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Day 30 -
Acceleron II Successful Flights - Launch Sound Analysis
Setting up Acceleron on the pad.
It is easier to colour the water at home and
bring it in measured doses.
Filling the segments through a funnel and
tube ensures only the lowest bottle is filled.
Attaching the in-flight camera with a small
The team poses next to the new rocket.
The payload section is filled with orange
coloured water to make it easier to see.
Frame from video showing the launch.
Peter is thinking, "hmmm that parachute has
more drag today"
Speed analysis - showing 19 frames of the
Inflight video frame - nice sun illumination
with ocean on the horizon.
A slightly different view of our park.
Two team members and their shadows.
Coloured streams of water can be seen top
The water from the payload starts draining.
Second launch showing the coloured water
Launch Sound Analysis, showing the different
features of water rocket noise during
Date: 31st March
2007 6:30am -
Denzil Joyce Oval. (launch site #4)
Clear skies and cooler temperatures. Light breeze.
(click the name for rocket details)
Team Members at
PK, GK, AK, John K and Paul K. Number
of launches: 3
We were eagerly anticipating this day
after having had to postpone last weeks launch of
Acceleron II due to the unfavourable weather. Although there was a breeze, it
didn't seem to affect the rocket's flight
Flight Day Events
On this day we were only going to
concentrate on the Acceleron II booster,
so we didn't bother setting up the other
launcher or bring other rockets. On any
flight day there are always too many
things happening and if you try to set
up and launch with different launchers
and rockets you are bound to forget
something or make mistakes in the rush.
It now takes us about 15-20 minutes to
set up Acceleron II for each flight.
We fitted our in-flight camera to
the side of the rocket, but this time we
used a little mirror to give a different
perspective on the launch (straight
down) while retaining a more streamlined
shape. The mirror wasn't quite long
enough and as a result the footage has a
small section of the video showing
On the first flight we had three
video cameras rolling plus a still
camera trying to catch the action. In
all the excitement we forgot to turn on
the onboard camera! We need that
checklist to go through before launch!
The rocket was filled with 1.5L in each
segment, about 200ml less than what the
simulator suggested. This was just
easier to measure out in 1.5L bottles.
Since we weren't trying to set any
records it didn't matter.
The rocket flew almost straight up and
the parachutes opened perhaps a
little early again, but close enough to
Both parachutes brought the rocket to a
safe landing. Only one bottle was dented
which was quickly fixed by blowing a
little air into it.
On the second flight we remembered
to turn on the in-flight camera. The
flight was excellent again and it looked
like a copy of the first flight. We
again used 1.5L in each segment. The
video turned out reasonably well, and
with a little repositioning and fixing
the mirror, should give us some nice
video in the future.
We also taped two high intensity LED
flashing broaches to the rocket, one
under the nosecone of one of the
booster segments and the other near the bottom
of the rocket, but they were still too
dim to see in the bright sunlight. They
will be good for night launches.
For the last flight we used 1.7L of
water since that was what the simulator
suggested should be used. The rocket
again flew a very straight path with a
slightly longer burn. The
parachutes again opened about the same
time so we were very happy with the
results. On the third landing a bottle again dented, but air should pop
it back into place and the rocket should
be good for the next launch.
Since the booster is a different
shape to the regular cylindrical
rockets, we also did some measurements
of the rocket performance to see how
closely the simulator predicts the real
rocket performance (see below).
Overall it was an excellent day of test flights,
and I think we are now ready to start
developing the second stage sustainer and
staging mechanism. If the booster continues
to perform the way it has we should be able
to get sustainer releases at close to
vertical. We believe the flights were about
70-80 meters in altitude, mostly due to the
early deploys, but this booster is not
designed to go high, it is designed to
accelerate and release a sustainer at about
12 m above ground.
I think the reason for the improved
stability of the booster over Acceleron I is due to its
increased length and having the ballast
further up in the rocket.
(If the video does not play, try the latest
Flash player from Macromedia)
We measured the approximate speed of
the booster to just after the start of the
air pulse. See diagram at left. We couldn't
quiet get all the measurements for the
booster well after the start of the air
pulse but we got a fairly accurate figure to
use in simulations. The speed measured at
~12 meters was a surprisingly low ~27.5
m/s. This means that a little time later
it may have peaked at around the 30 m/s.
Simulations predicted around 33 m/s at
burnout + 14.3 m/s increase due to air
pulse for a total of 47.3 m/s for
this rocket. There are a lot of factors that
influence the simulator predictions so the
result is not a great surprise.
This is why it is important to fly real
rockets and not just rely on simulations. We
will need to do this experiment again but
measuring the speed further up in the flight
The burnout altitude was within 30 cm of
what the simulator predicted, well within
the margin of error.
Launch Sound Analysis
We had a close look at the sound produced
by the rocket, as much more accurate
measurements can be made on the timing of
events on the rocket. (refer to diagram at
You can clearly see the click the
rocket makes at is leaves the launch tube.
With that you can accurately measure for how
long the launch tube is effective, and get
the rocket's initial acceleration.
You can also measure exactly how long
the water phase is when comparing your
rocket to simulators. There is a
distinctive transition to air pulse.
can also measure the length of the air
pulse to a certain degree but as the
pressure tails off in the rocket it is
hard to determine where the air pulse
When we zoomed in on the start of the
air pulse there was a distinctive set of
regular spaced pulses, with fairly high
amplitude. The camera easily picked these
up at a range of perhaps 20 meters. From the waveform we measured
the frequency to be 297 Hz ever so
slightly increasing over time, as their
amplitude decreased before being swamped
with noise. They are clearly visible for
about 20 ms (see diagram).
We do not know what these represent,
whether they are perhaps some sort of
sonic shockwaves, as the air accelerates
close to mach 1, or the resonant frequency
of the rocket or nozzle.
Since all objects have a resonant
frequency, we tried just blowing across
the nozzle of the rocket and recording the
sound. We couldn't quite get a pure note,
but when we removed the nozzle we got
quite a nice pure note. The resonant
frequency of the rocket segment in this
case was: 122Hz which isn't close to the
frequency we observed. The waveform shape
in this case was also very different -
more sinusoidal rather than the sharp
pulses. This is why we are leaning to some
form of shock waves rather than just
Thinking that perhaps this was a
unique case we reviewed the audio from a
number of launches and other rockets and
similar pulses were present (in both
stereo channels) although in this example these were the
Since the audio was gathered from the
ground there is likely to be a Doppler
shift in the audio frequency also, but
since we don't have a clear measurement
how far away from the rocket we were, it
may be difficult to accurately predict the
expected Doppler shift.
We don't know if these pulses at the
start of the air pulse and their frequency
are significant, but it may turn out to be
important when designing CD nozzles.
If you have any suggestions about the
cause of these pulses, we would love to hear
from you - contact us
A very good flight. Filled with 1.5L
of coloured water and about 600mL in
the dummy payload. The rocket went
up very well and the parachutes
opened just before apogee. The
rocket landed well. Had camera on
board but no footage.
A very good flight again. Filled
with 1.5L of coloured water and
about 600mL in the dummy payload.
The parachutes opened just before
apogee. The rocket landed well. Had
camera on board and we obtained
Very good flight again without a
camera but used 1.7 L of water in
each segment. Again the deploy was
Notes to Self
Food colouring is really messy,
especially the blue stuff in high