This blog covers the day to day progress of water rocket development by the Air Command Water Rockets team. It is also a facility for people to provide feedback and ask questions.

Monday, November 16, 2009

Keeping neighbours happy

This weekend we were going to do burst pressure tests on a range of bottles, but blowing up bottles in one's back yard isn't something to get your neighbours excited about. We are always looking for ways to lower the noise during these tests. Having the bottle full of water greatly reduces the noise, but a little bit of air can enter the bottle as it expands and so the resultant explosion can still be very loud.

Sound suppression attempt #1:
We filled the bottle with water, and then placed the whole bottle in a small plastic bathtub, filled it full of water, covered the bottle underwater with a towel and held it down with two bricks. ....
Result: Big boom, tub split in half, lawn received a healthy watering.

Sound suppression attempt #2:
So we drove over to dad's house, filled the next bottle with water and suspended it about 1/2m below the water in the concrete in-ground swimming pool.
Result: Quieter boom but what was amazing was the fact you could feel the concrete shake under your feet with the boom! This was a bottle full of water with about 100mL of air in a large pool and it burst at ~160psi. The slow-mo video standing on a tripod on the side of the pool shows the blur as the shockwave hits shortly later followed by tiny ripples on the surface doing interference patterns for about 1-2 seconds. Obviously the shockwave travelled through the water and echoed around the pool. .... WAY COOOL! ... but not the objective. Could the shock wave damage the pool? Unlikely, but we didn't want to take too many more chances.Tiny ripples on the water are the sound waves bouncing around in the pool

Sound suppression attempt #3:
Dad brought out an old SCUBA cylinder with the bottom cut off. This aluminium tank has a wall thickness of about 1cm, and so we place the bottle in it, stuffed old rags in the hole, placed it up against a rock wall, put a bench, lead weights and seat against it to stop the tank from flying in the other direction, wrapped the tank in two layers of foam and burst the bottle ....
Result: A barely audible thud.
Subsequent tests showed that you really only needed the rags in the end of the tank to suppress pretty much all the noise. There was no need for the foam wrap, or weights to hold the tank down.

Putting the bottle in the SCUBA cylinder

Stuffing in rags to suppress the sound.

Tank is under two layers of foam and bench with weights is meant to stop the tank from flying away.

In the end 9 bottles were blown up, and the neighbours didn't complain one bit.

Wednesday, November 11, 2009

So far so good ....

During the week we made up four new spliced-pairs of 2L bottles. Each has a capacity of around 3.15L. The splicing and reinforcing technique seems to be working well so far. The Sikaflex is doing its job sealing the splices.

Today they were all pressure tested to 140psi without any leaks, and the bottles and tape showed no stress marks. We are very happy about that. Having a good yield rate means we can produce more of these with less bottles.

Now they will all be assembled into a Polaron type rocket. The initial target launch pressure will be 130psi with a 15mm nozzle. While we are building this rocket we will continue to make up more spliced pairs for the rest of Acceleron. Some of these will also be converted into boosters.

I think I have an approach too now for the pressure switch, so I'll be doing some prototype work on that soon.

Friday, November 06, 2009

Investigating water behaviour in flight

At Doonside this weekend we flew the Axion rocket again 3 times with a camera on the boom to try and get a better shot of what is happening in the lowest bottle in flight. We used food colouring to enhance the contrast and no foaming agent this time. We also painted the fins a neutral gray to prevent the auto-iris on the camera making everything else too dark.

The full launch report is available here: http://home. au/~aircommand/ day82.htm

Acceleron V

Since a few people have now asked about when Acceleron V is going to fly again, I might as well do an update. We are in the process of getting Acceleron V back in the air. There were a couple of issues that we really wanted to solve before trying again. The number one priority is getting the spliced pairs reliable enough to be able to launch with them and not worrying about being very close to the burst limit. The last explosion showed that the spliced pairs just weren't there yet. Even though they were tested to 130psi, they were obviously close to that pressure. Although we could fly the rocket at lower pressure, the rocket's flight envelope was really not designed for anything lower than 110psi. Ideally the rocket needs 130-140psi.

The need for stronger spliced pairs is the reason we have been doing the splice tests recently. If further tests with the new splice technique continue to go well, we are hoping to replace the existing spliced pairs with the new ones on the rocket. We have 4 new spliced pairs curing at the moment and we will hydro test them to 140psi. We will then make a Polaron style rocket and test fly them at 130psi. If the spliced pairs perform well in real life situations then we will make up more to replace those on Acceleron V.

The other issue is the pressure switch. We want to have a reliable pressure switch to detect burnout. The pressure switch we used on the first flight just wasn't reliable enough, and the old pressure switch (TDD) leaks a little. When these issues are resolved, then we will try to put Acceleron V up again.