The Sherman Tank DSR2 Rocket Mass Heater

I am pleased to share my journey building "The Sherman Tank." Because builds need fun names. Watching how others have built heaters has helped me immensely, so now I'll share!

Goal- Make a powerful heater with a removable core for an poorly insulated shop. Able to be moved. Able to add mass that can be fine tuned.  

Core choice- Peter Van Den Bergs DSR2 (Double Shoebox Rocket version2) Selected because the shape lends itself to my build, it's shape is such that it can be removed, it has a fun window which could become a door and oven, it's super efficient, and it can be ran as a simple open system (no door or secondary air tube, BUT those can be added later in this design.)

Material choice- Ceramic Fiber Board for the core because it is light weight, lending itself to removal. And because it's easier to work with than castables or firebrick. A 300 gallon steel tank for the housing/bell because I had the big ugly thing in my yard already and it fits the bill of an inclusive shell that can be moved, rather than built in place from things like brick and/or cob.

Sizing- I chose a 7 inch system to use with 8 inch flue pipe for better guarantee on proper draw. Also liked 7 inch because it fit my firewood and my tank. The one measurement you can adjust in a core is the firebox length, which I took from 20 inches to 24 inches. This also matched the longest edge of the CFB sheets. Someday I will move a system into my home which has an existing chimney with 8 inch flue.

Problems- Despite the best advise not to, I ordered CFB online, shipped to me. It was damaged. I was able to get a partial refund, but freight or in-person buying is the correct way to go.
There is bad CFB out there, and that's what I got from Phoenix Supply. It did not take the flame well. Spraying the pieces with sodium silicate saved the day. Now instead of fraying, the CFB just gets harder. I did have to water down the thick 40% sodium silicate solution by a third to be able to spray it. A little blue dye helps you see where you've covered.

I'll tell the build story mostly in pictures, but here are a few videos also:
-The initial CFB material test results. This was rather disheartening https://youtu.be/lwmUGAVHQtk
-The smokeback at the beginning of the burn, likely caused by me still figuring out how to properly light the thing! I was able to fan it back to start the proper burn path https://youtu.be/JXbv9jMCT7k
-The roaring takeoff of secondary combustion. A great pleasure to behold in real life https://youtu.be/DS-EtrIa7iA
-Even smokeless burning at about the 20 minute mark. I bet it would have been sooner if I had lit properly (plus the tank residue) https://youtu.be/hI91G6SjNl8

Using an infrared temp gun, I got the following readings:
500 to 550 degree F Max top temp right above the exit port of the core. Otherwise tank top was 400 to 500.
200 degrees at flue base (where it meets the tank top) but 180 above that.
Towards the end of the burn the whole tank read more evenly around 300 degrees.

Conclusion- Well over a year ago, I started studying rocket mass heaters. And to build one, it takes lots of studying! You have to determine your goals, your style, your materials, then run into obstacles until you hammer out something that will then need a bunch more refinements. Special thanks to Peter Van Den Berg, Matt Walker, Thomas Rubio, Gerry Parent, and all the folks on Donkey's board. Most of this core came from studying the 31 pages of Peters DSR2 build here: https://donkey32.proboards.com/thread/3503/double-shoebox-rocket-mark-ii

Next up- I'll do an update post when I move it into the shop. This will include how I'll add thermal mass, and how the chimney will interface.
Any comments, criticisms, and questions are welcome!

 

That was an absolutely fabulous documentation of your Shop Sherman Tank Matt!
Thank you for taking the time to do so.
I don't have any personal building experience with the DSR2 project but look forward to watching yours!

I’m glad I scrolled down and found this.... exactly what I have in mind but on a lil smaller scale.

Andy Bhill wrote:I’m glad I scrolled down and found this.... exactly what I have in mind but on a lil smaller scale.

My scale was going to be smaller too. Two stacked 55 gallon drums was the original plan... and it wasn't until my foam mock-up that I realized the 7" core would never fit! Turns out the guy who designed this core (Peter Van Den Berg) was running a little 4 inch core for his tests in 55 gallon drums. Guess I could have still used drums, but I would have had to figure out how to mount them above the exhaust port on top of the core.

