The
Water Rocket FAQ

This FAQ (Frequently Asked Questions) is built up from questions that people ask me or the AWARS group from time to time. See also the Problems Page.

How powerful is a water rocket compared to a pyro rocket?
How much does it cost to build a water rocket compared to a pyro rocket?
How much does it cost to launch a water rocket compared to a pyro rocket?
Is water rocketry safe?
How does a water rocket work?
What is a reaction mass and how is this different in a pyro rocket?
Can I only use water as the reaction mass in a water rocket?
Can I only use air as the gas in a water rocket?
Why does a rocket need to be aerodynamically stable?
What is the Centre of Drag?
What is the Centre of Gravity?
What do you mean by a rocket diameter?
How do I check to make sure that my rocket is aerodynamically stable?
How much weight do I need to add to the rocket?
How big do the fins need to be?
What is a Drag Factor?
What is the Nozzle Constant?
How does changing the shape of the bottle help the rocket fly better?
What is the difference between speed and velocity?
What is a Launch Tube?
What is a T-Nozzle?
Why use a launcher?
What is a sustainer?
Why do I need to use a recovery system?
Which type of parachute should I make?
What type of cord should I use for the parachute strings?
Is it worth spending a lot of time making a parachute?
Why does the chute need to be released at apogee (the highest point in the flight)?
What is a Tomy timer?
What is PET?
Can you make water rockets using other materials?
Do I need a license to launch water rockets?
What do I need to take with me when I go launching water rockets?
What is a safe place to launch a water rocket?
What is a safe distance from a launcher?
Why should I use a computer model?
Will a computer model cost me anything?
Do you have to be a PhD to use a computer model?
Which computer model should I choose?
Do I need the HTML help pages to be able to use the computer model?
What do I need to do to install the water rocket computer model?
What do I need to do to register the water rocket computer model?
What do I need to do to uninstall the water rocket computer model?
What does AWARS stand for?
What is the AWARS WebRing?
Which countries have water rocketeers?
What is the altitude record for a water rocket?
How powerful is a water rocket compared to a pyro rocket?
Curiously, when people talk of the power of a rocket, they usually talk about impulse instead. Power is the rate of energy use per second which would give a 2 litre water rocket a power output of around 2kW (2 bars on an electric fire) whereas Impulse is measured in terms of the force the engine gives, multiplied by the time. The result of this is an answer in Newton Seconds but to make life easier, the impulse for various motors is put into bands. There are A and A, A, B, C and so on, doubling the impulse with each letter. The rocket motors that you can buy in the shops for a pyro rocket are usually A, B or C. A standard 2 litre water rocket gives an impulse rating of E, or 8 times more powerful than a B rating.
How much does it cost to build a water rocket compared to a pyro rocket?
Pyro rocket kits in the UK, based on a C motor and having a thin plastic chute for recovery, can cost 10 - 20. A 2 litre water rocket, made from two or three pop bottles (costing 15p each in winter or 25p each in summer) and a chute made from a bin liner and some cord will cost around 1 to make.
How much does it cost to launch a water rocket compared to a pyro rocket?
A pyro rocket uses a motor with each launch and these cost around 1.50 each. A water rocket uses around 600mls water and some compressed air so unless you are using expensive mineral water the price of launching a water rocket is negligible. In fact, you can build a water rocket for less than the price of a pyro rocket motor.
Is water rocketry safe?
If you follow the same types of safety rules that apply to pyro rockets, it is as safe. One thing that you can't do with a water rocket (with cold water in it) though is burn yourself on the fuel.
How does a water rocket work?
You put some water (roughly 33% of the capacity of the rocket) into the water rocket, place it on the launcher, fix it in place and then pressurise the gas above the water in the rocket by pumping air into it. When you release the rocket, the force caused by the nozzle accelerating the water out of the rocket provides enough force to make the rocket go up into the air.
What is a reaction mass and how is this different in a pyro rocket?
The reaction mass in a water rocket is the water. It is the force caused by accelerating this downwards that makes the rocket go upwards. In a water rocket, the energy that forces the water out is stored in the compressed air (you can prove this by filling up a water rocket to the top and then seeing if there is any significant flight). In a pyro rocket, the energy that forces the reaction mass out of the rocket is stored as chemical energy within the reaction mass. Water rocketry is therefore special in that the reaction mass and the energy storage can be separated.
