Archive for August, 2008

 

New Nozzle, Fairing and Assembly of 2 Bottle Rocket

Just getting the last few things done on the 2 bottle water rocket today to get it ready for its launch on the weekend. Two things built were a faring for the gap between the bottles and making a nozzle for the 1.25L cap and finally assembling the water rocket

Water Rocket Nozzle

The nozzle first. We had made nozzles for our 1.5L single bottle water rocket but it had a bigger cap so we had to make one to fit this size bottle cap. We used a gardena male adapter, its sold as a 12mm fitting in hardware stores to fit straight onto a tap. Its exit nozzle diameter is 9mm.

12mm Gardena male fitting

12mm Gardena male fitting

The part where it screws onto the tap was nearly all cut off leaving only a 2mm section. This 2mm section is still needed as it holds the fitting inside the bottle cap. It was cut pretty much right through the end of the last “A” in gardena in the pic above.  A file was used to then reduce the diameter of the ridge to get it to fit inside a bottle cap. After filing and shaping here is what I finished with.

Cut & shaped gardena fitting

Cut & shaped gardena fitting

Next a hole was drilled in the bottle cap to fit the size of the nozzle. The hole in the bottle cap needs to be big enough for the fitting to go through but still have the ridge that we just shaped inside the bottle cap. I estimate the hole in the bottle cap is ~ 18mm but I didn’t have a drill bit that big, so drilled a 12mm hole then used the end of a file to make it bigger. I had to make sure I cleaned up the hole of any plastic burs. I just used a sharp blade to do this.  Be careful !

Next I cut a washer from a bike inner tube, easiest way I found was to draw around the outside of the bottle cap on the rubber then cut just inside the mark so it will fit snugly inside the bottle cap. Also cut out a hole at least 9mm in the centre for the water to get out the nozzle.

Here is what we have

Nozzle Parts

Nozzle Parts - (L to R)Â washer, bottle cap, gardena fitting

Assembly is easy, no glue is needed – first put the gardena nozzle through the hole in the bottle

Cap & gardena nozzle

Cap & gardena nozzle

The put in the washer and screw it to the bottle

Cap, Gardena fitting and washer

Cap, Gardena fitting and washer

Here is what it looks like attached to the bottle. Just blow in the end to see if it leaks or put it on your launcher .. whichever is easier. If it leaks check the washer is in the correct place.

Nozzle attached to rocket

Nozzle attached to rocket

Bottle Fairing

I wanted to put a fairing over the space where the two bottles are joined to make it as aerodynamic as possible. I used a 1.5L P&N bottle and cut the middle section out of it. I realized the diameter of this bigger bottle would be slightly bigger than the 1.25L, so to make it a snug fit I needed to shrink it a little.

I don’t have any pics of this method as I don’t have a heat gun or a blowtorch so I used the hotplates to shrink the 1.5L bottle (this is not the recommended method). I filled a 1.25L bottle full with water, attached the pump and pressurized it to 20psi (the gardena quick release came in handy). This made the bottle nice and solid. I slipped the 1.5L section over the top of the 1.25L then rotated it over the top of the hotplate until it started to shrink. It did buckle a little but it did shrink. I will have to work on this procedure, but for the time being it worked. I depressurized the 1.25L bottle and slipped of the 1.5L section.

The 1.5L section was put in the space between the two joined bottles and taped on with strapping tape. Note there is only atmospheric pressure in this space, not pressurized air from the bottles. Once the fairing is taped in place the water rocket has a nice aerodynamic shape all along the length of the rocket. (see pics below)

Water Rocket Assembly

I finally got to the point of assembling the rocket. All the sections were put together with 3M strapping tape.

