Multifunction outdoor rocket system

Well, I see lots of folks wanting a system that does it all, and a lot of different ideas on how to accomplish that. Here's a system I built over the weekend. It's neat in that it can function as an outdoor rocket oven, a bbq, a big cooker for canning or boiling corn or crab or the like, a mass heater, and a cozy spot to sit around and watch the fire while being warmed at your feet, under your seat, and by the barrel. The mass responds very quickly so you can cook your meal and have a warm seat in the time it takes to cook, and the warmest spot is down low so it's lovely to hang around it while you cook.

Fantastic! I've been gathering the supplies for my outdoor kitchen project, which will get underway as soon as I can tear out an old dilapidated building that's currently in the way. The window is great -- I was planning to put a door in that spot so I could add smoke wood for barbecue, but I think after seeing your setup I'll try to scrounge up some glass because that's awesome.

Thanks Erik. Credit is due to Dan Murf for breaking some glass down there first. I've gotta say, the way it radiates heat down at your feet is a very, very pleasant feature.

Erik, I've been thinking about the smoke chips you brought up, and where to add them. I believe if you were to put them down in the burn tunnel you wouldn't get the flavor, but what if you put them up top? I have a weber grill on top of the barrel/riser right now, with a clay pizza stone in there to diffuse the heat. I bet a small amount of soaked alder or apple chips or the like underneath that pizza stone would provide a really good amount of smoke, and could be controlled with the weber grill's vents. I'm going to try it, great idea.

I'm not 100% confident it'll work out, but I have a massive pile of bricks out there to experiment with now so I'll give it a shot and report back on my results. The theory I was going on is that by the time the flames are starting on the path up the heat riser, most of the available oxygen has been consumed already. I'd add the smoke wood in there so it would make some smoke to flavor things. I think your setup with the weber is going to be great -- the chips would be right at the top of the riser then where the oxygen is pretty depleted and the heat is nice and concentrated. The setup I'm working on will be a bit different because I'm building the oven part out of bricks too, but depending on what kind of results I get with the smoke chamber I might try to emulate your setup to some degree (maybe put a door in top of the heat riser with a little metal rack to hold some chunks of wood). With a little bit of tinkering, I think these rocket-based setups will end up being the all-time ultimate barbecue rigs -- long heat retention, wood fired, radiant+convective+conductive heat transfer to the cooking goods, highly efficient, and easy to control.

Erik Lee wrote: I think these rocket-based setups will end up being the all-time ultimate barbecue rigs -- long heat retention, wood fired, radiant+convective+conductive heat transfer to the cooking goods, highly efficient, and easy to control.

I feel the same way, and am so excited about the possibilities. I got the pizza stone just blazing last night in a matter of minutes after starting, and was able to cook immediately. Quick start up is another great benefit. Getting the heat off the barrel and window while cooking is unreal, at least in my cold climate, it just makes outdoor cooking that much better. I'm looking forward to your experiments with the smoke flavor, keep us posted. I'll do the same.

That is really amazing Matt. I am scheming for down the line a bit. Posted another thread on my idea, which is to build a rocket powered homebrew rig. seeing that kettle steam up there gives me hope that my idea might just work!

Thanks Jonathan. Have you seen the Institutional Rocket Stoves from the aprovecho primer? Seems like what you want to do has been really well refined already.

Here's a starting point..

http://www.vrac.iastate.edu/ethos/ethos05/proceedings2004/presentations/scottnewrocketstove.pdf

Here's a little update on the system from today...

Here's another update, I talk about how the dual window is configured.

Awesome Matt! I'm considering building my first rocket stove this summer, without any hands-on experience or guidance, just going by Ianto's book. For safety I was thinking of making it an outdoor bench/grill much like this one. Questions: Is the cobb on your bench protected in some way or do you expect it to hold up to the rain in it's own? What did you use as a foundation under your stove and bench, if any?

Hi John, you'll do great with just Ianto's book as guidance. One thing I would encourage for anyone thinking about any of this stuff is to start experimenting with real basic set ups out of clay that might only work for a day or two. You can learn a lot and gain a bunch of confidence, and mostly it will free you of a lot of ideas about "stuff you need."

Before I built my first one I agonized over materials, where to get good bricks, clay, tubing, etc. By the time I built this one for the first time I was comfortable digging up some mud and building the stove with nothing more than the mud, some scrap wood forms, and one little piece of 6" tubing to mold the riser around. It worked great, and frankly, for a simple cooker with just a feed and riser under some kind of cooking surface, I think it's the way to go. Perlite and clay for the burn areas in my opinion create a better burning stove than one with bricks in there.

