Thermal insulation/ablator for combustion chamber

[Oinikis]

So I'm going bigger, and rolling paper tubes for the casing is no longer and option, due to weight, time required and being inconsistent, plus complicated technique for putting in plugs while not shattering the grain. So, I have bought 40mm diameter PVC (or possibly something else) pipes, and before you judge me to death, I do realize that it is dangerous, and that it shaters into sharp shrapnel which are invisible to X-rays, and I treat it like a potential pipe bomb. So, the pipes are fairly sturdy, and according to my calculation they should handle pressures up to 50 bar, but i'm not going far past 20 bar.

 

 

Now the problem is, that if you just poured in concrete for nozzle, dropped the grain and poured concrete on top (obviously, wrap the grain in tape first), it will burn though the side, and turn into a burning nonsense. Obviously, insulation/ablator is needed. After some test I have had some working results. I made grain, 2 mm smaller in dia than ID, sealed by wrapping it in tape (put an e-match before), wrapped it in paper with wood glue (like a tube, with 1 cm excess at each end) and fold over the excess (forms a wall 1mm thick, and by keepin it wet it works as a great ablator), wrap it in more tape to seal that, and glue it with epoxy/concrete dust mix (essentially, it's more rigid and heat resistant) to the nozzle and pour concrete plug on top (In case you're wondering, before pouring it in, using soldering iron I make series of grooves, for concrete to bite in, works like a charm). First problem, it's a bit over complicated, but I could still do it if I make them in bulk, but the second, and most important one is that the folds are inconsistent, and sometimes it does work, other times there might be a gap between fold, which will result in a leak, which will lead to a hole in the casing and turn into a nonsense.

 

 

Now there has to be a better way. Now the top isn't a problem, concrete surround the grain perfectly, and it never fails, the problem is the bottom. So by knowing this, I need something which molds and leaves no gaps, because if everything is filled, gasses will have nowhere to go. First solution might be just putting in the smaller unisolated grain, and pouring concrete on top, but I've done some calculations, and it would be quite heavy, plus prone to fracture. I thought I might wrap it in soaked wet (with water/wood glue combo) paper towel, and put it in unisolated, so it fills all the gaps, but i'm not sure concrete will like it, and it might shrink, leaving gaps, due to mild drying. Then I could use some silicone, but then how would I make sure I leave no gaps, especially with longer grains. So do you have any ideas on how to solve the problem? I have no access to thicker walled, denser pipes, because Eastern Europe, so I need to work something out.

 

Here is a pic of cut open motor which held the pressure (max Kn~130, estimated max pressure 16 bar, BATES grain). The reason it's so short is that I don't want to waste stuff on developing techniques. Today I did another test, rocket was of similar size, except it had a much smaller nozzle and was an endburner (like a sustainer motor, with a bit higher Kn), and it burned though in one place, probably due to a gap in a fold.

 

Also, during one test I put way, way too much ignition powder, and blew the motors, it sorta shatered, but only into a few big pieces, without small shrapnel, which I found interesting. The plug stayed in place, it was the tube which got ripped, showing the amazingness of my casting method. Also yes the is gray, suggesting it might be something other than PVC, maybe HDPE.

 

http://i.imgur.com/reJM9Vr.jpg

Edited August 9, 2015 by Oinikis

[stix]

Good to see you back on the rockety forum Oinikis. You've obviously been busy doing lots of experiments. I'm finding it a bit difficult to work out your grain making method, there's possibly too much info - can you summarise.

 

Is this part correct - You are you pre-casting grains, then wrapping in an inhibitor and then inserting into the motor casing - the flame sometimes propagates between the grain outer and the casing?. I've used RTV Silicone which is a reasonable inhibitor but too expensive especially for larger motors (40mm ID +).

 

I agree that going to larger motors requires a review of the casing material. I use aluminium tube. Not something that I like to use, but I don't have a economical alternative. I make sure that I am well buffered during testing.

 

An alternate method is to cast into pre-made sleeves which are already inhibited. This could Include a central rod which can be extracted when cured. This makes the whole process easier - although you do have to invest some time and cost into making the tooling.

 

Cheers.

