# Combining shock cord material

### Help Support The Rocketry Forum:

#### dford

I have about twice the length of my rockets length of kevlar. I'm wondering if I added an Estes rubber shock cord if it'd
A) Be strong enough for RMS on 29mm
B) Help prevent zippering

#### kjohnson

##### mox nix
1) depends on the velocity on ejection.
2) not if the Kevlar goes past the end of the tube.

Make the Kevlar into a leader to the motor mount, and long enough to end about an inch before the top of the tube, and tie the elastic onto the end.

#### cwbullet

##### Obsessed with Rocketry
Staff member
TRF Supporter
Global Mod
I often use a Y harness out of Kevlar each end with tubular nylon connecting them in the middle on my big birds. I am not sure if they decrease zippering but they do decrease cost. A 40-60 foot piece of tubular Kevlar is not cheap.

I certainly think your on the right track,
but why elastic ??
Why not enough light Kevlar so the parts can slow on their own
before they reach the end of the harness ??
You're always much better off doing something the right way in the first place
then cutting a corner and then putting a bandaid on it so it still works...

Teddy

#### T-Rex

##### Ordinary Average Guy
TRF Supporter
I am not sure I would use the Estes rubber shock cord, but you could use a length of wide elastic from the sewing section of your local market. There are a lot of variables that you didn't include. How big / heavy is the rocket? Keep in mind that the rocket will be suspended by the shock cord. Also keep in mind the ability to change out said shock cord. The heat damages the rubber, so at some point you will probably want to change it out. Put the elastic on the outer end of the kevlar to make that doable. Kevlar should last longer than the rocket.

As has been mentioned, you might consider folding that kevlar in half to make a 'Y' harness. (kevlar attaches to opposite sides of the rocket, then attach the elastic to the peak of the 2 legs) I have that on a couple models and it works well. You still have to potential for a zipper, but it is reduced significantly because the force is split between to 2 legs assuming that they extend beyond the end of the tube. If they do not, the zipper potential is only slightly reduce since the force is applied to the single piece of elastic, but the length is changed so the force is less.

I don't always convey my thoughts clearly, I hope this is clearer than mud.... Enjoy!

#### dford

Pictures tell a thousand words. She had to be in the photo with her rocket.
Anyhow this is what I'm working with.
The shock cord has already been mounted. Open Rocket says ideal delay at 8 BUT when I sim at 8 I get early deployment warning. So I sim it at 4, I get green light.
Thus I bought a G64W-4.
This is my second time using kevlar [first attempt didnt go so well (shock cord non related)] and the anchor "Y" application seems smarter.
My thought was using the Estes band was I had one available.
Thoughts towards an elastic band was it may not be strong enough.
The Estes band was designed for this, it must be better than the elastic band.

Ever since I've been using dog barf I have not seen one bit of scorching on plastic parasheets. (Or shock cords)
At this point it seems I could buy elastic cording, trim (and save) the Kevlar for another rocket.

Last edited:

#### Steve Shannon

##### Well-Known Member
TRF Supporter
I think a 4 second delay seems a trifle short. I would either use longer Kevlar only or combination of Kevlar and tubular nylon (my preference due to stretch of nylon). If you use elastic you will replace it often and it has very little strength compared to either Kevlar or TN.

[emoji1010] Steve Shannon [emoji1010]

#### dford

I think a 4 second delay seems a trifle short. I would either use longer Kevlar only or combination of Kevlar and tubular nylon (my preference due to stretch of nylon). If you use elastic you will replace it often and it has very little strength compared to either Kevlar or TN.

[emoji1010] Steve Shannon [emoji1010]
I was concerned about the delay also so I'm hoping OR is correct and the weight of the rocket is low enough it slows down enough.

Me thinks I'll run with stretchy stuff though. The chute will remain on the Kevlar and I'll add an additional third to the length of it with stretchy.

#### rharshberger

##### Well-Known Member
I was concerned about the delay also so I'm hoping OR is correct and the weight of the rocket is low enough it slows down enough.

Me thinks I'll run with stretchy stuff though. The chute will remain on the Kevlar and I'll add an additional third to the length of it with stretchy.
Can you post the OR file, the delay seems a bit short for me too.

#### rharshberger

##### Well-Known Member
Okay, I got the file and according to OR the Optimum delay is 8.2 seconds. I believe the issue you were having is that the motor delay is only 8 seconds and OR says 8.2 seconds therefore it says you have a early deployment, which is marginally the case. Next question is, Did you intend/plan to achieve a polished finished? That was selected is the only reason I ask. The "unspecified" weight is that extra for glue/paint or to equal out the simmed weight with the actual weight? Is this a Estes Panavia kit? The closer you can get to the actual weights of all parts the better the sim results will be.

When I use OR for non electronic deployments, I usually set my parachute to automatically open at Apogee, OR then ignores the motors actual delay, then using the Optimum Delay column in the simulation section OR tells me what it believes based on the information it has is the best or "optimum delay". By doing it this way and using reloadable motors I can get the ideal delay and then can adjust the motors delay to what I want. I don't believe there is any way to adjust a Single Use motor except for the AT DMS and LMS motors.

