Help with A Used Rocket Oven

Howdy folks!
Frontloading my question - how do I make best use of my available materials to create a relatively lightweight, portable Rocket Oven Riser?

My situation:
Sometimes I have more dollars then sense, I have a tendency to leap without looking at all.
I bought a used Rocket Oven from Craigslist... I even drove to Montana to pick it up (and spend the weekend at Wheaton Labs).

After talking with the builder, he used the design from the Rocket Oven Movies (which I've also watched) and went with an all-brick riser.
By the time I picked it up, he had dismantled it, separating the stand, oven, and taking apart all the brick.
The one piece of feedback he gave me surrounded the functioning of the oven - he couldn't seem to get it up to temperature very well.

I got it home, put it on its stand, and realized - the riser is a rather fixed and massive structure, moving this after construction likely won't be ideal...
I haven't quite figured out a permanent location to install it yet.

Immediately my mind went to portability and the "5 Minute Riser" concept, but I'm not exactly sure how to implement it.

I've read The Rocket Mass Heater Risers eBook , this Superwool Five Minute Riser thread and the Five Minute Riser post.
I saw the Twist on the 5 Minute Riser idea and this rather attractive Portable Rocket Oven build.
All of which have been informative and furthered me down the path.

My available materials:

I have the fire bricks, there are many, including multiple half bricks. Everything you can see in the detail picture below.

The builder was kind enough to also give me some refractory mortar. (maybe a quart?)

I also picked up roughly 12 feet of 2-foot tall ceramic fiber blanket off Craigslist several months back

I have several smaller sheets of stainless steel

And an inordinate number of 2 foot long cedar fence pickets

My laundry list of questions:
I'd like to build a container for the wood feed and burn chamber but I have zero welding experience.
Is there any safe way to use wood for the box or am I virtually guaranteed to char and/or create a flaming death trap?
Do I need to knuckle down and learn to weld?
What alternatives might I have in this space?

I figure reusing the bricks for the feed and chamber is likely the most sensible option (dissenting opinions?).
Can anyone point me to a good resource for how to orient the bricks?

Given this is a white oven, am I safe to use the ceramic blanket as is or will I need some stabilizing substance?
If the latter, does anyone have a preference over rigidizer vs slathering on layers of mortar/other refractory material?

Besides the general rule of the riser being 3 or more times the height of the wood feed, is there any advice for dimensions?
Should I measure the diameter of hole in the bottom of the oven and work backwards from that point for my cross sectional area?
Similarly, measure the distance from that hole to the ground to work out my riser height - and subsequent burn chamber/wood feed sizes?

Since this isn't a RMH riser and, the good Lord willing, won't experience extreme temperatures except inside, can I get away with 'any old stovepipe' I can find at Habitat for Humanity's ReStore?

What have I missed? What am I not thinking of?
Is there anything screaming "this is going to blow up in his face!" either figuratively or literally?

Thank you for reading!! :)

Hi Clay.
Nice find!
Since you have the core (riser, feed and burn tunnel) built to dimension of the rocket oven book already, I would just use that to begin.

My guess why it didn’t get up to temperature is that it wasn’t insulated, or was it before dismantling it?

Any exhaust pipe should be fine. We’re talking about the exhaust after the oven, right?

Thanks for replying, Benjamin!

Benjamin Dinkel wrote:
Since you have the core (riser, feed and burn tunnel) built to dimension of the rocket oven book already

I’m actually not sure if that was or wasn’t built to the specs of the book - the builder said he stuck to the video and avoided the extra coin of the book.
Maybe that’s something I ought to investigate.

How are you thinking that would work? Would I dry stack the riser and disassemble when I want to move?

Benjamin Dinkel wrote:
My guess why it didn’t get up to temperature is that it wasn’t insulated, or was it before dismantling it?

I was under the impression it was not insulated and operated as-was, per those pictures.
Would a five minute style riser be more or less insulative than brick?

Benjamin Dinkel wrote:
Any exhaust pipe should be fine. We’re talking about the exhaust after the oven, right?

The exhaust is already in place, I was more thinking of using the stovepipe to surround the ceramic fiber blanket and use that five-minute version in place of a brick riser.

Appreciate the feedback!

Hi Clay, you’re welcome.

Clay McGowen wrote:
I’m actually not sure if that was or wasn’t built to the specs of the book - the builder said he stuck to the video and avoided the extra coin of the book.
Maybe that’s something I ought to investigate.

It’s easy enough to measure. And you would have to measure the csa if you were to build another core anyway.

Clay McGowen wrote:
How are you thinking that would work? Would I dry stack the riser and disassemble when I want to move?