Matt Todd wrote:

Andy Bhill wrote:I’m glad I scrolled down and found this.... exactly what I have in mind but on a lil smaller scale.

My scale was going to be smaller too. Two stacked 55 gallon drums was the original plan... and it wasn't until my foam mock-up that I realized the 7" core would never fit! Turns out the guy who designed this core (Peter Van Den Berg) was running a little 4 inch core for his tests in 55 gallon drums. Guess I could have still used drums, but I would have had to figure out how to mount them above the exhaust port on top of the core.

Yessir. I just found the shoebox threads last night and read them. My thoughts was the same as yours with a insert slid in. I’m curious as to the outside measurements of your core. I was thinking after seeing yours if the firebox could be partially outside and the shoebox area turned at 180 degrees to be centered inside the tank because the tank I have is about 32 inches. Thank you for posting this!!!

Andy Bhill wrote:
Yessir. I just found the shoebox threads last night and read them. My thoughts was the same as yours with a insert slid in. I’m curious as to the outside measurements of your core. I was thinking after seeing yours if the firebox could be partially outside and the shoebox area turned at 180 degrees to be centered inside the tank because the tank I have is about 32 inches. Thank you for posting this!!!

Ohh boy, those 31 pages of DSR2 thread have a lot of juicy nuggets. I got to where I just started copying and pasting notes out so I didn't have to keep going back to it!

My 7inch system outside dimension are 35.5 inches long (the actual length of a 36 inch sheet of CFB), 14 wide but more like 14 & 1/8 to 14 & 1/4 actual. And 29 tall. Height was a pain because I had to add to full sheets that come as 24 inches, which meant cutting 3 inch strips to make a 2 piece wall.  

I'm having trouble picturing what you mean about turning the top box around 180 degrees.
The top exhaust of the shoebox goes 1/4 the length of the box from the front, so with a 32 inch diameter tank you'd still be able to fit. Mine only sticks out so much because I put the flue pipe behind the core. It could be beside it, or even outside on an elbow if you feel cool enough to cut a round hole in a curved surface (which I did not.) I should note that the 7inch system is designed with a 20 inch firebox, but I extended to 24 inches to match CFB size (firebox plus stub.)

Yeah those pages had me going back and forth. I don’t have all the 3D stuff but a quick sketch is something like this.

Edit. Pic needs turned 90 degrees. Haven’t figured this site out yet

Andy Bhill wrote:Yeah those pages had me going back and forth. I don’t have all the 3D stuff but a quick sketch is something like this.

Edit. Pic needs turned 90 degrees. Haven’t figured this site out yet

Ok, I guess that's kinda how I did imagine it. That should work just fine... but it makes the structure more awkward in needing support and you lose the ability to use the top box for an oven or just a window. And seeing the secondary fire burn is half the magic of this design

The Sketchup 3D program was too difficult for me to make anything decent with. But I did find it possible to use for measuring and scaling up Peters existing designs and verifying my calculations.  

Matt Todd wrote:Turns out the guy who designed this core (Peter Van Den Berg) was running a little 4 inch core for his tests in 55 gallon drums.

Not entirely correct, it happened to be a 5" system, quite tiny still.

Matt Todd wrote:

Andy Bhill wrote:Yeah those pages had me going back and forth. I don’t have all the 3D stuff but a quick sketch is something like this.

Edit. Pic needs turned 90 degrees. Haven’t figured this site out yet

Ok, I guess that's kinda how I did imagine it. That should work just fine... but it makes the structure more awkward in needing support and you lose the ability to use the top box for an oven or just a window. And seeing the secondary fire burn is half the magic of this design

The Sketchup 3D program was too difficult for me to make anything decent with. But I did find it possible to use for measuring and scaling up Peters existing designs and verifying my calculations.  

I’m just going to learn a lot more and figure out a size for the tank I have. Yes I do like the secondary burn is amazing. The metal work will be a piece of cake for me but those white boards and chemicals you did is where my ignorance comes in.

Peter van den Berg wrote:

Matt Todd wrote:Turns out the guy who designed this core (Peter Van Den Berg) was running a little 4 inch core for his tests in 55 gallon drums.

