Saturday, August 22, 2009

Project 3000 Update: 08-22-2009

It has been another amazingly productive week here at U.S. Water Rockets. We've made a major advance in our nozzle design which has been tested on the bench and proved to be the solution to a major impediment to our world record flights. The issue is an effect we have dubbed "Blowback".

The way we discovered this issue is thanks in part to a question posited by our new friend Steve Porter, who frequents the WRA2 Water Rocket Forum, and noticed something odd in our HD Video from Video Log #1.

Steve noticed that when the rocket was coming down from apogee, the view in the camera lens became blurred and distorted. We had noticed this ourselves back at the time of the launch and we had attributed the effect to fogging of the camera lens. We believed that the heat from the camera and the cold air had somehow condensed moisture and fogged the lens. Since the rocket took a long time to retrieve from the tree it landed in, there was no fog on the lens when we recovered it, so we had no proof. You can see how blurred the HD video image became near the end of the descent in the image below.
We were discussing the theory with Steve and he suggested an alternative idea that never occurred to us. His suggestion was that water dripping from the nozzle would have been blown along the outer skin of the rocket as it fell rapidly thanks to the partially failed parachute. The water drips eventually were blown back up to the nose of the rocket where they collected on the rear-view mirror we placed there. The drops of water eventually collected there and blurred the lens.

We consulted some of our more recent videos and noticed that there indeed was a good deal of water streaming from the nozzle after apogee. The water would drip off and fly down below the rocket. The water could easily be seen spraying around the downward looking views in the HD video. Fortunately, we had a recent launch which had a partially tangled parachute resulting in another rapid descent and just before splashdown we did notice a slight amount of fogging in the video. We believe that this confirms Steve's brilliant observation regarding the water.

We went back and looked at some of our other recent flights and we realized that there was a lot of water dripping from the nozzle during the descent phase. Notice how many water droplets are visible in this recent still-frame shown below.
That was not the end of our story, however... we discussed the dripping water and as you would expect we concluded that it would not be a problem for video in normal flights. We did decide that the amount of leftover water inside the rocket was excessive. It amounts to useless dead weight that needs to be carried to apogee, and it also equates to a loss of thrust that could have been used to lift the rocket.

We ran some static tests and realized that the blowback effect is caused when the water level becomes shallow enough for the air pressure to "punch" through the water, forming an air column through the water and actually splashing the water upwards above the air pulse and all over the inside of the rocket, where it would slowly run down and out long after the rocket had reached apogee.

We have made some changes to our nozzle inlet and added an inverted cone shaped baffle to the rocket just above the nozzle, and we have reduced the amount of blowback by 2/3 what it was. We were able to test this in the lab by weighing the rocket before test firing and then weighing it after test firing and subtracting the mass of the dry rocket to determine the weight of the water remaining. Subsequent test flights have much less water droplets visible, so we are confident we have solved the problem. See the improvement in the image below?
We will be using the new nozzle design in our next round of test launches. A report on the new nozzle performance will be forthcoming. Once again a big thanks to Steve for his insightful observations!

See you next time!


  1. Hi USWR,
    That is an interesting observation. I would not have expected that would be the case with a high power rocket. What percentage of the water weight was the left-over amount before the nozzle mod? We see a similar effect for Robinson coupled bottles during the air pulse, but there the stream of air is directed through the center of the bottle, so the water pushed up against the sides isn't all that surprising. Of course I'm assuming you don't have such a restricted orifice in the pressure chamber since you use a launch tube. I find it interesting that the water goes up the sides when I assume the air more or less acts evenly on the surface of the water if the chamber was only cylindrical. Perhaps it is a common phenomenon relating to water running out at the nozzle rather than an effect of a constriction in the pressure chamber. When we ran tests with baffles over the Robinson coupling it removed that effect. It will be interesting if you manage to eek out extra performance getting the extra water out.

    - George

  2. We saw around 5% of the water weight being retained by the rocket due to this effect. The problem seems to be that some kind of "vortex" is generated like when you open the drain in a bathtub. It's difficult to know for sure what is happening because we cannot see inside the rocket while it is fired and we can only speculate on the problem. One other thing to note is that because of our launch location and how we land our rocket, we like to launch slightly off 90 degrees to aim for a safe landing. The slight angle of the rocket may be causing the problem to be worse in some cases. There are a lot of possibilities.