Thermal insulation/ablator for combustion chamber

[stix]

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.

 

Ah yes - now I see... (I think). Then why are you concerned about the paper/card tube messing up the nozzle grout/cement?

 

I can't see the issue - If you are doing it this way: (image attached)

 

I guess you are doing something different. Can you please explain a bit more about your technique and grain manufacture.

 

btw. How do we insert a larger image into the post instead of these pathetic thumbnails?

[Oinikis]

Your schematic is just what I do (well, except that insulation doesn't go all the way, I leave some bare PVC and make grooves with a soldering iron for the grout to bite in). You see, if I the grout is in conctact with the paper, it does not harden correctly, I think because paper might be absorbing and evaporating water. Now I have tried putting aluminium foil on top and it works, I just had a misfire today (E-match failed me, but I have the solution), and due to the small nozzle I was unable to fire it externally.

 

Another point. In these tests the Kn was 250, and that is chamber pressure of about 40 Bar, bit too much, eh?, so right now I have construction underway and it will have Kn of about 120, or chamber pressure of about 16-18 Bar. Really, what was I thinking with making the nozzle so small. Anyway, we'll see after the test.

Edited August 27, 2015 by Oinikis

[FrankRizzo]

The convergent nozzle (cone shape) that Dag suggested will help immensely. The gasses are currently hitting the flat top of your cast nozzle becoming turbulent, and transferring a lot of heat to a very small section of area. With a convergent nozzle, the gasses flow out of the motor much more efficiently.

 

Also, you might try dipping your propellant grain in molten wax as an inhibitor, then rolling paper around the grain as an ablative outer liner.

 

If you have access to a hydraulic press, the nozzle would also do well to be pressed into place (bulging the case and locking it in).

[Oinikis]

Franky, first off, nozzle shape came up in this topic several times, and I know that it would help, and I made point why I want to get it working with a flush nozzle, so I can can calculate chamber pressure from thrust, because it has nozzle coefficient of 1. And no, gasses just bounce off, they do not become turbulent, because turbulence doesn't happen at high Mach and high Reynolds numbers. Gasses acts more like sand, or individual balls, rather than a viscous substance.

 

Wax might be an idea worth considering, but it was also mentioned that I can't roll my ablator around the grain, because tips of it have to be inhibited inside the nozzle/bulkhead, which are made out of concrete which brings me to the third point

 

I just make a mold and pour grout inside, how come can I use hydraulic press to bulge the tube? It is liquid-ish, not bentonite clay.

 

 

So I have just done a test. The tube split longitudinally. And I find this very weird, I even lowered the chamber pressure. Only think that comes to mind is that there was already a split, because it wasn't violent. And it wasn't burned though, it was a split, everything else held finely. Man these pipes really do not want to become rockets. I have no idea what to do. If it was just a split, then it wasn't my mistake, but I can't have that kind of inconsistency. I think i'll continue testing.

[stix]

On the positive side the inhibitor held and no burn through at the nozzle. As you pointed out it's not good to have unreliable casings.

 

These tube are reasonably brittle, so I'm thinking that under pressure the tube slightly bulged out in the middle and therefore a small split occurred and then continued along the length.

 

These tubes are extruded, so perhaps that has something to do with it, like some sort of grain like paper? dunno, just a thought. If that was the case, maybe you could roll and glue some card around the middle like a tight belt, therefore delaying the initial bulge.

 

If the tube was already cracked or weakened then what are you going to do? - thoroughly test each casing tube beforehand... How?

 

You said that you were using a Kn of around 250 and dropped back to around 120. That's still too high for these tubes imo. Perhaps the initial pressure split it therefore you lost pressure thereafter. Approx. how long did it burn for?

[Oinikis]

To clear thing up, i'm trying end burners with ridiculously small nozzles to have the Kn, so it is constant. I meant during earlier tests Kn was 250, while now I changed design a little so during this test it was 120. the grain is 2.5 cm long, so under working pressure i'd say burn time is about 2-3 seconds. And you see, even when I had the high Kn, casings held for a while, and burned though only after the pressure had built up, and didn't break even with a hole, so I think there might have been a defect, or something.