According to the information given by OR a G64-8W Single use motor would be a near perfect delay, the G76-10G with -2seconds removed on the delay would work as well, but it is a reloadable motor.

Last edited:

#### dford

When you simmed it with the 10 or 7 second delay selected as the motor did you get early deployment did warning? I see the optimum delay at 8 but I went less just for simplicity of the sim and it worked?

I do use reloads and am actually going to our hobby shop right now. I'll pick up a longer delay.
I will look into drilling the delay down more to be more accurate.

Sounds like I need a scale to be more efficient.

I designed it on my own, it isn't a kit.

I papered the entire BT, fillets and fins, so I added the mass (just a guess) to give it a little more reality to it. But again I should get a scale it sounds.

#### The_Lone_Beagle

##### Well-Known Member
I looked at the file, and it was behaving weirdly, at least to me. While it said the optimum delay was at 8 seconds, setting it to that delay resulted in really high deployment speeds, while your 4 second delay resulted in lower (better) deployment speeds.

A couple of the component weights seemed odd. I tried to work on it a bit, but I'm at work. Look at your body tube weight...it is less than an ounce. Yet, the inner tube you are using as a motor mount is over 3 ounces...not sure what is going on there.

HTH

#### dford

The body tube I'm using is a real cheap estes tube. I didn't know it when I bought them. I think I mic'd it out to .022 thick. I should say thin. So I papered it, twice. I'm sure it is quite heavier than the ork file reads. It certainly is stronger in comparison to the ones I have not papered.

I did mean to establish polished finish. I'm heading into town to get that also. I have sanded everything down to 1500 grit prior to painting. The polish should go on nicely.

I should mention the "fillets" aren't necessarily fillets. They are TTW, double glued, then triple glued inside MMT and outside. The last glue layer outside I papered the "fillet".

So, I added the mass there to compensate. How much mass? Well that was a guesstimate.

As far as the plot goes with the file as I read it, having a seven second delay it was descending too fast for chute deployment and the four deployed at a better speed.

My biggest concern within this thread was the Kevlar shock cord being too short, but hopefully lengthening it will help reguardless the delay time issue.

Any other thoughts on the ork file are still appreciated.

I need a scale.

Not worth it!!

#### dford

Closest I could get for the time being.

I picked up some "future" for polish.

Thanks again.

[Don't do the four second delay? No sir. I am not. I'll listen to a level three any day]

#### Sooner Boomer

##### Well-Known Member
You can "bridge" kevlar (or other non-elastic shock cords) with a piece of elastic.

#### The_Lone_Beagle

##### Well-Known Member
This is the scale I use...I use it to weigh individual components (it only goes up to 1kg, so you won't use it to weigh a Level 2 or 3 rocket, but it is ultra precise with small components).

For heavier weight items, I would use other things. If you have a house scale, and want to weigh a rocket, I would just weigh myself first, then weigh me & the rocket, and then get the difference...no need to buy another scale.

Last edited by a moderator:

#### AlfaBrewer

##### Well-Known Member
Even if you weigh every component, you still need to weight the finished product to account for glue, paint, etc. You can adjust the CG as well. Just click on "sustainer", then "edit" and override the weight and CG.

A decent kitchen scale is generally enough for our purposes.

#### dford

Beautiful flight outta site for my eyes.
We (people at the club) decided probably the delay was too long at 7. Most likely optimal around 5-6.

The crummy estes parasheet ripped away from the shrouds rending the bird into free fall.

On the flip side, had I used my rip stop parachute I probably would have caught a zipper.

The rocket believe it or not soared to around 2500+ feet and came back with a few scratches.

My build method proved on point. I need a scale. Estes chutes are only reliable in LPR, or under near perfect MPR conditions.

I bridged the kevlar, added a foot of paracord. Chute still ripped.
Anywho. It was fun reguardless.

#### Attachments

• 61 KB Views: 84

#### bobkrech

##### Well-Known Member
Any motor ejection recovery rocket you launch has to be able to withstand a 100 G deceleration.

Why you ask? Simply due to manufacturing allowances in the motor performance.

If you sim is correct and truly represents the rocket you built, the optimum delay for the motor you chose is 8.2 seconds. Your actual choices for the G76-4,-7 or -10. The allowable error on a delay is +/-20% with a minimum allowed deviation of +/-1.5 second for delays shorter than 7.5 seconds, and a maximum allowable deviation of +/-3 seconds for delay longer than 15 seconds.

The nominal difference for the 4 second delay is 4 - 8.2 = -4.2 seconds and worst case difference is (4 - 8.2) - 1.5= -5.7 seconds!

The nominal difference for the 7 second delay is 7 - 8.2 = -1.2 seconds and worst case difference is (7 - 8.2) - 1.5= -2.7 seconds.

