Cooking on a 4 inch batch rocket stove

Good on you for preserving but if you think about the size of fire needed to heat a full on pizza oven and the amount of BTUs required to saturate 250kg of solid mass to 500c, i think your little 4” rocket is doing ok!
Perhaps if you built a 6” air tight model it would be a lot closer to your goal.

Yes, a rocket stove has not yet been shown to be able to heat up a fire brick to 500 deg C which is why I switched to using a plate of cast iron with a terracotta tile instead.
The final problem I'm having is that the top edges are not browning but the bottom is.  This shows that there is sufficient heat to toast the bread but the edges are not getting close to that heat source.
When I open up the oven door the draft is broken and the fire shoots up to the top.  I want to draw the fire closer to the edge of the pizza bread to toast the edges.  So, I think I should try and make a deflector plate at the top of the riser to push the flame closer to the bread.


Correction: I missed this thread https://permies.com/t/88746/Rocket-Stove-Black-Rocket-Oven which uses two 6 inch rocket stove to heat the oven up.  The poster is MIA so we don't have a video of it in action.

And I forgot about this post https://permies.com/t/94502/rocket-ovens/pizza-stone-cracked-rocket-stove#775360 which shows browning of the edges of my pizza!  So, what's different ?  

Graham.  Don't yiu think it's about time to build something  mortared and a bit more on the normal troden path?

A batch box with the cooking plate above the firebox, and  an oven on top of the riser for example.

Or a bigish J with cooking plate above the riser, and an oven to the side?

Max, I have a house for the daughter being built at the rear of my section.  When it's finished we will build something that is permanent.
I even have Matt Walker's stove plans as a fall back.
In the meantime I get to play crazy fire scientist and cook!




I envision the final stove will go at the base of the hugelkultur to the right .. so approximately in the middle of the image where they're digging up some clay to move to under the deck.

Satamax Antone wrote:Graham.  Don't yiu think it's about time to build something  mortared and a bit more on the normal troden path?

If you aren't using mortar and your plaything is almost working, it will haul ass with mortar.
Have you ever seen how much of a negative effect a vacuum leak has on a gasoline engine?

The clay slip & sand come off neatly with a short bristled wire brush if you need to rearrange.

That's the plan.  Get the best working model with leaks, and then seal her up.
I have the fire clay, the sand, the ceramic fiber board, ceramic fiber blanket, the glass fire door, and the matrikote just arrived .. I just need to get access to the site once the builders leave.

Today I fired her up using Peter's start up method ( kindling near the Venturi, then more wood, and then fully load when everything is burning well ) and I think with the addition of some hard wood the fire was much better.
I had flames shooting up the riser across the top of the oven for a lot of the pizza cooking time and it wasn't hard to brown the edges.  I did burn the bases a few times but I was trying to juggle too many things at once as well as be the cook.  I'm very encouraged by this.  And in the start up phase, I cooked some chicken on skewers in the riser.  Wife didn't want me to use potatoes at the end of the skewer but after the first chicken wing slid off falling to the bottom of the riser, we reverted to using the potato trick and lost no more!

And an hour after we had finished eating the oven was still over 230 deg C.

It’s all good fun ah...
I cook tandoori style down my riser (cement riser) it is probably the most appreciated dish I do.
I use marinated chicken breast, works best when the fire is gasifieing and the riser top sides are around 400c.
I made a little grid to hang 4 skewers on and drop them down the riser, takes about 3 minutes to colour up and cook.

Duck legs un the rocket oven. 4 minutes each sise, approximately.

It's interesting that cooking in or over the riser seems to be much faster than over a hot gas grill. The temperatures must be much hotter

I bought a couple of balls of pizza dough from an Italian restaurant and split them into 4 balls to make pizza.  Although I had a lot of difficulty getting that dough into a round shape it still worked well enough.  The sides puffed up nicely.

But the "pizzza stone" cracked again so it's now in 3 pie-ces.  I guess I really need cordierite.  The cast iron griddle being used for the roof of the firebox is buckling under the heat which isn't helping.  The stone reached 350 deg C or 662 deg F.

