Move Zig, For Great Justice

Friday saw our first games day for some time, having skipped the Christmas break due to overseasedness and lack of interest. Anyway, as a lunchtime diversion and uncle duty I thought it time to introduce my young nephews to the ..erm... educational and scientific applications of pyrotechnic devices. Actually, the fun of firing model rockets into the sky. Having recently established that such devices are still, surprisingly, legal I bought half a dozen of the more powerful mid-sized C6 series motors. Actually, by today's standards it seems these are now baby motors, with some very serious pieces of kit out there..

Anyway, both rockets lifted off successfully (ie: no scary looping at ground level this time) with the first (long) rocket reaching a decent altitude. Unfortunately it was overcast, making the pale balsa rocket hard to see against the cloud. It was also rather windy, meaning that we got a faint glimpse of it drifting off into the next suburb once it's streamer had deployed. Just like old times. The second (short) rocket appears to have suffered a failure during the boost phase, probably fin flutter. A few bits floated downwind but again nothing was recovered. Oh well, I suppose I get to keep the remaining engnes for another day. It's been a slow Easter, so I've done a post-flight report, which some of you have already seen. I've included it below:

Some observations from Friday's inaugural Games Day Lunchtime Launching. Firstly, it was interesting to note the different survival tactics used by observers when faced with an unguided and untested high-speed projectile..





The approaches seen in frame 1 from left to right appear to be:

1. See what Mum does.
2. Leg it.
3. Cover your head and hope for the best.
4. See and avoid.

From frame 2:

- Connor and Chris stick with their original plans.
- Matthew and Ja find something large to hide under, in this case Chris.

As for the actual rocket flight, analysis of the video footage for the 2nd launch and application of high school physics reveals the following:

- Time to estimated 2m height = 0.2 seconds (3 frames @ 15 fps)
- Average acceleration at launch = 10.2 G (100 m/s^2)
- For the C6 series engine used, thrust is approximately linear from 0 at t = 0 to 14 N at t = 0.2 secs. This means that peak acceleration at t = 0.2 would have been around double the average, or 20.4 G (200 m/s^2) - it would actually have been greater than this but I've assumed constant mass for simplicity..
- Velocity at 2m height = 20 m/s (72 km/h)



C6 Series Engine Thrust vs Time Plot

The launch sequence for the short rocket follows:


Igniter Fires


Engine Start


Liftoff


We Have Liftoff!


Cleared Tower


Gone!

The video does not capture the breakup of the vehicle, however analyses of the audio track was conducted. It was found to correspond nicely to the thrust curve and revealed:

- An initial structural failure appears to be recorded, followed by the distinctive sound of the vehicle tumbling.
- Failure occurred 1.55 seconds after launch during the constant (4.5 N) thrust phase.
- Vehicle was within 0.25 seconds of burnout.

The audio track analysis was a little big to fit on this page, but you can still look at it if you like.

To sum up, lessons learned from this experiment were:
- Don't launch in too much wind.
- The 1.6mm thick balsa I used for the fins were probably of insufficient stiffness for use with these engines - I'll try to find some stiffer quarter grain next time.
- You really need a clear sky in order to see the thing.
- Long rockets good, short rockets bad, or at least harder to get right.
- 50% of recorded observers chose to hide behind someone.
- Rockets are fun, we'll do it again soon while it's still legal.

Yes, I think we will..