That’s an option. How often do you plan on moving it? Is it in one piece now and you were able to move it?

Clay McGowen wrote:
I’m actually not sure if that was or wasn’t built to the specs of the book - the builder said he stuck to the video and avoided the extra coin of the book.
Maybe that’s something I ought to investigate.

It’s easy enough to measure. And you would have to measure the csa if you were to build another core anyway.

Clay McGowen wrote:
I was under the impression it was not insulated and operated as-was, per those pictures.
Would a five minute style riser be more or less insulative than brick?

That explains the oven not getting to temperature I think.
Made from brick it would have more mass, so start a bit more sluggish but then keep the temperature for longer. In any case, the riser needs to be insulated.

Clay McGowen wrote:
The exhaust is already in place, I was more thinking of using the stovepipe to surround the ceramic fiber blanket and use that five-minute version in place of a brick riser.

That should work with any stove pipe material. Finding the right size and wrapping it around the riser can be a challenge though.

Hi Clay;
I'll try to answer some of your questions.

That appears to be a 6" rocket, made of heavy firebrick.
That style riser will take quite a while to come up to temps.
Insulated firebricks could be substituted for heavy bricks, but they are expensive.
A five-minute riser would quickly come up in temp.
Build your 5-minute riser using Superwool inside an appropriate size stove pipe (8" to make a 6" riser)

Please do not use the refractory mortar, once it is dry it is extremely hard to remove.
Core bricks should be assembled using a clay sand mortar (1 scoop clay to 3 scoops graded sand)
Bricks assembled with this mortar can be taken apart with a rubber mallet, and the mortar can be saved in a bucket to be rehydrated and used again and again.

The feed tube on your current stove is way too short.
Using your firebricks increase the height to a minimum of 12" preferably 16".
Use no metal on a core at all, it will heat up and cause problems.

Dimensions for a 6" J-Tube
Feed tube, burn tunnel, and riser must have the same internal size  (6"x 6")
Feed tubes are normally 16" tall  to accept standard firewood
The roof of the burn tunnel should be 10"-  12" in length
The riser should be no less than 36"

Quick question that might help other answer your question.

Are you planning on using while it is in that cart or do you plan to set it up once it is moved?

Thanks everyone for the ideas and patience as I educate myself and come up to speed!
I took some time to read more posts on this forum and Ernie and Erica Wisner's Rocket Mass Heater Builder's Guide

Addressing the last question first:

Anne Miller wrote:Quick question that might help other answer your question.

Are you planning on using while it is in that cart or do you plan to set it up once it is moved?

I definitely want my LoadHog back! Such a great little wheelbarrow.
I've finally circled back and set it up back on the stand as seen in the first photo.
In addition, I wanted to give this a go with things I either had on hand or could easily pick up locally.
Yet I didn't want to commit too much time into this project as I wasn't quite convinced I would be successful.
As such, I tried to dry stack the bricks and get a feel for the size...

thomas rubino wrote:
That appears to be a 6" rocket, made of heavy firebrick.
That style riser will take quite a while to come up to temps.

Good eye Thomas! Can confirm, these are the heavy ones.

Benjamin Dinkel wrote:
How often do you plan on moving it? Is it in one piece now and you were able to move it?

As a result of the above, I'm now thinking of moving this a lot less frequently than first imagined.
The former owner had disassembled it (the best he could) and my brain didn't immediately register how heavy they were all together.
As much as I'd like to move it to and from a safe storage location, I don't have a team of horses to drag it nor concrete to roll it on... it's probably going to remain in its current location for a while.

thomas rubino wrote:
Please do not use the refractory mortar, once it is dry it is extremely hard to remove.
Bricks assembled with this mortar can be taken apart with a rubber mallet, and the mortar can be saved in a bucket to be rehydrated and used again and again.

I didn't really understand what you were driving at here, Thomas, because "the former owner was able to get them apart"...
After trying to clean things up a little, I have a much more visceral comprehension of the difficulty involved...
I spent several hours with a chisel and mallet trying to scrape the bricks clean enough to dry stack.
With far too many chipped out sections and busted corners, the bricks are still pretty rough and I'm shocked the former owner completed as much as he did.
Suffice it to say, I will take your advice and avoid using the refractory mortar.

With respect to dimensions, I did the best I could to match yours as well as the Wisner's suggestions on CSA for a 6" system
Unless I dig down I'm not quite going to get the full 36" riser. I'm closer to 33" from the top of the foundation bricks to the outside of the oven.
Maybe with the distance from the corrugated metal to the actual inner chamber, I've got 36" but that's a stretch.


