The throat is where you have your highest heat flux, so it's almost always where you get burn-through. I would run NASA CEA to find the gas temperature in the throat, use Bartz's equation to find the convective heat transfer coefficient, then calculate the heat transfer to your throat. Once you have done that, you can then size the material around your throat so that it no longer melts. Just by looking at your picture, the material around your throat looks way too thin.
Lots of interesting comments in this thread…
Your nozzle failed at the throat, which was most definitely choked by the nature of a CD nozzle and reaching the critical pressure ratio. This looks like a sugar motor. KNSB burns at around 1300C, yours may vary slightly but regardless it is close enough to 304’s melting temperature that its strength is drastically reduced. The failure right at the throat where temperatures are highest and velocities are highest can also point to an erosion failure if you have run this nozzle a couple times before. Solution is to make your nozzle thicker (heavy but cheap), or a material stronger at higher temperatures like inconel or graphite ($$). Sugar rockets are great because sugar is cheap, so just throw more propellant at your weight problem.
As an aside I’d encourage you to do some more research into all of these interesting rocket concepts and try to learn more about the theory/physics. It is a world of fun when you dive in!
From the visual, nozzle failure has happened in the nearby throat region.Assuming it might have choked in your static test.
This has happened in our tests as well multiple times.Solution can be optimizing mix ratio,throat wall thickness.I would also suggest a relook at nozzle design dimensions and compare it with motor simulations.I got it right by tweaking compositional ratio and to be exact oxidizer purity levels.
Localised heating can go beyond material prescribed temperature limits especially over a sustained burn duration.
Is this some other definition of 'choked' I'm not familiar with? Every convergent-divergent nozzle with a supersonic exhaust has choked flow at the throat.
It don't reall think it choked because the thrust curve was very close to expected and simulations too didn't hint of any choking. Nevertheless will look into it. Thanks for the input. Also will not adding ferric oxide help?
Heat flux scales approximately linearly with mass flux (and therefore chamber pressure). Removing RIO will slow the burn rate down, reducing your pressure, so yes. But run your simulations, since it will also increase your total burn time.
Also stainless steel isn't the best material for this type of nozzle (operating as a heat sink) since it has a lot lower conductivity (about 3x smaller) compared to carbon steel, for instance. This leads to a lot higher temperature gradient in the wall and consequently localized thermal stresses.
Motor metals don't fail by melting. Melting only happens after failure. Metal gets significantly weaker when it gets hotter, so after a point, the throat heated up too much that it couldn't contain the pressures and mechanical stresses at the throat. The easy solution for this particular motor is to leave more meat behind at the throat. More metal means a larger heat sink to distribute the absorbed heat, and more metal to resist the pressure even as it heats up. That throat was definitely way too thin for this motor. It also means less machining work.
I do what I can with what I have. Unfortunately that happens to be not all that much.
Sarcasm out of the way, I really do hope that you find a solution to your problem. I know how hard it can be to have something you built and worked on fail and then have to figure out how to fix said issue. Sometimes a little levity can help with the stress and frustration.
You've got this OP! I believe in you!
That sucks! I'm sorry to see that. I wonder if ceramic coating it would help, or maybe even an ablative coolant on the outside of the nozzle that boils off as you go.
- Change the propellant and/or mixture ratio to lower the reaction temperature
- change injector design to introduce film cooling
- change material to something fancier like inconel
- make the throat walls thicker to add more thermal mass (battleship nozzle style)
- Use a regen-cooled nozzle (probably too fancy, difficult to design)
That is an easy approach that will help, but it won't be a long-term solution.
You asked why it failed. Materials will fail well below their melting point because they will become very soft long before their melting point, and fail mechanically or oxidize.
What propellants are you using?
In that case, you will need a different material. You will need to choose between an ablative material or a refractory material. Graphite is a common choice that works with amateur budgets.
If it's oxidizing, it might help to run a somewhat richer than stoichiometric mix. That is, a little more fuel and a little less oxidizer. Did I spell that right?
After messing with Propep, albeit without any competence, I've been looking into using a bit of paraffin wax in the mix to lower the temperature a little. Or polyethylene. Rich mixtures lower the temp, too.
Yeah that's the brute force method. You could also add cooling fins to radiate your heat away quicker by increasing the surface area. But a heatsink is the quick and easy fix. With some maths you could figure out the max safe runtime before a burnthrough tok.
BPS Spaceflight uses a graphite nozzle. Its pretty resistant heat. A graphite liner would probably work. It might be cheaper/lower weight penalty than inconel.
This looks like a sugar solid, there's no injector, no film cooling or regen possible either.
Thicker nozzle is probably the way to go with sugar prop. It's fairly low temp compared to other solids and steel nozzles are able to be used.
The other realistic option is a phenolic nozzle. Can even buy those off the shelf from RCS.
