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CCAT rocket mass cobb bench heater

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Engr305 Appropriate Technology page in progress
This page is a project in progress by students in Engr305 Appropriate Technology. Please do not make edits unless you are a member of the team working on this page, but feel free to make comments on the discussion page. Check back for the finished version on May 23, 2019.



The existing space in which our team is to build this rocket mass cobb bench heater was the foundation for a previously existing home on the CCAT grounds. CCAT is a student run instructional home that demonstrates appropriate technology to promote sustainable living and design. The cement foundation and plastered brick chimney is all that remains at the current site, just to the left of the cobb oven.


This spot is currently underutilized as it could potentially be a communal workshop space and interactive recreation area that demonstrates appropriate technology. By interacting with a rocket mass heated bench, students and community members can learn about how appropriate technology and design stacks functionality.


The Cobb Rocket Mass heated bench will provide functions of heated lounge space, cooking, and a place to bring community members together. Cobb is a building material consisted of all natural components: clay, straw, sand, and water. Thermal mass heating has been used all over the world to heat beds, floors, and earthen home structures.


Team Rocket Power consists of Ben Nguyen, Aidan Belleau, and Michael Perez.


Team Rocket Power will design, prototype, and implement the aforementioned technology before the end of the semester.

Problem statement[edit]

The purpose of Rocket Power is to design a locally sourced, aesthetically pleasing, and fully functioning rocket mass cobb bench heater.

We hope this demonstration of appropriate technology will have both a utilized purpose for accessible and comfortable seating, as well as an educational tool for future generations of students.

Literature Review[edit]

Rocket Mass Heater[edit]

A rocket mass heater, or rocket stove, consists of a right angle ‘J’ shaped combustion chamber inside of an insulated container. This insulation container creates a draft in which the heat gasses are drafted and then pushed through piping into mass. This concept can be applied to heat earthen built furniture in homes that contain high thermal mass. [1]

Rocket Mass Heater Testing[edit]

Testing of rocket mass heaters for emissions efficiency is very difficult to come to conclusive data as each system will vary. Not one scenario will be exactly the same as fuel, operation, and structural design are variables. The testing would also be very expensive, ranging from $8,000 to $30,000 and would be targeted for builders who are in the business of licensing and patenting particular designs. [2]


Materials Wood fuel Labor

Basics of Cob[edit]

Cob is regarded as the first earthen material used for construction. Cob buildings from the 1400’s are still standing to this day and serve as an important reminder that cob does have longevity.[3] In its most popular form it is a combination of clay for strength, sand to reduce cracking by shrinkage, for strength and water to create cohesiveness. [4] Together these four material create a medium that is easily malleable and self-drying. It must be noted that while straw does provided other tasks such as decreasing weight of the mixture and aiding in the drying process its main purpose is to provide tensile strength. Thus many materials can be used in place of straw as long as it has tensile strength properties (i.e. herbs, pine needles, various types of grasses).

Creating and Mixing of Cob[edit]

The creation of cob is specific to location, materials at hand, what makes environmental and economic sense, preferences, time allowance, and desired outcome. Due to the extreme variability of the medium there is no universal standard of cob mixture. [5] From review it is understood that mixtures containing more straw and water tend to see the most amount of shrinking.</ref>O. A., ., C. J., ., A. O., ., O. D.-S., and ., A. E. (2006). “Engineering Properties of Cob as a Building Material.” Journal of Applied Sciences, 6(8), 1882–1885.</ref>We will therefore be mindful of out straw and water content and create sample bricks of different mixtures to determine the best combination for our task, materials at hand, and desired outcome. The large scale mixing process will be done by foot on a tarp. I

Impacts of moisture(concerns)[edit]