The nominal difference for the 10 second delay is 10 - 8.2 = +1.8 seconds and worst case difference is (10 - 8.2) +2 = +3.8 seconds.

It should be clear that the 7 second delay is the proper delay. For every second you are off of the optimal delay, the rocket is moving 32 fps faster than the zero optimum velocity. That's 22 mph per second off apogee. The 7 second delay could result in an 86 fps (59 mph) deployment and still be in spec. If you used a 4 second delay, the average deployment speed would be 4.2s x 22 mph/s = 94 mph and the worst case would be 5.7 s x 22 mph/s = 125 mph! That's probably faster than you car could go, and about the cruising speed of a Cessna 172! A model rocket parachute will shred at those speeds. The peak opening shock in G's is approximately the speed in mph. If your rocket weighs 12 ounces, that's 125 x 3/4 pound = 94 pounds. Every part of the recovery system must withstand that load. Certainly thread shroud lines and taped shroud/fabric attachment points won't survive. When you are launching a rocket that weighs more than 1/2 pound, you really need a nylon chute and sewn braided shroud lines.

Additionally Kevlar is not a shock absorbing cord because is does not stretch. It will either fail in jerk, or will destroy whatever is attached to it. You need a much longer Kevlar shock cord so the aerodynamic forces on the separated rocket slow everything down before the chute inflates fully. 10' would be the absolute minimum, and 15' would be preferred. A 1' - 2' length of elastic cord tied to stretch upon deployment of the Kevlar will also absorb a lot of shock.

When you launch a near high power motor in a rocket with a low power recovery system, something's gona give.

#### Tonimus

##### Well-Known Member
...The crummy estes parasheet ripped away from the shrouds rending the bird into free fall.

...My build method proved on point. I need a scale. Estes chutes are only reliable in LPR, or under near perfect MPR conditions.
I've intentionally late deployed Estes parasheets. I've even used one as a drogue repeatedly for a 7# high power bird. They do have their limitations, but I suspect that your delay was more than 1-2 seconds longer than would've been optimal.

#### rharshberger

##### Well-Known Member
The Estes PSII parachutes are known for the shroud line connections to fail, even on nominal flights. I can't quote any one thread, but there have been a number of folks here on TRF that mention them failing in flight.

#### dford

When you launch a near high power motor in a rocket with a low power recovery system, something's gona give.
Well said sir. Well said.

4 seconds seemed awfully short and a lot of experience on the forum pointed it out.

I did bridge the kevlar once only to relieve some tension and added a foot or so of paracord.

Obviously wasn't long enough.
Seems like the safest bet is to get a delay drill tool to set delays as close as possible with simulations. Also to put in values for a sim accurately.

I will certainly keep your formulations in mind with reguards to ejection charges, rocket velocity and strength and quality of recovery systems.

I should have used my nylon parachute I had made with 34" shroud lines.
I hadn't tested it yet so I didnt.

I need a scale.

#### bobkrech

##### Well-Known Member
You need to do a sanity check anytime you use a sim simply to make sure you have not accidently clicked an option you don't want to use. When you simmed you rocket with a G76-4 and have an optimum delay time of 8.2 seconds, a warning should have appeared. Either it was turned off or disregarded.

Look at the Top Flight Recovery style thin mil chutes. https://topflightrecoveryllc.homestead.com/thinmil.html The lightweight 24" is all you need. It should also withstand a +/- 5 second error (100 g) deployment delay error with out shredding.

I simply played around with Newton's laws of Motion.

g = ~32 ft/s/s

a = delta V / delta t

v = a x t, v = sqrt (2 x a x d)

d= v,average x t, d= 0.5 x a x t^2

For example if you're rocket deploy 4 seconds before or after apogee, it's vertical velocity is v = a x t = 32 ft/s/s x 4 s = 128 fps. If it takes 0.1 second to slow down to 20 fps, then the deceleration is a = delta v / delta t = (128 - 20) ft/s / 0.1 seconds = 1080 ft/s/s in G units g = a / 32 ft/s/s = ~ 34 G.

In reality, the parachute is ~ 5' long and if it open in one length, the time is 5' / 128 fps = ~ 0.04 s. In this extreme, the peak deceleration would be 128 ft/s / 0.04 s = ~3200 ft/s/s. In Gs = 3200 ft/s/s / 32 ft/s/s = 100 G load. If your rocket weights 0.75 pound, the load is 100 x 0.75 pounds = 75 pounds.

These simple checks can be done to make sure your sims make sense.

Bob

#### Buckeye

##### Well-Known Member
TRF Supporter
The easiest thing to do is look at an entry-level cardboard MPR kit from LOC, Aerotech, or Madcow and note the recovery materials and design. When jumping skills levels like Estes LPR to MPR, I think it is always good to start with kit and learn the standard techniques.

Anyway, some larger flat elastic tied to a length of kevlar (at the hot end) would be just fine. If you have a baffle, all elastic can work, too. I have a 20-year-old Aerotech Cheetah with the original stock elastic tied to the cooling baffle. No problemo.