3d Printed Pulsejet Project

[RoadRunnerAerospace]

So I was thinking (in the middle of german class), somehow I got from how much I wanted out of there to pulsejets. From there I started to go through ideas of possibly making my own pulsejet, eventually leading me to the question: can I 3d print a functional, and integratable pulsejet for R/C models, etc? The short answer is sort of, yes. 
The biggest problem with the entire concept is cooling it. I'm saying that a 5 minute run time with throttle to the firewall, preferably 10 at 3/4. SO, after thats out of the way, lets get "technical".

PROBLEM 1!

• Cooling

- Pulsejets get hot, like, really hot, so I need to figure out how to insulate the plastic from the extreme temperatures from the combustion

PROBLEM 2!

• Pulsejets are weird, they "pulse" the fuel flow to allow the suction/blow of a normal running pulsejet, I want to keep the jet simple, I.E. no computertrons beep-bopping around with timers, mainly because I cant code. 

Current progress is stuck at how to insulate the 3d-printed structure from the heat, however I'm formulating an idea, and I'm not great at cad.

 

 

 

[ment18]

Its probably easier to connect metal piping together than trying to use plastic, though I'm not familiar with how well shaped pulsejets need to be. 

How much in the way of tools do you have access to?

Edited October 1, 2018 by ment18

[RoadRunnerAerospace]

1 hour ago, ment18 said:

Its probably easier to connect metal piping together than trying to use plastic, though I'm not familiar with how well shaped pulsejets need to be. 

How much in the way of tools do you have access to?

Hand tools, drill, hammer, and the likes. I can get to a 3d printer at school.

The plastic would be easier to manufacture labor-wise, but it would be a little more complex to insulate from the heat

Edited October 1, 2018 by RoadRunnerAerospace

[ment18]

I think the temperatures involved mean the plastic is useless, easily liquified far before you get any performance.  It needs to be easy to melt to print, unless you have a special printer. I think joining metal components is really the only option.  Valves or no?

[RoadRunnerAerospace]

37 minutes ago, ment18 said:

I think the temperatures involved mean the plastic is useless, easily liquified far before you get any performance.  It needs to be easy to melt to print, unless you have a special printer. I think joining metal components is really the only option.  Valves or no?

Valveless, I think I can tweak the mixture to reduce temp

[Gargamel]

You do understand how most 3d printers work right?  They heat plastic filament till it almost starts to melt.   Often just slightly hotter than boiling point, sometimes more.  

Pulse jets build up a LOT of heat.  You're going to need some pretty serious insulation material to keep the plastic from melting.

And then there's getting the material inside.  I don't know if your going for a straight or U shaped pulse jet, but either way, it's going to be pretty difficult get a perfectly even coating of whatever this material is, inside a pre-printed tube.  Especially if it's curved.    

Your best bet is to just get a Steel or Ti pipe.    A thick wall Aluminum pipe would probably work well too, as long as you kept the duty cycles low.   

Unless you're going with a sintered metal printer, there are some thing's that 3-d printer excel at, making combustion chambers is not one of them.  I have my own printer and use it all the time, and this is not a project I would be using it for.  

I'm a machinist, woodworker, welder, and maker in general.  So I have a lot of tools on my skill set I can tap for a project like this.  So my point of view might be slightly different than yours.   But with that in mind, what's your budget for this project? 

 

Edited October 2, 2018 by Gargamel

[ment18]

http://www.pulse-jets.com/phpbb3/viewtopic.php?t=2701 -- Pulse Jet Tube Temperature >800C

https://www.lifewire.com/specs-on-3d-printing-materials-2232 -- ABS Melting Point: 240C

Its gonna require a massive performance hit to make up for that disparity, physics of engine might not even allow it because the air flow rate it based on the power of the engine.

 

[RoadRunnerAerospace]

21 minutes ago, Gargamel said:

*snip*

I'm not saying its easy, but if I can get the temp within 300 degrees of the melting temp of the extrusion material, I can use gasket sealant used in cars to cover the inside of the pulse jet, maybe more if I can find a better insulator, I'm keeping flight times low for this exact reason, eventually it will melt, I just want to make it usable before it does

[ment18]

45 minutes ago, RoadRunnerAerospace said:

I'm not saying its easy, but if I can get the temp within 300 degrees of the melting temp of the extrusion material, I can use gasket sealant used in cars to cover the inside of the pulse jet, maybe more if I can find a better insulator, I'm keeping flight times low for this exact reason, eventually it will melt, I just want to make it usable before it does

does that require a metal surface and no air to seal?