 

As one time you said, maybe I should step back. I just checked on the internet, and the same hardware shop I bought these pipe from, should have 40mm dia tubes with wall thickness of 2mm, and one meter of it costs around 4 dollars. 1 meter would make at least 4 really powerfull boosters, or 13 small motors for research like getting fuel performance data (Kn vs. pressure vs burnrate) or various nozzle coefficients. If that tube will solve my problems, i'm down for that, so now I think I'll try that. Plus after i'm done, I can extract the tube from the motor and sell as a scrap, getting some money back, and helping the planet. And I don't feel bad about that unused pipe, because it will find it's uses, like a light weigh casing for electronics and science to put on rockets in the future.

Edited August 28, 2015 by Oinikis

[stix]

End burners are always going to be problematic in larger motors. As I'm sure you are aware, the burn profile results in a larger combustion chamber at the nozzle end, therefore more casing being exposed to flame over time, which leaves more of the casing vulnerable to heat, pressure and then failure. I wonder if it would be better to use smaller tubes like 3/4" ID (19mm) then extrapolate that data for larger core burner models.

 

Anyway, although it can be very frustrating a times, testing is always lot of fun.

 

Good luck with the new tubes, We await your results from your latest experiment.

[Oinikis]

Alright, so now I tried aluminium tube. What's the obvious way to lock it in? holes, but guess what, the groat refuses to harden in them, maybe due to adhesive of the tape. For PPH tubes I just made grooves with soldering iron and they worked like a champ, but now I can't do that on the aluminium. So, I need to make some big bumps/grooves inside for the groat to bite in, or something like retainer ring. First though is to take an angle grinder, and make some cuts, forming flaps, and then bending them inwards 180 degrees, but I don't know. Do you guys have any ideas?

[stix]

I use pressed waxed fireclay and drill 4 small 1.5mm holes roughly evenly spaced around the circumference.

 

I did this because a nozzle blew out once - not due to over pressure, but because I didn't ram it well enough. I also rough up the inside at the nozzle area with heavy glass paper. With the aluminium tube I use, I beleive that the nozzle will still blow first before the tube - therefore a safety release - sort of.

 

However, of course as usual you would want to make sure it's not pointing at anything especially if the tests are done horizontally. A heavy nozzle flying through the air at speed would be lethal.

 

Be Safe. Cheers.

Edited August 31, 2015 by stix

[FrankRizzo]

Franky, first off, nozzle shape came up in this topic several times, and I know that it would help, and I made point why I want to get it working with a flush nozzle, so I can can calculate chamber pressure from thrust, because it has nozzle coefficient of 1. And no, gasses just bounce off, they do not become turbulent, because turbulence doesn't happen at high Mach and high Reynolds numbers. Gasses acts more like sand, or individual balls, rather than a viscous substance.

 

You're not getting high Mach inside your tube; the flow velocity doesn't increase to that level until the nozzle throat. You're also not producing a pure gas stream, so any flat surface that the combustion products are flowing against, is going to produce a turbulent flow. High turbulence at the nozzle/tube interface is going to increase heat conduction and local weakening of the tube. Anyone whose made an endburner rocket motor with crappy tooling (and paper tubes) will tell you about the burn-thru problem in that area. Adding even a simple 45-degree angle will minimize that.

Edited August 31, 2015 by FrankRizzo

[FrankRizzo]

Alright, so now I tried aluminium tube. What's the obvious way to lock it in? holes, but guess what, the groat refuses to harden in them, maybe due to adhesive of the tape. For PPH tubes I just made grooves with soldering iron and they worked like a champ, but now I can't do that on the aluminium. So, I need to make some big bumps/grooves inside for the groat to bite in, or something like retainer ring. First though is to take an angle grinder, and make some cuts, forming flaps, and then bending them inwards 180 degrees, but I don't know. Do you guys have any ideas?

 

Use a pipe tap to form an internal thread where the nozzle is being cast. Alternatively, cut a groove and use a snap ring to retain the nozzle.

[Oinikis]

Well, I think the eating problem is simpler than that. Gasses bounce off at all directions evenly and hits the tube, while having a 45 degree part more or less "flushes" the gas in to the center, but now with aluminium tube that shouldn't be a problem.

I tought about threading, but I don't have many tools; my main tool is an angle grinder.

[stix]

Oinikis, don't let yourself become complacent when using aluminium tubes, it can easily lull you into a false sense of security.

 

I don't like the fact that I use Al tubes. I would prefer cardboard, but I have to roll them myself - rather spend that time on other things. However, I do have many years of experience making rocket motors, so in my mind, it's an educated and calculated risk that I take. I've had my fair share of cato's and hopefully that is in the past, but there is always that present reminder.

