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rocket stoves

Feeder/tunnel/riser dimensions --- 1/2/4 vs. 1/1.5/3.5 ---

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posted 7 months ago

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Greetings RMH community!

After more than 12 years of reading, planning and preparing the groundwork, I am getting ready to move into the construction of what I think of as the heart of the house.

There are some final considerations I need to make.

First, I needed to dig out the space for the construction. I made the bench in natural stones. And, I made a stone wall where the core will be built. So it will be partly built into a structure of stones. Within that space, I can fit a burn tunnel of one meter. That will leave enough space to insulate the walls, so that the heat will not damage the cement, and tumble the walls.

I will cast the core. I will make a mix of puffed silica (bought in bags, used here on the sidewalks in winter, to prevent people from falling on the ice). I will use water glass as the binder, and I will mix a small amount of refractory cement (resisting 1000 Celsius) into the mix. And some sand.

The considerations I need to make now is what dimensions to choose. I find it easier to navigate when I understand the principals - for example, the distance between the top of the heat riser and the barrel, how would that be affected by the two different dimensions - or rather, how can I calculate the length based on how I expect the build to function? Should I for example make a shorter and smaller core, to leave even more space to separate it from the stone walls? I will also use leca (puffed clay) to protect the stone walls, and glass.

I will make a chimney on the outside to ensure a good draft. And I will include in this chimney a place to make a small fire, to be able to heat it before I start a fire on the inside.

Best regards

Fox James

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What style of heater are you planing, J tube or batch box?

Scott Weinberg

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posted 7 months ago

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Oystein Tandberg wrote:Greetings RMH community!

After more than 12 years of reading, planning and preparing the groundwork, I am getting ready to move into the construction of what I think of as the heart of the house.

There are some final considerations I need to make.

First, I needed to dig out the space for the construction. I made the bench in natural stones. And, I made a stone wall where the core will be built. So it will be partly built into a structure of stones. Within that space, I can fit a burn tunnel of one meter. That will leave enough space to insulate the walls, so that the heat will not damage the cement, and tumble the walls.

I will cast the core. I will make a mix of puffed silica (bought in bags, used here on the sidewalks in winter, to prevent people from falling on the ice). I will use water glass as the binder, and I will mix a small amount of refractory cement (resisting 1000 Celsius) into the mix. And some sand.

The considerations I need to make now is what dimensions to choose. I find it easier to navigate when I understand the principals - for example, the distance between the top of the heat riser and the barrel, how would that be affected by the two different dimensions - or rather, how can I calculate the length based on how I expect the build to function? Should I for example make a shorter and smaller core, to leave even more space to separate it from the stone walls? I will also use leca (puffed clay) to protect the stone walls, and glass.

I will make a chimney on the outside to ensure a good draft. And I will include in this chimney a place to make a small fire, to be able to heat it before I start a fire on the inside.

Best regards

Fox James asked the all important question, simply being "Which TYPE are you intending to build?" All the dimensions are readily available for the two main types. J tube and Batch box.

After 12 years of reading-planning, this mixture you talked about? Someone has proven it already for casting? Just asking

you said, "make a shorter smaller core" but you understand, these known and proven dimensions, detail the SIZE of the core as per size of the stove, again I am just asking, if your planning on taking proven results and change them to match what you think you might need to make it all work? Over the years, there have been many attempts to "prove a new shape and design" only to find out something does not work out well. Just pointing that out.

The size you choose, is generally generated by the size you need.

Lastly, starting two fires, vs using a by-pass for a few minutes? again I am just asking, but the by-pass has proven to work well, and really simplifies much of your build.

https://permies.com/t/241976/Batch-Rocket-single-bell-build

Oystein Tandberg

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It's a J-tube. I've used the mix in another mass heater and it works. In that build I used the standard dimensions, but since the barrel will be partly covered by rocks this time, I expect it will change things - and I'm trying to figure out how. And how to use it to my advantage. (the second fire has other functions as well, to produce woodchar, to have a fire place outside, in case of inspections, and more)

So what I want to know is if I should extend the distance from the top of the heat riser from the barrel, make it 10 cm -or do the opposite, make it 5 cm.

