Shock Loads

[Space Oddity]

I've planned my medium/high power rocket design for a level1 attempt, early next year. I've just bought some 750lb Kevlar cord and am looking at swivels for the parachute. Up till now on low power rockets I've used swivels that are usually used by anglers, with no problems.

I was wondering what load I really need to account for. At level 1, is 750lb a bit over the top? Are angling swivels actually strong enough?

On OR I usually go for a delay that gives me an ejection velocity of between 5 to 20 mph. Is this reasonable? I don't like using parachutes that are too big because I'm not a fan of long countryside walks to recover the rocket.

Yes, I could calculate the loads but would rather rely on the experience of you all than my jaded and revised mathematics.

SO.

 

[markkoelsch]

How heavy is the rocket? How long is harness? Can you guaranty optimal recovery ( hint you cannot)?

I generally do not use swivels except the swivels that come on my B2 and Tac chutes.




Mark Koelsch
Sent from my iPhone using Rocketry Forum

 

[Handeman]

The only swivel I use is on the fin can of my L2 rocket. The drogue charge is small so the load isn't large.

I haven't found it necessary to use swivels on HPR chutes. I just attach them directly to the shock cord. Usually form a loop in the cord, slide it through the shroud lines, around the chute and snug everything up.

As for the 750 lb Kevlar, what size is that? I've been using 1/8" tubular Kevlar on my L1 birds. It's worked great.

 

[codysmith]

The 750 kevlar is really thin. I used this for my L1 bird, but I'mm now discovering TN and wondering how I didn't zipper my rocket

 

[G_T]

For a crude number, I'd assume shock loads could go up to around 30G. Design for that as a minimum.

Kevlar is tough but it can shear if shocked against an edge. It would be a good idea to protect the Kevlar, or any recovery harness material, where it can contact the edge of a tube. This is particularly true if the tube is fiberglass or metal. Even a few wraps of masking tape or gaffer's tape can help a lot. You might be surprised how much wear the protective tape gets.

I don't trust most small swivels. I gave one of the smaller ones a yank test once using a vice, and it failed. If one looks over a swivel and it is not obvious that it can take the load with overkill, then I'd treat it as obviously not being able to take the load.

In any event, most chutes (exception SkyAngle) don't spin and therefore don't really need a swivel. SkyAngle comes with swivels already installed by the manufacturer and they should be fine.

Gerald

 

[Handeman]

For a crude number, I'd assume shock loads could go up to around 30G. Design for that as a minimum.

I don't trust most small swivels. I gave one of the smaller ones a yank test once using a vice, and it failed. If one looks over a swivel and it is not obvious that it can take the load with overkill, then I'd treat it as obviously not being able to take the load.

In any event, most chutes (exception SkyAngle) don't spin and therefore don't really need a swivel. SkyAngle comes with swivels already installed by the manufacturer and they should be fine.

Gerald

Very good advice. I think 30G is a large shock load on any recovery system. If it works as designed, I don't believe you should see more the 5G loads. With DD there should be very small loads at apogee and if you have the right drogue and flight/falling profile when the main deploys, not more the 3G - 5G when the main deploys. It's when things don't go as planned that you get the high G loads. I believe electronic deployment helps a lot in reducing loads on the recovery. Miscalculated delay times for motor ejection seem to cause the biggest shocks on the recovery systems. Drogueless DD flights where the fincan is way above the payload when the main deploys is another high load situation.

As for swivels, I only trust the large ball bearing swivels. Gerald is right about the chutes not spinning, they don't. The fin cans, that's a different story. I think the fin can on my rocket will hit 50 rpm while falling. I use a ball bearing swivel on the shock cord where it attaches to the fin can. That has worked great for me.