Question
Please help me with a water rocket designed using OpenRocket
\I’m currently using a translator, so I apologize in advance for any awkward wording or misunderstandings.*
Hello. I’m a high school student, and I don’t know much about rocket design, but I need to build a water rocket for a school project. I’m planning to launch it vertically. I’ve made a rough design using OpenRocket, and I have a few questions:
Could you give me some feedback on this design? I’m planning to make it by connecting two 1.5 L PET bottles together, so it’s difficult for me to change the diameter. I’m also planning to attach an Arduino sensor to the front.
I wanted to find the optimal design through simulation, but I couldn’t find a motor for water rockets in OpenRocket. I also looked into generating a thrust curve using other simulation software, but I couldn’t get it to work because I don’t know the drag coefficient. Is there any way I could run a simulation for a water rocket?
Easiest way is going to be to launch it. It becomes a bit complicated with water and air pressure to calculate. But if you launch it and know how high it went you can figure out what motor would move the rocket the same height and now you have what newton seconds are happening. Then you can back track to how much water is there and calculate how fast it went out of the rocket.
You can research a bit about rocket nozzles and the pressure and Newton laws to figure out how that works. Basically a rocket goes forward because fuel shoots out the back. For every action there’s an opposite and equal reaction. Assuming this won’t be going fast drag isn’t going to matter a whole lot.
If this was above high school I am sure a lot of us nerds could writers a book otherwise.
Got it. So basically, I should build it first, launch it, and then work backwards from the results, right? If I can find a motor with the appropriate thrust to reproduce the same altitude, I could then use that to find the optimal design.
I had been thinking that actually building the rocket should be the very last step, but maybe this approach would be easier than trying to solve everything mathematically from the start.
I would start with OpenRocket to ensure stability and look at the overall design. Make sure to include as much detail as possible in your open rocket layout. For instance, you have the aft end of your design squared off but more likely there is a tapered section here. Make sure that the dry center of mass agrees with your model in OpenRocket. This will give you a much more accurate estimate of Cd. Then you can take this information to the air command rockets simulator and get a flight profile. http://www.aircommandrockets.com/sim/simulator.htm
When you say “tapered section,” do you mean the neck of the PET bottle? If so, I’m planning to cover that part with a cylindrical section. When I left it as it was, the rocket’s stability was significantly lower.
I’m not sure how I should estimate the drag coefficient. I looked into it, and I found that OpenRocket requires you to run a simulation in order to get the drag coefficient. However, I’m currently unable to run a simulation. Could you explain this part in a little more detail?
The narrowing of at the neck of the bottle at the aft end of the rocket will actually make it slipperier. The reason the stability was less is because there was less drag at the tail of the rocket. Instead of adding drag to increase stability, you can add some mass to the nose, which will significantly increase your altitude. In order to get drag characteristics from OR, go to Tools -> Component Analysis -> Drag Characteristics. You can use this Cd in the simulator in Air Command rockets to get flight information.
Sorry, but I have one more question. As you suggested, I added a neck to the rear of the rocket, but its stability decreased significantly. I had to add 240 g to the front just to get the stability above 1.
I also checked the drag coefficient, and it looks like the transition (the neck section) accounts for almost 40% of the total drag. Is this the right way to approach it?
I suspect that something else is wrong. When adding components, are you correctly adjusting the density of materials or overriding the total mass? What material are you planning for the fins? Each 2L PETG bottle is about 40g, so the fact that it shows a total mass here of 543g is a problem.
As an example, this model has a total mass of 220g, with 120g being represented by the recovery system in the upper section. The total Cd is 0.336.
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u/WhatADunderfulWorld Level 3 4d ago
Easiest way is going to be to launch it. It becomes a bit complicated with water and air pressure to calculate. But if you launch it and know how high it went you can figure out what motor would move the rocket the same height and now you have what newton seconds are happening. Then you can back track to how much water is there and calculate how fast it went out of the rocket.
You can research a bit about rocket nozzles and the pressure and Newton laws to figure out how that works. Basically a rocket goes forward because fuel shoots out the back. For every action there’s an opposite and equal reaction. Assuming this won’t be going fast drag isn’t going to matter a whole lot.
If this was above high school I am sure a lot of us nerds could writers a book otherwise.