@Max, what thickness is the steel you're using for your cooking plate?

I decided to remove the steel from the firebox roof and replace with standard firebrick. As expected it took several hours to get the brick up to 230 deg - 300 deg C for cooking pizza.  But then it takes 15 minutes to cook as opposed to when I had the metal over the firebox and pizza stone on top.  Then it took 5 minutes.    But now the floor of the brick oven remains at 220 deg C for the next couple of hours even though no more fuel has been added.  It's the heat from the residual coals which are keeping the bricks hot.

So, I guess for any brick wood fired oven you have to plan what you want to cook - roasting potatoes and chicken wings as the fire comes up to heat, pizza when it's maximally hot, and then use the residual heat to bake bread.  The only advantage over a traditional wood fired brick oven is that I generate far less smoke as it fires up.

Graham, just curious...do you know about how much wood you burned for the video where you baked the chicken, the potatoes, pizza, and boiled your kettle? That would be a nice data point.

Phil, no sorry I didn't weigh the wood but can do this next time. I'm really only using pallet Wood except for kindling that I bought. And I doubt I would have used more than half of a pallet.  The home stores, as you know, no longer sell firewood since it's the middle of our summer.  One has to go to a specialist retailer to find it.   So, I'll be scouring the city today for free pallets.  But I've noticed a trend. Many pallets are now made of plywood which I presume we can't burn because the adhesives might release formaldehyde.

I'd expect formaldehyde to be fully combusted in a rocket setting. Probably the most environmentally responsible thing to do with ply and MDF.

I was in town yesterday and grabbed a couple of HT treated pallets from Bunnings. I forgot how long it takes to pull these apart!  I burn them with the nails.  And even one pallet has a decent amount of fire wood but it all has to be cut to size.  And split finely otherwise it tends to smoke.  I guess that's because these pallets are often left out in the rain.

If I have to cut up a pallet, I use an electric jigsaw to break it down and a chopper to finish it off.
Luckily my own workshop produces quite a bit of hardwood off cuts and my local joinery shop sells bags of hardwood for £1.50 a bag.

Sadly I don't have access to virtually free hardwood. I've seen this neat tool to rip pallets apart but until I get one of those I'll just keep using my battery powered hand saw.

Phil, I roasted some potatoes tonight just for you! I wasn't able to weigh the wood consumed but it was just under a shopping bag full of pallet wood. See the video for details.

My wife wants me to try roasting a Peking duck.  But that means the duck needs to be hung in the oven.  So, I raised the roof by another 50% but I think that might be reaching the limits of this 5" batch rocket.  After 3 kg of wood, and 50 minutes of burn, I was only getting 220 deg C on the roof of the oven, and 120 deg C on the floor.  Maybe I need to line the oven with some insulated steel?

Just more data points.  I made a wooden door from hard wood ( sustainably harvested left overs from the deck construction ), and lined the hot face with a used aluminium tray.  That has seemed to stop the top part of the door from burning from the heat but it's early days. I found that soaking the door in water didn't stop it burning.

After 1 hour I'm hitting 220 degrees on the brick floor of the oven, and at 1.5 hours I'm reaching 290 deg C.  The burn chamber gets to about 800 deg C.  Smoke seems to be consumed mostly but it's tricky because there's very little riser when cooking in the top box of a DSR2 like appliance.  After 4 hours with no added fuel the brick floor is still at 100 deg C.

I'm still wondering if I should attempt to roast the Peking duck I prepared over the last 2 days.  At 33 cm from neck to tail it's actually too long to hang in my electric oven  Fortunately duck only needs 180 - 150 deg C to cook.

Well, I'd never roasted any bird in my life before let alone Peking duck and in a rocket oven.  But here's the video warts and all.  I learned that

1. The oven is too small even though I enlarged the height by 50%
2. Two meat hooks can't substitute for a proper poultry hook
3. There's too much temperature differential between the top and bottom of the oven

So, I ended up taking the bird off the hooks and roasting it lying at various angles in a small flat roasting dish as I kept turning it to avoid burning it.



I also leaned that it's cheaper for me in terms of ingredients to buy a Peking Duck freshly cooked from the deli downtown though they only sell them once a week.

