- Joined
- Mar 16, 2016
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- 274
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I've been spending the past few days coming up with a mitigation for a mistake I made on this build. After I glued the middle CR to the MMAS mother tube, I did a trial fit of the fins and had trouble getting them to line up correctly. Upon further investigation, it looks like the middle CR, which has notches for the fins, was shifted/rotated slightly (maybe 1/32" or 1/16") so the notches in it don't quite line up with the notches in the bottom CR. I'm not sure how this happened, but, I think maybe the lines I used to align the middle CR were just a bit off. I put the whole MMAS assembly into the slotted body tube to confirm the mistake. Below is a picture and you can see the top CR notches not quite lining up with the slot. They look to be about 1/8" off here because any mistake made at the inner part of the circle (CR) gets amplified as you go further out in the circle.
Fortunately, I caught this before I glued the fins in place. Unfortunately, this has to be fixed as it would cause the fins to be out of alignment along the length of the rocket. That is not acceptable as it would likely cause significant spin and I'm aiming for a straight flight with no spin.
My plan to fix this is to file the notches in the middle CR so that they line up with the bottom CR and then use a few jigs/guides to ensure the fins are straight. I'm also going to confirm the lengthwise alignment using the slotted body tube. I'll put a shim in each middle CR notch to fill the gap I created when I filed them. Below are some pictures of the jigs I'm going to use. One is made from some spare 2x4 and modelled after others I've seen here on the forum and the other uses foam board and a pattern from PayloadBay at https://www.payloadbay.com/index.php?page=Tools&action=FINGUIDES. Note that the wooden jig will be held in place with some elastic wrapped around the MMAS mother tube. I've also put together two short pieces of 1x3 into a basic 90 degree angle guide and used it to mark new fin root lines on the MMAS tube.
I'm planning to start gluing fins once I've looked this over a few more times to be sure the fins will be aligned (within a few microns) in all directions.
Fortunately, I caught this before I glued the fins in place. Unfortunately, this has to be fixed as it would cause the fins to be out of alignment along the length of the rocket. That is not acceptable as it would likely cause significant spin and I'm aiming for a straight flight with no spin.
My plan to fix this is to file the notches in the middle CR so that they line up with the bottom CR and then use a few jigs/guides to ensure the fins are straight. I'm also going to confirm the lengthwise alignment using the slotted body tube. I'll put a shim in each middle CR notch to fill the gap I created when I filed them. Below are some pictures of the jigs I'm going to use. One is made from some spare 2x4 and modelled after others I've seen here on the forum and the other uses foam board and a pattern from PayloadBay at https://www.payloadbay.com/index.php?page=Tools&action=FINGUIDES. Note that the wooden jig will be held in place with some elastic wrapped around the MMAS mother tube. I've also put together two short pieces of 1x3 into a basic 90 degree angle guide and used it to mark new fin root lines on the MMAS tube.
I'm planning to start gluing fins once I've looked this over a few more times to be sure the fins will be aligned (within a few microns) in all directions.