what do you guys think of my first rocket stove design?

I just posted on here yesterday inquiring about the science behind maintaining a draft on a rocket stove.  Because I am wary about building something that may not work as I hoped, you will see that not only is my design ridiculously simple but is meant to have an initial phase (simple chimney straight out the roof indicated by the dashed line) and then I will transition to an experimental 2nd phase in which I add progressively longer horizontal runs.  I initially intend to encase a simple 6" wye stovepipe flue connection in concrete.  Once I find a configuration that I like, I will probably encase the whole thing in either concrete or gravel or sand.  Am I missing anything?

I should mention that this is for a greenhouse with very little monetary value, far away from the house, and only for use during the early spring when I'm trying to get seedlings growing. I don't need a lot of heat, it would just be burning for a couple hours each day max.  Also, it is not to scale.

Being your first build, I would highly recommend getting some bricks and making a simple J tube in your back yard.
Fire it up, and experiment with it for several days.
Also, pick up a copy of the Rocket Mass Heater Builders Guide by the Wisners and/or a copy of Ianto Evans book on Rocket Mass Heaters.
You can learn so much from these two resources.
Steel anywhere in the flame path and the use of concrete usually don't have much to do with the rocket stoves that people build here.
Search the rocket mass heater forum here on Permies. There are lots of peoples builds that have been documented and many lessons learned that can really help a person get started and avoid common mistakes.
Good luck and have fun experimenting.

Thanks for this response.  I should clarify, this is not my actual first time ever rocket stove.  I did throw one together a few years ago, a simple J design using a paint can and tubing.  It worked well but due to it's simplicity I barely count it as a build. This is the first one I am putting any brain cells to work towards.

There are some significant differences between what is commonly known as a rocket stove and a rocket mass heater. Most rocket stoves I've seen are like you mention, several tin cans assembled in an L format, sometimes with some insulation between them and intended as a small cookstove, sometimes made from cinder blocks or steel pipe and little to no insulation. The design doesn't burn very hot, so the metal or concrete can handle the temps.

A rocket mass heater on the other hand is well insulated and can only be made from materials that can withstand much higher heat like masonry or refractory materials, and burn temps go well above what cement/concrete/steel can survive for more than a couple burns. People have tried aircrete and variations and have learned when properly insulated it burns out really fast, and steel spalls as well. While you could add a finish coat of concrete to the outside, you'll want something more durable like brick on the inside, surrounded by insulation like perlite around the burn chamber.

If you plan to build a 6" system, you can start with 20-30 feet of ducting depending on the number of elbows used, or will you make an open bell/stratification chamber?