  • PPNC (Ping Pong Nose Cone) Nosecone connected to pressure bottles
  • Two bottles – Robinson coupled were tightened and pressure tested to 40psi
  • 9mm nozzle put on the bottom pressure bottle
  • Removable fin assembly (RFA) was taped to the bottom of the rocket.
Assembled 2 bottle water rocket

Assembled 2 bottle water rocket

Last thing to do is fit another speed flap and test it on the timer prior to next weekends flight :)

Posted by on August 26th, 2008 3 Comments

Robinson Coupling

I finally got the pieces together this weekend to do my first Robinson coupling. I used pretty much the parts as Bill Robinson (the guy who came up this method) suggests. Parts are

  • 1 x piece Threaded Lamp rod approx 2.5cm
  • 2 x nuts for the rod
  • 3 x rubber – washers made form bike inner tube
  • 1 x bottle cap
  • 1 x 1.25L P&L empty bottle

Also the tools needed were

  • 1 x 10mm drill bit
  • 2 x 14mm sockets
  • 3 x socket extenders
  • 2 x socket wrench
  • 1 x metal coat hanger
  • 1 x glue
  • 1 x sharp scissors
  • 1 x sharp blade

Once you have everything together its quite simple. The hardest part was getting the threaded lamp rod. I visited a lighting shop and was going through the lamps etc. When I asked the shop owner he came out with a jar full of 2.5cm length lamp rod sections and a jar of nuts to fit. The nuts I found in the hardware stores were all too big to fit in the bottle cap so finding all of these pieces together was great. Better still the guy gave them to me .. no charge :) I will have to go back and get some more, it might cost me a few $ but not much.

First step was to drill the 1.25L bottle. I drilled a 2mm hole through the very centre, the thickest part of the bottle. I then used successive drill bits, stepping up 2mm at a time until I got to the 10mm bit. I used this method so I didn’t gouge out a bit of plastic and to keep the hole edges very precise. I also used the sharp blade to clean up the edges.

1.25L Bottle - 10mm hole

1.25L Bottle - 10mm hole

Next step was to drill a hole in the bottle cap. I used the same method. I don’t have a vice (I should get one) so I held the cap with a pair of wide jaw pliers so if the drill slipped it didn’t get my finger.

I then prepared the washers. I used a bike inner tube, cut a section and opened it flat, drew a circle using the outside of the bottle cap, then cut just inside the line I drew. I did this for 3 washers. I then cut out the inner circles. For the one that will fit in the cap of the second bottle, I cut out the hole just bigger than the hole in the cap. For the other two I did it just smaller, so it would be a tight fit around the lamp rod.

"Robinson Coupling" parts

"Robinson Coupling" parts

You can see the washer top right is slightly smaller in diameter. I found that when I tried to push it in the bottle opening of the bottle with the 10mm hole cut in it, it wouldn’t fit through. So I trimmed it to fit. As long as its bigger than the nut, it will be ok (The nuts are 14mm BTW)

I used a drop of superglue (should use thread lock but I don’t have any) and glued one nut to the end, then added the (smaller diameter) washer. I then came across a interesting problem, how to insert it into the bottle and get it through the hole so I could attach the other end. The solution ended up being simple. I used a metal coat hanger and slipped it through the bottle hole and up through the bottle opening, I then just slid the lamp rod and nut down and into place.

Metal coat hanger to the rescue

Metal coat hanger to the rescue

Lucky I have a few socket sets around, some of them minus a few sockets, but they all had the socket extender bars, so I just clipped three together and put the 14mm socket on the end and inserted it into the bottle to hold the nut.

socket extenders

socket extenders

You can see here how the socket extenders and the 14mm socket, hold the piece of threaded lamp rod and nut in place.

Lamp Rod held in place

Lamp Rod held in place

Next on goes another washer, you can see the washer is tight around the lamp rod.

Middle Washer in place

Middle Washer in place

Then on goes the bottle cap, the last washer and the final nut. These are then tightened down with a socket wrench with another 14mm socket on it. I just held the other socket extension bar in place with another socket wrench while I tightened the top one down.

bottle cap washer and nut in place

bottle cap washer and nut in place

Only thing I noticed is that the rubber washer in the bottle cap will tend to deform a little as the top nut is tightened down. I will probably need to make this slightly bigger for future Robinson couplings as the first leak test I did, it was leaking a little in one spot in the cap where the washer folded up a little and didn’t seal properly. I pushed it down in place, fitted on another bottle and it held pressure fine.

