Each flight log entry usually represents a launch or test day, and describes the events that took place.
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Unfortunately it was too windy this weekend to fly more altimeter flights. Since there are no flights to report, we thought we would do a progress update of what we are currently working on in the workshop.

Polaron IV

From our thrust tests earlier in the year we noticed that the Polaron rocket produced thrust for around 7 seconds using Jet Foaming and a 7mm nozzle. Compare this to full bore nozzles where the thrust lasts typically only hundreds of milliseconds. Here is the video of the test (see the last test in the video). We thought this was pretty interesting and would make for some great footage in flight of a long duration foam trail.

From past experience we know that launching even smaller rockets with a 7mm nozzle and foam can produce unpredictable results(second flight) mostly due to the low initial thrust. The resultant low take-off speed causes the rocket to be quite unstable with the rocket not having sufficient speed for the fins to work properly. The slow release of water also increases the amount of time the center of gravity stays near the tail end of the rocket, further compromising stability.

Polaron III has flown twice with foam before using a 9mm nozzle. The first flight was excellent, but the second leaned over too much and for some reason the parachute did not deploy and the rocket was completely destroyed. Although it did not survive it still travelled over 200 meters from the launch pad. A 7mm nozzle has around 40% less cross sectional area when compared to a 9mm one.

So we decided to build Polaron IV to explore this long duration burn. To overcome the low thrust on take-off we are using three booster segments that will passively fall off when they stop producing thrust. These will work similarly to how Trevor's Green Ant Shuttle rocket worked. Although the retention of the boosters will be slightly different.

In order to keep things simple, there will be no staging mechanism and the Polaron rocket will also fire at lift-off. The individual booster segments are constructed from two 1.25L spliced bottles and use 13mm nozzles. The segments are designed to return to earth without a recovery system again for simplicity. The whole rocket is being designed so that we can extend the main rocket as well as the boosters for increased capacity.

Launcher

Due to the booster configuration we have to build a completely new launcher for this rocket and others like it. It will be similar in design to the Acceleron launcher but will replace the central release mechanism with a gardena release mechanism. The launcher uses 12mm launch tubes only for the boosters. Most of the plumbing is made of brass and the hoses going to each nozzle are rubber with external braided steel sleeve. It is rated to at least 450psi.

Rocket

We are currently building the nosecone for the rocket. It is almost identical in design to what we have been using for the Hyperon rockets except it is 110mm wide. We are also building a second one in case the first one gets damaged. It is easier to build two at the same time then to go back and build it later. It will also give us the ability to switch the nosecones in the field.

The nosecone uses V1.3.1 of the flight computer and will be equipped with a camera and an altimeter. A new bigger chute will also be made, as Polaron III used a pair of smaller ones.

We have the boosters already glued and the fins will be reused from the original Polaron rocket.

Our new acceleration switches arrived today too from RS Electronics. (see photo on left) These things weigh less than a gram and are supposed to activate at around 5Gs. We are keen to fly them and will be ideal in reducing the footprint and weight of the flight computer.

Hybrid Splice

On the Yahoo water rocket forum, Pat LeBlanc had a good suggestion for an alternative way of joining two bottles. The hybrid splice attempts to overcome the limitations of regular glue-only splices which require special glues such as PL premium to provide enough strength to hold bottles together under pressure. It also attempts to overcome the limitations of Robinson couplings that typically have a small hole for air/water to pass through leading to efficiency losses.

The idea was that you could have a full bore coupling between bottles without the need to seal the coupling where it passes through the bottle. Instead, another sleeve between the two bottles provides the pressure seal. The hybrid splice also allows pressure to exist on both sides of the coupling, meaning that distortion of the lobes of the bottles would be virtually eliminated. After a number of discussions with forum members we had a go at constructing and testing a hybrid splice.

