PD mid-body chute-attachement question

[wscarvie]

Hi all,

I'm working on a Parachute Duration design, and plan to attach the root of the parachute's shock chord at the CG of the rocket, so that it floats down with the rocket horizontal...hopefully adding drag.

I'm looking for advice on a means of anchoring the shock chord (probably Kevlar line) at the CG.

Right now, the CG is right in the middle of the body tube. No coupler or anything internal to attach the shock chord to. Is there a means of attaching the chord to the outside of the body tube that won't affect drag too much, and will still hold? Should I put a coupler in there, to more firmly anchor the chord?

Hope that makes sense.

Thanks.

 

[illini]

Mylar tape?

What engine class?

Are you sure the CG will be in the middle of the tube? Did you take into account the weight of the engine casing and the lack of weight of the nose cone and chute?

 

[Micromeister]

Will:
We usually start with an EXTERNAL fin root mounted shock line, of 70 to 130lb kevlar.
I drill a hole that will just pass the kevlar through one of the fins about 1/4" above the rear end. pass the line through the hole and CA that last 1/4" in the fin/body joint line. then epoxy the line up the other side to the forward end of the fin making this the fin fillet. also fillet the other side. run the line along a pre-drawn marker line stuffing the remainder of the line Min 30" into the body with whatever chute or streamer your using. to get the model to hang install a spent motor casing as if the model had just flown. tape the shock line to the body at various points until you find your balance point and mark it. I use 1/2" silver mylar tape, two wraps around the body at this point with the shock line tight along the centerline. that's it. NO the tape doesn't tear, it usually outlasts the model.
I'm not sure how well the tape shows up in this pic but these models balance out pretty close to the fins.

Another way to do this is with the Famous Apogee Lariat loop varient, actually in reverse.
Hope this helps a little.

 

[wscarvie]

Originally posted by illini
Mylar tape?

What engine class?

Are you sure the CG will be in the middle of the tube? Did you take into account the weight of the engine casing and the lack of weight of the nose cone and chute?

It's for A Superroc PD, at next year's NSL. The model is LONG, so the CG is roughly in the middle of the tube, even with the expended casing inside and the nose off.

Thanks!

 

[wscarvie]

Hi Micro,

That makes sense to me. Any suggestions as to where to get the adhesive (I assume) mylar tape?

Thanks much.

 

[jflis]

There are a number of ways that you can do this.

As many have mentioned, you need to find the CG of your model in "recovery mode". Meaning, nose cone and recovery device (including wadding) REMOVED and a spent motor casing installed as you would normally install a motor (eg: if you use tape to hold it in, then tape the casing in the same manner).

In most small PD and SD models, this will give you a CG that is quite close to the fins. I would be quite surprised if it wound up near the middle of the body tube.

I have attached a series of figures of how we do exactly this with our Cougar 660 model. We use #90 Kevlar and it seems to work fine. Point of note, when preparing your model for flight, you want to be sure that the kevlar is held very tightly against the body tube so that it reduces the flutter during boost.

Please let me know if this is or is not clear. HOpe it helps!

jim

 

[JRThro]

Originally posted by jflis
There are a number of ways that you can do this.

As many have mentioned, you need to find the CG of your model in "recovery mode". Meaning, nose cone and recovery device (including wadding) REMOVED and a spent motor casing installed as you would normally install a motor (eg: if you use tape to hold it in, then tape the casing in the same manner).

In most small PD and SD models, this will give you a CG that is quite close to the fins. I would be quite surprised if it wound up near the middle of the body tube.

jim

I have a 32.5" x 0.787" rocket whose CG is at the 22" mark, *with* the nosecone, all recovery items, and new C6-5 motor installed. I really should re-do it in VCP w/o the NC or parachute, and using the expended motor weight, and see what I get.

EDIT:

Okay, here's what I got. The CG barely moved at all when I removed the NC and used the "burned" weight of the motor. It's still 10.3" from the rear, versus 10.5" before launch.

Can anyone tell me how much a 12" Quest parachute weighs, just to confirm things?

 

[wscarvie]

Originally posted by jflis
In most small PD and SD models, this will give you a CG that is quite close to the fins. I would be quite surprised if it wound up near the middle of the body tube.