Not entirely correct, it happened to be a 5" system, quite tiny still.

Since you are here and I’m just in the process of learning. I do have one question,  when the p traps are installed to induce air, have you tried one that would come out in the center of the secondary burn camber at the bottom?  The only reason I ask is due to what I know about car injection systems. The carburetor came first then there was throttle body fuel injection, then port fuel injection and now cars have direct port injection where fuel is delivered straight into the cylinder.

Andy Bhill wrote:I do have one question,  when the p traps are installed to induce air, have you tried one that would come out in the center of the secondary burn camber at the bottom?  The only reason I ask is due to what I know about car injection systems. The carburetor came first then there was throttle body fuel injection, then port fuel injection and now cars have direct port injection where fuel is delivered straight into the cylinder.

Good question, I'll try to provide an answer.
Yes, I did try feeding air directly into the riser. In fact, that was the point I started developing the floor channel. My red bell heater was set up to allow this type of modifications, the first try was a single rectangular duct at the bottom of the firebox  and tried that at different positions in front of the port, inside the riser and so on. None of those worked as well as the original overhead p-channel. Trying 4 different sizes of duct, twelve different combinations with vertical parts and so on, I came to the conclusion the best spot to introduce air at the moment is the top half of the port in front of the rear wall. In reality, I spent the entire first heating season testing a multitude of possibilties, quite a long list actually.

Overall difference as compared to car systems: injection (in cars) is done with fuel and air already mixed in the right proportions. In this batchrocket system mixing is done in the riser, helped by a whole lot of aggressive turbulence. Combustion is more or less taking place in every stage, although most of the power is generated in the afterburner area, i.e. the riser. In my opinion this heater core bears more resemblance to a wood gasifier although it doesn't sport a forced air distribution. It relies entirely on aerodynamics and under atmospheric pressure, that's why it looks so deceitfully simple.

It relies entirely on aerodynamics and under atmospheric pressure


That makes sense to me. It’s not a internal combustion engine. Doesn’t have the ability to change rpm’s unless it’s forced with air.

I think part of the DSR combustion effectiveness is the change in direction from firebox to top box, and rotating the top box might reduce that effect. Flow would be able to go more directly from firebox to exhaust and create less beneficial turbulence.

Glenn Herbert wrote:I think part of the DSR combustion effectiveness is the change in direction from firebox to top box, and rotating the top box might reduce that effect. Flow would be able to go more directly from firebox to exhaust and create less beneficial turbulence.

Fair enough. I’m only a couple  weeks into learning and the reason I ask is due to the size tank I’d like to use. With the firebox on the outside would allow more area in the bell. I have to consider the bell is 3/8 think and could use a bigger riser. But it’s my thinking based on what I know now. That could change if it’s all wrong.


Sorry Todd!! Not trying to throw your thread off.

Andy Bhill wrote:
With the firebox on the outside would allow more area in the bell.

The important factor here is less about how much space you are occupying in the bell, and more about the internal surface area. That surface area is where heat is exchanged, either into mass for storage and slow release or into metal walls and quickly radiated out. So naturally you don't want to stuff your bell full of core... but as long as you have enough clearance for the exhaust to stream past/exchange with those internal surface areas then you're good.  

Here are the ISA formulas. I believe these are from Peter. Keep in mind that the floor of the bell and the walls of the core do not count.
12.5 cm (5") ISA 3.7 m² (39.8 sq ft)
15.0 cm (6") ISA 5.3 m² (57 sq ft)
17.5 cm (7") ISA 7.2 m² (77.5 sq ft)
20.0 cm (8") ISA 9.4 m² (101 sq ft)
22.5 cm (9") ISA 11.4 m² (123 sq ft)
25.0 cm (10") ISA 14.7 m² (158 sq ft)

And in this forum there's a discussion on how all metal bells can have about 20% less ISA, presumably because they shed heat so quickly.
https://donkey32.proboards.com/thread/1365/inch-batch-box-link-permies

I was very lucky that the tank I used happened to be just about perfect in ISA, since I had already build the core!

