Smaller Nozzles on Water Rocket Cars & Controlling Thrust

[thampson]

Over the last few months, our knowledge of water rocket cars has been expanding and some of the things we have learned on water rockets dont always apply equally to water rocket cars.

One thing we have found is that for larger volume water rocket cars ... you dont necessarily want the water to be expelled as fast as possible and get as much thrust as you can in the first second or so of the launch.

The water rocket cars arent fighting gravity directly like the water rockets and carrying additional water is not such a big penalty. Also getting the car rolling and having a long thrust period is better for stability than a short higher powered one.

For those that have been following our progress you will know we have had stability and thrust steering issues with our 6.75L water rocket car. One of the ways we have tackled solving this problem is by controlling the thrust by using a reduced diamter nozzle.

We use the gardena 9mm nozzle for our rocket cars and launcher system and have been trying for a while to get a reduced diameter nozzle working. Some of our failures are here

http://wrocket.hampson.net.au/?p=816

After George from Air command posted a video showing how to use other bottle caps to make a longer threaded nozzle cap, then our solution of using disc inserts in the bottle cap worked as there was plenty of thread left to secure the bottle cap to the bottle. Thanks George.

There is a writeup here of how to make the reduced nozzle disc and longer threaded nozzle cap.

http://wrocket.hampson.net.au/?p=948

and short video here




Georges excellent video is available here




Then we gave the new nozzle a good test today on a vertical test stand (aka strapped to a tree). Tested it at 80 psi and 140psi with reduced nozzle disc in and out.

Results + writeup are here

http://wrocket.hampson.net.au/?p=963

Video is here



With the 6mm reduced diameter disc fitted it took 2.3 times longer to expel the water .. giving us a longer thrust phase with lower magnitude thrust .. just what we are after.

This may come in useful for others having stability issues with water rocket cars and is very easy to make

-todd-

[U.S. Water Rockets1]

Over the last few months, our knowledge of water rocket cars has been expanding and some of the things we have learned on water rockets dont always apply equally to water rocket cars.

One thing we have found is that for larger volume water rocket cars ... you dont necessarily want the water to be expelled as fast as possible and get as much thrust as you can in the first second or so of the launch.

The water rocket cars arent fighting gravity directly like the water rockets and carrying additional water is not such a big penalty. Also getting the car rolling and having a long thrust period is better for stability than a short higher powered one.

For those that have been following our progress you will know we have had stability and thrust steering issues with our 6.75L water rocket car. One of the ways we have tackled solving this problem is by controlling the thrust by using a reduced diamter nozzle.

We use the gardena 9mm nozzle for our rocket cars and launcher system and have been trying for a while to get a reduced diameter nozzle working. Some of our failures are here

http://wrocket.hampson.net.au/?p=816

After George from Air command posted a video showing how to use other bottle caps to make a longer threaded nozzle cap, then our solution of using disc inserts in the bottle cap worked as there was plenty of thread left to secure the bottle cap to the bottle. Thanks George.

There is a writeup here of how to make the reduced nozzle disc and longer threaded nozzle cap.

http://wrocket.hampson.net.au/?p=948

and short video here




Georges excellent video is available here




Then we gave the new nozzle a good test today on a vertical test stand (aka strapped to a tree). Tested it at 80 psi and 140psi with reduced nozzle disc in and out.

Results + writeup are here

http://wrocket.hampson.net.au/?p=963

Video is here



With the 6mm reduced diameter disc fitted it took 2.3 times longer to expel the water .. giving us a longer thrust phase with lower magnitude thrust .. just what we are after.

This may come in useful for others having stability issues with water rocket cars and is very easy to make

-todd-

Hey Todd, that looks like a really effective way to make nozzle size changes out in the field, so you don't have to stick with just a single size and build cars around that. Now you can build a car and then tune the nozzle for the car you built instead.

Has anyone written a water rocket car simulator program yet? That would be something useful.

[thampson]

Yes absolutley .. we will be making a few different discs this week to try out. My feeling is the 6mm disc is a little small for this setup, but it did give us the longer thrust we were after. I will post some videos of the 6mm disc in action on the car shortly.

We havent tried them with foam yet but it will be interesting to see how this goes as well. We will do some testing on the vertical stand to get some data.

I havent seen a water rocket car simulator online anywhere yet .....

-todd-

[thampson]

I just thought I would add to this post. We have completed more testing with reduced nozzle inserts and have the results posted on our website.

We tested 6mm, 7mm, 8mm and 9mm nozzles and also an insert with 5 x 2.5mm holes in a star pattern. All using a 1.5L bottle and 600ml water at both 80psi and 140psi. Results include

- time to empty the water from the bottle
- the hole area
- pic of each nozzle
- test pressure

http://wrocket.hampson.net.au/?p=976

The intersting test was the 5 x 2.5mm insert. Ive attached a pic of the insert disc to this post. It fits in the longer threaded nozzle between the 9mm gardena nozzle and the bottle

It looks to mix the air and water better than a normal nozzle, although not so well as foam I would think. I did a video as well, its not brilliant quality unfortunately .. but here it is



Id be interested in the communities comments on this method of mixing the air and water

-todd-

[zetez]

Videos from nozzle tests are great! Like tests of solid propellant motor from rocket. I'm impressed.

[U.S. Water Rockets1]

I just thought I would add to this post. We have completed more testing with reduced nozzle inserts and have the results posted on our website.

We tested 6mm, 7mm, 8mm and 9mm nozzles and also an insert with 5 x 2.5mm holes in a star pattern. All using a 1.5L bottle and 600ml water at both 80psi and 140psi. Results include

- time to empty the water from the bottle
- the hole area
- pic of each nozzle
- test pressure

http://wrocket.hampson.net.au/?p=976

The intersting test was the 5 x 2.5mm insert. Ive attached a pic of the insert disc to this post. It fits in the longer threaded nozzle between the 9mm gardena nozzle and the bottle

It looks to mix the air and water better than a normal nozzle, although not so well as foam I would think. I did a video as well, its not brilliant quality unfortunately .. but here it is



Id be interested in the communities comments on this method of mixing the air and water

-todd-

Impressive videos, Todd! Your scientific approach to these car designs is very interesting and fun to watch t the same time. In looking at your "star" pattern nozzle reducer, it looks like there could be a problem after using it repeatedly. Do you think the center part of that pattern will fatigue and blow right out? How is it holding up?

[thampson]

Thanks for the comments US Water Rockets

The 5 Star shape disc is holding up well, I have probably done 8 tests with pressures going up to 140psi with it so far and so signs of fatigue yet. Its possible it could blow out eventually, I will keep an eye on this and come back with an update at some stage if there is some noticable change

-todd-

[RaZias]

The gardena nozzle has that advantage, it allows to inserts things to modify it, contrary to the open nozzle that has to let an air tube with an o-ring to enter in the throath of the rocket.

That´s why I prefered a gardena system to a open one, I am trying to work in a special system like yours.

The trick I have in mind is like this: trying to find a way that by some holes it only gets water out and by other only air out.

So you will have a flux of water with a flux air in the center.

The central holes should have been connected to straws and the others around without straws.
The straws goes inside the rocket.

The problem is that when the rocket is filled with water it shouldn´t enter water in the straws.

So you fill the bottle vertically, then you put the nozzle.
When you want to turn it up you should make it very quickly so the water doesn´t enter them.

JUST DO IT !