Rocket cob oven

[Gulf]

Re: Rocket cob oven

A bucket full of large marbles will weigh the same as a bucket full of small marbles.

I am breaking my own personal rule of posting after three drinks, but what the hey:

My 9th grade science teacher presented our class with a container full to the brim with large rocks.

He asked us "if the container was full"? We all agreed that it was.

He then, transfered the contents to a larger container and poured in some pea gravel. He shook it up and poured all of the contents back into the original container and again it was full to the brim.

Same question was posed and we all agreed that it was full.

He then poured all of the contents back into the larger container and then added some sand, He again shook it up and then poured the contents back into the original container and again, all of it's contents filled the container to the brim with none left over.

He asked the same question again. By this time nobody would utter a word.

He then poured in some water.

I may have missed something from what he was attempting to drive in to our youthfull heads, and I will admit that we did not weigh the container after each fill but, I bet that a bucket of small mables weighs more than a bucket of large marbles.

Just Sayin"

[david s]

Re: Rocket cob oven

The large rocks in the jar and then the gravel poured in to fill the spaces between is exactly the principle used for making concrete where the large aggregate leaves spaces between which are filled with sand and the spaces between the sand are filled with cement so the resultant total volume is much the same as the volume of the heavy aggregate alone. In the case of vermiculite the course stuff I get has both course medium and fine particles, which makes its total density much the same as the fine grade. (Both bags weigh the same) If you could get a course grade that didn't have the fine grains in it as well then it probably would be a better insulator. Maybe this is why perlite is a better insulator than vermiculite because most of the grains are of a more uniform size (at least the ones I get are)

A bucket of small marbles still weighs the same as a bucket of large marbles.
Any physics experts out there to confirm this?
I'll shout you a drink if I'm wrong.

[Gulf]

Re: Rocket cob oven

Well, I have been told many times that I have lost my "marbles" and will nolo contendere on that subject I have no "data" or "physists" to consult. However, I came to the understanding that course vermiculite was the better insulator compared to fine by reading many threads on this site. It seems to be the consensus. So far I have only installed a 4"-5to1 portland vermicrete floor under my dome. For the top 1/2" I used horticuture grade (fine vermiculite) only because I ran out of the course grade. I later sought out and was given enough 2" CalSil to correct what I thought was an inferior 1/2 inch of insulator. Do you have any data to confirm that their is no differnce? I am refering only to vermicrete, not loose fill.

[david s]

Re: Rocket cob oven

No, I don't have any data on this that's why I asked "is there any data on this?" I've tried both fine and course vermiculite to make vermicrete and it seems to perform the same to me aand as the bags of fine and course weigh the same they therefore have the same densityand should provide the same insulation value,lthough I've not conducted any tests, that's why I was asking.

[arthuritus]

Re: Rocket cob oven - back to topic

Beautiful oven there mark. I've attached my design for my cob oven mk II. Let me know what you think. Nothing is to scale, just to give an idea of general layout.

Does anyone know about optimization of a rocket in terms of height to diameter? There will be a tradeoff whereby increased height will allow more complete burning, but more heat to escape.

Also, I'd like to find an alternative to stainless. It will be expensive to get that fabricated. I'm already using mud because I'm too poor to buy bricks.

I couldn't figure out how to do it in my diagram without doing a full set of orthographic drawings but I'll try to explain it here; I'm thinking I'll make the hearth component (metal plate and fire bricks) flush with the cob walls on the front and maybe the sides too, so all the hot air from the rocket will be coming up the walls at the back of the oven and have to travel all the way forward to get out the door, heating the oven etc as it goes. This also means I don't have an annoying gap at the front.

I'm aware that there are proper plans out there for rocket ovens but I haven't managed to find any that use cob with rocket technology. Any thoughts or suggestions on the matter will be much appreciated.

[Archena]

Re: Rocket cob oven

Try looking for rocket stoves as heating- I've seen it combined with cob that way.

I'm sick as a dog right now but if you don't find anything let me know and I'll hunt down a link or two when I'm better.