After I stacked the burn chamber, I wanted to test fit the stove pipe.
I put that on top and it was a fairly tight fit.
At this point, I couldn't help myself, I wanted to test fire things.
I forged ahead using the ceramic wool I had in the garage, donned a face mask, cut a piece to fit in the stove pipe, and put the pipe back in its place over the burn chamber.

From there, I primed the chamber twice, then started feeding in some wood.

I did get some smoke-back and smoke-thru (all the leaky cracks between bricks) but I proved to myself the efficacy of the design.

Benjamin Dinkel wrote:
That explains the oven not getting to temperature I think.
Made from brick it would have more mass, so start a bit more sluggish but then keep the temperature for longer. In any case, the riser needs to be insulated.

Benjamin, I fully agree here.
The fire was slow to get started and remained sluggish for a while.
Once the bricks warmed up a bit though, things started to go a lot smoother.

The fire burned cleaner, faster, and in the right direction (most of the time).
There was still some leaking, and the draw was less than I would like, but again I attribute that to the cracks between bricks.

I didn't quite get it up to the temps I wanted, but in less than an hour I went from the ambient temperature of ~40°F to a nice 250°F.
I'm anticipating better results with a less janky burn chamber.


Let me know what other points of feedback you all have!
I'll be working on creating the clay/sand mortar in the coming days and am very much still open to feedback.

Edit to add pics:

I did it I did it!!

I might have done it the other day, but definitely reached temp last night.

I dug some clay and screened some sand, mixed these by hand in a wheelbarrow, and used the resultant mortar to level the bricks and seal the gaps between them.

If you’ve found this post and are going to attempt this in the colder months, do yourself a favor and keep a pot of warm water near by. After working until I couldn’t feel my fingers, this trick allowed me to keep pushing on without numbness or even pain. 10/10 would recommend.

Once I laid the bricks, I started a fire and fed it for over an hour. I kept referencing the thermometer in the door and kept feeding because it was still very low.
I opened the door at one point and was hit with a blast of air so hot I couldn’t accept it was only 250… that’s when I looked at the thermometer hanging from the racks - pegged at over 600°F.

I then realized the door thermometer had a probe so short it didn’t extend past the inner wall of the door and was sunken into the insulation… of course. This wasn’t reading the true temperature.

I have a new one on order, with a 4” long probe, and hope for more accurate results when I install it.

In the meanwhile, I’m planning to bake some bread and maybe even a pizza or two, doing my best with the rack thermometer.

Huzzah, I have cooked!!

It was not the most amazing nor delicious, but it was my first time ever making pizza dough from scratch. With a largely grain-free, and resultantly gluten free, diet I was simply glad my creation turned out to be edible.

Will be submitting this as the twofer, but I’m still hoping to bake some bread in here, too.

Nice work, Clay! Love the photos.

Excellent work on all the construction and impressive documentation, Clay! I'm glad to see that everything is working out well with your new oven.

You might recall that over here at WL we heat a bit differently than what you picture in this thread (our wood feed is shorter, and we process wood into "toothpicks") but it seems like you have no problems reaching temperature.

Once your new thermometer arrives, please share with us how much time is required to reach baking temperature. I'd love to learn more about this, your set-up, and how we might tweak things here to bake even quicker.

Ask and you shall receive, Commander!

TL;DR: 17 minutes from first stick added to reaching 500°F

Cassie and I meal prepped tonight, I took charge of the sweet potatoes.

I wanted to both use this for part of a BB (roast 4 pans of food) and test out the timing.
Additionally, I wanted to run a test on the quantity of wood used.

I put all my wood in a tote to weigh it and only used those sticks. The starting weight was 23.4lbs.
I did weigh my paper starting materials, but I didn’t like my selection and ended up using stuff which came from a different box. I believe this was less than 2lbs, but make no official claim.

I primed the riser, added sticks and a few more starting scraps of paper, until the rocket took off.

Below are the times and temps:

time	activity	temperature
17:20	priming the riser	ambient ~35°F
17:23	first stick in	<200°F
17:37	three loads of wood through	340°F
17:40	five loads of wood through	520°F

From there, I fed the fire while roasting whole sweet potatoes, trying to keep the temperature around 500, but getting close to 600°F a few times. I found opening the door to check on them was also a good way to maintain temperature, in addition to changing wood size (larger chunks lead to a slower burn) and limiting airflow with bricks partially covering the wood feed (also slowed the burn and kept temps down).

I put the sweet potatoes in at 17:45 and took them out at 18:41. They were a touch over-done, which I attribute to both the time and temp. I’d’ve liked to keep the oven closer to 450, or even 400, but this was my first time trying to limit the burn (and first time roasting whole sweet potatoes).

