Thrust Measurement Experiments

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air.command
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Thrust Measurement Experiments

Postby air.command » Thu Oct 09, 2008 10:07 pm

Last weekend we did a number of water rocket thrust measurements on our new test stand and have done a bit of a write up here:

http://www.AirCommandRockets.com/day68.htm

There were 17 tests covering 6 different experiments. A lot of these tests involved the use of foam, as there isn't much data available on the net. One of the things that we were surprised about was that foam generally had a higher total impulse than water alone although the average thrust was always lower. We only used regular straight through nozzles for these tests. CD nozzles will be done in the next round.

Image

Comments, suggestions for improvement, and requests for specific tests are always welcome.

We are mostly trying to test with restricted nozzles as it tends to draw out the thrust curve and it is easier to identify characteristics rather than full bore nozzles that only last several 10s of milliseconds. We want to try running higher pressure tests as well as we can do around 600N on the stand.
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Postby b-h » Thu Oct 09, 2008 11:19 pm

Hi George,
That's pretty interesting research. Have you thought about a choke for the nozzle that works on g-force or floats on the water level in the casing . You may be able to throttle the motor on lift off or after lift off. I find it interesting that at the .7 mark all your tests have the same thrust remaining" well almost".

Steve

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Postby Andrew Burns » Thu Oct 09, 2008 11:49 pm

b-h wrote:Hi George,
That's pretty interesting research. Have you thought about a choke for the nozzle that works on g-force or floats on the water level in the casing . You may be able to throttle the motor on lift off or after lift off. I find it interesting that at the .7 mark all your tests have the same thrust remaining" well almost".

Steve


I'd say that's because from the 0.7 mark onwards it's just ejecting air and no water, the air is more homogeneous than the air/water mix being ejected before so there's less variation.

From that looks of that graph you're getting 40-50% of your total impulse from the air pulse is that about right? But I'd imagine the air would only weigh a tiny fraction of the total rocket with water. Wouldn't that mean the effective ISP of the air in the water rocket is much greater than the water. Have you ever considered trying a water rocket with just gas?

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Postby b-h » Fri Oct 10, 2008 9:15 am

Andrew,
Do you mean a gas like LPG that could be in a liquid state and filling the same volume as the water normally would?

Steve

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Postby air.command » Fri Oct 10, 2008 10:04 am

b-h wrote:Hi George,
That's pretty interesting research. Have you thought about a choke for the nozzle that works on g-force or floats on the water level in the casing . You may be able to throttle the motor on lift off or after lift off. I find it interesting that at the .7 mark all your tests have the same thrust remaining" well almost".

Steve


Hi B-h,

That concept has been discussed quite a bit on the water rocket forums. I am not sure if it has been actually attempted though. There is a design called a T-nozzle http://au.youtube.com/watch?v=tTNR4Biqdis that has been used quite a bit, but that is only effective for launch tubes where you use the large nozzle for takeoff while still on the launch tube and then the nozzle reduces to a smaller diameter for the actual flight.

You should consider using a launch tube for your steam rocket. It's a way to give your rocket momentum without loosing too much steam/water in the process. Launch tubes can get you an extra 10 - 15% more altitude.

I believe that the thrust should be fairly large on take off to get the rocket up to stable speed and then continue with a lower but longer sustained burn. The reasoning is to keep the top speed down in order to reduce drag.

We have done some initial trials of a variable nozzle that we developed which is large on takeoff and then reduces as the pressure drops in the rocket.
This will be going on the test stand next. Here is a video of how it works:

http://au.youtube.com/watch?v=cugLxF7bJRw

Andrew - That's correct we find that the 'water phase' provides around 60% of the total impulse and the 'air phase' about 40% Although the air has a much lower mass, it's velocity approaches Mach 1.

We have tried the air only rocket and although it works, it does not go as high, perhaps 70% when compared to water/gas. Every rocket has an ideal amount of ratio between water/air to get the most performance out of it. We always use a simulator to figure out the optimal amount of water for a particular rocket, pressure, nozzle size, drag etc.

The maths behind water rocket trust is not trivial, and we don't understand all of it, but here is a great read on what is involved:

http://www.et.byu.edu/~wheeler/benchtop/pix/thrust_eqns.pdf
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Postby astroboy » Fri Oct 10, 2008 4:08 pm

Nice data George, can I assume you are using your new software to transfer the DATAQ data to excell for processing??

Nice idea of using a launch tube and 'captured' reduced nozzle for a steam motor. This may be one way to solve the problem of fitting inlet and outlet tubes through the nozzle for an external water heater.
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Postby Bones » Fri Oct 10, 2008 6:22 pm

Love your work George.
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Postby air.command » Sat Oct 11, 2008 7:40 am

astroboy wrote:Nice data George, can I assume you are using your new software to transfer the DATAQ data to excell for processing??


Hi astro. Yes I'm using the exporting software. I've extended the functionality and am now doing all the processing in the software as it was faster than trying to do it manually in Excel. I still export it to excel as it has very good graphing tools.

Did you ever get a chance to test the software on your files?

astroboy wrote:Nice idea of using a launch tube and 'captured' reduced nozzle for a steam motor. This may be one way to solve the problem of fitting inlet and outlet tubes through the nozzle for an external water heater.


That was my thinking too, that all the heating elements could be in the launch tube to simplify the rocket body construction. You also don't need to fly any of the heating elements.
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Postby astroboy » Sat Oct 11, 2008 10:28 am

No I haven't had the chance to use the software yet (Its sugar crushing season, so I dont have a chance to scratch myself. The slack period of the year is coming soon so hopefully I will have some time to fiddle.
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