I'm heating a small place, I don't need the oven to be as big as possible.

thomas rubino

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posted 7 months ago

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Hi Oystein;
5 cm (1.9") is much too small
10cm (3.9") will work well.
20 cm (7.8") will also work.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
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Oystein Tandberg

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posted 7 months ago

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thomas rubino wrote:Hi Oystein;
5 cm (1.9") is much too small
10cm (3.9") will work well.
20 cm (7.8") will also work.

Thank you! And, what would be the effects of a bigger distance?

And - should I make the heat riser twice as tall as the burn tunnel? And what would the combined effect be - bearing in mind that pars of the system will be built in, leaving less barrel surface to be exposed.

From my reasoning, it would be good to have a longer heat riser, as the gap between the riser and barrel increases. I reason this because I remember having read that a small gap makes for an added bottle neck effect. Now that I am making a very good chimney to go on the outside of the house, I will have a good draft at the exit of the system, and that will take some of the stress off the heat riser, as it will have a good pull from the external chimney. And that is another reason I go through the stress of making a fire place at the bottom of the external chimney, because I want to reduce the time I have to burn at a low temperature, and get the fire going on the inside. I'd rather freeze a moment or two making a strong fire at the base of the external chimney to get things moving right from the start. And then under normal circumstances, that wont be necessary as the temperature in the house normally, hopefully, would be much higher than the outside temperature, and draft is no problem - windy as it usually is as well..

thomas rubino

rocket scientist

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posted 7 months ago

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The effect of a larger distance from the riser to the barrel top will be, lower radiant heat released from the barrel and more heat passed into the mass.
Riser height should not be more than 48"(122cm) but not less than 36" ( 91.4cm) measured from the floor of the core.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com

Oystein Tandberg

Posts: 53

posted 7 months ago

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thomas rubino wrote:The effect of a larger distance from the riser to the barrel top will be, lower radiant heat released from the barrel and more heat passed into the mass.
Riser height should not be more than 48"(122cm) but not less than 36" ( 91.4cm) measured from the floor of the core.

Thank you, really appreciate it. That solved many issues. So I will make it 120 - and the burn tunnel will be 60 cm, and the feeder 30cm. And I will aim for a 20 cm distance, as its very beneficial to heat the bench and not the air.

Oystein Tandberg

Posts: 53

posted 7 months ago

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thomas rubino wrote:
Riser height should not be more than 48"(122cm)...

I took some pictures for illustration, the heat riser is too tall and the burn tunnel too long in this picture..

I have some questions. Since I'm casting the core, it will be quite easy to create a passageway for air to be feed into the system along the length of the burn tunnel. I would make them "bottle neck style", and maybe two is enough, one on each side. I would make it in a way that I can cover them easily, and I'm planning to do the same with the feed tube, so I can strangle the fire easily If there should be a blow-back, and for other reasons as well. Any thoughts on this?

I don't know if the pictures become more clear by me explaining them at this point, but its one showing the front, and one showing whats to the right of the fireplace. I've wanted to create an oven under the core, but I doubt it will get very hot, maybe to dry something... I've left a big opening to be able to remove ashes.

Then to the question - is it better to have the feed tuber slightly tilted, will it effect draft?

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Øystein Tandberg

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posted 6 months ago

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Oystein Tandberg wrote:

thomas rubino wrote:
Riser height should not be more than 48"(122cm)...

I took some pictures for illustration

here is an update

74A4ED86-0452-4F9F-8142-58443D4D029C.jpeg

Fox James

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posted 6 months ago

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You might find a few things of interest in this video.

