Foundation for rocket oven

I started designing and building my rocket mass oven about two three weeks ago. At that time, I had some free red bricks that I picked up from someone locally and I was thinking about using lime to mortar the bricks. Thanks to some advice I got from folks on here, I changed around my plans. I managed to scrounge some free (and unused) fire bricks. I also managed to source some very pure local clay right out of the ground when I initially thought I had none. These are all good things. Thank you!

Now, on to the next issue: the foundation. I lived on sloped piece of land. It's a pretty dramatic slope. I live in NJ, so winters can be harsh here. Frost heaving is a consideration. My original plan was to dig out about 4 inches of soil, level it, create a perimeter with the red brick, and then fill it in with a bag of 3/4 in gravel and then build the oven directly on top of it. I have two questions here:

Is this enough structurally? Not that there is much structure - the intake and burn tunnel will be brick and the heat riser will be steel pipe, but still.

Is this enough insulation on the bottom of the stove? Someone mentioned (on another thread) that heat could be lost to the ground. This doesn't make tons of sense to me since heat rises, but I'm no expert.

If you want it to last I would fill the hole with 8" of gravel and compact it then I would pour concrete slab with rebars (a must, otherwise it will eventually crack). At least 6" thick.
You definitely need insulation under the floor. Without that it's almost impossible to bake anything properly. Heat radiating from the dome/vault will char the top of the bread/pizza/etc, but the bottom will remain soggy. I built in the past a temporary test oven, from dry stacked red bricks and baked in it tens of breads/pizzas/casseroles. Without insulated floor I had to put the baking dish on the coals, otherwise the floor would suck all the heat, preventing proper baking.

I'm building myself a bread oven (equivalent of 20 cm batch box rocket) with a smoker/(lower temperature oven). I used 6 cm of perlite concrete and then insulating firebricks 23 for the next layer.

For perlite concrete I use:
-10 part of perlite by volume
-1 part portland cement by volume
-not too much water so it will remain thick.
It cures quite slowly.

I also use clay perlite:
30 parts perlite
5 parts clay
7 parts water
Mix the clay with water to create clay slip, add perlite and mix. The resulting material can be also fired and I'm going to try it this week.
The resulting volume of the mix will be around 65% of the used perlite volume.

Cristobal Cristo wrote:
You definitely need insulation under the floor. Without that it's almost impossible to bake anything properly. Heat radiating from the dome/vault will char the top of the bread/pizza/etc, but the bottom will remain soggy. I built in the past a temporary test oven, from dry stacked red bricks and baked in it tens of breads/pizzas/casseroles. Without insulated floor I had to put the baking dish on the coals, otherwise the floor would suck all the heat, preventing proper baking.

Sounds like you're talking about a regular dome oven. I'm building a rocket oven. I'll need insulation in the baking chamber when I get to that point, but I'm mainly asking about insulation between the ground and the bricks that will form the burn chamber (the J-tube part). Sorry for any confusion there!

Also, a rocket oven is almost certainly much less massive than a regular oven. That's probably important for foundation considerations.

Andrew Lubrino wrote:Sounds like you're talking about a regular dome oven.

I'm not building a dome oven. It's a batch box rocket with little bit different proportions and rectangular heat riser - it worked awesome and burned cleanly in primitive dry stacked version. The firebox is my main oven where the most heat will be absorbed from the coals - I like it really hot for proper pizza.
The floor is two layers of firebricks (the second dry stacked with fine sand in joints) and the ceiling is a sectioned vault built from #2 wedges. The heat riser will be heating the second chamber above for lower baking temperatures, so I hope I will be able to use it to finish breads and bake pastry. I will also use it as a smoker. The second chamber also has the vaulted ceiling with a chimney exit close to the door.

If you want to use J-tube then it will be lighter. What will constitute the oven?

Cristobal Cristo wrote:
I'm not building a dome oven. It's a batch box rocket with little bit different proportions and rectangular heat riser

I'm confused. Does the perlite concrete go between the concrete pad and the J-tube? Or is the concrete pad itself made of the concrete perlite? Or does it go somewhere else?

