The 'Pepsi Bottle Rocket' Page

Gutless Disclaimer

This web page contains information describing a potentially dangerous project. Children under the age of 18, adults without prior knowledge of 'shop safety', and anyone who is unfamiliar with chemicals and/or high pressure air and their potential dangers should NOT attempt any of the things mentioned on this web page. If you build or attempt anything described on this web page, it is at your own risk, and not mine, no matter what. This page is for entertainment purposes only, and if you think you might be breaking any laws by doing anything described herein, you'd better find out beforehand.

Some time ago I thought it would be cool to make a liquid fueled rocket out of a Pepsi Bottle. As it turns out, there are 2 ways to do this. The first method is slightly dangerous, but the second method is rather safe as long as you aren't too aggressive with air pressure.
Plastic 2 liter pepsi bottles work well because they have no "cap" on them, and the resulting shape is quite aerodynamic. You can also peel off the label pretty easily. The resulting clear plastic bottle will then 'light up' rather well at night if you use the 1st method, making for a cheap fireworks display.

METHOD #1: The liquid fueled pepsi bottle rocket


A liquid fueled pepsi bottle rocket should fly at least 30 feet into the air on a good day, glowing white hot for about 1.5 seconds. Not bad for a cheap model rocket. I've flown a few of them, and I found them interesting, at the least.



METHOD #2: The pressurized air and water pepsi bottle rocket

WARNING: high pressure air could make a pepsi bottle DETONATE. Wear hand and eye protection if you do this, and expect that the bottle will detonate if you exceed 90 psi. There is no guarantee that staying under 90 psi will prevent detonation, however. Shaking a pepsi bottle will pressurize it to around 50 or 60 psi by my estimate, so it's capable of handling quite a bit of pressure. But 90 psi is clearly above the maximum you'd expect from a pepsi bottle in normal circustances, and soda bottles in general have been known to detonate when left inside of a car on a hot day with the sun shining on them.

NOW, with the 2nd gutless disclaimer out of the way, after testing the 2 liter version of this rocket in flight, I can verify that the calculations (below) that show its altitude peak at between 70 and 100 feet are relatively accurate, but of course, your flight tests may vary. I have ALSO successfully tested a 24 oz bottle half filled with water. It accelerated for a good 1-1/2 seconds, and I lost track of it immediately and for several seconds following launch, after which I noticed it tumbling back down (from straight up) at a height of about 25 or so feet above my head. So it's quite possible that the bottle exceeded 45 feet during the flight. This bottle was half full of water (to reduce it's total flight) but was pressurized to about 80 psi. As it had no 'fins' this was actually a better choice (see below). The hole diameter was slightly larger than 1/4", which ended up giving it a rather tremendous (but short) acceleration period.

This is a photograph of the 'test bottle'. Estimated peak altitude on maiden flight: 45 feet.

And, here are some simple calculations and observations, based upon a slightly greater than 1/4" diameter hole (which is what I have - diameter must be around .31 inches for 0.5 cm^2), 90 psi, and a 2/3 full 2 liter bottle.

For the 24 oz bottle (appx 700 ml) filled half way, pressurized to 90 psi with a slightly greater than 1/4" hole, here are the following observations and calculations:
With these 'pepsi bottle rockets', it's not so much "how fast do I go" because once you run out of water, the bottle has no momentum at all (weighing so little) so that the air quickly slows it down. Rather, it's how LONG you can continue accelerating effectively. An ideal balance exists between fuel (water level) and hole size. Too much fuel and pressure drops too low at the end of the flight, causing the water to 'drain out' instead of spray out. Too small of a hole and the acceleration is too small. Too large of a hole, and the fuel runs out too quickly. In this example, we double the hole area (slightly over 3/8" diameter).
So in this case, doubling the area of the hole gives you slightly more altitude, and a shorter acceleration (not counting wind resistance). But somewhere is probably the 'maximum height' value for an 'ideal hole size'. If you reduce the area to 0.7 cm^2 (84% of previous diameter), you get an average acceleration of 1.0 G's for approximately 2.1 seconds, for a maximum height at the end of acceleration of 21.6 meters, just slightly smaller than the previous one. If you increase the area to 1.3 cm^2 (114% of previous diameter) you end up with about 1.1 seconds of acceleration at an estimated average of 3.8 G's of acceleration. This yields in a maximum height at the end of acceleration of 22.5 meters. But 1.5 cm^2 (122% of previous diameter) gives you an average acceleration of around 4.3 G's for the flight, but for only 1 second, with a maximum height at the end of accelration of only 21 meters, and it goes down from there. And because mass is so small, wind resistance takes over near the end of the flight, most likely causing the peak height to be lower than expected. In short, smaller velocities at the end of the flight are probably more practical than trying to rely on inertia. Given this, peak height after acceleration becomes the most important determining factor on the maximum height attained during the flight.

