Rocket stoves are a type of fuel-efficient device, named in the 70's, but dating back millenia in concept.  A super-hot chimney above the fire draws the flames sideways and up, blending hot fuel and air into a quick, hot, clean-burning fire that takes little wood, leaves little residue, and has lots of uses.

Variations on the same concepts include Fox-stoves or Badger-stoves, cooking Rocket Stoves, Dakota firepits, traditional furnaces, kilns, modern incinerators, smokestacks, bell-stoves, and millions of humble tea-stoves and household hearths developed by resourceful people all around the world.

The Rocket Mass Heater takes this useful, clean-burning heat, adds a self-feeding wood box, and channels the exhaust through a masonry block to store touchable warmth for hours or days. (Masonry heat storage is also seen in Roman hypocausts, Chinese 'kang' stoves, European masonry heaters, and ancient masonry dwellings and hearths).

Rocket Mass Heaters are a specialized adaptation of incredibly powerful Fire Science concepts that are worth knowing in their own right.

• Heat people and things, not space.  Masonry or water holds hundreds of times more heat than air, stores it longer, and releases it at safer temperatures for household comfort.  Mass can be heated by sun, by geothermal, water or steam, fire or flue gas.
• Smoke is wasted fuel.  It's also poisonous.  Good mixing, high-temperature combustion, and the right balance of draft and drag create a clean-burning, efficient stove.  Efficient stoves use less wood, as well as promoting healthy air for everyone in the neighborhood.
• Fire needs attention.  Staying warm all night is a major safety concern with solid-fuel devices. Smoldering 'banked' fires put out smoke, creosote, and carbon monoxide, and increase the risk of conflagration.  Mechanically controlled fires (as in pellet stoves) become unreliable if storms knock out the electrical power.  Masonry heaters solve this problem by storing heat from a quick, hot, supervised fire, providing hours or days of comfort without further attention.
• Waste into Resource: Rocket Mass Heaters are designed around common, affordable materials that can be down-cycled from the waste stream, or obtained in almost any location at 'dirt cheap' prices.  Once built, they are a solid-state technology that will last for decades (and with one or two replacement parts, centuries) with minimal maintenance.
• Passive Comfort is Awesome: The fire roars merrily in its insulated chamber, fuel feeds itself slowly downward, and the masonry soaks up the heat and transfers it gently toward your feet, butt, or other appreciative surfaces.  Radiant heat from the metal surfaces provides instant gratification, while stored heat warms the house gently like a spring thaw.  No moving parts, minimal wood-prep and cleaning, and very low risk of creosote or chimney problems.

Picture albums from some Rocket Mass Heater projects:

Test beds, earthen oven, Dana Annex 2008: http://picasaweb.google.com/eawisner

Video:
Trackers Rocket Stove Intro: (2 min of Ernie)
http://www.youtube.com/watch?v=kNv-WX8-Ymc

Rocket Stove Article by Paul Wheaton, with pictures, animation & videos:
http://www.richsoil.com/rocket-stove-mass-heater.jsp

(Other video is available too - like this interesting all-masonry bell stove variation:
http://www.youtube.com/watch?v=-jzKKIHhTU0 
Compare the search results from "Rocket Mass Heaters" and "Rocket Stoves, Cooking;" you'll quickly pick up on the differences in design and intended use.)

Rocket Stoves.. Experimenters corner. Answers questioned!
(An edge-exploring discussion forum set up by researcher Kirk Mobert.)