I promised to write up some of the rocket details for a few people, so here goes…

I’ll start with some pictures of the actual launches, then talk about the technical detail.

Preparing

Ready for pressurisation

Launch

Drogue parachute deployed perfectly, but no main

Probably a good job the main parachute didn't deploy, else we'd have ended up in the trees rather than the brambles. Also learnt that if you stand around pondering for long enough there's bound to be a sillier idiot than you willing to go climbing through the brambles.

Oh, and we did have some minor mishaps too...

Launch video (which I seem unable to embed)

Rocket body pressure chambers

The failed 'Anna Chapman' rocket

After the impressive sight, but inevitable failure of the Anna Chapman last year (launch video) I fancied building something similar but hopefully more successful. Last year we did find that 500ml bottles seemed to have a lot more bang for their buck than 2L bottles but this didn’t deter me too much. I’m still not really sure why it was, but I imagine it had something to do with cross sectional area versus volume. I surmised a longer rocket might well work better.

Inspiration from a rather impressive looking Air Command rocket using asymmetrically spliced bottles.

Air Command Water Rockets is an excellent resource, with some amazing launch videos which I wanted to try and at least go some way towards replicating. After some research I followed their Asymmetric Splice #5 Tutorial to build a number of spliced double-ended bottle sections. I shan’t repeat their tutorial, but I ended up using different glue as PL Premium is all but impossible to get hold of in the UK. I used Sikaflex EBT from Screwfix to seal the splice and a 2:3 Gorilla Glue:Polyfilla mix for the sleeving (more details on this Water Rocket Forum thread with links to Flickr pictures). Needless to say my garage is now covered in Polyfilla and glue; it gets everywhere.

IMPORTANT TIP: Wear gloves when handling Gorilla Glue; and replace said gloves if they break. Even the small amount I got on my thumb through a hole in my glove took over a week to wear off. It’s evil stuff.

The benefit of making a series of pressurisable ‘units’ rather than one long one is that they can be individually static pressure tested and the failures thrown away without affecting the rest. Having said that I am tempted to try and make a three or four bottle long ‘unit’ at some point.

Connected spliced bottles ready for launch

Bottle joins

The plan was to then join three or four of these together into a longer rocket using Tornado tubes from Hawkin’s Bazaar; but at £3.50 each they do work out rather expensive and the Northampton branch doesn’t seem to stock them.

Foreground left: Joined bottle connectors with various sized holes to allow air/water through. Foreground right: Gardena nozzles made to fit bottle top. Rear: Failed bottle top connectors.

From a thread on the Water Rocket Forum I came across a bright idea that Dan Brooker from British Water Rockets had to fuse two bottle tops into a coupling. He has made a demonstration video of how to do this. Despite his lack of testing at that stage I thought it worth a go. My success rate was less than one in three and I began to run out of bottle tops, but I found that some of my attempts withstood a static pressure test to 100psi. I’m not sure how much higher they’ll go, but Dan has reported some failures since. YMMV.

Talking of failures… when carrying out static pressure tests be wary of how much water leaks and air is let in – I had an inadvertent launch at 10 o’clock at night into next door but one’s garden.

Launcher

My old Clarke cable tie based launcher

Martin's Clarke cable tie based launcher, with 21.5mm launch tube

New launcher with optional boosters (annotated)

Having read about possible options on Air Command Rockets, I decided to have a go at a Gardena (Hozelock) launch mechanism. Reasons were two-fold: a. it might seal better; b. the smaller nozzle may sustain flight longer (albeit at a lower force, but that comes in handy when using boosters). I could always revert to the old launcher if all else failed.

Safety

A safe distance from the launcher - hosepipe and string allows remote compression and launch

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.

Others may not have thought ahead so much...

Martin getting a tad soggy. Note: Some of the photos I've used, including this one are from one of the 2010 Ballistics events

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.

Booster with hooks

Hook receptacles on main body bottle

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.

Drogue (40cm) and main (1m) parachutes

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.

Activated parachute mechanism

Parachute mechanism atop the rocket. Upper half of the top bottle contains the drogue chute and R/C release mechanism; the lower half the main chute which failed to deploy

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.

Despite some initially poor weather, it dried out, brightened up, and the wind died down to allow us to launch some stuff!  Check the flickr feed here.  For those interested in the telemetry data from the payload we sent up, it looks like this.

Rocket

Quite a short run as the rocket didn’t take long going up or down.  Acceleration data isn’t terribly readable – the peak acceleration is obviously off the +- 4g scale of the accelerometer.

