DSR (Double Shoebox Rocket) revisited


So it’s getting pretty cold outside, and when I built the last version of my stove I promised a breakdown analysis after I had used it for a season and had a chance to sort out my experience. Now that I have rebuilt the beast it seems the time is here to write about it.

First I would say that overall it performed pretty well, the weakest part of the thing being the door. The visions cookware lid I started with did work, but I would not plan on using such a thing except in a pinch. Eventually the first one cracked, primarily from physical shocks because it really does get too hot to handle at times leading to bumps and drops, and once hot it will use any excuse to crack.

Over the three to four month life of the stove there were numerous tweaks and while the final product worked, it would never have lasted another four months without many more overhauls.

My building style was primitive, using lots of clay and perlite, which was always sufficient in the original rocket stoves I built, but with the more compact and intricate design of the DSR bricks are really the way to go for all vertical surfaces, not just inside the firebox/batch box. Also bricks give an element of speed to the construction, and the finished product is structurally more stable. With clay walls on much of the old stove, cracks were commonplace, and I often worried about them just falling completely apart. If you do want to use clay, make sure it is a continuous mold type process. Forming one wall one day and then adding to it or building an adjoining wall the next does not give a reliable bond.

The water tank was the next area of interest, primarily because it was an issue I had to revisit many times, especially after one spectacular failure. My theory that pex could withstand temperatures in excess of water boiling was good only to a point . Hot, therefore soft, pex under extra pressure will blow out. Use all metal fittings inside the heat chamber and for a distance outside the chamber, and of course, when heating water, always have the system open so steam can escape safely. Pressure relief valves are a real good extra insurance in case one forgets to depressurize the system.

Soot was present everywhere the exhaust touched , except the parts that got the hottest in the burn chambers. The thought occurred to me that I had basically turned the entire downstream part of the stove where heat was extracted into a creosote trap, and future builds will have to accommodate an easy access for periodic cleaning.

Future builds will also need to be more efficient, and since start up and cool down phases of the burn are the most problematic, shortening those periods compared to overall burn time will be a major concern. Quality dry fuel will become a major goal for operation of the next iteration of the stove.

Recently I read more about the continued development of the DSR II and it has taken some interesting turns. I’m not sure I will be using all the developments, but at some point I will likely be adding a short riser at the back of the firebox. That will likely wait for me to get some ceramic fiber board, so for now I’m just using the old DSR design.

original visions lid, the crack is visible on the left side running under the handle, and I continued to use it till it cracked into several smaller pieces later on
this hodge podge is the water tank enclosure As the winter closed in, my hurry to get the stove functional meant many makeshift connections and very iffy seals. The forced draft kept a negative pressure inside the stove so leaks weren’t really a problem, but it would always be best to have everything tight as a drum


I broke down and bought a 10×10″ piece of neoceram glass–expensive (50$) but a good investment
this is before the old stove was finished being built
taking the stove apart, note the soot in the oven chamber but not the firebox area
getting a start on the base of the next DSR, note the exhaust has been extended and turned toward the left in preparation for the changed position of the water tank and the exhaust exit from the stove

I was a bit remiss taking pictures as I dismantled the old DSR, and obligations elsewhere meant much of the new construction was not overly documented, but look at the next post to see how the new DSR changed.

8 thoughts on “DSR (Double Shoebox Rocket) revisited”

  1. Clay walls. What do you mean, “clay walls”? Don’t you use cob, which is mostly sand? Sand is the aggregate that gives strength. Clay is the cement that holds the sand together. Builder’s sand is sharper and better for masonry purposes compared to ordinary sand. Adding a bit of refractory cement would help, per Matt Walker. He tends to use refractory clay from a bag instead of whatever clay one might find lying about.

    1. Hi Jake, You are right of course about the use of sand and refractory cement for some of these types of uses, cobb also might have a place, but I never felt the need for it.

