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#181 - Acoustic Apogee 2

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#145 - Harness

#144 - Water vs Foam

#143 - Whalan Reserve

#142 - Doonside

#141 - Windy

#1 to #140 (Updates)

 

Dark Shadow - Build Log

Introduction

Dark Shadow is a serious attempt at building a high power water rocket. Many lessons were learned from The Shadow and Shadow II prototype rockets. We are exploring new construction techniques and new materials for us in this rocket that we haven't used before. This rocket is also being put on a diet to minimize weight where possible. The target launch pressure for this rocket is 600psi. (~40 bar)

The log is in chronological order so to see the most recent post you need to Jump To The Bottom. You may need to refresh this page to see any latest updates.

CAUTION: If you are going to attempt to build rockets such as these, please exercise extreme care when testing and flying them. This rocket uses very high pressures that can potentially cause severe injury to yourself and those around you. Always double check your equipment and review safety procedures before every test and flight. See more information on Safety Guidelines.

Build Log

Design

While the overall size and shape of Dark Shadow will be similar to The Shadow, there are a few differences for improved performance.

  • The pressure chamber will be reinforced with carbon fiber.
  • The payload section will be smaller to reduce weight.
  • The rocket will have a tail cone for better aerodynamics.
  • The fins will be thinner
  • The pressure chamber bulkheads will be integrated into the pressure chamber design.
  • The nozzle will be smaller to reduce the overall acceleration to levels similar to The Shadow.
  • A new launcher will be built based on the Polaron G2 electronic launcher.
  • The rocket will use a tower launcher and will not have rail buttons.

12 June 2014 - Made a sample tube with 3 wraps of 85 gsm e-glass cloth. This will form the inner liner for the test rocket body.

The tube measures 525mm long and weighs 50 grams.

   

31 July 2014 - Made the tail cone plug and forward closure plugs from wood then spray painted them with putty, We also trimmed the ends of the tube.


Machining tailcone plug

Machining forward closure plug

Plugs spray painted with putty.
 

 


Tube as it came off the mandrel

Ratty end

Trimmed end
 

1 August 2014 - We cut threads into plug to enable it to be mounted during fiberglassing process. we also sanded back the putty for a smooth finish and then applied first coat of 2-pac clear varnish Estapol 7008.


Applying 2-pac to the plugs
     

2 August 2014 - Sanded the first coat with 600 grit paper and applied second coat of 2-pac to both.


4 August 2014 - Sanded the plug with 1000 grit paper, put on 3 balloons and using only a little bit of talcum powder as lubricant between layers, cut out 13 pieces of 85 gsm glass and applied them around the plug, this used 1 pump of epoxy. For a smoother finish, we added a 2 cm strip around the base of the plug that will overlap inside the tube.


First forward closure
     

5 August 2014 - Pulled the forward closure off with difficulty. The talcum powder doesn't work very well as lubricant on the balloons. The forward closure is just a little too big to fit inside the tube.


12 August 2014 - Made up a new forward closure with 10 gores of 85 gsm cloth and a 2cm base strip. We also made a new nosecone with 10 x 200gsm gores bias cut and 3 veil 85 gsm gores for a better finish. Used 3 layers of balloons again with silicone grease between the balloon layers.

 
 

13 August 2014 - Took the forward closure off the plug. With the grease it goes off a lot easier. The outer most balloon was not covered in anything and as a result the balloon had glued itself well to the inside of the forward closure. There was no way to remove it. Hot water, and even a steel wire brush in the dremmel could not easily remove it. The good news though is that the 10 gores and one strip were the correct size to fit snugly into the tube.

The nosecone also pulled away easily from the plug, and this time the outer most balloon had an very thin coat of the silicon grease. This made it possible to remove the balloon from inside the nosecone fairly easily. We will try using silicone mold release on the balloon on the next attempt.


16 August 2014 - The nosecone felt a little heavy for what it needed to be. We can use this nosecone for other projects, So we made up a new nosecone using only 6 x 200gsm gores + 3 x 85gsm veil gores. This time we gave the outer most balloon a thin coat of the silicon grease. We had also put only two balloons on the plug.


17 August 2014 - Pulled the nosecone off the plug very easily. So easily in fact that the outer most balloon stayed in place. We set up the nosecone next to a heater to get it to fully cure


19 August 2014 - We trimmed and sanded the nosecone down with sandpaper under a running tap. This part of the nosecone now weighs 19 grams. The final nosecone we are aiming at being closer to 30 grams. The current Shadow II nosecone weighs 67grams. The entire Shadow II payload section including all electronics, cameras and parachute weighs in at 380g. We are aiming at around half of that for Dark Shadow. Running simulations on Shadow II, That much weight saving would result in another 140 feet altitude.


