'Chute failure: theories welcome..

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Tim51

Tim51

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After 3 perfect flights (CTI H125, CTI G115, and CTI I195) my 4" Loc Fantom came a cropper yesterday when the chute failed to fully deploy. It was flying on a CTI I236 with motor ejection, and, as with all the previous flights, the chute was a 44" Loc Angel, which has performed well on the previous occasions. The separation occurred at apogee but the chute was not pulled out, and the
two halves, still tethered together bounced. Damage was relatively slight (the field was bare dry soil, recently ploughed) resulting in a curved crump on the lip of the airframe. There was no other damage to either fins or airframe, which was something to be grateful for, given the altitude it was falling from.

WP_20151004_20_01_57_Pro.jpg
So I'm left wondering what exactly happened... Did the harder acceleration of the I236 wedge the chute into the airframe with such force that the stock CTI ejection charge couldn't push it out on this occasion...? I'd welcome any thoughts/theories.

John

--
UKRA #1895 L1
 
Most likely guess, without being there, is that you packed the chute too tightly into the airframe on this occasion.

I'd like to know about your harness, too. What are you using? (Kevlar, nylon, elastic, something else?) After a few deployments, Kevlar will start to get sticky and can bind up in the airframe. There are ways around this, of course.

Also, are you using a Nomex or Kevlar cute protector, or dog barf, or Estes-type wadding?
 
Bat-mite said:
Most likely guess, without being there, is that you packed the chute too tightly into the airframe on this occasion.

I'd like to know about your harness, too. What are you using? (Kevlar, nylon, elastic, something else?) After a few deployments, Kevlar will start to get sticky and can bind up in the airframe. There are ways around this, of course.

Also, are you using a Nomex or Kevlar cute protector, or dog barf, or Estes-type wadding?
Click to expand...

Thanks for the response. I don't remember packing the chute more tightly than before (but I fully accept that reads like a classic 'I don't think I drank that much..' denial :D ). The harness I'm using is pictured below: kevlar shock cord, stitched with kevlar thread, 12" Nomex blanket, and a Nomex sleeve (stitched with kevlar thread) that button holes onto the quick link to keep it in place. The sleeve length is sufficient to extend beyond the forward end of the airframe when fully extended.

WP_20151005_17_13_38_Pro.jpg
 
What did the chute look like on post mortem? Did it look like it was wedged on anything? Expanded in the tube once packed? How cleanly/clearly did the nose cone separate? How much shock cord was left in the tube?
 
Banzai88 said:
What did the chute look like on post mortem? Did it look like it was wedged on anything? Expanded in the tube once packed? How cleanly/clearly did the nose cone separate? How much shock cord was left in the tube?
Click to expand...

That's an interesting question. Almost all of the chute was still in the airframe, but shoved forward, so that a small portion of the still folded chute was protruding from the mouth of the airframe. When I removed and inspected it, it looked fine - no signs of melting or burning. It wasn't wedged on anything solid (there are no projections inside the frame). The NC was fully separated - we could see it falling as a separate object, connected only by shock cord, so that made it fall slower and it didn't core sample. Approximately a quarter of the shock length cord was out - that portion that leads from the chute quicklink to the NC anchor point.
 
Did you have knots in the shock cord, or places where it was stuck together?

My only guess was that somehow, despite you doing everything that you normally do for a successful deployment, somehow the chute got jammed. Could the shock cord have gotten between the chute and the airframe wall? Do you Z-fold your shock cord?
 
Tim51 said:
That's an interesting question. Almost all of the chute was still in the airframe, but shoved forward, so that a small portion of the still folded chute was protruding from the mouth of the airframe. When I removed and inspected it, it looked fine - no signs of melting or burning. It wasn't wedged on anything solid (there are no projections inside the frame). The NC was fully separated - we could see it falling as a separate object, connected only by shock cord, so that made it fall slower and it didn't core sample. Approximately a quarter of the shock length cord was out - that portion that leads from the chute quicklink to the NC anchor point.
Click to expand...
Did previous ejections seem energetic, assuming they were at altitudes where you could tell? Was the unejected 'chute easy to extract post-flight? How long is the shock cord between the nose cone and the 'chute's attachment point.

The first question relates to the possibility of a weak charge, the second to jamming, the third to the ability of the nose cone to pull the chute out of the airframe if ejection gasses leaked past the 'chute to too great of an extent to actually push the 'chute out of the airframe.
 
Bat-mite said:
Did you have knots in the shock cord, or places where it was stuck together?

My only guess was that somehow, despite you doing everything that you normally do for a successful deployment, somehow the chute got jammed. Could the shock cord have gotten between the chute and the airframe wall? Do you Z-fold your shock cord?
Click to expand...

There were no knots and the cord wasn't stuck together. I suppose the shock cord could have inadvertently been squeezed between chute and frame, but my normal method (I hope) would tend to avoid this: I fold the chute around its shroud lines, and the chute is pushed into the airframe, pocketed in its nomex blanket, pushing the lower portion of sleeved shock cord back to the base of the chute bay. I try to make sure the remaining forward section of shock cord sits on top of the parachute in a loose coil with no knots. (I'd be interested to hear about Z folding - I'm not sure whether that's what I'm doing already or not..)
 