Burra Maluca

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posted 6 months ago

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Scott Weinberg wrote:

Oystein Tandberg wrote:
I will cast the core. I will make a mix of puffed silica (bought in bags, used here on the sidewalks in winter, to prevent people from falling on the ice). I will use water glass as the binder, and I will mix a small amount of refractory cement (resisting 1000 Celsius) into the mix. And some sand.

After 12 years of reading-planning, this mixture you talked about? Someone has proven it already for casting?

Waterglass is sodium silicate which is used in CO2 sand casting.

I tried to raise the subject here years ago as a friend of mine used to use it for casting experimental parts for jet engines, but no-one ever really showed any interest.

I'll be fascinated to see someone actually trying it for casting rocket cores.

Here's the thread, from seven years ago - shippable cores - could CO2 sand be the answer?

The guy is now my neighbour and I'd be happy to pick his brains about it if anyone want me to.

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Scott Weinberg

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Location: North East Iowa, USA

posted 6 months ago

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Burra Maluca wrote:

Scott Weinberg wrote:

Oystein Tandberg wrote:
I will cast the core. I will make a mix of puffed silica (bought in bags, used here on the sidewalks in winter, to prevent people from falling on the ice). I will use water glass as the binder, and I will mix a small amount of refractory cement (resisting 1000 Celsius) into the mix. And some sand.

After 12 years of reading-planning, this mixture you talked about? Someone has proven it already for casting?

Waterglass is sodium silicate which is used in CO2 sand casting.

I tried to raise the subject here years ago as a friend of mine used to use it for casting experimental parts for jet engines, but no-one ever really showed any interest.

I'll be fascinated to see someone actually trying it for casting rocket cores.

Here's the thread, from seven years ago - shippable cores - could CO2 sand be the answer?
Ok, I will ask this differently. in one place it quickly told the ratio was 500 grams of sand to 30 grams of Sodium Silicate. And then CO-2 method of quickly curing.
I did not find any reference of adding other items for insulating purpose, or ratio's of such.??
Nor did I find how durable any of this would be Day after day of fire, with the included wood loading. and for years.??

Maybe I have missed this somewhere?

The guy is now my neighbour and I'd be happy to pick his brains about it if anyone want me to.

https://permies.com/t/241976/Batch-Rocket-single-bell-build

Glenn Herbert

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posted 6 months ago

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A few comments...
You show a mockup of cardboard tubes for formwork. The original experimenters found that a square cross section was better than a round one because it gave more scope for beneficial turbulence to form. Peter van den Berg in particular made many versions of mixing enhancers, and found the best one was the P-channel with trip wire in the ceiling (which you can find detailed specs for in many places.) If you want something that will be sure to work, I would suggest going back to that formula rather than experimenting with air feeds in various places... especially as you will not be making multiple feed versions to test them against each other, but one version which may or may not be optimum.

I think 48" is not an absolute maximum, but is probably long enough for full combustion to occur with good draft. A 30cm feed tube is pretty short, as you would be safest making the feed able to fit the fuel completely inside so as to be able to cover it airtight quickly in an emergency. If you have 30cm fuel readily available, fine, if not I would make the feed tall enough to hold the fuel you will have. I have found with my well-drafting system that 1:1.5:3 works fine, specifically 16" tall feed, 24" burn tunnel floor, and 48" riser (which due to installation clearances ended up more like 42", and still the flames seldom reach the top of the riser.)

Slanting the feed tube will not be beneficial; it will only give the fuel a wall it has to slide against with the chance of hanging up on any irregularity.

Øystein Tandberg

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posted 6 months ago

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Thank you guys - great video! I will watch it again a few times before I comment.

Regarding the mix - I actually made a "prototype" and burned it every day, twice, for a whole winter. And yes - the mix needs to be stronger in the feeding hole. And I think I will copy this video and place a metal screen to stop the wood from entering too much into the burn tunnel. But - the chimney looks like I just caste it. No sooth and no fractures of any kind. I'm very pleased with my mix - but what I will do is that I'll put a little more sand and fireproof cement in the mix that goes in the feed tube. And make sure it's properly compacted.

