In the spirit of full transparency and to ensure we can take advantage of all of the combined wisdom of the community, I wanted to start a discussion about all of the engineering related design decisions that went into the PYRIKX rocket stove kit. There will be a different post about the purely design and feature related considerations, as well as the build/assembly related process. If anyone has suggestions for improvement, variants or general ideas to contribute please add to this discussion!
What Were We Trying to Achieve?
We wanted a stove that’s light and simple enough for camping, yet powerful enough to boil water or simmer a meal in under ten minutes. No fans, no fancy parts – just geometry and gravity doing the work.
Our goal: reliably deliver at least 8,000-12,000 BTU/hr of cooking heat using only sticks you can split in the woods.
Tube Size and Riser Height: Why 4″×4″ at 18″
We settled on a 4″×4″ square-tube riser because it:
- Fits trail wood. A 4″ interior clearance lets you burn 1″×2″ split sticks – no special fuel required.
- Remains portable. It’s light enough to carry, yet robust.
- Delivers the right draft. At 18″ tall, the riser naturally pulls air so you can feed about five sticks per hour and see ~10 kBTU/hr at your pot.
Here’s how the numbers line up:
- A batch of 2-3 sticks every 5–10 minutes should deliver about 30,000 BTU/hr of fuel energy.
- Natural buoyancy in the chimney exhausts only ~30,000 BTU/hr of heat. (The theoretical limit for this size.)
- About one-third of that exhaust couples into your cookware—hence 8,000–12,000 BTU/hr of cooking power.
We also looked at variations on the height of the stove. Shortening the riser to 12″ drops draft by ~20 % (≈8.5 kBTU/hr). Lengthening to 24″ gains ~15 % (≈12 kBTU/hr) but adds bulk. So I thought 18″ was a good compromise.
The larger 6″×6″ variant doubles heat capacity (≈23 kBTU/hr) but demands more fuel. So use it if you need to do a lot more cooking, but be prepared to feed the beast!
Feed Angle: Why 60°
We set the fuel-tube input angle at 60° so sticks slide in slowly, burning off their gases on the grate before dropping into the riser.
This gentle angle gives volatile gases time to mix with the primary air below, producing a clean, smoke-free flame. Steeper angles (45°) speed reloads but risk “tunneling” (cold spots/clogs). Those angles are easier to cut when you have to saw the tubing, but our CNC laser doesn’t care – so we optimized for it.
Airflow Control: Front-Mounted Damper and Primary Air Only
- Damper location: At the front of the bottom air-intake tube, so you can adjust by sight and feel.
- Primary air only: All combustion air enters through the grate under the fuel bed; there are no secondary-air ports to complicate things.
- Ash-bin leakage: The removable ash drawer leaks about 2% extra air, but the front damper gives you full control over total intake.
One damper, one air path, one grate = simple and fool-proof.
What You’ll See at the Pot
In the real world, there are a lot of variables! On a mountain in Denver the results will be different than on a beach in Florida due to elevation, temperature, etc. But very generally:
- Feed rate: 2-3sticks every 5–10 minutes should provide about 30,000 BTU/hr of chemical energy.
- Draft cap: The 4×4×18″ riser exhausts ~30,000 BTU/hr, of which 8,000–12,000 BTU/hr are captured as cooking heat.
- Boil time: One gallon (8.34 lb) from 70 °F to 212 °F requires ~1,183 BTU. At 8–12 kBTU/hr that’s ~6–9 minutes.
Potential Improvements: Insulation, Swirl Vanes and More
We can spend forever improving the efficiency and performance, but at the cost of more expense and a more challenging build. However, there are a few things you can do to squeeze more performance out of one of these stoves.
- Insulation: wrapping something like a Titanium Exhaust Heat Wrap with Lava Rock Technology around the riser and chamber traps heat in the flame zone, raising exit-gas temps from ~600 °F to ~800 °F and boosting draft by ~12 %. It also radiates heat back into the fire, cutting smoke and improving burn completeness.
- Swirl vanes: Three thin helical fins (3″×12″, twisted 30°) inside the riser can force gases into a corkscrew path, breaking up cold pockets and extending residence time. In theory this adds ~6–8 % efficiency (≈ 600–800 BTU/hr more at your pot).
- Hardwoods (oak, hickory, maple) pack more energy per pound than softwoods (pine, fir, spruce). At a typical 20 % moisture content: Softwood LHV ≈ 6,500 BTU/lb, Hardwood LHV ≈ 7,320 BTU/lb
- Dryness Matters Even More. Every extra percent of moisture content (MC) in your wood steals heat to evaporate water. Oven-dry (0% MC) ≈ 8,600 BTU/lb, 10% MC ≈ 8,040 BTU/lb, 20% MC ≈ 7,320 BTU/lb, 30% MC ≈ 6,700 BTU/lb.
It should be noted that all of these improvements can be stacked. So if you insulated and added swirl vanes, which using extremely dry hardwood, you’d get a massive improvement of heat out of the stove.
Hopefully that wasn’t too boring or TMI. Love to hear everyone’s thoughts…
John P.
Texas Metal Works
Founder
1 user thanked author for this post.
Noe Fontes (April 28, 2025) - You must be logged in to reply to this topic.