Cob and brick oven with rocket technology

Thanks Thomas.
First I’ll build a J-tube with stratification chamber in November.
Then maybe a batch box in our house, once we finally signed the contract.

A little update from the pizza rocket.
I am very pleased with the performance. Heats up in roughly 15 min, fairly easy to start.
There is some smoke sometimes and some soot collects on pots ect.
But I really enjoy baking and cooking in and with it. Bread in clay pots, oven vegetables on a sheet and some beans or lentils all at the same time with the oven rack system.

Oven packed

In the picture you can see the slits where the hottest air comes out from under the "pizza tile". When using a tray to bake potatoes or other vegetables I like to prop it up with 2 of the T-bars so it's not in direct contact with the hot tile.
And the bread goes up on the rack.

Oven rack system

The rack system are just bricks with cuts and t-bars that span. Like this I can put something on the bottom and something on top. And switch it up according to need.


When making pizza I noticed the pizza didn't get enough heat from the top. Understanding physics was key to come up with a solution.
The hot air does collect in the vaulted ceiling and needs to exit at the bottom of the oven. So the ceiling does get nice and hot. So far so good.

Oven vaulted ceiling

But the ceiling is far away from the top of the pizza and heat radiation is governed by the inverse square law.
So I took a thick metal plate, cut it in half to fit through the oven door and put it on the upper rack when making pizza.

Metal sheet for higher pizza radiation

Metal sheet in position

This way the top of the pizza gets crispy cheese at the same rate as the crust gets perfectly browned.

Dear Rocket Scientists,
a neighbor want's me to build an oven for her. Pretty much a copy of the one that I built.
I'm excited about it but I have a doubt.

My oven gets pretty black on the inside. I know, it's a black oven. But I was wondering whether that could be alleviated with a longer riser? It is a sign of incomplete combustion after all, correct?

In the current design the riser is 85cm (33"). I could get away with making it a bit higher at the cost of burying the core some 20cm (7").
Any thoughts?

Ben, in my Walker BBQ oven, I have two exits.
With one up high to create a white oven and a second at floor level for a black oven.
A brick blocks the upper exit when smoking food.
Being all brick, it holds cooking temps well, no matter which exit I use.
Of course, Matt's riserless core design burns very clean, no matter where it vents.
I actually add hardwood slivers on a cast plate inside the oven to smolder.

To answer your question, a taller riser will undoubtedly help.
Is there a way to add a second exit and still maintain oven temps?

Hi Thomas,
I don't really know what you mean by second exit. In the oven itself? Or from the riser into the oven?

In my design, the hot exhaust hits a clay tile that sits about 1,5" above the riser and then spreads the heat around the oven.
The hot gases rise, give their energy to the brick, cool down and are allowed to exit through the chimney that is open at the hot tile level.

Is there a minimum riser height? Apart from being 3 times taller than the feed? Could a longer burn tunnel also help?
It increases the Time (3 Ts: Time, Temperature, Turbulence) the gases spend in the core if I understand it right.

Hi Ben;
My 6"x6" riser exits through the floor and into the brick oven.
At the ceiling of the oven, there is an 8x4" brick that is easily removed.
With it out it allows any smoke to quickly vent.  (With a riserless core, there is very little smoke)
If I insert the upper brick, any smoke exits at the floor of the oven, thus creating a black oven.
Allowing the oven to vent from the "white" exit does not significantly lower the oven temperatures. The heat is stored in the bricks, not the hot air.

As far as J-tubes, when used as an oven versus a mass heater.
I have not heard of a minimum riser height, although I would guess 30" or so.
Do not extend the burn tunnel; it is a terrible idea... (I tried & had to rebuild in January)
Burn tunnel roof length should be 10" - 12", absolutely no longer.

The neighbor/client came up with the idea of an ash clean out for the "Pizza rocket". It is quite important for them.
I was thinking of leaving out a single fire brick in the front of the core. Any opinions on the feasibility of that?



I would then mold the cob around a tile or something blocking that opening.

I understand that they would like to be able to sweep out the ashes on the straight path through such a opening instead of digging in the firebox. I would only suggest to carve some two finger cavities in the face of the brick, so it can be pulled out with two fingers, without attempts to pry it out (that would chip it sooner than later).

I understand that they would like to be able to sweep out the ashes on the straight path through such a opening instead of digging in the firebox.

Yes, that's exactly what they have in mind.

Good idea with the carving of the brick. I might add some kind of handle and insulation maybe.

Hi Ben,

Great work!

I have been looking to do something similar for classic neopolitan pizza cooking. This needs oven base to go around 370 - 400 deg C.

I would be interested to know if you can achieve such temperatures with the j-style rocket burner?

Loz Harrop of the UK is a great guy - he's a knife maker but has made some good rocket stove heaters and a pizza oven too.

https://www.youtube.com/watch?v=YUkJHIx43M0&t=928s

See the above - his little 5" L-tube has  15" riser and does a fantastic job. His is a very light weight setup so its easy to run up to 400 deg C in about 15 mins.

I was wondering if your oven can reach such high temperatures?

Do let us know.
Thanks.
Ankit

Hi Ankit,

I reach normal working temperature in about 15 mins.
For Pizza I heat up about 30 mins and then the oven is ready to go.

With the addition of the steel plate I get a crispy crust and browned cheese in about 4 mins per Pizza.
But I can't really tell you exact inside temperatures.