One bad data point on the pressure causes the smoothed graph to have a small dip in it, but otherwise the data appears consistent (if unverified).

Temperature data isn’t useful, the sensor is cooling from a previous high of 35C from being wedged into the rocket nose cone.

 

 

Plane

29th August 2010!

That’s when the Racecourse Park will echo to the bizarre sounds of our tennis-ball lofting machines. Meet at 2.30pm, and we’ll start cooking at 4pm. There will also be chemical and water rockets again, with material for those wanting to build water rockets.

There’s no specific plan for specific categories of launch. If anyone requires this, leave a comment here, and one will be written.

As well as the standard pub meet this month (Tuesday 17th August) NortHACKton is holding TWO other events. The dates for these events are up for selection by doodle poll until the 27th July where I shall pick the highest-voted days out of a hat of some kind, moderated by any mitigating factors that become apparent (site availability etc). Vote now!

Hack Evening Workshop

Our first workshop will be a series of short informal talks. These will not be held in the public area of a pub, but either a back-room or a community centre. Contact the mailing list or me directly if you’d like to give a talk. There will be time for questions and networking after each talk, and a projector for those with slides.

Ballistics Day

The previous rocketry day was a success. So we’re going to repeat it, but with the addition of trebuchets, or other catapults or methods of propulsion. Bring tennis balls, and then show how far you can propel them. Again, there will be a barbecue, and some raw materials for hacking on the day.

A good meet today. We kicked off with some reminiscing about the rocketry day and some of the more entertaining lessons learnt (such as not launching chemical rockets into the wind). After this everyone agreed that we should do it again. Rather than merely repeat the rocketry day, it will instead be extended to also include siege weapons capable of throwing tennis balls, such as catapults, slingshots, trebuchets and the like. More information and a doodle poll soon.

Second event we would like to hold is an evening of talks, with an eye for holding this regularly. Location submissions are accepted for somewhere cheap and quiet. We’ve got a projector already, if a bit old. Again, I’ll release poll information tomorrow. Suggestions are also welcome on the list or as a comment here for places to advertise either or both of these events.

Finally, I am very pleased to announce the donation of a number of electronics boards. I’ll write this up in a separate post, as it’ll take a while to enumerate them and provide links to the right websites. But we now have toys that will go on the wiki and people can book them out and do what they like with them.

With the success of the (probably annual) NortHACKton rocketry day just behind us and the ever likely possiblility that there will be another day shortly to launch the chemical rockets; I thought I would take the opportunity to write up my two little rocket launcher designs so that other people can copy and expand upon should they wish. Alternately it’s only 363 days till the next rocketry day so get going now and consider that to be tinkering time.

Cork and Valve Launcher

I wanted to start with something so simple that anyone could make their own and then go play with it. You will need:

• A cork (I bought Youngs wine corks from Wilkinson)
• A schraeder valve (Just ask at your local tyre garage)
• Some selotape

1. Cut one of the corks in half
2. Drill a hole through the cork a little smaller than the neck of the schraeder valve
3. Insert the valve into the cork
4. Wrap the cork in one or two layers of selotapec
5. Fill the rocket with water and then insert the cork into the neck of the bottle

• To Launch turn the rocket upside down and fill with water, wedge the launcher into the neck of the bottle and then prop the bottle into a vertical position, I did this using some metal camping pegs. Start pumping, when the pressure inside the bottle is sufficient it will force the cork out of the bottle and the rocket will take off.

The reason I think this simple launcher was so effective is that it takes time for the cork to slide out of the neck. If you pump quickly then you can get a suprising amount of pressure into the bottle before it launches. Mine were often getting to 80 PSI. If the cork becomes too loose in the bottle then just wrap another layer of tape around the cork.

Cable Tie Release Launcher

The cable tie release mechanism is a rather elegant solution to the problem of holding the bottle onto the launcher.  This design is a very much minimalist version of the system but was employed with great effect on the rocket day. All the fizzy drink bottles have a little lip of solid plastic just below the cap. This launcher makes use of that lip by gripping it with the cable ties, holding the bottle down until it is ready for release.

