Planning DSR2 Build With Cooktop -- Have Questions!

Right, so. Still considering replacing my Fisher Baby Bear with an RMH.
Design constraints:
*It has to fit into a tight corner with clearance from wooden walls, so footprint no more than 36x36.
*It has to heat a 500 ft2 cabin, presently burning 2-2.5 cord a winter with the Fisher. Good insulated 6” chimney stack, straight up 14’ inside the house.
*I want it to do a little of everything: at least a small cooktop; at least enough oven to shove a loaf pan into (I would consider just using the firebox); then charge a mass of about 30 ft2 ISA (which is about all I can fit in this corner). So that’s a 5” system, probably.
*I suck at metalwork, so doors need to be _simple_.
*I’m not thrilled about the notion of constantly taking the thing apart to fiddle, tweak, and replace parts. I want simple, reliable, long-lasting, idiot-proof design.

I spent a looooong time falling down rabbit holes on Donkey’s forum. Looked at the Walker stove, but I’d prefer a narrower, more centered core. Fell madly in love with Trevor’s Vortex, but … it looks awfully complicated and seems to require a lot of post-build fiddling to tune the airflow to the mass. So that leaves me looking at Peter van der Berg’s DSR2, open door, for sheer simplicity of build. Although if he gets the DSR3 to where he wants to publish – and if it runs as reliably as the DSR2 – I’d be interested.

Anyway, thinking DSR2 at the moment, but I have Questions….

But first, sketches. Again, a 5” open door system, firebox made of hard firebrick splits inside CFB, top box wholly insulative, exiting out the top port as recommended then doubling back under a cookplate and into the bell. Suppose if I don’t want delicious ceramic fibers in my daily bread, I’d better plan to bake in the firebox, pre-riser. The lower bell a brick box to cooktop height, the upper portion made of 15” flue liners, 3’ tall. Gases exiting from back of cookplate, then stratifying to an exhaust channel beneath the core and back up to exit at the right. All to be cob plastered 3” thick + sculpting.

SO NOW ABOUT THOSE QUESTIONS
1) Thinking of running an open system: no built-in air channels, no door during firing, just a spark screen and a slanted hood. But this would be in the house. So… anyone know the present status of smokeback research? I know the Sherman Tank had issues, at least at first. Not clear whether anyone else has done.
*Also, the DSR3 can’t do open system, can it? Or do we just not know yet?

2) Math Problems:
A. That top exit port. The spreadsheet I downloaded says for a 5” system it needs to be 7.25” wide (same as whole top box, right). Then it says “Height 2.09” and “Depth 1.45.” I’m confused. I read that as a slot 7.25 x 1.45 in a 2” thick ceiling, which seems awfully narrow. It would make more sense if it were 7.25 x 2 in a 1.45” thick ceiling. I’m quite sure Peter’s tests are accurate, but I’m not sure my understanding of this spreadsheet is?
B. I know some people have had issues with the DSR2 exit gases cooling too much to run a cooktop. Suggested fixes seem to be, Insulate both the top box and the cooktop under-channel, and, Make the under-channel as shallow as possible. CSA on a 5” system would be 2.66” – but that makes a skinny rectangle of a CSA, not an open circle, so would that create too much restriction? What’s the shallowest height could I get away with without bogging down core performance?
C) Seriously considering a casserole door, because an open system would have oodles of airflow to cool the glass. BUT. It looks like the molding of the frame would have to create a lip over the top of the firebox, restricting firebox opening. Does a hanging lip screw up the ability of air to enter or to flow over the top of the fuel load as open-door operation requires? Also, as an open system, do we have any ideas about how *much* of a gap should be left at the lower edge for air intake? Because Peter seemed to have his best results with at least some restriction.
D. Another possible restriction point in the bell path: Gases will stream out from beneath the cooktop unrestricted and rise into a 15” diameter round bell made of flue liners. But as they sink, they’d need to squeak around a 3” corner gap. This wouldn’t need to be otherwise blocked, so we’re talking about a 3”x24” slot, which is 3.75 times CSA but narrower than I like. So: how much back pressure might a restriction like that make? It would then pass into a channel of just about 1.3 CSA beneath the firebox and out the exit stack.

3) Shattering casserole doors: I wouldn’t do it on any closed system, but I’m impressed with Gerry Parent’s brilliant fix. https://permies.com/t/71575/Casserole-Door-innovative-door-batch  Only he’s got airflow on both sides and (depending on smokeback research?) I might want the upper rim closed.
Still, while oval lids would be the best fit, ALL of those seem to be merely soda-lime glass these days, urgh. Looks like EU-sourced Visions skillets are available that are actually borosilicate, though not cheap. Thing is, if it’s an open door system, does it matter? The glass might have direct contact along its upper rim with hot cob near the firebox (if the upper rim should be closed), but full and free airflow over the remainder. So is soda-lime good enough? Because an oval really would make a better shaped firebox opening.

