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rocket mass heaters

Cottage Rocket: Any experience with building them?

Benjamin Dinkel

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posted 7 months ago

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Hey fellow Rocket Scientists,
the plans for my first J-Tube RMH with bench in October fell through, it's probably going to happen in January.
So while I'm waiting on that I thought I could build a Cottage Rocket for a friend that is renting a house with a (shitty) woodstove.

From some kickstarter or something I got the plans from Paul, Uncle Mud and the Bernal Bros. (permies.com/t/193729/Plans-Cottage-Style-Sauna-Rocket#1607390).
They look great, I'll go off what's shown there.

I was interested to see if someone built one already (according to those plans?) and wants to share some pics and some findings/lessons learned.
I found this thread with some helpful pictures (https://permies.com/t/190054/Seeking-plans-oil-drum-rocket)

The plans mention insulative fire brick. I haven't seen that in Spain yet. Do y'all think that dense firebrick (1" thick) plus 1" insulation would work instead?
Same question for the riser, as I've not been able to get my hands on Superwool. Rockwool wouldn't hold up to the temp and flame abrasion in the riser, would it?

I'll try to find 3 steel barrels soon and start by cleaning them, I'll post pics.

A good starting point to all RMH research: https://permies.com/w/rmh-resources and here https://www.permies.com/t/41635/Top-Questions-Rocket-Mass-Heaters
A good starting point for permies.com questions: https://permies.com/w/how-permies-works

Benjamin Dinkel

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posted 7 months ago

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Dear Rocket Experts,
any opinions on the material questions?

Could 1" dense firebrick plus insulation instead of insulating firebrick work?

As for the RockWool in the riser I found a pretty clear video that that's not an option, thank you Peter Peterson.

Any reasons why 1" dense firebrick plus insulation would be a lesser option?

Phil Stevens

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posted 7 months ago

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Firebrick with insulation around it will work. It will take a little while for the riser to come up to temperature, so the initial part of a burn won't be super efficient. But after that it should go great. Fox James in Guernsey uses vermiculite board, but I don't know whether that is something you have access to in Spain.

Fox James

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posted 7 months ago

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You can definitely use fire brick backed with insulation.
Dense material will take longer to heat up and can cause start up issues if there is not a strong drafting chimney.
Insulating material will offer very quick start up but many of the products are very bad for your health!
The most effective method is, just ceramic fiber wrapped inside a metal tube.
However this could (will) result in dangerous, loose fibres, potentially polluting the air.

Benjamin Dinkel

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posted 7 months ago

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Thank you Fox and Phil!

Ana Chorlito

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Location: NW Spain

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Hello, you can find these bricks in Spain in ceramic supply stores, they are used for ceramic kilns. Sorry for my English.

Benjamin Dinkel

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posted 7 months ago

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Hey Ana. Thank you for your reply.
What stores exactly do you mean? I can find dense firebrick in some of the building material stores, but have not seen the insulative ones. Do you know what I should ask for in Spanish?

Ana Chorlito

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posted 7 months ago

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Hello, in Spanish they are called "ladrillos refractarios aislantes", here is a link to a store where they have them.
https://anper.net/productos/415/construccion-de-hornos/re005631-ladrillo-refractario-aislante-230x114x64-1400-c-t-26

Ana Chorlito

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Hello, in Spanish they are called "ladrillos refractarios aislantes", here is a link to a store where they have them.
Oh, I think you can't add links, it won't let me, the store is called Anper dot net, I'm sure there are more sites that have them.

Benjamin Dinkel

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posted 7 months ago

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Muchas gracias Ana!

Ana Chorlito

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No hay de qué

Benjamin Dinkel

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posted 3 months ago

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I was finally able to continue with the cottage rocket.
I picked up a couple of barrels that have clamp tops, burned the paint off and slowly fabricated the parts for the cottage rocket.

Here they are waiting to be assembled:

Opening the manifold (1/3 of a barrel) for the exhaust pipe

The interior "shelf" that the core will rest upon. I used a lid of another barrel. If I were to do it again i would flip it around, because it concave now, looking at it from above. That means with the weight of the cob on top it kind of springs down and back up again.

Getting ready for the first cob

Side panels in place

5 cm of cob in place and the side panels secured by the main barrel.

Next step is building the core on top of the cob.

Benjamin Dinkel

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posted 3 months ago

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I'm basing the build mostly on the plans permies.com/t/193729/Plans-Cottage-Style-Sauna-Rocket#1607390 availabe here on permies.
I have to say it's not as straight forward as it seems at the first glance. The plans do not provide a lot of measurements, also they would change with materials available.

