A question about rocket stove mortar...

Hello there.

I was hoping to gleen some advice regarding mortar. I'm planning on building a rocket oven in shed at the bottom of garden. I aim to build the burn chamber, and heat riser from some reclaimed storage heater bricks. These are roughly 9" square, and I have thus far chopped these in half using a hammer and bolster. I have thus pieced these together roughly like so to form the elbow of a J-rocket stove. I aim to maintain a rough 3" internal diameter, because I want to use steel cans to complete the last parts of the chimney.

I will extend the height of the heat riser a little more before it reaches the oven. Around the burn chamber and heat riser I will build an outer wall to create a cavity, to which I am very tempted to fill with perlite, glass jars with lids on, and/or airtight tin cans. My question is, what is the best type of mortar, if indeed any, for me to use on the backside of the storage bricks (away from the face of fire) to effectively seal them, and try to make airtight. Would I get away with using a mixture of sand mixed with cement and mud concoction? Is it really necessary for me to add lime? Would I be better off slapping a bit of pure clay over the seams? I seem to lack clay in the garden, but am happy to get my hands on a bag of bentonite kit litter to do the job. Or indeed, is it actually necessary for me to seal the outside of the bricks at all?

Great to see a forum devoted to rocket stoves, and I hope to add more pics as the project moves along. Thanks for any advice!

Ooops.. I should add that I'm trying my best not to create a heat sink around the core, so I'm trying to keep mortar (or clay) down to a minimum.

Those bricks are not going to be the best for the combustion core. They are designed to be high-mass heat sinks, exactly the opposite of what you want in the core. They will be great for the oven floor or dome where you want to absorb and hold heat (though I don't know if they have any properties that would make them problematic for a floor to bake food on).

Bentonite clay is the wrong type to use for any part of your project, as it swells significantly when it gets wet. Is there any place you can get to where there is clayey soil? It doesn't have to be perfect, just sticky and strong enough to hold together. You can form a perfectly good combustion core for a rocket oven from cob with sand and straw mixed in; the heat of the fire will eventually get all but the feed tube fired into soft earthenware, and the feed where you have sticks rubbing wants some stronger lining anyway. Empty glass bottles built into the fill around the core will help considerably with insulation.

Oh, no glass jars/bottles with tight lids - that is asking for them to explode when they get hot. Put bottles with the mouth down so no water can collect, and they can be bedded in sand which is a fairly decent insulator. Perlite is better but will settle some over time if poured in loose, so don't depend on that to hold anything up. Have you read Kiko Denzer's book "Build Your Own Earth Oven"? That has a lot of information you need if you want to avoid mistakes that will make your oven fail.

For a look at how I started my rocket oven, see this thread in Donkey's forum (starting at October 30, 2014). The rest of the thread has lots of good info too.
http://donkey32.proboards.com/thread/1301/all-adobe-mud-cookstoves-ovens?page=3


Last bit for now: 3" diameter is probably too small to give enough heat for an oven larger than a toy. An L-tube rocket stove like you show can be scaled down, but when you try to run it anywhere but straight past a pot it will likely lose force and not burn well. A J-tube is trickier to scale down, and requires knowing exactly what you are doing; even then experts are reluctant to build systems smaller than 4".

Marcus Miller : I agree with what Glenn is telling you, only wanting you to start out with at least a 6'' J-bend RMH system, Generally anything smaller is an advanced
build, and when an if you get the smaller size working it is too small to do much good ! Big AL

allen lumley wrote:...start out with at least a 6'' J-bend RMH system, Generally anything smaller is an advanced
build, and when an if you get the smaller size working it is too small to do much good ! Big AL

Wouldn't that depend a lot
on what you are trying to accomplish? What about a 4" core in a mass heater for a small, very well insulated space? Better w/ a 6"
core and just build smaller fires?

Geoffry Levens : As new members come to these pages everyday- it is important that any message we share we fine tune to be useful to them !

