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Rocket Blog


Bottle Splicing

posted 29 Sep 2017, 07:50 by Andy Lakin-Hall

It's time to try making a bigger capacity rocket by splicing two 2 litre bottles together.

There's some good guidance to be found online for this, but I'm following the US Water Rockets Guide.

I finally sourced some PLA adhesive to try this, which has proved difficult in the UK.

I've prepped two bottles, removing the labels and stickiness with WD40.

I cut the bottoms of both bottles as straight as I could, removing all the burrs.

One bottle needs to fit inside the other tightly, and to do this it needs shrinking in hot water. My first attempt was a disaster. I boiled some water in a saucepan and waited for it to cool down. But when I dipped the bottle in, the water was way too hot and the bottle shrank way too much. 

For my second attempt, I started from cold water and then kept dipping the bottle as the water heated up. In this way, I was able to rapidly dip the bottle and then check to see if it had shrunk just enough to fit inside the other bottle.

At least. That's what I should have done. In fact I cheated and used two different bottles. I found that a PET water bottle was already fractionally larger than the lemonade bottle, and they fitted together snugly enough. I smear of PLA glue and the bottles were fixed together.

I used a length of PVC pipe to make sure the bottles were aligned correctly and support the splice as I let it dry over a week.

In the spirit of Werner Von Braun, I decided to go for some all-up testing, pressure testing both this splice and my new 3D Printed nozzle. This also taken from a US Water Rockets Design on Thingiverse.

I forgot that failure is always an option. As I pumped up the rocket to 20psi , everything seemed to hold securely. I left it there for 5 minutes, and the pressure remained stable throughout.

Then there was a sudden loud bang. The splice had failed, sending the upper bottle chamber straight up into the ceiling, and making a neat hole.

  

The upper chamber was completely destroyed, and the lower chamber was bent back. The fins were recoverable, so I can use them again.

Now I have repaired the hole in the ceiling, I can look at what happened. Clearly the problem was the splice was not strong enough. The two different bottles obviously were too different in size for the splice to hold.

For my second attempt I am starting with two identical bottles and shrinking one very gently until the two bottles only just slide together. The slide together with just a bit of water to lubricate, and then are quite difficult to pull apart.

I've roughened up the inside edge of one bottle and the outside edge of the other, and then spread PLA adhesive around the area to be joined. Not too much - just enough for a thin consistent layer.

This is now drying as before. I have also reinforced the splice with some coloured Duck Tape. 

Box Fin Design

posted 15 Jul 2017, 07:25 by Andy Lakin-Hall

I've been looking for a new design for rocket fins, and came across the Box Fin from US Water Rockets. These have the advantages of being adjustable, and they self-align to the side of the rocket, so I don't need the fin gluing jig to hold things in place in straight alignment.

The template that USWR provide needs scaling to fit the diameter of the bottles I'm using. I used some string to wrap around the circumference of the bottle I will be using, and then used that to calculate the diameter, and then the width of the fin shape. I opened the PDF template in Inkscape and copied the fin shape from there. That was then pasted into a new Inkscape file and scaled to the correct dimensions. My printer scrambles direct prints from Inkscape, so I exported the file as a PDF and then printed that on card at 100% scale.


 I cut out the card fins and tested them against a bottle, to make sure they fit correctly.

Satisfied with the card templates, I printed out three new copies and used repositionable spray-mount adhesive to temporarily glue them to sheets of yellow corriflute.

I cut out the fins with a craft knife, scoring through the central line to make a hinge. Then I used blue PVC tape to cover the edges and join the three parts together. Here they are fitted up against a bottle to test their fit.

The new fins can be mounted the the eventual rocket, either with velcro or with bamboo skewers as described in the USWR guide.

If you're interested in the template I made, which fits standard 2litre fizzy bottles, you can find it here.

3D printed Pikachu

posted 29 May 2017, 13:03 by Andy Lakin-Hall   [ updated 29 May 2017, 13:42 ]

Duncan found a Pikachu model on Thingiverse. I needed to make a couple of changes to the design, including mounting the model on a larger base, that also had a hollowed bottom to avoid warping. I added lettering to the edge of the base too. Duncan added a hat with a peak to top things off.

The model was smoothed, and then primed with white spray primer, before a base coat of acrylics.


I added a wash of thinned orange to darken shadowed areas, and then added some highlights and detail to finish.




3D printed Froglets

posted 29 May 2017, 12:41 by Andy Lakin-Hall

Here's another project I've been working on with my 3D printer. Little model froglets which I can use as Geocaching treasures. 

The project started with a design in Tinkercad. 