I'll try to document the build as well as possible. Maybe it'll help the next person building a cottage rocket. But I often get "in the zone" and forget to take photos...
Any questions are obviously welcome.

Rico Loma

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posted 3 months ago

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Looking great thus far
Thanks for documenting this for all of us aspiring to build one!

Benjamin Dinkel

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posted 3 months ago

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Yesterday I put the core on top of the shelf. With the weight of more cob and the refractory brick the shelf (made from a lid) was constantly sagging inward/downward. Like the middle was lower than the sides.
If I were to do it again I would put something underneath to prevent that. It was hard to get a level foundation.

Foundation made from 4cm thick refractory brick. Also visible another layer of cob including some "urbanite".

Hard to see but the middle was sagging about 5mm down. I could correct it a bit, but the core definitely has some issues because of this sagging.

As level as I managed

Core almost complete

Core complete. I took off the corners of the core to leave a little bit of space

Core with "5 minute" riser

A first little test burn. A windy day and all of the stones damp, but it worked alright.

thomas rubino

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posted 3 months ago

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Hi Ben;
That is looking good!
I wonder why you have a double wrap of superwool to make your 5-minute riser?
That looks to be 1" or 25mm thick wool.
If using 25mm material, a single wrap inside of a sheet metal tube is the standard way to build one.

I noticed in your cob-making photo that you are doing the barefoot stomp to mix.
I also started out making cob that way because that is how the early books showed it being done.
I quickly modified that method to a much cleaner, easier way.
I use a tarp with dry clay and sand, slowly adding water. I fold the tarp over the mix, and leaving my shoes on, I simply stomped the tarp.
I add water and roll the mixture to get the consistency that I want.
Fast, easy, and much cleaner.

I have some friends in North Idaho who are currently building this as a sauna rocket.
I will send them a link to your thread, perhaps they can collaborate with you.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Benjamin Dinkel

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posted 3 months ago

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Hi Thomas,
I somehow thought it needs 2" of super wool, thanks for the clearing that up.
So with 1 layer I need a tube for the outside?
In this case it will all be surrounded by cob. Do you think it will do without an outer shell?

I honestly like stomping the cob. Good massage and peeling for the feet. But I'll keep your technique in mind in case that feeling changes in the future.

thomas rubino

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posted 3 months ago

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Yes, I think you could carefully cob that, but... using a section of stove pipe or sheet metal wired on first before cob would be more durable.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Nathon Daellenbach

Posts: 3

Location: Sandpoint, Idaho

posted 3 months ago

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Hi Ben,
We are currently doing a build using the sauna rocket plans. We used the superwool and wrapped it with wire remesh to hold its shape. I will be getting more pictures and documenting our build as well, hopefully in the next couple of days I will post them. For now here is a pic of our riser.

PXL_20250130_013943230.PORTRAIT.jpg

Nathon Daellenbach

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Location: Sandpoint, Idaho

posted 3 months ago

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I noticed that your side panels are kind of short. In the sauna rocket plans it says they should match the height of the riser. Can anyone weigh in on this? What would be the result of a too short or too tall side vs the riser height?

thomas rubino

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posted 2 months ago

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Hi Nathon;
I noticed that your core support is cut directly from the barrel bottom.
Bens was cut from a lid and bolted into place. His is flexing, is yours?
Your side plates are taller than his.
It may just be the differences between a cottage rocket and a sauna rocket, or it could be that the plans were updated from Ben's version to yours.

EDIT) I've written to Mud asking for clarification.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Chris McClellan

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posted 2 months ago

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Hey everyone,
Your heaters are looking great. I am glad you are publicly shaking out the problems with the plans. I hope Andrés can find time to update them based on your experiences. I will see what I can do to help you too. Some elements of my original designs didn't seem to translate well into the 3D renderings. Thank you Thomas for flagging me on your thread. The main difference between the SaunaRocket and the CottageRocket is the internal mass in the CottageRocket and none in the SaunaRocket. The side panels or baffles as I call them are only needed to hold the cob or pebble mass from blocking the down tubes, as such you don't need them in an empty saunarocket. They should be about as tall as the riser in the cottagerocket to maximize the the mass storage around the riser where it gets hottest. The 5 minute riser with 1" ceramic wool compressed inside an 8" diameter piece of stovepipe inside the cob mass will be strong, easy to build around, and easy to fix later.

You can follow my latest adventures at
https://www.unclemud.com
https://www.youtube.com/unclemud
https://permies.com/u/164246/Chris-McClellan
Buy my stuff at https://unclemud.com/store/

Benjamin Dinkel

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posted 2 months ago

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Nathon Daellenbach wrote:I noticed that your side panels are kind of short. In the sauna rocket plans it says they should match the height of the riser.

