Retro-Fitting Elastic Shock Cords for Safety

[Buckeye]

I read this article in the latest issue of SR and found it quite peculiar. The author claimed 20 years of success with elastic shock cords but was compelled to change them to nylon and kevlar to be "compliant" with Tripoli-MN and NAR. I see nothing in the NAR's safety codes that dictate choice of recovery materials.

So, the author proceeded to outfit his Estes and Aerotech kits with reinforced bulkheads, multiple screw eyes, tubular nylon, and quicklinks. This probably doubled the weight of the rocket. The strangest part was using two shockcords in parallel - one kevlar, one elastic. I never heard of this. The author then complained that the redundant shockcords are prone to tangling. Well, no kidding.

What am I missing here? Seems like way over-building for MPR birds. My 20 year old Aerotech Cheetah still has the original stock elastic strap tied to the cooling mesh baffle!

 

[samb]

Interesting.

Tripoli-MN spells out these requirements in pre-flight inspection documents on their website:

https://www.tripolimn.org/wp-content/uploads/2016/03/Mid-Power-Inspection-Requirements.pdf

https://www.tripolimn.org/wp-content/uploads/2016/03/Low-Power-Inspection-Requirements.pdf


I agree that these stipulations are local additions to the national organizations safety codes. I'll assume that they added them for a reason.

 

[bill_s]

^^-- I wouldn't fly there with those rules. I consider elastic a must with lighter built rockets, also some of mine require several pieces of actual wadding (no room for dog barf).

Engineering says that just because they're doing it with big tough fiberglass rockets doesn't mean everyone should do it.

 

[Oberon]

The wadding thing might be a "littering" issue (I too prefer actual wadding for tight fits, I get bad seals with dog barf alone unless I put in a lot)? But I agree that elastic cords are fine (and sometimes preferable) for most mid power birds. Yes, they need to be sufficiently long and in good condition, but that's true for all harnesses.

 

[Buckeye]

Why does an F motor dictate shock cord material?

I guess I won't be flying with Tripoli - MN any time soon.

 

[manixFan]

^^-- I wouldn't fly there with those rules. I consider elastic a must with lighter built rockets, also some of mine require several pieces of actual wadding (no room for dog barf)...

I use straight Kevlar cord in everything from a Baby Bertha to a 7.5" 60lb. N powered rocket.

I can't think of a single reason why elastic would be a must for any rocket. The cons far outweigh any pros: it burns through easily, is damaged by heat, can cause the infamous 'Estes dent' and has poor strength and longevity.

Kevlar has none of those issues. If zippers are a concern they can be easily mitigated - I've never zippered a rocket with a Kevlar shock cord.

Just a different view I guess.


Tony

 

[samb]

I wonder how many clubs have documented material and/or construction requirements like this for their flight operations ?

 

[Buckeye]

I can't think of a single reason why elastic would be a must for any rocket.

Tony

Nobody is suggesting this. However, the club is demanding just the opposite. They say elastic must never be used, based not on mass nor acceleration, but on the spurious criterion of motor impulse. Huh? If you build stock Estes, LOC, and Aerotech kits, you are SOL in the land of 10,000 lakes.

The club rules also mention that parachutes must not tangle, yet the author has dual shock cords (to be compliant with the first rule) that will inherently tangle. MPR should not be this hard!

 

[manixFan]

Nobody is suggesting this....(that elastic is a must)

Hmmm, Bill S. clearly says he considers it a must for his rockets per post #3. My reply was in response to his post as I quoted, not the criteria listed by the club the OP referenced. I'm not trying to pick on Bill - I'm just voicing my opinion that I feel Kevlar is suitable for rockets of all sizes and that elastic is a poor choice for a shock cord.

I agree that dual shock cords is a crazy idea. I don't get SR so I can't read the article and his reasoning but I bet he is using he elastic as the 'primary' shock cord by making it shorter than the Kevlar cord.

I know a lot of folks think that Kevlar is too inelastic to work as a shock cord. But there are simple ways to add shock absorption to a Kevlar line without compromising its strength, same with avoiding zippers. But nothing can be done to overcome the inherent weaknesses of elastic as a recovery material.


Tony

 

[Buckeye]

Post #3. You are correct. I missed that!

Yes, the author had one cord shorter than the other.

 

[samb]

.... I don't get SR so I can't read the article and his reasoning but I bet he is using he elastic as the 'primary' shock cord by making it shorter than the Kevlar cord.

...
Tony

I'll bring my copy to Gunter this weekend if you want to come up and have look. :grin: Appropriate recovery harness material discussions aside, I'm more interested in a club's decision process in coming up with rules like this.