portable rocket mass heater

I have measured the barrel temp.  My thermometer goes to 800 and the temp is greater than 800. 

I have called ernie and erica many times about this.  My impression is that my ideas are actually good ideas that have not been tried.  Neat!

As for smokeback:  ernie suggests adding a layer of brick at the wood feed. 

I want it to be even rockety-er - stronger draw.  Ernie suggests a taller combustion chamber and closing the gap between the heat riser and the barrel by a quarter of an inch.

Frankly, closing the gap makes me think that it will slow things down, but ernie says it will make things go faster. 

Decreasing the gap will mean the air has to move faster through that space thus creating a better draw.  There are limits to this, at some point frictional forces and energy loss due to increased turbulence overcome the benefits but a 1/4 inch closing of the gap shouldn't get you to that point.  Just recall that a chimney works by not only creating a stack height but also constricting the exhaust as compared to an open fire, same concept here just looks different.

it dont look any different; Paul's just not squinting right.

mekennedy1313 wrote:
Decreasing the gap will mean the air has to move faster through that space thus creating a better draw.  There are limits to this, at some point frictional forces and energy loss due to increased turbulence overcome the benefits but a 1/4 inch closing of the gap shouldn't get you to that point.  Just recall that a chimney works by not only creating a stack height but also constricting the exhaust as compared to an open fire, same concept here just looks different.

I don't think that is an accurate and complete description of what is going on here. Yes a smaller gap means lower cross-sectional area and higher gas velocity, but just to the extent that it compensates for the smaller area, all other things being equal (and no friction/turbulence) the volume of gas per unit time would be the same.

What is primarily responsible for that effect is how efficiently you evacuate the heat from the hot exhaust gases after they exit the heat riser. Hot Air rises, and that what generates the draw and powers the rocket stove, however on a rocket stove you have to force your hot gases back down the outside barrel and the hotter they are the more energy it will take to do that. When the heat riser gets nearer to the top of the barrel that convects those hottest fresh combustion gases closest to the barrel surface, where the heat is conducted to the steel of the barrel and then radiated out into the room. Additionally in a barrel topped rocket stove a heat torus is formed where the gases slow down temporarily. Moving the riser higher does increase velocity (as was said before) which decreases the pressure within the center of the cross-section and makes a tighter torus, which rests higher in the barrel, again radiating the heat before you have to force the hot gases down.

The more heat you ditch from the top of the barrel the less you have to pay (with draft energy) to force to the bottom of the barrel. This makes it less efficient as a mass heater, but it ensures that it is an efficient combustion device.

So, a few more bits from ernie:  I get the impression that after adding some bricks to the feed, that this is doing plenty good. 

So Paul I don't remember linking to your rocket stove page, (does this forum/ do you) do that automatically? Or have I lost my mind?

I have modified the software to do that automatically.  Kinda like wikipedia does.

I have moved.  And I am now living in a place that does not have building codes.  And the place I am in has a spot for a dryer, but I dry all of my clothes using a clothes line or drying rack.  The room that the dryer would go in isn't a place where you would really sit on a bench - but it is a place where if there were a RMH, the heat would probably end up all over the house. 

This place currently has exclusively electric heat. 

A variation:  Arround the seal of the bricks and barrel, use a mix of sand, perlite, ash, DE, etc.  Stuff that will help make a seal.  Around the duct, put just rocks (golf ball size to melon size), held in place by 2x4's. 

The output can then be the dryer exhaust.

How does this sound?

Another question:  what about putting rocks on the barrel?  I suppose that if the barrel gets hot enough, rocks could change the shape of it and that could be dangerous.  So maybe I would need something metal that sits on the rim of the barrel to hold rocks off of the flat part?

paul wheaton wrote:
I have moved.  And I am now living in a place that does not have building codes.  And the place I am in has a spot for a dryer, but I dry all of my clothes using a clothes line or drying rack.  The room that the dryer would go in isn't a place where you would really sit on a bench - but it is a place where if there were a RMH, the heat would probably end up all over the house. 

This place currently has exclusively electric heat. 

A variation:   Arround the seal of the bricks and barrel, use a mix of sand, perlite, ash, DE, etc.  Stuff that will help make a seal.  Around the duct, put just rocks (golf ball size to melon size), held in place by 2x4's. 

The output can then be the dryer exhaust.

