Design: Five Block Rocket Stove

The Five Block Rocket Stove design and the text of this article first appeared in the PNZ Autumn 2010 Members Newsletter produced quartly for the members of Permaculture in New Zealand.  As the author retains the rights to any article published it is republished here with minor changes for further dissemination and feedback

The principles involved in the design of rocket stoves were invented in 1982 by Dr. Larry Winiarski whilst he was Technical Director of the Aprovecho Research Centre.  His work is recorded in the Design Principles for Wood Burning Cook Stoves [1]

A rocket stove is a simple to implement and highly efficient fuel combustion and heat transfer design, utilised in the design of cooking appliances and space heaters

A typical rocket stove consists of a short chimney sitting on top of a fire chamber with a front fire box.  The external design and function of rocket stoves varies considerably depending on their use as either: stoves, hot plates, ovens, space heaters, static installations, mobile devices, etc. Designs also vary considerably between manufactured and handmade stoves

My interest in rocket stove design focused around its use as a stove, primarily for base camp kitchen cooking during site work, courses, etc.  My requirements differed from those of the Koanga Institute team, who have implemented a static rocket stove design in their course kitchen[2]

Generally all stoves have similar characteristics: A firebox at the front base of the rocket stove divided into separate air (bottom) and fuel (top) intakes which allows fuel to be added above the air intake at the base of the chimney in the fire chamber.  Heat in the fire chamber rises in the chimney and cause a draft at the fire box encouraging hot and efficient combustion of the fuel at high temperatures.  At the top of the chimney heat is focused directly on the cooking/heating surface.  Stoves designed in this way are very fuel efficient, produce very little smoke and ensure optimum heat transfer efficiency from fuel to food

My design objective was to produce a simple stove based on rocket stove design that fulfilled the following criteria:

• The design can be implemented by anyone with access to the materials
• The design utilises actually available and affordable materials
• The design requires the minimum number of tools and addition inputs to implement
• The design is simple to construct, use and maintain
• The construction can be broken down and reassembled in different locations
• The construction can be quickly built on spec
• The construction is ‘safe’
• The fuel must be accessible and sustainable
• The design works as a component of the larger design/philosophy

The first stage of my project was research.  I carried out an internet study, found many interesting articles and designs and watched some inspirational YouTube videos.  I also began assessing what materials I would use to build the stove, initially focusing on those used in the articles or videos of my research i.e. purchased or recycled.  The more I studied the less I knew and I had begun to focus on details!  At this point I moved my point of focus from the internet to a review of both my criteria and what I was attempting to achieve, the sketch pad and the physical modeling of ideas 

During my review I realised that I had not included the fact that this first design implementation was only a test to see if I could make a rocket stove and assess how they worked.  The design did not need to be perfect, it just had to provide the learning material to take me to the next level of design/complexity. Therefore the cheaper and quicker I could make the stove the better.  Through this design analysis I realised that the hollow masonry/concrete block I have used in my pottage garden layout would be the ideal resource to construct the stove 

My initial block stove layouts and models were large, cumbersome and obviously dysfunctional!  The wonderful thing about blocks of any sort is that they can be assembled and reassembled in many different ways, and the more I handled the blocks and became familiar with their characteristics the better I utilised their features and the leaner the design sketches and models became.  I finally settled on a minimalistic five block model, which fulfilled my design criteria, and I arranged to build and test the design

I originally intended to test the stove on the beach, where I collected the fuel twigs and sticks, but unfortunately this location and several others fell under the personal revelation that you cannot just light fires anywhere you like.  Testing eventually took place at home!

The blocks were easy to position and assemble on a concrete base (See Figure 1. Base block and chamber and Figure 2. Base block and fire chamber with chimney blocks and fire box added to form five block rocket stove)

Figure 1. Base block and chamber

Figure 2. Base block and fire chamber with chimney blocks and fire box added to form five block rocket stove

Once the blocks were in position a fire was lit in the fire chamber a fire box divider added and fuel added (See Figure 3. Lighting fuel viewed from fire box and Figure 4. Lighting fuel viewed from chimney, note scale of fuel being used)

Figure 3. Lighting fuel viewed from fire box

Figure 4. Lighting fuel viewed from chimney, note scale of fuel being used

Once the fire was set I was able to heat a tin of baked beans from opening to serving in five minutes (See Figure 5. Baked beans cooking on the stove) and boil 2 litres of water in the thermette in less than seven minutes (Figure 6. Thermette boiling over on the rocket stove chimney)

Figure 5. Baked beans cooking on the stove

Figure 6. Thermette boiling over on the rocket stove chimney

Following practical tests the temperature at the chimney exit was measured to be in excess of 300 Deg C the maximum range of my thermometer

Overall, my five block rocket stove design was successful in allowing me to cook and heat water quickly with very little fuel.  I have now added it unchanged to my base camp kitchen equipment, along with the thermette, a smoker and a cob ove - which will cover any cooking/kitchen requirements with fuel that I can grow myself in the form of coppice.  I have no doubt that once installed in situ design modifications could increase the efficiency further.  The design fulfils all of my criteria and several friends plan to implement identical designs. Further pictures and notes can be found on the following public album[3] 
 
Notes:

• To be close fitting and stable the designed stove needs to be constructed on firm level ground.
• Although the thermette boiled quickly and fiercely, its conical chimney shape did reduce the draft into the chimney. When the thermette was removed, flames literally jumped out of the chimney as the pressure changed.
• In a semi permanent installation the chimney could be clay lined and the whole stove insulated
• The draft in the chimney could be improved by removing the cavity mould taper of the block to create parallel sides for the chimney
• Replace the tin can sheet divider with a piece of suitable steel. Design sketches and more photos and details can be found on my Picasa online albums page created during the project[3]

1. http://www.rocketstove.org/images/stories/design-principles-for-wood-burning-cook-stoves.pdf
2. Koanga Institute Rocket Oven! Permanent Wall Mounted Kitchen Oven
3. https://goo.gl/photos/pjhMyh8pGJa6MQfg6

Rocket Stove Materials:

• *4 standard plain end concrete block 390*140*190
• *1 standard open end block 390*140*190
• *1 large tin can sheet to make firebox divider
• *1 small oven rack to act as post stand above chimney
• * Lots of small twigs/sticks for fuel  
• A small helper to keep the fire stoked is helpful!

Chew