4) Capping materials. I could probably find a nice flat round rock for the upper portion of the bell. (We have lots of gneiss; how does that hold up to heat? I’d tack a blankie to the underside anyway.) But the cooktop-height bell cap is more complicated. I could make it fit with 9” firebricks. We have a kiln supply store nearby that sells “medium duty” ones cheap, and while I wouldn’t trust that grade in the firebox, a cap is well outside the inferno. Does capping with bricks work? Or are there too many gaps opening with thermal movement, and thus leakage? How would I cut them to get the chimney through, that’s a complicated cut? Does anyone have a better idea? Whatever I use will probably wind up a work surface so should be firm and flat and reasonably finished looking.

5) I’m having trouble designing the bypass. I can see the place it would go – right out the side of the cooking channel to the chimney. But if I cap the box portion of the bell with firebricks, yet make the cooktop under-channel as shallow as I possibly can … that doesn’t leave me much of anything I can cut through to make one. How big exactly does a startup bypass need to be? Can I get away with just a slit? (Not likely to be firing it up in summer.) Anyone see a better idea?

6) Baking in fireboxes at coaling stage. Who’s done it? Are split firebricks in a casing of CFB likely to hold enough heat? I know people have had good oven luck with full size firebricks, but those would rob too much heat from startup on a 5” system. I’ve also heard of people who abandoned baking in their firebox because it was “not for them.” Which I don’t know how to troubleshoot.

… I could probably think of more questions but that’s plenty for now! Thank you all for your considerations.

That is not a cheap build so I would use ceramic glass for the door and and consider insulating fire brick for the top box. You may be able to  successfully cook in the top box while the fire is running on coals but you would need door for that too.

The hot plate is best in thick steel or cast iron, you can buy BBQ cast iron plates 12 x 16” maybe use two.

Also you could use a vermiculite box on top of your hot plate to cook in ( I have a video) even with a Rotisserie!  
You can buy wood oven doors with glass, I paid around $200 for the last one I bought or you could get someone like Thomas to build you a custom one.

Hi Fox!
You make beautiful stoves. Thanks for taking the time to look at this as I flounder through planning.

You’re right, a top box oven is a great solution, and wider than the narrow firebox. I’m afraid your nice Skamolex does not seem to be available on this side of the pond, but no doubt I could find something that would work. For anyone else looking, here is the link: https://permies.com/t/181437/vortex-rocket-cook-stove

I know you mentioned back in June that you were having serious problems with your 4” Vortex running too hot to be safe. https://donkey32.proboards.com/thread/703/vortex-stove?page=72  
I *don’t* need cracked glass and charred rafters! But I do want to charge a post-cooktop mass, which you are not trying to do, so a certain amount of dragon fire seems needed. Even cast iron probably can’t take the sort of temps you were getting, but I suppose some sort of diffuser could perhaps be placed beneath. In fact, if I used a top plate of ½” CFB but drilled it full of good sized holes, hm….

Living with a metal box stove, I am used to knowing that if it starts glowing red, I can simply shut all air intakes tightly and it will cool right off, although it will make a lot of smoke. Doesn’t happen often, but it’s nice, from a safety standpoint, to have the option. But a box stove isn’t running anywhere near rocket temperatures, so does that work with these?
I suppose it would mean, yes, one needs a real door, not a casserole lid.

I see you’ve tried a wide range of air options, including standard suggestion, door open, heated secondary air, no secondary air…. I’d love to know where you settled, or at least the present status of your experiments!

And finally, speaking of materials available here in the US, why are these people http://skylinecomponents.com/ offering “bio-soluble” CFB for half price and free shipping? I’m intrigued, but old enough to be skeptical. Anyone have experience with them?

Hi April, I have found the 4” vortex to run fine without any secondary air, there is a slight temperature difference but that is probably a good thing in my case!
It also runs well without a door but it burns out very quickly that way so I would recommend a door.
Yes you can control the fire by restricting the air but then you will quite likely get smoke on the viewing glass and out of the chimney.
I found that by using the BBQ cast iron under the ceramic glass the cooking temps are just about right, around 350-400c at peak burn but mainly around 250-300c.
To be honest compared to a electric or gas stove or oven, rocket stoves are nowhere as easy to cook on, they are designed to burn hot and fast, perfect for heating a mass but a bit volatile for cooking!
However with my outdoor vortex stove the surrounding bricks and granite worktop get lovely and warn.
The vermiculite board is still looking fine, no noticeable Deterioration so far but I dont expect it to last for ever.
In my case, if the board fails after a couple of years I can just replace the whole unit for £150 and a couple of hours work.

I'll take a crack at some of these. I've been where you are, just knowledable enough to see the gaps in your knowledge!

April Wickes wrote:SO NOW ABOUT THOSE QUESTIONS
*Also, the DSR3 can’t do open system, can it? Or do we just not know yet?

I can quote Peter as saying this on the Donkey board:  "no separate secondary air provision. Size of air inlet is less important in this design, simply because the core is slowing down by itself when a fuel overload is imminent. It seems to be scalable, I expect it to be capable of running without a door at all." But this statement was some time ago and he may have a different opinion now.