[Oinikis]

Okay, first of, I roll a thin tube, in which I cast the grain, if I'm making BATES, I put the tube on a base with a rod sticking out (trought the tube), and I put the fuel around the rod, inside the tube, and go around with a stick pressing it in, it is reliable, consistent and easy to do. So the grain has an outer inhibitor (or a sleeve) and it's not the problem.

 

The problem is that the pipe i'm using doesn't like being heated (it melts), so I need to protect it from the heat, thus I need insulation between the grain/combustion chamber and the pipe, thus all the wrapping.

 

Not sure if I answered clearly, but I need something to protect the pipe from the heat, not the grain.

 

If silicone works, i'm down for that, because it's only going to be lining, but it has to remain rigid and heat resistant, so the seal doesn't break.

 

Wait, lemme make a drawing.

EDIT: Here's the drawing, I need to protect the pipe from combustion

http://i.imgur.com/mC6hpzy.png

Edited August 10, 2015 by Oinikis

[stix]

Ok, meanwhile whilst you make your drawing I'll make some comments.

 

I'll assume by "pipe" you mean the motor casing tube. The nozzle end is the main area to be insulated. You could apply aluminium tape to the inside if the casing tube - it's not that expensive. That is the main area of concern.

 

PVC pipe will easily deform under heat and pressure compared to aluminium of the same thickness.

[stix]

Just saw your drawing - It's getting late here so I'll check it out tomorrow. The RTV Silcon that I've used is high temp but mainly used for car engines, Possibly not good enough but maybe worth a try. My current method doesn't need it because I don't have big issues with exposed areas.

[Oinikis]

Yes, I mentioned in original post that nozzle area is the main problem. I don't see how aluminium tape would work, since it's easy for it to burn though, plus, even if it doesn't, it's a heat conductor, and will melt the tube. I put the drawing in the first reply of mine.

 

Using full metal casing would be ideal, but I can't do that, yet.

I just had an idea, I could insulate the tube before casting the plugs (like inserting a tube inside the pipe/casing), that way the insulators ends will be inhibited by concrete, making it super reliable, because the problem is the transition between insulator and the nozzle. The question is still there, what should I use. Not sure whether I could use a paper tube, because i'm not sure concrete is going to like that, also, i'd like there to be no gap between insulation and the pipe, so maybe coating it with something, like silicone.

 

http://i.imgur.com/V4ZS8oM.jpg

[MrB]

Looks like your average PP plumbing pipe. (Polypropylene)

Not what you want to go ask them to pull out of your body as shrapnel, but you say your aware of the dangers, so i wont nag.

 

Oh yeah. It should have black text on the outside to identify it...

B!

[Oinikis]

Well, first of, this is sewage pipe. On the sides it is written in Polish, so I can't exactly understand everything, and it's mostly dimensions and address. What I understood though is that it's max working temp is 95C, which is higher than PVC, although no near 1700. Also there was PP-H written on it, and google tells me it's the stiffest form of PP. But it's strong and sturdy, and with insulation it can be used. I googled it's tensile strenght, and the pipe should hold up to 50 bars, but as I said, i'm not exceeding 30. That one time I put in way too much ignition powder I described, it blew up into 3 big chunks, 2 of them contained nozzle, and one of them was a big section from the middle, so it didn't shatter into thousands of deadly shrapnel, although it should still be treated with respect.

[stix]

. . . I just had an idea, I could insulate the tube before casting the plugs (like inserting a tube inside the pipe/casing), that way the insulators ends will be inhibited by concrete,

 

That idea seems good to me. Where you have shown the insulation in the drawing would be ideal.

 

Agreed about the aluminium tape - it would act as a conductor of heat and therefore completely the opposite of what you need. I keep getting confused with fire retardant and insulator. I've used aluminium tape to stop burn through of a grain - it worked.

 

What about this:

• Soak some thin card (similar to manilla folder, 200-300gsm) in a solution of borax.
• When dry, roll 5-6 layers? around a smaller tube.
• I don't think you will be able to glue it, but when inserted into the motor tube it should spring out.
• Position it as indicated in your drawing.
• Cast the nozzle.
• Insert the grain

Borax acts as a fire retardant and paper/cardboard is a good insulator.

