Ever since our first rocket stove workshop with Tim Barker in Victoria I’ve been secretly coveting a rocket oven to call my own.
So you simply cannot imagine my excitement when in a NZ tipshop (the famous seagull centre in Thames) I came across an old, rusty, yet solidly constructed rocket oven core that had been build almost exactly (yet seemingly independently) to the specs of Tim’s design. Right down to the two sets of baffles to force the exhaust gases to conduct an S path up each side of the oven core, slowing the transit time and allowing more heat transfer (though Tim tells me he’s gone off baffles these days, a story for another time…).
This oven core would have taken me at least three days of material sourcing and fabricating. So to pick it up, along with some bits of flue, for $10, I was rather stoked. The only obvious issue was the abrupt reduction in the cross-sectional surface area of where the flue exited the top of the oven – an easy fix. The cavity gap or the distance between the two skins through which the flue gasses would travel was also quite small – about 25-30mm, so I wasn’t sure how well the gasses would actually flow, but it certainly seemed like there was scope for a scone or two. I loved that it looked like a bit of rusty junk but that thanks to Tim I had the eyes to see it for the precious jewel that it was.
The thing kept rusting for the next 10 months or so until we found ourselves relatively settled in our new housebus park up. Relatively settled yet ovenless. The time had come to pull it out and get it working.
After another visit to the tip shop I returned with a stand made for working on outboard motors. My mate Pete and I soon grinded (me & Pete’s son Toby) and arc welded (him) this into a perfect little stand for the core. A quick visit to my dad’s shed yielded a length of stainless steel flue pipe long enough to make up the heat riser and the flue out the top of the oven. I even got his permission before nicking off with it.
Within a day we had ourselves a stand and I had riveted on the flue top and bottom. About ten minutes later I’d put it outside and fitted into into a perfectly sized bit of concrete block (which I’ve found falls to bits after a few months – fire bricks or old red bricks much better).
First pass (before insulation). Note the awesome home-made charcoal kiln in background (for my mate Pete’s home-made forge).
I lit it expecting it to fail dismally, or to barely work, and for us to start a long process of tweaking it to get it operational. Imagine my surprise when it started drawing straight away, and within ten minutes was hot enough to bake a tray of scones in no time at all!
The next day we took it to our house bus site and set it up again. I fitted a thermometer (again from the trusty tip shop – from an old bbq lid) in order test it out without any insulation and then to keep testing it as we gradually insulated different bits.
The fist thing I insulated was the oven core by wrangling & riveting an old hot water cylinder shroud. Two finger cuts later we filled it up with pumice and fired it up again (thanks Ciela & Nikka!).
Next we insulated the riser with a pumice-filled larger flue section which we squished into an oval profile for easier dismantling when we come to move the unit (thanks Brad!).
Then the burn tunnel and base of the riser which was shedding much heat (Thanks Manda!).
Meantime whilst I was collecting data it was obvious that what the thermometer on the door said was very different to the actual temperature inside, which was obviously a lot hotter. I estimated by about 50 degreed celsius. So in the name of science I ordered a couple more thermometers, one magnetic one for the outside and one that sits inside the oven exactly where the actual food goes, and both measuring up to a very uncool 400 degrees celsius. The pizzas kept coming and often we’d do several loaves of bread, enough pizzas for ten people, and some sort of desert in one go.
The most impressive timed session I got out of it so far was getting to 180 degrees celsius in 6 minutes, 225 degrees C in 7 minutes, then maxing out at 365 degrees C after 13 minutes. By this point the base of the oven inside was glowing red hot. We were cooking pizzas start to finish in 3 minutes 50 seconds and the only smoke was when we left them in too long. All on a handful of twigs I tell you, a handful of twigs! The thing I really love is that is faster to heat up than a conventional electric or gas oven. So in that sense, twigs aside, it is actually more convenient than its conventional, expense, non-renewable energy gobbling alternative.
Shortly thereafter Tim visited. Things were going quite well till the moment of truth and it came time to light the oven and I got a bit nervous and overloaded it and there was smoke everywhere and flames going the wrong way, and, oh gosh it was not ideal. The student humiliating himself in front of the sensei. Only for a few minutes though – luckily the rocket then started rocketing, the awkward silence broke, and on a few willow twigs we took it up to 355 degrees c at which point the thermometer appeared to jam up. As for the pizzas – oh gosh – delicious.
The apprentice and the sensei enjoying rocket pizza.
And that’s it. I’m done here. What, you want some kind of wrap up? Fine – here you are: Then, dear readers, me, my family, and our brand new rocket oven lived happily ever after. If you would like a similarly happy ending and a much better built oven get yourself along to our next rocket stove/efficient combustion workshop. The end.