Z-folding is folding the cord accordion style, and then sticking a rubberband around the folded section. It prevents knotting and tangling.

Z-fold.JPG
 
Winston said:
Did previous ejections seem energetic, assuming they were at altitudes where you could tell? Was the unejected 'chute easy to extract post-flight? How long is the shock cord between the nose cone and the 'chute's attachment point.

The first question relates to the possibility of a weak charge, the second to jamming, the third to the ability of the nose cone to pull the chute out of the airframe if ejection gasses leaked past the 'chute to too great of an extent to actually push the 'chute out of the airframe.
Click to expand...

Thanks for your consideration. Of the four flights, the lowest apogee was on a G115, and that was suitably energetic. The chute pulled out ok, much as it went in, in the sense that it was a snug fit but not stuck / glued / melted to the side of the airframe.

It's 25" / 65cm between the NC loop and the chute loop.
 
Are there any marks on the nose cone that would match up to the damage to the airframe? I'm thinking of the possibility that the airframe could have collided with the nose before the chute was deployed.
 
Tim51 said:
Thanks for your consideration. Of the four flights, the lowest apogee was on a G115, and that was suitably energetic. The chute pulled out ok, much as it went in, in the sense that it was a snug fit but not stuck / glued / melted to the side of the airframe.

It's 25" / 65cm between the NC loop and the chute loop.
Click to expand...
Taking that information into consideration, if it wasn't a weak charge, the higher g-force compression of that snug fit may have been at fault along with a chute pack diameter increase caused by the opposing ejection pressure and 'chute inertial forces, something that would be a much more common problem with chute packs in smaller diameter HPR rockets that are both a snug fit and fill one or more IDs of airframe length.

The chute pack's compression (length decrease and resulting diameter increase) at ejection causes a friction drag effect against the airframe's inner wall and delays ejection long enough to allow gas leakage without full ejection of the pack while during the same instants in time preventing the nose cone from pulling the pack out. After that very short effect has ended, the chute can be extracted with no more friction that that experienced when it was inserted.
 
First I'm assuming the kit is the LOC Fantom 438, and not the EXL. It uses motor deploy and not DD and was built pretty stock.

From what you have described, the protected shock cord was near the motor, than the wrapped chute, then all of the shock cord, then the nose cone.

First, the ejection charge will not push the chute out of the tube unless the chute is large enough and firm enough to keep its shape and to act as a piston. It has to be very snug in the tube. If it isn't, the gases will go right past it.

Since the gases don't push the chute out, it is up to the nose cone to pull the chute out. Several things can go against you on this. If the nose cone is a very light plastic LOC nose cone without any added weight, it has little momentum/inertia to pull anything out of the BT unless it is traveling at a high speed. If the fit of the nose cone on the BT is relatively loose, the nose cone will move off the BT with little ejection charge force and very little energy will be transferred to the nose cone. With the large 4" BT to fill, the pressure of the ejection gases will not be very large. Also, since much of the shock cord was above the chute, the nose cone had to pull all of that shock cord out, expending its momentum, before it pulls the chute out.

The following is my speculation of what happened:
  • You had a relatively loose fit of a light nose cone so at ejection, it left the BT without a lot of momentum.
  • The ejection happened early in a nose up position so the charge had to push the nose cone upwards against gravity and the air drag caused by the speed the rocket. The earlier the charge, the higher the speed, the higher the air drag.
  • The relative large volume of the 4" BT reduced the overall pressure created by the ejection charge and since the nose cone left early, didn't impart much energy to the nose cone.
  • The nose cone did not have enough energy to pull the shock cord and the chute out of the BT against the forces of gravity and air drag so the chute was left in the BT.
If that is even close to the circumstances, my suggestion is to z-fold the shock cord and wrap it and the parachute in the burrito. If not that, then put all the shock cord into the BT before the chute so the nose cone will pull the chute out along with the shock cord.
Consider putting several X of masking tape on the nose cone shoulder to give it more friction in the BT. The higher friction will keep it in place while the ejection charge pressure builds and eject the nose cone with more energy.
Consider adding a little BP to the ejection charge. That really shouldn't be needed with HPR motors, but would be a definite consideration with MPR sized motors.

If your flight profile wasn't anything like I assumed, please disregard all of the above!
 
Winston said:
Taking that information into consideration, if it wasn't a weak charge, the higher g-force compression of that snug fit may have been at fault along with a chute pack diameter increase caused by the opposing ejection pressure and 'chute inertial forces, something that would be a much more common problem with chute packs in smaller diameter HPR rockets that are both a snug fit and fill one or more IDs of airframe length.

The chute pack's compression (length decrease and resulting diameter increase) at ejection causes a friction drag effect against the airframe's inner wall and delays ejection long enough to allow gas leakage without full ejection of the pack while during the same instants in time preventing the nose cone from pulling the pack out. After that very short effect has ended, the chute can be extracted with no more friction that that experienced when it was inserted.
Click to expand...