Anyway, it's not protected from the rain yet, and does need to be if it's going to last. I suspect the hydrated lime treatment will not work here due to thermal shock, and I'm not really keen on linseed oil, as it's food for mildew, and the location of the stove guarantees mildew would love it there. I'm currently thinking of thin slate or flagstone pieces to create tops for the bench and feed, with generous overhangs. We'll see.

I used rocks from my garden to pile up a little "foundation" just so the cob didn't wick up moisture from the ground.

Good luck with yours!

We don't have rocks here, and we don't have clay We just have sand, loads of it.

Where are you Roy? I found bags of fire clay for $12 for a 50# bag. One of those mixed with your sand and some perlite would make a fine stove.

Er, in the UK There is currently no fireclay available in the UK as the coal mines are shut down. Apparently ball clay would be just as good, since "fireclay" is a misnomer, and no better than other clays if mixed correctly. I must try some local blue clay. That price is amazing, is that for powdered clay? the lowest price I can find here is £9 for 25kg.

Yes, that was powdered clay. I think you are right about the ball clay, I'd give it a try.

Hello Your rocket stove looks great! I am in the process of setting up my first rocket stove and was just wondering if you used ducting in the cob bench? You dont mention it in your list of supplies I have lots of stone, slate and fire bricks. The thing Im struggling with is clay Im wondering if theres anything else I could use to hold it all together? Im just thinking does it have to be a solid mixture? Could I use vermiculite banked up and just the top layer in something more clay-like, say vermiculite mixed with sand and cement where the temperatures would be cooler, to hold it all together

Thanks Carol, there is no ducting in this bench. It's a chamber which heats evenly like a bell in a masonry stove.

I think you are going to want some clay. Loose materials would be hard to work with, but clay is pretty much everywhere it seems. Surely there is some mud near you. Try building a quick outdoor system with mud and straw and see how it holds up.

Thanks for the help. Now what you have said is just what I needed to hear! This idea started off simple, then gained problems. I find that if things get complicated, I tend to overthink it and then abandon the project! The idea of 'just do it' suits me fine! I need to man up and jump in. A little bit of advice I need though Mud Ive got a lot of that, I just moved a stack of it from the bottom of our pond So Im guessing it would be termed silt, rather than mud? Would that be suitable, or would it be a bit, erm, odorous? Also, is there any sort of duct leading from the riser to the thermal mass, or is it more of a channel? I can see you have used stone as mass, would there be any 'route' for the warmth to travel along, or does it permeate the stone and mud? Is it like, solid mud and stone? Last question (well on this post at least!), when the mud is dry, wouldnt it be a bit dirty to sit on? Would I dirty my new white Yves Saint Laurent trousers if I sat on it? (Just joking, Ive never owned anything YSL, let alone trousers, let alone white ones) So I guess my overall question is, am I likely to end up with a smelly, dirty RMH?
Thank you again for re-motivating me
God bless

I've found that my mud turns out just fine, mix in some straw, and you've got cob. This project I totally stacked functions and dug a drainage ditch around my garage, and used the mud to build the stove. I don't think it will smell or be dirty, no more than any other cob. Seriously, cob is pretty much just mud and straw. I think once you build something with it you will be amazed at how simple it really is. Dig up some mud, stomp in some grass clippings or straw or pine needles or whatever you have, and build.

My bench is a hollow chamber, no flue, but totally hollow. It heats very quickly and evenly due to that fact.

Thankyou for your help Im happy now to go retrieve that mud and make it into something useful Now that you mention it, when I cleared it from the pond last year, the last few mounds I removed I left at the side of the pond as I was tired by then and couldnt make the last few trips to where I was moving it. When I moved if a few weeks ago, it was pretty much in the same shape I had left it, so I can see how it could be used as clay. Great discovery! I find that, in operations like this, its a much better practice, and a greener one, to use what you have rather than trekking around looking for a specific item. I believe we are usually provided with what we need, quite often right under our noses! So, going back to the bench, is it like a support sort of framework of stone, covered in cob? Given that its hollow, what holds up the the seat part of the bench? It couldnt be something combustible could it? I have an area on the patio that is just crying out to be turned into something like you have built. It is made from brickwork and is a bbq area with a bench frame at the side.

BTW If you dont have a flue, where does the emissions exit the set-up, or doesnt it? Isnt there steam in the emissions and so wouldnt it soften the cob bench if not vented out somewhere? Also what sort of size glass panels did you put in? Im not sure Ill be as adventurous as to put the glass in but, just in case. And is it special glass/ Would ordinary glass not withstand the heat? Also, you mention perlite in one of your postings, is the feed tube and burn tunnel made from some sort of mixture of perlite and mud, or is the whole surrounding structure just good ol plain mud? And lastly, did you have to wait for the mud to set or will the heat set it? Questions questions!