Too many Potential Rocketeers Break their hearts on systems that are Too small to work well . 4'' and even 3'' Rocket (cook) Stoves can be made and do work

However, trying to down size any Rocket Mass Heater Build down to a Working 4'' model is very hard, and its performance minimal, this makes drying your Cob
Thermal Bench a longer much drawn out process! Add in the likelihood of Math errors creeping into the build, and it is even likely that by encouraging those types
of builds we will actually be creating a future group of Non-Rocketeers who will go away and tell all and sundry that ''I tried to make one of those Rockets,'' ''It's
a bunch of Crap, they don't work "!

I don't think that there are too many people who have said Don't build a 4'' system, most have said build a 6''system, learn how to live with one as your primary
heat source, you will probably be satisfied with Your Rocket ( our expectation is that they will be so satisfied that they will stop there or go bigger )

Again, Scaling down is very problematic, The Sq. area inside an 8'' circle is 50.25sq. inches, A 6'' is 28.26( less than 2/3rds) The 4'' is 12.5 (1/4 not 1/2 !!!)

And the percentage of surface area to volume shifts with the smaller pipe losing more 'flow' to friction by percent than the larger piping! So you lose much more,
Trying to shrink pipe

And if this violates your sense of the order of things check with any Fire fighter, paid or volunteer, one of the very 1st things we learn is 1/2 the size of the
firehouse cut the flow of water to a 1/4!

I hope this clears up your question, as it is an important thing to address to be able to move on For the Craft! Bi AL

Thank you Allen, I do get the dramatic size/power decrease. I plan to build at least a couple J-tube test stoves in my back yard, starting simple and get the feel. Eventually I am after a mass heater for my house which will be super insulated and only about 300 sq feet so I have some concern about overheating the space. Once I have done some test set-ups in my yard I can try to ferret out appropriate size info.

Thanks again

Geoffrey Levens wrote:Thank you Allen, I do get the dramatic size/power decrease. I plan to build at least a couple J-tube test stoves in my back yard, starting simple and get the feel. Eventually I am after a mass heater for my house which will be super insulated and only about 300 sq feet so I have some concern about overheating the space. Once I have done some test set-ups in my yard I can try to ferret out appropriate size info.

Thanks again

I believe I read another thread you have going. In such a small space, provided you are going to be there all the time (as opposed to a cabin or somewhere you only visit for a day or two and then leave, or you are an over the road truck driver who is only home one or maybe two days at a time) in that case, I think there are two basic ideas to control getting "too much" heat going (and do make sure you have high windows that can open, taking out heat if needed).

1) Use a smaller barrel - less radiating surface area.

2) Cob over the barrel, or modify it to be a masonry bell. This will slow the heat transmission. And if you creep up on this, you may find a happy point where you have x-percent of the top barrel not covered by cob. There are pictures of these kinds of builds floating around, showing the barrel partially covered in cob. If doing this, I'd add two points as very useful: a) be sure to include a large manifold with clean-up access, and b) make the top of the barrel removable for cleaning and inspection.

There are barrels that have a clamp kind of thing that holds the lid on. These strike me as very practical.

As for as the size, I too would suggest the 6-inch system. I've built several of them, and really they seem to be pretty easy to build and run. The only point of difficulty I've had, is the expected need to add a chimney. What I have not tried, and which Peterberg suggests as a possible alternate to try, is increasing the height of the top of the barrel to one foot above the internal fire riser. In another thread he mentions this worked on a test build in which he was involved. I've not tried that, but I hope to remember it the next time I'm having draft problems, just to note the effect it has.

The point has been made that the dimishing Cross Sectional Area (CSA) is a big deal. Not only does the math get tighter (an 1/8" difference is a bigger percentage of error, the small the CSA) but the surface area increases which creates more air friction. These are some of the reasons the smaller builds are more challenging, and has less room for error just by their nature.