For informational purpose's;  
Bell ISA can be oversized by utilizing bypass gates.  
By preheating part of the system including the chimney, a larger ISA bell can be heated.  

Matt thank you for the link. I’ve read about the isa but haven’t saw the thread about steel and the 20%. Good to know. What I have been doing is the weight of the tank. I’m going through the numbers several times to convince myself I’m even close. But maybe others will have the correct answer. Let’s say I have 480# mild steel, raise the temperature from 50 to 400 degrees in a hr, that will take 7-8kw hr.... convert to btu. I think I guess a 6” box would hold about 15# of wood, that’s around 80k in btu depending on type/moisture etc. But I could be way off on my thinking, it’s happened before

Morning,

I've been looking at a DSR2 design for powering a charcoal kiln and the size of the Sherman seems spot on for retrofitting my kiln.

Problem is I've seen so many "designs" and I'm fairly slow to catch on on what is being discussed that I'm at a loss at understanding what works and why...and I'm one of "those" people that learns best by trying to reinvent the wheel.  Anyhow, I realize I don't have enough days in an hour to do that and I'd love to get something built and operating and the Sherman, at least size-wise fits the bill.

So, that begs a few questions:

Can you post the specific dimensions you have come up with for your core?  ie firebox length, height and width?  rear port dimensions and top box dimensions along with "chimney" port?

Also, I don't see where you have put in a P-channel or any other secondary air tube.  Is there one or does this unit simply not use one?

Lastly, in reviewing other DSR sketches in these forums I see that the port from the firebox to secondary box is located at the back of the firebox and not in the roof.  Why is this?

Thanks!

Hey Rick. I have no idea what your kiln might look like would or how it would function, but I can answer your three questions. Would you be loading wood into the tank around the core and firing the core to "cook" your wood into charcoal?

Dimensions- this is a 7 inch system based on Peter VanDen Bergs work at batchrocket.eu
Since he did not publish the specs for the DSR2, I had to follow the breadcrumbs in the donkey pro-boards to find the additional details that differentiate this core from the batchbox core. https://donkey32.proboards.com/thread/3503/double-shoebox-rocket-mark-ii
I have attached my adaptation of the core sizing spreadsheet with the added DSR2 dimensions.
A 7 inch design just happened to fit the width of the flat weld on the fuel tank I'm using (the former top, now side)

Secondary air- Peter co-developed this DSR2 core with a secondary air version and an "open system" that does not use a secondary air tube. BUT to run this properly as an open system, you have to have a door that restricts air to 60%. Please see this thread on how that all got sorted out https://permies.com/t/152831/Open-System-DSR-Smoke

Port location- The port is in the back of the firebox on the DSR2 by design, just like the standard batch box. The original DSR1 design (abandoned) had a top port, and Peter is developing a new and improved DSR3 core with a top port also. (I'm really excited about the DSR3,  https://donkey32.proboards.com/thread/3710/dsr1-vortex-aspects-dsr3)

Hope that answers your questions. Throw any more at me if you have them.

Matt,

Thanks for the info.  Looks like I had most of it save for the top box and stumbler.  Speaking of which, can't say I've seen that word before.  Can you explain its purpose and where in your pictures it is shown?

As for using the DSR2 to fire up the Kiln, basically consider the bell nothing more than a giant oven and the DSR2 as its heat source.  I will have a "retort" charged with hardwood that will be placed inside the "oven" and will "cook down" (distill) the hardwood until it is charcoal.  There is a bit more to it than that including directing the distillates (steam, "wood vinegar" and volatiles) out of the retort and back into the fire.  The big thing is to heat up the "oven" cleanly and without a lot of smoke which in a traditional kiln isn't as easy as it looks.

Rick Wood wrote:
Looks like I had most of it save for the top box and stumbler.  Speaking of which, can't say I've seen that word before.  Can you explain its purpose and where in your pictures it is shown?

The stumbling block is not visible in any of my pictures, but I've circled it in this diagram of the stove. My understanding is that it provides a bit of flow re-direction that prevents flames from shooting out of the top port and is essential for a stable burn. Not much to it really, just an extra bit of material on the top box ceiling.