Regarding turbulence - I've seen the "small V shaped recess" that Ernie and Erica made at the bottom of the burn tunnel, and I thought I'd make one as well. Regrading Peter van der Berg's P-channel, I will have to watch that again, to see what I can do. I like what I saw in the video - I'm deep into vortex math, by the way, and esoteric knowledge, Egyptian, Chinese..

I've placed the oven next to the door, so I can get plenty of air fast. And, I'm making a tall chimney on the outside. The prototype worked despite the outside chimney being noting but ventilation pipes. When it snowed or rained they would get cold very fast, and the draft was, well, you can probably imagine. I burned a lot of wet wood as well, and pallets with a lot of nails - come to think about it.. It was Covid times, and I usually travel to warmer places during the winter..

So I will make the feeder a bit taller, and not tilt it. It was not the intention, but it was difficult to make a clean cut of the cardboard using a old was and a pair of scissors. I still have enough length to make another cut, and the hole I'm making for the core is deep enough. So it's all good. I might coil a 8mm copper coil and cool down the top of the feeder - I recon it can stop the flames from wanting to rise up, and make sure the logs burn more like matches. I was originally planning to make two more coils as well, one where the hot air is pushed down in the bench, and one (not touching the barrel) closer to the top of the barrel - I've made a bathtub on the other side of the build, so I can keep the water running - there is an issue there with water exploding inside the tube - I'm still in the thinking box on the whole water issue - making some big water storage tanks to be able to have enough water as well.. Would be nice though.. And I think it would be useful to cool down the feed tube.

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450247140_482333314382781_3831177373172181218_n.jpg

Glenn Herbert

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posted 6 months ago

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For your feed tube, I think you would get the most life by making at least the top 10cm or so of brick or metal, as that will have more wear than anywhere else. As for cooling the feed top, I made a liner of 1/8" steel for the upper 6" which contains the secondary air supply to the P-channel; it certainly preheats that air, but does not at all stop the fire from creeping up the logs. If your draft is strong enough to keep that from happening, I think the air would be moving so fast that you would have a ton of excess air which will only cool the system. I cover my feed at least 3/4 all the time I am burning and still get plenty of air for full combustion.

The feed interior will get to several hundred degrees at least, far above boiling, and I would expect any coil near there to be flashing to steam.

Fox James

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Øystein Tandberg wrote:Thank you guys - great video! I will watch it again a few times before I comment.
.

There are lots more videos on my channel including info on casting.
I think this was the last video about the vortex J tube here …. https://youtu.be/LbZTkuiMBRU?si=LS1mh3FdcECU5ERF

Øystein Tandberg

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posted 5 months ago

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Glenn Herbert wrote:... I think the air would be moving so fast that you would have a ton of excess air which will only cool the system. .

Thank you Glenn - most appreciated input! It is interesting what you say, I am trying to anticipate the draft I will need and also be able to create by various manipulations - I opened another thread to discuss this in a separate thread, but it is of course very connected, like the universe itself. https://permies.com/t/261567/heated-bench-cross-sections#2548605

Without getting too philosophical - I try to cover all bases in this system. I'm making a rather massive fireplace at the base of the outside chimney - and I plan to line it, and the pipe, with a fireproof material consisting mainly of puffed silica, and puffed clay, using waterglass as the binder. That can sustain much higher temperatures than bricks, and it also have much higher insulation qualities. So, when the exhaust leaves the house in a horizontal pipe, it will connect to this rather large oven, and the chimney will have the same diameter as the inside oven/core. That will give me the possibility to regulate the initial draft substantially, and I am also contemplating other solutions, to what you propose, reduce the air that flows into the system. I have placed it next to the door, so I can just open the door and get as much air as I want - it is next a source of abundant water - so now you made me think of the other extreme, allowing the very minimum of air - I see now that is very important.

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