Sorry, I meant no flue path inside the bench. I did stick a 4' long piece of flue pipe into it for the exhaust, so there is a little flue pipe.

The glass is ceramic stove glass, regular glass would not hold up there. The windows are roughly 12"x 8".

Yep, perlite and mud for the burn areas, with some straw to hold it together. Just mud and straw for the rest. I always just burn 'em dry, seems to work fine. I light them up as soon as they are the right shape and continue to cob while it burns.

Hi Matt,
I love your work with the glass. You have been playing fast and loose with the 'basics' and getting great results. I like the multi-functionality of this setup.

I do have to say I notice a little more smoke coming out of your flue / grill on the second video than I am used to seeing with a well-insulated burn tunnel. Can't tell on the 3rd video if the exhaust is smoke or steam.
This is a common issue with fireplaces with two open sides - you lose the clean-burning benefits of a hot fireback / draw path for the combustion gases to channel along. I have wondered about using multiple layers of mica (or your ceramic glass) so that the inner layer can get nice and hot and provide some of this clean-burning effect, without so much heat shock it cracks the glass.

Where are you getting the ceramic glass?

Regarding rain protection:
I would not expect flagstone on top to give enough protection to the cob, unless you're on the rainshadow side of the Olympics and don't get much rain. But as you say, sometimes it's better to build and re-build fun things and not agonize over doing it 'right.'
If you do try the flagstone approach, of course you can make a slight dome to direct water toward the edges; there's another issue where even with an overhang, the water can wick down the surface into the cob sides, instead of dripping off at the overhang. One of the Portland bench builders was doing a 'bead' of aquarium cement set back slightly under the lip of the overhang, to create a specific dripline that interrupted the water's tendency to wet the whole surface.

Most of the Portland builders eventually agreed that a cob bench just needs a roof. There were several moisture-related collapses of well oiled and waxed benches, and problems with moisture not drying out from under capping materials like slate or tile. It's not the surface, but the core of the bench that needs to be kept dry, and 'waterproof' materials tend to trap interior moisture as well. I like the bell bench. If your trusses are wood, you may need to keep them dry as well for strength.
Adding a roof over an existing cob bench sometimes involves some damage from dropping roofing bits on it, or re-designing the base to accomodate roof supports. So if you love your fire-quarium-grill enough to roof it, you might want to do the roof before the final plaster.

Thanks for sharing your work.
Would love it if you can continue posting updates on this as long as you use it - I suspect it will continue to improve, and any maintenance issues will be fixed with your customary artistry.

Yours,
Erica W

Thanks Erica, I appreciate your input and the kind words.

Regarding the smoke, and the burn, I have been playing with balancing a lot of factors, and am concluding that the system can be tuned to favor different features. In the second and third videos, the burn tunnel is completely blocked and the only flame path is through those tight gaps at either side. That does make the stove a little finicky, especially if trying to get much heat to the bench. But for sitting around it while cooking, it's wonderful. Bright and warm down low, and plenty of heat to cook on up top. When I restrict the hole in the barrel top though, it kinda chokes and slows way down. With the center of the burn tunnel open, lots of heat to the bench, and good powerful draw even with the top closed, but not as much heat down low and more soot/less flame visibility.

Currently I'm running in between, I removed the outside bricks in the burn tunnel obstruction, widening the two gaps at the windows. It's running very smoothly right now, and gets some good heat to the bench with the top hole closed. Due to the windows though, it will never run like a well insulated stove, I don't think. They bleed so much heat it's amazing, and can't be helping the burn any. It is worth it to me though, it's not just nice to look at, it's really a pleasure to sit around. I'm spending much more time outside in the evenings than I would be otherwise, and I'm toasty warm. That warmth down low is amazingly pleasant.

I do think that protection for this cob bench may be different than other non-heated cob structures. I understand the stone caps typically don't work because of the moisture underneath them, and the wet cob core, but in this case those factors may not apply due to the heat inside the bench and the hollow nature of it. I dunno. I'm ok with a few failures on the way to figuring out what works for this, and like you said, I'm really enjoying the process. Good info on the drip edge, I've built many boat parts that had them, and they make good sense here as well.

The glass I found on ebay. I mounted it in some steel I had here.