You will need:

• 21.5mm outer diameter pipe (this is tricky to get hold of and I resorted to the internet)
• 40mm diameter waste pipe (available from B&Q etc and the 40mm I believe is the inner diameter)
• 20-30 cable ties (these need to fit around the edge of the bottle but inside the waste pipe)
• A schraeder valve (might as well get several from the tyre place, they’re handy)
• Hot Glue
• Selotape

1. Place the 21.5mm pipe into the neck of a bottle, I used a 250ml coke bottle as it’s easier to wave around at this construction stage.
2. Loosely pull the black cable ties together and place them around the pipe, these will be used to hold the white cable ties in place. (It’s only black and white so your know what I’m talking about)
3. Add white cable ties around the pipe making sure they all line up with the lip around the bottle neck. My design used 15 cable ties and I found it much easier to space them by using an extra cable tie between every third tie.
5. Pull the black ties tight, this will hold all the other cable ties down. There should be roughly 50mm between the first outer tie and the top of the horrizontal ties. Then carefully remove the spacer cable ties, try not to move the other ties at this point.
6. Trim the cable ties to the desired length and apply a liberal coating of hot glue to the pipe and the ties. Make sure to get the glue right down into the gaps where the spacers were and also into the end of the outer ties to prevent any possibly loosening.
7. Cut a length of waste pipe to go over the cable ties, locking the bottle into place. I pulled mine manually so I kept mine long to grip it, but it could be as little at 20-30mm for other methods as long as it covers all the cable ties.
8. Finally hot glue the schraeder into the bottom of the pipe. A better design would be to pack the valve with a cork to make it fit the gap better/tighter but hot glue will do the job adequately.
9. Add a few layers of tape to the make the pipe fit the neck of the bottle snuggly, this will allow you to build up the pressure inside the bottle before release.

• To launch the rocket, fill the bottle with water and connect the launcher to it while it’s upside down. Slide the waste pipe down over the neck of the bottle and the cable ties securing it in place.  Turn the rocket the right way up and prop it in a vertical position (again with the tent pegs). Pressurise the bottle and then when you are ready to launch simply pull the waste pipe down from around the neck.

Feeling the need to create something needlessly complicated to short a battery across the ignitor of the chemical rocket motor I turned to my Arduino Mega.

I already have a dot matrix display for my Arduino and have been using it to scroll the word ‘NortHACKTon’ when we go to the NortHACKton pub meets so that those people looking for geeks in a pub know who we are ”(cos we would’t stand out enough without it )”. I figured the dot matrix could be used to provide a visual countdown and the Arduino could activate a relay which connects the 9V supply directly to the rocket and then whoosh off it goes.

Schematic
It’s a very simple little board, consisting of a driver chip and a relay and the connection to the dot matrix display. I chose to use the ULN2003 to switch the relay primarily because the package already contains a freewheeling diode and can be directly driven from the arduino and secondly because I have 3 tubes of them in the shed already due to a misorder on a previous project. The dot matrix was also used on the same project. It is the 0832 from sure electronics and can be brought pretty cheaply on ebay, for only £7 inc P&P.

Arduino Program
Being an arduino most of the code is already written in other projects, (that is after all why we use arduinos isn’t it?). The program needs to read in the input from the micro switch to determine when the countdown should be running, it should only count down while the switch is depressed. When the countdown reaches zero the arduino should trigger the relay should be activated. Both of these functions are standard IO type code using digitalRead and digitalWrite. The DotMatrix is driven by the “Matrix Display” Library. The display is initialised when the program starts and images displayed by turning single pixels on and off.

I modified the Font.h file slightly to incorporate more of the ascii characters so I have included my version of the Matrix display library in the source code attachment. If someone were to be really keen I would suggest moving the character mapping function into the header too and creating the rest of the ASCII characters for the display. It’s on my list of things to do along with a variable width font, but that list is already pretty large so this won’t be done for some time.

The scrolling display displaying the NortHACKton url is created using the drawString functions provided as part of the library. The rocket and abort animations are defined as byte arrays at the top of the code and drawImage function simply calculates the visible part of the array and writes that data to the display.

Arduino Source Code

Steve setting up the rocket

The arduino rocket launcher ready for action

Demo Video

More photos of the rocketry day can be found here

On a cloudy Independence Day, over 30 people gathered on the Racecourse Park to make and launch water bottle rockets. This is our largest group event by an order of magnitude, and thanks to everyone who came. We’re currently waiting what look to be an excellent crop of pictures from @alanjames. There’s a few pics on the @northackton feed at the moment as well. When new sources of pics turn up they’ll be linked from here. The wiki page will also be updated.

To summarise the day then; we started setting up at just after 2pm, to a rain-free but very windy day (22mph according to the BBC). The first rocket was launched at about half two, and rockets were launched frequently until 4pm when the barbecues were started, which proved to be deliciously distracting. A number of families turned up, and bottles, card, and cardboard were used, under careful supervision, to make rockets for the kids, which were also launched. The wind proved to be a bit troublesome, as rocket fins would get caught by the wind and blow the rocket off course quickly.