[RoadRunnerAerospace]

12 minutes ago, ment18 said:

does that require a metal surface and no air to seal?

nah I've used it on model rocket engines

:edit: I've started work on the calculations 

Edited October 2, 2018 by RoadRunnerAerospace

[MaverickSawyer]

Y'know, just for grins and giggles, I'm curious to see how well it turns out. I highly doubt it will work, for reasons explained above, but if nothing else, it'll be an... educational experience.

[RoadRunnerAerospace]

16 minutes ago, MaverickSawyer said:

Y'know, just for grins and giggles, I'm curious to see how well it turns out. I highly doubt it will work, for reasons explained above, but if nothing else, it'll be an... educational experience.

if anything it'll be quite fun

[RoadRunnerAerospace]

OK so if someone can "sanity" check me that would be great.

The thrust caused by a pulse jet is of course, the pressure acting against the walls, etc. So if I were to make it small, and I mean like, 5 inches long, could I possibly reduce the required pressure for combustion/thrust, and therefore heat? I know it works that way with rocket engines (in a perfect world), and pulsejets use a variant of the DeLaval nozzle. 

As of current i'm looking at a safe run time of 1 minute at 124°c/minute, so at 2 minutes I'll be exceeding maximum operational temperature of the plastic, through the insulating epoxy (Quiksteel), and melt the extrusion material. I'm wondering if during the mixing process I can put some insulator in with the quicksteel to decrease its thermal conductiveness.

[Gargamel]

11 minutes ago, RoadRunnerAerospace said:

As of current i'm looking at a safe run time of 1 minute at 124°c/minute, so at 2 minutes I'll be exceeding maximum operational temperature of the plastic, through the insulating epoxy (Quiksteel), and melt the extrusion material. I'm wondering if during the mixing process I can put some insulator in with the quicksteel to decrease its thermal conductiveness.

Alternate Idea.   Use the 3d printed part to act as mold for the epoxy.  I'm not familiar with that particular epoxy, but a lot of them can be thinned with Denatured Alcohol to make them more viscous (less viscous?  Runnier).    Print a 'negative' mold of the engine you want, pour in the epoxy, add steel wire as mini re-bar if needed.    Then you can burn out the plastic, or use a water soluble filament and just dissolve the mold.    This will give you the material you need, in the exact shape you want.    I doubt you can make a breakable mold while keeping the ID's as you want, so burnable/dissolvable may be your best route.      This will give the exact wall thicknesses and shape you desire at each location.     Still uses 3d printing and your epoxy. 

Since the plastic was just a form for the epoxy anyways, this will save you mass and size, and extend your run time. 

[RoadRunnerAerospace]

10 minutes ago, Gargamel said:

Alternate Idea.   Use the 3d printed part to act as mold for the epoxy.  I'm not familiar with that particular epoxy, but a lot of them can be thinned with Denatured Alcohol to make them more viscous (less viscous?  Runnier).    Print a 'negative' mold of the engine you want, pour in the epoxy, add steel wire as mini re-bar if needed.    Then you can burn out the plastic, or use a water soluble filament and just dissolve the mold.    This will give you the material you need, in the exact shape you want.    I doubt you can make a breakable mold while keeping the ID's as you want, so burnable/dissolvable may be your best route.      This will give the exact wall thicknesses and shape you desire at each location.     Still uses 3d printing and your epoxy. 

Since the plastic was just a form for the epoxy anyways, this will save you mass and size, and extend your run time. 

That is a very good idea, it's sort of like carbon fiber if you really squint

[Nuke]

dont use pla. you cant even sand that stuff without it melting. 

as for how to keep it cool idk. most pulse jets tend to glow red hot while operating, and i dont think there is a plastic that can handle that. there are filaments that contain metal particles or carbon fiber but i dont think that would help much as the plastic that makes up the bulk of the material will melt/vaporize. if you have access to a forge you could print positive molds, encase them in casting sand and fill it in with whatever liquefied metal you have on hand. i also heard there are powder printers that can print in something that you can stick in an oven to give you a metal part (hp's new 3d printer). going somewhat exotic perhaps you can print your parts with channels for liquid nitrogen cooling, but then you end up with embrittlement issues. you might be better off buiding it out of sheet metal and pop rivets. 