23 August 2014 - Today we machined up the first forward closure from a chunk of PVC for the forward closure.

We need to bring the end of the rocket to a close and attach a mounting point for the parachute. This part of the rocket may experience a significant load during parachute deploy. Probably close to an hour later of deciding on the shape and machining it from a lump of PVC. We ended up with a cap that was a bit bigger than what would be ideal. The shape is difficult to create by hand. It also weighed in at 25g which was pretty heavy.

 

 

 

So we started talking about alternatives, and we finally did a quick prototype of using the top of a PET bottle and heat shrunk over the tail cone plug. It looked like it was going to work if some of it was filled in with epoxy. With the carbon fiber over the top of it, it should be strong enough to hold in place. At this time we turned to see if we could also use the same concept for the nozzle end. We heated up the top of a PVC pipe and widened it so that it would not pass through the neck. The idea was that we would fit the final nozzle into that.

 

We are looking at using the same technique for both the top as well as the nozzle. The first shrinking worked fairly well although it was a little uneven. So we tried again with another top but this time with a larger section of the bottle.

We also tried making one for forward closure. For this we found that the bottles with the conical top work better because they have a wider top. We used the heat gun to shrink it but as it was shrinking it started moving up. So that was a fail.

Finally we put the plug with the fiberglass forward closure over it into the lathe, and then using the tail stock held the top of the bottle against the plug to prevent it from moving up and also used the PVC pipe in the throat to stop it from collapsing. Set on about 200rpm, we used the heat gun to evenly shrink the bottle over the fiberglass. This worked really well and gave us a well aligned bottle. The uneven bottom edge was just trimmed off. The cap now only weighs 7 grams. We trimmed the big flange on the bottle down with a dremmel tool too, and the thread and other flanges on the neck will be used to hold the carbon fiber sleeve over the top.


24 August 2014 - We fiberglassed another forward closure, as well as the first tail cone today. They both used 10 gores of the 85gsm cloth with 1 wrap of 2cm wide cloth for the gluing edge. We again used a couple of balloons on the plug and a thin coat of silicone grease on top.

We also bought a roller cutter for cutting the fiberglass, and it works like a charm. I don't know how we ever managed without it all these years.

We mixed the remainder of the epoxy with microballoons to make it easier to sand, and we covered the nosecone with the mixture to fill in any holes. We then used a scraper to remove as much of it as possible to help reduce the sanding time.


25 August 2014 - We pulled the tail cone and end forward closure off the molds and they again came off easily. The forward closure was a perfect fit into the tube, but the tail cone was a little on the small side. We will make the tail cone again, but this time we'll use 12 gores, 3 balloons and perhaps 1.5 wraps of the 2cm wide strip, for a better fit.

 

 

   

30 August 2014 - We made a nozzle insert today from a piece of PVC pipe and enlarged one end of it to fit inside a bottle neck. We also cut a thread in it so that we could screw in nozzles. Having the ability to replace the nozzle is important in case it gets damaged on landing. If the nozzle was there permanently we would have to replace the whole rocket. This time the nozzle will not be protected like it was with the Shadow and Shadow II, so there is higher likelihood of damage.

We also machined up the nozzle from alumnium. An o-ring will be sandwiched between the insert and the nozzle. The final nozzle diameter is 15.5mm which is about 65% of Shadow's cross sectional area. The reason for the reduced nozzle is that with the expected higher pressures we want to keep the overall acceleration down. At 600psi with the smaller nozzle the rocket will have acceleration similar to that of Shadow - around 55G+. The launcher will use a standard 15mm aluminium launch tube.

 

We then heat shrank a bottle neck to fit over the tail cone. We again did this on the lathe to get it aligned.

Next we machined up a hose adaptor so that we could extend our hose another 5m just to add some more distance between us and the rocket. We now have about 17m of hose. If the hose has a 6mm diameter internal hole, then the total hose volume will be in the order of 480mL. We need to take this into account when calculating how much air to bring to the launch site.

Lastly for the day we pressure tested the hose to 600psi (~40bar). The intended pressure for the rocket. The hose was terminated with a quick release connector connected to the head of regulator. We made sure we sat behind a screen door in case the hose intended to let go.

   
 

2 September 2014 - We received the carbon fiber sleeve samples from Soller Composites. The sleeves look very good. The heavy grade looks like it will do the job. It will be interesting to see how many layers are needed to achieve the desired strength. I would expect that even one may be enough, but probably two. We tried threading the tube inside the sleeves, and it looks like the 2, 2.5 and 3 inch sleeves could be used. We'll need to see what angle the fibers make when the sleeve is pulled tight over the tube. The 1.5" sleeve is too small to fit on the tube.