I should note that the measurements I gave in my last post are for an elongated firebox, because I wanted to fit 20 inch wood. This is well within the "It can be 25% longer than original specs" tolerance that Peter said is ok. Which is a discrepancy you might run into if you do your own math for a different core size. Regardless of size, the stumbling block goes in the middle of your top box length.

Awesome application you described! Seems like a great fit. If I had any concern it might be accidentally beating up a CFB core with all that wood loading and shifting so  you might consider other materials. CFB just worked best for me as something I could slide into the bell. Since you would presumably have a big access door of some sort to load your wood into for the charcoal process, that gives you more room to work with bricks or castings inside.

Below is a very rough sketch of what I'd like to do.  


The upper part is what I have now.  A bell, retort, and stack.  the whole thing sits on top of a tire rim with a fire built under the retort so the flames hit it directly.

I'd like to replace the fire ring (inefficient and smokey as hell) with a cleaner burn.  since it takes 90 minutes to three hours to cook the charcoal I'm leaning towards the DSR2 but a j-tube is an option.

With the J-tube I just need to center its stack on the bottom of the retort.

With the DSR2 I'd just remove a section of the top part of the upper tunnel and use the bottom of the retort for that ceiling.  The exhaust gases can exit next to the bottom as shown to heat the bell.

Clear as mud, right?

Hi Rick;
You got pretty good mud over there!  I can see rite thru it!
I think either rocket design would work for you.
The batch design would burn longer and a touch hotter than a J tube.

Rick Wood wrote:Below is a very rough sketch of what I'd like to do.  

Oh cool, diagrams are helpful. I originally thought your bell WAS your retort and, hence my concern about wood cooking/steaming/breaking down all around a CFB core.

You mention using the retort itself as part of the ceiling of the core. I think that would work! You'd have direct heat on the bottom of the retort and the exhaust stream would heat all around it too. One thing you might run into is space inside the bell though. My core occupies a lot of vertical space in my bell. I have room on either side of the core for thermal mass, but little clearance between core and bell ceiling.  Granted, you can use a bigger bell as long as you have a bypass to get the flow started.

So, I'm looking at the actual bell I have to use and you may be right...not a lot of room.

If I had to scale down the batch box to fit it I know I can go the table that peterberg put up for standard BB builds, but what what ratios would I be looking at for the top box.  For example how tall should it be, where should the chimney/exit port be, size of stumbler, etc. for that matter are there rules of thumb for those things?

Also you mentioned that you made the firebox a bit longer than the table specified, that there was a bit of flexibility in the firebox length, something like 25%.  Is that plus or minus 25% or just plus?  Right now my fuel wood is 20 inches

Rick Wood wrote:If I had to scale down the batch box to fit it I know I can go the table that peterberg put up for standard BB builds, but what what ratios would I be looking at for the top box.  For example how tall should it be, where should the chimney/exit port be, size of stumbler, etc. for that matter are there rules of thumb for those things?

The top box width equal to that of the firebox, height same as width, i.e. square.
The internal ratios up to the top of the firebox are conform the published table, I'd recommend to build just a rectangle box, 1" deeper than the riser would require. The short riser, called riser stub in this case, can then be formed with Superwool at three sides, left and right plus rear. The wool stops at the top of the firebox' deck.
The top box is err... on top, as wide and long as the lower box. This way, there will be a sudden step wider above the wool. The stumbler for a 6" version is about 2" wide and high, situated halfway in the depth of the top box. Spanning the width of the box, mounted at the ceiling. The core's exit is situated roughly at a quarter from the front and in the ceiling, as wide as the box and its cross section area is 80% or less than the riser's csa. That last one is assumed to be round, not square so you need to take the csa of a circle, its diameter equal to one side of the square.
Did I forget something? Don't hesitate to ask.