How does this sound?

Sand, perlite, ash, etc. may not make a good enough seal, if DE is diatomaceous earth it all sounds like a lot of nasty-sharp, rock dust without any binder to hold it together.  Maybe you meant to use some clay and those are the aggregate of your cob mix; if so, that's some mighty expensive cob. Why do you keep changing things from the basic design?  You don't like mud?  Parts of Montana are ready-mix, you just dig it out of the ground after a good rain.

Just Rocks: Limited thermal contact, reduces efficiency of system, the air gaps will still insulate a lot of your heat.  But if you don't mind a less-efficient system, as the tradeoff for not having yucky mud to deal with... I guess I can keep playing with the idea, just because it's you.  What have you got against mud? 

Main problem is with a less-efficient system, you need a bigger one, and if you get much bigger than 8" (and you have extra insulation where the original design doesn't) you may be looking at localized temperatures too high for the materials.  And at moving a lot more rocks, or just building a big contraption that doesn't give you much benefit except emergency / supplemental heat.

Other thing to watch for is uneven support / wear on your ducts, I suppose they're designed to be exposed but I really like the extra support the cob gives them.

No problem with dryer exhaust as output, unless your system is too hot because it can't dump heat efficiently into the thermal mass.

If you don't have anything against mud, you just want to be able to dis-assemble and re-assemble it: Ever consider using adobe bricks, or even regular bricks, instead of lumber, to make the frame and footing?  You can make 'modular cob' (adobes), and pour the core with monolithic cob to make good contact along the curving pipe.  You'll get the thermal contact; you can mortar or dry-stack the adobes depending if you want more thermal contact or quicker removal; and your thermal mass will be more compact and take up less space, while capturing more heat because of the better contact.

I realize these earthen materials are dense and moving them involves work, but rocks are too.

paul wheaton wrote:
Another question:  what about putting rocks on the barrel?  I suppose that if the barrel gets hot enough, rocks could change the shape of it and that could be dangerous.  So maybe I would need something metal that sits on the rim of the barrel to hold rocks off of the flat part?

Putting rocks on top of an old metal drum while it's hot doesn't seem to appreciably change the shape of the metal drum.  People put all kinds of things on top of RMH barrels while they're hot.  Kettles of water, rocks for a sauna, cookware, accidentally abandoned clothing.

I guess if you're building a weird system it might not remain at normal "hot woodstove" temperatures like most RMH's do.  But at normal temps, deformation from weight is not a huge problem.  I'd worry more about the ducting, which is thinner and hidden.

Issue with mud:  part of this mission is "portable".  I want to be able to take it apart and put it on a truck in less than an hour.  And then go somewhere and take it off the truck and put it back together in an hour.

With the rocks:  I'm thinking that if I keep the size to an inch or bigger then air will rise through the rocks.  I'll get more instant heat and less heat storage.  But I guess this is an area I'm exploring:  how to come up with something that will work in some situations. 

The problem with the sand was that it was being insulative, so heat was leaving the house.  With rocks, the insulative factor is almost eliminated.  You get more instant heat, but you do still get some thermal mass. 

Just getting into RMH'ers myself and like the idea for camping and such. Just an idea, why not use water as your mass? I'm not sure how but it just seams as a nice way as you can drain the water when you want to move it and then fill it when you get it moved. it may even be used as warm shower water or something, forgive me if I'm way off on this just a thought.

Paul,
I can't thank you enough for creating these sites. I spent this entire day watching your videos and reading posts. Felt like I'd been to a day long seminar on all the things I want to know more about. I have a multitude of questions, but I'll read more forums before I ask-in case the answers are there.
Keep it up-please!

Hydrow wrote:
Just getting into RMH'ers myself and like the idea for camping and such. Just an idea, why not use water as your mass? I'm not sure how but it just seams as a nice way as you can drain the water when you want to move it and then fill it when you get it moved. it may even be used as warm shower water or something, forgive me if I'm way off on this just a thought.

Well, do you have a drawing, perhaps, of your idea?

tony21752 wrote:
Paul,
I can't thank you enough for creating these sites. I spent this entire day watching your videos and reading posts. Felt like I'd been to a day long seminar on all the things I want to know more about. I have a multitude of questions, but I'll read more forums before I ask-in case the answers are there.
Keep it up-please!