A 6-inch system is as far as I can tell very forgiving and robust. And if you want less heat, build a smaller fire And after the mass warms up and dries out, you should be living nice and cosy!

Erik Weaver : As Ianto Evans has pointed out in his book with two rocket mass heaters built exactly the same, except for barrel size, they both have to shed
the same amount of heat energy to create the push me pull you magic that allows Your RMHs super draft. Counter-intuitivly, the small barrel will radiate that
heat energy off of the barrels surface at a Higher Temp not a lower one! You may need to give the smaller barrel more space in the room due to its higher
temperatures

This is the same reason when your 4 yr old is tired and wants up in your lap they are such a good cuddle With a higher mass to surface area than yourself,
in order to maintain a Nominal 98.6ºF they must radiate off the excess heat energy at a higher temp !

Partially covering the barrel will work, as will smaller fires, we still need to plan on reaching the higher Temps that together with our refractory material re-
radiate the heat energy to our combustion core and allow for hot clean fires !

I believe that I understand the Taller gap above the Heat riser is a function of the Horizontally fed Batch rocket, I may be in error there !

For the Good of the Craft ! Big AL

allen lumley wrote:Erik Weaver : As Ianto Evans has pointed out in his book with two rocket mass heaters built exactly the same, except for barrel size, they both have to shed
the same amount of heat energy to create the push me pull you magic that allows Your RMHs super draft. Counter-intuitivly, the small barrel will radiate that
heat energy off of the barrels surface at a Higher Temp not a lower one! You may need to give the smaller barrel more space in the room due to its higher
temperatures

This is the same reason when your 4 yr old is tired and wants up in your lap they are such a good cuddle With a higher mass to surface area than yourself,
in order to maintain a Nominal 98.6ºF they must radiate off the excess heat energy at a higher temp !

Partially covering the barrel will work, as will smaller fires, we still need to plan on reaching the higher Temps that together with our refractory material re-
radiate the heat energy to our combustion core and allow for hot clean fires !

I believe that I understand the Taller gap above the Heat riser is a function of the Horizontally fed Batch rocket, I may be in error there !

For the Good of the Craft ! Big AL

Good point, thanks for mentioning that so quickly! I wasn't thinking that through. Strike the smaller barrel! That only works if one also reduces the system size, which is *not* what was being suggested.

Guess that reduces the options to one: 2) Cover in cob

At some point, be that partially covered or completely covered and several inches thick (way, way, way overkill I suspect!) you'll find that happy medium between free radiation and cob-thermal mass slowing the heat release.

The temp/surface area makes perfect sense. If you put a 500-gallon drum over a 6-inch rocket heater, instead of the 55-gallon barrel, you would not expect to see the same temperatures at the surface. There is a lot more area to be heated, and the same heat being input into the system, so it has to be lower in temperature on the surface. Well, the opposite is true too Less surface area and the same heat to dissipate, would require higher surface temps.

The only other possibility I can think of is more heat flowing into the thermal storage or out the chimney. I'm not knowledgeable enough about fluid dynamics to predict the ratio of heat dissipation, through the surface of the barrel or out the duct work. But if Evans make a duel test and measured the heat differences (which I now vaguely recall reading) that demonstrates the difference in effective radiation into the room is significant, if not the only effect.

I think covering with cob is better anyway. For an entirely different reason I almost didn't mention changing the barrel - that also changes the system size relationships, area around the fire riser, manifold area, etc. Each of these points can become problem areas if the flow of air is restricted.

So far, far better, just cob over as much of the barrel on the six inch system as needed to moderate the heat immediately radiated into the room.

I do wonder if this might be a case where a masonry bell would be highly effective? But that too seems like it changes a number of factors, and would need to be researched and tested. I'm sure I've read threads about that over on Donkey's forum, so one might browse there if interested in researching that option. One of the down-sides of full masonry construction is access to the innerds. Popping a barrel off is easy to do, even if enclosed in cob. Not so much with traditional masonry So I much higher degree of certainty in the final outcome is required, for my tastes anyway.