 

If you want to measure pressure, then there are other methods that are much more controlled - and safer.

 

Take it easy. Cheers.

Edited August 31, 2015 by stix

[Oinikis]

I did not get the tought that the motor is a potential pipe bomb out of my mind, I still treat it the same, as a potential bomb. But this time I think nozzle would give up first, and at the worst would take a piece of tube with it. As for retaining the nozzle/bulkhead, I put 4 rivets where the groat will be poured in. The rivets sit really firm, and knowing the hardness of the concrete I use, It should do it, however I can always add more, like for example doing higher pressure test.

 

As for the pressure measurement, I think the method of using an end burner and then calculating the pressure from the thrust should do it, because it burns at constant rate for quite some time.

 

As for the safety, I'll put a temporary brick wall on one side of the stand, so it will stop any shrapnel in case of the worst, plus, I won't be near it when it's hot.

[stix]

The safety warning was more directed at others reading this post that may be new to making rocket motors - and it's always good to remind ourselves and others about the possible dangers. I think Al tubing is more safe that using pvc. To be honest, I can't even remember exactly what happens to the tube after cato, I know it doesn't shatter, but may split and send a large piece in your direction.

 

So back to your motor. Are you still going to use an insulator? - also, what's the ID and how thick are the walls?

[Oinikis]

the tube is 36 mm ID 40 mm OD, so wall thickness is 2 mm. I'll try without insulation, because even at 500 degrees centigrade the tube is strong enough to withstand working pressure with ease.

[stix]

the tube is 36 mm ID 40 mm OD, so wall thickness is 2 mm. I'll try without insulation, because even at 500 degrees centigrade the tube is strong enough to withstand working pressure with ease.

 

That's very similar to the tube I was using for my "beta series" of motors. 36.7mmID, 40mmOD leaving a wall thickness of approx. 1.65mm. There are also various strengths of Al tube

 

https://www.onlinemetals.com/merchant.cfm?id=947&step=2&top_cat=60

Aluminum Round Tubes are available in alloys 6061, 6063 and 2024. Aluminum 6061 is the most widely used alloy, offering better corrosion resistance and weldability than other Aluminum Round Tube, but less strength. Alloy 6063 offers high corrosion resistance and is commonly used for outdoor structural applications such as Aluminum Tube railings and trims. Aluminum Round Tube 2024 is high-strength and commonly used in aerospace applications.

 

My motors don't have the flame exposed to the wall for the reason that it may soften under heat and pressure, then cause a blow through. But I've never tested it that way, so I don't have any actual experience.

 

One thing I do know is that the Al tubing I buy now is softer than about 3yrs ago - possibly a better thing than brittle.

 

Looking forward to seeing your results.

 

Stay safe. Cheers.

[stix]

Oink's, are you still alive?.... ........ gulp!!

[Oinikis]

Yes, i'm alive, or undergoing basic bodily functions at least. There is one, and hopefully the last problem. The grout. Upon hardening it doesn't really seal, I sand it, but still, it refuses to harden at the very edge. I remember using the same grout in aluminium mold (thus, it wasn't reacting, althou it was from soda can, and I think they are lined with something) and it hardened very well, it was like glass, it reflected light, so maybe i'm adding too much water now, and the excess is evaporating , creating gaps and stuff, so i'll try less water, because I noticed it slightly decreases in volume, so water is escaping, and if there is perfect amount of water, it should all remain in, forming various crystalline complexes and hydrates. Well, we'll see.

 

Man all over the internet I see you guys in The West have some special stuff, which expands upon hardening, man that would help me.

[stix]

. . . Man all over the internet I see you guys in The West have some special stuff, which expands upon hardening, man that would help me.

 

Which West is that ?!?! Never heard of it, sounds promising though. - Good to hear you are alive.

 

The nozzle is a big part of the motor - as I'm sure you are well aware of, and even more made apparent with your current issues.

 

I don't really have any more advice except to say that you need to change the way you are making your nozzles. With bigger motors it becomes harder to make pressable nozzles especially using waxed clay - doable though, because I've done it with similar sizes that you are doing. You need good tooling and a good press to get good results - it works.

 

However, I did a lot of experimenting about 3yrs ago when I was ill (but never ill enough for pyro) I had about 6mths off work, almost killed me financially but during that time I came up with a nozzle mix that can be formed without requiring large pressures, set in a conventional oven, then post inserted (or cast in place) into the casing tube, and has great refractory qualities. The ingredients aren't anything special.