Wife thought it was great and I'm supposed to do a replication test in another couple of weeks!

Good effort Graham, I do find your vids a little stressful though, I end up wishing you had a nice rocket stove and a nice wood fired oven!
How much longer before you can make a more permanent version?

I'm still just trying to figure out what's possible. I think I'll reduce the size of the oven again and keep it like that. I can roast duck on its side.

The issue facing me at present is that I'm using what is really the secondary combustion chamber of a DSR2 for cooking. So that causes smoke. But if I insulate the really short riser with ceramic fibre blanket will I then lose the benefit of mass, and gain very little?

I recall you mentioned something like using CFB means there's no thermal inertia in your system.

As an additional data point, I fired up my electric oven today, and put in two pizza stones.  I could get the stone up to 200 deg C and the pizza, although it rose, was doughy in the middle, and the base was not browned after 15 minutes.  I had my pizza sandwiched between the two stones.  The cheese did brown though.  My electric oven maxes out at 250 deg C and I waited 25 minutes before putting the pizza on the stone.  So, in that respect the rocket stove is looking much better!

I merged your stuff with the following thread. I hope that is okay by you.

After spending a lot of my time on my hands and knees tending a batch box, there are definite advantages to having a top loading rocket stove!  But the constant tending to for a J tube is a pain, and the possibility of smoke/fire back puts me off them.

The standard batch box dimensions aren't great either for cooking as for a 5 inch batch box, you need a riser 3 foot tall.  So, you then have to build a platform to stand on to cook.  If you shorten the riser so that the gases are taken sideways for the rest of their length, then the brunt of the gas flow is now no longer directed at the bottom of your cooking pot or surface.  And you've stolen heat from the secondary combustion so your fire burns less efficiently.

So, maybe there's still a place to look again at the mag rocket stove?

Graham, you just need a bigger fire box with a lid!

Fox, I watched that video but looks like a lot of metal work involved.  So, it seems he has a primary insulated combustion chamber which he uses to gassify the wood, and then he burns the gases in the riser using secondary air from somewhere.   My home's commercial wood burner is also a gassifier and that much wood would certainly not last all night let alone a hour!

He has lots of videos including one with all the dimensions, the riser is only 29” from the base to the top so it would fit in a barrel just fine.
However he does use quite a high chimney but even then he says it will still work fine with a much shorted one.
Anyway that format suits me and it has been tried and tested for 5-6 years now!
My new fire will be based around that design and as I don’t weld, I won’t be useing much sheet metal, more ceramic board and bricks.
I have been useing my fire in gasification mode for a few months now, I blast it with full air for an hour or until I have done my cooking and the room is warm and then seal it down with just secondly air. I just wish I had a much bigger fire box!

Graham, you want a cooking rocket. Build one like mine.

Max, I have several working rocket stoves for cooking now. I just want to refine the design to suit cooking instead of space heating.  I think you use the roof of your combustion chamber for cooking, and have a black oven inside. I'm not enamoured with the way a batch box has to be fed, and I'm wondering why I can't seal the front leaving only primary air open, remove the roof, refill with wood and close it again. I'm using these stoves outside so it's less hazardous than if I were inside. I'd imagine a removable lid made of steel with the hot face covered with ceramic fibre. So that would make cooking over the combustion chamber tricky! Since these stoves act in batches, we're only talking of loading fuel every 40 minutes or so.

Oh, and what prompted this was that the other day while my batch box was burning, I took the fire bricks off the roof to place a piece of ceramic blanket there. I was a little surprised to see that the flames were still running horizontally diving through the Venturi rather than changing direction to head to the sky. So that makes a removable lid a more viable option.

This 2015 vid talks a bit about secondary air and also shows it working but not until right at the end...

Thomas, now you have your P plate fitted, you could easily play around with gasification just by fitting a lid.
In my case I just have a piece of glass that fits up to the plate.

Fox James wrote:This 2015 vid talks a bit about secondary air and also shows it working but not until right at the end...