Thanks for the considered response. That all sounds like a very accurate diagnosis - everything you've said above fits with the circumstances / what I saw.
 
Handeman said:
First I'm assuming the kit is the LOC Fantom 438, and not the EXL. It uses motor deploy and not DD and was built pretty stock.

From what you have described, the protected shock cord was near the motor, than the wrapped chute, then all of the shock cord, then the nose cone.

First, the ejection charge will not push the chute out of the tube unless the chute is large enough and firm enough to keep its shape and to act as a piston. It has to be very snug in the tube. If it isn't, the gases will go right past it.

Since the gases don't push the chute out, it is up to the nose cone to pull the chute out. Several things can go against you on this. If the nose cone is a very light plastic LOC nose cone without any added weight, it has little momentum/inertia to pull anything out of the BT unless it is traveling at a high speed. If the fit of the nose cone on the BT is relatively loose, the nose cone will move off the BT with little ejection charge force and very little energy will be transferred to the nose cone. With the large 4" BT to fill, the pressure of the ejection gases will not be very large. Also, since much of the shock cord was above the chute, the nose cone had to pull all of that shock cord out, expending its momentum, before it pulls the chute out.

The following is my speculation of what happened:
  • You had a relatively loose fit of a light nose cone so at ejection, it left the BT without a lot of momentum.
  • The ejection happened early in a nose up position so the charge had to push the nose cone upwards against gravity and the air drag caused by the speed the rocket. The earlier the charge, the higher the speed, the higher the air drag.
  • The relative large volume of the 4" BT reduced the overall pressure created by the ejection charge and since the nose cone left early, didn't impart much energy to the nose cone.
  • The nose cone did not have enough energy to pull the shock cord and the chute out of the BT against the forces of gravity and air drag so the chute was left in the BT.
If that is even close to the circumstances, my suggestion is to z-fold the shock cord and wrap it and the parachute in the burrito. If not that, then put all the shock cord into the BT before the chute so the nose cone will pull the chute out along with the shock cord.
Consider putting several X of masking tape on the nose cone shoulder to give it more friction in the BT. The higher friction will keep it in place while the ejection charge pressure builds and eject the nose cone with more energy.
Consider adding a little BP to the ejection charge. That really shouldn't be needed with HPR motors, but would be a definite consideration with MPR sized motors.

If your flight profile wasn't anything like I assumed, please disregard all of the above!
Click to expand...

Thanks for the detailed response and suggestions. It is the Fantom 438, not the EXL. My earlier build thread is here: <<https://www.rocketryforum.com/showthread.php?127275-LOC-Fantom-modifications>>. The I236 flight was probably the highest and fastest I'll fly it with just a single chute and motor ejection, as I'm in the process of building and fitting an EXL extension and an electronics bay before flying again. My flight profile was different than what you've suggested, but as you kindly took the time, I'd like to go through your points and clarify rather than rudely ignore them:


  • You had a relatively loose fit of a light nose cone so at ejection, it left the BT without a lot of momentum. A: It was actually a reasonably tight fit (as always, I did the 'hold the nose / upside down shake' test) and it had a couple of bands of masking tape as with earlier flights. During the build I'd added 60g weight in the form of epoxied lead shot to the the nose cone (see build thread) as a precaution to maintain the stability margin because I'd made custom fins. In addition the NC has an eyebolt, nuts and washers as an anchor. The NC is therefore reasonably heavy.



  • The ejection happened early in a nose up position so the charge had to push the nose cone upwards against gravity and the air drag caused by the speed the rocket. The earlier the charge, the higher the speed, the higher the air drag. A: Some error here is certainly possible. The I236 is a 17A. Open Rocket suggested a delay of 10.6s so I set the DAT to -7 and adjusted down to 10s (as you'll appreciate my other option was -5 which would have given 1.4s over). There was a very slight tear on the rim of the airframe which could have been the start of a zipper, so mis-timed deployment is certainly possible I suppose, but the main damage seemed to be impact damage from the bounce.
  • The relative large volume of the 4" BT reduced the overall pressure created by the ejection charge and since the nose cone left early, didn't impart much energy to the nose cone. A: Bear in mind I had a larger (44" Loc Angel) chute than the 36" that came with the kit. If anything, it was a tighter fit.
 
astrowolf67 said:
Are there any marks on the nose cone that would match up to the damage to the airframe? I'm thinking of the possibility that the airframe could have collided with the nose before the chute was deployed.
Click to expand...

Thanks for the response. The NC was remarkably un damaged. Apart from dirt from the ground, there were 3 tiny paint chips, but no other scuffs or marks. I got more chips on the nosecone from the G115 flight than this, which surprised me. The airframe has just areas of damage: what might have been the beginning of a small zipper (just a very small break at the rim of the airframe which hasn't run down the side), and a larger parabola shaped kink, extending back from the lip of the airframe, consistent with the bounce when it hit the ground.
 

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