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J4 and Hyperon pressure vessels completed and tested on the right.

This week we have been mostly building and pressure testing various rockets and launchers. We also revisited an old friend - Acceleron IIIb and are preparing the booster for launch again, although modified.

Splicing Bottles

We have made a total of 8 spliced pairs of bottles now. We will be using these bottles to make up rockets and boosters in the next round of development and so we thought we'd share how we go about making these. The video tutorial shows what materials are required and the process used to make them. The same technique can be used to make longer rockets simply by repeating the splice a number of times.

(If the video does not play, try the latest Flash player from Macromedia)
Also available on MySpace here

Testing

We pressure tested a splice pair yesterday to 180psi (12.4 bar) before a section of the splice delaminated and sprung a leak without failing destructively. Since glue is involved and different amounts end up being used in each splice the strength predictability is a lot lower, and the bottles should always be tested to above the operating pressure before use.

We were still happy with the 180 psi as the bottles normally fail at around 190psi. Both ends of the bottles were quite distorted and we would normally not operate at these pressures. It does give us confidence though that they should be okay to operate at 140psi (9.7 bar) as the upper limit.

Polaron IV & Launcher progress

We have now also pretty much finished the Polaron IV launcher. We have painted all the wooden sections a bright yellow so it will be visible from altitude. The release head mechanism and pulley & lever system for the string is also complete. The reason for the lever is to reduce the amount of lateral force put on the launcher when the string is pulled as the release head under pressure needs quite a bit of force. This helps prevent the launcher from tipping over when someone pulls the string.

We have also pressure tested the launcher using the new pressure regulator up to 215psi. This was still well within the rating limits of the system but since the first few launches will be in the 110-130psi range it was enough to satisfy us. We tested both bleed valves for the main stage and the boosters with success.

When pressurising, we really have to think about what valves get open or closed and in what order.

We made dummy plugs for the booster nozzle seats for the test. This allowed us to test that side of things without having to put the boosters on the launcher. We fitted a strapping tape reinforced small 300ml coke bottle to the main stage release head and used the 7mm aluminium nozzle that the main stage will use. 

We are now working on the booster retention mechanism. This consists of a skirt that is slipped over the bottom of the rocket body. The skirt includes the tubes the boosters attach to as well as the fins permanently bonded.

If the weather is good next week we should be able to test the boosters off the new launcher with a Polaron IV dummy rocket to see if the booster release mechanism works.

Acceleron IV

Acceleron IIIb has been sitting patiently in the workshop for quite a few months for us to find a much bigger launch site. Now that we may be able to fly it at Doonside, we decided to pressure test it since a lot of the Robinson couplings had been tightened for so long. We didn't know how that would affect stress on the bottles in the longer term. We disassembled the booster and decided to test one segment at a time rather than all at once. We didn't bother filling it with water since this wasn't a burst test, but covered it with a number of towels and a piece of foam held down by bricks. Dad stood with his back to it and I had my hearing and eye protection on. As soon as we got to 110psi (the last launch pressure) ... BOOM. The segment exploded. It was a loud boom again, but the neighbours didn't even come out. They must be getting used to us.

Pretty much all the debris was contained under the towel so we found most of the bits. We didn't even try to find the failure point. We pretty quickly decided that the 2 Liter bottles just don't cut it, and we would have to fly with a very low safety margin to make them effective.

So on the spot we decided to rebuild the Acceleron booster using 1.25 Liter bottles instead. We have found that the bottles are quite happy at 130psi. Reinforcing the 2 Liter bottles would have achieved only slight improvement in performance due to the added weight of the reinforcing.

Within 3 hours we had disassembled the Acceleron booster segments and removed all the couplings. We then assembled 3 new segments with new seals. The lowest section of each segment now is made from a spliced pair of bottles with three 1.25L bottles Robinson coupled on top. This configuration gives us almost 6 Liters per segment. The spliced bottles give us more volume for water in the bottom and should also help reduce the blow-through effect. We don't use foam in the booster so foam generation is not an issue.

The new booster segments are a little taller and so we also extended the central aluminium pipe. All other aspects of the booster remain the same. We need to make a number of spacers out of foam to fit on the central pipe in order to keep the correct spacing between the now smaller bottles.

We ran a number of simulations to see what will happen with the reduced volume, and we found that we will get approximately the same performance as with the 2L bottles simply by increasing the pressure from 110psi to 130psi. In the extreme case we could possibly fly at 140psi but we will wait with that.

The one added advantage of the increased pressure (from 110 to 130psi) is that the sustainer will also be able to launch at the higher pressure since it is also based on the 1.25 Liter bottles, and so go higher. The sustainer is at the same pressure as the booster.

We will extend the launcher filler tubes to allow more water to be put into the lowest section, but other than that, no other changes to launcher or rocket are necessary.

Miscellaneous

Tornado Couplings

We experimented with making some Tornado couplings this week out of bottle caps and gluing them together inside a short length of T8 FTC with PL Premium glue. We let it cure for a number of days, but on pressure testing we found that the coupling let go at around 110psi which is not very good. The main problem is the adhesion to the polypropylene caps. It came off cleanly. The glue held well to the FTC. We are looking at some alternatives.

PCBs for Flight Computer V1.5

Rather than continuously building our flight computers on prototyping board we have designed new compact PCBs for the next iteration of the flight computer. We are now in the process of having a small run of those manufactured and will be testing them shortly. When the flight software is finalised we will publish the full design details again.

J4 and Hyperon

We have rebuilt the J4 and Hyperon pressure vessels since the crash landings and have pressure tested them up to 120psi. They are now ready to have the nosecone's fitted.