I started watching near the end as it's 30 mins long.  I didn't really see the secondary air - he needs a big arrow pointing to what he means!  His temperatures aren't that high either; 400 deg C at the top of his riser when I believe Peter was getting 1300 deg C.  And his explanations were also a bit inaccurate I believe on what happens in the riser.  But that was taken in 2015.  But I'm still tempted to try gassifying!

Graham Chiu wrote:His temperatures aren't that high either; 400 deg C at the top of his riser when I believe Peter was getting 1300 deg C.

Not entirely true: 900º C (1650º F) at the top of the riser and very, very close to 1200º C (2190º F) 25 mm in front of the wall opposite the top end of the port. Difference is still huge, though.

Hi Graham, getting 400c while gasifing small amounts of wood is pretty good in my books!

I don’t know what you want to see from your stove?
A top loading compact stove that has a big fire box, good cooking hight and economical wood consumption all appeal to me.
Like I have said Potty’s videos are well documented and there are lots of them but if you want gas analyser results with more technical  evidence, then stick to Peters designs.
There are other videos where his has his stove at max load with full air and the top plate, through under plate baffle and the top steel is over 1000f
In my own stove I can get over 1000f too and that is through 6mm steel, if I try to measure the actual top of the riser,  my gauge just goes off the scale.
I have tried pointing the temp gun down the riser but that goes off the scale too.

If you look at the video at 29.00 you can quite clearly see the secondary air working.

Hi Fox, I want all those things but I want much higher temperatures than that video shows because at 200-400, and 500-800 deg C you get de novo synthesis of dioxins and furans in the riser from the chlorine atoms in the wood. And since I'm standing at the top of the riser cooking, I'd rather not be exposed to endocrine disrupters!

Peter has temperature probes in his riser so he can measure the temperature accurately. I don't believe those IR guns can accurately read what's happening at the bottom of the riser. I might try to see if I can melt copper at the bottom which would tell me I would be getting over 1085 deg C.

The formation of fly ash also promotes the production of these chemicals so maybe gassifying is the way to go to minimise these risks.

Ok so I am a bit confused now!
So you want to cook over the exposed top of the riser and that temperature at the top of the riser needs to be over 800c?
Or am I missing something?
Like I say on my fire a can get just over 500c on top of the plate but I have no idea what the temps is at the bottom of the riser or mid riser or even under the plate or at the top of the riser?
In any case I won’t be breathing in any fumes... will I ?
Sorry if I am being a bit thick but I still don’t quite get what you are aiming for?

Hi Fox, that's correct.  You're okay as you're cooking on a steel plate, and the gases are being vented out a chimney away from you.

But if you want to cook on a wok, you need a raw fire stream so that the flame hits as much of the wok as possible.  So, I want as high as temperature as I can get at the bottom of the riser to destroy these compounds, and preferably a temperature over 800 deg C at the top of the riser.  I'm currently only hitting 700 deg C but I haven't sealed my combustion chamber yet.  And maybe I need a double wrap of the riser like you do.  As the gases cool from 800 deg C the dioxins/furans can form again (de novo synthesis) but this time they're out of the riser and hopefully are being blown away by the draft.  The dioxin/furan synthesis is much less at the higher temperature window than it is at the bottom window.  And I gather this is one of the problems of catalytic stoves which promote combustion at the lower temperature window.

Ok well I guess we are all trying to learn on this forum but I was under the impression that the high temperatures required, were achieved at the end of the tunnel and the bottom of the riser, not at the top of the riser?

Anyway, anything that I can cook in a wok I can cook on my hot plate. 530c is extreamly hot to directly cook on! In fact i avoid the centre hot spot and cook on the 200-300c area.
It is crazy hot, so hot it frazzles virtually anything that touches it, I think that even the best high temp oils combust at 270c.

However a flat bottom wok works fine or I can cook directly on the plate for anything without a sauce. I can also expose my riser and place a round bottom wok on that.
Although I can understand your concerns, I must say that at this stage in my life, after a life time of cooking over open fires, barbecues, pizza ovens, washing machine drums and now rocket stoves, personally I don’t share your worries and I am not is the remotest bit concerned about a few stay fumes!