Rick Wood wrote:Also you mentioned that you made the firebox a bit longer than the table specified, that there was a bit of flexibility in the firebox length, something like 25%.  Is that plus or minus 25% or just plus?  Right now my fuel wood is 20 inches

The firewood doesn't need to fit, rather 10 cm (4") shorter so there's space in the front and rear for air. Since you are planning to scale the core down I think lengthening the firebox in order to fit 20" fuel in there is overstretching the situation quite a bit. I'd think cutting your firewood down to 10" would solve this potential problem. Remember, you'll get much more heat out of the fuel as compared to a bog standard wood stove. Possibly twice as much, so cutting again isn't as daft as it sounds.

Regarding the plus or minus tolerance: I tested a longer firebox, not a shorter one. So the 25% length tolerance is plus only. Mark that the core doesn't like fuel length exactly the depth of the firebox, shorter is no problem. Even half the depth didn't throw up a wobble, at some point I fed the system a bunch of small pieces which was gracefully accepted.

I was honestly surprised at how big the 7" core was when I assembled my mock-up out of foam. The measurements were one thing, but seeing it was another. Have you checked out the DSR3 thread yet? It will be much less long, maybe leaving room for a retort behind it in your tank.   A DSR3 sized for 20" wood would only be 20" long (plus material thickness) since there is no riser stub behind the firebox like in DSR2.

Yes, those batchrockets scale up very quickly. When every linear measurement is scaled up, say, by a factor of one third (from 6" to 8") the volume jumps up a factor of 1.33x1.33x1.33=2.35, meaning the size of the firebox is more than doubled! A ten incher is huge, a 12-year-old kid is easily able to hide in there. A one foot system's volume is a factor 8 larger as compared to a 6 incher, imagine that! Never saw such a biggie, by the way.

OK all,

Thanks again for the input!  I've had some time waiting to change my circadian rhythms from night owl to day worker to read through some of the links and while I'm better informed I'm still a bit combobulated.

Am ready to start construction and am now in the planning stage.  I like the 7 inch CFB model we've been talking about as it seems the path of least resistance.  However, CFB is not readily available and I don't know enough about it to be confident ordering it online.  Standard firebrick, however, is cheap and local.

In my mind, simplicity and use of commonly available materials is preferred.  So let's start with a couple basic questions:

If I were to dry stack and use a standard brick length for the firebox floor and ceiling, what size DSR2 would that produce?

Will Dry Stacking be sufficient or do I really need to use mortar/cement between any joints?

If I wanted a wider firebox I assume I could cast "slabs" for the roof and floor while using standard firebrick for the walls and riser stub and port.  I've had poor luck searching for videos of the casting process...Any recommendations to videos/links that go into that process in a bit of detail?  Also, what sort of lifespan do casted slabs generally have?

Lastly, I need to be sure to reach and sustain attain an internal bell temperature of 750*f.  Any gut sense as to what size DSR2 will be best suited for that?f   Obviously bigger is more likely to do that, but I am limited to a 55 gallon drum for the bell and 30 gallons of that will be occupied by the retort vessel.

Below is a revised layout...keep in mind the DSR2 size in the image is not necessarily proportional to the bell and retort. the size of the DSR2 is what needs to be determined primarily based upon ability to heat the bell/retort to 750 for up to 90 minutes.   the DSR2 doesn't have to fit within the bell, rather the bell and retort will be set on top of it such that the exhaust from the DSR2 impinges on the retort bottom. a mating flange of sorts will be sorted out later.

I would think you would need to heavily insulate the drum to maintain that sort of temperature?

Absolutely,

In fact the bell is heavily insulated as I mentioned that in an earlier post. The design shown in the picture is the same configuration I've been using for about 10 years, the only difference is I was using an open burn fire underneath it.  Essentially I converted a truck rim fire ring into a "woodstove" and set the retort and bell on top of the ring.  Fire underneath directly impinging on the bottom of the retort.

What I am looking to do is upscale from the current version which is a 15 gallon retort inside a 30 gallon Bell. I'm upscaling to a 30-gallon retort  and and 50 gallon Bell.

I also want to use the DSR2 to as the heat Source because of more efficient and cleaner burn

For a true bell you will want to move your exhaust down to the side about 1/4 or 1/3 from the bottom, but above the top of the fire box. Then the super heated gases will stay in the top of the upper barrel while the relatively cooler gasses escape out the side near the bottom.