Disappointingly the height competition did not go well as the theodolite I constructed couldn’t be used quickly enough to measure apogee well. It also may have moved when used for measurement as the angles it read seems surprisingly low. Martyn’s alternate device appeared to work more successfully, if it was in pre-war units!

After the barbecue the few remaining expressed their disappointment that it was too windy to launch chemical rockets. And then we launched some anyway! The first rocket had been launched before on a D class rocket engine. Unfortunately the C class used this time did not give it sufficient altitude, and the wind pushed the fins so that the rocket flew horizontally, and then landed before the parachute ejected. As it landed on its nosecone, this ejection charge merely blew the rocket apart, quite satisfyingly. The second rocket was much more successful. So successful in fact, that we don’t know where it landed, it was still afloat on its parachute when it left the park! So now we know, if it’s so windy the launchpad blows over, it’s too windy to launch. Everyone seemed to have a great time, and were all keen that we run another, so perhaps before September, maybe we shall run this again.

The poll is now closed, and the most popular date is the 4th of July. Football fans will be relieved to hear there is no World Cup match on this day. The start time will be 2pm. It’s a bit difficult to say exactly where we will set up as it will depend on who’s playing sports or sunbathing when we arrive, but I suggest that arrivals aim for the racecourse sculpture-thing which is opposite the main entrance of the Avenue campus of Northampton University. It’s about half-way down St George’s Avenue. We’ll have a gazebo in case of rain, so that should be visible, and if not, look for rockets.

We’ll start the barbeque towards 4pm. In order to minimise food wastage I’d suggest bringing your own food, as otherwise we’ll either run out or have too much. There’s no shops within staggering distance.

Suggested things to bring:

• A rocket, or a bottle to make one – preferably with water in it.
• Food for the BBQ
• Sunscreen/raincoat
• Friends!
• Anything you like

See (or edit) the wiki page for further information. I’ll keep an eye on the weather, and I suggest the backup day is the 10th if the weather threatens to be horrible.

The third meeting of NortHACKton took place on the 15th of June 2010, after Martin and I dropped in to see the NSME again, as they were having a group meeting in the park. A smaller meeting this week, but still rather productive. It would be difficult to point fingers at the World Cup as we were meeting during the Brazil vs. NorthKorea match.

Point of note for Tuesdays, the Malt Shovel does not serve food. Igor and and his girlfriend arrived and we had a discussion of the comparison of the Spanish and English climates, and sources for Arduino hardware. We also compared different embedded microprocessors, and talked about the FEZ platform, which is an open source microprocessor that is programmed in .NET instead of C. It’s £55 at cool components, which is a little more expensive than the other new entry, the mbed, but is a faster processor.

Yet another discussion was had about a high altitude balloon launch, and I think a long term goal of the group will be to support such an endeavour, as it certainly captures the imagination effectively. It may also be a good recruiting tactic, particularly if we were to send up something that had room for a secondary payload. Importantly though, it would be a technically complex job that requires some financial investment. It was agreed that postponing this until at least next year would allow for practice and research in the area.

Upcoming event: Rocketry

In lieu of a balloon launch, NortHACKton will have a rocketry day in the Racecourse park. This gives a chance to do something easy and fun. We’ll launch a range of water bottle rockets and hopefully some chemical ones as well. People can bring their own bottles and turn them into rockets and have them launched. This is relatively family-friendly, but under 18s must be accompanied by their parent or guardian. We will

PLEASE ENTER YOUR FREE DATES HERE. This will be closed in a week. If you’re planning on bringing kids or plus ones, then tell me to weight your votes appropriately.

We’ll hopefully have some guides on producing your own rockets if you’d like guidance or you’ll be able to make them on the day. There will be a separate blog post once a date has been chosen.

Upcoming event: T-shirt hacking

Another idea that was considered at the meeting was the idea of sewing microprocessors and LEDs to some T-shirts. Rather than buying Lilypads that are nice but cost more than the T-shirts, it was suggested we use PIC10s or ATtinys that are small and cost £1-2. These would need dedicated programmers, but be easier to sew.

We could have an evening trying this with some pre-programmed processors, and interested people can bring their own T-shirts to modify. A second possibility is that we have shirts printed with the NortHACKton robot logo on it, and add LEDs to the robot face and background.

Positive and negative feedback on this idea is welcomed. This would be run in the Community Centre on a Monday in July-August.

Links from the discussions

1. Seeed studio: open source hardware. Thanks to Igor
2. FEZ open source microprocessor running .NET. Thanks to Igor
3. CUSF balloon launch this week.

Date of next meeting: 20th July 2010.