Glenn Herbert

The Wheaton Labs videos are rough gravity-load timber framing. A true traditional timber frame has mortise and tenon joints, pegged to withstand tension as well as compression loads. The number, size and species of pegs makes a huge difference; mortise and tenon joints do not use tenons like nails, but like through bolts.

The trusses you draw are not "on top of" the tie beams, the tie beams are a critical part of the trusses, and all have to be pegged together so the joints cannot pull apart. If they are joined properly, the tie beam is largely in tension holding the bottom ends of the sloped top members from spreading. If you depend on the tie beam just supporting the gravity load of the upper members and roof, then it must be really strong to resist bending.

I agree with framing the trusses (as proper trusses) in place rather than trying to raise timber frame bents. I also think 10' spacing is much safer. My approach would be  to start with longitudinal eave beams the full length with all posts diagonally braced, then building the trusses on top of that base with structural tie-down from top members to posts. Maybe it would even work to extend that tie to deadmen buried in the ground, to counter hurricane uplift.

I think the extra weight of roundwood purlins spanning between trusses rather than 2x purlins would be beneficial to high wind stability.

You're not thinking of 2x6 flat purlins spanning 10', are you? Those would sag dramatically just from the metal roofing load, and collapse with serious snow load. If you use good sized beams spanning across the truss bents at ridge and eaves with intermediate rafters spaced 3 or 4 feet apart, 2x6 or 2x4 purlins would work fine.

Those purlins would be a lot of lumber to buy, and if you have a surplus of roundwood, I would use those as purlins and skip the purchased lumber. This would require quite a bit of flattening time to give a good surface for metal roofing. If you have some 20' logs for purlins, using those staggered to span multiple bents would increase the stability of the structure. You will need diagonal braces in both directions in any case unless the posts are set several feet deep in the ground, which depending on your climate and soil would likely rot in several years without effective treatment and placement. 2' deep post setting will barely keep them upright during construction, never mind live loads. If you are in a hot moist climate with wet soil and little stone, I would try different foundation methods than earthfast posts, assuming you want this structure to last.

Looking at your sketch, your trusses need to have the tie beam directly secured to the top members of the trusses, and the longitudinal beams tied into those - there are several ways that could be done. The diagonals in the trusses would be necessary at the span you show unless the top members are really strong. The truss joints need to be serious load-bearing types; again, there are numerous possibilities.

More detail on the sizes of your available logs would help us give the best advice.

Stone in a buried structure will quickly transmit heat from interior to ground or vice versa, depending on which is warmer. The issue for modern civilization is that ground temperature in any place that needs serious heating in some part of the year is around 55 F at most, considerably cooler than comfortable. Dry sandy soils may allow heat to build up from a heated interior and eventually approach comfort levels, but wet soils will never get warm.

Wood as the underground structure will effectively insulate the interior from the ground, losing heat much more slowly than when exposed to the cold air and wind. Stone holds more heat per unit volume, and transmits it faster than wood. Soft, lightweight woods will be better insulators than dense woods, though they will most likely rot much faster if exposed to damp.

If you have a vertical coil, every loop will want to form an airlock. I think the best way to get around this would be to fill it while it is horizontal, close it off, and then lift it to the vertical position and secure it. A bleed valve at the beginning of the coil should help get rid of any air that finds its way into the coil, as long as it is not so much that it gets into the second or following loops.

Wow, that is a small TC liner :O
I had a 12 x 12 which was so big I added an 8" galvanized duct outer skin to fill around the 6" SS to avoid spending a ton of money on perlite, and an 8" TC which was a tight fit for the 7" SS I used to allow for a future 7" batch box RMH. (I had to slightly flatten the four sides of the 7" SS to fit it into the 8" TC.) My experience with terracotta flues is that an 8x8 is about 6 1/2" to 6 3/4" square inside.

I put solid stainless steel liner inside the old terracotta chimney liners for new wood stove installations, and poured perlite into the cavities (after blocking the bottoms), and that insulates them nicely without tearing out the original liners. Much less expensive, too.

Steel barrels over the riser do not wear out in less than decades of normal use. Thomas Rubino is in an extreme climate and building situation where the RMHs had to burn almost all day every day, and his barrels did not wear out; one was retired because the RMH design was changed, and the other is still going strong as far as I know.

Ernie and Erica's book is still the best reference for J-tubes and what surrounds them, but the piped mass concept has been superseded by the bell concept.

You can make a bench-style bell in about the same footprint of a piped mass bench. We can give you specifics if you have your core design solidified, and construction materials basically figured out.