2 Bottles in robinson Coupling attachment

2 Bottles in Robinson Coupling attachment

I haven’t done a 100psi pressure test yet, only about 20psi one. Reason being it was late and I didn’t want to wake the neighbours with an unexpected bang :) But I am happy with the connection. Big thanks goes out to Bill Robinson for the idea.

Posted by on August 25th, 2008 6 Comments

Removable Fin Assembly

I like the idea of making the rockets modular so that I can swap parts from rocket to rocket. This will hopefully reduce rebuild time when there is a crash, as well as letting me mix and match to get the config I want. Part of this modular approach is to build a Removable Fin Assembly (RFA), which is what I have been doing this week.

I decided to go with a trapezoidal fin design to see how they perform. Here is a diagram of the design I came up with, this was based on other diagrams I came across on rocket websites.

Trapezoidal Fin Design

Trapezoidal Fin Design

I am using the P&N 1.25L bottles and the diameter (D) of the bottle is 87mm. The size of the fin is proportional to the diameter of the  rocket body. The trapezoid is D wide, 1/2D at its furthest point from the rocket body and D high where it sits against the rocket body.

I used my leftover corrugated plastic to cut out three (3) fins. I decided to run the corrugations up/down so that the air would channel down through the fins in the normal upright position. My assumptions are

  • it will reduce drag in the vertical position as there is less surface area cutting through the air
  • it will increase the surface area when the air is pushing the fin (and therefor rocket) back to the vertical

we will see if this assumption is right I guess.

Firt cut the fins out of the corrigated plastic

First cut the fins out of the corrugated plastic

Here is a close up looking down from the top of the fin, you can see down the corrugations

looking down corrigations

looking down corrugations

I also cut out six (6) sections of corrugated plastic to use to glue the fins to the RFA body. These were eight (8) corrugations wide and approx 2/3D in length. I used a sharp knife / blade to slit down one side so it would bend back at approx 90 degrees.

fin attachments

fin attachments

The RFA body is the centre section of a 1.25L bottle. This is the same diameter as the rocket body. I cut it to allow for a lip of a few mm to make it easier to attach to the main pressure bottles of the rocket. This lip was already in the bottle where is widens a little near the base of the bottle. You can make it out in the below pic (click the pic for a higher resolution picture)

removable fin body section

removable fin body section

So what I have to work with is this, three (3) fins, six (6) fin attachments and 1 body section.

Removable Fin parts

Removable Fin parts

Next thing to do is measure the exact positions for the fins. I used a piece of paper to wrap tight around the bottle, temporarily tape it and marked where it overlaps. It is then taken off the bottle, unrolled and the distance from the start of the paper to the mark I made (external circumference of the bottle) is measured. Its then divided accurately in three and a ruler used to mark the 1/3 and 2/3 (fin) locations.

The start of the paper and the end mark is the other (3rd) fin location. The paper is put back on the bottle 1cm up from the bottom and the locations marked on the bottle with a permanent pen. Lines are then drawn vertically up the body section. This gives the exact locations to mount the fins.

fin locations marked on body section

fin locations marked on body section

Next the fin attachments are glued to the fins. In order to get these flush, when the glue was setting I held the attachment and fin together against a hard surface. This will ensure when they are attached to the fin body the spine of the fin as well as the fin attachment will fit snugly against the body surface.

glueing fin attachment

gluing fin attachment

I did one side at a time and let it dry before doing the other side. So now I had three (3) fins with both the fin attachments connected. Here are some pics.

both fin attachments connected

both fin attachments connected

fin attachments looking from the bottom

fin attachments looking from the bottom

As the body section of the RFA is not flat, but instead round, the movable fin attachments allow the angle to be more like 100 degrees rather than 90 degrees to follow the curve of the bottle.

