Rocket Bread Oven Design Advice

Hi! This is my first post on Permies, though I've been reading along for a while now.

Last summer, I built a traditional cob dome oven to bake large batches of sourdough bread in. I could fit 8 loaves at a time, and could squeeze 3 loads out of a big (3+hr) firing. It's fun to use, but the amount of smoke and inefficiency are starting to bug me... I'm now thinking about building a "rocket deck oven", roughly following the plans for a white oven from Tim Barker & Joel Meadows' The Rocket Powered Oven. I figure that I can get two ~20" shelves in my drum, and if I build it 24" deep, that gives me 8-10 loaves per bake. What I'm really excited about is the ability to heat up the oven between batches (no more loaves getting overproofed while I re-fire the cob oven!!). I also like the idea of not having to sweep coals & ash out when I want to bake bread!

Due to space constraints, I'm planning on having both the chimney and the J-tube on the back side of the oven. This means that I would probably have to add some baffles between the barrels to force the hot gasses to the front of the oven. This would create a series of ducts that force the air back and forth in the oven as it works its way up, then down the back and out the chimney. What I am wondering is, should I try to keep the cross-sectional area in each duct the same as the chimney and J-tube? Will adding too many baffles interfere with the ability of the oven to draw properly? I've attached a few design pictures - the CAD ones are for the single baffle design, and the hand drawn sketch is for the multiple baffle one.

Any advice will be greatly appreciated!



The entire oven, with outer drum attached

Outer drum removed to show the baffles and vents. This is similar to Tim & Joel's design, but with a bottom vent to accommodate the J-tube in the back.

Just the baffles and chimney / heat riser.

A sketch of the multi-baffle system to heat the oven more evenly. Will this hamper the flow/draft too much?


Cheers,
Will

Welcome to Permies Will!

Thanks for taking the time to make such a great first post!  The cut away diagrams are really helpful.

As on owner of a Barker design rocket oven, I think making bread in it is a great idea.  We mostly use ours for pizza, but bread (and chicken - I want to cook a chicken!) should work well.

A few thoughts that may (or may not ...) be helpful:

1) That may be more baffles than necessary.  A single baffle that forces the air to the front and then up should work.
2) your final exhaust tube should be much closer to the bottom of the divider panel ... maybe 2".  Where your quick sketch shows the exhaust, it is too high and the back of the oven won't be heated well
3) Not sure what your space constraints are... but remember, this stuff doesn't have to be co-planar!  you could have the feed tube and burn tube at a 90 degree angle to the barrel, and that would keep you closer to the original plans (HIGHLY recommended for a first build)
4) flow area between baffles ... This is planned as a 6" system - be sure the feed tube, burn tube, riser conform to the formulas and make sure the final exhaust is also 6".  In between the two, so long as the cross section area is greater than 6" you're fine.  
5) I regularly hang out at a bakery with a massive wood oven... commercial sized, hundreds of loaves a day.  They do a daily burn ... and then it sits for about 16 hours before the bread goes in.  Its all mass.  The rocket oven is a great pizza oven because it gets hot and you don't worry so much about temperature fluctuations while it cooks.  But bread .... you might find that adding a bunch of mass to this will help you maintain a temperature.  Something  as simple as filling the bottom with pea gravel, firebricks or cob.  You might find that you even want to make the inside of the oven square and fill the edges with some mass.

Thanks for the quick reply Eliot!

I'm trying to make my design closer to the Barker plans, but also use the base of my existing cob oven. My base is a 3.5" thick 46"x46" reinforced concrete slab on top of cinder blocks. I was considering cutting a hole in the concrete for the heat riser, but I have no experience with that, and there's a good amount of rebar in the pad. Right now, there is 8 inches of pumice and old bottles on top of the slab, followed by a layer of bricks and the cob oven. Here's a picture of the whole deal (excuse the melting cob! A light rain caught us by surprise...):





To keep the rocket stove as close to the Barker plans as possible, I could place the J-tube on the side of the existing base, and then add an elbow at the top of the heat riser to direct the heat into the bottom of the barrel oven. Here's a picture of that too:




Does anyone see any problems with the extra turn in the J-tube? The picture makes it look longer than it will be – it should only be ~8” of tube, but 24” from one vertical wall to the other.

