Eyebolt came loose and rocket separated, what caused this damage?

[billdz]

Launched my modified 3" LOC Tweed B yesterday. Flew normally to 1744'. After ejection, the payload section/nose cone and the lower part of the rocket separated. The lower section came down with the chute and landed softly not far from the pad. The top part came down hard but surprisingly with no damage to my Nexus 5 cell phone that was in the payload section, encased in a swimming pool noodle. On the ground, right next to the payload tube, were the washer and nut that secured the shock cord eyebolt to the bulkhead. Apparently the nut came off and the eyebolt pulled out, which caused the two parts of the rocket to separate.

See below, there were two areas of damage, fortunately not too bad in either case. There's a gash in the bottom of the payload section, presumably that's where it hit the ground hard. More difficult to figure is the damage to the bottom section near the fins. This part landed softly in grass on a chute. What could have caused this? Is it somehow connected to the eyebolt issue?

 

[Nytrunner]

How long is your cord and what material is it?

Also, did you use thread-lock or epoxy on the Eyebolt nut? Unscrewing in flight is not uncommon. At the very least, I recommend a nylon locknut or a springy lock-washer.

 

[mpitfield]

I use red or blue loctite on all of these plus in some cases I lock the nut down with second nut. One thing, if you use red loctite be repaired to have to heat the nut up to a high temp to remove the nut. If that is a concern then use the blue loctite.

 

[blackjack2564]

Once again...how long is your shock cord?

To me it looks like NC [tip punch hole in it?] or payload section smacked the fincan pretty hard. Looks like the tube was pushed in, down from fin, by whatever hit it. A too short or tangled shock cord would cause the front half recoil and go back into fincan.

Fact of fincan landing softly re-enforces this theory. Scuff marks on paint by tear shows something hit it hard, very hard.

 

[jsdemar]

I always use a secondary point on the bulked to attach a short leader of wire or kevlar from the primary eyebolt. If the main bolt starts turning, the short line will tighten and stop it.

 

[billdz]

My bad on the eyebolt, I bought this as a built rocket and did not check carefully enough.

The shock cord is 12' long, too long to whip anything into the lower airframe. It does look like something smacked into the lower airframe just above the fin. Perhaps the eyebolt somehow? The NC separated with the top section, not sure how it could have hit near the fins.

 

[rharshberger]

The gash in the payload bay looks like it might have landed on a fin possibly causing the damage to the fin section. And 12' really isnt all that long

 

[billdz]

"The gash in the payload bay looks like it might have landed on a fin possibly causing the damage to the fin section."

The lower section with the fins landed far away from the payload bay.

 

[cavecentral]

Guessing fin cut hole in payload. Could have happened in the air?

 

[jsdemar]

The damage looks like stress to the airframe at the centering ring from landing hard on a fin.

Never trust an eyebolt to NOT unscrew itself. See my previous message about the 2nd tie point. Or use U-bolts. Or use a kevlar loop permanently epoxied through the altimeter bay.

 

[cavecentral]

Airframe stress on impact was my first thought. The slice in the upper made me think the fin did it, and the above fin damage was from the payload.

 

[billdz]

I'm surprised people are suggesting that the payload tube and the fin hit each other. The two sections landed over 1000' apart. I suppose it is possible they hit in the air but how could that happen? With the eyebolt out, the ejection charge would have sent the payload section flying far away from the lower section. The upper section came down a lot faster than the section with the chute, so they could have collided on the way down, but that seems unlikely.

I'm also surprised the payload/NC part of the rocket did not sustain more damage. This was in freefall for 1744' and I was assuming it would be a total loss.

 

[cavecentral]

It's just that slice. Now that I see it on my computer (not phone screen), I doubt it was the fin that made that slice. The hole is too wide.

 

[billdz]

On close inspection, it sure looks like some fairly sharp object hit the lower airframe just above one of the fins.

 

[qquake2k]

Whether an eyebolt, eyenut, or u-bolt, I always coat them with epoxy so they can't loosen.

 

[mpitfield]

I'm surprised people are suggesting that the payload tube and the fin hit each other. The two sections landed over 1000' apart. I suppose it is possible they hit in the air but how could that happen? With the eyebolt out, the ejection charge would have sent the payload section flying far away from the lower section. The upper section came down a lot faster than the section with the chute, so they could have collided on the way down, but that seems unlikely.

I'm also surprised the payload/NC part of the rocket did not sustain more damage. This was in freefall for 1744' and I was assuming it would be a total loss.

I think the theory would be that the two components were together long enough for them to hit each other before they decided to part ways.

 

[Steve Shannon]

I'm surprised people are suggesting that the payload tube and the fin hit each other. The two sections landed over 1000' apart. I suppose it is possible they hit in the air but how could that happen? With the eyebolt out, the ejection charge would have sent the payload section flying far away from the lower section. The upper section came down a lot faster than the section with the chute, so they could have collided on the way down, but that seems unlikely.