 

Edited October 2, 2018 by Nuke

[YNM]

20 hours ago, RoadRunnerAerospace said:

how to insulate the 3d-printed structure from the heat

Please don't make a fireplace from plastics.

[MaverickSawyer]

1 hour ago, Nuke said:

dont use pla. you cant even sand that stuff without it melting. 

as for how to keep it cool idk. most pulse jets tend to glow red hot while operating, and i dont think there is a plastic that can handle that. there are filaments that contain metal particles or carbon fiber but i dont think that would help much as the plastic that makes up the bulk of the material will melt/vaporize. if you have access to a forge you could print positive molds, encase them in casting sand and fill it in with whatever liquefied metal you have on hand. i also heard there are powder printers that can print in something that you can stick in an oven to give you a metal part (hp's new 3d printer). going somewhat exotic perhaps you can print your parts with channels for liquid nitrogen cooling, but then you end up with embrittlement issues. you might be better off buiding it out of sheet metal and pop rivets. 

 

Hmmm... 3D print a wax positive and do investment casting?

[RoadRunnerAerospace]

I think I'm rolling with the epoxy mold idea

(I did a couple calculations off paper too if you're wondering where I got those numbers

[Nuke]

2 hours ago, MaverickSawyer said:

Hmmm... 3D print a wax positive and do investment casting?

yes but ive seen it done with pla positive molds. the plastic burns away because of the hot metal and outgasses (there are flames involved so be ready for that) leaving behind the desired part.

Edited October 2, 2018 by Nuke

[MaverickSawyer]

Hmmm... Might want to be careful with that on something like this. It might be sensitive to the quality of the casting, which, if not done very carefully, the burnout method could damage the mold or leave voids in the final part... That's why I'm kind of curious about using a wax, which, with the right mold medium, you could bake it and the wax would melt and run out.

[RoadRunnerAerospace]

1 hour ago, MaverickSawyer said:

Hmmm... Might want to be careful with that on something like this. It might be sensitive to the quality of the casting, which, if not done very carefully, the burnout method could damage the mold or leave voids in the final part... That's why I'm kind of curious about using a wax, which, with the right mold medium, you could bake it and the wax would melt and run out.

Yeah was is looking like the best idea, baking would also harden the epoxy

[Gargamel]

My main issue with using wax for an epoxy mold is that epoxy is exothermic.    I don't know what tolerances you are building this engine too, but epoxy under normal conditions, will usually heat up enough to melt or at least deform wax.    And this will happen while the epoxy is still fluid enough to be affected by the wax deforming. 

A lost wax casting is usually a good method for casting, but if you have very tight tolerances, you might want to triple check the product before firing it up. 

[RoadRunnerAerospace]

25 minutes ago, Gargamel said:

My main issue with using wax for an epoxy mold is that epoxy is exothermic.    I don't know what tolerances you are building this engine too, but epoxy under normal conditions, will usually heat up enough to melt or at least deform wax.    And this will happen while the epoxy is still fluid enough to be affected by the wax deforming. 

A lost wax casting is usually a good method for casting, but if you have very tight tolerances, you might want to triple check the product before firing it up. 

I would have never thought that epoxy was exothermic, I'll have to look further into it

[Gargamel]

43 minutes ago, RoadRunnerAerospace said:

I would have never thought that epoxy was exothermic, I'll have to look further into it

It can get blisteringly hot.   I know of of podcaster for a well known woodworking podcast who was using cheap styrofoam cups to mix stuff in.  He mixed up some epoxy and used what he needed, and let the rest harden up in the cup, as you do for extra you have no use for.   The heat from the reaction actually melted the cup and caught it on fire.  Granted, there may have been some other chemical reactions occurring, but I know first hand that epoxy gets very hot when setting up.    I've done large pours of epoxy for Hulls on RC racing boats, and those suckers stay hot for a couple days (the 6-7 pounds of lead shot in them tends to hold in the heat too).   

It shouldn't get hot enough to deform PLA/ABS, so using a 3d printed mold should be ok.   Using plastic as a positive to create a lost wax mold would work, but I have two main issue with it.   If you have any larger sections in the design that taper down on both ends, getting wax mold out of the plastic one will be an issue.   And for similar reasons, it will be very hard to verify that the ID's of the final product are to spec. 

There's a bunch of tricks you can try, check the interwebs for a variety of them.