Our first idea for the hybrid splice involved using 5 screws in the lobes of the bottles holding the bottles together, with just a big hole in the middle to let the air pass, but finally chose an alternate design that would make things a little easier to construct. We replaced the Robinson coupling in Pat's idea with just a simple bolt in the middle. We then made 10 holes in the base of each bottle ( 2 in the side of each lobe ) with a heated 10mm rod. The air holes were aligned with the corresponding ones in each bottle so that the air could flow easier from one bottle to the next. The hole for a bolt was just drilled in the middle. We used a metal bolt for the test, however, a nylon one should be used for safety and weight reduction reasons.

To tighten the bolt we just used a socket wrench with an extension and a long screwdriver. No other special tools were needed.

The total cross-sectional area of all the holes was equivalent to a 32mm Robinson coupling. That is about 15 times bigger than our typical 8mm coupling.

The idea behind this hybrid splice was that no special components such as threaded lamp rods or tools were needed. The other idea is that even an inexpensive glue could be used to provide the seal in the sleeve since the bolt in the middle would be providing the holding force.

Test Results

We spliced two 1.25L bottles and let it dry for about 4 days and then performed a hydrostatic burst test on it. The splice held up to 130 psi. The sleeve was held down by a combination of PL Premium and VISE glues. After we put the PL glue in we noticed there were a couple of minor leaks, so we poured the runny VISE glue in to fill those, and that sealed it well. 130psi is not all that great for this particular hybrid design when you consider the VISE glue-only splice held 170psi+ and our Robinson couplings hold also around 170psi+ with bottle burst pressures around 190 psi. A 130psi splice means about 100psi operational pressure.

It is unclear what the initial failure point was but both bottles cracked between the holes in the bases and the sleeve also ripped all the way around. It did not delaminate from the glue, the plastic failed. After a number of discussions with forum members we now suspect the failure was at the sleeve first and when that failed the bottles did. Normally a sleeve like that should hold at least 180-190psi. The bottles still remained together and no shrapnel went flying. It looks like the holes in the sides have weakened the bases too much.

It was a really unusual failure because the sleeve edges are still attached to the bottles all the way around. The straight edge seen on the torn sleeve photos is from the scissors when I cut the sleeve away to photograph the inside.

Conclusions

If the sleeve failed first that means that the bolt wasn't doing a good job of holding the bottles together. When we first bolted the bottles together we noticed that there was a certain amount of give. The bottom of the bottles flexed a little when you pulled on the two ends. This was seen before gluing the sleeve on. With the one bolt, this flex in both bases was probably enough to put most of the strain on the sleeve when pressurised. Because the pressure was the same on both sides of the lobes you didn't get the typical crack propagating from the central bolt hole but rather the circumferential cracks between the weakest points.

The reason the sleeve may have failed at a lower pressure is that because the sleeve is only really held down by the ends. The middle could have bulged out under pressure, and placed uneven strain on it. With a normal splice the sleeve is completely held down along its full length by glue and so this bulging is unlikely to happen.

The cross-sectional area of the bottles is 63.6 cm2 and at 130 psi you end up with a force of 581Kg! pulling one bottle in one direction and the same in the other direction. No wonder you get a bit of flex in the base of the bottle.

As a result it may be better to try the Robinson coupling Pat suggested in the first place for the hybrid splice. Although the coupling may experience the same flex at the base of the bottle, putting the strain on the sleeve again. It may achieve better results since the bottles are not weakened by the holes in the lobes.

Richard Wayman from TOR water rockets is also having a go at building a hybrid splice that uses nylon bolts in the lobes of the bottles, with a hole in the middle. His bottles have a much nicer shape for this, and we're looking forward  to his test results.

Miscellaneous

In the background we are also working on an T-8 FTC rocket to see how they perform first hand. The rocket will also carry a flight computer for deployment and an altimeter, but will not be designed to carry a camera.

We have almost finished constructing a new stager concept. While theoretically it should work, we have no idea how well or if it will work at all in real life. We will publish full details again when it has been test flown.