Each flight log entry usually represents a launch or test day, and describes the events that took place.
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Team Members at Launch Event: 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 path.

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 normal view.
• 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 apogee. 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.

Big water rocket 18.75 Liters - booster


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Speed Analysis

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 path.

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 left)

• 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.
  
  You 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 finishes.
• 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 resonance.
• 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 clearest.
• 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 here)

Flight Record

Notes to Self

• Food colouring is really messy, especially the blue stuff in high concentration.