Now that is mighty thoughtful!

Look at what this guys did!

http://www.iwilltry.org/b/build-a-rocket-stove-for-home-heating/

yep look what he did

paul wheaton wrote:
Look at what this guys did!

http://www.iwilltry.org/b/build-a-rocket-stove-for-home-heating/

There you have no mass at all. You want 2 have the smallest cross sectional area that still works and you want to burn twigs or small diam. wood and not much at a time, because as the guy says, the room cools fast after the fire goes out. So you want to keep it going all the time, but not get too hot.

Either use it to heat a big room so you can make it run at normal wood volumes.... or do some experimentation to see if you can get a smaller pipe diam to work ok-ish.

My other thought, is that this could feed a mass bench/block. Because the whole thing is welded steel (besides the insulation in the riser) and sealed right to the output (which is round and fits a pipe) this could be the first part of your portable heater... I could get this into my p.u. on my own. If the mass heating pipe could be lifted as a unit or stay in the box, that gives you unit two. Now you don't have to worry about sealing, just add gravel and put 24"x24" pavers on top for your bench. I would dry stack brick as close to the barrel as (touching) as I could at least half way up. If you are worried about them falling, line up the holes and drop short pieces of re-bar through. Or... you could go soldier with the bricks and run chicken wire through the holes to hold the bricks tight to the barrel... maybe not too tight as the barrel being hotter will expand faster than the chicken wire.

Assembly and disassembly...
1)Lift box with pipe from truck and position.
2)Lift tank into box and connect to pipe with big SS hose clamp
    (you attach exhaust at this point too... dryer hose?).
3)Fill box with gravel (or whatever mass you choose.....).
4)Put pavers on top of box.
5)Dry stack bricks around tank and secure with chicken wire.
6)Sit down far away from heater for a cool one because you worked hard to get this done

Some where around step 3 you should be able to start it burning if you were in a hurry to get the room warm, the lack of mass at that point would be a good way to get "quick" heat.

I would do the transfer piping different too (at least try). I would run the hot gas from the tank into the top pipe instead of the bottom. put a T soon after that connects to the bottom pipe... see if this really bad picture helps:


Anyway, I've laid it out without the big fold in the middle for clarity. The idea is that the gas stratifies by temp and the coldest gases are what you exhaust. In the masonry heater world they would (sort of) call this a bell. They did some experiments with the flu on the floor with just some capped flu pipe sticking up and found that the top of these capped flu pipes was warmer than the bottom. They were only 24 inches high I think. Points A and B could be other places to connect the top and bottom for easier/better stratification. This would mean that the gas would be moving a lot slower in the heat transfer tube than what you have now but the back pressure would be the same because the exhaust would be the same. This does put the exhaust at the other end of the mass from the tank and you might want to run it back through.... I'm running out  of ideas about now.

I've looked through this a few times.... and one of the things missing is what size truck you are going use to cart this thing around? Either you are making a design as a prototype... think 1/2 ton PU, or you have a truck you use. I would suspect the later. Anyway, The mass has to fit, plus whatever tools you need to assemble, plus fuel.  Maybe with the size you are making it's not that heavy..... but even a half pallet of clay tiles overloaded my old 3/4ton.

I am thinking - why not figure a way to skip the mass when transporting for display? People can watch the heater work and visualize that the mass will work as heat storage and butt warmer.

Then, if you want to use it for the winter somewhere, add the mass.

ronie wrote:
I am thinking - why not figure a way to skip the mass when transporting for display? People can watch the heater work and visualize that the mass will work as heat storage and butt warmer.

Then, if you want to use it for the winter somewhere, add the mass.

Or try two gas water heater tanks in parallel. The center tube of two at 4in diam is still less than one 6 inch (24sqin vs 28sqin) , but if you could find something with 5 inch tubes... I have seen tall skinny water heaters.... but not new. The main problem with this idea is that even though you could fill them on site, they would be a huge bench, too big I think. Also they might take a long time to heat up even with "only" 40 gallons each.