Hello Matt Well I bit the bullet and set up a RMH in the garden today. Im so glad I came across your post and talked to you regarding the cob/mud. It was literally a matter of digging some mud, mixing in some grass clippings and away to go! What fun! Reminded me of making mud pies when I was young. The bit I felt a bit iffy about was when the little insects and tiny worms started wriggling out of the cob as it heated up! And then the chickens decided they wanted to join in and de-worm the cob. Anyway, I think you have to use cob to understand the concept of it. So thankyou for removing some of the technicalities and giving me the confidence to 'just do it'. I used fire bricks for the feed tunnel, burn tunnel and the heat riser. Ive been experimenting with the whole rocket system for a few weeks now. Mostly I have used a length of stove pipe, and have found that immediately its placed on the burn tunnel, the draw takes place and the rocket action starts up. I have found with the brick riser that this doesnt seem so apparent. I think it could be down to a few reasons, the fact that the metal is smoother on the inside, the fact that it doesnt have any gaps as the brick riser has, albeit small ones, and the fact that brick takes longer to heat up. When I have referred back to your post, it sounds as though you actually made your riser by forming it around a pipe? Maybe I have got that wrong. Could you explain what you used as a riser and how you did it if you made it?

Hi Carol, sorry for the delay, I missed this post. I create my risers with metal flue. The cheap, thin kind. I use an inner and outer pipe, say a 6" inner and 10" outer, and fill the space between with a wet clay/perlite mix. I think this is Ernie's technology, and it's a wonderful way to get up and running quickly. You will burn the inner liner away rather quickly, so it's important to try to get the mix wet enough to stick everything together fairly well. Hope that helps, glad to hear you are playing with it. It's a lot of fun and helps to show you that you don't need all the answers before you start. Just dive in and you'll answer a lot of your questions yourself as you go. Good luck!

Well, here's the latest and hopefully final configuration for this thing. I had a rare treat and got to demonstrate it to Paul and Jocelyn in person yesterday. I truly enjoyed the visit you two, thanks so much for making time to come see me.

Anyway, this one now features a front loading door that enable batch-loading for longer burns.

Matt, that's very nice!

Hello Matt Thanks for your reply. You have been very busy, and your stove/heater looks great, very professional. The sausage and leeks made me feel very peckish! I love how your chicks and cat are attracted by the heat, but I must say that I think its very dangerous letting your dog sit on the burn tunnel at 1:43 Doesnt he get too hot?! Also your stove is obviously much bigger than it looks in the vid to fit a Retriever in there. We are enjoying superb weather in Wales at the moment so the RMH has been bumped into second place as I have been playing around with solar water heating. Had very good results too. Mind you, the weather is abnormally hot right now. I hope it continues.

Carol, I'd like to think the dog would have enough sense to get down once it got too hot. Ha! I believe the dog you're seeing is actually a reflection in the glass! If you watch the video before that you'll see the dog do the circle thing and then lay down in the grass.

It's actually an incredibly tiny dog, about the size of a hummingbird. I find if I bake them in the burn tunnel for just 20 minutes or so, they come out perfect!

Ha! I thought she was being serious. My apologies.

My apologies are in order if she was! Heath, thanks for the compliment above, I appreciate it.

Hello,
Yes I was trying to be humorous! I thought the reflection looked just as though the dog was sat there, unperturbed. Aww the very thought of that lovely dog being even near the burn tunnel! Sorry to take the thread off point. Time to get off the K9 subject and back to RMSH's? This design looks very efficient. The slabs you have used to finish the bench look like just the job. I find that what is so good about these sytems is that the possibilities are endless and they can be changed and improved upon time after time. My plan is to do just that outdoors and then maybe get a set-up indoors. To go back to the heat riser question, thankyou for your reply. When you say that the inner metal burns away quite quickly, do you then replace the metal thats burned with another length of metal, or do you run it just through the chimney thats been formed by the clay/perlite mixture?

The latter Carol. The clay/perlite mix seems to hold up just fine once the metal liner is gone.

I need to start setting up a RMH indoors and have decided the garage would be the best place to do it. It is a big double garage, built from single brick construction. I think it would benefit from insulating the walls, probably with insulating plaster boards. I would like to set the RMH up first, but wondered, would the cob bench be better against the bricks, or should I insulate the immediate area of the RMH set up and then build against the plaster boards? Or is there something else I could do/use?

If possible I would insulate the outside of the garage as I did with our solid brick single storey extension. I put vertical battens on the wall the thickness of the polystyrene (F- board I think) sheets. Covered that with Klober paper. Then put 10mm thick tiling battens nailed onto the first lot of battens, then nailed the cladding boards. Mine were waney edge douglas fir. Putting the insulation on the outside gives you the mass of the bricks as a heat store. Not much, but every bit helps.