Thank you Allen, great info. I am definitely going to "be there" much of the time. My current, very uninformed thought is to have the entire barrel inside of the first bell (if not be the bell itself) and only have a thick steel or iron plate on the top for a cooking surface. Also see if I can build a light weight enough but functional, removable oven to sit on top of that so that I can have either oven/mass, or cook top immediate heat as I choose. Lot of tweaking to get in range I imagine, and windows, plenty of windows.

In one of these threads someone showed a really nice build, with nice photos, and in good detail. On top of their barrel they added some kind of cob mixture and set into that a pizza cooking plate, or whatever those are properly called. They said they get nicely roasted veggies off that! Although, I believe their main objective was to moderate the temperature, and reduce the radiation to a nearby wall/ceiling.

Certainly interesting. I'm thinking I may cob over at the very top of my next build, which I am currently modifying anyway. I don't plan to cook any veggies up there (too tall in my set up) but I would like to reduce the heat radiating off the top.

To that end, I think I'll also add a little extra distance between the top of the fire riser and the top of the barrel. I seem to recall seeing in a video somewhere, in which Ernie Wisner remarked that increasing the distance between the top of the fire riser and the top of the barrel, tends to push the "heat ring" lower down the sides of the barrel.

I'm thinking that too might help reduce the hottest temperatures seen on the surface of the barrel, by spreading the hottest gases out over a larger internal surface area (but that just a guess, when trying to imaging those flames and hottest column of air rising out of the fire riser, and moving into the barrel, hitting the top of the barrel, and rolling down to the bottom of the manifold, and into the duct work).

These might be additional options to consider as well.

I don't plan to cook any veggies up there (too tall in my set up)

Yeah, I was actually thinking of building a platform of some sort in front, raised area to stand on, funky split level kitchen...

Thank you guys for the informative replies. Much appreciated.

Glenn Herbert ~ I realise that these bricks will act as a heat sink, but my hope is that I will have insulated the core so well from the outside, that the heat shall not be able to radiate out. If it cannot radiate out, then it shall be forced to radiate in, and in theory at least, no heat shall be lost. This is why the cavity which is being created around the core must be as close to a vacuum as possible, or at least as close to a space with lots of trapped air, and why I'm trying to avoid any direct contact to ground (ground being the biggest heat sink there is!)

Ultimately, my aims are to use materials that are easily reclaimed, and more often than not I find storage bricks, and sadly, not firebricks. And I want to make something that is going to last a long time. I fear if I make a core from cob, it will only deteriorate over time. I'm also trying to experiment with a build for those that don't have access to lots of clay/soil.

As said, I'm not trying to warm a huge mass, just the core and an oven chamber, so I'm hoping to keep down the size of the internal diameter to 3", so that it configures with the diameter of tin cans, as I think these are a great source of easily accessible (practically free!) material. The oven chamber is notsomuch a toy, but rather an old microwave oven (beware of toxic insulators inside!) I recently made a fire in it, to try and get the paint off, and in the process discovered it makes a great fire-pit!

I think I would like to start a different thread regarding glass bottle/tin can insulation. Do glass bottles really explode when heated directly behind the face of fire? What is the maximum heating temperature of a wine bottle? If the contents of a sealed bottle contained only air, and these were heated, then the air would turn into water vapour, which has much less volume (around 1600 times less) - thereby creating a vacuum. The bottle will then try to draw more air in to compensate (nature abhors a vacuum). I wonder if the danger is in the bottle imploding? As long as I'm not throwing ice water on these warmed bottles, I wonder what types of stress these bottles are capable of taking. Does anyone know first-hand what happens to these bottles when they act as insulation? From what I have seen thus far, people tend to bury them without a mechanical seal, and rather plug them with mud, but under a further layer of mud/sand, does the bottle suck the plug in? (Which reminds me of that school experiment where an egg is sucked into a bottle)

On your advice then, I shall endeavour to make a mortar made from mostly clay to smear over the core (to essentially seal it.)

