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Use higher pressures. As long as
the rocket's pressure chamber remains within
safe limits, increasing the pressure can have a
significant impact on altitude gain. It may be possible
to reinforce the rocket to hold higher pressures without
adding too much extra weight.
-
Keep weight to a minimum. Every
rocket has an optimal weight. Small and simple single
bottle rockets may sometimes be under their optimal
weight, and adding a little weight to the rocket may
increase altitude. Due to construction techniques,
larger rockets typically come in above their optimum
weight and as a result need to be built as light as
possible. Use a
simulator to calculate the optimum weight for a
particular rocket. Also keep any payload weight to a
minimum.
-
Increase rocket volume. Generally
increasing a rocket volume will also increase altitude.
The best way to increase the volume is to make the
pressure chamber longer. This doesn't increase drag
significantly, although there is a corresponding weight
penalty. Increasing the diameter of a rocket to increase
the volume will not only result in more weight, but more
drag, and generally lower the maximum pressure the
pressure vessel can hold.
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Streamline the body of the rocket to
reduce drag. Avoid any unnecessary protrusions into
the air stream. Keep the body of the rocket as smooth as
possible, avoiding sharp transitions. The ideal shape is
an elongated teardrop. Depending on the construction
materials available, a minimal diameter rocket can
reduce drag significantly, at the cost of volume. A
smaller diameter rocket can also hold higher maximum
pressure.
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Use a launch tube on the launcher.
A launch tube can have a significant
effect on the apogee of a rocket. The longer the launch
tube the better. The diameter of the launch tube should
be as big as possible and should be about the size of
the nozzle to reduce the amount of water loss as the
rocket accelerates up the tube. Note that a maximum
sized nozzle may not be the most optimum size after the
rocket leaves the launch tube. Consider using a
T-nozzle
for better efficiency.
-
Use the right amount of water.
While a third the volume may be a good approximation, every particular rocket will have an
optimal water fill based on its weight, drag
coefficient, pressure, nozzle size etc. Use a
simulator to predict the best amount of water to use
for each rocket configuration.
-
Use an optimum sized nozzle. The
nozzle diameter should be optimized based on the various
rocket parameters. Use a
simulator to figure out the optimal nozzle size.
There may be limitations on changing the nozzle size due
to the type of launcher and launch tube used.
-
Use multiple stages. Correctly
designed multi-staged rockets can increase the altitude
of the sustainer over single stage designs. Consider
your highly optimized rocket sitting on top of a
booster.
-
Optimize stage release timing.
Releasing the next stage of a multi-stage rocket is
critical in maximizing the altitude reached. Use of
real-time in-flight measured flight parameters for
initiating staging can achieve best results. The best
time to release the next stage is just after booster
burn out just as the booster starts slowing down.
-
Use a boat-tail on the rocket. A
smooth transition from the rocket body diameter down to
the nozzle will assist with
base drag reduction.
-
Allow the air to cool inside the
pressure chamber. As air is compressed inside the
rocket it is heated. As the air cools, the pressure will
drop in proportion to the temperature decrease. You can
trickle fill the rocket before launch to
make sure the optimal pressure is achieved.
-
Streamline the leading and trailing
edges of your fins. To reduce the profile drag of
your fins they should have an aerofoil profile. See
this document for more details.
-
Use 3 fins instead of 4 or more.
If the launcher allows it, and the rocket is otherwise
designed to be stable, the use of less fins should
result in less drag and less weight on the rocket.
-
Use optimally shaped fins. The
fins should have an optimal shape. See
this document for more details. The optimal shape
will vary based on the rocket design and the rockets
flight profile.
-
Use optimally sized fins. While
having the correct fin profile and shape is important,
it is also important to not make the fins too large.
Fins that are larger than what they need to be add to
the drag and weight of the rocket. Large fins may also
cause the rocket to be over-stable.
-
Ensure smooth internal water flow
through the nozzle. Increase nozzle efficiency by
ensuring non-turbulent flow of water and air from the
pressure chamber and through the nozzle. There should be
no sharp transitions in the flow. Polish the inside of
the nozzle.
-
Fly on a windless day. Wind will
cause the rocket to weather-cock into the wind causing
it to fly in an arc and achieving a lower altitude than
if it went straight up. The amount of weather-cocking
will depend on the rocket's stability design and the
wind speed. A rocket that is
over-stable will tend to
weather-cock more.
-
Use a rounded nosecone. Parabolic
nosecones are the most efficient for water rockets as
they travel well in the subsonic range. Here is a
nosecone shape comparison
document detailing common nosecones used by model
rockets.
-
Use a less dense liquid. Lower
density liquid can have a positive effect on raising the
apogee of the rocket. However, using a liquid other than
water may mean that the rocket may not be considered a water
rocket. Changing the density of water can be achieved by
aerating the water such as in a foam. Use a
simulator to predict the altitude of a rocket with a
lower density liquid.
-
Use a heavier gas. Some gasses
like CO2 can provide better performance due to their
heavier molecular weight and hence provide a greater reactive
mass.
-
Align the fins properly.
Misaligned fins can cause more drag and potentially
excessive rotation of the rocket. The rocket looses
energy due to drag and some of the energy goes into the rotation
of the rocket. Fins should also be as rigid as possible
to prevent fins fluttering.
-
Make the rocket stable. An
unstable rocket will not fly straight and achieve a
lower altitude. Rockets should be designed to be
stable when they are dry. The boost phase is
generally very short with larger nozzles and so the
rocket spends most of its ascent dry. When using smaller
nozzles, the rocket should be designed to be slightly
more stable to account for the longer duration of the
water being in the tail of the rocket.
-
Remove internal obstructions.
Streamlining the internal water and air flow adds to the
efficiency of the rocket. If the construction techniques
allow, consider removing flow constrictions such as
couplings/baffles to insure most efficient flow and
prevent water being retained during the thrust phase.
-
Fly from higher elevation launch
sites. Starting at a higher altitude means the air
is less dense and therefore the rocket will experience
less drag. For example on average the air in Denver,
Colorado is ~15% less dense than at sea level. See here for
elevation vs air density graphs.
-
Launch rockets into thermals.
Flying a rocket in a thermal can add extra tail wind to
the rocket reducing drag. Here is an
extensive document on thermals, how they work and
how to find them. Thermals are also useful for
increasing the air time of your rocket.
-
Point the launcher as vertically as
possible. All things being equal, a rocket that
flies 2 degrees away from vertical will fly about 0.5%
lower, and a rocket that flies 5 degrees away from
vertical will fly about 2-3% lower.
-
Optimize direction of second stage
after staging. Ensuring the next stage of a
multi-stage rocket leaves as close to vertical as
possible can be tricky, but is essential in reaching
maximum altitude.
-
Launch rockets on a humid day.
Humid air is
less dense than dry air. Lower density air will
lower the drag on the rocket. At standard temperature
and pressure at sea level, 100% humid air is
approximately ~1% less dense than dry air. See the
air density calculators for more information.
-
Launch rockets on a hot day.
Higher air temperature means lower density. Here is a
document relating air pressure, air density and
temperature. See the
air density calculators for more information.
-
Grease the launch tube for less
friction. If you are using a launch tube with your
launcher that has a relatively tight fit on the nozzle,
make sure friction is reduced by lightly lubricating the
launch tube. Less friction will result in higher
take-off velocity.
