I'm an idiot -- and I want people to shame me so I don't do this again.

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Bat-mite

Bat-mite

Rocketeer in MD
Joined
Dec 5, 2013
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At Saturday's launch, I discovered an interesting problem with my FG 4 rocket. It lives in the basement, which stays at about 70 degrees F. On Saturday, outside temperature at launch time was in the 90s.

I assembled my rocket, but when I tried to insert the shear pins, the holes were off center. They were actually too shallow, so there was no way I could get them into the NC unless I redrilled. So apparently the expansion of the FG due to heat caused my holes to become misaligned.

I had three choices: 1) fly without pins; 2) go bug AMW to use their electricity to redrill (and lose an hour of flying time); 3) not launch it at all.

Now, because it was hot and the FG expanded, that also meant that the NC was very snug in the payload tube, and I could not easily pull it off.

So, which of the above options do you think I chose? You're right. I launched it without shear pins. And would you believe ... the main came out at apogee? No, really! Who'da thunk it?

So it drifted about 3400', and everyone thought for sure it was in the trees. Fortunately it drifted so far that it came down in a wheat field. I had to drive a mile and a half to get to it, but I did recover it, and did not get shot at by any farmers.

Your job is now to tease me, taunt me, shame me, and humiliate me so that I don't ever do this again. Please?

The picture below shows the recovery path.
XC-landing.JPG
 
You are viewed favorably by the rokit godz. They merely reminded you that a successful flight must have the necessary shear pin sacrifice or they will be displeased. For us mere mortals to cast aspersions on one who is viewed favorably by the godz would be to invite our own personal trials.


I'm not gonna tempt that with a 15K shot coming up at LDRS.
 
Last edited:
I'm glad you got it back! And, if you are like me, you will make other bad decisions on the field, where launch fever is at its worst!
 
Bat-mite said:
At Saturday's launch, I discovered an interesting problem with my FG 4 rocket. It lives in the basement, which stays at about 70 degrees F. On Saturday, outside temperature at launch time was in the 90s.

I assembled my rocket, but when I tried to insert the shear pins, the holes were off center. They were actually too shallow, so there was no way I could get them into the NC unless I redrilled. So apparently the expansion of the FG due to heat caused my holes to become misaligned.

I had three choices: 1) fly without pins; 2) go bug AMW to use their electricity to redrill (and lose an hour of flying time); 3) not launch it at all.

Now, because it was hot and the FG expanded, that also meant that the NC was very snug in the payload tube, and I could not easily pull it off.

So, which of the above options do you think I chose? You're right. I launched it without shear pins. And would you believe ... the main came out at apogee? No, really! Who'da thunk it?

So it drifted about 3400', and everyone thought for sure it was in the trees. Fortunately it drifted so far that it came down in a wheat field. I had to drive a mile and a half to get to it, but I did recover it, and did not get shot at by any farmers.

Your job is now to tease me, taunt me, shame me, and humiliate me so that I don't ever do this again. Please?

The picture below shows the recovery path.
View attachment 265646
Click to expand...

Your explanation for the bad fit doesn't make sense. If your pihholes line up at 70 F, they will line up at 90 F. Materials expand with heat. Assuming the NC and the airframe are the same material, as the temperatures increase, the clearance increases while the angular spacing remains the same.

Did you have a scribe mark match line on your airframe and NC to confirm orientation? Unless you use a rotary index to drill your holes or have an orientation mark to insure alignment, you have a 1 out of N chance of aligning a N hole array correctly.

Bob
 
I never assume I can get it right so, when I have drilled these such holes, I drilled through both pieces at the same time, pinned it and then moved on to the next. I also make a hard to see mark by one set to make sure I get them in the right spot on assembly.
 
I agree with Bob, your holes should align regardless of the temp. One other issue that might be causing this is if you have a nosecone with a separate coupler for the shoulder, some filament wound fiberglass nosecones do, and if the coupler is not glued or otherwise secured, it could move up into the N/C and cause this problem.
 
Yeah, I have a mark. It's not that they didn't line up around the circumference. They actually didn't line up vertically. It would be logical to assume that the nose cone may have been slightly high when I drilled. But the opposite was the problem. The holes in the NC were too high! What I mean is, I would have to push the NC in deeper than it can go in order to match the holes. Strange, I know! Temperature is the only variable that I can see.

pinholes.JPG
 
Bat-mite said:
... but I did recover it, and did not get shot at by any farmers.

Your job is now to tease me, taunt me, shame me, and humiliate me so that I don't ever do this again. Please?

The picture below shows the recovery path.
View attachment 265646
Click to expand...

I do hope you checked with Bob. I don't know who owns that property but if they weren't apprised and saw you on their field, it could be bad for the club.
 