When I weighed the bucket at the end, I found there was 14.6lbs left; I had used less than 9lbs of wood for a burn lasting 78 minutes.

Stephen, back atcha - I’d love to get some calculations from the WL oven to compare. It seems really good, thinking of hunks of logs going into a cast iron wood cook stove, but I have no real baseline.

My sticks were all dimensional lumber, a small percentage was oak, a little more was cedar, and the majority was pine. Most of the wood burned was suboptimally shaped - triangles from 3/4inch thick fence pickets about 4inches wide, thicker 1inch square 10inch long cutoffs, and a few of those triangular prism shaped cedar cutoffs from making siding.

All in all, pretty happy with the result!!

Not only did I learn about my oven, but I learned a valuable life lesson too…
Never, ever, allow glass to experience thermal shock.
I made the mistake of putting my Pyrex dish on an extremely cold stainless steel shelf, and it explosively shattered in less than a second.



As I say in the video, it was definitely no fun to clean, but thankfully the potatoes still had their skin, so I was able to salvage the food, if not the dish nor my pride.

I really do need some stainless steel sheet pans - if anyone has some they like, with thick enough gauge to handle high heat (unlike that poor shelf which is now quite deformed) please post a link. All the ones I’ve seen have poor reviews for warping.

First of, sorry for loosing you Pyrex pan! Lucky you caught it on video though, it's quite something to see.

9 lbs of wood still seems like a lot to me for an hour of burn time. I wonder how much more efficient you could get the oven if you insulated the core and riser?!

Great job!

I built a rocket oven a few months ago and have been using it. Myself along with other people have posted what they have built here.

I used fire fiberboard for my rocket and it does work but it is showing signs of ware and tare. I have thought about building another rocket oven using a five minute riser but I need to find the time to make that happen. To insulate my oven I did use Morgan Superwool from Dragon tech (here is the link https://dragontechrmh.com/morgan-superwool-plus-non-ceramic-fiber-blanket/ ) and it is great stuff.

I have a few questions:

Are you getting any smoke from the font of the oven? From the pictures it looks like there is no cob around the font of the barrel

The insulation on the oven looks to be compacted at the bottom of the oven or is it just the picture?

Another issue I have had is getting the right temperature. If I put a thermometer on the door and one on a rack on the inside I get a +/- 50 different. Do you have the same issue?

In this video you can see how I put a 2 by 4 on the bottom and used wheels to help move it. Also I used rivets along with aluminum angles to hold the rocket together.  

Benjamin Dinkel wrote:First of, sorry for loosing you Pyrex pan! Lucky you caught it on video though, it's quite something to see.

Ya know, I'll say "it was worth it" - I had heard of this happening, but I learn best through doing and teaching, now I know. And it was indeed an experience!
I ended up finding another one at the thrift store for $6 this past week, so it all worked out in the end I guess

Benjamin Dinkel wrote:I wonder how much more efficient you could get the oven if you insulated the core and riser?!

Again, Benjamin, spot on here.
I fired it again to make dinner last night, it reached temp in less than 15 minutes, but the heat from the sides of the burn chamber was incredible.
Eventually, it got so hot that I couldn't hold my hand on the bricks for more than half a second.
I could feel it from feet away and even started using it to dry some wood for next time.

I think my next major improvement will be insulation.

Thanks for sharing, T!
A lot of interesting builds in that link, and I appreciate the video too.

T Blankinship wrote:Are you getting any smoke from the font of the oven? From the pictures it looks like there is no cob around the font of the barrel

The insulation on the oven looks to be compacted at the bottom of the oven or is it just the picture?

Yes and yes...
I'm only getting a little smoke, right at the start of a burn, but the amount is definitely non-zero.
The insulation is also definitely compacted a bit. I put my hand on the bottom on the metal near the riser and the top near the chimney, both were about the same temperature, but I'd expect the top to be hotter if the insulation was equal.
Both are symptoms of the same experience - a 500 mile journey from Arlee, to Wheaton Labs, then back to my house, then a grand tour around my backyard in a wheelbarrow.
I'd like to cob up the front but haven't made time for that.
I'm not sure what to do about the insulation concern

T Blankinship wrote:Another issue I have had is getting the right temperature. If I put a thermometer on the door and one on a rack on the inside I get a +/- 50 different. Do you have the same issue?