Thanks again.

3" Cans and Fire Brick

Two points caught my eye as I read the previous post. I guess they both involve the idea of cutting costs, but I ask myself, at what costs to effectiveness, and are these really saving work/energy in the long run?

Of course, long run is defined by one's own time frame. A test build that is only going to run one heating season is not the same as a permanent heater for one's home. Green houses I suspect are in between. Nurseries at least, seem to modify often, heheh. So time is a variable everyone has to address on their own terms, and may arrive at very different design parameters as a result (of those which fall within fire safety concerns, etc).


3-Inch Cans

How long are these going to last? Not very long. Off the top of my head I'm thinking aluminum melts around 1800 F and mild steel a little higher, a little below 2000 F. I've measured 1710 F at the entry of my burn chamber (6" system), and it is doubtful that is the hottest point. Under ideal conditions wood is said to burn at roughly 3400 F. The bottom line, is that I think the cans are doomed to fail, and fairly quickly from a heating use (as opposed to some of the flaming cans of death seem on picnic tables in some YouTube videos, heheh).

Doesn't this mean that for the 3-inch can idea to work as a model for space heating, a pourable / castable core material will have to be poured in around the tin can mock up? That would only be using the cans as the form, and letting them burn out is fine after the casting material has set up.

What comes to my mind, is that if one is going to the trouble of learning to make castable, and getting the required materials together, and performing a successful pour, how likely is it they also needed to limit themselves to tin cans as form material? I'm dubious. Or I missed the point, which is also possible.


Fire Brick

To my mind this touches on a lot of topics. Yes, fire brick is fairly expensive (and for sure do *not* buy it at Lowe's, Home Depot, or similar chain stores - get it from a brick store, mine was about twice as expensive at the box stores, at $5 per full brick, vs. $2.25). But it also lasts a long time, and has the temperature characteristics that are needed for the extreme heat and rapid rise in temperature created in a rocket stove core. And it is super simple to work with.

And there are designs which minimize the use of fire brick, due to its cost. Using an alternative material for the fire riser for example, or using splits on the sides and top, instead of full brick, etc.

So I just wonder about costs vs. efficiency vs. lifetime vs. safety when I hear about substituting X for fire brick.

Of course, I also do not really want to re-build my RMH after I install the permanent build. (Right now I'm running a prototype as a test this winter - that I do not mind re-building, in fact I'm in the middle of another re-build right now; but this is for testing, and fine-tuning, not my permanent build).

I don't think I'm saying that avoiding the use of fire brick is right or wrong, in your case, or anyone else's. But I am thinking out loud about the overall bottom line effects such substitutions make, not just from a cost perspective, but all other angles, ranging from suitability of the materials for the task, to lifetime, to ease of use, etc, etc.


And I'm still working on my first cup of coffee, so I may be totally missing the point too heheh As I say, just thinking out loud here.


Fire Clay Mortar

As I mentioned above, I am in the middle of a re-build. I tore everything down to the floor brick. I had made the "floor" of the fire box (feed tube + burn chamber + bottom turn of the fire riser (the rest of the riser is perlite-clay between two forms)) out of full fire brick, set above 3.5 inches of perlite-clay insulation. I then "cobbed" on more perlite-clay around the fire brick, anywhere from several inches thick to only an inch above the "bridge" or "roof" of the burn chamber, to as little as 1/2-inch or so near the top of the feed tube. Then outside this, another layer of perlite-clay with some sand added to help it hold together and add stand up. Then the final layer had cellulose insulation added instead of the sand, as the "fiber" material to help counter tension cracking.

From what I could tell in the initial short test, these all worked ok. At some points where this got its hottest, the perlite-clay showed discoloration, dark brown to black. But it was on the order of 1/8-inch maximum, and in most places much thinnner. And it was only present at what must have been the hottest points of the burn.