I'm new to rocket ovens, and I'm totally unfamiliar with a batch rocket oven. I spent the last ten minutes reading about it and I'm still unsure where you're putting your pizza from your description. Any good links to learn more about batch rocket ovens?

Pictures will explain it better.
I have used used firebricks so the joints seem thicker than they are. I was laying them on 2 mm joint using mesh 20 sand as the aggregate.
When looking at these images I realized I have poured 12 cm thick perlite concrete. I like to have a lot of insulation in the floor.

Okay, I see what you did here and this exactly answers my question. The answer is 100% yes, I need some insulation  under the oven. Is the insulated fire brick necessary? Or will the perlite do the trick on its own?

Also, your oven looks great. Thanks for sharing!

Conductivity of IFB-23 is around 0.16 W/mK.
1:10 cement to perlite is around 0.06, so 2.5 times better. It's also very cheap as long as you have access to bagged perlite - I get it from a irrigation supply store. Of course the perlite concrete can not work in the flame path, but as a backing, it will work reliably

Hi Andrew,
Christobal is right on point here. Do absolutely use insulation under the core of your rocket, whether J-Tube or other desig. “Heat rises” applies to one of the ways that heat transmits called convection.
It also goes all directions as radiation and through matter as conduction.
Without proper insulation on the core it can not provide the proper temperatures for complete combustion and your oven will probably not work as great and use more wood than necessary.
In the original post you’re saying your riser will be made from steel pipe?
Steel pipe won’t last long, this is the hottest part of a rocket. Firebrick plus insulation is what I use on my ovens.
Do you have a thread about the design of your oven?

Benjamin Dinkel wrote:
Do you have a thread about the design of your oven?

I don't. Should I create one of those? I was originally thinking of building one of the designs from Tim Barker's book. That book's design is very simple, but it lacks some detail and as I read more about other designs, I'm thinking it might not be exactly what I'm looking for.

Any resources on how to pick a design? After some reading, it seems like there are three major contenders: Walker riserless core, a batch rocket, and a traditional J-tube. This oven / stove will mainly be for us to cook with outside during the heat of the summer. The walker oven appeals to me because a. it's all masonry with no ugly looking barrel and it sits closer to the ground instead of having a masonry tower. The drawback is that it seems to produce a lot of radiant heat. For me, that would be lost heat - I'd want everything to go to the cooking chamber.

I could stick with the original J-tube, but it's just so ugly. Not sure the wife will accept that thing sitting in the yard. Thoughts? Sorry for the brain dump.

60-80% of wood combustion energy is radiation.
Out of that: 50-70% is from wood and coals and the rest is radiation coming from the flame.
Dense woods have higher radiation percentages. They burn less violently and may leave more coals slowly radiating energy.

Assuming mid first value of 70% and the second to be 60%, we get:
0.7*0.6=42% coming as radiation from the wood/coals.
So flame and convection carries away remaining 58%. Let's assume pessimistically that one fourth of that will stay in the firebox, so it's 58*0.25=14.5%.
The total energy remaining in the firebox would be 42+14.5=56.5%.
For denser woods it would be more. For longer fireboxes it would be also more, because they would extract more energy from the flame. I use very dense eucalyptus that produces a lot of coals.

It's only theory, but the fact is that all professional bread ovens in bakeries have long baking chambers that are also fireboxes. Also, all the forges have the most heat where the coals are, not the flame.
Thinking about that I decided to built the oven in a traditional way, but at the same time I wanted to have a good clean burning of the batch box. Since around 40% of energy would be carried away with the flame, I decided to direct it to the second chamber which should reach lower temperatures.

Bakery ovens have two chambers. The first one "freezes" the bread at 250 C for 15 minutes and then the top one is used to finish it at 200 C for another 40-50 minutes. With one chamber it wold be difficult to have continuous production. A bread oven with one brick wide walls (230 mm), with 4 layers of bricks for the floor (260 mm) and the vault of two rows of wedges (230 mm), insulated with 200 mm of perlite on all sides, can store enough energy after one heating to bake even 7 batches of bread.