 

A better option would be using Sodium Silicate Solution - It can be picked up from a pottery supplier. That way you can glue whilst making the inhibitor sleeve. Come to think of it, I don't see why this method wouldn't work using PVA or similar glue. I've used it with photocopy paper to inhibit the nozzle end of a grain - it didn't burn through, although this was a short burn time of around 2secs.

 

In any case the pvc tubing is not ideal, and I don't mean because it could shatter, but just for the simple reality that it will deform at a low temperature. But I hope you can get it working.

 

Sometimes it's best to cut your losses and take a step back and re-evaluate. I know it can be very hard to do, but in the long run it can lead to a much better result.

 

Have ever you tried to get some cardboard tubes from odd sources? I have some nice tubes that were cores for a roll of shrink-wrap film. These are around 50mm ID with a wall thickness of 5mm and 500mm long. Very strong and I reckon I could drive my car over it without deforming it. I'm hoping to use these one day for "the big one" but just to test on a thrust meter, because launching it would be a big big problem without proper authorisation (which would be unlikely).

 

Anyway, if you are really set on using your current tubes, then perhaps borax or sss could be worth trying?

 

Cheers.

Edited August 12, 2015 by stix

[Oinikis]

Well, I think it would be better to roll a tube of dry paper, and let it spring out, use a finger to further unroll and press it into the tube and then wet it.

 

 

The reason I want to there be no gap is that if there is a gap, the pressure might blew the insulation. If it is pressed against, the tube will hold the insulation.

 

You see, I can't get my hands on a lot of stuff, so I might try just plain diluted wood glue.

 

 

As for the paper tubes I can't get any, plus, I don't think you could cast concrete into them. Also paper tubes do not seem to be very consistent for me.

Edited August 12, 2015 by Oinikis

[deer]

I must apologize for not reading the whole thread, but from that picture of cross section I'd say you just need to get pipe with thicker walls.

I've used 25mm Polypropylene (PPE) water pipe and it has 20mm ID. so it's 20% walls. Mine are easy to bend after recovery but never completely plastic or too hot to touch.

 

Also, the motor probably burns too slow. Most issues should come from pressure (bulkhead / nozzle blow-outs , CATO etc). The grain just burns so fast it ejects everything and chars the insides just slightly, before the heat transfers to tube.

 

Another thing you might look into is opacifiers (e.g. carbon) - they don't let the whole grain to transmit heat, so just the very burning surface becomes hot. *Basically* rcandy to heat is like glass to light - the heat shines right trough it. Opacifier makes it like tinted glass to heat.

Edited August 13, 2015 by deer

[Oinikis]

Well, first of, I mentioned somewhere that I can't acquire other types of pipe, but that's not the point. You see, PPE is flexible, and is more of a hose than pipe. I use PPH, which is stiff, strong, and a bit more heat resistant. There are charts online, which show tensile strength and tensile modulus of various plastics.

 

And what do you mean it burns too slow? The pressure is up, and the grain burns at the correct speed. You see, the temperature of combustion is almost always the same, but when you have high pressure too, due to density, combustion gasses become extremely aggressive and much better heat conductors.

 

The pipe can hold up to 50 bar, while most of the rockets don't go past 20 bar, plus the plugs are fixed in so well, that the tube rather rip, than the plugs blow out.

 

Also, the grain is fine, it has no problems, the problem is protecting the pipe from hot, high density aggressive combustion gasses, and Stix and I have tought up of potential solution, which is in testing. I don't think you grasped the problem.

[deer]

I just noticed I’ve made a mistake - I use PPR cold water pipes.

 

As for the burn speed, if you're up to pressure it must be fine. Sometimes I've had cases where burn speed is so slow my coreburner becomes endburner near the nozzle and then the flame can reach walls during most of the burn time. If everything is as expected, grain should cover and protect walls so long until it has been almost completely consumed (except for the bulkhead / delay).

 

Anyway, keep up the good work. This is a common issue with plastic pipe engines and solid solution would be really welcome.

[chand]

Guys please help me out in this problem I am going in science fair this time but I don't know practical uses of model rocket however I know uses of rocket but I want to know what are practical uses of model rocket please help me out this is ny mail addres

 

chandsureja@Yahoo.Yahoo . com

Please help me

[deer]

You do realize that your question is completely off topic and would require a separate thread?