 

3 September 2014 - Did research today on carbon sleeves and how they are applied. It looks like we will need to get the slow hardener to give us enough time to work on the sleeve for the entire rocket. This is going to be a big job, and may need an additional person as well. The ideal winding angle is 55 degrees from the axial axis. We will need to put the sleeves on and measure the fiber angles the different sized sleeves give when pulled tight.


4 September 2014 - Stretched the sleeves over the tube today and took pictures of the filament orientation. We measured the 2", 2.5" and 3" heavy sleeves and found them to have fibers at 60, 43 and 34 degrees respectively. This means that we will most likely go with the 2" sleeve as it is the closest to 55 degrees. If we find that we need a second layer, we'll then apply the 2.5" sleeve over the top of that.

 

5 September 2014 - Plugged launch tube. One of the main things that we are concerned about is the amount of heat generated during pressurisation of the rocket. The glass transition temperature if fairly low of the West systems epoxy so we need to keep under that to reduce the chances of failure on the pad. This means a fairly slow fill. Unfortunately the thermal conductivity of fiberglass is very low and carbon fiber isn't much better, so cooling the outside of the rocket won't help much with reducing the temperature on the inner wall, We need to cool it from the inside. Simply bubbling the air through the water won't cool it very much at all as we've seen in our other experiments. So we are going to try using foam to cool the air and rocket due to the large surface area, it should transfer the heat into the water. The other advantage is that foam may be able to provide additional performance boost. The problem is how to generate the foam inside the rocket when there is a launch tube? So we are going to plug the launch tube just above the nozzle and below that we will make a series of small holes that will bubble up through the water to make foam. When the rocket launches and gets above the fill holes no air will come down the launch tube. If you didn't have the plug, the air would escape through the top of the launch tube and out the fill holes. The fill holes also allow you to depressurise the rocket if needed.


6 September 2014 - Machined up a small aluminium cylinder that can be used inside PVC pipe for better grip in the lathe. We also machined up an expander for PVC pipe out of brass bar stock. At first the expansion angle was too great and was causing the PVC pipe to buckle. So we reduced the angle, We also tried preheating the expander before putting on the PVC pipe and also heating it up. This time the PVC pipe expanded nicely.

 

Procedure for making PVC nozzle insert:

  1. Cut 60mm of PVC pipe

  2. Insert support cylinder and trim one end on lathe.

  3. Counter sink the end of the tube by hand.

  4. Make a hole 17.6mm diameter - some PVC is already this size.

  5. Cut thread using a tap on the lathe. Supporting the end using the tail stock and using an adjustable spanner so it keeps it from turning. Go about 15mm deep.

  6. Cut off 33mm from the threaded end with a cutting bit in the lathe.

  7. Hand counter sink other end by hand on the lathe.

  8. Insert nozzle into PVC thread.

  9. Put expander in the lathe and preheat with a heat gun for 40 seconds (Black II, Red III) and rotate on low RPM

  10. Put PVC on expander and continue to rotate and heat

  11. When end of PVC changes shape slightly, remove heat, stop the lathe and push PVC onto expander up to the stop.

  12. Blow on it to cool it down.

 

8 September 2014 - Made a new tail cone today using 12 gores of the 85gsm cloth and wrapped the base 2 times. The end of the tail cone gave me a little trouble as it wouldn't quite adhere to the plug. So I made a small paper cone and sat it onto the tail cone to keep all the fibers together. Although the paper will stick to the tail cone it will be easy to sand off.

We weighed a number of the different components today.

  • Nozzle insert - 5g

  • Aluminium Nozzle - 21g

  • Tail cone - 13g

  • Tail cone bottle - 9g

  • Forward closure bottle - 7g

  • Forward closure - 8g

  • Nosecone - 19g


9 September 2014 - We pulled the tail cone off the plug, and the paper cone keeping the fibers down looked like it worked well. However, when we tried it inside the tube, the tail cone was still just a little too small. It maybe needed another 1 or 2 wraps of fiberglass to make it fit right. This is undesirable because there would be too much of a step between the inside and outside helping to concentrate stress. The diameter of the forward closure plug is 59.1mm while this one is 58.3mm. Not wanting to make up another plug, my wife suggested, why not just wrap the plug in a couple of layers of fiberglass to widen it. And so we did. We added two wraps of a wider strip of the glass, and 2 layers of a narrower strip so 4 layers all up. This should bring it up to about 59.2mm or so which can then be sanded back. This will allow us to then go back to using the 10 gores and single wrap for the tail cone.