April Wickes wrote:
That top exit port. The spreadsheet I downloaded says for a 5” system it needs to be 7.25” wide (same as whole top box, right). Then it says “Height 2.09” and “Depth 1.45.” I’m confused. I read that as a slot 7.25 x 1.45 in a 2” thick ceiling, which seems awfully narrow. It would make more sense if it were 7.25 x 2 in a 1.45” thick ceiling.

My 7" DSR2 has an exit port that's 10"x3" (10" is my total box width) so the math is 7" system divided by a 3" port= 2.33. Your 5" system divided by a 2.09" port= 2.39 so by this crude math I can at least say that we are proportionally close! When I built this core, I struggled through the forums for breadcrumbs and ended up getting a lot of my dimensions by taking Peters DSR2 sketchup file, scaling it to a 7" core, and using the measurement tool to extrapolate what some of the dimension would be that I couldn't confirm otherwise. I've never actually seen a spreadsheet that has all the DSR2 details aside from the one I made.  I wonder if some of the data gumming you up here is actually numbers for the stumbling block.

April Wickes wrote:
B. I know some people have had issues with the DSR2 exit gases cooling too much to run a cooktop. Suggested fixes seem to be, Insulate both the top box and the cooktop under-channel, and, Make the under-channel as shallow as possible. CSA on a 5” system would be 2.66” – but that makes a skinny rectangle of a CSA, not an open circle, so would that create too much restriction? What’s the shallowest height could I get away with without bogging down core performance?

I believe the standard answer to the "minimum height above exit port" question is "same as system size". So 5" in this case. Not sure. But you've seen the Sherman tank pics where there is tons of height between the exit port and tank top, and that tank top gets plenty hot for cooking! Peaking at around 400 degrees F at 30 mins into the burn (ambient air temp 45 degrees F.)

April Wickes wrote:
C) Seriously considering a casserole door, because an open system would have oodles of airflow to cool the glass. BUT. It looks like the molding of the frame would have to create a lip over the top of the firebox, restricting firebox opening. Does a hanging lip screw up the ability of air to enter or to flow over the top of the fuel load as open-door operation requires? Also, as an open system, do we have any ideas about how *much* of a gap should be left at the lower edge for air intake? Because Peter seemed to have his best results with at least some restriction.

As you saw in another post, it's not truly a completely open system since the opening needs about 60% coverage to work properly. Peter advised the air opening to be in the sides of the "door/cover," rather than a bottom gap. Hence my janky T shaped door.  So I don't think you'd have an issue with molding for your casserole door IF you left air gaps on sides.

April Wickes wrote:
D. Another possible restriction point in the bell path: Gases will stream out from beneath the cooktop unrestricted and rise into a 15” diameter round bell made of flue liners. But as they sink, they’d need to squeak around a 3” corner gap. This wouldn’t need to be otherwise blocked, so we’re talking about a 3”x24” slot, which is 3.75 times CSA but narrower than I like. So: how much back pressure might a restriction like that make? It would then pass into a channel of just about 1.3 CSA beneath the firebox and out the exit stack.

I believe the answer to this is also "same as system size" so don't restrict more than 5 inches.

April Wickes wrote:
5) I’m having trouble designing the bypass. I can see the place it would go – right out the side of the cooking channel to the chimney. But if I cap the box portion of the bell with firebricks, yet make the cooktop under-channel as shallow as I possibly can … that doesn’t leave me much of anything I can cut through to make one. How big exactly does a startup bypass need to be? Can I get away with just a slit? (Not likely to be firing it up in summer.) Anyone see a better idea?

That's a though one with your current design, but if you had the 5" gap for your cooking channel it would be easier. From what I've seen, your bypass should actually be at the top of your bell though. And you could achieve that with a T in your stovepipe into the bell. The hole does not need to be large, and in fact some designs leave a small bypass open all the time because the vast majority of gasses will follow the proper path when everything is warmed up.

April Wickes wrote:
6) Baking in fireboxes at coaling stage. Who’s done it? Are split firebricks in a casing of CFB likely to hold enough heat? I know people have had good oven luck with full size firebricks, but those would rob too much heat from startup on a 5” system. I’ve also heard of people who abandoned baking in their firebox because it was “not for them.” Which I don’t know how to troubleshoot.

I thought half the fun of the DSR2 was the built in oven, aka the top box! Granted, that's another door to figure out. Something I have not done myself.

Disclaimer: I always hesitate to speak for Peter, so when I do it's direct quotes from a board and even then things may have changed. Maybe he'll visit this thread like Santa Clause and provide better guidance.

Fox, thanks for your reply. I’ll accept the word of wisdom from you and Matt both and stick a door on the thing, or at least something door-adjacent. I’m still considering a cast iron casserole lid, which could have two positions, closed airtight and then turned horizontally and placed on a shallow cob lip which could be calibrated to provide a certain amount of primary air. Then I could close the thing hard when unneeded (or about to burn the house down). The only catch I’m seeing is that such a design would still make it a lot easier to introduce air at top and bottom than on the sides.

