the science of bottle rockets

[J_Blues]

Let me begin by saying I am -not- a science person (nor a math person, but thats irrelevant i think), and I was recently witness to an incident at a Cub camp that I cannot explain. The youth in my group are looking to me for an answer, and so I in turn look to the teeming millions to help me out. Here's what happened.
One of the events at this camp was "bottle rockets". For those who don't know, to make a bottle rocket you fill a two litre coke bottle a quarter of the way full with water, flip it upside down onto a rubber cork attached to a bike pump and fill the bottle with air pressure, as much as you can before the bottle flies into the air, drenching the poor fool who's holding the bottle down (Me). Anyways, after all the kids had a go at pumping the bottle rockets and after I was drenched from head to toe, another one of the leaders decided to see if he could clear the barn with a bottle, and so he told me to hold on to the bottle as long as I could, and so I did. What happened next is the part i can't explain, the pressure built up so much that i lost my grip on the bottle and all the water and air burst out the bottom and we were left with an empty bottle.. and no rocket (so to speak). The bottle however was filled with smoke ((at first glance the other leader and myself we're laighing hysterically at the uncanny resemblance to an oversized BT bottle... i swear it looked that much like smoke)) I thought perhaps it was steam created by the magic of science and pressure somehow (like i said, i'm not a science kinda guy), would this be the case?? and if so, where did it come from? pressure?? and if it's not steam, what was it.. and where did it come from??

thanks for any help
J

[Chronos]

Well, I can give a partial reply, at least. What you saw was not smoke, nor, strictly speaking, was it steam (steam is gaseous water, and is invisible). What it was was a bunch of tiny droplets of liquid water suspended in the air... I don't recall the scientific term for this off the top of my head, but you could call it "fog" and not be too far off. As to why and how it formed, I'll have to leave that to someone who was paying more attention in fluid dynamics.

[Oblio]

The compressed hot air picks up moisture and is somewhat cooled by passing through the water. Pull the plug, water escapes, pressure drops, air cools below dewpoint. Voila ! Fog.

[Boris B]

I think this was just a plain old aerosol. I might be wrong, since there is some difference between an aerosol and a suspension that I am forgetting. Anwyay, I imagine that it occurs even in the successful rocket flights, only it's not as visible because the bottle is further away.

My theory: the compressed air that is at the top of your rocket will try to escape. Naturally, this pushes the water out of the mouth of the bottle. Since the water is not solid, some of the air passes through the water, leaving less pressurized air behind along with water. The air pressure equalizes (leaving your rocket out of fuel) before all the water is gone, leaving a teensy bit of water spread out everywhere in the bottle's interior. There will probably be some vapor (invisible as Chronos said), and some drops on the sides (regular old liquid), along with a smokey-looking aerosol cloud ... not too different from the cottony-looking stuff which covers the sun before it rains.

[Sam Stone]

An Aerosol is possible, but my money is on 'fog'. Gases under pressure heat up, and they can absorb much more moisture. Release that pressure suddenly, and you'll get a rapid drop in temperature and the instant release of water vapor.

If you want to see this effect right now, go buy a bottle of Coke with a crimped top. Pop the top, and you'll see the instant formation of a small cloud inside the bottle, with wisps of vapor leaking out of the top.

Incidentally, what we called 'bottle rockets' when I was a kid were actual firecracker thingies that had a little rocket stage on them and were attached to a thin wooden stick. They were 'bottle rockets' because you used a bottle as your launch pad. Just drop stick inside until it touches bottom, light the fuse, and voila. They'd shoot maybe 50 feet in the air and then go off like a regular firecracker. We had much fun shooting them at each other in mock 'wars'. We could have put someone's eye out.

[Boris B]

Yeah, we called the gunpowder-powered things bottle rockets too. The plastic compressed-air powered thingies were "water rockets", and we never made ourselves. Instead, we got them at the toy store, ready made.

I even remember a two-stage one that was a neat idea, but didn't work very well. Fill up both rockets, which attach by a gasket, and theoretically the upper stage is supposed to start accelerating after the lower one is spent. In practice I think both started at the same time, which made the whole launch wetter than usual.

Gosh. Now I want to go down to a toy store and get a bottle rocket. They would probably work okay underwater too, which would be good for fishing. No, not really, they would travel too slowly. But the bubble trail would be pretty impressive. Or, here's a good one. Swim to the bottom of the pool with the thing already pumped up. Then launch it, and see if it breaks the surface like an SLBM. Try not to hit sunbathers near the pool.