Edited August 17, 2015 by deer

[Oinikis]

So, I have tried the new design, and it failed. Now crosscut showed me that at the area where insulation meets the concrete, concrete is not correctly set. This could be fixed with applying tape on the insulation.

 

Another problem I think is that springed out paper roll with glue applied to it isn't rock hard, and it is possible that gasses push against it, making a gap. Now I can make thicker insulation, wet the roll with diluted glue, till it's all wet, and dry it, but it will take a lot of time, and i'm not sure how reliable it could be (air gaps), In other words, I'm not sure what to do. It did hold for a second, but not all the way.

 

Here's the pic. The half of top plug fell out during cutting. it revealed tiny channels between insulation and concrete, proving my theory. The only burn-though area is near the nozzle, also notice how much more material ablated near the nozzle. I need something which won't leave air gaps in the insulation, because air gaps are fatal.

 

http://i.imgur.com/xmPwcnd.jpg

[stix]

Agreed with rolling and gluing the inhibitor sleeve so there are no air gaps - wood glue/pva should be fine. I wouldn't dilute the glue too much. Also letting the concrete set for much longer to be sure. I know it's hard to be patient, but sometimes we just have to wait.

 

The area where the motor failed is always going to be the weakest and most prone to attack by hot gases because of it's sharp corner. If you were able to smooth it out (while wet) to a 90deg radius, I'm sure that would help - but it may not be possible with your motor making method.

 

The other possibility is that under pressure the tube has slightly flexed. Any slight flex of the tube will result in a slight separation of the nozzle from the wall, leaving a tiny gap for hot gasses to escape. Either way, that corner is very vulnerable.

 

The more I think about it, the more I think that you should be able to get something workable out of that tube. Although making longer ones might be a bit more of an issue with flex.

 

I'm interested in how you are assembling these motors and are they "true" bates grains? because it looks like in that last image, the grain is inhibited at the top (head). Knowing the method will help us understand the issues.

 

-

 

What about rolling and gluing the sleeve around the pre-cast grain, leaving an overlap on each side, then inserting into the tube?

 

Sort of like this:

 

------------------- Inhibitor Sleeve

-------------

| |

| GRAIN |

| |

-------------

-------------------

 

Sorry about the crap graphic, without the courier font it may be difficult to see it (and to think, I'm a graphic designer by trade)

 

If you pre-plan and rule up your inhibitor material, you should be able to get overlap on both sides of the grain and the correct amount of turns around the grain could give you a nice "snug" fit into the tube casing.

 

Cheers.

Edited August 19, 2015 by stix

[Oinikis]

First of, letting concrete to sit longer won't help, because it gets more viscous with time and after about 8 hours it heats up and hardens quickly, and sets fully within the next 4 hours or so, so if it didn't harden during that moment, it never will. It's an interesting kind of concrete. Althou I just ran out of it. Dad says I could use basic concrete he uses and has, but I don't think it's as good, and I have experience with the old one, plus it served me well, and it is actually designed for high temp applications, so I'll probably buy that, but I went off-topic.

 

Yes, I considered smoothing out the concrete near the nozzle, and it will be done with motors I'm going to use for stuff (basic de laval), and It will be easily done with molds. The reason I want to get it to work with flush nozzle is that it will guarantee me better performance, and I need a flush nozzle for determining the chamber pressure of a motor when it has a constant burn (flush nozzle has a nozzle coefficient of 1, and thrust = pressure x crossection area x coefficient). For that I will need an endburner which brings me to another point

 

The rocket this time was endburner, with much smaller nozzle, but with the same Kn, but I don't think insulation cares about it as long as the pressure and the temperature are the same.

 

 

Then, I can't roll insulation around the grain and then insert, because I need to insert a mold for the nozzle.

Edited August 20, 2015 by Oinikis

[dagabu]

Interesting concept, not a new one but I haven't seen it for a while. Ultimately, you need a sleeve for your casing with enough length to allow the nozzle/bulkhead grout to cover the sleeve on both ends.

