Matching shear pin holes on new booster to existing coupler.

[Bat-mite]

Mr. Klett at Madcow has just shipped me a package containing everything I need in order to rebuild the booster for my Formula 200.

The existing coupler has nice, tapped shear pin holes for #8-32 screws. The new booster will need new holes.

I have been trying to think of a way to not have to drill and tap new holes in the existing coupler, but rather try to match the booster to what is already there.

Of course, that is easier said than done. Even a fraction of an inch off means that the holes won't line up.

So I am looking for ideas. I have a couple myself, but maybe you have some tried and true solution that I haven't thought of, since I know I am not the first person to have to fit a new booster to an old rocket. :wink:

Idea Number One

1. Insert the coupler into the booster up to the switch band.
2. Insert a bright light into the coupler.
3. The light should show where the holes are; mark them with a Sharpie, and drill them slightly large to accommodate inexact drilling.

Problem with Idea Number One

1. Light may not be bright enough to shine through the fiberglass, rendering this idea moot.

Idea Number Two

1. Carefully measure the height of each hole in the coupler down from the switch band.
2. Carefully measure the distance between the hopes in the coupler with a tailor's tape.
3. Recreate these measurements in the booster and drill slightly large.

Problem with Idea Number Two

1. Booster has larger OD than coupler, and thus measurements would need to be modified to accommodate.

Idea Number Three

1. Wrap a sheet of paper around the coupler, with the top at the switch band.
2. Mark the shear pin holes.
3. Transfer the paper to the booster.
4. Drill through the paper into the booster; drill slightly large.

Problem with Idea Number Three
Same as Idea Number Two. Booster has larger OD than coupler.

 

[Dave A]

Mr. Klett at Madcow has just shipped me a package containing everything I need in order to rebuild the booster for my Formula 200.

The existing coupler has nice, tapped shear pin holes for #8-32 screws. The new booster will need new holes.

I have been trying to think of a way to not have to drill and tap new holes in the existing coupler, but rather try to match the booster to what is already there.

Of course, that is easier said than done. Even a fraction of an inch off means that the holes won't line up.

So I am looking for ideas. I have a couple myself, but maybe you have some tried and true solution that I haven't thought of, since I know I am not the first person to have to fit a new booster to an old rocket. :wink:

Idea Number One

1. Insert the coupler into the booster up to the switch band.
2. Insert a bright light into the coupler.
3. The light should show where the holes are; mark them with a Sharpie, and drill them slightly large to accommodate inexact drilling.

Problem with Idea Number One

1. Light may not be bright enough to shine through the fiberglass, rendering this idea moot.

Idea Number Two

1. Carefully measure the height of each hole in the coupler down from the switch band.
2. Carefully measure the distance between the hopes in the coupler with a tailor's tape.
3. Recreate these measurements in the booster and drill slightly large.

Problem with Idea Number Two

1. Booster has larger OD than coupler, and thus measurements would need to be modified to accommodate.

Idea Number Three

1. Wrap a sheet of paper around the coupler, with the top at the switch band.
2. Mark the shear pin holes.
3. Transfer the paper to the booster.
4. Drill through the paper into the booster; drill slightly large.

Problem with Idea Number Three
Same as Idea Number Two. Booster has larger OD than coupler.

If you are using #8-32 screws for shear pins, I think those are too large.
I use #2-56 screws for my 6" and 9" rockets, never an issue.

 

[Exactimator]

I was going to suggest #1 before I read your choices.

Is the rocket large enough to reach in from the top of the coupler and hand spin a drill bit through the existing hole, working from the inside out?

 

[Steve Shannon]

Personally I wouldn’t aggravate myself by trying. If forced to retain the threads in the coupler, I would put the coupler in and then drill the BT from inside, using my fingers to turn a bit that just slips through the existing threads. I would tap it the same way.
Otherwise just drill and tap new holes and fill and paint the old ones if the bother you.

 

[tim cubbedge]

I had similar conundrum last week in 4" rocket. Had to replace payload section. I just drilled new holes.

 

[Bat-mite]

If you are using #8-32 screws for shear pins, I think those are too large.
I use #2-56 screws for my 6" and 9" rockets, never an issue.

Thanks, Dave, but I have flown the rocket four times with three #8-32 screws in each end, and never had a problem.

 

[mpitfield]

I successfully did this last fall on my 3" Darkstar.

I simply used paper to mark the holes, no tools, then transferred the template to the new section. The trick is not to do all the holes at once, in my opinion.

Do the first hole, check it for accuracy by pining the coupler with the sheer pin or equivalent, adjust if necessary, then mark the next hole.

This is very much a measure twice drill once, or in my case I likely drilled a dozen times.

Good luck.

Sent from my ONEPLUS A3000 using Rocketry Forum mobile app

 

[Nytrunner]

If you go with #2 or #3, some simple trig may come in handy.