[arthuritus]

Re: Rocket cob oven

I did manage to find this; Rocket Oven.mov - YouTube
Its a bit long winded but there are some very interesting ideas in there.

Building all the nuts and bolts out of clay sounds like fun BUT theirs is tiny. If anything cracks they can just lift the whole dome off or knock out some bricks at the bottom and they have access to everything. A decent size oven would need proper foundations and getting access for repair job could be a nightmare. Plus I want everything to look closed in, just like a normal oven.

At some point they comment that the bottom of their pizzas were getting burnt, so they had to reduce oven temperature and take almost 5 mins to cook their pizzas. I think this is because all their heat is going direct to the underside of the hearth. It shouldn't be a problem if I just use the rocket for original heating and still have a normal fire inside the dome as well.

Also, take note cobbers; they add sugar or potash to make their clay more workable. I hadn't heard of that technique before.

Anyway I was very impressed with what they managed to do with basic materials. Looking forward to your observations.

[shuboyje]

Re: Rocket cob oven

The oven in the video does look pretty similar to what you described. By constricting the rocket exhaust to the rear of the oven and venting out the front I think you will get better performance then they do. I'd also go with a very low dome because odds are you are still gonna lack top heat, but it will turn out a much better pizza then a home oven. You should look at the little black eggs people build from old weber grills. The heat source and materials are very different, but the path of the fire and heat through the oven is exactly what you proposed. You will find they struggle with top heat, but with enough tweaking do work good enough to produce a 2 or 3 minute pizza.

[Archena]

Re: Rocket cob oven

Rocket Stoves

Homegrow Evolution

Design Tool - Firefox Only

This should be the one you need:
Rocket Baking Oven Construction Manual (PDF, 1.6MB)

[shuboyje]

Re: Rocket cob oven

That design is for a very basic white oven, as is every other design I've seen for rocket stove ovens which generally use metal drums. I don't think that is what the people in this thread are looking for, and I certainly don't see it producing pizza any better then a home oven if that is the goal.

[Archena]

Re: Rocket cob oven

Um, yeah - the advantage to a rocket stove is that it uses relatively little wood. In places where wood is scarce that's extremely important. It wouldn't be better than a black oven or another white oven - it's only advantage is fuel consumption.

I suspect an off center configuration would be better. I don't see any way to not have a hot spot so the better bet is to make that work for you. I'd put the hot spot to the back where the fire would be in a black oven. I'd also mark it in some way so I knew where it was for certain. The oven should work normally and you don't have to cook around a big hot spot in the middle. (Hmm, maybe to the side would be better - it could be used for searing and other really fast cooking...)

If working time isn't at issue, anything they did in brick can be done in cob - it's just no where near as fast. I'd also leave an access (unmortared brick, maybe) to the open section under the hearth. If the rocket stove doesn't work out, insulating that section would be simple and you'd have a standard black oven.

Hmm, you could probably substitute a chimney flue for the brick upright chamber...

[arthuritus]

Re: Rocket cob oven

Most of this video here Permaculture Our Urban Design Part 7 - Rocket Oven Pt1 - YouTube is not really of interest to me but at the 8-9 min mark he shows how hes attaching the rocket stove to the oven.
He uses an cradle slung underneath the main horizontal slab (the base of the hearth) with adjustable screw thingies, I cant remember what they're called. If you already have a woodbox space underneath it would be a very simple matter to remove the cradle and access everything for repairs. To me, that tiny bit of genius made watching all 20 minutes (part 1 and 2 : O) worthwhile.

With this cradle design in mind, I think my plan of attack to make the chambers of the rocket part would be to either use chimney flue covered in cob or cast something out of perlcrete.

Regarding the hot spot, I think a combination of a flared opening under the hearth, a ceramic deflector sitting on top of the opening and the metal base to the firebrick hearth should help to distribute the heat evenly. And as you point out Archena, a hot spot can be put to good use...

[arthuritus]

Re: Rocket cob oven

Also, if anyone does take the time to watch that (and part 2), I was very surprised he could only get it to 250 degrees C.

First of all let me state that I only intend to use the rocket part as a turbo, to speed heating and to increase my overall fuel efficiency. I will still have the main fire going up top.