I'm also surprised the payload/NC part of the rocket did not sustain more damage. This was in freefall for 1744' and I was assuming it would be a total loss.

Body parts typically strike each other when the main charge fires and the chute compartment is forced away from the nosecone or when the two parts are at the ends of the shock cord equidistant from the drogue chute. Whenever I’ve seen eyebolts unscrew it’s always after the main deployment while the assembly is spinning and swinging. Either scenario would allow one body tube to strike the other before the eyebolt unscrewed, thereby damaging both parts. Once the eyebolt unscrewed the remaining part connected to the parachute would have greatly reduced descent velocity and could easily drift 1000 feet away from the part that fell freely.

 

[qquake2k]

It's been my experience that you can't always figure out how and/or why things happen. Take this damage for instance. The top of the body tube obviously caused it, but why didn't it damage the tube? The nose cone is solid basswood.

 

[rharshberger]

It's been my experience that you can't always figure out how and/or why things happen. Take this damage for instance. The top of the body tube obviously caused it, but why didn't it damage the tube? The nose cone is solid basswood.

My guess qquake2k is that the CA reinforced airframe and the fins on your Bullpup (iirc the rocket correctly) is what prevented the airframe from being damaged, and as you know that while basswood is a hardwood (so is balsa) it really isnt all that hard. Nice variation on the Estes Smile btw...

 

[K'Tesh]

Like what was already said, use U bolts, lock the nut down with a second nut, and epoxy it in. In the highly unlikely event one side manages to break loose and back off, you've probably got a lot more problems with the rocket than that.

 

[qquake2k]

My guess qquake2k is that the CA reinforced airframe and the fins on your Bullpup (iirc the rocket correctly) is what prevented the airframe from being damaged, and as you know that while basswood is a hardwood (so is balsa) it really isnt all that hard. Nice variation on the Estes Smile btw...

You're probably right, but it still surprised me. And yes, it is the Bullpup.

 

[billdz]

Here's a pic of the same rocket on a prior, normal descent. Guess what you are saying is that the descent started this same way, the nut then worked its way off the eyebolt during descent, then the top part started descending more rapidly and struck the bottom part as it passed it.

There's cell phone video of the bad flight at https://youtu.be/AbYwjzATzXU . There was some weathercocking prior to ejection, perhaps caused by the extra weight of the cell phone in the payload. On the video, shortly after ejection you can hear my son exclaim, "I see something falling!" So whatever happened, it happened quickly.

 

[Cabernut]

Looks to me like there was still quite a bit of velocity at deploy due to the weathercocking, the two halves happen to smack together, leaves the fin can dent while jarring the eyebolt loose or nearly loose. Separation immediately or shortly after.

I once had a flight where the nose cone cracked from smacking the fins during a speedy eject. It's deceiving how much energy gets flinged around up there, apparently.

 

[Bat-mite]

Harness is too short or ejection force is too high.

 

[djs]

No additional advice other than what was already given, but why did you put a cellphone in the av bay?

 

[Homer_S]

No additional advice other than what was already given, but why did you put a cellphone in the av bay?

Simple GPS tracking? Use "locate my Google" or whatever and see where it wound up. Either that or giving ET a way to phone home...

Homer

 

[billdz]

I use the Insane Rockets app. With a phone on the rocket, it provides GPS tracking, altimeter, flight data, and flight graphs. It also allows remote control of the phone's video camera, but I have not tried that yet, would need to put a hole in the av bay.
See:
https://play.google.com/store/apps/details?id=com.insanerocketry.insanerockets

 

[rcktnut]

If you have a sim of that rocket, sim it with the motor and delay you used and launch it at 20, 30, 40 degrees off vertical and you will see the speed at deployment. Looks to be 20-30 degrees in the video. Believe me, it will be at a speed that you would not like for deployment.

 

[billdz]

Doesn't weathercocking shorten the normal delay time? The below simulation includes a 25 degree launch angle, and the sim suggests the ejection should have been late rather than early. In the video, the rocket starts at the 5 second mark and ejection is around 16 seconds, so 11 seconds from liftoff to ejection. This is a long burn motor that burns for 7 seconds, and delay was set at 6 seconds, total of 13 seconds. So the ejection occurred a couple of seconds early, which was apparently enough to cause the problem.

 

[Steve Shannon]

Doesn't weathercocking shorten the normal delay time? The below simulation includes a 25 degree launch angle, and the sim suggests the ejection should have been late rather than early. In the video, the rocket starts at the 5 second mark and ejection is around 16 seconds, so 11 seconds from liftoff to ejection. This is a long burn motor that burns for 7 seconds, and delay was set at 6 seconds, total of 13 seconds. So the ejection occurred a couple of seconds early, which was apparently enough to cause the problem.

Weathercocking shortens the height of apogee, but more importantly you have horizontal velocity which remains fairly high throughout the flight. Any significant velocity when a parachute opens can cause problems.


Steve Shannon