T=tape length

Tbooster = Tcoupler*(ODbooster/ODcoupler)

 

[markjos]

Personally I wouldn’t aggravate myself by trying. If forced to retain the threads in the coupler, I would put the coupler in and then drill the BT from inside, using my fingers to turn a bit that just slips through the existing threads. I would tap it the same way.
Otherwise just drill and tap new holes and fill and paint the old ones if the bother you.

I agree with Steve. There's no harm in creating a new set, and just be done with it. Sloppy holes created by fixing a misalignment just cause new problems with shear pin reliability. (If you're determined to try aligning with the old ones, and can't reach through the coupler with it in place, then at least you can insert the coupler until the holes are at the top edge of the new airframe tube, mark the airframe at the correct areas around the circumference, and even match the distance from the edge - but really, just drill new ones).

Mark

 

[Nathan]

I agree with above posts. Just drill new shear pin holes. Chances are that if you attempt to match old holes then at least one will be off just a tiny bit, making the process of inserting shear pins even more aggrevating than usual.

 

[woferry]

Not that there's anything wrong with the other suggestions, but personally I don't see why #1 wouldn't work, the RW/Madcow tubes have had no trouble showing light shining through in my experience. If you don't see it your light isn't bright enough.

 

[ecarson]

Just throwing this out there. Is there a way to clamp the coupler to a drill press table?

I am thinking you could set up the coupler, so the non-powered drill bit comes down perfectly inside the current threaded hole. Then slide
the booster section onto the coupler to the desired orientation, and then lower the powered bit to make just a small divot in the booster tube.
After the location is set, you could then drill the booster tube through, and then tap.

This assumes you can get the coupler fixed in such a way that the booster can still slide on.

 

[Dave A]

The time it would take layout and drill the holes, you might as well drill and tap new ones, J-B weld the old ones.

Sent from my SM-G935V using Rocketry Forum mobile app

 

[Bat-mite]

This raises an interesting question. If I were to drill new holes and leave the old ones unfilled, what does that do to the altimeters once the coupler is out at apogee?

All the way up, the coupler has had four 1/4" holes for sampling; but once it is out at apogee, there are suddenly a bunch more smaller holes. Does the sudden appearance of additional holes goof with the sampling?

 

[woferry]

Dave has a good thought on just plugging the old holes if you're concerned, but I don't see any issue with extra holes. The point of the holes is to ensure that any air pressure deltas between the outside and inside are quickly resolved, the more holes the better off you are. The port sizing guidance is really to ensure a minimum hole volume, the maximum volume is effectively unlimited so long as you don't care about what it might do the aerodynamics, which post-apogee shouldn't matter anyway. Consider an AltimeterOne/Two/Three that is generally out in the open-air post-separation, it works just fine that way.

 

[FredA]

This raises an interesting question. If I were to drill new holes and leave the old ones unfilled, what does that do to the altimeters once the coupler is out at apogee?

I NEVER bother to line up the parts to re-use the old holes.
I just drill and tap new holes for every flight.
Using 4-40's you could fly many, many flights before the strength is compromised.
I flew one NC about 25 times before giving it to a friend who continues to fly it....looks like swiss-cheese on the shoulder, but who cares...

With this line of thinking, you can focus on a good seating and smooth separation and not have to worry about alignment.
As parts take a beating from many flights, this become more important.

 

[jd2cylman]

I did this on a 4" FG airframe. I used a Dewalt 90° drill adapter with a 1/16" drill bit. Drilled each hole from the inside one at a time then finished drilled from the outside, adding a pin after each hole was done. Worked great. Any airframe larger in dia would be easy peasy...

 

[djkingsley]

This raises an interesting question. If I were to drill new holes and leave the old ones unfilled, what does that do to the altimeters once the coupler is out at apogee?

All the way up, the coupler has had four 1/4" holes for sampling; but once it is out at apogee, there are suddenly a bunch more smaller holes. Does the sudden appearance of additional holes goof with the sampling?

I doubt the extra holes will impact the air flow through the av bay after apogee nor will it affect your altimeters in any measurable way.

If you don’t want to epoxy them closed, put some clear tape on the inside of the couple to cover them.

 

[OverTheTop]

For fiberglass rockets I have been known to illuminate the inside of the coupler with a light bulb or LED. The location of the holes is then visible on the outer airframe. Positions are marked and starting with a small drill and working up the hole is enlarged. Stepping up the hole a bit at a time allows my to use a needle file to adjust the centering if my pilot hole was a bit off. Seems to work pretty well and repeatably.

 

[blackjack2564]

Idea Number One

1. Insert the coupler into the booster up to the switch band.
2. Insert a bright light into the coupler.
3. The light should show where the holes are; mark them with a Sharpie, and drill them slightly large to accommodate inexact drilling.

Done this several times, works flawlessly!

See here for explanation & pics:
https://www.rocketryforum.com/showt...-manual-you-asked-for-it!&p=294454#post294454

https://www.rocketryforum.com/showt...-manual-you-asked-for-it!&p=295109#post295109