Firebee Q-2/KDA-1 (3D Printed Parts + CNC'd Plywood Fins)

[BigMacDaddy]

Playing around with a Firebee design... Going for the original Q-2/KDA-1 version. Just very early prototype at this point, not happy with how the nosecone came out (I have a different version that I did with lofts in Fusion 360 so will likely switch back to that one). I am considering a 16 degree engine cant (the angle of the thrust of the original model) instead of 7 degrees (the angle of the original engine) to try to get the thrust line closer to the CG.



This mock-up is with a 7 degree cant. Forgive the rough print, this model still needs some work (and I printed with bed too cool in my rush to get a prototype printed). Also I just grabbed the first BT-50 tube I found (it will be cut about flush with the plastic part in the real build).


I am struggling a bit with how to mount a canted traction motor like this to reliably push/pull the rocket and also eject the nosecone. In my current design, the gray transparent part below the BT-80 tube would be glued to the motor mount tube and to the BT-80 tube and a motor block would be installed inside the motor mount tube. The engine would be cantilevered (for lack of a better word) into the nosecone and an centering ring / block would be used inside the nosecone (the pink part) to hold the front of the motor. Do you think I can get the gray part mounted strong enough that it does not rip off the main body tube? I could maybe include a small 3D printed part inside the BT-80 tube to give support above and below the motor mount tube. Most of the weight in the model should be in the nosecone. Planning to launch with D12 or E12 motors.


I am trying to use an internal motor block inside the nosecone to allow the ejection to push nosecone off (The pink part will be glued inside the nosecone). This large angle means the motor mount tube is somewhat in the way of the nosecone ejection. I was thinking of a partial / top half shoulder inside the BT-80 tube and small tabs on the bottom to hold it in place for launch but allow it to eject diagonally. I would like to get an 18" or 24" chute in the BT-80 tube section to be pulled out by the nosecone. What do you think? Other options or is this a good plan?

 

[jqavins]

Hmm, complicated.

I'm clearly missing something about the gray shroud; why are you worried about it? The shroud to BT-80 joint doesn't seem to be part of the load path. I guess I'm missing something, but here's how it looks to me:

• There is a chunk removed from the bottom side of the BT-80.
• There is a "centering cradle", attached to the outside of the BT-80 that holds the MMT in position.
• The MMT is glued to the centering cradle and to the cut edges in the side of the BT-80.
• The gray part is a shroud covering up all that.
• The load path is from engine block, through MMT, and into the BT-80, in part directly and in part through the centering cradle.
  • Thus, the shroud is not structural.

You might try assembling that much of one with the shroud left off, then holding the BT-80 in one hand and pulling on the MMT with the other.

Ejection and 'chute deployment are the really complicated part. I don't like the idea of the diagonal deployment, as I think it would strain the edge of the BT-80 no matter how you do it, leading to rapid wear out even if it works for a couple of flights.

What about moving that red part further forward and adding a bend to the MMT. That might be a 3D printed special coupler, or a mitered joint between tubes. Then the front of the "bent" MMT can enter the red part at a normal angle* and push the nose straight off.

Oh, and one more thing: make the MMT engagement depth in the red part greater than the NC shoulder engagement in the BT-80. That way, the shoulder is guaranteed to be clear of the body tube before the red part is clear of the MMT, assuring that the nose is pushed all the way off before the ejection gas is vented into it.

* See what I did there?

 

[BigMacDaddy]

Hmm, complicated.

I'm clearly missing something about the gray shroud; why are you worried about it? The shroud to BT-80 joint doesn't seem to be part of the load path. I guess I'm missing something, but here's how it looks to me:

• There is a chunk removed from the bottom side of the BT-80.
• There is a "centering cradle", attached to the outside of the BT-80 that holds the MMT in position.
• The MMT is glued to the centering cradle and to the cut edges in the side of the BT-80.
• The gray part is a shroud covering up all that.
• The load path is from engine block, through MMT, and into the BT-80, in part directly and in part through the centering cradle.
  • Thus, the shroud is not structural.

You might try assembling that much of one with the shroud left off, then holding the BT-80 in one hand and pulling on the MMT with the other.

Ejection and 'chute deployment are the really complicated part. I don't like the idea of the diagonal deployment, as I think it would strain the edge of the BT-80 no matter how you do it, leading to rapid wear out even if it works for a couple of flights.

What about moving that red part further forward and adding a bend to the MMT. That might be a 3D printed special coupler, or a mitered joint between tubes. Then the front of the "bent" MMT can enter the red part at a normal angle* and push the nose straight off.

Oh, and one more thing: make the MMT engagement depth in the red part greater than the NC shoulder engagement in the BT-80. That way, the shoulder is guaranteed to be clear of the body tube before the red part is clear of the MMT, assuring that the nose is pushed all the way off before the ejection gas is vented into it.

* See what I did there?

Thanks very much for thinking through this. I need to read what you wrote more carefully but just to clarify: the gray part is the "centering cradle" - I guess it has inside and outside walls so the shroud and the centering cradle are the same part in my initial design.

I was just not sure how strong a part surface-glued to a BT-80 tube would be (could it support a 24mm motor mount tube).

I am becoming more confident as I realize I can also glue a part inside the BT-80 tube (and maybe, as you are describing, I could make this a solid part that slips onto the bottom edge of the BT-80 tube (may require cutting the BT-80 tube a bit more but not much).

Your idea to put a bend at the end of the motor mount tube so that it ejects straight off is a good one. That might work. I did not think I had enough length but I think I do (did not really consider 3D printing an insert that goes inside the motor block to turn ejection charges).

I wonder how deep of a shoulder I actually need if I have the motor tube going inside an inner part of the nosecone.

 

[jqavins]

OK, so rewording my thought on the gray part: think of it as (in ship building terms) a rib and a hull section, printed as one. If the rib portion of it is glued to the (cut) BT and the MMT, and MMT is also glued to the BT, I don't think there's a problem. If the hull section portion of the printed piece is also glued to the BT, so much the better. So, (I'm repeating myself now) print that piece, take a bit of BT-80 and a bit of BT-50, build it up, and see how it feels.