Each flight log entry usually
represents a launch or test day, and describes the
events that took place.
Click on an image to view a larger image, and
browser's BACK button to return back to the
Day 165 - Liquid Density Experiment - Part 2
Location:Whalan Reserve, NSW, Australia
Conditions:Calm <~5km/h, sunny
Team Members at Event:GK and Paul K.
Liquid Density Experiment - Part 2
To investigate how different liquid densities (at the same
volume) affect water rocket performance.
Looking at the traditional thrust equation below, we see that
thrust or the force produced by the rocket is proportional to
the mass flow rate (m-dot). So if we keep everything else
constant in the equation, and increase the density, it should
result in greater thrust and therefore it should also follow
that the rocket goes higher. However, greater thrust doesn't
represent the whole picture!
If you increase the density of the liquid you may produce
greater thrust but you are also making the liquid that's still
inside the rocket heavier and so you have to lift more weight
which works against the greater thrust.
In the following example say if you have 1L of water the total
weight you are trying to lift is the dry weight of the rocket
plus the density times the water amount which in this case gives
you 1.2Kg. However, with a liquid density of say 1.5 and the same
volume of water you are actually needing to lift the dry
weight of the rocket plus 1.5 times 1 Liter which gives you
Other than weight there are a number of other factors that
affect a water rockets final altitude, such as the drag
coefficient, launch pressure, nozzle size, launch tube size and
length, nozzle efficiency, air density etc.
The actual mathematics behind how a water rocket's maximum altitude can be estimated is a lot more
complicated and I highly recommend reading
Dean Wheeler's detailed analysis on this subject.
We first ran a number of simulations for the following rocket
that we fly regularly. From past experience we know that
Heath's simulator is fairly accurate in predicting the
Here are the simulation results.
Each of the points on the surface of Graph 1 represents
one simulation run. Along the bottom axis we have the rocket's
dry weight, while the other axis is water fill amount as a
percentage of the total volume and vertically is the
predicted altitude. The liquid density for these first
simulations was set to 1 representing water.
The red line represents the optimal
water amount needed to reach maximum altitude. Note that the
optimal percentage changes depending on the dry weight of the
rocket. For this rocket that weighs
550grams, we can see that we need close 29% water fill to reach
maximum altitude. (Graph 2.)
We ran the simulations again, but
this time for each point on the graph (Graph 3.), we let the simulator run
another whole series of simulations that varied the density
between 0.4 and 1.6. The simulator then found which density
achieved the greatest altitude. Here we are plotting the
altitudes again in relation to the dry weight of rocket and the
percentage water fill.
At the same time the simulator not only gives us the maximum
altitude, but also tells us what density would give us that
optimal altitude. So we plot the optimal density on Graph 4. The graph again has the dry rocket
weight along this axis and the water fill percentage along this
one with the optimal density on the vertical axis.
The red line represents the density of water. Anything on
the upper side of the line means a higher density liquid achieved
greatest altitude while everything on the lower side achieved greatest
altitude with a lower density liquid. Each point on Graph 4
corresponds to a point on Graph 3.
So for this particular rocket that weighs 550grams, we can
see that the optimal fill percentage this time is closer to 31%
and it corresponds to a density of about 0.75. (Graph 6) However, the difference in predicted altitude is
only about 4 feet higher (362 feet [110m]) with the optimized density vs plain
water (358 feet [109m]) That corresponds to only about 1-2% of
the overall altitude so its quite small. You can also see from
this graph that if the rocket weighed more that 1Kg then the
rocket would perform better if it used a liquid denser than
Practical Experiment - Method
For the purposes of this experiment, the performance of the water
rocket is measured by the altitude it achieves. The higher the
rocket flies the higher its performance.
A rocket with the following parameters was made for this
experiment. A small volume rocket was chosen to reduce the
amount of different density liquids that would be needed for multiple flights. It also represents a typical small water
rocket flown by many students.
The rocket uses a parachute deployed by a timer to bring it
back safely after each launch. The timer was adjusted so that
the parachute opened well after apogee allowing a clean maximum
altitude reading to be obtained. The altitude was measured using
the AltimeterOne barometric altimeter with a resolution
of 1 foot.
The following liquids were prepared for comparison.
D = 450g /
D = 556g /
D = 360g /
D = 196g /
The sugar solution was prepared by dissolving 700grams of
sugar in warm water, and adding enough water to make up to
1100mL. Three drops of food dye were added to each liquid to
make it easy to identify which liquids were used in photos and
The ice cream was first melted and measured also by volume
and it had the lowest density because it was full of bubbles.
The problem, however, was that by the next day a lot of the
bubbles escaped and so the 450mL of measured volume was actually
closer to 250-300mL. For this reason we did not use it in the
comparison flights. We did fly it a couple of times just to see
what would happen though.
Weighing sugar solution
Mixing sugar solution
All equal volume
The rocket was launched at 120psi (8.3 bar). Consistency
between launches was achieved by setting the pressure regulator
to one setting and leaving it there for all of the flights.
450mL of liquid was used for each launch as it is close to
the ideal 1/3 capacity of the rocket. We measured the exact
amounts into individual bottles the night before launch to make
it easier to conduct the experiment on the day. Six flights were
performed with each liquid type and the results were recorded.
Following is a table of all the flights and their recorded
Fail - early deploy
crash - parachute deployed, but failed to
due to sugar.stickyness.
The results have been grouped by fuel in the
graph below. The graph also shows the average altitude for
The average altitude for the water was
186.8 feet. The altitudes varied by 10.7%.
The average altitude for the alcohol was
192.3 feet. The altitudes varied by 13.5%.
The average altitude for the sugar
solution was 179.4 feet. The altitude varied by 17.3%.
Here are some photos from the experiment:
Pouring liquid into rocket
Putting rocket onto launcher
Back to the launch site
Doing it all again
This time with alcohol
Sources of Errors
The variations in the
measurements for each fuel type were due to
a number reasons including the
due to environmental conditions such
as cross wind.
differences due to temperature and
air supply setting.
For this particular
rocket configuration and launch
pressure, the obtained results
seem to confirm the simulator
predictions that higher density liquids
can perform worse than lower density
ones at the same volume. The sugar
solution which was 25% more dense made
the rocket fly 4% lower than normal
water. The alcohol which was 20% less
dense, made the rocket fly 2.9% higher
than the normal water.
The differences in
altitude were small when comparing the
different liquid densities to pure water. If you are looking to get peak
performance from your rocket, there are
better ways to create a low density
liquid than using volatile liquids like
alcohol. See foam vs water comparisons
- Day 144.
When placing the
alcohol filled rocket onto the
launcher it always sprayed a little
more than the water does. Not sure
why that was.
through the system creates an
incredibly sticky mess on
everything. If you are going to try
the same experiment, bring lots of
fresh water to wash everything down.
What you miss, the ants will eat.
We suspect that the
early deploy was because the break
wire was coated in sugar and
provided poor contact. When setting
up the rocket for the next launch we
found that the break wire was
unreliably triggering the timer.
After washing the contacts with
fresh water, the breakwire worked
We opted not to try
salt for increasing density, as salt is corrosive and also kills