It is a well-known fact that Arcata, California has high humidity and precipitation for more than half of the year.[6] This is due to the fact that Arcata is located in northern California on the western coast of the United States. Moisture and humidity can affect rates of drying and can alter mixtures causing unexpected shrinking and drying outcomes mostly seen in plasters. </ref>O. A., ., C. J., ., A. O., ., O. D.-S., and ., A. E. (2006). “Engineering Properties of Cob as a Building Material.” Journal of Applied Sciences, 6(8), 1882–1885.</ref> Again plaster samples of varying recipes will need to be created and subjected to tests that will be determined on a later basis. The fact of the matter is the climate of Arcata will permit a structure made out of cob to be successful in enduring heavy use for several years.</ref>“Where Does Cob Work?” (n.d.). Green Home Building: Earthships, <>(Feb. 19, 2019).</ref>

Understanding R-Values[edit]

R-value measures the thermal resistance of a material. This can also be expressed as the temperature difference that will cause one unit of heat to pass through one unit of area over a period of time.

InspectApedia provides a comprehensive R-Value table with different insulation materials and various common construction materials. [7] [8]

Basics to Clean Burning[edit]

The following was adapted from the book Designing Improved Wood Burning Heating Stoves by Mark Brayden et al.[9]:

“A good combustion chamber changes wood or other biomass into heat without creating much smoke or creosote (condensed wood tars). Complete combustion of wood results in two byproducts: carbon dioxide and water vapor. In contrast, incomplete combustion creates unburned particles that cause pollution and creosote that fills chimneys and can cause chimney fires if it ignites.”

1.) Metering the fuel– Splitting wood up into small pieces (no bigger than wrist and 16 inches long) and feeding them at a consistent rate. 2.) Creating a hot fire-Managing the combustion zone where fuel, flame and air are mixed to create a fast burning and efficient coal reservoir. 3.) Insulating the combustion chamber- Crucial for mass stove heaters that store energy in materials with high insolation properties (see Understanding R-Values) 4.) Igniting escaping smoke- To ensure that the ignition chamber reaches optimal efficiency, the exiting gases from the exhaust should be just CO2 and water vapor. 5.) Providing sufficient oxygen– Starving the fire slows it, cools it down, and produces smoke. 6.) Warming and increasing velocity of cold air entering the fire– Air is warmed as it passes through a small opening (usually the same entrance where wood is fed) into the combustion chamber. For systems without a fan, make enough small holes under the door into the combustion chamber so the holes have as much cross-sectional area as the chimney exiting the stove. Position the holes so that primary air is sucked into the coals and up into the combusting wood. 7.) Creating sufficient draft– Use a tall enough chimney, double the height of the width of the internal vertical combustion chamber. An insulated chimney creates much more draft than an uninsulated chimney. High air movement entering under the fire, up though the coals, to create mixing which reduces emissions. Do not use a damper in the chimney. Design the stove to run efficiently with enough air entering and leaving the stove to burn fuel cleanly.

Do not block the air intake, reducing primary air and blocking the necessary amount of intake volume will create articulate pollution

Types of Mass Heated Rocket Stoves[edit]

Different designs of the J-shaped combustion chambers have been refined by hundred of permaculturists and homesteaders worldwide. To assess the success and meet the main goal of longevity for an overall design, the below project references display ideal qualities and strategies. The examples begin with low cost and minimum design complexities and advance to high expense, technical, and professionally engineered.


Solunit Rocket Stove[edit]

Overall cost = 700$ Materials: 110 firebricks, a 55 gallon drum, an old water heater core, ~20 ft 8” diameter ¼” Steel Pipe, concrete, pumice, and multiple tons of cobb

[11], advantages and disadvantages, and/or have a comparison matrix. 

