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Day 159 - Measurement Errors
28th March 2015
Location:Whalan Reserve, NSW, Australia
Conditions:Clear skies, calm to light winds 0-5km,
Team Members at Event:Paul K and GK,
This week we look at measuring water rocket flight performance. When
doing experiments that affect the performance of a water rocket
you need to be able to measure how much better or worse the
rocket performs with each change. The problem, however, is that
the errors in your measurement may be greater than the effect
you are trying to measure.
There are multiple performance parameters you can look at
such as altitude, acceleration , speed, flight time, etc.
depending which parameter you are trying to optimize.
Typically peak altitude or flight duration are commonly
and so we'll focus on those in this experiment.
To look at the variability in measurements
between similar altimeters on the same flight and the
of those measurements between successive flights under the same
conditions. We also compare the flight duration variability on
the same flights
For this experiment we used our
Axion II rocket and installed
4 x AltimeterOne's next to each other.
Each launch was pressurised to 120psi. This was done by
setting the pressure regulator to one setting and then leaving
it on that setting for all launches. The rocket was
launched after air stopped flowing into the rocket.
Each flight used 1500mL of water measured on a scale to
minimize performance variation due to the water amount.
Weather conditions remained mostly consistent during the
experiment with calm to light winds.
Flight durations were measured using recorded video.
The measurements were taken from the rocket leaving the pad to the first
contact with the ground. The landing area was level.
We performed 5 flights with water only, and two additional
with 1500mL of water/foam solution.
Here are some photos from the experiment:
Setting to constant pressure
Ready to go
Measured water amount
Paul was a great help on the day
Here is a summary of the flight results:
1 - water
2 - water
3 - water
4 - water
5 - water
6 - foam
7 - foam
Differences in Altimeter readings
As we can see from the data
the altimeter readings for two of the flights (#1 and #4) were
within 2% of each other, with the rest of the flights within
For these flights that represents a difference of +/- 3 feet.
This is fairly good considering the
altimeters are only measuring air pressure. It also means that
the readings are fairly consistent between the altimeters.
Differences between individual flights
To compare the
differences in altitude between individual flights (#1 to #5), we use the
average of the 4 altimeter readings on each flight, and then
The difference in altitude between individual flights is more
significant and when we look at the averaged altitudes we see that they were within
12% of each other. The first flight was out considerably
so looking at flights #2 to #5 the difference was only 4.5%. For
flights #6 and #7 with foam the difference was about 7%. This
can be significant if you are trying to measure subtle changes
to the design of the rocket. The variations can be due to any
number of factors such as slight differences in the launch pressure, temperature,
launch angle and environmental conditions such as cross wind,
thermals and wind gusts. It would not be unreasonable to expect
errors as much as 10% when comparing separate flights
under the same conditions.
Differences in flight durations
Although the rocket used the same
parachute deployment time delay for all flights
the differences in flight durations were even more significant.
This is due to a number of factors such the time it takes for the parachute to fully
inflate which in turn can be caused by variations in packing
technique, and aerodynamic environment around the rocket at the
time of deploy. Other factors include thermals changing the
descent rate, and the attitude of the rocket under
parachute which can change the the total drag coefficient
resulting in different descent rates. The descent times
varied as much as 29%!
Although the numbers given here are specific to this rocket, it
does give a
representative idea of what to expect when performing your own
experiments. The errors can be significant and may completely
swamp any minor performance differences you are trying to
measure. This means for expected minor changes in performance,
you typically need to perform many flights and average those to
see the differences.