Your two-layer cooktop makes a lot of sense to me! I was concerned about using a single layer of cast griddle, because while I know it bows and warps less than steel, if it did move around it seems it would certainly release gases directly into the room. But by putting a hunk of cast underneath ceramic glass with a bit of wiggle room as you did, the cast bits could shimmy, warp, and expand as they like while still deflecting enough heat to make the glass safe.

I’m sure you’re right that a rocket cooktop would be challenging for a chef, but I’m really just after boil-and-sear, plus the baking of bread. I have been using a gas range as a backup or in the summers. You are clearly a more skilled and ambitious cook!

Matt, I really appreciate your digging into some of these questions. Is that your DSR2 spreadsheet I’m using? I think it must be. Since it’s no longer possible to get the free 2017 version of SketchUp and all new versions seem to be “register, log-in, and use online only,” I don’t seem to be able to access any .skp files. Even if I weren’t creeped out by the registry, we don’t have stable internet out here in the sticks. I do all my thinking offline! So being able to get some of this data as a spreadsheet instead is very, very helpful (deep obeisance).

Please let me know when you have one for the DSR3!

> I can quote Peter as saying this on the Donkey board:  "no separate secondary air provision. Size of air inlet is less important in this design, simply because the core is slowing down by itself when a fuel overload is imminent. It seems to be scalable, I expect it to be capable of running without a door at all."
Yes, I saw that, but the wording of it seemed … not very nailed down yet. Still sounds promising.

Okay, yes, a deeper top box would leave room for a bypass in the side. So would the DSR3, and the DSR3 would also, being shorter, give me more room inside the bell to avoid restrictions. I like the concept of the DSR2 having a top box oven too … but on a 5” system, or even a 140 mm one, you’re only talking a 7” or 8” square anyway, and that partially restricted by the stumbler. Too small to be quite useful, though I guess it could do ONE loaf pan at a time. What I’m most concerned about in choosing between designs is idiot-proofing the burn (and the build).

April Wickes wrote: Is that your DSR2 spreadsheet I’m using? I think it must be.
Please let me know when you have one for the DSR3!

I vaguely recall uploading the batch box spreadsheet with additional values for the DSR2 features somewhere.
And since it's a real slow day at work and you've got me curious, here are the measurements from Sketchup on the DSR3 core that Peter shared!
I believe this would be called a 6 inch system since he said the diameter of the tube is 100% of system size or slightly larger.

Like you, I'm excited by this design because there are no secondary air provisions to worry about and it seems pretty safe as far as being nearly impossible to put into fuel overload.
I doubt I will build anything this season, but I'm on the hunt for a 6 inch ceramic riser sleeve to play with
And Peter if you read this, thanks again for sharing that Sketchup file!

Matt, I could kiss you. I couldn't get to the .skp file even if I had a way to open it; it's gone 404. You're my new fairy godfather.

April Wickes wrote:Matt, I could kiss you. I couldn't get to the .skp file even if I had a way to open it; it's gone 404. You're my new fairy godfather.

HA, no worries. All I know came from studying others and asking good questions so it's only fair to pay it forward. Heck it looks like you might just build a DSR3 before me and then I'll be asking YOU questions.
And for Sketchup, you have to log on to their website and open the file from your computer in their online program (and can save it to your online library there too.)

Yes, the process you describe for accessing SketchUp is precisely what I don't expect would work with our minimal, spotty rural internet, aargh. But you've fixed it.

Interesting. It looks like Peter has come much closer to Trevor's Vortex proportions on the firebox than his usual deep-and-skinny Batch Boxes tend to run.

Oops, I went and converted your .jpg to a scaleable spreadsheet to attach for anyone else who needs it ... and now I see why you did it as .jpg. Permies won't let me use .xls. I guess all y'all will have to do your own conversions.

Oh well. This is very exciting.

Of course, by now Peter has probably found a better way to do it....

Huh. All right, now that I can actually look at the DSR3, I'm seeing it presents me the same challenge as Trevor's original Vortex: how do you span the firebox roof? The great advantage of the skinny batch box dimensions is that in anything 140mm or less, you can just span with ordinary firebricks. But for these, it would have to be a 4" system or less to pull that off, and the 140 mm sizing I was looking at would need to span 11".

Trev used castables, but the more I read about these, the less they sound idiot-proof....
Fox used a rugged, high-quality grade of vermiculite board, but if there's anything of comparable quality available in the US, it's not popping out at me.
I'd really prefer not to have CFB directly exposed in the firebox at least, if I can avoid it.
So what does that leave? Kiln shelves possibly?

Here's my confusion about kiln shelves: there seem to be about a dozen different materials and grades. Silicon carbide (which sounds too crack prone and hard to cut), cordierite, mullite, high alumina (is that just mullite?), etc etc.  I know some people have experimented with some of these as firebox roofs -- how are they holding up? Thomas?