 

Firstly I would abandon the grout idea. Surely you can get some kitty litter (add melted wax if needed) and pound the shit out of it into the casing and then drill a hole. Maybe you are just being stubborn and want to stick with your idea. Fair enough, I get it, but sometimes the smart thing to do is recognize that a particular method or idea doesn't work, then change it.

 

Are you trying to achieve making a castable nozzle, or making a workable consistent sugar rocket motor? Think about it.

 

Cheers.

Edited September 8, 2015 by stix

[Oinikis]

I'm trying to make a castable nozzle, because for the serious rockets you need de laval nozzle with a washer, and making it with grout is almost the only way, plus if I get it working it will be consistent. Also don't forget that you could ram the nozzle, but what will you do with the bulkhead? These motors aren't basic sugar motors for tiny headers anymore, these are more like high power booster for sounding rockets, well, maybe not exactly sounding rockets but still.

Edited September 8, 2015 by Oinikis

[stix]

Been there - done that. The size you are making is my "beta" series. It works very well for me.

 

The "castable nozzle" is always the big one, the ultimate idea - let me know when you get it sorted. If you can't work out what to do with the bulkhead (how to secure it without blow-though) then you should consider what it is that you are doing, or wanting to achieve.

 

Sorry if that sounds a bit terse, but it's late here, and I'm done.

 

Cheers.

Edited September 8, 2015 by stix

[Oinikis]

So, I've googled a bit, and turns out my local construction stuff store has expanding concrete, so I went and bought it. It is Portland cement, expanding kind, with chromium, further googling showed that it is quite common, and everyone could find it. There is even writings on it stating that is is used for sealing stuff. Another fun fact, once you mix it, you have 4 minutes. For those 4 minutes it is runny and nice, but after 5 it's like thick clay, and after 30 mins it's solid, and warm. The data sheet states, that after 6 hours it handles 15 MPa crushing pressure, after 24 h 25 MPa and after 28 days 40 MPa, so leaving it overnight is enough, so now the exiting part.

 

The test wasn't a full success, but a breakthrough nevertheless. At first it left me confused, (first 2 pics), especially the bulkhead, how did that happened without leaking? because even thou they shifted a bit, it didn't leak, which is very good. biopsy showed, that rivets turned a little. the expanding bit of the rivet is more or less is of spherical shape, so it acted like a ball socket, but since they aren't perfect, bumps have an enormous leverage against the socket, so that's how it cracked it.

 

So now that's the final problem, and I think it can be solved by using longer rivets, so there isn't as much leverage against the concrete it sits in, and they shouldn't turn.

 

Also, it had quite some erosion, because it isn't designed for handling heat, but that's what washers are for, plus I could mix in a small amount of the other grout I used, which is designed for fireplaces, plus i'll get some extra working time, but I don't think that's really necessary.

 

It feels like i'm close.

 

>>pic related

 

http://i.imgur.com/wVKaXVU.jpghttp://i.imgur.com/mQZbHIN.jpghttp://i.imgur.com/4bwM6cR.jpg

[stix]

Looking very promising.

 

I've heard of others using washers to stop erosion, so that sounds like a workable solution. I've never heard of expanding concrete, it sounds very interesting.

 

I'm just wondering if the reason for the crack may have been heat related, ie. the whole motor including the rivets heated up very quick - therefore expanding the rivet ball and crack! just a thought.

 

Instead of rivets, the other method (that I use) is drilling holes which allows the nozzle material to force through the hole and therefore becomes it's own rivet. Also it's important to ensure that the inside of the tube where the nozzle and bulkhead are, is roughened then cleaned with denatured alcohol.

 

How long did this test burn for?

Edited September 13, 2015 by stix

[Oinikis]

Alright. It definitely was rivets twisting, because if you look at their heads, you can clearly see they are twisted. Ok maybe not so clearly, but yeah.

 

The method you described isn't really the most reliable, because you could easilly make a mistake, plus if a small amount of gasses do leak, the first place they are coming out is the hole.

 

The inside of the tube was roughened up with 60 grit sandpaper, and it was done at a right angle to the tube, so if a tube is standing, the grooves are horizontal.

 

The burn time was about 1-2 seconds, it is an endburner with grain lenght of 2.5 cm.

Edited September 13, 2015 by Oinikis