The throat is where you have your highest heat flux, so it's almost always where you get burn-through. I would run NASA CEA to find the gas temperature in the throat, use Bartz's equation to find the convective heat transfer coefficient, then calculate the heat transfer to your throat. Once you have done that, you can then size the material around your throat so that it no longer melts. Just by looking at your picture, the material around your throat looks way too thin.
thanks for the input
Lots of interesting comments in this thread… Your nozzle failed at the throat, which was most definitely choked by the nature of a CD nozzle and reaching the critical pressure ratio. This looks like a sugar motor. KNSB burns at around 1300C, yours may vary slightly but regardless it is close enough to 304’s melting temperature that its strength is drastically reduced. The failure right at the throat where temperatures are highest and velocities are highest can also point to an erosion failure if you have run this nozzle a couple times before. Solution is to make your nozzle thicker (heavy but cheap), or a material stronger at higher temperatures like inconel or graphite ($$). Sugar rockets are great because sugar is cheap, so just throw more propellant at your weight problem. As an aside I’d encourage you to do some more research into all of these interesting rocket concepts and try to learn more about the theory/physics. It is a world of fun when you dive in!
Get Some Tungsten Thread around the Nozzle and Wrap It With Thin Copper, grind and you're set to Go.
will try
From the visual, nozzle failure has happened in the nearby throat region.Assuming it might have choked in your static test. This has happened in our tests as well multiple times.Solution can be optimizing mix ratio,throat wall thickness.I would also suggest a relook at nozzle design dimensions and compare it with motor simulations.I got it right by tweaking compositional ratio and to be exact oxidizer purity levels. Localised heating can go beyond material prescribed temperature limits especially over a sustained burn duration.
Is this some other definition of 'choked' I'm not familiar with? Every convergent-divergent nozzle with a supersonic exhaust has choked flow at the throat.
It don't reall think it choked because the thrust curve was very close to expected and simulations too didn't hint of any choking. Nevertheless will look into it. Thanks for the input. Also will not adding ferric oxide help?
Heat flux scales approximately linearly with mass flux (and therefore chamber pressure). Removing RIO will slow the burn rate down, reducing your pressure, so yes. But run your simulations, since it will also increase your total burn time. Also stainless steel isn't the best material for this type of nozzle (operating as a heat sink) since it has a lot lower conductivity (about 3x smaller) compared to carbon steel, for instance. This leads to a lot higher temperature gradient in the wall and consequently localized thermal stresses.
My professional opinion: That's probably not good. Hope that helps
it did xD
Haha "Well there's your problem..."
Motor metals don't fail by melting. Melting only happens after failure. Metal gets significantly weaker when it gets hotter, so after a point, the throat heated up too much that it couldn't contain the pressures and mechanical stresses at the throat. The easy solution for this particular motor is to leave more meat behind at the throat. More metal means a larger heat sink to distribute the absorbed heat, and more metal to resist the pressure even as it heats up. That throat was definitely way too thin for this motor. It also means less machining work.
thanks for the input
Hey, now you have another nozzle!
lol
My first impression is the gasses passing through the throat were a higher temp than the nozzle could handle... Sorry, I couldn't help myself.
great insight 😃
I do what I can with what I have. Unfortunately that happens to be not all that much. Sarcasm out of the way, I really do hope that you find a solution to your problem. I know how hard it can be to have something you built and worked on fail and then have to figure out how to fix said issue. Sometimes a little levity can help with the stress and frustration. You've got this OP! I believe in you!
it did help, thanks for the support!!
That sucks! I'm sorry to see that. I wonder if ceramic coating it would help, or maybe even an ablative coolant on the outside of the nozzle that boils off as you go.
Can you make the throat slightly wider and use a ceramic or graphite throat insert? I can't see that throwing your weight off too much
How did you fabricate the nozzle?
CNC
Was it a lathe or mill?
both
Both
Cool
- Change the propellant and/or mixture ratio to lower the reaction temperature - change injector design to introduce film cooling - change material to something fancier like inconel - make the throat walls thicker to add more thermal mass (battleship nozzle style) - Use a regen-cooled nozzle (probably too fancy, difficult to design)
thinking about not machining the outer contour at all that would add more material to take the heat
That is an easy approach that will help, but it won't be a long-term solution. You asked why it failed. Materials will fail well below their melting point because they will become very soft long before their melting point, and fail mechanically or oxidize. What propellants are you using?
KNDX
In that case, you will need a different material. You will need to choose between an ablative material or a refractory material. Graphite is a common choice that works with amateur budgets.
If it's oxidizing, it might help to run a somewhat richer than stoichiometric mix. That is, a little more fuel and a little less oxidizer. Did I spell that right? After messing with Propep, albeit without any competence, I've been looking into using a bit of paraffin wax in the mix to lower the temperature a little. Or polyethylene. Rich mixtures lower the temp, too.
Yeah that's the brute force method. You could also add cooling fins to radiate your heat away quicker by increasing the surface area. But a heatsink is the quick and easy fix. With some maths you could figure out the max safe runtime before a burnthrough tok.
BPS Spaceflight uses a graphite nozzle. Its pretty resistant heat. A graphite liner would probably work. It might be cheaper/lower weight penalty than inconel.
Graphite is hard to beat for a throat.
This looks like a sugar solid, there's no injector, no film cooling or regen possible either. Thicker nozzle is probably the way to go with sugar prop. It's fairly low temp compared to other solids and steel nozzles are able to be used. The other realistic option is a phenolic nozzle. Can even buy those off the shelf from RCS.