 

Two items that will help a LOT! Put a convergence in the nozzle, the gasses are swirling and undercutting the casing, that is sugar rocket 101 and the easiest thing to change right away. Simply take a wooden dowel and shape the end to a blunt point, twist it a time or two to shape the convergence. The second is to add the sleeve to the casing before adding the fuel grain. One idea that I have seen work time and time again is a soda can aluminum and thick paper. Simple white glue will dry hard and allow the paper to insulate the aluminum so it cant act as a heat sink to the layer beneath it.

 

Make sure the grout (you may call it concrete over there but over here, concrete is a different animal and would not work for nozzles) covers both ends of the sleeve so that the gasses remain inside the chamber.

 

Good luck!

[chand]

guys I am in problem I don't have induction stove but I have gas one can I make rocket candy propellant I tried once and my propellant didn't burn on stove but it didn't delivered expect performance and my sugar didn't caramelised

[dagabu]

guys I am in problem I don't have induction stove but I have gas one can I make rocket candy propellant I tried once and my propellant didn't burn on stove but it didn't delivered expect performance and my sugar didn't caramelised

 

Chand, DO NOT make R-Candy inside your house!!! You need an electric skillet at the very least, an oil bath cooker would be even better.

[stix]

Interesting concept, not a new one but I haven't seen it for a while. Ultimately, you need a sleeve for your casing with enough length to allow the nozzle/bulkhead grout to cover the sleeve on both ends.

 

Two items that will help a LOT! Put a convergence in the nozzle, the gasses are swirling and undercutting the casing, that is sugar rocket 101 and the easiest thing to change right away. Simply take a wooden dowel and shape the end to a blunt point, twist it a time or two to shape the convergence. The second is to add the sleeve to the casing before adding the fuel grain. One idea that I have seen work time and time again is a soda can aluminum and thick paper. Simple white glue will dry hard and allow the paper to insulate the aluminum so it cant act as a heat sink to the layer beneath it.

 

Make sure the grout (you may call it concrete over there but over here, concrete is a different animal and would not work for nozzles) covers both ends of the sleeve so that the gasses remain inside the chamber.

 

Good luck!

 

Good one dags, I like your idea with the Al and thick paper. Which way though - ie. thick paper to the casing wall side first?

[Oinikis]

Alright. Dag, It might not have been clear, but we already considered putting in the sleeve beforehand, and then casting, so the ends of the sleeve are inhibited, like in one of my drawings. Then I also know the benefits of convergent part of the nozzle, and how to cast it, I've already done it before.

The reason I want it to get to work with a flush (basic) nozzle is that I then can calculate the chamber pressure from the thrust data, to make various fuel characteristics graphs (like Kn vs Pressure, burnrate, Isp, etc.). Also, If good enough insulation is used I don't think it's a problem.

 

Now the main problem I have. The concrete/grout is an interesting beast. It's designed to be a binder for high temp mixture for making fireplaces, and in it's pure form it works best against something non reactive and non absorptive like plastic. Now I have tried multiple times, and it doesn't harden well around the paper/glue sleeve, it just turns into dust, probably due to absorption. I could for example put some aluminium foil around it, so it won't bother the grout. But I think that is overcomplication, and not reliable, because there are so many ways air pocket could emerge and rek it.

 

I need something that could stand the heat, not mess with the grout and do not have any air pockets, and I have no Idea what to do. Al and paper sounds sweet, I could insulate the very ends with hotglue, to prevent it messing with my grout, but I'd need to then start drinking. A lot. But it gives more ideas, like having basic paper glue sleeve covered in something to prevent the grout problem. I think I should investigate this possibility.

 

And Stix, I think he meant having paper between the tube and the Al, so Al would be the lining, but not a heatsink.

Edited August 25, 2015 by Oinikis

[stix]

. . . And Stix, I think he meant having paper between the tube and the Al, so Al would be the lining, but not a heatsink.

 

Are you sure? I would think the other way around. ie. paper/cardboard in contact with the pvc tube as the heatsink/inhibitor and aluminum toward the flame front. It has heat reflecting properties as well.

 

Now I'm just confused.

[Oinikis]

Well, that was what I meant, aluminium on the flame front, and paper between the two, so aluminium doesn't act as a heatsink. Anyway, I need to think of something to coat that paper tube so it isn't messing up the groat.