Garens Earthbag Bedroom=[edit]

An earthbag is... Design Materials [12]

3rd example[edit]

Outdoor mass heater example

Efficiency of Combustion, and Heat Transfer Efficiency[edit]

The Rocket Stove, created by Dr. Larry Winiarski, features an insulated fire box and short chimney (14”). Low mass insultaion such as wood ash help to keep combustion temperatures high and improve burning efficiency. The high combustion temperatures also greatly reduce smoke and leads to saving of wood, making a more efficient stove. [13]


Heat, Temperature, and Thermal Energy Thermal Energy is an energy of the system due to the movement of its atoms and molecules as a form of energy. In this sense, a system can still sustain thermal energy even in an isolated environment. According to the first law of Thermodynamics, work or heat that changes the thermal energy also changes the pressure, volume, temperature, and other variables. This explains how high pressure inside a rocket stove chimney generates extreme heat and forces it up and out into the isolated container surrounding it. This is crucial to gaining high combustion temperatures in a small space. [14]

Extra info:

“The work of a stove can be divided into two parts: 1.) efficiency of combustion and 2.) efficiency of heat transfer to the pot.” (p.1) This along with operator skill are the three essentials to make the system work. Of the three, “improving the efficiency of heat transfer can result in greater savings of firewood.” (p.1)   .[2]

Increasing Heat Transfer Efficiency[edit] The transfer of heat from the stove to pot is potentially be the greatest loss of energy in the system. [3] Insulation[edit]

“Insulate and separate the fire and heat from the stove body with a good, low mass insulator” (p.1) [4],. 

Fire Flow[edit] “shorten the fire flow path as much as possible without creating excess smoke.” (p.1). [3] Proximity of heat to pot[edit] Try to have the flame touch or nearly touch the pot. The greatest differences in temperature between the heat source and the pot results in the highest efficiency of heat transfer.” (p.1). Designing interpretive materials[edit] The design of the rocket stove should incorporate these methods above to gain the highest efficiency possible.



  1. Evans, I., and Jackson, L. (2006). Rocket Mass Heaters: Superefficient Woodstoves YOU Can Build.
  2. Wisner, E. (n.d.). “Rocket stove efficiency/certification .”, <> (Feb. 21, 2019).
  3. Hazeltine, B., and Bull, C. (2003). Field guide to appropriate technology. Academic Press, Amsterdam.
  4. Elizabeth, L., and Adams, C. (2000). Alternative construction: contemporary natural building methods. Wiley, New York.
  5. Elizabeth, L., and Adams, C. (2000). Alternative construction: contemporary natural building methods. Wiley, New York.
  6. “” (2015). Climate Sahara: Temperature, Climograph, Climate table for Sahara -,, <> (Feb. 20, 2019).
  7. "R-values of Insulation and Other Building Materials." Accessed February 21, 2019.
  8. (2018). R-Values of Materials: Table of Insulation R-Values and Properties for Various Insulation Materials & Building Materials (brick, block, wood, soil, air gaps, etc). [Accessed 21 Feb. 2019].
  9. Bryden, Mark, D. Still, D. Ogle, and N. MacCarty. "Designing improved wood burning heating stoves." Aprovecho Research Center (2005).
  10. This is an example of a named reference. You can use these named references to repeat citation content throughout the document.
  11. This is a third example of a plain footnote.
  12. Moving from City to Country, M. (2014). Earth Bag House Construction with Rocket Stove Mass Heater & More (Garen's Earthbag Bedroom). [online] YouTube. Available at: [Accessed 21 Feb. 2019].
  13. Still, D., Kness, J., Billesten, B., Cox, G., Espenan, M., Nael, J. B., Nicholas, D., Subramanian, M., and Zettler, D. F. (1996). “Efficiency of combustion, Operator Expertise, and Heat Transfer Efficiency.” Fuel Efficient Wood Stoves and Hayboxes.
  14. Knight, R. d. (n.d.). Physics for scientists and engineers: a strategic approach.
  15. Still, D., Kness, J., Billesten, B., Cox, G., Espenan, M., Nael, J. B., Nicholas, D., Subramanian, M., and Zettler, D. F. (1996). “Efficiency of combustion, Operator Expertise, and Heat Transfer Efficiency.” Fuel Efficient Wood Stoves and Hayboxes. .