Posts Tagged ‘hydrostatic’


3 Bottle Wrocket Hydrostatic Testing

We decided to add another “robinson coupled” bottle to H2 to make a 3 bottle wrocket and hydrostatic test it.  We were interested to see how this coupling would go and whether we would see the drip leaks we encountered on the 2 bottle hydrostatic test.

Adding the third bottle used the same technique as joining the first two bottles, which was surprisingly fast when you know what you are doing (well have done it more than once…. :) ). I also created an additional fairing for the third bottle using the “low tech” hotplate technique.

Additional Safety Precautions

I decided that even though we are not burst testing these bottles, there is still significant energy inside the bottles at 125psi, so I will be using my safety goggles on these hydrostatic tests.

Safety Goggles

Safety Goggles

Hydrostatic Test

I took Georges advise and filled the hose leading to the pump with water, but it only filled back to the 10mm section and wouldn’t take anymore (air locked possibly). Evidently this turned out to be not enough as it stated out fine and no air was being pumped into the bottle,  but as the pressure rose the air just went around the water in the hose and got in. I will have to find a way around this problem for future tests.

You can see here the amount of air that made its way into the first bottle

air in first bottle - 3 bottle test

air in first bottle - 3 bottle test

The test itself went well. The bottles held pressure well, all the way up to the maximum on the pump gauge, which was 125psi. The first to second bottle still had the drip leak (about 3 drips every 2 seconds) but the second to third showed no leaks. This pressure was held for about 3 minutes.

125psi - hydrostatic test

125psi - hydrostatic test

here is a pic of the bottles at 125psi (you may need to click on the pic for the full size to see detail)

3 bottles at 125psi

3 bottles at 125psi

Just out of interest I weighted the 3 bottle assembly, including the 3 x 1.25L bottles, 2 x robinson couplings, 2 x fairings and nozzel. It weighs in at 193grams

Final Thoughts

Our next launch is to test the parachute deployment, so we will probably go back to 2 bottles – most likey take off the first one with the dripping connection. Once we have the parachute system working properly we will add the third again and go for some height :)

Posted by on September 18th, 2008 Comments Off

2 Bottle Wrocket Hydrostatic Testing

During our Launch day 6 to test the nosecone and parachute deployment we suffered a leak in our H2 rocket which limited launch pressure to 70psi. So we spent this weekend doing hydrostatic testing of the bottles including the robinson coupling where it was leaking.

As is usual with leaks it looks like it was the washers, so we made a new set of 3 washers and fitted them. We stripped the wrocket back to only the pressure bottles. This is the benefit of a modular design, the nosecone and the removable fin assembly just came off (so as not to damage them) and the 2 bottles and nozzle were left to test with.

With the 2 bottles totally full of water it weighs in at around 2.75kg, way to heavy to balance on the launcher. We used our gardena quick release and just connected that straight to the wrocket nozzle in a horizontal position.

2 bottle hydrostatic testing

2 bottle hydrostatic testing

We found that there was a leak in the one way valve coming from a molded join section at around 90psi. I fear the pressure may have been too much for the plastic valve, so we removed it. We will have to look for a brass one to use instead.

We had a few small leaks in the tubing connections which were tightened then we pressurized until around 120psi and left it there for a few minutes. There was a small leak between the cap and the bottle on the robinson coupling, but not enough to stop a launch, I think this was from the bottles swelling in this area. Here is a pic at 120psi

120psi - bottles swelling

120psi - bottles swelling

just on 120psi

just on 120psi

We are happy that we have a pressure body thats ready for launch, we just need to reassemble the rocket and do some timing tests on the nosecone and we are ready to fly. Hopefully with good weather this week we will get a late afternoon mid week launch  :)

Things we learned

  • Dont try to use a launcher for a hydrostatic test … the wrocket is too heavy
  • At 120psi full of water the wrocket is too heavy to go more than 10cm using a manual release.
  • Always check for leaks :)
  • We are probably going to have to look for a new pump soon :)
  • When fully filling a 2 bottle wrocket for hydrostatic testing the water tends to stay in the top bottle instead of making its way into the bottom bottle. The way to get the water into the bottom bottle is to gently squeezing the air out of the bottom bottle. This forces the air into the top bottle and water down into the bottom bottle. Careful not to squeeze too hard so you dont put a crease the bottle

Posted by on September 15th, 2008 2 Comments

Bottle Testing + Longer SRT III

Bottle Testing

While I have been looking around for some pieces we need for a “robinson coupling” to make a multi-bottle rocket, we have been developing the single 1.5L bottle.

It is fun to blow up a bottles on the pad but its something we want to minimize, especially now that we are increasing the pressures. It also tends to destroy nozzles as the rocket is pushed down into the launch stand.  We have starting using  the better practice of hydrostatic pressure testing the bottles. This is accomplished by filling the bottle with water and pressure testing. The idea is that when the bottle lets go there will be less of an explosive force due to minimal amounts of air in the bottle under pressure. Also as the water cant be compressed the bottle comes up to pressure faster.

George over at Air Command has a great page on this procedure with some excellent videos and burst pressures from different sizes of bottles.

We made a new pressure vessel and redid the nozzle, this time with a rubber seal made from a bike tyre tube.  The bottom of the bottle was taped with 2 layers of cloth embedded duct tape and 3/4 of the bottle was taped tightly.

The bottle was filled with water and connected to the pump female quick release and the pressure raised to 110psi. At this stage there was a air leak which seemed to be coming from the 1 way valve threads. I had to do a manual release.

The good thing is that the bottle held at 110psi so we will still have to do a proper hydrostatic test to see where it bursts.  I will need to put in a release valve as it was a bit hairy doing a manual release. Also the foot pump we have been using only goes to 120psi so I am going to have to look for another pressure meter and install this in line as well. Sees we have a bit of work to do in this area.


As SRT II was lost on the pad, we built SRT III, we used the pressure vessel (bottle) and nozzle we tested above and used the fin assembly from SRT II and made a new nosecone from another 1.5L bottle. This time we made the nosecone longer (by using more of the second bottle) and taped it to the pressure vessel. We also decided to try putting the fins back further behind the nozzle to see if this would provide better stability. To do this we used 2 lengths of wooden skewers. This was achieved by actually using 5 skewers for each mount (15 altogether). Two pairs of two skewers taped together then a single taped to both pairs. These were taped to the rocket body and the fin assembly.

Posted by on August 3rd, 2008 Comments Off