   

 


12 September 2014 - We epoxied a small plastic ball into the nosecone of the rocket as this had the right diameter. We sanded back the tail cone to smooth out the fiberglass. This doesn't need to be polished, as we have a balloon that sits over it.

   

13 September 2014 - We made a new tail cone using the 10 gores and single wrap around the base. Only two balloons were used for this plug. This time we again used the paper cone on top to keep the fibers together.


14 September 2014 - We removed the tail cone from the plug and it separated fairly easily. Test fitting it inside the tube gave very good results and should be good to use for the test chamber.

   

Day 150 Plugged launch tube experiment. See Day 150 update.

We wanted to see if we could get foam generated inside a rocket during filling while using a full bore nozzle and full length launch tube. So we adapted a normal Clark Cable-tie launcher and drilled 6 holes near the nozzle inside the rocket. This would allow air to bubble up through the water and form foam. Even back in 2007 Damo was using holes in the bottom of his launch tube, but the launch tube was not plugged. There are two problems with this approach. The air wants to flow up the launch tube instead of through the holes, and once launched the air can escape out of the holes through the launch tube. So we simply plugged the launch tube above the little holes. This way as the rocket moves up the launch tube, the air can't escape from the rocket.

One disadvantage is that you can't use ARB boosters with the plugged tube.


3 December 2014 - We received 2 carbon fiber plates today from hobby king. These are 1.5x300x100mm and should be sufficient to make up to four fins. (3 fins + 1 spare)

     

10 December 2014 - We prepared the shrunken plastic bottle for gluing by cutting little notches around the edge. The idea here is to help spread the forces along the interface between the plastic bottle and the fiberglass shell. We also drilled a number of small 1.5mm holes into the plastic bottle to help get it to "key" with the epoxy. We removed most of the bottle flange with a sander in the dremmel which will allow the carbon fiber to conform better to the shape. We left a little bit as it will help grip the CF sleeve better. We also sanded the entire inside and outside of the bottle with 120grit sandpaper for better adhesion. We then glued the bottle top to the fiberglass shell with 24hr Araldite. We filled the top of the bottle with more epoxy this time mixed with microballoons to help provide support for the top of the pressure chamber. We then added a weight to the top to help compress the join.

We also glued the PVC nozzle insert into the tail shrunken bottle neck. Again we mostly removed the bottle flange for better sleeve conformity, and cut a series of notches along the edge. We also used the 24Hr araldite for this.

 

11 December 2014 - We glued the bottle neck with PVC insert to the fiberglass tailcone and again added weight on top to help seat it properly.

     

17 December 2014 - Glued tail cone to body.


18 December 2014 - We sanded the joins, and filled the gaps with epoxy and microballoons. We are trying to reduce the chance of little air pockets developing after we put the carbon fiber sleeve on.


Length of sleeve compared to
pressure chamber

19 December 2014 - We wrapped the fiberglass pressure chamber with 2" heavy CF sleeve and, used 3 pumps of the epoxy which was not enough. We used peel ply in a spiral fashion around the CF to give it a better finish. We held the sleeve down with wire while pulling it tight and used fiberglass strands to help hold CF sleeve in place. Next time we will use wire on everything, and then apply the fiberglass after the CF has hardened.

We used the tap to hold the nozzle end of rocket and a piece of PVC pipe to hold the other end while laying it up. We also made the nozzle seat for the launcher.


Preparing to pull the sleeve on

Sleeve in place with peel ply applied

Machining nozzle seat

Nozzle seat completed.

20 December 2014 - We trimmed the excess CF from test pressure chamber. This was just done with a hacksaw. We also cut out the forward closure gores x 20. But when we tried making up another forward closure with them, we failed miserably. This fiberglass is just too stiff to conform nicely to the shape like the previous glass. We'll have to get more of the original 85gsm glass. We also filled the top of neck with extra epoxy. We wrapped extra fiberglass threads around end of the sleeve to replace wire that was there holding things in place temporarily.


Test pressure chamber with
peel ply removed.

Excess CF at one end
 

And at the other.
 

Getting ready to fiberglass
forward closure.

23 December 2014 - We made a temporary nozzle retainer for doing pressure tests. This basically consists of two plates with a pair of bolts holding the nozzle down against the nozzle seat. We also tapped 2 mounting holes in the bottom of the nozzle seat for later mounting on the launcher.

   

24 December 2014 -  Made up more laminated balsa sandwiches These have a 200gsm cloth overlayed with bias cut 84gsm cloth over the outside. We use any left over epoxy for these sandwiches. We also made the payload bay body tube with 6 wraps 84 gsm e-glass. Length of glass 1150mm. Finally we added glass filament reinforcement to test chamber ends to strengthen them.

