(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.
powerful is a water rocket compared to a pyro
||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 ¼A and ½A, 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.
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.
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
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.
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.
is a reaction mass and how is this different in a
||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
I only use water as the reaction mass in a water
||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.
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.
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).
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.
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.
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.
do I check to make sure that my rocket is
||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.
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
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
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