I did. He told me how to get there. He also said he would disavow any knowledge of me if I got caught. :wink:
 
Evidently, he is not that worried. If he doesn't, I don't. Unless it's hunting season and forested areas are involved ;)
 
Bat-mite said:
Yeah, I have a mark. It's not that they didn't line up around the circumference. They actually didn't line up vertically. It would be logical to assume that the nose cone may have been slightly high when I drilled. But the opposite was the problem. The holes in the NC were too high! What I mean is, I would have to push the NC in deeper than it can go in order to match the holes. Strange, I know! Temperature is the only variable that I can see.

View attachment 265650
Click to expand...

One piece or two piece NC? If the NC coupler is a separate piece, I've had experience with it moving and holes not lining up.

Nate
 
I'd bet that even if both parts were FG, differences in material thickness and possibly shape may cause asymmetrical expansion.

Glad you got the rocket back.

Now, don't make the same mistake twice! :wink:
 
I'm tempted to concur, heat seems unlikely with FG, especially over a 20 degree range. I drilled my darkstar's pinholes at about 25 degrees and they lined up at 100.


But the lesson is, carry a cordless drill, or know someone who does :)
 
Did you twist the two portions perchance?. I drill and number the holes on both pieces so #1 lines up with #1 and #2 lines up with #2 and so on. Some birds have three or four shearpins. If the tubes are rotated off there is no way I can drill holes so they are
completely symmetrical. That's why I number them so they line up every time. I've never had any trouble at any temperature with misalingnment as long as the number matched. Kurt
 
John,,
Check your witness marks rotation-ally....
You had to have it rotated out / wrong.......
Temp change will make for a snug fit,, OK,, been there.....
But the holes will still line up....
You can check this and duplicate at home what you saw at the field when you're at your leisure and not rushing...
Assemble in your basement correctly and see the marks line up,,,
Then assemble incorrectly and see if it looks like what you saw at the field....
I'm betting you had it rotated wrong..

Teddy
 
Mine never align up circumferentially so can never mistake the orientation. LOL. But I do mark one so I don't have to iterate the parts alignment at prep time.
 
nute said:
One piece or two piece NC? If the NC coupler is a separate piece, I've had experience with it moving and holes not lining up.

Nate
Click to expand...

Here's our winner. That makes total sense. It is a two-piece, glueless NC. My guess now is that I cranked down harder on the eyenut when I drilled, and at the field I didn't make as many turns. Brilliant!

Thanks, Nate. I'll count my turns until I can screw into it, and then use that number of turns. I also want to drill the payload holes slightly bigger, to give me a little leeway. As long as it is smaller than the screw head.... :smile:
 
Oh, and thanks to everyone else who made suggestions!

I still didn't get that shaming I was looking for.... :wink:
 
Bat-mite said:
Oh, and thanks to everyone else who made suggestions!

I still didn't get that shaming I was looking for.... :wink:
Click to expand...

You sir, are a dumb ass...

Don't be a dumb ass...

Is that better...:grin:
 
Bat-mite said:
Here's our winner. That makes total sense. It is a two-piece, glueless NC. My guess now is that I cranked down harder on the eyenut when I drilled, and at the field I didn't make as many turns. Brilliant!

Thanks, Nate. I'll count my turns until I can screw into it, and then use that number of turns. I also want to drill the payload holes slightly bigger, to give me a little leeway. As long as it is smaller than the screw head.... :smile:
Click to expand...


Glad I could help!

What I did was to use some rivets to hold the coupler in place. After jamming it in once, I swore never to have to pull that out again. Still glueless, just a little bit more permanent. with a 4" rocket, you could probably drill, tap and countersink something to make it a little prettier without too much trouble.

Nate
 
nute said:
Glad I could help!

What I did was to use some rivets to hold the coupler in place. After jamming it in once, I swore never to have to pull that out again. Still glueless, just a little bit more permanent. with a 4" rocket, you could probably drill, tap and countersink something to make it a little prettier without too much trouble.

Nate
Click to expand...

I don't know if I'm willing to go that far just yet, but if my idea doesn't work, it's good to have a back-up plan. Thanks.
 
Glue a small 3/4in. vent band to NC coupler or glue a slice of coupler inside the NC to act as a stop for shoulder/coupler.

I also found this out the hard way...BUT....you can always use 1/4 wide strips of masking tape in lieu of shear pins. Shear-tape I call it.
 
Was there a vent hole? I'm just curious because I'm building my first larger rocket and I can't decide if I should shear pin the NC or not.
 
It didn't separate due to pressure (yes, there's a vent hole). It separated when the apogee charge went off, and the back pressure from the explosion jerked it loose. Shear pins (or something to do the same job) are a must.
 
Bat-mite said:
Oh, and thanks to everyone else who made suggestions!

I still didn't get that shaming I was looking for.... :wink:
Click to expand...
UNCLEAN! UNCLEAN! How DARE you launch without the required shear pins?

SHAME! SHAME! You must NEVER leap to unlikely conclusions about the root cause of a failure!

UNCLEAN! UNCLEAN!
 

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