Can Cords Be TOO Long?

[deandome]

Launching my first 4" DD rocket this weekend, so I'm overthinking everything...

Recovery=wise, I built my Thor with about 23' of 1/2" Kevlar cord on the booster (better safe than sorry). Despite being advised that cords can't bee TOO long...that seems a bit much. So would you advise I cut 5-10' off...knowing that might end up being unused scrap, or stick with the 23' I have just because I'm cheap?

Most 4" birds appear to have about 15' or so between booster & payload and maybe a bit more for the main.

I'm also debating drogue vs. no drogue. Wildman says no drogue, but I dunno.

Thanks!

 

[rabidsheeep]

Unless you've got 800 feet of cord, and your main is popping at 800, you cant have too much.

For the two 4" rockets I have that I've flown D/D, I've always gone drogueless.

Good luck!

 

[Handeman]

As for the cord, until you have a use for the part you cut off, I would leave it the way it is.

As for going without a drogue, only if the upper part of the rocket is lighter then the fin can and burned out motor. I have DD rocket and the alt bay, main laundry, and upper tube and nose cone are heavy enough that without a drogue, they drag the fin can down ballistic.

 

[FROB]

Better to have more cord- i'd leave it as is. 23' doesn't sound excessive to me, i'd have used 25-30', but the 1/2" kevlar is overkill for a 4" rocket IMHO. 500-1000 Lb rated cord should do for a 10 lb rocket, more is just extra weight.

 

[MarkM]

Better to have more cord- i'd leave it as is. 23' doesn't sound excessive to me, i'd have used 25-30', but the 1/2" kevlar is overkill for a 4" rocket IMHO. 500-1000 Lb rated cord should do for a 10 lb rocket, more is just extra weight.

Agreed on the 1/2" is overkill. I use 1/4" kevlar on my 4" stuff which are 7-8 feet tall and weigh 8-10lbs dry. The length is fine...in fact, I go longer. I use about 40' for the apogee event, whether drogueless or a small chute, and 25' 1/4" kevlar for the main. Sure there is a thing as "too much" but what you want is by the time the parts separating have reached the end of the harness almost all energy from the ejection charge has dissipated so there is less shock/stress on the parts.

Just an FYI: I'm using 1/2" for my 6", 30lb L3 bird!!

 

[llickteig1]

Handeman is spot on. You don't want the fin can/booster going stable. Drougueless is usually OK unless you're using a big/heavy motor.

23' is a good length - don't cut. I usually put the longest piece on the apogee deployed section as that is the most likely to be going the fastest in case the rocket weathercocks or the altimeter is either early or late. Also, the 'chute cords on the main effective add length to the main recovery system.

Just what I do, YMMV.

--Lance.

 

[blackjack2564]

Long cord on apogee = not shaking out the main.
Shaking out the main at apogee = long walk!

I use 30-40 on my 4in Thor, you never know when apogee event can be late, and if it is, it's usually going downhill at a pretty good clip. The extra length helps scrub off speed and prevent zippers.

If your cord is too short, the nose cone will snap taught, when it reaches the end of cord, the chute will slam against the NC, popping it off, causing the proverbial "shaking out of the chute" and = the long walk.

Shorter cord can be used for main, because by then, your rocket should be falling at a rate of app. 65-100 ft per sec.

 

[jadebox]

If you have telephone or power lines near the launch site, longer recovery cords can make your rocket more likely to snag a utility line. But, generally, longer is better.

-- Roger

 

[sylvie369]

If you have telephone or power lines near the launch site, longer recovery cords can make your rocket more likely to snag a utility line. But, generally, longer is better.

-- Roger

In fact I believe Deandome is planning to fly this thing at Bong, which means no power lines, but plenty of rocket-eating trees. Longer cords mean bigger chance that the heavy part hangs down near the ground, where it can be yanked on repeatedly in anger and frustration until the tree - which has somehow developed an iron grip - yields.

A longer cord also increases the odds that some part of the rocket will land on the ground rather than in the middle of one of the small lakes, reducing the odds that you'll need a rubber raft, or hip waders, and a PFD (yup, believe it or not, it's required).

 

[pyrovette20]

23' sounds right to me. 1/2'' is by no means overkill. I just had a 3'' fiberglass rocket blow/burn through 1800 lb. kevlar. Admitingly it was a blow thru at 200'-300' on an AMW I motor.

 

[FROB]

23' sounds right to me. 1/2'' is by no means overkill. I just had a 3'' fiberglass rocket blow/burn through 1800 lb. kevlar. Admitingly it was a blow thru at 200'-300' on an AMW I motor.

Are you suggesting we should design our rockets to withstand CATO's and burn through etc. ? Makes me think of a Sherman tank... :surprised: