Rocket Tandoor Oven!

I've been planning to build a rocket oven based on the kickstarter for a while. As I was thinking about it, I came up with an alternate design, an Indian-style Tandoor oven. I've always been fascinated by those, this would combine the traditional style with rocket technology. I think it has many advantages.

My plan is:

To use a Matt Walker style 6-inch j-tube (ceramic fiber board), but modify the top of the J to point horizontally

The J-tube would feed into a hole cut into the top/side of the barrel

The barrel and J-tube would both sit on a solid foundation on the ground

Inside the barrel, I would form a clay vessel out of cob (instead of straw, I'll use hair to make it a little porous)

The inside of the barrel and outside of the inner clay pot would form the chamber where the hot air would flow

Instead of an exhaust, there will be a 4" hole in the bottom of the clay pot. The heat would exit through there, circulating inside the clay pot. There would also be a removable top to the lid to allow exhaust to flow while the oven is heating up. So this will be a black oven. I'm thinking optionally, you could also burn wood or coal inside the bottom of the clay pot.

I'm attaching a rough sketch of what I'm thinking. Eventually I'd also build a brick surround around the barrel and fill that with perlite for insulation.

I can document my progress here. Any thoughts or suggestions are welcome.

The things I'm a little unsure about are: the hole in the bottom/top of the clay pot, would 4" or so work? The cob mix: I'm planning on making a form and building it up from the bottom, any special considerations for the mix to ensure stability?

Nathan Strumfeld wrote:The things I'm a little unsure about are: the hole in the bottom/top of the clay pot, would 4" or so work? The cob mix: I'm planning on making a form and building it up from the bottom, any special considerations for the mix to ensure stability?

4" at the bottom may prove to be too much of a restriction in the draw of the stove at a place where the exhaust has to make a 360 degree turn.

When you say a form, do you mean a base for your idea to sit on? If so, lots of rock, rubble, urbanite whatever to make it hold a solid shape, get you up to the height you want in a very short time (without having to wait for cob to dry) and straw rich for tensile strength.

Look forward to your documentation Nathan.

Thanks Gerry!

Good point, I think I'll expand the hole to 6". Or I might do three 2" holes.

For the form, I mean for the inner clay pot. I'm planning on maybe using basket reed to create the shape of the pot and then coating it with paper mâché. Then I would cob a 3" layer around the form (inside the barrel) to create the clay pot (and lid). I'm planning to use hair instead of straw, apparently you want this clay to be porous so that dough will stick to the clay walls. Do you think this will work?

I did start a foundation using urbanite, like you're suggesting. I'm still filling in some of the gaps and I'd like to create a level platform of cob with lots of straw mixed in (figure this will help insulate the bottom too) to build on.

Nathan,   I'm really not sure how well your homemade inner clay pot will hold up especially right at the neck where the heat riser is blasting flames into it and much less elsewhere. The temperature variances may promote excessive cracking and/or failure...but not sure as this is something new for me too. This may be something that you need to experiment with unless someone more experienced in pottery chimes in.
Since it is all encased in a barrel, there really is no need for a certain shape so perhaps the oven part can be made with dense firebrick which can certainly handle and hold the temperatures for a long time that your looking for?

Just as a side note, have you seen the rocket cooker the Tim Barker made?  

Rocket-Powered-Oven-build-super
rocket-ovens

The high heat blasting it is definitely a concern. I'd hate to spend all that time building it and have it crack apart.
I've seen smaller home-made tandoors made by putting together large terra-cotta flower pots. Maybe I could reinforce the neck part of it with a clay pot. I guess another option could be wrapping that part in some ceramic fiber blanket.
Let me know if anyone else has thoughts on this.

Your foundation looks great! It's always good to use reclaimed material.

I am not a rocket anything designer, but I'm bothered by the U-turn you are asking the heat to make, after the J-tube.

In a rocket mass heater, the heat riser is strongly insulated and is inside the barrel. The heat goes straight up and hits the top (in my mind the bottom of an upside-down barrel) and then splits in all directions down and then out into the mass.  The top of the barrel gets very, very hot.

In my rocket oven, the heat riser empties up into the (sideways) barrels and goes around the inner barrel to heat it.

Your drawing has the heat getting directed sideways into a barrel and then going down?  It's odd for heat to go down.  I don't know if that's going to work.  I mean, I know that it goes down in a rocket mass heater, but it doesn't need to spread out evenly, it just needs to leave the barrel and heat the mass.  I don't see how the intense heat coming out of the heat riser is going to heat anything but the very top (and not just the top, but one side of the top) of the inner clay oven, and I can imagine flames making their way up around the clay oven at the top.

> 4" hole maybe too small... maybe a 6" hole or two 2" holes

I believe that two 2" holes have about 7" square area; one 4" hole has about 13" square area; one 6" hole has about 28" square area. Rough ballpark.


Rufus

Nathan,  In case all goes south on you, Julia's rocket oven is a piece of art that is definitely worth a look at: quest-beautiful-rocket-oven

Julia Winter wrote:Your drawing has the heat getting directed sideways into a barrel and then going down?  It's odd for heat to go down.  I don't know if that's going to work.  I mean, I know that it goes down in a rocket mass heater, but it doesn't need to spread out evenly, it just needs to leave the barrel and heat the mass.  I don't see how the intense heat coming out of the heat riser is going to heat anything but the very top (and not just the top, but one side of the top) of the inner clay oven, and I can imagine flames making their way up around the clay oven at the top.

Hi Julia, Thank you for the input! I really enjoyed watching your rocket oven come together by the way. Love the mosaic!

I was picturing the barrel functioning more like a bell or stratification chamber, where the hottest air would stay at the top of the barrel and only the cooler air would flow up through the bottom of the clay pot. Maybe it wouldn't have enough draw though, I'm not sure either.
Another alternative I had thought of, but requiring a little more work, would be to make it a white oven. So there would be no hole in the bottom of the clay pot, but a another 6" hole would be cut at the bottom of the barrel opposite the J-tube, then a stove pipe exhaust coming out there. The advantage would be you could create a strong draw by extending the stove pipe much higher.
I'm gonna think about this one, maybe I'll do some experimenting before getting too far into it.

I love your idea for this!
I don't think you are asking too much of the rocket, after all there isn't a bench to push exhaust through, just the barrel.
Your idea isn't far off the basic J rocket exhausting into a barrel.
I'm curious though, is deep and narrow ideal for a tandoor oven?
The bottom reaches seem like they would be hard to use without burning the crap out of yourself!
A little googling make me think the deeper part of the tandoor is there to hold the coals, which should be unnecessary in this design.

William Bronson wrote:I love your idea for this!
I don't think you are asking too much of the rocket, after all there isn't a bench to push exhaust through, just the barrel.
Your idea isn't far off the basic J rocket exhausting into a barrel.
I'm curious though, is deep and narrow ideal for a tandoor oven?
The bottom reaches seem like they would be hard to use without burning the crap out of yourself!
A little googling make me think the deeper part of the tandoor is there to hold the coals, which should be unnecessary in this design.

Thanks William! It makes sense in my head, but I have no idea if it will work. I've never built a rocket anything. I might try my first idea (hole in the bottom of the clay) but have a backup plan (add a stove-pipe exhaust) if it's not working.

As far as the depth of the clay, I was trying to maximize the space inside. I was going to set some bricks in the bottom of the barrel to elevate the base of the clay pot- spaced apart to allow air flow. But maybe I'll elevate the bottom a little higher. If the oven does work, I'll have to buy some Tandoor accessories like the long metal skewers to cook meat, it's a different style of cooking. I would also have notches for cross-bars at the mouth of the oven, so you could hang things to cook in the middle of the oven, and maybe even set a grate on the mouth for grilling. I've talked to an Indian chef who said it takes some practice to cook naan, even in a traditional Tandoor, without burning yourself.

I was also thinking that when firing up the J-tube, it might help to start a small fire inside the bottom of the clay pot to start pulling the heat through.
Thanks for the Input!

I added a layer of cob, this is roughly the footprint the oven will take up including a brick surround with a 3" gap for perlite insulation in between the barrel and the brick.

Does anyone have experience using powdered clay such as fire clay for making cob? I was debating using that for the clay pot. The clay I have is very hard and dense, takes a lot of work to get it mixed. Also, I would think fire clay would stand up to heat and resist cracking more than regular clay.

Hi Nathan;
Yes , I have made bunches of cob using dry fireclay. No rocks, it is a joy to work with.
3 part sand to 1 part clay. Here #50 sacks of fireclay cost from $9-$20
I usually buy bagged builders sand as well.
It will crack if you do not have enough sand but it is much more heat resistant.

thomas rubino wrote:Hi Nathan;
Yes , I have made bunches of cob using dry fireclay. No rocks, it is a joy to work with.
3 part sand to 1 part clay. Here #50 sacks of fireclay cost from $9-$20
I usually buy bagged builders sand as well.
It will crack if you do not have enough sand but it is much more heat resistant.

Thank you Thomas! Good to know. It seems worth the extra cost to buy fireclay for small applications. Lincoln 60 is a good one to use?
I didn't realize there were so many types of sand. Instead of masonry sand, just buy all-purpose sand? In this picture, the one on the right mentions silica sand, will that work?

Thanks everyone for the expertise. This has been a learning experience for me, a good distraction during the lockdown. At least hardware stores are still open to get supplies. Actually, I went to an industrial supply place to get the ceramic fiber board and fire bricks today. It's pretty morbid, but they said they've been very busy lately since they make and supply parts for cremation furnaces.

Hi Nathan;
Yes, Lincoln 60 is what we get here in the NW.  Hawthorn is a common brand in other parts of the country.
Wait till you put your hand in a bag of fireclay... it is super soft and silky!
It also will get all over your clothes and the bottoms of your shoes... your home will have clay boot tracks if your not careful.
Either of the sand bags you showed me will work fine.  Main thing is they are uniform with no rocks.

Hi from Tandoor-land, Nathan!

We bought a tandoor liner a couple of years ago for our school. I'd say it was almost 2 feet in diameter at the widest part, and 1.5 feet maybe, at the top. It was a fired clay pot, bottomless, so we built it into an adobe and cob structure, with the outer gaps filled with sand.

Unfortunately, right when we finally installed it, our previous cook, a young guy who had worked at his uncle's tandoor bakery, left and a new cook who had never used a tandoor started. The new cook (who had hairy arms -- which might make a difference!) basically wouldn't use it because he said he was getting burnt (even though I think it was only coals in the bottom, not flames, when you slap the dough in). So in your design, one issue I see is that if everything else works (and I have some of the same concerns as people up above on this page), you'd have to slap the dough in while the fire is raging up through the baking chamber. Is there a way to have a bypass that you could open while putting dough in, and then close again for baking? Or are you thinking you'd preheat the whole thing and then let the fire die and bake?

For tandoor-roasting things on skewers, a local friend of mine has started a business with an Australian partner making Himalayan Rocket Stoves. These are to be used like woodstoves for heating in winter, and are not mass heaters, as the houses here tend to be adobe so the house itself can serve as the thermal mass. They kept two removable round iron plates on the top for cooking: one over the horizontal burn tunnel, and one directly over the heat riser. My local friend takes the top off the heat riser and sticks skewers of yummies down. They roast in a minute or two, and then we pull them out, close the top again, and devour the yummies. He roasts marinated paneer, green pepper capsicum, mushrooms, onions, red meat, or chicken, and other veggies, in it. Here's a video of roasting paneer in the rocket tandoor.

Rebecca Norman wrote:Hi from Tandoor-land, Nathan!

We bought a tandoor liner a couple of years ago for our school. I'd say it was almost 2 feet in diameter at the widest part, and 1.5 feet maybe, at the top. It was a fired clay pot, bottomless, so we built it into an adobe and cob structure, with the outer gaps filled with sand.

Unfortunately, right when we finally installed it, our previous cook, a young guy who had worked at his uncle's tandoor bakery, left and a new cook who had never used a tandoor started. The new cook (who had hairy arms -- which might make a difference!) basically wouldn't use it because he said he was getting burnt (even though I think it was only coals in the bottom, not flames, when you slap the dough in). So in your design, one issue I see is that if everything else works (and I have some of the same concerns as people up above on this page), you'd have to slap the dough in while the fire is raging up through the baking chamber. Is there a way to have a bypass that you could open while putting dough in, and then close again for baking? Or are you thinking you'd preheat the whole thing and then let the fire die and bake?

For tandoor-roasting things on skewers, a local friend of mine has started a business with an Australian partner making Himalayan Rocket Stoves. These are to be used like woodstoves for heating in winter, and are not mass heaters, as the houses here tend to be adobe so the house itself can serve as the thermal mass. They kept two removable round iron plates on the top for cooking: one over the horizontal burn tunnel, and one directly over the heat riser. My local friend takes the top off the heat riser and sticks skewers of yummies down. They roast in a minute or two, and then we pull them out, close the top again, and devour the yummies. He roasts marinated paneer, green pepper capsicum, mushrooms, onions, red meat, or chicken, and other veggies, in it. Here's a video of roasting paneer in the rocket tandoor.

Hi Rebecca!

Thanks so much for all this info. It’s great to get your perspective on this.

Bummer about your chef burning himself. One suggestion I heard was that you can wear heat-resistant gloves when making naan. It doesn’t work so well in a restaurant setting because it’s a pain to keep taking them on and off when switching tasks. I guess it takes some practice to slap the dough in there without touching the super hot clay.

I’d like to try making naan, but I don’t think I’ll be making it too often, my wife has gluten issues. I’m mostly interested in roasting meat/veggies. I’m thinking that I’ll fire it up and probably wait until the fire dies down to cook anything. Once the clay gets really hot, it should maintain the temperature for a while, which would leave time for baking or whatever. This would be similar to how you’d use a regular brick/clay oven but I’m hoping mine will heat up much faster. I’m gonna try experimenting with my first idea, and I can always add a bypass or different type of exhaust if needed.

The Himalayan Rocket Stove looks really cool! I enjoyed the videos. Seems like it doesn’t require much work to install, which is great.

I’ll keep you posted on how things go.

Happy Friday everyone!

I started cutting the ceramic fiber board. Hopefully it all fits together. It's softer than I expected, when I try to make pencil marks sometimes the tip of the pencil seems to tear the surface, making the marks a little tough to see. But I'm getting the hang of it.

I have a barrel that had motor oil in it, I can still smell some of the residue. Any recommendations for cleaning/prepping it before grinding the bottom off without blowing myself up? I'm thinking I'll have the side without the bungs facing up. I'm not sure if just swishing some soapy water, then dumping it out is enough? Filling the whole thing up with water seems like a pain, especially since I'd need to then flip it upside-down.

Also, my base I'm going to set this all on has about a couple inches of cob with plenty of straw mixed in. Should I do another inch of perlite/clay slip for insulation on the bottom? Or is the cob enough?

Thanks!

Finished my modified J tube, it was a little late but I couldn't resist giving it a test run. Worked great!
I was a little disappointed because the bottom part was slightly bowed in the middle, leaving some gaps. And the fire bricks are a bit loose, but I guess once it's all sealed up it will be fine.

Nathan Strumfeld wrote:I have a barrel that had motor oil in it, I can still smell some of the residue. Any recommendations for cleaning/prepping it before grinding the bottom off without blowing myself up? I'm thinking I'll have the side without the bungs facing up. I'm not sure if just swishing some soapy water, then dumping it out is enough? Filling the whole thing up with water seems like a pain, especially since I'd need to then flip it upside-down.
Also, my base I'm going to set this all on has about a couple inches of cob with plenty of straw mixed in. Should I do another inch of perlite/clay slip for insulation on the bottom? Or is the cob enough?

The oil will probably burn somewhat but it's not going to blow up on you. If your concerned, you could use a jigsaw with a metal blade or a drill powered nipper like the one Thomas used here: Mocking-batch-box-core which also includes him burning the paint off the barrel as one way to do it.
Nice job on the cfb core. I would go with the extra perlite/clay if I were you.
Peter Sedgwick has build a core like yours and gave his impression of extra insulation here: Advice-RMH-build-Hokkaido-Japan

Thank you Gerry!
That power nipper looks like a great way to go. I'm going to remember that for future reference, if I end up building another type. For this, I'll still plan on grinding the bottom (top) off my barrel from the rim. This is because I need to cut a hole in it and set it back down once the clay pot is built inside the barrel. Good to know about the oil though, I may just swish some dawn/water around a few times before grinding.

Makes sense about the insulation. I'm going to add a couple inches of clay/perlite under the barrel. This will work out well too because I need to raise the height of the barrel relative to the J tube. Right now the J is slightly higher than the top of the barrel.

It's great to see what everyone else is doing and exchange ideas!

Here's the J tube in action. Stupid idea to set it on my deck! It charred the wood below a little. That's my dog Windsor at the end.

And I added a couple inches of perlite/clay. I need to get some fireclay, the stuff I have I dug out of a construction site. It feels very pure, almost like modeling clay, but it's so dense. Really hard to mix.

I ground the lid off my barrel yesterday. It took longer than I anticipated to grind through that steel! I then burned a fire inside the barrel for a couple hours to burn off the remaining paint and oil.
Today I sanded the paint off the lid, just around the edge since I'm going to cut a hole out of the middle. I found that some rough sand paper also works well for smoothing out the jagged, sharp edges on the rim and lid of the barrel.

I was watching some videos of Uncle Mud making cob. Wish I had known the trick about soaking the clay hunks in water for a couple hours before mixing. It makes the clay soooo much softer and easier to mix. I found a pottery supply place that sells Hawthorne bond fire clay. I think for my mix, when it comes to making the pot, I'll start with a 7:5 ratio of sand to fire clay with some dried grass. They also sell clay bodies- one has grog added to resist cracking, and another that's meant for stoneware has mullite which they say resists thermal shock. These are meant to be fired though. But I thought maybe I could find some of these ingredients to add to my mix? I'm inexperienced when it comes to clay mixes.

I would definitely do some test cookies with various proportions of ingredients, dry out and then fire to see which ones survive, crack the most/least etc.  
Nice job on the barrel by-the-way!

Thanks Gerry!
Yeah, makes sense to test out some different mixes prior to building the whole thing.

Quick update:
I successfully got the hole cut in the barrel where the J tube will feed into! Like everything else, it was a bit trickier than I thought.
I've been doing this project in baby steps. Work, wife, kids, dog, etc... Trying to keep plugging away though when I can.

I got 4 bags of Hawthorne bond fireclay, and 1 50lb. bag of grog. The grog is supposed to prevent cracking since it's basically crushed ceramic that's already been fired. I'm going to try a few mixes, make some small bricks, and see how they hold up.

Also found this metal tube thing lying in an abandoned parking lot. It fits inside my barrel, so I thought of maybe throwing together a quick prototype- just form a base/top out of rocks and clay just to see if it actually draws heat like I'm picturing, or if I need to modify my design. Not sure if it will take more time than it's worth though.

It was freezing last night, but it's beautiful today in Chicagoland! How is everyone holding up?

I made some test bricks with different mixes of clay/sand/grog. The grog is interesting, it's gritty like sand, but I guess it's chemically the same as clay. It does add a lot of texture to the mix. Love the fireclay! Much easier to work with than the stuff I dug out of the ground. It's too early to tell, but I think my favorite ratio was 1:1:1 clay to sand to grog. It wasn't too sticky or too crumbly. I'll let them sit for a while and then I guess put them in a fire to see how they hold up.

I also added this lip of clay slip and perlite around the base of my barrel. I'm thinking it will give it some support, and then if I have to remove the barrel for some reason, it will be easy to see where it sat.

To be continued...

Hey Nathan!

Cool to see what you’re doing. Been working on a number of different projects myself. Playing around with clay and other materials.

One of the projects is an outdoor kiln made with a oil drum. I will post more details on the project later in a separate thread. Point is, inside the base of the can I used a mix of sifted high clay content soil, sifted fire ash and charcoal. Let it dry for about 5 days before firing it.

Base is about about 3in thick. Zero cracking and seems to be fired and has ceramic properties.

After that experience, I would suggest that you try a 50/50 mix of fire ash and clay. Maybe incorporate some rockwool fiber or basalt fiber shards into the mix to increase the tinsel strength.

What I believe you are looking to crate is more or less a high density refactory terra-cotta pot that can withstand thermal shock and extended high temperature exposure.

Here are a few images of what I did along with some reference of fibers that might work.

You’ll have to experiment and see what consistency will work best to build your pot, but maybe this will help.

Peter✌️🤓✌️

Hey Peter!