Undoubtedly. I don't quite trust the thermometer inside, but I do trust my own senses as well as the cooking times and food behaviors I've observed.
There is significant stratification inside my oven and, given the fact that it's not quite level, that's both top to bottom and front to back.
I'd love to come up with a solution for this as well, but I'm really struggling to think of a viable option.
Maybe a fan like in a convection oven? But I have no desire to add electrical power to this delightfully off-grid solution.
Maybe one of those heat-powered/Peltier fans that go on a wood fired stove? But the clearances don't seem like they'd work very well...


If anyone has thoughts on how I could insulate the burn chamber, I'd appreciate hearing.
For reference, the outside dimensions are roughly 30inches long x 9inches tall x 9inches wide
I'm leaning towards ceramic fiber board for the portability but open to other materials too.
Especially with the price, fragility of that material, and non-standard dimensions I need...
Superwool with standard clay brick beyond that?

Results and pictures of last nights dinner!

I tried something new, I wanted to get this to hold a relatively stable temperature, eventually so I can bake things with more accuracy.

I noticed towards the end of the sweet potato roast that the coals kept the fire nicely and allowed me to feed only a few sticks at a time, which slowed and steadied the temp.

I fed this a lot at first, both warming the entire structure and to generate some coals.

Once I had a nice bed going, I covered the wood feed almost entirely, save a 2” gap.

Through the gap I put in smaller chunks which lit up instantly and took the heat up the riser and to the oven.

I was able to keep this up for a good while and roasted some chicken, potatoes, and broccoli, without burning a thing.

I see bread and possibly cookies in my future!

Clay McGowen wrote:

T Blankinship wrote:Are you getting any smoke from the font of the oven? From the pictures it looks like there is no cob around the font of the barrel

The insulation on the oven looks to be compacted at the bottom of the oven or is it just the picture?

Yes and yes...
I'm only getting a little smoke, right at the start of a burn, but the amount is definitely non-zero.
The insulation is also definitely compacted a bit. I put my hand on the bottom on the metal near the riser and the top near the chimney, both were about the same temperature, but I'd expect the top to be hotter if the insulation was equal.
Both are symptoms of the same experience - a 500 mile journey from Arlee, to Wheaton Labs, then back to my house, then a grand tour around my backyard in a wheelbarrow.
I'd like to cob up the front but haven't made time for that.
I'm not sure what to do about the insulation concern

T Blankinship wrote:Another issue I have had is getting the right temperature. If I put a thermometer on the door and one on a rack on the inside I get a +/- 50 different. Do you have the same issue?

Undoubtedly. I don't quite trust the thermometer inside, but I do trust my own senses as well as the cooking times and food behaviors I've observed.
There is significant stratification inside my oven and, given the fact that it's not quite level, that's both top to bottom and front to back.
I'd love to come up with a solution for this as well, but I'm really struggling to think of a viable option.
Maybe a fan like in a convection oven? But I have no desire to add electrical power to this delightfully off-grid solution.
Maybe one of those heat-powered/Peltier fans that go on a wood fired stove? But the clearances don't seem like they'd work very well...

If anyone has thoughts on how I could insulate the burn chamber, I'd appreciate hearing.
For reference, the outside dimensions are roughly 30inches long x 9inches tall x 9inches wide
I'm leaning towards ceramic fiber board for the portability but open to other materials too.
Especially with the price, fragility of that material, and non-standard dimensions I need...
Superwool with standard clay brick beyond that?

I think putting cob on the front would help a lot.

As for the stratification I have the same issue. If I put a temp probe pointing down I get 600F but if the probe is parallel to the bottom of the oven I get around 350F to 400F. I think some work needs to be done on probe placement and find good spots.

I have used fiber board for my rocket it does work but will over two to three years show signs of ware. I have had my rocket fiber board for about two year now it still works but if I am using the oven for over an hour or more the core will heat up. Also keep in mind that fiber board must be protected from water.

A new/old idea:
Small and fast, big and slow. The smaller the wood the faster the burn and the bigger the wood the slower the burn. I keep different pieces of wood for controlling temp in the oven. Someone needs to invent a phrase to help with temp control.

Oven stratification isn't unique to rocket ovens. I get the same thing with my wood fired range, and why do you think fan ovens were invented  :P ? I'm trying to come up with a non-fan solution, and the only thing I've thought of so far is to adjust the insulation so you get more loss through the top than the bottom of the over, which doesn't sound particularly neat. I suspect the trick would be to create a thermocycle creating a circulation within the oven to minimise the temperature differences, not trivial I suspect.

The stove top fans I've seen need a temperature difference either end, to get the electric power difference. Maybe there is a market for a smaller internal-oven fan? I'm thinking it would be best if the cold side was outside the oven, which takes more engineering. A stirling engine is another that uses temperature difference to create movement, but no electric. That may be even more tricky to set up to run reliably though.