The model is made from a sphere, stretched in the Z axis, supported by four parabaloids. The model is not manifold, so I had to fix this by uploading the .stl file to https://service.netfabb.com

I arranged six duplicate models on the bed for slicing, and then printed them. The array of six created a large enough area that adhered to the bed very well.

One of the limits of my printer is of fine details. These froglets are small, so any imperfections in the print stick out like a small thumb. These blemishes need filling off.

Once smoothed down, the next step is priming. I used three coats of white spray primer. The paint fills in many of the remaining blemishes.

After the primer, next comes the base coat. These three are painted with acrylic, while another three were sprayed with green car spray paint. The car spray made a very glossy smooth surface, while the acrylic was better for detailing.


3D Printed MicroBit clip

posted 27 May 2017, 13:56 by Andy Lakin-Hall

The plan is to eventually 3D Print an entire parachute pod for the rocket. I've started with a clip for the MicroBit.

I have designed the clip in Tinkercad using a model of a MicroBit as a guide.

The clip will allow a Microbit to be snapped into position easily, and then remain in position firmly during launch and flight.

The first print lifted a little in printing, so the base is a bit bent. That's down to me still learning about how to get prints to stick to the printer bed properly. My basic 3D printer does not have a heated bed, so this is a bit trial and error at the moment.

I also need to improve the side clips, to give them a little more flexibility. They've come out a bit rigid, but it's going in the right direction.


3D Printer

posted 27 May 2017, 04:49 by Andy Lakin-Hall

I have been fortunate enough to be able to get myself a 3D Printer. I bought the CoLiDo DIY, which I got as a self-assembly kit for just over £200.

https://www.colido.com/en-uk/product-uk/colido-diy/

This is very much an entry-level printer. It was quite easy to assemble and get started with. You have to level the bed manually using a card, which is a bit hit-and-miss. The bed is not heated, and it is tricky to get the first layer of print down without lifting off.

Adhesion to the bed has been a problem. I've found that frequently a model will reach 80% finished, and then start to slide about. I've tried to remedy this with sticking fine-grade sandpaper to the bed with repositional spray-mount. I've also been able to pause the print and then add blobs of blu-tac to the base to hold it still, with varying results.

 


Evaluation of MicroBit Pod

posted 5 Apr 2017, 08:40 by Andy Lakin-Hall

The new rocket pod is not as good as I hoped it would be. There are one or two problems which are causing the parachute pod to fail to open properly.

Battery Box

This is currently at the top of the pod. With three AA batteries, this puts a lot of mass forward into the nose section. It would be better if i could use a smaller battery, or move the pack to the bottom of the pod.

Servo Position

With the servo positioned centrally inline with the door, the elastic band often gets fouled, and fails to release from the servo arm.

It would be better if it was lower down, where the MicroBit is now. This would allow the servo arm to rotate further and properly release the elastic band.

MicroBit

Currently at the bottom. It could be moved to the top to free up space for the servo.

Parachute Bay

The parachute bay has turned out to be smaller than I hoped, which means the parachute gets too compressed to open successfully. I need to make this bay broader somehow.

LS08m assembled

posted 10 Jan 2017, 08:16 by Andy Lakin-Hall

Following the completion of my new parachute pod with the micro:bit controller, I can assemble a new rocket; this one named LS08m, with the m standing for micro:bit.

The nose cone is taller than usual, but this is intended to bring the centre of balance backward to try and get the rocket to fall sideways. The problem being, the new parachute module has the heavy batteries at the top, making the rocket nose-heavy. Ideally I will be able to swap the 3xAA pack for something smaller as some stage in the future. 

I still need to make a connection between the parachute and the rocket body. Previously I did this by fixing a loop of shock cord to the parachute module, and I still have a reel of shock cord on hand.

I also need to make a small access hole in the nose section to allow the battery to be turned on and off.

New Parachute Module

posted 4 Jan 2017, 05:46 by Andy Lakin-Hall

Time to build a new parachute module for my water rockets.

This unit will combine the side releasing design I previously used with radio control, but this time use a microcontroller system to detect apogee and release the parachute automatically.

I have written about the build and shared photos of the under the Parachute Systems page.

The microcontroller I am using is a BBC micro:bit, and I have shared the code for that in my Recovery Systems section.

St Denys' CF16 Midi Controller Repair

posted 13 Dec 2016, 12:46 by Andy Lakin-Hall   [ updated 13 Dec 2016, 12:53 ]

The sound system at St Denys' Church in Rotherfield is remotely controlled by a Kenton CF16 MIDI controller. It's done many years of service, but recently has been sending spurious signals to the master mixer, causing the faders to dance up and down. Finally it was time to replace the sliders.

 

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