April Wickes wrote:Here's my confusion about kiln shelves: there seem to be about a dozen different materials and grades. Silicon carbide (which sounds too crack prone and hard to cut), cordierite, mullite, high alumina (is that just mullite?), etc etc.  I know some people have experimented with some of these as firebox roofs -- how are they holding up? Thomas?

During various development stages of the DSR3 I used cordierite for the top box. In the final stage I used the same thin (10 mm) cordierite kiln shelfs thoughout, no other materials. The material looks a bit like yellow firebrick although it seems to be finer, with a closed surface. All the pieces were stacked dry, with Morgan superwool around the outside. All shelves are still in one piece, no cracks whatsoever, despite the rigors I put them through.
In short: are kiln shelfs worth their money? Yes, in my humble opinion they are. Pricey, but worth it.

Edit: ceiling of top box with 10 mm (0.394") shelve is 26 cm span, equal to about 10", I'd say. The ceiling of the firebox is done in 15 mm (0.591"), same span. Resting on top of that slightly thicker shelf (supported left and right only) is the ceramic tube, weighing in at 3 kg (6.61 lbs). Mark that the ceramic tube is devided into three different pieces, necessary for to avoid cracks.

I'll have to add that the DSR3 is scaled up in the mean time upto 200 mm (8"). Have a look at the report at Donkey's: https://donkey32.proboards.com/thread/3899/dsr3-build-200mm-system-bench
This system is built using considerably thicker material: firebrick slabs sized 1'x 2' x 2".

Thank You, April for asking and Peter for answering!
This is wonderful news!
I have been experimenting with roofs this summer.
I am currently using a large 16"x24" cast iron griddle on my 7" shop batch and a 2" thick 12"x24" heavy firebrick on my 6" Batch.
A friend in Canada is trying a kiln shelf roof this season.
I currently have the highest hopes for the cast iron.
I have been intrigued by the kiln shelves as well, but with so many types I did not want to spend the money.

I am delighted to hear about Peters's experiences with cordierite shelves!
This gives us a solid answer about them!
And only 10mm thick!  I was looking at a minimum of 18mm (3/4") thick shelves.  
As he noted they are spendy but so is cast iron, heavy firebrick, or ceramic fiber board!
I'm hoping both my new roofs perform flawlessly this season.
Both are in uninsulated buildings and run long and hot all day. (back-to-back fires)
If both the roofs hold up then we will have 3 good choices for builders to use.

Peter and Thomas, thank you both for taking the time to reply! It’s good to hear that 15mm cordierite is working out, because that does seem to be available in this area, and (relatively) affordable, at least compared with the other kiln shelf types. I’m also glad to see this 200mm DSR3 build; I had not found that thread. And pleased to know that “riser sleeve” is the preferred term (I was trying to look up “refractory tube,” which mostly seemed to get me to wee little pipettes.)

Peter, if I recall correctly, you said your 130mm system could reasonably charge about 3.3 m2 ISA. Does anyone know by what proportions one should reduce ISA if there is a radiative surface before the bell? I presume a cooktop must steal a fair amount of heat, but I’m not sure how much.

… Gosh, I might be running out of questions. This is great!

Okay, riser sleeves.
I contacted half a dozen suppliers and most of them spit in my face, unless I were to order a whole pallet. They didn't offer a pallet price or minimum order or anything, just, "Nope, bug off." I hear this experience is not unusual.
Then I heard back from these people https://www.carpenterbrothersinc.com/foundry_products/molding-and-core-making/riser-sleeves/insulating-riser-sleeves/ and they offered to send me a box of 18 6"ID 12" long insulating riser sleeves for $5.20 each.
That's a fantastic price! It's so fantastic I'm pretty sure we cannot possibly be talking about the same thing.
Does anyone know what the heck they just offered me?

April Wickes wrote:
Does anyone know what the heck they just offered me?

Well that would certainly be amazing. Best thing you could do is ask them for a specification sheet that includes temperature ratings so you/we know how hot these can get. There's not much too them. From my understanding it's just an insulating sleeve to help keep molten metal hot inside it's container during casting. Perhaps there's a difference between re-usable and disposable riser sleeves as far as durability, so that might be a question to ask too.  

Yes, definitely ask for a specification sheet.
I suspect they would not like the open flames and the extreme temps in a batch riser.
The other issue I see is connecting the riser pieces to the length you need.
I doubt that just sitting them on top would be stable enough and each section could leak at the joint.

Consider using insulated firebricks or a Morgan super wool 5-minute riser.
You can also cast your own with fireclay and perlite.

The Carpenter Brothers sales guy is hunting for the spec sheet on his $5 "insulating riser sleeves." Yes, Matt, you're right, they were designed for single use. The sales guy didn't see any reason why they WOULDN'T work for what we're trying to do, as it should be within their temperature range and even open flame he thought would be less of a shock than puddles of molten metal. I'm still dubious. I will post whatever I get for their specs when he sends them along.