But it is the clay mortar I really wanted to mention. I put this on very thinly. And I tried to squish it out when setting the fire brick, feeling until I felt brick-touch-brick. I did slather on a bit of extra fire clay at the intersections of the bricks. All in all, this appears to have held up well. There was one crack that was a little larger and not filled in as well, apparently, which resulted in the largest black area from heat escaping into the surrounding perlite-clay. But given there is nowhere for the heat to go that is able to cause damage, it really didn't do anything except darken the insulating material near the crack.

I'm pleased with this result. Now when I was reading some masonry heater sites, they mentioned that 3-mm was the thickest fire brick mortar ought to be (that's roughly 1/8-inch if I worked that out right -- 25.4 mm per inch, 12.7 per 1/2-inch, 6.35 per 1/4-inch, and 3.175 per 1/8-inch). So I think this next build, I'm going to try making clay "pudding" (my previous mix was very watery, and I had to fish clay out of the bottom and slather it on the brick faces), and aim for a mortar joint closer to 1/8-inch. Not that I am dissatisfied with the previous jointing, I am, I just want a point of comparison for future reference.

A little microwave like you show might be small enough to heat with a 3" rocket... but you will need a bunch of mass and insulation around it, and you will need to cut an entrance in the bottom for the hot gases to enter, and make an insulating door. All in all, it seems like a lot more work to make an oven the way you are trying than to make the whole thing of clay/soil cob. The dense bricks would be the right kind of material for the oven floor.

No matter how well you insulate them, the heavy bricks will not work as well as cob or firebrick or insulating refractory. A tiny 3" system needs all the help it can get to work well.

Have you ever seen Kiko Denzer's book on earth ovens?

"Do glass bottles really explode when heated directly behind the face of fire? What is the maximum heating temperature of a wine bottle? If the contents of a sealed bottle contained only air, and these were heated, then the air would turn into water vapour, which has much less volume (around 1600 times less) - thereby creating a vacuum. The bottle will then try to draw more air in to compensate (nature abhors a vacuum). I wonder if the danger is in the bottle imploding? As long as I'm not throwing ice water on these warmed bottles, I wonder what types of stress these bottles are capable of taking. Does anyone know first-hand what happens to these bottles when they act as insulation? From what I have seen thus far, people tend to bury them without a mechanical seal, and rather plug them with mud, but under a further layer of mud/sand, does the bottle suck the plug in? (Which reminds me of that school experiment where an egg is sucked into a bottle) "

Glass bottles explode when they are sealed tightly and the air inside tries to expand. It takes a lot of heat for this to happen. In my first rocket oven firebox, I had bottles set mouth down in soft clay, and one of them ended up being very close to the surface of the back end of the firebox. When I put the first fire in it to dry out the core, this bottle got hot fast and the clay plug kept it from venting, so it blew out the back face with a loud pop/bang. Fortunately it didn't damage anything important.

When the air in a bottle gets hot, it expands. If there is any water inside, it will boil and expand 1700 times, so this is a serious danger. Imploding is never an issue. Water vapor is already expanded, and will only expand more if heated.

Glass that is heated fast and unevenly can definitely crack, which may or may not be a problem in any particular situation. Bottles buried in sand or cob are likely to heat up slowly enough to not be damaged.

When the air in a bottle gets hot, it expands. If there is any water inside, it will boil and expand 1700 times, so this is a serious danger. Imploding is never an issue. Water vapor is already expanded, and will only expand more if heated.

Hi Glenn. I'm a little unsure, but from what I have gleened thus, vaporisation pressure only increases if there is a source of water. More water has to vaporise in order for the water vapour pressure to increase. In other words, I don't think that it is the water vapour which is expanding more and more from an increase in heat. I really need to do some more research on the precise behaviour of water vapour when heated.

When I put the first fire in it to dry out the core, this bottle got hot fast and the clay plug kept it from venting, so it blew out the back face with a loud pop/bang. Fortunately it didn't damage anything important.