Hey Nathon, I'm also building a cottage rocket after the sauna rocket plans. I will add on to the side panels once I've cobbed up a bit.

Thomas Rubio wrote:I noticed that your core support is cut directly from the barrel bottom.
Bens was cut from a lid and bolted into place. His is flexing, is yours?

I noticed that too. With the ring going all around his solution might be better.

Thank you Mud. The difference between the Sauna and Cottage Rocket sound very logical. The plan I got is called a sauna rocket and features mass. Would be great to add to the description of the plans. And maybe not only call it sauna rocket, but point out that one plan does both and the mass makes the difference.

Benjamin Dinkel

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posted 2 months ago

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Chris McClellan wrote:The 5 minute riser with 1" ceramic wool compressed inside an 8" diameter piece of stovepipe inside the cob mass will be strong, easy to build around, and easy to fix later.

I have another question about the riser. I just revisited the Risers E-Book. The rebar or outer stovepipe is for stability so the wool doesn't sag? When using a stovepipe, I would wrap the wool around my desired diameter stovepipe and then push the whole bundle inside a bigger one, then take out the interior diameter stove pipe?

Benjamin Dinkel

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posted 2 months ago

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And before I continue the build, a question to the experts.
Due to availability I'm using dense firebrick. Which part of the core should definitely be insulated?
The riser is the most important from what I understand, but is fine in this case since it's made from insulation.
Next most important the burn tunnel roof?
Then burn tunnel sides? Lower part of the riser?

My reasoning was to just cob around it, giving the whole heater more thermal mass. Or would that effect the clean burn too much?

Fox James

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Hi Ben, I am just checking that you know the risk involved with ceramic fibre?
Just be very aware that if you should ever need to expose or manipulate the riser once it has been fired, it will release millions of toxic particles into the air.
If it is sealed in and not accessible then you are good?
There are products available to seal the surface that work to an extent.

Benjamin Dinkel

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Hi Fox, thanks for your advice.

I was under the impression that that was not the case for the super wool!? And thought that was one of the upsides of that material.

Or is it just safe to handle before being fired?

thomas rubino

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posted 2 months ago

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Hey Ben;
Superwool is safe until heated to riser temperatures, 1400F and over.
At lower temps, the Superwool remains safe to handle.

After heating, it will form a crust on the surface. If you disturb the riser enough, it will throw dust, so be sure to have a mask on.
As Fox mentioned, coatings are available to minimize that dust if it concerns your customer.

I have opened my cores numerous times and needed to remove the riser. I avoid sticking my face over the riser and simply set it aside while making modifications or repairs.
Then set it back in place and cob it down. I use a paper mask while removing and replacing.

When creating a 5-minute riser, with stove pipe, no inner form is needed.
The Morgan is carefully cut with an angled edge. It is self-supporting on the inside, and the stovepipe makes it easy to handle.
A 6" riser takes less than 24" to form, so your riser can be as tall as you desire it to be.
It uses more than 24" for an 8" riser, so you must Stack two sections to get the desired height.

It would be nice to have the burn tunnel, its roof, and the lower riser base insulated, but the entire thing can be heavy brick.
The feed tube and the floor of the burn tunnel should always be heavy brick to protect from abrasion.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Chris McClellan

gardener

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Location: Burton, Ohio

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posted 2 months ago

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Benjamin Dinkel wrote:

Chris McClellan wrote:The 5 minute riser with 1" ceramic wool compressed inside an 8" diameter piece of stovepipe inside the cob mass will be strong, easy to build around, and easy to fix later.

I have another question about the riser. I just revisited the Risers E-Book. The rebar or outer stovepipe is for stability so the wool doesn't sag? When using a stovepipe, I would wrap the wool around my desired diameter stovepipe and then push the whole bundle inside a bigger one, then take out the interior diameter stove pipe?

Correct

You can follow my latest adventures at
https://www.unclemud.com
https://www.youtube.com/unclemud
https://permies.com/u/164246/Chris-McClellan
Buy my stuff at https://unclemud.com/store/

Benjamin Dinkel

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posted 2 months ago

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Thanks Tom and Mud.
I'll insulate the core as well as possible and try to find a piece of stovepipe 200 mm diameter to put around the riser.

thomas rubino

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Location: latitude 47 N.W. montana zone 6A

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You can use just sheet metal, with rivets or screws to create a 200mm or any size form.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Chris McClellan

gardener

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Location: Burton, Ohio

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I like to use ratchet straps to tighten the outer layer of sheet metal or stove pipe around the ceramic fiber blanket. I have found that compressing the ceramic fiber between the inner (removeable) pipe and the outside layer gives me a more durable riser. Paul likes the wire cage around the ceramic wool but I prefer the steel outside liner because 1) it protects and supports the ceramic wool and minimizes my contact with the dangerous dust and 2) it separates the riser from the surrounding mass making it easier for me to move the heater or change its configuration adding or removing cob or pebbles without damaging the ceramic wool.

