I was planning on building a 25lbs Valved Pulse Jet Engine but couldnt figure out how much thick should the tail pipe and the combustion chamber be? Can I use a #25 Gague Mild Steel Sheet rolled out for my given diameter for the same purposes? Will it be enough?? Plz help!

Yikes, welds are likely to be the weakest points, regardless of metal thickness. You need to determine how much fuel and air will be in the combustion chamber, and calculate the energy of that explosion. The maximum force will depend on how fast the explosion occurs and how complete the combustion. From that, you can determine what kind of metal and thickness.

 

I recommend you start with an engine in a wind tunnel with no one nearby, everything in a hole in the ground and test your engine before you use it anywhere that might damage anyone or anything. And, run an endurance test many hours to make sure it will not fail.

 

 

The welds should be tested with appropriate equipment, CT scan, x-ray or ultrasound, IDK I'm not an expert in that field.

 

If you cannot do the calculations, I recommend you should not make the engine.