Something smaller might be needed like a box of tallow or lard (um maybe not, rancid fat smells bad) or paraffin. These phase change in the right area to make a smaller mass go a long way. For that matter, raise the lip of the barrel 4 inches and fill it with printers metal or solder (probably a collar around the barrel lower down would be better as the solder may have the effect of keeping that part of the barrel cooler and we need a temp differential to make the rocket work). once that melted it should store more heat than mass alone would indicate. Most phase change systems are based on lower temperatures to work with solar systems, but with a wood burning appliance  something with a higher temperature may work just fine. It could be enclosed to keep fumes from being a problem if it's boiling point is high enough. When it cools it is solid and won't spill anyway.

All of these things may start to cost more though, not the idea here.

Well, this is my first post, and i'm on 3 hours of sleep, so ... well, you know.

I've spent many hours researching this stuff in the wee hours, watched all the vids and read bunches of sites, and somehow managed to still not order the books- not about money... just keep spacing out...

ok, just sent the request to my financier... may order the pdf today, as i'm doing a "workshop" (rather just showing what i've learned and playing) tomorrow for students.

I've so far made a j-tube unit out of regular vent piping. The center tube melted. http://www.flickr.com/photos/divb/5390521505/

I bought a stovetec stove- we'll fire it up tomorrow the first time.
experimented with a couple other things, including some big pipes, ceramics and various kinds of bricks.

I prototyped this in various configurations: http://www.flickr.com/photos/divb/5390476429/



I found paul's think about the portable unit- which is exactly what i need/want for my project: http://ecobox.me - we're in shadow almost all day during the winter, so solar gain sucks-was going to be totally solar, but can't right now. Need some heat: http://www.youtube.com/watch?v=QxsxGT_ZSFY


So... i want to be able to move it in a couple hours, not have messy (or wet stuff) which rules out cob, though i did buy a couple bags of lava rock to play with.

My idea is to build the combustion chamber into this can: http://www.flickr.com/photos/divb/5390578459/

Then use the triple-wall pipe: http://www.flickr.com/photos/divb/5390476429/ as BOTH the draw tube and outer cover- i played with this by putting the wok (see picture) over the top and actually got steam out the bottom that i was able to inhale - no pain, no eye burn (which i'll get immediately in the smoke)... and that was with a pretty crappy way out at the bottom, since it's temporary and i don't want to cut the $100 pipe until i know what i need.

I'm not really clear on the dimensions of burn tube- and am concerned that the metal that holds the outer metal onto the insulated double wall inside will obstruct the flow (sort of 4 "spokes"):  http://www.savemoneywithus.com/images/Img742.jpg

i have a combination of thin heavy firebrick and the thicker very light refractory brick. I need the individual components to be either easily moved by two folks (up and down stairs) or packaged up(i.e. gravel can be bagged, whereas cob is more of headache).

So i've very much got Paul's mission in mind, but far less experience. I also don't mind spending a little to make it work, and would love to have the visible stainless- even thought of putting a big pot over the top and using that as a barrell... but don't know how short i can go before things stop working.

Open to ideas and suggestions. I'd have more pictures of my experiments so far, but they've all been in the middle of the night... as in 2 am, really...

best,

d.i.

This is Erica on Ernie's login ....

I'm late for work on our actual rocket mass heater project, but I just wanted to drop the idea that for portable systems where you have plenty of money, brick might be a good thermal mass.  You can line certain sections of the pipe with brick and a small amount of mortar, or cast little corner wedges from refractory / concrete.

Welcome, Divonbriesen, and good luck with your projects.  i think you'll find most of the proportions you need in the book.  Height is mostly determined by H>L+h, where H is the heat riser, L is the length of the horizontal portion of the burn tunnel, and h is the height of your feed tube.  In practice, most heat risers are 3 feet or more. You can make a large part of this from brick or masonry, but for a portable system a whole barrel is more convenient.

Yours,
Erica

divonbriesen,

I cannot see your pics without creating an account there.  Feel free to upload your pics here (click on "additional options" when posting).

Ernie and Erica:  I'm thinking about a combination of rocks, bricks and pea gravel around well sealed ducts.  I think you are right about the sand.  To get the sand to be a thermal mass instead of an insualator, you gotta pack it super tight, which just takes out a lot of the benefits.  With pea gravel and rocks/bricks, then you get enough air flow that it is not insulative.  And you get something that does not require packing. 

Great idea Paul!
I’m working on a design for a custom built, very small (200 square feet) house on wheels, and would love to include an RMH. With this type of RMH design, the thermal mass material (rock, adobe bricks, earth bags, glass beads, water, wax, etc.) could be removed before taking the house on the road.