In my case the second chamber can also work as a smoker. Once I finish it I will share my findings of how it performs. My firebox volume is 73.5 l. The top chamber is almost 100 l.
20 cm batchbox of standard dimensions is around 71.5 l.

We bake a  lot and it's our only way to bake anything.

Andrew Lubrino wrote:
I don't. Should I create one of those?

You could. It has helped me to gather information, get questions answered and then also to document the build for later.

To understand better what you're going for, are you looking for an oven or a stove?
I have seen few designs that combine those two things well.

I would choose the oven/stove design first and then find out how to best "power" it or if there are height restrictions etc. and pick the core from that.

A J-tube is also not ugly. Check out my new build if you want.
permies.com/t/280509/rocket-ovens/Cob-brick-oven-rocket-technology#2995590

I'm sorry it has taken me so long to reply on all this. I'm definitely going to start a rocket oven build thread to document everything I'm doing. I've made a few decisions over the past few days and I think I'm really close to starting on this. I've decided to build the oven itself with all masonry - so core + oven and an additional stove top on the core. To do that I'm going to use Matt Walker's riser-less design, which also seems pretty easy to build and sits closer to the ground. One more question I have on the foundation:

I'm on a pretty tight budget with this build. I looked into pouring concrete and it's just too expensive for me, so here's what I'm thinking. I basically live on top of a mountain. The soil is basically pure gravel with large boulders mixed in. The sloped land also assists with drainage.

I'm thinking of using some of these giant granite stones I have laying around for the foundation. I'll dig to a depth of three or four feet (the frost line in NJ) and throw in some of those boulders as flat as I can and then sand on top of each course of rocks for leveling. Does that sound like it will work? Some sources say you should really put down gravel as a base layer. I could do that, but gravel also is not cheap at all, and my soil is already excessively well-drained. Seems silly to put more of that in there.

Where I live, pretty much every single house, including mine, has a stone foundation and was built 100 years ago or more. I doubt they used gravel back then and every house around here seems pretty rock solid.

are you looking for an oven or a stove?

I'm looking for a cooking apparatus with a stove top to cook with pans and then an oven. Something similar to a regular oven that runs on natural gas in the kitchen, just outside and powered by wood.

Andrew,

Stone foundation will be good if done right.
Walker core needs good chimney.
It's difficult to have all in one device with one firebox. I would recommend that you first try different designs for few months, before you commit to something more serious. I have been cooking outside for four years using various stoves made from dry stacked bricks. Observing their behavior and seeing the shortcomings helped me to decide which specific designs to choose. People use different cooking techniques, ingredients, cooking vessels, etc, and all of it affects which stove design fits one's needs the best.

Cristobal Cristo wrote:Andrew,
I would recommend that you first try different designs for few months, before you commit to something more serious.

I didn't think of this, but now that you mention it, that is a great suggestion. Thank you!

For a dry stacked version that I'm playing around with, is it even worth building the foundation? Or should I hold off on that for now while I'm still testing?

I'm planning to build this dense fire brick (not the insulated kind). Will that stand up to the weather (rain, wind, etc.)?

It's good to have some flat base. You could use some paving slabs, or grade the dirt and pave it first with regular bricks, or make soil concrete: 9 parts of soil and 1 part of Portland cement mixed, some water to make it workable, but not too wet and placed like concrete, and smoothed to be level.
To protect from elements, I would leave it covered with tarp when it rains. Once the bricks get wet it takes a few hours to dry ti well with fire.
Experimenting with dry stacked bricks has its limitation, because some design only work if made from proper materials like Walker's core, which is sensitive to the quality of structure, since it has the 180 degrees turn that slows the speed of gases. Other designs burn violently even if assembled poorly. Making these experiments you will quickly find out what works for you.