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Flight Log Updates

#186 - Level 1 HPR

#185 - Liquids in Zero-G

#184 - More Axion G6

#183 - Axion G6

#182 - Casual Flights

#181 - Acoustic Apogee 2

#180 - Light Shadow

#179 - Stratologger

#178 - Acoustic Apogee 1

#177 - Reefing Chutes

#176 - 10 Years

#175 - NSWRA Events

#174 - Mullaley Launch

#173 - Oobleck Rocket

#172 - Coming Soon

#171 - Measuring Altitude

#170 - How Much Water?

#169 - Windy

#168 - Casual Flights 2

#167 - Casual Flights

#166 - Dark Shadow II

#165 - Liquid Density 2

#164 - Liquid Density 1

#163 - Channel 7 News

#162 - Axion and Polaron

#161 - Fog and Boom

#160 - Chasing Rockets

#159 - Measurement

#158 - Dark Shadow

#157 - Polaron G2

#156 - Foam Flights

#155 - Down The Barrel

#154 - Revisits

#153 - ClearCam

#152 - Mullaley, Axion G2

#151 - Competition Day

#1 to #150 (Updates)

 

Inverter - Build Log

Introduction

This is the build log of our rocket project called "Inverter". 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.

Design

The Inverter rocket is based on a central pressure chamber that mostly contains water and two side mounted air-only pressure chambers. They are joined together at the top using an air manifold made from PVC tubing. The rocket's name comes from the fact that the flow of air is inverted inside the rocket.

This approach allows one to build a large capacity single nozzle rocket out of regular bottles without the rocket being too long. This concept has been used over the years by water rocketeers. Here are some examples:

Because the rocket only uses a single nozzle it simplifies the launcher design and allows us to use our existing one without modification. The flat rocket is also easy to attach to the guide rail.

We chose to use 2 air chambers in order to reduce the overall drag and since water rockets are also usually filled to 1/3 water it meant we could fill the central segment almost completely with water. Having a longer central segment also meant that water could be distributed vertically in the rocket and hence helping to increase the stability.

 


Build Log

12th December 2012 - Designing the manifold. Since this was going to be an important part of the rocket we decided to start with the manifold. We wanted to have an opening on the top of the central segment so that we could fill it with water once it was on the pad. It would be difficult to turn it upside down to fill it and not have the water flow into the air-only segments. The other alternative would have be to fill the rocket through the nozzle, but our launcher isn't set up for that, so we chose to simply add an opening at the top of the rocket.

We wanted to use the tornado couplings so that we could easily assemble or replace the segments as needed, and use our standard components. We added enough spacing between the manifold pipes to allow us to use wider segments in the future.

The manifold looks funny the way it is because we wanted to have a 4 way connection above the central segment, but they don't sell them at the local hardware shop. They only sell T-connectors. When I asked the sales guy, he said "This is the plumbing section, this isn't the furniture section." A little bewildered I said "OK" So either there is something about plumbing that only needs T-connectors, or somewhere out there there is furniture made from PVC piping. Checking on-line, I can see there are X pieces available for PVC pipes... hmmmm.

Based on the layout below we bought the necessary components at the local hardware store.


13th December 2012 - We cut out all of the pieces and assembled them with the tornado couplings, and weighed everything... 235 grams was way too much for just the manifold! It was time to re-think the overall design to reduce the weight so we decided to change the design to just a T and 2 elbows. We also decided against using the tornado tubes and would attach the captive caps to the PVC tubes. Sealing would be achieved by soft rubber washers under the caps. We were going to loose the ability to fill the rocket from the top, but would still be able to if the whole manifold was removed.


Original manifold plan turned out to be too heavy,
but did have access to the center bottle.
     

15th December 2012 - Machined up 3 PVC rings that would hold the captive caps. We glued these with epoxy to the PVC tubes. We also obtained the bottle caps from the local supermarket. We got these from hand wash detergent pump packs. Now we have at least a 6 month supply of hand washing detergent. The pumps in the caps were cut out and the hole in the cap enlarged on the lathe so it would fit on the PVC tube.


Parts cut out for the lighter manifold

Test fitting the whole thing together
   

18th December 2012 - Finished making 7 new 90mm spliced pairs with the narrow sleeve. We had used up most of our spare spliced pairs last year for the commercial and so we had to make a few for this rocket. We want to use 3 - 4 spliced pairs per segment on this rocket.