Glad to hear no-one was hurt! But I wonder if the problem lies in liquid water having contaminated the bottle in someway, and does not necessarily come from air per se. I have only a vague understanding of these things, and have trawled a few web pages to try and learn a bit more, but as I understand it , a bottle filled with air at 0 c, at something like 15psi, will find that when it is heated to 100c, the pressure will increase by a third, to something like 20 psi. Can a sealed glass bottle keep up with these increases in pressure? One hopes so ... I've included a tantalising link below where the guy compresses the air in a wine bottle to 260psi!

Gas Pressure Increase with Temperature
In 1702, Amontons discovered a linear increase of P with T for air, and found P to increase about 33% from the freezing point of water to the boiling point of water.

That is to say, he discovered that if a container of air were to be sealed at 0°C, at ordinary atmospheric pressure of 15 pounds per square inch, and then heated to 100°C but kept at the same volume, the air would now exert a pressure of about 20 pounds per square inch on the sides of the container. (Of course, strictly speaking, the container will also have increased in size, that would lower the effect—but it’s a tiny correction, about ½% for copper, even less for steel and glass.)

Remarkably, Amontons discovered, if the gas were initially at a pressure of thirty pounds per square inch at 0°C, on heating to 100°C the pressure would go to about 40 pounds per square inch—so the percentage increase in pressure was the same for any initial pressure: on heating through 100°C, the pressure would always increase by about 33%.

Furthermore, the result turned out to be the same for different gases!
http://galileo.phys.virginia.edu/classes/152.mf1i.spring02/ThermProps.htm

Like I say there are huge gaps in my understanding, and when I get some time, I would love to start another thread which goes into more detail about bottle/ tin can insulation.

A little microwave like you show might be small enough to heat with a 3" rocket... but you will need a bunch of mass and insulation around it, and you will need to cut an entrance in the bottom for the hot gases to enter, and make an insulating door. All in all, it seems like a lot more work to make an oven the way you are trying than to make the whole thing of clay/soil cob. The dense bricks would be the right kind of material for the oven floor.

No matter how well you insulate them, the heavy bricks will not work as well as cob or firebrick or insulating refractory. A tiny 3" system needs all the help it can get to work well.

I don't plan on any of the gases entering the oven chamber ~ they will move only around the outside, much like the one seen in the diagram below...

3-Inch Cans

How long are these going to last? Not very long. Off the top of my head I'm thinking aluminum melts around 1800 F and mild steel a little higher, a little below 2000 F. I've measured 1710 F at the entry of my burn chamber (6" system), and it is doubtful that is the hottest point. Under ideal conditions wood is said to burn at roughly 3400 F. The bottom line, is that I think the cans are doomed to fail, and fairly quickly from a heating use (as opposed to some of the flaming cans of death seem on picnic tables in some YouTube videos, heheh).

Doesn't this mean that for the 3-inch can idea to work as a model for space heating, a pourable / castable core material will have to be poured in around the tin can mock up? That would only be using the cans as the form, and letting them burn out is fine after the casting material has set up.

Hi Eric. I hope to use the cans late on the exhaust, where temperatures are lower, but would try to design something where they are easily replaced, or cast in something cheap like everyday concrete, rather than anything refactory.

Much thanks.

Okay, you're looking for a white oven, and one that depends on continuous combustion rather than building up stored heat. Maybe your 3" rocket will do the job... try it and find out, and then come back here and tell us what worked and what didn't - we want to know.

The bottle in my case was embedded in wet clay, so there was plenty of water in the plug even if not right in the bottle. The temperature would have increased from around 60 degrees F to maybe 500 degrees or more, so that is a lot of increased pressure even without water effects. (1.33 x 1.33 x 1.33 x 1.33 x 1.33 = 4.16 or more). Four times the internal pressure could cause a problem in a cheap bottle.