I’ve enjoyed watching your rocket adventures in Japan! It's fun playing with clay isn't it?

Are you using those new firebricks you got for the kiln? Sounds like a cool project, look forward to seeing how that goes.

Thanks for the suggestion with the fire ash. You’re exactly right, it’s basically a giant pot that needs to withstand the high heat. I don’t mind if it cracks some, as long as it holds together, I’m planning to make it a couple inches thick. I do remember Matt Walker saying that adding a little ash to his mortar mix gives it some refractory properties. Probably a good idea to try adding some to my mixes.

For the fibers, I actually got a small bag of hair trimmings from my barber a couple months ago. This was right before the pandemic started shutting everything down, wish I had gotten more. I also dried some grass clippings, it’s smaller/finer than straw. I'm hoping the combination of these works.

Thanks for the help! Talk soon.

-Nathan

I've had to put this project on the back burner for a while due to a major flood in our basement. I was busy rebuilding and I took on another project- rerouting my sump pump into a dry well. Tons of digging! But that is finally finished and I'm back at it!

I got some more materials and made the lid out of my fire clay/grog/sand mix. I built a sand form and put about a 2" layer of the clay mix around it. It's tough to get it perfectly even, but it's taken several days to dry which has given me time to go back and smooth out some imperfections. I've been adding hair, dried grass, and fire ash to the mix too.

I created this indented lip around the rim- I used basket reed and cardboard to create the circular shapes. For the actual pot, I'm not going to use a form. It's just too big and nothing seems to hold up. I'm going to form a circular slab for the bottom and then rectangular slabs that I'll just wrap into semi-circles and hand-mold them together. It's kind of hard to describe but I've seen Tandoor makers use this method.

Here's a few pictures of the lid.

So I started building up the bottom of the clay pot inside the barrel, thinking it would be easier.
And after sitting overnight, there is rust inside the barrel- the wet clay formed condensation inside. I didn't think it would rust so quickly.

Any suggestions for removing the rust? I'm not sure how dangerous it would be for cooking, the food wouldn't be in direct contact with it.

I think I'm gonna need to remove the clay bottom once it dries enough and build it inside my garage, outside of the barrel. Then assemble everything once it's dry. It will just be really heavy to lift.

Any rust that forms on my barrel, I just hit it with a grinder with a wire wheel on it. Being that its very minimal surface rust in your case, steel wool or even a brillo pad would probably suffice. A light coating of vegetable oil and it will look spiff again. I highly doubt rust would cause any health issues especially since your not cooking on it.

Thanks for all your help with this Gerry! I'm relieved to hear that.
I wasn't expecting the clay to give off so much moisture. Lesson learned! I'll wait till it's mostly dry to place it back in the barrel.

I cleaned up the barrel on the inside with some steel wool and applied a little olive oil. Worked great!

I've built up the clay pot about 21", I should just need one more layer to get it up to about 32". You can see the 6" hole in the bottom. Once it's up to height, I'm going to mold the rim to fit the lid.
I'll need a few people to help me lift this into the barrel once it's dry, it's really heavy! It will be awkward to lift it up a few feet and then down into the barrel without much clearance on the sides. I guess that's why I wanted to build it inside the barrel to begin with.

If I did want to add a chimney bypass later, could I just use a 6" hole saw to cut a hole in the side of the barrel? And then push the stove pipe into that? Seems like it would be the simplest way to do it, but it is a rounded surface.

Looks great Nathan!  

Sounds like a feasible plan to add a bypass later if needed. A small grinder or jigsaw would probably be easier to use. No problems being a round surface either with these tools.

Thanks Gerry! I really appreciate all the technical support I've gotten here.

I've got the inner clay pot built, just waiting for it to dry. Still thinking how I'm gonna lift this into the barrel, it's very heavy!
I cut some notches into the rim of the pot and then ground a couple pieces of rebar so they fit together to form temporary cross-bars. There's lots of cool accessories I'm thinking about for this, but I want to make sure it works first.

The lid doesn't fit perfectly- it's slightly bigger than the mouth of the pot, but it seems like it will still work well.

That looks super cool Nathan. Great job!
I used to lift my barrel onto my rocket mass heater with a small hand operated pulley suspended from the ceiling. Perhaps something like that could be used?