There may be some that would scoff at the idea of having an RMH in a mobile home (house on wheels in this case), but I don’t see why the trailer frame couldn’t be custom built, if necessary, to support the weight of the RMH when the house is shored?

Given the tiny size of the house, the RMH won’t have to be very big in order to heat the space; I’m just wondering if there is a minimum size that an RMH should be for best results? I guess that it would just depend on being able to find the right materials in a smaller size.

Even if a RMH built for a very small house on wheels lacks some degree of thermal mass and is not “super” efficient, there still seem to be a number of advantages in using this type of heater. And I still haven't ruled out the possibility of using plenty of mass - as long as it can be disassembled/removed/drained when the house is to be moved.

I’d love to hear any thoughts on this type of application. And if anyone would like to see my small-house-on-wheels with an RMH design plans (Google Sketchup) in order to know what I have in mind, then let me know.

Thanks to all for sharing your experiments and thoughts!

Ernie Wisner wrote:

Welcome, Divonbriesen, and good luck with your projects.  i think you'll find most of the proportions you need in the book.  Height is mostly determined by H>L+h, where H is the heat riser, L is the length of the horizontal portion of the burn tunnel, and h is the height of your feed tube.  In practice, most heat risers are 3 feet or more. You can make a large part of this from brick or masonry, but for a portable system a whole barrel is more convenient.

Yours,
Erica

Thanks Erica! That's a great little formula to keep in mind. I've not read the book cover to cover yet, but given some attention to the diagrams, and in particular noted that the burn tube should be the fattest part of the thing. I also saw that it said that the shorter the tube the better- so if the barrel allows, would you ever want to just not have a burn tube at all? I.e. if it was burning right up into the pipe would that be OK?  I was amazed by the performance of my jtube (just two 90 degree pipes put together to make 180) and am curious if the channel is simply needed to get the burn over to the bottom of the chimney.

More questions, but should probably finish book first.

I think between your suggestion of brick and pauls of pebbles and such we can figure something out. My concern is getting a tight enough connection between the pipe and the surrounding material to xfer the heat well... hard to imagine anything bonding like cob- and as paul said, sand will just insulate. Maybe flat plates welded to the exhaust pipe? They could sit on top of and under rows of bricks...

Breh- I'd love to see your plans

Paul- flickr is the world's number one photo site and you should be able to see most all (and all of mine) without any account whatsoever.

Having said that, i just checked, and some of those were listed as private &*(&%$($&*^%($)(32-$!~! - fixed - sorry. Getting used to my new phone, which i've indoctrinated by breaking, twice.

d.i.

Hey Paul, I just saw the video on RMH on the main page of this site and I noticed that you were using ductwork as your riser, or am I missing something? If it is ductwork, will it work long term or did you just do it as a test? Im trying to get local materials for a RMH and obviously ductwork would be alot easier to get, but I have been told that it will not stand up to the heat. Thanks for your help and the vids, they are awesome!

I just had a concern with the pea gravel:  what about leaking ducts or ducts decomposing?

Rob S. aka Blitz wrote:
Hey Paul, I just saw the video on RMH on the main page of this site and I noticed that you were using ductwork as your riser, or am I missing something? If it is ductwork, will it work long term or did you just do it as a test? Im trying to get local materials for a RMH and obviously ductwork would be alot easier to get, but I have been told that it will not stand up to the heat. Thanks for your help and the vids, they are awesome!

My impression is that the duct inside might eventually burn away.  That's why the stuff is made to hold its own shape.

Ah I think I see, so basically that tube was more of a form? Will the non metal riser get as hot as the other materials in there? And I assume a barrel will burn away too eventually or maybe its just rust that makes it fall apart?

I get the impression that if any of the inner duct burns away, it will probably be closer to the bottom.  And that while it is possible that the outer barrel can eventually burn away, it would take many years of heavy use - and then you just replace it.