Thomas, the DSR3 core under discussion has a short horizontal afterburner, so there's no need to "stack risers." It might be possible to make something out of carved up IFB or perlite, but I would have concerns about those holding up.
Your suggestion of the 5 minute riser is a good one, and Peter's first experiments adapting Trev's Vortex design did use exactly that. They were only temporary, though. My concern is, while your standard vertical riser tube positions the metal sleeve well away from and insulated from the heat of the rocket, in *this* design, the sleeve would be fully exposed to the gas stream. It seems inevitable a metal sleeve would warp under those conditions, and I'm concerned that as it did so, it would deform the superwool as well. Is that a reasonable concern? Does anyone see a way that would work? Might superwool dipped in rigidizer hold up long-term without any sleeve?

The other thing I thought of is to try to use a round clay flue liner as a sleeve, but if I went that way, in order to leave room for the superwool, I would need to use an 8"ID clay liner (expansion joints pre-cut as per Peter's pattern). A trip to the local building supply place suggests the 8" clay liners available here are fairly thick walled. So we are now talking about a honking big and heavy object to cram into a small, lightly built top box. With a bunch of extra mass, no less.

Other ideas? Or are there reasons why these ideas would work just fine?

Hi April;
Having never built a shoebox rocket myself  I am not familiar with all the differences between them and a batch box.
I would think that the external temp of the metal skin should be significantly less than inside the riser even though that riser air has now entered the shoebox.
What has Peter used in his latest versions?
I have not used the ridgidizer product so I can not offer an opinion on how well it would work.
High temp insulating firebrick (IFB) is always a good option.
Away from any wood abrasion, an IFB is a long-term install.

These two pages from the development thread cover the round "riser" (really, an afterburner chamber?) and why the choice was made to not use superwool.

https://donkey32.proboards.com/thread/3710/dsr1-vortex-aspects-dsr3?page=7
https://donkey32.proboards.com/thread/3710/dsr1-vortex-aspects-dsr3?page=9

As far as using clay chimney liner... can that handle the intense heat of 2100 degrees?

Ah, there it is in a tiny aside: Yes, the metal sleeve was unstable in this use, and also the superwool "shrinks." So that would make the 5 minute riser difficult.

Peter's comment
>There's one catch with Superwool though: when it has been heated to a temperature above 800 ºC (1470 ºF) it will form crystals (Kristabolite) and those resulting fibers won't dissolve at all. This does sound real bad and given the fact that superwool will shrink dramatically when exposed to temperatures above 1000 ºC (1830 ºF), vacuum formed Superwool is off the table. The higher specced Ceramic fibers, also from Morgan Ceramics, doesn't show these behaviour but the best quality binder will give away much earlier. To this end, there's clay added to the binder in the vacuum process which will bake into ceramic while the normal binder is baking off. This clay addition will lower the insulation value quite a bit, by the way.<

Urgh. Back to needing to find a durable riser sleeve. This is proving to be the major sticking point of the DSR3 design so far

I think you're on the right track with Carpenter Brothers. The one documented 3rd party build used a "vacuum formed ceramic fibre riser sleeve made by Quinsis in Belgium." Which sounds like what you found IF it's ceramic fiber.
Maybe you could contact that Belgium supplier for specs for comparison (even though you wouldn't likely buy from them because of shipping.)

https://donkey32.proboards.com/thread/3899/dsr3-build-200mm-system-bench?page=2

The upshot of my riser sleeve search so far:
The most inexpensive ones seem to be heavy on sand, light on ceramic fiber. Their SDS did not include a temperature classification. But if they don’t insulate, they may not work in this application. Haven’t got a price quote on higher fiber sleeves yet, but they do exist. Still waiting on a bit more information.

If pre-made riser sleeves are just an impossible bust and superwool can’t stand up to the chemical gas attack or afterburner temps without shrinking or slumping inside a too-expansive sleeve, Thomas has got me thinking about IFB. As I understand it, the design of the DSR3 depends on the passage of air around the wunky shape of negative space in the top box to slow the gases and keep the burn under control. The wood gas passes through the port in the top of the firebox, roars around in the short horizontal afterburner (concentrating its greatest heat in the upper rear quarter, hence the need for expansion joints), and then, at the end of that tube, makes a sharp switchback and swirls through an awkward path made of curvy triangular leftovers on its way to the exit. Trev’s Vortex tries to achieve a similar slowing effect by adding baffles to a shallow rectangular top box.

So 2600* IFB is soft and easily cut, yes? (Once, when I was a teenager, we found a chunk of IFB discarded in an alley. We had no idea what this object was. We ate some of it trying to find out, which probably wasn’t a great idea, but we’re all still alive. My sweetheart took a pocketknife and carved it into a sculpture of a snail. His name was Zippy. His eyestalks broke off, but I think he’s still in a box in the attic somewhere. I should dig him out and stick him in a stove.)
Anyway, if you can make a snail with a pocketknife, you can probably cut IFB with any woodworking tool you’re willing to dull, like, say, a holesaw, or maybe a jigsaw. So I could make a tube from IFB, as pictured below. In its simplest form, this would make a square of the outside edge, and two tall straight skinny channels for exiting gases. Would that lose me the baffle effect of wunky triangles?

