With the NSWRA launch day being delayed a week due to bad weather we had just enough time to put together the larger booster and sustainer for the next test of the Mk2. stager. Great pyro rocket photos of the launch day are available here. We also did some experiments with a simple variable diameter nozzle (see below).

The sustainer has 4 fins made out of 3mm plywood glued directly next to each tube. This not only provides extra strength to the tubes, but also adds extra rigidity to each fin. Everything was glued with PL premium, but due to the bubbly nature of PL, fillets were finished off with Selley's Sikaflex glue.

The pressure chamber consists of a 2.1L spliced pair of bottles Robinson coupled to a 1.25L bottle. The Robinson coupling is there to generate foam using the Jet Foaming technique.

The recovery system is our standard one with V1.5 flight computer. The nosecone is also fitted with a Z-log logging altimeter and a FlyCamOne V2 video camera.

Baryon II Booster

The booster is based on two 2L spliced pairs of bottles coupled together with a 22mm tornado coupling. It uses a 15mm nozzle for higher takeoff speed and so that we can use a launch tube with it. The fins are long but fairly narrow, in order for them to clear the launcher ring. The top bottle of the booster is fitted with the Mk2. stager. Around the stager is a collar made from a PET bottle. The collar has 4 thin walled aluminium tubes glued to it. These tubes act to stabilise the sustainer during the boost phase.

 

The parachute is mounted between the two bottles half way along the body. The parachute release mechanism is the same as the Baryon I booster - a flap held together by a wire attached to the sustainer. During staging this wire releases the parachute. There are a few extra loops of the main line wrapped around the parachute so that the booster has a chance to slow down a little before the parachute fully opens.

Launch Day Events

• It was fairly calm when we arrived at the launch site, but the weather forecasts predicted a strong breeze for the day. We quickly set up the launch site in order to get the first launch in of the 2 stage rocket. From previous experience we knew that 2 stage flights are best performed in calm conditions. By the time we were ready to launch the breeze had picked up a bit, and so we angled the rocket into the wind.
• We pressurised the rocket to 120psi and launched. The rocket weather-cocked into the breeze and by the time the second stage released it was angled by perhaps 20 degrees from the vertical.  Staging occurred right on cue with good parachute deployment on both the sustainer and booster. Due to the arced flight, only 381 feet (116m) was achieved which was significantly lower than expected. We'll try this combination again in calm conditions.
  
  I thought I heard a small leak somewhere from the rocket during pressurisation but I am not sure where it came from or whether it was from the launcher. We will need to investigate to make sure everything is okay for the next launch.
• Next off the launch pad was a repaired Tachyon IVb after the lower bottle failed last month. We played it safe and only filled it to 160psi. We wanted to get at least one flight in before trying higher pressures. This rocket was also fitted with a baffle over the Robinson coupling to prevent the blow-through effect which is usually more evident with higher pressures. The rocket took of visibly faster and had a nice straight flight. For these first test flights we didn't fit it with an altimeter or camera in case of failures.
• Due to the high wind conditions we skipped over flying the long Axion rocket and flew the trusty old J4. The rocket also arced over soon after liftoff due to the wind and flew a long way down range. It landed just in front of some trees, with the parachute hanging 30cm from the ground on the lowest branch. ... a close shave.
• In light of the conditions we decided to fly the smaller Tachyon IVb rocket at 170psi, but as we approached 140psi a leak developed at the nozzle cap and water started draining. We had a video camera set up on a tripod close to the rocket so we were able to see exactly how much water was lost ~150mL , and where the leak had occurred. Since we were at 140psi, we decided to launch as that was enough pressure for a good flight.
  
  The rocket had a nice flight again although a post landing inspection showed that the Gardena nozzle o-ring had come out of its groove and was threaded further up the nozzle! We suspect that the gap between the nozzle and launcher is perhaps just a fraction too big and with the higher pressures the o-ring could have been squeezed in there more than it should. During launch the pressurised water flowing past would have finished pushing it up. The leak was likely due to an improperly seated washer in the nozzle. We have seen this on a number of occasions and the remedy is to take off the nozzle and put it back on again.
  
  The rocket seems to fly well with the baffle, but it will be a while before we can evaluate what sort of a performance hit/gain it has.
• There were some great pyro rockets flown on the day by other members of NSWRA.  

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A day that you can walk away from without damage done to rockets is a good day.

Variable Nozzle

Over the past month we have been working on a way to remove residual foam from inside the rocket as it flies. On a previous occasion we found that a significant amount of foam can remain inside the rocket which ends up being dead weight, reducing the peak altitude.

Tim Chen had recently discussed the concept of a variable nozzle for water rockets, although his concept was quite different to that below, it lead us to adapt the de foaming design to the variable nozzle shown below.

The original de foaming concept consisted of a rubber membrane with a small hole in it in the highest upper bottle. A larger plastic ball would sit over this membrane and act as a one way valve. Upon launch it would seal against the membrane and as the pressure continued to drop in the lower bottles the ball would start getting forced through the membrane until there was enough pressure difference to cause the ball to drop through the membrane causing a blast of air from the upper bottle to help force the foam out of the lower bottle while providing further thrust. The ball would be caught in a small cage under the membrane.

A rocket typically needs to have a large diameter nozzle on lift off to provide enough thrust to get up to speed, but once stable flight is achieved, a smaller nozzle can be used to sustain the thrust for longer.

The variable nozzle simply consists of a rubber membrane with a small hole in it. As air and water are forced out through this hole, the more pressure you have the membrane stretches and the bigger the hole gets. This means that on launch with high pressure the hole becomes very big and as the pressure drops the hole gets smaller automatically.

The rubber membrane adds less than 1 gram to the rocket and works with existing Gardena nozzles. The membrane is simply pinched between the nozzle and the bottle.

The rubber membrane was made from a swimming cap, but rubber kitchen gloves, or bicycle inner tube could be used as well. We are yet to test fly it, but ground tests so far have looked good. We are going to have to experiment with different hole sizes and rubber stiffness in order to see what the best combination is.

Update: The static test stand experiments have now been carried out. See Day 75 for details.

The elasticity of the material controls the relationship between pressure and nozzle size.

It should also be possible to use a launch tube with this design.

Here is a video of how the nozzle works. The Gardena nozzle has been removed so that it is easy to see what the membrane is doing. In the video the bottle is filled with water with the nozzle at one end and an air inlet at the other. We tilt the bottle back and forth to either let only air flow or water flow for the demonstration. And no the green fingernail polish is not mine. :)

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We'll publish the two techniques we are currently exploring for clearing the foam from the rocket in upcoming updates.

Flight Details (day 65)