7 new spliced-pairs
     

28th December 2012 - We glued the whole of manifold together today using PVC cement.


Partially glued together
     

1st January 2013 - Pressure testing the manifold. We tried pressure testing the manifold today but have had several problems with getting good seals. We would fix one leak, but when pressurised to a higher pressure another would leak. We decided that this was going to be too unreliable especially if we were going to pull the manifold off to fill the rocket on the pad. So it was decided to switch to plan B and go with the o-rings.


Ready for first pressure test - had some leaks

Test fit of what the Inverter rocket will look like
   

7th January 2013 - Machined up 2 of the three o-ring sleeves that will fit into the end of the manifold. Each sleeve is made of aluminium and weighs 2 grams. These will be glued into the ends of the tubes with epoxy. The hole in the sleeve is 16mm for maximum airflow. The wall thickness is around 0.75mm.


Stock aluminium

Sleeves machined to fit

..with o-rings

Test fitting rings into the manifold

8th January 2013 - Machined up the third o-ring sleeve. We glued all the the o-ring sleeves into the manifold with epoxy.


All 3 sleeves glued in

Close up of the sleeve in place.
   

12th January 2013 - We pressure tested the air manifold to 120psi today. There were no leaks and the manifold was easy to put on and take off again.


Pressurised to 120psi - no leaks
     

15th January 2013 - Gathered up all the tornado tubes, nozzle, guide rail lugs necessary for the rocket. We are using 22mm tornado tubes on the central segment since it is going to be passing water through them. These should be hopefully big enough not to choke the flow too much since the nozzle is only 16mm. The cross sectional area of the tornado tubes is roughly double that of the nozzle. We decided to make up three more spliced pairs of bottles for this rocket so we prepared the bottles for those as well.


Fairings, tornado tubes with seals, nozzle and guide rail buttons.
     

16th January 2013 - Spliced the the 3 new spliced-pairs with Sikaflex today. Only 10 more days to launch so we need to hurry up so the glue has enough time to fully cure. Prepared the sleeves for these as well.


17th January 2013 - Finished gluing the sleeves on with PL premium today. We also did some preliminary simulations to get an idea of how the rocket is expected to perform. It looks like it should do around 500 feet at 120psi. It is hard to enter this particular rocket's parameters accurately into the simulator, but at least we have a general idea. It's a little higher than what we are actually expecting, especially with a high drag rocket like this.


19th January 2013 - Cutting out corriflute fins. We first made a cardboard template to trace them out evenly on the sheet. There is a huge Bunnings store that has just opened up about 10 minutes away from our place. They now sell the 5mm and 3mm corriflute sheets, so very easy to get. We also cut out the fin cans that the fins will be glued to. The fins are attached to the lowest single bottles on the air segments. The 2 fins on each side are at 90 degrees to each other. Because the rocket needs to be screwed together, we allow the fin can to rotate so that once everything has been tightened we can rotate the fins to the correct position. These will be then simply taped in place like we do with all of our fins.


Fins cut out from 5mm corriflute

Fin cans

Gluing fins to the fin cans
 

We cut out the 4 plywood brackets that will hold the segments together. We test drilled a couple and tried sandwiching a tornado tube between them and then secured it with a cable tie. I think this system should work. If not then we'll have to think of something else. One concern that we have is that the rocket may twist on itself. Well that's part of the fun of trying to fly this rocket.


Cutting out struts from a sheet of plywood

4 plywood struts. 40mm x 260mm.
   

20th January 2013 - Gluing the second set of fins to the fin can. When these cure we will add Sikaflex fillets to the fins. We also arranged the bottles to go on the rocket as the black tornado couplings are about 4mm shorter than the commercial ones. We need to make sure the tornado tubes remain in line so we can attach the brackets. Luckily the spliced pairs of bottles turned out in various lengths (+/- 5mm) , so we picked the longer ones for the central segment and the shorter ones for the outer segments. We need to get cracking before Saturday because we still need to pressure test all the bottles, and add the parachute deploy mechanism.


Gluing the second set of fins
     

21st January 2013 - The boys helped pressure test the 10 spliced pairs today to 125psi. To much of John's disappointment none of the bottles exploded. :) With the pressure tests done, we can now assemble most of the rocket. We added the Sikaflex fillets to the fins as well.


