The tone of some of the posts on this thread has become needlessly pugilistic. I’ll try to summarize – and maybe add something Richard can work with.
>> If the BB Ultra had a fan as an OEM option who would be interested?<<
Richard, you have certainly open up a can of worms with your invitation for folks to comment on your plan to modify a nearly flawless design.
As far as I can see, adding a fan to this stove would only take away from all of its advantages (simplicity, elegance, no dependence on gizmos, batteries, or manufactured fuel) to gain . . . what?
Richard, you stated that the fan would add flexibility – but did not expound upon that. I honestly don’t know what you mean by this, but I’m certainly willing to hear you out.
Throughout this thread, the only specific advantage postulated for adding a fan is that it might make lighting the BushBuddy easier. However, several posters have pointed out that – with practice – the BushBuddy is already easy to light without a fan. As a BushBuddy owner myself, I agree.
Thus, without some specific examples of increased flexibility, adding a fan for ease of lighting alone would add dependence on packed-in technology, weight, complexity, and (probably) awkwardness – for a minuscule or nonexistent gain. This is a legitimate point – one not easily dismissed.
The type of backpacker who would buy a BushBuddy in its current form yearns for a simpler form of backpacking and enjoys honing his bushcraft skills – but does not mind the convenience of well-designed “low-tech” (non-gizmo) equipment like the BushBuddy. This type of backpacker would not enjoy a “teched-up” BushBuddy with a fan. I know I would not.
On the other hand, a tiny version of the Zip stove (which it appears you are trying to make) could definitely have broad appeal. A potential pitfall with this type of tiny, fan-driven BushBuddy is that it would consume fuel at a VERY high rate, and would therefore need a large firebox. As Mike said, “The Zip stove is as big as it is for a reason. If you fan the fuel it will burn that much more quickly and if there is a downside to the BB it's that you have to keep feeding it fuel.”
That said, I see no problem with mounting a fan (the small type that cools computer chips – not the whole “box”) in the bottom of the BushBuddy below the ash bin. It should move the air through both pathways (up through the center and up between the outer and inner walls). I bet it will burn really hot.
Richard, let us know how it works.