It’s easy to picture pumping a bottle full of air and then hitting a button to make it shoot into the sky. Compressed air launchers do exist, so why do we add water to these bottle launchers? There are two reasons: stability and propulsion.

Stability: The first reason is stability at takeoff. Two-liter soda bottles are very light. Even with the addition of fins and a nose cone, the slightest breeze can either topple them over or blow them off course as soon as they’re launched.

Propulsion: The second, and more important, reason is to act as a propellant, the “rocket fuel,” if you like (this is where some of the science comes in). Water is a lot heavier than air, so when it’s pressurized and suddenly expelled through a narrow aperture at the tail of the rocket, it generates much more thrust than air could alone. As a result, your bottle rocket goes higher — claims of 90 feet and more are common. Some have been known to exceed 250 feet, though there’s limited choice when it comes to rockets of this power available commercially.

Let’s first look at the things you don’t get and will need to provide yourself: most bottle launchers don’t provide the soda bottle or pump. The first is simple. Just finish that soda in the fridge and give it a quick rinse. The second is usually something you probably already own: a bicycle tire pump. Alternatives include a small electric tire compressor or tool compressor.

For what does come in the kit, cheap DIY bottle launchers provide you with a set of fins to stick on your bottle, an air valve that screws on the top, and a hose to attach to your pump. There’s no kind of launchpad; the fins rest directly on the ground (so you need to find somewhere level). There’s also no launch control. The bottle rocket takes off when you’ve pumped enough air in to reach a preset pressure.

Pay a little more and you’ll get a bottle launcher that incorporates a few more features. These look better, they’re more durable, and they offer more control in general. These additional components include the following:

• Launchpad
• Check valve: You’ll probably get a brass check valve that prevents overinflation
• Launch mechanism: This is likely a pull-cord that releases a firing pin. It may not look like much, but at 10 to 15 feet long, it allows you to get much farther from the bottle rocket than the length of the inflation hose to keep you safer.
• Ground stakes: You’ll get one or more ground stakes, so your bottle rocket launcher won’t get blown over by a gust of wind, a common problem with the bottle-only type of launcher.
   

Pay more for a high-end model and it will be more durable and potentially offer greater control over performance. The maximum height we’ve seen claimed for one of these systems is 350 feet. What’s more, the robust construction makes performance very repeatable. This is important for science projects in which you want to be able to monitor the effect of small changes in water volume and air pressure. These more expensive kits have the following features:

• Brass and steel components: Chrome-plated steel prevents rust from all that water flying around.
• Pressure gauge: The pressure can be as high as 120 pounds per square inch (psi), and a gauge is often included so you can monitor inflation more accurately.
• Hose: A braided steel hose and quick-detach fittings may be included for connecting the launcher to a compressor.

There are also models that launch more than one bottle, with valves that allow for pressurization at the same time and trigger mechanisms that offer a choice of separate or combined launch. These require some assembly, but all the parts are included. They’re usually made of PVC pipe which is strong enough to handle the pressures involved and tough enough to take the kind of beating dealt by enthusiastic kids.

Most bottle launchers are designed to launch the rocket vertically, but that’s not always the case. Rail systems exist that launch at a fixed angle, and some can be adjusted. From a teaching point of view, this incorporates ballistics, projectile engineering, Newton’s laws of motion, and more. However, great care is needed with this type of bottle launcher. You need a clear flight path and plenty of margin for error in your safety calculations. It’s generally recommended that an adult be present when launching bottle rockets, and it’s even more important with this type.

Inexpensive: The cheapest bottle launchers that cost less than $25 are DIY kits. They are quick and simple to build, but they don’t have any kind of launchpad and offer limited control over pressure. They’re fine for a bit of low-cost fun, but offer little in the way of educational merit.

Mid-range: The majority of bottle launchers cost between $25 and $50, and most low-power models designed for smaller children are within this bracket. Also included are those that can launch multiple bottles simultaneously (minimal assembly is required).

Expensive: There are a few bottle launchers made with high-quality chromed steel and brass components. These offer tremendous durability, and they cost between $60 and $100.

• Read the instructions carefully. Read them twice and make sure you understand them. If in doubt, seek clarification.
• Use glue in a well-ventilated area. If you need to use glue to put your rocket together, work in a well-ventilated area. Some products have very unpleasant odors and can be harmful.
• Never use glass bottles. Only use plastic water or soda bottles. Never use glass, and never use bottles that once contained household or garden chemicals or any kind of flammable liquid.
• Wear eye protection. It is very important to protect your eyes when pressurizing the bottle.
• Launch in a clear area. Keep people and pets at a safe distance. If the bottle doesn’t launch successfully, be very careful when approaching it. Objects under high pressure can be dangerous. If possible, allow the pressure to dissipate before investigating.
• Check the wind direction. Before launch, consider how wind might affect the bottle rocket. Never launch if you can’t reasonably judge the flight path and landing area.
• Do not rely on a parachute for safety. It can fail.

Q. Is there any risk of a bottle exploding?
A. It’s unlikely. Most modern soda bottles are designed to withstand at least 90 psi (though it’s likely they can exceed that). We’ve seen some bottle launchers that run to 120 psi, but they’re less common. Most max out at less than 60 psi.

It’s also worth noting that when deliberately tested to destruction, almost all bottle failures result in a loud bang, but no flying debris. If you establish a safe perimeter before launch, there should be no danger.
 

Q. Could a falling bottle cause damage or injury?
A. It’s possible but unlikely if you follow the recommended safety precautions. Your neighbor might not be happy with a bottle falling on their car or prize rose bushes, but an empty soda bottle weighs a couple ounces, so the impact should be negligible. Unexpelled water will add weight (though the farther it flies, the less water will be left), which is why it’s important to establish a safe area around the launch site, follow the instructions carefully, and only test variations when you’ve gained some experience with your launcher’s standard performance.
 

Q. Can I make a bottle launcher myself?
A. Of course! There are lots of instructions online — from basic to complex — and plenty of informative videos. It can be a lot of fun. Putting it all together takes time, and you need to be patient if you want to build one you can use again and again. The advantages of buying a premade bottle launcher or bottle launcher kit are that you know all the parts have been designed and tested for you, and it can very quickly be ready for flight.