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.

2 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.

Leave a Reply

Your email address will not be published. Required fields are marked *