losing shear pins at altitude

[Zebedee]

So using redundant systems reduces the risk for the impact it can increase the risk of a premature ejection.

Rocketeers tell each other that premature ejection can happen to anyone - not sure I believe it.

 

[llickteig1]

You are crazy.
Proper design and recovery technique trumps super long cordage any day.

The early deployment may have been from improper venting of recovery bay.
under sized sheer pins can be another.
I use just one 2/56 on a 4" fg rocket flying on L motors.

JD

Mr. JD is the only person who has mentioned the the thing that many flyers aren't fully aware of ...

High altitude and particularly high thrust/weight flights will cause the internal pressure trapped inside the main parachute bay to remain at ground level which can be much higher than the air pressure of a rocket at high altitude (especially one traveling very fast so it can't equalize fast enough via leaky joints).

Tolerances between the parts of modern fiberglass airframes are so close that the internal pressure can't bleed out unless you provide a pressure equalization port for this purpose. There can be several pounds of force pushing on the upper section (usually the nosecone) which can cause the shock of the apogee deployment to shear the shear pins and push the nosecone off.

If you're having trouble "shaking out the main", make sure you have a pressure equalization port in all airframe sections. I really does help.

--Lance.

 

[UhClem]

Mr. JD is the only person who has mentioned the the thing that many flyers aren't fully aware of ...

I cobbled up a web page on the subject once upon a time.

 

[rharshberger]

Mr. JD is the only person who has mentioned the the thing that many flyers aren't fully aware of ...

High altitude and particularly high thrust/weight flights will cause the internal pressure trapped inside the main parachute bay to remain at ground level which can be much higher than the air pressure of a rocket at high altitude (especially one traveling very fast so it can't equalize fast enough via leaky joints).

Tolerances between the parts of modern fiberglass airframes are so close that the internal pressure can't bleed out unless you provide a pressure equalization port for this purpose. There can be several pounds of force pushing on the upper section (usually the nosecone) which can cause the shock of the apogee deployment to shear the shear pins and push the nosecone off.

If you're having trouble "shaking out the main", make sure you have a pressure equalization port in all airframe sections. I really does help.

--Lance.

+1 pressure bleed ports.

 

[MarkH]

Leaky av-bays help too. This might sound crazy and goes against common practice of sealing the av bay, but I usually put a small hole in av bay lids to help equalize pressure to top and bottom sections, in the event the vent ports in those sections get blocked or are inadequate. In doing so, with a small hole in the lids ~1/16", the pressure spike seen by the altimeters from charges isn't all that much ; probably on the order of a few hundred feet.

 

[rharshberger]

Leaky av-bays help too. This might sound crazy and goes against common practice of sealing the av bay, but I usually put a small hole in av bay lids to help equalize pressure to top and bottom sections, in the event the vent ports in those sections get blocked or are inadequate. In doing so, with a small hole in the lids ~1/16", the pressure spike seen by the altimeters from charges isn't all that much ; probably on the order of a few hundred feet.

What about BP residue entering through the holes in the AV bay?

 

[mpitfield]

I drill pressure equalization ports in all my rocket sections; I even use them in my mid power rockets that just use motor eject and don't exceed more than a couple hundred mph. The rocket I just had a main @ apogee failure on, has 3 pressure relief holes 120 degrees apart, in both the drogue and main section. So obviously I am on the same page as those who believe in using them as a best practice, after all I learned all of this from you forum members.

However as in my case I believe that any pressure differential likely had no appreciable effect or contributed to the root cause.

 

[mpitfield]

What about BP residue entering through the holes in the AV bay?

I was thinking of that as well as any pressure spike that the altimeter may sense and what if any effect it would have on the altimeter performance.

 

[Zebedee]

What about BP residue entering through the holes in the AV bay?

One should have separate vent holes for the AV bay and the Chute bay. I use some putty (not epoxy putty) to seal around where the ejection charge wires go through the bulkhead between them to prevent pressure spike and BP residue reaching the altimeters. Based on the data from a stratologger it's mostly successful (and I can see the difference between using the putty and not).

 

[llickteig1]

Leaky av-bays help too. This might sound crazy and goes against common practice of sealing the av bay, but I usually put a small hole in av bay lids to help equalize pressure to top and bottom sections, in the event the vent ports in those sections get blocked or are inadequate. In doing so, with a small hole in the lids ~1/16", the pressure spike seen by the altimeters from charges isn't all that much ; probably on the order of a few hundred feet.

I don't think I would recommend this practice. The pressure spike isn't the worst problem, but the crud from the ejection charges is. Over the long haul, the residue can have a really bad effect on the pressure sensor plus the residue is highly corrosive.

--Lance.

 

[wighty44]

So far my builds (all FG) use 2-56 plastic screws (3 main section, 2 in the drogue section). After marking their locations and drilling 1/16" pilot holes, I remove the inner tube/NC. I then enlarge the exterior holes with a #50 drill and tap those holes. The inner mating holes are then slightly enlarged with a #46 drill.

So far each DD launch has realized the proper section separation and shear of the nylon pins. Perhaps I've been fortunate but I've not had a problem losing shear pins and I don't tape over them.

 

[kramer714]

I use https://www.mcmaster.com/#90136a572/=z84g63 for fiberglass and carbon fiber rockets, works great shears well. Plus they dont come out unless you pry them out or shear the heads off with a razor.

Mike K

 

[Bat-mite]

I always drill pressure relief holes in my airframes, but I had forgotten why until just now....

 

[ChrisAttebery]

FWIW: I'm using these rivets from McMaster:
https://www.mcmaster.com/#91010a014/=z8hugj