physics.illinois.edu/qa/listing.php?id=17080.

The key idea is that you need air space, to store energy, and you need water space, to store mass to be expelled to provide momentum from that energy. If those were the only criteria, the optimal fill would be 1/2.  The water also increases the weight that needs to be lifted, so that reduces the optimal filling to roughly 1/3. The exact value is best determined by experiment.

Mike W.

(published on 05/15/2015)