Next step is to glue the fins + fin attachments to the RFA body, this was done one at a time and the line drawn previously was used to line up the spine of the fin. These were held in place while the glue dried. I have them 1cm from the bottom so that when the rocket is standing up while not being used, the weight of the rocket is not on the fins.

fins attached to RFA body (you can see the RFA body lip more clearly in this pic)

fins attached to RFA body (you can see the RFA body lip more clearly in this pic)

Due to the type of glue I’m using, I’m not very confident in the strength of it under load, so I decided to strengthen the connection points to the RFA body with wire. The wire is 1.5mm tin wire. I used a drill to drill 2 holes (1.5mm) in each fin attachment and through the RFA body. I then bent a section of tin wire and secured the fins to the RFA body. I used 6 wire fixings in all, one for each fin assembly. This pic shows one already done as well as the next one ready to go it.

fin assembly reinforcement

fin assembly reinforcement

Once all six fin attachments are done it looked like this, ready to be fitted to the main rocket

finished RFA

finished RFA

finished RFA - top view

finished RFA - top view

Posted by on August 23rd, 2008 2 Comments

Nosecone Reliability Updates

I have been working on trying to increase the reliability of the release mechanism on the nosecone over the last day or so and have disassembled and reassembled the nosecone a few times.

I found today that the cardboard L brackets holding the nosecone support structure together had separated and the ‘V’ shape of the nosecone was narrower and as such the elastic bands on the deployment plate didn’t have enough pressure to kick out the parachute. I think this was probably caused by 3 things

  1. The glue I’m using isn’t strong enough. I’m using a Sellys “shockproof” superglue but I don’t think this is going to cut it. I’m going to have to try to get my fingers on something better.
  2. It didn’t like being assembled and disassembled a few times
  3. The elastic bands on the deployment plate were possibly strung too tight

I decided to strengthen the structure by wiring the uprights of the nosecone support structure to the circular top and bottom plates. I used what I had available which is some tin (I think) wire which is about 2mm thick and while able to be bent with your fingers, retains its shape very well. To do this I made 4 big staples out of the wire and wired each upright in 2 locations to the base and top plates.

Bottom uprights wired with 4 wire pins for top plate ready to go

Bottom uprights wired with 4 wire pins for top plate ready to go

Its easier to do all 4 at once, I found out when doing the bottom plate. You can see here the 4 pins are ready for the top plate to be wired on.

4 Pins in place in uprights

4 Pins in place in uprights

The top plate is put on, i pushed a piece of the wire easily through the corrugated plastic beforehand so the holes were in the right locations, then the wires are just bent to either side and trimmed to remove access

pins in place and bent over (excess not trimmed yet)

pins in place and bent over (excess not trimmed yet)

here is a pic from the inside section where the parachute sits when its ready to be deployed

pic inside the parachute area before deployment plate is put back in

pic inside the parachute area before deployment plate is put back in

I did realize that I now had things that the parachute could possibly snag on .. (Murphy’s law at work) .. so I covered all of the exposed wire with tape to make it smoother and eliminate (or reduce as much as I can) the chance of the parachute snagging while being deployed.

exposed wire taped up

exposed wire taped up

I taped the wire at the top and bottom of the plates so there was nothing to snag on.

Another small change to increase reliability was to invert the wire I had holding the door onto the nosecone. When the elastic band unraveled it caught on the exposed ends a couple of times during testing, so I took them out and reversed them so the bent sections were inside. I covered these with tape as well on the inside.

inverted wire pins

inverted wire pins

An additional change was to remove a small amount of corrugated plastic I was using to mount the timer on. I found that during testing the bolt on the timer would hit the inside of the plastic nosecone and in some cases stopped it. I only noticed this issue when I first packed the parachute. Removing the extra piece moved the timer closer to the middle of the nosecone and allowed more room for the bolt on the timer to spin freely.

Further testing showed an improvement in the reliability of the parachute deployment. I realize the extra wiring will add weight to the rocket and reduce the height it will reach but the nosecone support structure is very solid and can easily handle the tight rubber bands on the parachute release plate.

I think the increased reliability will be worth a little less height :)

Posted by on August 22nd, 2008 Comments Off