10 spliced-pairs ready for testing

Paul helping test the splices

Sikaflex fillets on the fins
 

22nd January 2013 - We test assembled the rocket today so that we could see what it will be like when set up on the pad ready for filling with water. We will definitely need the ladder for this one as well. The rocket is still missing its parachute deployment system which is scheduled for tomorrow. We weighed the rocket as it stands without the recovery system and it has come in at close to 1500grams.

While assembling the rocket I noticed that the bottles were scratched in one place. This must have been caused by inserting the bottles into the test cylinder. We will line it next time to avoid any further scratches. Hopefully the scratches aren't enough to weaken the bottles.


Assembled rocket
     

23rd January 2013 - Today we built the deployment mechanism for the rocket. Because of its size the rocket is using the larger 800mm diameter parachute. The deployment mechanism is very simple and consists of the parachute being pinned down by a strip of PET plastic held against one of the plywood struts. A servo motor is connected to the STII to release the door. There is no spring to eject the parachute, it simply just let's it go into the air stream. The parachute will be attached to two separate points on the rocket so it comes down sideways. It will be interesting to see how much it will swing side to side on the way down. We have attached the struts to the tornado tubes with cable ties.


Struts with deployment mechanism

The reverse side has the battery
   

24th January 2013 - Finished preparing the rocket, We added drinking straws to the leading edges of fins to make them more aerodynamic. KFC straws are great for this as they fit nicely in the 5mm Corriflute.


Fin leading edges
     

26th January 2013 - Launch day. The full launch report is available here: Day 129


Filling with water

Ready to go

Maiden flight

Onboard video frame

27th January 2013 - We took the rocket completely apart and discarded the damaged bottles. We inspected everything for damage and really the only things that need to be replaced are the bottles and the parachute. We started preparing the 6 new bottles for splicing.


Damaged bottles

Only the bottles are damaged
   

28th January 2013 - We spliced the six bottles with Sikaflex today and prepared the sleeves that will go over them. We also re-glued the retaining ring on the manifold tube with the 24 hour epoxy. This retaining ring holds the captive cap on the tube and stops the bottle from flying off. The parachute also got an inspection, and we will replace it with a different one for the next flight.

       

2nd February 2013 - Today we pressure tested the manifold again to 130psi to make sure it wasn't damaged during the landing. Luckily the top bottles acted as a shock absorber so it's fine. The bottle splices were also finished today with the sleeve being attached with PL. We'll wait 4-5 days for the splices to fully cure before testing them. I also looked over the deployment mechanism and realized that the one main difference between this one and our previous successful incarnations of it were that they used the hinge pin method to hold the parachute in place. This is a much more secure connection of the door and does not have the same loading problems. While the rubber band stretches under load, the hinge pin doesn't. The hinge pin method will not work easily in this setup but we'll implement something more secure.


Re-testing the manifold to 130psi

Making more splices
   

4th February 2013 - We made some improvements to the deployment mechanism today. The servo was epoxied to the plywood strut so that it is a lot more solid. Before it was just wired to the strut. We also replaced the rubber band with a loop made of nylon rope. This now prevents the parachute door from stretching open under load. Once we assemble the rocket we will also add a little platform under the parachute so it can't slide out from under the door.


Epoxied Servo

Nylon loop replaces the rubber band
   

5th February 2013 - We made a new parachute today out of an old umbrella. The old one was ripped in too many places. Instead of gluing pieces of reinforcing fabric and using eyelets we went with gluing sections of a ribbon for the shroud lines to attach to. We haven't used this technique before so we're not quite sure how well it will work. We just used regular contact glue.


New parachute

Detail of the glued ribbon and
shroud line
   

7th February 2013 - We pressure tested the 6 new spliced pairs and re-assembled the rocket. We added a little colour to the outside since the parachute was going to be blue and yellow and the water was also going to be blue.

We added a small support platform under the parachute to stop it from sliding down during acceleration. This was just a piece of PET bottle taped to the side of the bottle.


Arrow shows the support platform

Parachute held down by door

...and rests against the platform

Assembled rocket ready to fly

10th February 2013 - Launch day. This time we had a much better success. Full launch report is here: Day 130.


Blue food colouring

Apogee deployment

Gentle landing

30th March 2013 - Launch day - Painting the sky. This time we put differently coloured water in each of the spliced pairs in order to change the water stream colour mid flight.  See the full launch report from day 132 for more details.


Multi-coloured

Flight 1

Flight 2

Air-pulse during flight 1

 

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