FLIGHT AERODYNAMICS

One of the first major problems in the design is the tendency for the bottle to "go wild" without some kind of aerodynamic stabilizer. The "long skinny" nature of the 24 oz bottle made it fly very well when less than half full of water, but when it was 3/4 full, it "went wild" and plowed straight into the ground. The 2 liter bottle wasn't much better when 3/4 full, and actually did a complete loop while airborne, but near the end of its flight (and very near the ground) it rapidly righted itself and shot about 10 feet straight up.
Rockets are (by nature) quite unstable, similar to holding a broom on the end of your finger. Since there is only a single "engine" and no controls, the aerodynamics of the rocket must stabilize it. The easiest way to accomplish this would be to attach some kind of 'fins' to the rocket, or simply limit the total amount of fuel in it. As it turns out, filling the bottle HALF WAY with water, and using a smaller diameter hole (0.5 cm^2), gives you the following characteristics:

So it looks as though USING LESS WATER (which gives you a higher thrust to weight ratio) may be the best method when the hole size is relatively small, AND there are no 'aerodynamic stabilizers' on the rocket to keep it from going 'wild'. According to "The Hitchhiker's Guide to Model Rocketry" (see link below) you have to keep the 'center of gravity' behind the 'center of pressure for the rocket to be stable. So it explains why "less fuel" flies more stable, because the center of gravity is significantly LOWER than the center of pressure.
Of course, if you want to add MORE stability, you can put 'fins' on the rocket. This will basically help keep the engine pointing down, and the top of the rocket pointing up, by adding a small amount of 'drag' to the bottom of the rocket. Alternately, you can add a pointed 'nose cone' which would not only improve the aerodynamics, but would have the added benefit of moving the center of gravity DOWN, with respect to the 'center of air pressure', and thereby stabilize the rocket even more. And fins will work best if they also cause the rocket to spin (like a bullet in a rifle). But you still need rapid initial acceleration for this to work properly, as I've found out the hard way - when the G force is too low initially, the aerodynamics won't help keep the rocket from going wild, because the "motor's" inherent instability (no hole is perfect, plus turbulent water flow causes inconsistency in thrust direction with all of that splashing water) becomes too significant of a factor when compared to the affect of air flow. So you need to get the thing going fast as soon as you can, or else the rocket will start wobbling in the air, and maybe even do a complete loop (as my 3/4 full bottle did).




LINKS

The Hitchhiker's Guideto Model Rocketry
The 'rocketry.org' web site archived www.rocketry.org
A Wikipedia page on water rockets
Water Rockets dot com
Instructables dot com Soda Bottle Water Rocket
A recently submitted link, Model Rocket Maintenance and Technology


I have also seen a few videos on the 'intarwebs' showing other people using various soda bottle rockets, rockets made from steel soup cans, and so forth. Basically, ANY pressure vessel with some kind of a nozzle can be turned into a rocket. On Mythbusters they turned a water heater into a rocket [in a spectactular and dangerous manner]. Care should always be taken to avoid being hurt by explosion. (gutless disclaimer, yeah)

I have seen suggestions online for 'hydro testing' your pressure vessel by filling it completely with water, then applying pressure to an amount that is 50% more than you expect to use for flight, to make sure that the pressure vessel does not leak, and does not explode. If it DOES explode during the 'hydro test' when completely filled with water, the damage from flying shrapnel would be minimized, since pressure drops VERY fast when it's basically "all water". But keep in mind that 'flying shrapnel' IS a possibility here. Safety is important. (It never hurts to wear some kind of eye and hand/arm protection)

Somewhat MORE recently, a water rocket may have come dangerously close to an aircraft. Just like problems with remote controlled aircraft in general, these things should NOT be flown ANYWHERE near a designated flight path or airport. The downside of 'no responsibility' from flying any kind of aircraft (including rockets) is, unfortunately, GOVERNMENT REGULATION, bad press, and actual injuries [or worse].

So, please, FLY responsibly!