Also, I was planning on making an 8" system, instead of 6" like in the book. A few reasons for this:

A) I'm making a deeper oven (mine will be 24" vs Barker's 17")
B) I'll have a bit more mass in the oven, with 3 shelves of 1/2" thick clay tile. All together I'll have something like 30 lbs of clay in the oven.
C) I really like pizza, and I love how my cob oven right now can make perfect pizzas in 2 mins. I'd like to have this functionality in my rocket oven, so I need the oven to get hotter than 700f. From what I've read, this seems totally possible, so long as the J-tube is big enough.
D) I bake bread at 500f and want the shortest possible reheat time between batches.

Does this seem reasonable, or would a 6" system be sufficient? I'd hate to build the whole system and then realize that I made it underpowered...

Finally, does anyone have any suggestions on what to build the heat riser out of? I'll make the burn tube out of old clay bricks, but I don't have enough to make the heat riser out of them as well. I figure I can recycle all of my cob that hasn't vitrified, add lots of pumice or perlite, and cast a riser. My only gripe with this is that the sand from the cob will make the mix less insulated...

I am located in San Luis Obispo, CA if anyone knows of any great material sources around here!

Cheers,
Will

Wow - that's a beast!  I'm going to joke that you want to reuse the structure mostly because if you don't, it has to be hauled away!

I have no experience cutting through rebar and concrete - but yeah, if you could do that ... then the rocket core could probably* be sited in the current wood bin.  The barrel could be nestled into your pumice and bottles - maybe strapped to the slab.  The * is dependent on the height.

If not ... making a turn AFTER the riser tube... I'm going to say maybe and hope one of the rocket gurus chimes in.  If the horizontal part is the same cross section, I'm wondering if this effectively makes the riser longer than optimal.  Typically after the riser the exhaust is allowed to expand into a higher volume area, to slow down.  I'd have to think about that aspect some more ... However the general idea of making the exhaust go through a few bends is fine - RMHs with a barrel go up, down, sideways, wander through a bench and then finally turn up a final elbow and escape.

For your oven specs, I'd worry more about insulation than an 8" system.  8" is a beast - nearly twice the cross section of a 6" - and actually too much heat is a problem for you!  My 6" wanders past the 700 mark with good fuel, no problem.  I'd suggest the size increase you are considering is entirely in the range of a 6" core.  And everything on an 8" is more expensive.

We also have 1/2" stones for the pizzas - the oven really doesn't seem to care.  I suppose they slow it down a little, but they also don't provide that much residual heat/mass/temperature stability.  Since you want to bake and flash pizza I'd suggest some mass that you could easily add for bread, remove for pizza.  And by mass I mean a lot more than clay shelves!

For risers, consider the 5-minute riser... a metal tube lined with fancy insulating roll.  It takes like ... 5 minutes ... to make one, the insulation really helps the core get going when cold and its easy to inspect and replace if necessary.

Haha yeah I don't want to do any more sledgehammering than absolutely necessary!

I like the sound of the 5 minute riser, thanks for showing me that! As far as insulation on the rest of the oven, I was just planning on using an insulating cob mix. Do you think that this will be insulating enough, or should I go high tech and get some extra ceramic fiber/superwool? My only worry is that costs will add up quickly with the ceramic fiber... What did you use for insulation in your oven?

Mine is insulated with simple rockwool.  And sheltered from the rain. Its only the really high heat areas of burn tunnels and risers that need fancy insulation.

Cob is not so much insulating as absorptive - it can be a heat shield, but it will steal a lot of heat from the oven.

Hi again!

I've been working on and off on my oven for a bit now, got a test fire yesterday and the bent rocket riser worked! When you say simple rockwool, are you talking about this stuff? (https://www.acehardware.com/departments/building-supplies/insulation-and-house-wrap/insulation/5027137) I had read that the binder in rockwool offgasses around 600f, but that might not matter if it is contained... I am planning on wrapping the whole oven in insulation and then putting some chicken wire and cob on top of everything, for durability and looks. Do you think that this will work?

Here's a picture of my oven so far, it's been an absolute blast to build!



Cheers,
Will

Re. insulation...

I used ceramic fibre blanket on the inside of the outer barrel:





I also used it on the outside of the barrel, and then covered it in a layer of building 'foil' insulation. The cfb is 25mm thick - in the future I'll use just one 50mm layer on the inside of the barrel.

Your situation is a bit different, but the ceramic fibre blanket is quite cheap and will work for you.

If anyone ever takes this cfb approach to insulating a rocket oven, note that the blanket holds its shape inside the outer barrel perfectly. I.e. no support is needed if it is worked in pretty tight...