 

25 December 2014 - We worked on the deployment mechanism prototype today. We wanted to make some improvements over Shadow's deployment mechanism. The one biggest drawback with the previous design was that it was friction dependant. The nosecone had to be tight enough to not separate on burn out, but not too tight so the piston could push it out. The friction properties varied based on temperature and humidity. The other problem was the shock cord got in the way of the piston and so had to be attached to the payload bay wall. There was always a danger that the payload bay could be ripped off the pressure chamber. The piston mechanism was also quite long and heavy. The new system gets rid of the piston, allows the shock cord to be attached to the main pressure chamber, is positively retained (not friction dependant) and is shorter and lighter. We'll do a detailed writeup of the entire mechanism once the final design is complete.

We also pressure tested the test chamber to 100psi but 2 leaks in the body were found. These must have come from tiny holes in the rocket body. So in an attempt to fix this we applied another coat of epoxy to the outside.

We made a nosecone bulkhead and glued it into a coupler we had made previously for the Shadow project. We also added the swinging grapple arms to the deployment mechanism prototype.

   
Getting ready for low pressure leak test
 

Pressurising to 100psi - couple of small leaks
in the body itself

Painting the surface with more epoxy
to stop the leaks.

Parachute grapple arms

Grapple arms open

Grapple arms closed

26 December 2014 - After the epoxy had cured we pressure tested the test chamber to 100psi and to great relief it was without leaks. We also tested prototype deployment mechanism to see if the arms would let go of the parachute. Originally we were going to use a spring mechanism to swing the arms open, but in testing it was found that this is unnecessary and the arms let go of the parachute freely.


27 December 2014 - We bought aluminium 30x50x2mm box section today along with another PVC pipe. The box section will become the basis of the launcher and the PVC pipe will be machined down to form the long tube coupler mandrel.


28 December 2014 - Today we tested the test pressure chamber to 650psi (45 bar) and it still held. :) We couldn't go higher because we only tested the hoses to 600psi previously. There were no visible stress markings on the pressure chamber. So we are not sure how close to the limit that is. We also rolled the first of the long tubes that will make up the basis of the rocket. The tube is made of almost 3 wraps of the 84gsm glass cloth The cloth is 1150mm long. We used 3 pumps of epoxy. We also used a cheap roller that ended up losing bits of it. We also started gathering up all the components for the launcher.

 
Putting the test chamber in a scuba tank.

We put the connection against the house
in case things went flying

Ready for the test

100psi

400psi

650psi!

Rolling new full length tube

Making up fiberglass-balsa sandwich
for bulk heads
   

29 December 2014 - We ordered a pair of Aerocon 36" parachutes today. With the higher performance of this rocket we wanted to have a more sturdy parachute for potentially higher speed deploys. We also ordered 30 feet of the 2" heavy carbon sleeve from Soller Composites.

We also tested a prototype of the release mechanism. After spending several days of trying to figure out a latch, lever and servo mechanism to release the grapple arms, we decided to just use a loop of wire hooked over the servo motor horn. By hooking the wire over the axel of the horn it can hold quite a bit of force. When the servo motor turns, it just slips the wire off releasing the grapple arms. We can also easily adjust the length of this loop.

We made some more progress on the launcher making a few of the smaller components and the lever arm.

We pulled the long tube off the mandrel ,but are less than happy with the result. Because of the size of cloth we weren't able to quite get 3 complete wraps (about 1 cm short) and the frilly edges left a few bubble pockets along the length.. While this would have been fine for a shorter tube, we need a bit more rigidity when we pull the sleeve over it. The cheap roller also left quite a few little bumps on the tube, presumably bits of the foam. So we will make up another tube with the frills cut off, and then add another single wrap as a second piece. This should also help reduce the chance of porosity in the tube.


Parachute inside grapple arms

Hold down wire

Components for launcher

Less than satisfactory first tube

30 December 2014 - We rolled a second full length tube today with 4 wraps of the 85gsm cloth. This time we used the better quality foam roller and the results were much better. We also continued to work on the launcher. The main base components are now mostly complete. We are using a brass insert in the lever arm to add extra strength to the nozzle contacting surface.


Rolling full length tube

Starting launcher construction
   

31 December 2014 - Rolled second full length tube the same way as the first. Made up the base of launcher and attached it to the bottom of the box section.

Second full length tube

Old and new tubes
   


Continue to Part 2...

Dark Shadow Parachute Deployment Mechansim

 

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