      The first rocket I built I brought in bucket after bucket of raw “earth” with a high percentage of clay–no real organics to speak of, today that original mud continues to be recycled in each new design. Cobb might help prevent some surface cracking, but would complicate recycling, cement is more expensive and make changing the design impossible without bringing in more “clay” . My design has changed each year for about the last five or six years, and much of the time I simply treat the earth/clay like I would concrete, only keeping the mix as dry as can still be formed in a mold, so this actually makes handling the stuff much easier than cobb. I think I might have thrown in a bit of sharp sand early on, but I don’t really have much sand like that, however I do have lots of clay for free.

      Some times for certain features like risers, refractory is preferable, but right now I have a garbage bag outside full of an old riser -perlite refractory mix that I’m hoping to find a use for since the riser pretty much fell apart when I removed it making way for the first DSR model. I may use that as an aggregate when I build another stove in the octagon. Clay, even mixed with perlite can go right back into the garden if I stop using it.

  2. Hello Bob,
    Since several months I am following donkey, wheaton, Peter van den Berg etc. They talk about the double shoebox and there is even made a video of it (Allerton Abbey) and I think it is all beautiful and I would like to make such a thing with an oven and heated thermal mass.
    Peterberg even posted a sketch-updrawing of the double shoebox but it is without oven.

    Before trying to make it I read the Wisner’s-book, just to understand a little bit more.

    And then I discoverd your site with this DSR that you demolished and rebuilt again? And you start talking about the fact that DSR1 was abandonned by Peterberg, but that it didn’t matter for your purpose at that time and then later on you write that you recently read more about the continued development of the DSR II and that you will use some of the changements etc…… and I am curious wich changes you will use and why.
    I tried to find information about DSR2, but I can’t find it.
    Paul Wheaton also publishes some nice foto’s of the making proces in wich I see (I think) an air inlet down at the right etc.

    I am puzzled…

    I thought I’d give it a go to write you and just ask. Because the way you made your DSR seemed to be the most rudimentary/straightforward (I don’t know the word) without to much industrial things. Later you explain why you will change to firebricks and it seems logical to me and your way of building/thinking seems to be close to the way I would like to build things.

    Maybe you are able to answer my questions (above) and maybe you even have some drawings, foto’s of the making proces etc?

    I am so eager to start building, but I also want to make a nice thing that works (A cooktop with oven and eventually a heated bank that is rather efficient).

    Hopefully you are able to answer my questions,

    best regards,

    Pieter

    1. Hi Pieter, sorry for the long delay in response, I’m usually pretty busy, and so many spam comments, it’s refreshing to see a genuine one. I’m not sure from your comment if you are following the donkey forums, but most likely not, since you haven’t see the DSR2 yet which has been in development over a year now. I like to play with these stoves, and often the slow rate I move has my plans moving in one path, just as a new innovation is coming around.

      The main difference (in my words, not theirs)is the double shoebox rocket (DSR)has no vertical riser as such, and the DSRII has a short vertical riser that improves combustion and temperature before it reaches the horizontal afterburner at the top shoebox. There’s a couple other interesting additions to their version which I plan to incorporate, an overhead stumbling block–sort of a pause in the flow to allow greater combustion, and in the next iteration I plan an internal top to the double shoebox with the exit in the top instead of on the side. The glass stove top will sit about 3 inches above that internal ceiling. This allows for greater temps in the final combustion stage, since a great deal of heat is lost through the glass top right where the gas is entering secondary combustion. I mostly run my stove with a ceramic fiber blanket over the glass, which dramatically increases the temps (over 1300 F) which of course allows cleaner burning.

      When I start the retrofit I will keep photos and post them when I’m finished, but for now I’m still busy outside and last years stove is still working pretty good, so there’s no hurry.

      In the meantime , here’s a link for lots more info, even talk about ovens

  3. Hello Bob,
    thank you very much for your reply. I was curious to here from you, but I think 4 days isn’t that big of a delay 😉 I am very pleased with you replying!

    I know donkey’s forum (that is how I found you) and for some reason I then didn’t find the thread but 3 days ago I found out about DSR2 being developed and the outcome.