Thanks Paul, so do you think I would be ok with just using ductwork for an outdoor RMH, its more so for small scale use than home heating at this point. BTW, Im listening to your podcast with Jack right now, while Im at work I love America!

paul wheaton wrote:
I get the impression that if any of the inner duct burns away, it will probably be closer to the bottom.  And that while it is possible that the outer barrel can eventually burn away, it would take many years of heavy use - and then you just replace it.

the bigger issue might be that the ducting holding in your insulation around the riser appears to be galvanized.  when that gets hot, it's going to vaporize the zinc (or otherwise cause some nastiness), which is not nice.  won't be a long term issue, just be careful not to do the it's-just-CO[sub]2[/sub]-and-steam/face-in-exhaust demonstration for the first few firings.  or the first year of firings.  unless it isn't galvanized at all.

don't know if this has been mentioned, but instead of lots of elbows (90 degree turn slow down the exhaust), you need to make some box ends for the pipes.  I can't remember what these are called, but instead of an elbow, it is a big box, so there is considerable volume right there, and you avoid the issues with gasses and lots of 90 degrees.

It is basically like this:
                  ______
--------------|          |
tube                      |
--------------|          |
                  |          |
--------------|          |
tube                      |
--------------|          |
                  |--------|

Does that make sense?

I built an RMH to fit in my converted semi trailer, the heater is18"x30"x28"t the mass is a 4'x6.5'x18" platform under my bed filled with sand to 14" air vents and fans to circulate heat.

The first Mod was 16ga SS fire box and chimney welded together insulated with rock wool dumping into a 20 drum with 1/4"steel replacing the thin bottom/top.
I wanted to burn wood chips as fuel due to cost and availability, this worked real well until I built a hopper for a larger fuel supply with out any regulating of how much fuel could burn at a time, depending on gravity to feed the fuel. oops
Within a few minutes of dumping in a 5gal pail of dry chips I had a run away with the entire works barrel, heat shield, fire brick and all glowing bright red, I could smell organic mater in the sand bed burning.
After opening windows and doors and the nuclear meltdown narrowly avoided the heater didn't work as well as it did before? upon investigation and a tear down I found the fire box carbonized and pillowed in on it self, the inner chimney (16ga SS) broke off like potato chips completely carbonized.
Wonder how hot that was???
I've since rebuilt it with the more conventional materials, and have been sorely disappointed with its performance.
Will do it again using high temp ceramics or inconel
This unit is relatively portable, two men can pick up the heater portion, the mass/sink is a bit more problematic

@ Tarkus,
Yikes!
I hope you took some pictures of all that?
By "more conventional materials" do you mean fire brick and cob?
I look forward to hearing about your next build!

velacreations wrote:
don't know if this has been mentioned, but instead of lots of elbows (90 degree turn slow down the exhaust), you need to make some box ends for the pipes.  I can't remember what these are called, but instead of an elbow, it is a big box, so there is considerable volume right there, and you avoid the issues with gasses and lots of 90 degrees.

It is basically like this:
                   ______
--------------|          |
tube                       |
--------------|          |
                  |          |
--------------|          |
tube                       |
--------------|          |
                  |--------|

Does that make sense?

That does make sense.  But does it work? 

@Breh, When I can get my Camera software to function properly I'll document and post the next build of the RMH. Conventional meaning Firebox made of fire brick and stove pipe for chimney. My supposition is that the fire brick box absorbs a lot of the heat first off slowing down the rocket draw, were as the first incarnation absorbed very little heat due to the materials. The 1/4 steel "kettle plate" would glow red within 10 minutes of start up and bring the kettle to a hard boil a few minutes later. Next incarnation will have a much thicker "Kettle plate" to hopefully  avoid the warping that the 1/4"plate experienced.
Fuel is also a consideration with wood chips large surface area more energy is released much faster, may incorporate an auger to control fuel feed.
Thanks
David

paul wheaton wrote:
That does make sense.  But does it work? 

Yes, it does work.  That is what pros use for air conduits and such when there are lots of curves.  The key is giving a lot of volume at the bend, and you avoid the majority of the friction losses.  The cooling earth tube folks could give you an education about that.

The box wants to be ab out 2X the cross section area of the tube.

This will drastically improve the flow through a long pipe with lots of 90 degree bends.

What about just upgrading to a larger tube while doing the 180?

What about just upgrading to a larger tube while doing the 180?

As long as you upgrade to a big enough tube.  If the tube starts off at 8" diameter, you would need to upgrade to about a 12" diameter.

What folks do for cooling tubes is put a small barrel (like 25 gallons) on its side, and the tubes just connect to the side of the barrel.

You could probably make a metal box, 10" on a side pretty easily.  Or make a 12" diameter cylinder.