This week is only a short update. We have spent quite a bit of time in the workshop working on a number of projects.

Polaron IV - upgrade

Polaron IV is currently getting a capacity upgrade. Each of the boosters is getting an extra 1.25L for a total of 3.35L each and the main stage is getting an extra 2L bottle added to the stack for a total of ~10L. Eventually all the bottles in the main stage will be replaced with spliced 2L pairs.

Because of the new longer boosters we will have to add additional pin supports to each to keep them in place during ascent. At the moment the boosters are based on the 90mm 1.25L bottles, but as we get more of the 2L spliced pairs made up, we will replace the boosters with these larger capacity ones. The launcher was designed for this capability so there won't be any need to change it.

Because the boosters are now getting a bit too heavy for just regular tumble recovery, we will be adding small parachutes to each of the boosters. The parachutes will be released shortly after separation. The parachute release mechanism will be very similar to how we do our side deployment except the door will be just a strip of PET. Instead of the pin, we will use either tiger-tail or just fishing line threaded through the latch to keep the strip held down. The parachute is just held under this strip against the booster. The other end of the fishing line is attached to the main stage. As the booster drops away, the fishing line slides out of the latch and the parachute is released.

The booster parachutes have been made from the lightweight rip-stop nylon we got from the UK a few weeks back. The parachutes really wrap up into a tiny space.

2L Splice Tests

We tested a spliced pair of 2L bottles as we wanted to know if they can withstand higher pressures than when they are Robinson coupled. We've had 2L Robinson coupled bottles fail at pressures as low as 110psi, which didn't instill too much confidence in using them for larger rockets. We have launched with them at 130psi before but we knew we were close to the limits. (Burst pressure of a unmodified bottle is around 170psi)

The 2L bottles fit exactly into a sleeve made from a 2.25L bottle (Thanks Damo for pointing this out) which simplifies things with shrinking the sleeve, otherwise the splice procedure is exactly the same as described here.

We also added a little reinforcement to the necks of the bottles where typically stress marks appear when pressurised above 120psi. These were made from a conical section of another PET bottle and glued in place with PL premium.

We hydro-statically tested the spliced pair using our new control panel up to 140psi and held it there for a couple of minutes. Over that time the pressure dropped by ~10psi which may have indicated a tiny leak somewhere but not wanting to approach it we weren't sure where it was. We couldn't hear anything because we were testing it outside during a storm. It may have been a hose connection or something. In any case it was still a successful test because the bottles held the pressure and upon inspection showed no signs of stress. The neck reinforcements seem to be working well.

We will now make up a number of these 2L spliced pairs for new rockets. We intend to go back and re-replace Acceleron's booster segments with these spliced pairs to get back to the higher capacity. Each spliced pair has 3.6L capacity. They will be joined together with the new tornado couplings.

Tornado Couplings

Last weekend we finally managed to produce a number of good Tornado couplings. Tornado couplings connect bottles neck to neck. These ones are easily made out of gardening supplies from the local hardware store. We modify a couple of different fittings for the purpose. Although we used a lathe for trimming the pieces to make life easier, they can be made with just a hack-saw and some files.

Some features:

• They have a 15mm hole
• Weigh 13 grams
• Require no glue
• Have been tested to 130psi, but can most likely hold more.
• Require no special tools
• All plastic construction - no metal.

The plastic fittings are normally used for garden irrigation purposes. From one of them we just make a nut by cutting off the hose adaptor, and the others we simply use as the hollow screws that fit within this nut. We trim off the hose adaptor on these and sand down the outer diameter so that it fits inside the neck of the bottle. We also file a slot into this section to allow us to tighten it once it is in the cap. We cut a hole in a couple of regular bottle caps including the internal seal. This hole is just a little bit smaller than the thread of the screw. This allows us to get a maximum grip on the small lip.

We then take a couple of soft rubber washers and sandwich them between the lids and each side of the nut.

Miscellaneous

We are also working in the background on a couple of projects involving an all-mechanical staging mechanism as well as our first FTC rocket.

These are coming along slowly, but will feature in future updates. We started building the prototype of the staging mechanism yesterday, but half way through we realized that the integrated non-return valve was in the wrong place. *Doh* But all was not lost, as it can be easily moved up in the mechanism so we didn't loose any work there.

The FTC rocket will be made from a 6' T8 FTC tube. The nozzle will be 15mm in diameter. We haven't decided what kind of reinforcement it will have yet, but we will most likely start with a couple of wraps of the glass fiber strapping tape. This is low priority at the moment. The plan is to eventually use it as a second stage on an Acceleron derivative. Further down the track we may also make a very small FTC dart to go on top of a Polaron like rocket, but this again is very low priority.

We started a small production run of 90mm payload nosecones that will be ready to be swapped in should we have a crash on the day. They will all use V1.5 of the flight computer. These will be utilized for all future 90mm rockets.

Included are a number of pictures from the last launch event taken by Andrew E. from NSWRA. He has a much better camera and took some excellent pics of the staging in progress of both the Polaron and Acceleron rockets.