With not much more work, I could nip off the corners into an octagon and re-do the top box math to keep 1.66 CSA of negative space (a hair more than 9” high for a 9” exterior diameter tube), but thinning the IFB like that leaves me wondering about the potential for cracks. I believe Peter said he had trouble with IFB cracking to begin with. Anyone have thoughts? Would it help to coat the inner surface with fumed silica/fireclay to repel some of the chemical attack? Going the IFB route would not be cheap (although it would be cheaper than buying an entire carton of riser tubes, especially if they didn’t work) so I’d like to have some confidence in it.

IFB cuts very easily. I used a handheld jigsaw with a fine tooth blade to cut mine.
A pocket knife works great for fine shaving.

Honestly, I am not convinced the IFB would survive the rigors of a DSR3's afterburner. In my development model literally all of the tubes I tried got cherry red, inside and out. Even that German made ceramic chimney liner tube I settled upon did.
Having said that, the ceramic fibre vacuum formed tube from Quinsys in Belgium did quite well. This wasn't a Superwool tube, but a higher quality ceramic fibre one with added clay which baked rock hard.
Even the simplest riser sleeves were up to the job, although I don't know for how long it would survive.

It is pretty easy to get ahold of some 3000F rated castable refractory cement if you're willing to try your hand at making forms. I'm contemplating this option myself. It could be as easy as centering a 6 inch tube inside of a 7 inch tube, pouring the cement, and making the cuts after it dries. The only 7 inch tube I can find is metal though, made for ducting. Could still work as a cement form though.

Thanks, Matt. I’m afraid it’s not the form creation that gives me pause about castables, but the packing/ vibrating/ curing/ slow-firing process, which seems like it has a lot of places to go wrong. I feel like I read a lot of disaster stories about newbies trying their hand at castables and winding up with a handful of gravel. Maybe it’s not as bad as it sounds?

Peter, I want what you describe, but I’m striking out on finding any reasonable source. It leaves me wondering about the earlier experiments you mentioned on the DSR3 thread (Jan ‘21). You were soaking superwool in various mixtures of clay slip, waterglass, and/or Moviset, trying to pin something down before you outsourced anything. Did you come upon any formula that worked well for you, and lasted?

I don’t think an uncoated superwool tube would work long term here, but I wonder about coating or soaking one with other things: rigidizer, clay slip, or, if necessary, satanite or IR-reflective paint? The design challenges are:
-if the tube is not rigid enough to stand horizontally without a sleeve, no metal sleeve (or wire wrapping) is likely to tolerate those top box conditions. So it’s got to be a solid piece in itself.
-if superwool “shrinks dramatically” above 1830*F, is there a coating that would stabilize it? I really don’t want to be replacing it constantly.
-and then of course there’s the fibers.

I’ve found Forsythe’s thread on these topics over at Donkey’s, but he’s deeply engaged in ceramic chemistry theory, maybe less engaged in “this is what I tried and it worked.” Can anyone point me to other threads on impregnating superwool with something more durable?

*Finally, there’s one thing about the DSR3 compared with the Vortex that I don’t quite understand. Trev said he had trouble when he brought the edge of the afterburner top shelf too close to the front. The gases pitted and corroded the glass, and took it outside its safe temperature range. For this reason, he wouldn’t bring the shelf of his 150mm version closer than 110 mm, nor his 100mm version nearer than 72mm.

But according to the spreadsheet Matt kindly extracted from the DSR3 .skp, the gap between the afterburner tube and the front of the top box is much, much closer: 50 mm on a 150mm system! Am I understanding incorrectly, or does something about the airflow in the DSR3 protect the front glass more than the Vortex did? This seems important.

… All right, I guess I’ll never run out of questions.

April Wickes wrote:Peter, I want what you describe, but I’m striking out on finding any reasonable source. It leaves me wondering about the earlier experiments you mentioned on the DSR3 thread (Jan ‘21). You were soaking superwool in various mixtures of clay slip, waterglass, and/or Moviset, trying to pin something down before you outsourced anything. Did you come upon any formula that worked well for you, and lasted?

Sorry to say, but no. I was searching for a quick and easy recipe and couldn't find it. Maybe there is, like clay slip soaked ceramic paper in several layers around a tube. In such a way the tube could be placed in an oven to dry it out. I don't have a suitable oven at hand so I skipped that stage.

April Wickes wrote:*Finally, there’s one thing about the DSR3 compared with the Vortex that I don’t quite understand. Trev said he had trouble when he brought the edge of the afterburner top shelf too close to the front. The gases pitted and corroded the glass, and took it outside its safe temperature range. For this reason, he wouldn’t bring the shelf of his 150mm version closer than 110 mm, nor his 100mm version nearer than 72mm.
But according to the spreadsheet Matt kindly extracted from the DSR3 .skp, the gap between the afterburner tube and the front of the top box is much, much closer: 50 mm on a 150mm system! Am I understanding incorrectly, or does something about the airflow in the DSR3 protect the front glass more than the Vortex did? This seems important.

