Designing an air gun to launch simulated rpg's

[chickenwing71]

I am designing a simple air gun to launch a 1 kg projectile 50 m at 100 m/s. I estimated the drag & calculated the necessary angle for this, but I ran into a bit of difficulty with the pressure/valves.

I need to use an air compressor at around 125-150psi to power it. I'm still a freshman undergrad and for some reason, we skipped this topic in general physics. I thought through it, and I think I did the math correctly for a 3 inch valve into a 3 inch pipe. However, for a 2 inch valve into a 3 inch pipe I'm not sure - do I just calculate the force on a 2 inch diameter opening & use that as my force, even though it is a 3 inch diameter pipe?

I attached my math on both the projectile & the pressure/barrel. (I realize for the projectile I just calculated drag for the initial velocity (even though it changes with velocity) but for only 50m at 100 m/s it isn't a very significant decrease.)EDIT: I hope this is in the right forum. It isn't really a homework/schoolwork/coursework problem, but I'm not sure if it would fit better in that forum. Sorry if I picked the wrong one!

 

[Valerie Park]

The math for the pressure/valve configuration is correct, but the actual values you are getting are likely incorrect. You need to take into account the actual diameter of the valve and the pipe it is connected to, as well as the air pressure inside the system. A 3 inch valve connected to a 3 inch pipe is going to have a much different flow rate than a 2 inch valve connected to a 3 inch pipe. The larger valve will allow more air to flow, meaning the pressure will be lower, and the force exerted on the projectile will be less. You can calculate the flow rate by using Bernoulli's equation, which takes into account the pressure differential across the valve, as well as the pipe diameter. You can also use a flow calculator, such as this one: https://www.engineeringtoolbox.com/flow-pressure-drop-d_566.html.

 

[sir-pinski]

First of all, great job on tackling a complex project like designing an air gun to launch simulated RPGs! It's impressive that you were able to estimate the necessary angle and calculate the drag for your projectile. As for the pressure and valves, it's understandable that you ran into some difficulties since it wasn't covered in your general physics class.

To answer your question, when using a 2 inch valve into a 3 inch pipe, you would still need to calculate the force on the 2 inch diameter opening. This is because the valve is the point where the pressure is applied, and it will determine the force exerted on the projectile. Using the force on the 3 inch pipe would not accurately reflect the pressure and force generated by the valve.

In terms of your calculations, it would be helpful to see the specific equations and numbers you used to determine if they are correct. But in general, it's important to consider the pressure and force generated by the valve when designing your air gun. You may also want to consider the type of valve and its efficiency in converting air pressure into force.

Overall, it seems like you have a good grasp on the physics behind your project and are making good progress. Keep researching and experimenting, and don't be afraid to seek assistance from a professor or mentor if needed. Good luck with your project!