Can I only use water as the reaction mass in a water rocket?
No, you can use any liquid you like. On the computer models, you can change the density of the liquid to anything you like. Before you try out using liquid Bromine or vintage Champagne, I suggest that you consider the impact on the environment (including any observers) and your wallet (not only in terms of paying for the reaction mass but in cleaning up afterwards). It is best to use water in that doing so does not incur excessive cost either to yourself or the environment.
Can I only use air as the gas in a water rocket?
No, you can use any gas you like. Again, you can change the gas on the computer models to see how well different gasses will work. You should not choose a gas that will react with any parts of the launcher or gas delivery system or rocket. It turns out that when considering cost and performance, Carbon Dioxide can be very effective although air does perform very well. For most circumstances air will suffice. Considerations should include cost, safety of handling, long term cost in terms of maintenance of equipment and so on.
Why does a rocket need to be aerodynamically stable?
If the rocket is not stable, there is no reason for it to point in any particular direction and its flight will be erratic and inefficient. If you launch a rocket that is slightly unstable, it will wander in an unpredictable way and is almost funny to watch (assuming that you do this in a sufficiently safe way). For a rocket to be stable, there is a rule of thumb that states that the Centre of Drag (CoD) should be between one and two rocket diameters behind the Centre of Gravity (CoG).
What is the Centre of Drag?
The Centre of Drag (CoD) is an imaginary place on the rocket that represents the drag of the rocket if it was all positioned there. You can find this position by making a cardboard cut-out of the sideways view of the rocket and finding its centre of gravity.
What is the Centre of Gravity?
The Centre of Gravity (CoG) is an imaginary place on the rocket that represents the mass of the rocket if it was all positioned there. You can find this position by putting the rocket on its side and balancing it there. If you suspend the rocket by a string, the CoG will be below the position where the string it tied. You can find the position of the CoG of the cardboard cut-out of the rocket sideways view in this way.
What do you mean by a rocket diameter?
A rocket diameter is the diameter of the body of the rocket at its widest point. If you have a two stage rocket with different diameter stages then the rocket diameter is the larger of the two.
How do I check to make sure that my rocket is aerodynamically stable?
Having determined the position of the CoD of the rocket, you can mark this on the side of the rocket. If the rocket balances at least one rocket diameter in front of this point (ie, its CoG is at least one rocket diameter in front of this point) then the rocket should be stable. Ideally it should be two rocket diameters. If the rocket needs further weight adding to the nose to meet this balance requirement then add it but remember that a rocket that weighs too much will not fly as high.
How much weight do I need to add to the rocket?
With the CoG and CoD in the correct places, you have what is effectively the minimum weight for the rocket. Next, you should run the computer model and see if adding extra weight makes the rocket fly higher (or further or whatever you are trying to acheive) and if extra weight is required then add this. If the ideal weight is less, you are effectively stuck unless you can move the CoD backwards by making the fins bigger.
How big do the fins need to be?
The fins need to be big enough to separate the CoD and CoG by one to two rocket diameters. You can shift the CoD backwards by making the fins bigger or by putting them further behind but there are limits simply because the fins have a weight of their own. They also add some drag.
What is a Drag Factor?
Most things in water rocketry can be measured. Even the drag factor can be measured although this is difficult and well beyond the means of only the most well equipped water rocketeer. Essentially, the drag factor is a fiddle factor that takes into account a number of factors and expresses them all in a number that gives the drag force on the rocket at a given speed and rocket diameter. Wind tunnel experiments have been done and example drag factors are on the computer models so there is no need to worry about getting a number for the computer models


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