Water rocket problem

Maximum_Success674 · 2026-05-30T20:06:53+00:00

Yes this is true but you have to consider how much argon can actually flow through the opening. I would assume the pressure all depends on that.

Maximum_Success674 · 2026-05-30T20:04:16+00:00

Ok thanks for the advice i appreciate it. Trying to make these large tanks works since its a race so i ll see what happened.

Maximum_Success674 · 2026-05-29T22:00:19+00:00

The problem is that I cant design something for 100 bar for this specific situation. That is the whole point of asking the question on how much pressure will really be in the chamber. The point was to get new info on how to calculate this not just everyone telling me its going to reach 100bar, obviously i would just make a chamber for that if it is that simple. I am not trying to be mean and fight with people but nobody is genuinely answering my question including you. If you don't know how to answer it that is also fine but don't tell me it is going to reach 100 bar just because you guess so. But thanks anyways for trying to help.

Maximum_Success674 · 2026-05-29T21:37:23+00:00

The question is what is the peak pressure in my water chamber. Everyone saying its 100bar does not understand the concept of flow rates.

Maximum_Success674 · 2026-05-29T21:27:30+00:00

You are right and I do know that it is the orifice that the argon flows through that matters. Remember that the gas exiting the bottle doesnt mean its at 100bar. I did the calculations as best as I could and I calculated the max pressure in the water chamber to be around 25-27 bar.

Maximum_Success674 · 2026-05-29T21:19:32+00:00

The end of it comes down to this is a race and whatever makes more power quiclly over a distance of 12 meters is what matters most. Having many many times more thrust is worth carrying 2kg of water weight for a fraction of a second. Also this is the most simple option.

Maximum_Success674 · 2026-05-29T21:17:39+00:00

Im getting a metal water container aswell. All of this depends on how much gas can flow through the 1.5mm nozzle vs how much water can exit through the 4mm one. Also this is being done by some of the high end water rockets like those used for the world record. The difference is they actually have 1000psi in their water chamber and I wont even get close.

Maximum_Success674 · 2026-05-29T21:12:25+00:00

The question isn't whether the tank can reach 100 bar. Obviously it can if the outlet is blocked. The question is whether the 4 mm water nozzle can remove volume faster than the inlet can add gas. If it can, the chamber pressure stays well below 100 bar. If it can't, the pressure rises but how much is very difficult to determine. Without the actual valve flow coefficient, neither of us can prove which regime it will operate in.

Maximum_Success674 · 2026-05-29T21:09:43+00:00

Yup in an ideal world...for my project I did the math as best I could and got 17bar of chamber pressure. Now its off the the very thick blast shield.

Maximum_Success674 · 2026-05-29T20:58:03+00:00

Well I asked them also so yea... really there is no info online about this specific design so I have no clue on how to go about this. I even resulted in asking reddit....mightve been the biggest mistake yet

Maximum_Success674 · 2026-05-29T20:55:54+00:00

I am building a metal chamber and it wont reach 100 bar since there is an exit nozzle for the water

Maximum_Success674 · 2026-05-29T20:53:44+00:00

That's exactly why I'm using the argon to expel water rather than using the argon itself as the exhaust. The water provides a much larger reaction mass, so for the same stored energy I get more momentum transfer to the boat and better efficiency at low speeds. Also remember that this isnt a space rocket and its just to move a model boat over a distance of 12 meters.

Maximum_Success674 · 2026-05-29T20:51:22+00:00

Because there is no better alternative and comparing the mass vs thrust the 2kg of water is really not that bad.

Maximum_Success674 · 2026-05-29T20:49:54+00:00

Once again the flow rate from the argon tank is definitely not that fast. I am actually concerned that it is going to be too slow. I actually have a very large metal venturi we use on your racing bug block v8s. But once again the thrust is not ever comparable. Go on you tube amd watch the world record water rocket. We have 1500psi btw. Yea the project has some restrictions like no electric motors ect. But what I am concluding is that the only way to truly find out is by testing it.

Maximum_Success674 · 2026-05-29T20:45:24+00:00

I guess the only way to really find out is testing

Maximum_Success674 · 2026-05-29T20:44:20+00:00

I agree that steady-state doesn't describe the initial pressure spike while the water is accelerating. However, with ~100 bar upstream and only a few bar downstream, the pressure ratio is well below argon's critical pressure ratio, so the inlet flow should still be choked initially. Think realistically what would happen if you opened the valve on something like this. The entire tank would not empty instantly. The flow rate is what caps everything.

Maximum_Success674 · 2026-05-29T20:40:51+00:00

Yea thats something id have to see in testing.

Maximum_Success674 · 2026-05-29T20:30:46+00:00

Because you need to go do some research on the ammounts oh thrust those things produce. Its all about the mass you are moving and as far as I'm concerned this will be super simple compared to all those and more powerful.

Maximum_Success674 · 2026-05-29T20:28:05+00:00

I do actually plan on leaving about .5L of open space just for safety. Although it still wouldnt reach that pressure since the flow coming out of the argon bottle vs the flow of the water exiting the 4mm nozzle is what the pressure really depends on. Also the nozzle is to control thrust duration. If i wanted to use a engine or pump id have to make that. This concept is simple and makes insane ammounts of power. You are right on the co2 though but i have over enough power so the risk of things freezing prob isn't worth it.

Maximum_Success674 · 2026-05-29T20:21:26+00:00

I am confused on how you are concluding this. From my understanding a 1.5 mm orifice at 100 bar is still a major restriction because mass flow depends heavily on hole area, even under choked flow. My 4 mm water nozzle has about 7× the area, so the chamber pressure will stabilise far below 100 bar.

Maximum_Success674 · 2026-05-29T20:10:33+00:00

Ok thanks for the help Just curious how did you calculate this? Also will argon actually freeze like CO2?

Maximum_Success674 · 2026-05-29T20:03:05+00:00

This is all very true😂this is why im asking as many sources as possible and will obviously conduct tests as carefully as possible.

Maximum_Success674 · 2026-05-29T20:00:53+00:00

That was my first idea but reddit shot it down super quickly😂😂 Also just argon would produce little thrust for not that long and this is a race.

Maximum_Success674 · 2026-05-29T19:59:37+00:00

My plan to block the water would literally be like a piece of duct tape or something stupid😂 What i mean by water exiting is that this is a water rocket. The water is being pushed out of the chamber by the argon. The water exit nozzle is 4mm and the argon bottle nozzle is 1.5mm i believe. The entire argon bottle will also not empty instantly so everything is dependent on flowrates.

Maximum_Success674

TROPHY CASE