However, I expected a bit more grunt from the rocket. Something that maxes out at 250 C would hardly be worthwhile. From what I've seen in other videos Rocket Oven.mov - YouTube they can do much better, apparently getting up and over 750 degrees C.

I will take a few guesses as to why his didn't work too well. Please feel free to shoot me down (you won't even have to watch the video for this) and to come up with suggestions of your own.

1) Chimney in the centre of dome. Most ovens have them at the front. Given that heat rises, wouldn't the hottest air be escaping if the hole is at the top?

2) Deflector plate at bottom of chimney made of tin. Seems like this would do little to reduce heat transfer up the chimney (such a good conductor) and would reduce draft.

3) Too small an intake for the rocket.

4) Small thermometer about 3/4 up the dome and a paver instead of a firebrick hearth. The heat is concentrated at the bottom of a rocket oven anyway, but because he has an ordinary paver for a hearth I think this problem is compounded because the paver is not absorbing and radiating the heat properly. That paver could be scorching hot and he'd never know until his veggies caught fire.

Anyway. Thats my 2 cents. Looking forward to reading a more educated analysis.

[Archena]

Re: Rocket cob oven

Most of this video here Permaculture Our Urban Design Part 7 - Rocket Oven Pt1 - YouTube is not really of interest to me but at the 8-9 min mark he shows how hes attaching the rocket stove to the oven.
He uses an cradle slung underneath the main horizontal slab (the base of the hearth) with adjustable screw thingies, I cant remember what they're called. If you already have a woodbox space underneath it would be a very simple matter to remove the cradle and access everything for repairs. To me, that tiny bit of genius made watching all 20 minutes (part 1 and 2 : O) worthwhile.

With this cradle design in mind, I think my plan of attack to make the chambers of the rocket part would be to either use chimney flue covered in cob or cast something out of perlcrete.

Regarding the hot spot, I think a combination of a flared opening under the hearth, a ceramic deflector sitting on top of the opening and the metal base to the firebrick hearth should help to distribute the heat evenly. And as you point out Archena, a hot spot can be put to good use...

Sounds good!

Do you have "Earth Ovens"? It has a chapter on insulating with cob that would probably be of interest.

[Archena]

Re: Rocket cob oven

Also, if anyone does take the time to watch that (and part 2), I was very surprised he could only get it to 250 degrees C.

First of all let me state that I only intend to use the rocket part as a turbo, to speed heating and to increase my overall fuel efficiency. I will still have the main fire going up top.

However, I expected a bit more grunt from the rocket. Something that maxes out at 250 C would hardly be worthwhile. From what I've seen in other videos Rocket Oven.mov - YouTube they can do much better, apparently getting up and over 750 degrees C.

I will take a few guesses as to why his didn't work too well. Please feel free to shoot me down (you won't even have to watch the video for this) and to come up with suggestions of your own.

1) Chimney in the centre of dome. Most ovens have them at the front. Given that heat rises, wouldn't the hottest air be escaping if the hole is at the top?

2) Deflector plate at bottom of chimney made of tin. Seems like this would do little to reduce heat transfer up the chimney (such a good conductor) and would reduce draft.

3) Too small an intake for the rocket.

4) Small thermometer about 3/4 up the dome and a paver instead of a firebrick hearth. The heat is concentrated at the bottom of a rocket oven anyway, but because he has an ordinary paver for a hearth I think this problem is compounded because the paver is not absorbing and radiating the heat properly. That paver could be scorching hot and he'd never know until his veggies caught fire.

Anyway. Thats my 2 cents. Looking forward to reading a more educated analysis.

Unedumacated analysis:

From what I've read a true white oven shouldn't have a chimney at all. I suspect all of your objections have merit but that #1 is the primary culprit. No ovens are built with the chimney centered - at least none I can find (and around here it'd be the first thing people would try talking you out of). Tin or otherwise, no deflector would overcome the heat loss - the best it could hope for would be to slow it down a little.

If I were him, I'd stuff an insulating blanket in the flue and see what happens. I suspect he'd be filling in the chimney a short while later...