    As a result of this info I got the idea idea to use the DSR2 as a motor for the cooktop that was made in Allerton (and what you made), with DSR1 as a ‘motor’. And I wanted to know if that woud be possible. Therefore I placed this idea (drawing included) at a dutch forum where I hoped Peterberg would read it (Dutch is my language). His answer was that he doubted if the DSR2 would give enough heat to cook on and he advised me to look for a proven design since my experience in stove building is almost none.

    But his answer makes me even more curious. First of all I didn’t understand why it would not heat enough to cook on. What is your experience?

    The DSR2 cooktop, as said before, I want to make with an oven and a bench all separately manageable; when I don’t need the cooktop I would insulate it (like you with a ceramic fiber blanket?) so the heat goes either to the bench or the oven. There will be a lot of ‘tubes’ with valves to direct the heat.

    As a profession I build with clay, lightstraw and wood and I know masonry. Do you think it is a big deal to built this? And most important; do you think it makes sense to try and build this? I want a thing that heats a room while cooking or just after that.

    I understand that your idea is to insulate it from the inside? Why not from the outside? like you already do? ….if I understand it well…..

    Curious to hear from you…take your time 😉

    Best regards,
    Pieter

    1. Hi Pieter
      If you want to send a link to your plans, I’ll take a look at the pictures. From your description, it’s difficult to understand what “valves and tubes” you might be using. “Tubes and valves” sound like complications where soot might build and complicate things. Clean straight runs are a good goal wherever possible- except where you need to concentrate heat in a bell, and even then the actual clearances necessary are more than you might think relative to your csa. A bell with an opening at the bottom might need an opening two or more times the actual csa size of the system to avoid back pressure just depending on the shape of the bell and internal obstacles. There are discussions of this on the donkey forums, and I can’t encourage you enough to keep reading there,

      My plan when (if) I incorporate an oven will be to use the first chamber space past the dsr, this would have an insulated door to prevent heat loss, and provide a more or less straight flow of air over the metal airtight oven top and down the side going from there to the second /water tank chamber. This eliminates any actual moving valves or complications and relies instead simply on concentrating the heat with the CF blankets

      External insulation would still be used to focus or direct heat, but the internal ceiling to the dsr I plan is for the purpose of isolating and raising the combustion temps in the top shoebox independent of pots or pans on top of the stove. There should be an actual net increase of power or heat to the cook top, and that heat will likely be more uniform, except over the actual exhaust through that internal ceiling.

      As far as cooking, Matt Walker does a lot of innovation with that, and getting familiar with his stuff is a great idea.walker stoves

      The DSR is not super powerful at small sizes, but not that bad, a normal load runs a half hour to an hour, and can be refueled while hot, although this sacrifices efficiency–dirtier burn. so I can cook sweet potatoes, cook rice, boil water. In refueling, one good trick is to just put in a few sticks at a time during the coaling stage which can actually help the coaling stage burn cleaner. Another trick if you just want to load it and walk away is to concentrate the coals in one small place toward the back, then load as much as possible with a smaller surface area of wood touching the coals. The aim is to get the next load igniting in a small space as directly as possible with the back port–thinking about it, that would be even easier with the DSR 2 which has the port level with the floor of the stove. Peter did some measurements on this, but most is my own speculation/crude observation. just chucking a bunch more wood in on a bed of coals leads to oxygen starvation and dirtier burns.

      Perhaps it was the duration of cooking time Peter was referring to. Again, Matt Walker has also been doing lots of developing and measuring as well, more focussed on your type of application

  4. Hello Bob,
    Because of your advice I dicided to give it a try and -if its true that the price for his plans ánd help (eventually) is 80 dollars- I am going to contact Walker Stoves.
    Since my experience with building this sort of stuff is almost ‘0’ I think I’d better build a thing that has proven to work. Aside I can always experiment.

    I am excited to start.
    Thank you very much for your effort. I already learned a lot only communicating with you. Thank you.

    Pieter

    ps
    Maybe you would like to know, but I never receive emails that you posted a reaction (after spam controle). I can only see them when I go to your site.

    1. Hi Pieter,

      glad to be of help, Please stay in touch, I’d love to know how things progress for you. I’m still outside most of the time, but thinking more and more about updating my own stove when I finally come in for the winter.

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