You can follow my latest adventures at
https://www.unclemud.com
https://www.youtube.com/unclemud
https://permies.com/u/164246/Chris-McClellan
Buy my stuff at https://unclemud.com/store/

thomas rubino

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Chris, are you using the 6# or the 8# density Superwool?
I've only used the 8# and never felt it needed anything.

The five-minute riser on My Studio Dragon is now in its 5th season.
I hope to do a completely new build out there this summer.
The piped mass will be removed, and the existing core will be moved into a brick bell, perhaps double skin if I can collect enough brick in time.
The riser will be reused as is.

Insulated firebricks are the best choice for a riser if you can locate and afford them.
Superwool is a good second choice.
Heavy firebricks, either whole or split and wrapped with an insulating blanket, are a good third choice.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Nathon Daellenbach

Posts: 3

Location: Sandpoint, Idaho

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Hi Tom,
The barrels we used were not removable lid type. So the barrel bottom i used for the base plate has a bit of convex curve and when the J-tube is set on top it becomes flat.
@Chris,
I am planning to use fire clay to seal in the J-tube area like it's shown in the sauna rocket plans. Will my tall baffles be an issue?
Going to try and get some detailed documenting done on the project today and posted asap.

Thanks everyone!

Chris McClellan

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You will be fine as long as you have 4 or more inches (10 CM) between the top of the riser and the top of the baffles, which is also about the top of the mass inside the CottageRocket.

You can follow my latest adventures at
https://www.unclemud.com
https://www.youtube.com/unclemud
https://permies.com/u/164246/Chris-McClellan
Buy my stuff at https://unclemud.com/store/

Benjamin Dinkel

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With the advice from Tom and Uncle Mud I upgraded/changed the 5 minute riser.
To keep this thread simple I'll only post the result here.

Some more can be found here: https://permies.com/t/265673/Morgan-Super-Wool-minute-riser.

Benjamin Dinkel

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Today I made quite some progress, but it's slower and more complicated than I expected.
I don't want to complain too much about the "sauna rocket" plans. I didn't buy the plans and I'm using them only as guidelines. But my whole day today is like 1,5 of the 10 steps in the plans. And there are not a lot of measurements or photos or examples given. If you commit to building one, be sure you don't mind improvising and problem solving.
The measurement for the cutout is given. I made mine bigger and wish I would have gone even bigger. Maybe the plans are for a different size barrel?!

Step by step... cobbed around the core

Attached the upgraded 5 (now 35 or so :D) minute riser

Made the baffels longer and put half a barrel lid down. And the cutout behind. (You can see how its not long enough to reconnect to the barrel)

The other half lid on top of the cutout space. All of this will be filled with cob.

A look at today's finished work

Jamey Sturgill

Posts: 13

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I am building the sauna rocket right now, too. I bought the plans. I am having the most difficulty with figuring out how to lay the bricks. I am using the fire brick splits that are 4.5x1.25x9. I am hoping I can get by without cutting very many. I haven’t cut any yet, though. I have been worried about that. Could someone share a diagram of sorts? In the pictures it looks like there is a picture of the feed tub sitting up in the 4.5 side and the bottom of the riser laying on the flat side, the 1.25 side. Is that the way to go?

Thanks in advance,
Jamey

thomas rubino

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Hi Jamey;
Don't be overly concerned about cutting bricks, it's not that hard.
Here is a thread I posted talking all about it.
https://permies.com/t/261066/cut-Brick-RMH

The trick is to soak your bricks.
A low-cost masonry blade on a side grinder can be used for trimming or quick cuts.
The diamond blade on a chop saw is very quick and accurate.
Always wear a dust mask.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Chris McClellan

gardener

Posts: 651

Location: Burton, Ohio

305

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Hello Jamie,
The SaunaRocket plans didn't stick too closely to my design, probably due to what materials were on hand. How many of the splits do you have? I will try to draw you up something that works with what you've got.

You can follow my latest adventures at
https://www.unclemud.com
https://www.youtube.com/unclemud
https://permies.com/u/164246/Chris-McClellan
Buy my stuff at https://unclemud.com/store/

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