Trevor is right about the proximity of the glas to the afterburner. You misread my drawing in the sense that there's an air frame in front of the ceramic chamber. This is 60 mm deep, so the distance from tube to glass is 110 mm again. Mark that in my final 130 mm design the glass > tube distance is also 110 mm. Just a bit more space wouldn't hurt.

Oh, good catch! I will update my spreadsheet!

My apologies for missing the extra depth that comes from the air frame! I have edited my post about measurements above.

Regarding afterburner tube materials: It seems that superwool isn't a viable option, but I see that ceramic fiber blanket is a different animal than superwool. I found this page on ceramic fiber blanket: https://www.ceramaterials.com/ceramic-fiber-blanket/

The most intriguing sentence to me is:
"The manufacturing process results in flexible insulation that can be formed to complex shapes in place and air dries to form a hard, rigid structure."  
Meaning it ships wet, wrapped in plastic to stay wet. Then you form it and it dries to retain whatever shape you for it into, presumably becoming even more rigid after firing. And with temp rating options of 2600 and 3000 degrees F, I wonder if this has promise!

Good Find Matt!
This looks very promising for the DSR stoves!
With a 3000F rating and shapable to fit I think it will do the Job!

Interesting. The funny thing is that somehow when I looked up CeraMaterials, I wound up on their wholesale page, not their retail ... and they wouldn't talk to me. Even though they apparently DO have some solutions for this problem sold in retail quantities, even through their website. Guess I was talking to the wrong person?

A few things I note reading through, though:
The 3000* blanket and the WetWool product are not the same. The WetWool and its binders are only rated to 2000* max, which rocket stoves tend to exceed, and the 3000* blanket contains no binders at all, probably because they'd burn off at that temp. Also, uh, a 24'x24"x1" roll of the 3000* stuff starts at $2,375, and the WetWool at $460 for 12.5'x24" of 1/4" roll. Either is kinda out of my price range. By an exponential factor.

But it's still a good find, Matt, because they do other stuff too. For example, under https://www.ceramaterials.com/fiber-glue-rigidizers/ they have a "Moldable Mix," which is apparently a ceramic fiber castable, 2300* classification, not fussy about temperature range for curing, $95/gal. Just the stuff we'd been discussing. A 1" thick afterburner tube would be a bit more than 1 gal, but 3/4" thick should work. Still be $95 to make the tube, ouch, but getting better....  Or it might be possible to glue a spiral of 1/4" kiln paper, as Peter suggested. The glue is colloidal silica, $35 a quart, more or less the same stuff as rigidizer. You'd need a bit more than 7 ft2 of paper and it might be worth looking for somewhere that sells it by the foot rather than the $250, 100 ft2 roll.

So yes, you've found the most helpful sales website yet, by far.

I do wonder whether Forsythe is right about being able to extend a material's working temperature range with zirconia paint. But I have no idea.

I did write a little bit about making a tube here https://permies.com/t/177125/rocket-mass-heater-riser-precast

Good thoughts, Fox. I hear your Vitcas is great stuff, for anyone within shipping range of the UK. I'm dubious about how much any metal sleeve would surely want to shimmy, even stainless steel, but I considered a sort of square-bottomed octagon with vertical sides extending down to the base to support itself. Still thinking a tube of spiraled paper might actually *want to be a tube.* Which would make life easier.

I'm sure you're right about the fibers. Definitely an outdoor job, followed with a fixative.

Speaking of moldable fibers, it does occur to me that for $95, I could fail and fail and fail again with Kast-O-Lite ... and still come out ahead, at least financially. (Though perhaps frustrated!)

Deleted, irrelevant post

After catching up on some reading on the Donkey board and threads here, I have reached what I think might be the lowest cost/best solution to build a DSR3 horizontal tube afterburner.
Two components: High temp rated ceramic fiber blanket (with spun alumina and silica with a minimum 2400F rating for good measure) and a refractory coating to meet the standards of rigidizing, sealing fibers, reflecting, and adding heat resistance.

Ceramic fiber blanket of this nature is available BY THE FOOT on ebay (no giant expensive wholesale purchases needed!) One example: High temp CFB on Ebay

Since the Vitacas Zircon coating is not available in the US, I found this alternative that looks very similar: Heat Guard on Ebay

For a 6" system: To make a 6" ID tube out of 1" CFB, I would cut blanket to 19x14 inches (the end circumference x length of tube.)
Cut the port shape out of the blanket, roll it, stick it in a 8 inch diameter form tube of any material (to make a 6" internal diameter)
Sray/brush apply the coating to the inside. Let the coating dry, remove the form tube, then coat the outside.
Then maybe pre-cook it with the propane yard torch for good measure.
$20 to $40 in blanket plus $55 in coating should get you a tube for under $100.

Now please poke holes in my plan before I decide to run with it :)

Hi Matt;
I think it will work.
Each piece may not be equal in quality so some may fail.
But you're a rocket scientist... we experiment... sometimes we fail... sometimes we win...
Go For It!