Simplest attempt is using a modified version of osmo-pocsag.  It writes out baseband to a file.  Create a fifo, and direct the the cmdline app to its output.

A more complex version is based on this (by bafe), and has some good waterfall pictures to give a better indication of what’s going on.  Confusingly it’s called gr-pocsag, and thus confusable with the POCSAG decoding GRC block by patchvonbraun here, which needs compiling and installing.

I’ll update this once I’ve actually either confirmed that what I’m seeing is not POCSAG or which/either/both/none works.

So for example, you can pick up FM radio, the pager network, transmissions from the ISS, or aircraft transponders.  Working GRCs will be posted on the blog

We’re going to try doing some of these things in the space over the next few months, the exact dates will be discussed on the list.  If you’d like to try this, here’s some links:

• Gnuradio.  You will probably need this build script for latest release
• The RTL binary itself
• The GnuRadio Companion source block for RTL

Another useful resource is the subreddit for it.

Onto newer things, we printed a NortHACKton stamp for visiting passport-holders.  After learning how to etch rubber, I then provided an object lesson in the power of cyanoacrylate glue by adhering to the stamp rather effectively.

Secondly I had made an SVG of a bokeh rig for my camera for Martin to convert and cut, which produced a passable first attempt at a kit that can fit to my D3100.  Not all of the pictures were good, but it was certainly a learning experience.  See Martin’s blog for the design, and the results are here, in the February pictures of this set.

March’s meetings – we meet in the space as normal for Monday 5th, and on the 19th there will be another pinball evening in Bedford.  See the mailing list for more details.

The polymorph was slightly more successful than the engine which due to some presumed error in construction, does not self-propel.  This is due to the piston being unable to turn the flywheel, itself due to the axle somehow jamming in place.  It still looks moderately elegant.

Next meeting 23rd January 2012.

• Light painting with a strobe
• Knitting
• Conkers fights
• CNC planning
• Raspberry Pi tinkering
• Kinect tinkering
• Workshop planning.

I may have even forgot some things!  Most importantly, thanks to Nicholas for bringing the Raspberry Pi board for us to have a look at.  We couldn’t do too much with it as unfortunately the monitor used was having resolution issues on the HDMI interface, but it booted, and we were able to login and have a brief look around.  Connected to a network (it supports networking over USB  too) would allow more interesting things to happen.  This rather overshadowed the Kinect Nicholas also brought, and I think we’ll need an evening just for that.

The NSME has a rather large horse chestnut tree overshadowing the clubhouse, so knitting wool was graciously donated to the cause, and an in a textbook case of over-engineering, a pillar drill was used for turning conkers into implements of war, and soon geeks were hitting themselves very hard while missing target conkers.

Light painting with the strobe did not work so well, as it is not as bright as a camera flash, and outside it is not able to light up targets at any range.  Strobes are very good for inducing headaches and nausea though.  If nobody wants it, it will be thrown away at the end of the month, as it’s reasonably bulky and has no (useful) purpose at this time.

Due to renewed interest in chainmail, Pat will be running a chainmail workshop on the 3rd of October, from 8pm.  If you have rings or pliers, please bring them along, and let us know on the list, so we can ensure everyone has enough.

We also discussed what events we could run at meetings.  Some suggestions:

• Nanode workshop
• Photography (light painting)
• Kinect hacking
• Revisiting OpenCV hacking

The idea is for very limited scope projects that allow a sense of accomplishment within a few hours.  Suggestions are welcome on the mailing list.

Date of next meeting: 19th September from 7pm

Safety

Oh, and being the sensible one, I elected to put a length garden hosepipe between the launcher and the pump. This reduced the risk of personal injury in the event of catastrophic failure and also kept the operator somewhat drier.

Nozzle

Air Command Water Rockets has a nice tutorial and videos about preparing a Gardena nozzle, so I shan’t dwell on that much. As they note, I had trouble finding a nozzle with a seal, so tried using some Sikaflex as a seal with mixed results. Ultimately, it was the nozzle that limited launch pressures to about 80psi, so this is where more work is needed to get a good seal between the Gardena nozzle and the bottle top.

Rather than the direct string pull I sought some mechanical advantage from a level arrangement for the launch release. In use we found that it still needed quite a tug to release and on one occasion the string snapped. Again, this needs more work.

Boosters

As if I’d not created enough trouble for myself already, I thought it would be cool to have more than one stage. Direct staging mechanisms seemed a little complex for a first go at anything more than the plain bottles we launched last year, so boosters seemed a sensible bet. Again, Air Command Water Rockets provided much of the inspiration with their Drop Away Boosters and Cluster Launcher pages.

The principle behind the boosters was pretty simple, but also very critical with a number of single points of failure (no manned versions of this bottle rocket design). The boosters were set up such that they would produce more force than the main rocket until exhausted at which point they passively drop behind the main stage. That’s the theory at least, my design wasn’t quite as sophisticated or refined as Air Command’s. The pictures below illustrate the coat hanger hook and pen cylinder receptacles making up the mechanism. They didn’t operate particularly smoothly, though maybe a neater effort would have better results – I was pushed for time.

Overall, the boosters didn’t work out too well. Even with PTFE tape around the booster launch tubes it was difficult to get a good, consistent seal on each of the three bottles; invariably, one let go and began to leak under pressure. And the minor mishap illustrated in the first series of pictures at the top of this post was due to one of the hooks detaching from its bottle.

As well launch efficiency, the reason for the launch tubes was to allow the compression line to remain dry/ This would allow the pressure to equalise between the boosters. However, looking at the failed launch with hindsight, we missed out a non-return valve. This meant that water from the main rocket filled the booster compression lines and prevented the pressure from equalising. I don’t think it was causal to the catastrophic failure, but it can’t have helped.

Parachute

I went a little overboard on the parachute. In the past I’ve found charity clothing bags ideal for building parachutes, but while light, they aren’t particularly resiliant and the my choice of tether line (dental floss!) got tangled easily. So I splashed out on some Ripstop Nylon from Ebay and got sewing.

Leo Singleton‘s Bottle Rocket Handbook (PDF) had a nice section on parachute design. I chose to build an elliptical design, calculated using his Elliptical Parachute Calculator (36KB ZIP)

My first parachute took the defaults from the tool and ended up a rather too large 1M diameter, still it would be a nice gentle float down. As I didn’t think I’d be able to deploy this on its own, I built a drogue of 40cm.

Parachute mechanism

Again, drawing on ideas from Air Command Rockets I built a Parachute Side Deployment Mechanism, with their elastic band release modification. I didn’t have time to acquire a Servo Timer II nor much faith in guessing the timing accurately so decided to use a 35MHz radio control receiver to control the server.

As the main chute was too large to fit within the elastic deployment pocket, I bargained that the drogue would lift the whole mechanism from the rocket top and deploy the main. On both parachute-recovery launches the drogue deployed perfectly, but the main chute failed. With hindsight, I should have attached the drogue directly to the main and had a hole or flap for it to deploy through.

Next time, I think I’ll build a second deployment mechansim, attach a medium sized (70cm?) parachute and trigger it shortly after apogee with an altimeter. The existing R/C drogue parachute can act as a back up.

I’ve written enough for now, feel free to ask any further questions in the comments below, or in person.