Hi Jim,

I don't think I'll be giving anything away here if I describe the model. It's not exactly an innovative approach to the event. The event is A Superroc PD. I'm planning to build a model about 50" long, out of BT-5, and fly it on an A10-3T. Even with the nose and wadding deployed and the motor burned out, the CG is (very roughly) a bit past half way down the BT...right between the two tube couplers it'll take to make the BT long enough, using Fliskits 18" tubes (plug, plug).

The length is the trick here. I'm concerned that having a lot of line running down from the mylar tape (for instance) to the fin root will add unneeded drag (and weight, though negligible). There is a tube coupler aft of the CG...I could run the line through the BT as you suggest for the Cougar, and tie off at the coupler.

Do you ever have problems with the Kevlar zippering the BT, Jim?

Thanks.

 

[Micromeister]

Will:
Superrocs generally don't hang horizontally that well. Even using black shaft, the length make a single balance point subject or ever tiny breath of air. if you really intend to use a horizontal suspension scheme. you should be looking at some type of bridle 2 point hanger with the chute attached at a central point. sort of like the method I use to bring my UpScale OT down horizontally.
This model is about 60" long by the way.
Hope this pic will give you some ideas.

Almost forgot:
ASP sells a roll of 1/2" adhesive backed mylar tape. Great stuff, Highly recommended
Silver Adhesive backed "trim" Monocote will work also. Silver is thinner than other color Monocote vinyl films.

 

[jflis]

A couple of things:

1) I didnt' realize this was superroc (duh ) so that explains the CG point in recovery mode

2) Nope, have never had a zipper, but then the cord is quite long and the model very light. When the streamer or parachute get to the end of the cord the model just comes along for the ride

3) I agree with John about using some sort of bridle (though for a superroc I wouldn't hook it up front to back as that is too much length and may fold the body tube). A bridle 12-18 wide, over the CG would work great.

4) drag. There are a number of ways this can be reduced. One that I have seen is to use monofiliment (fishing line) for the portion that is on the outside of the body. much smoother and can be held tight against the body during boost. very little drag

5) *any* kind of exterior connection of the shock cord will allow you to put in a much larger recovery device because you don't have to worry about it getting hung up on an internal shock cord mount. This would more than offset any problem with drag caused by the cord itself (imho )

hope this helps
jim

 

[Micromeister]

Will I have to admit I've done a lot of Super roc flying but have never even given horizontal recovery a second thought. for many of the reasons you mentioned, Added drag of the external shock cord and the possibilty of zippering. Zippering really isn't a major concern as a wrap if that thin silver mylar tape helps.
You have a choice in Superroc, minimum length or Maximum length. for A motor class you can be 75cm to 150cm (29-1/2"" to 59-1/16"). Most Bt-5 cardstock tubes will give you a fit crimping at the maximum length. transitions and smaller tubes get very touchy in a slight breeze. If you've ever seen an Estes Mini-Mean Machine model it is a great qualifing enter in A SRD. Heres one of my A SRD models

A couple other things to keep in mind with super-rocs. SuperRoc Duration events Really are more luck then skill Well except for finding thermals It's more important to get a straight boost and good deployment then how the model hangs. I've had A SRD models thermal away on an 18" mylar chute and No deployed a 24" in an identical model. Totally depends on the air you catch, way the chute is packed and how loopy your boost ends up That's way many competitors call them "stupidrocs" ... cause they are so LOOPY!!! remember to have fun!

 

[powderburner]

Jim, you gave away the fishing line 'secret,' now I've got nothing left up my sleeve!

Will, I was just about to post a suggestion about using two anchor points to make a yoke when I got to the post already put up by Micro. I agree that trying to put something as long as a superroc on a single tether is asking for the opening shock of the parachute to snap the BT in two. You might want to use your existing 'hard points' (the reinforced BT joints with the couplers/doublers) as attachments for your tethers and join them together far enough away so that the knots will all stow (internally) into the front of the rocket's BT.
As far as how to attach the tethers to the rocket, my experience is limited to small (non-superroc) PD and SD rockets. A heavy linen or monofilament line was plenty adequate and I just epoxied the line to a sanded spot on the exterior (It doesn't hardly take a single drop of epoxy or glue, but do it neatly and smoothly). I don't remember ever having a separation failure.
I guess what I am saying is: be careful of over-building, especially on a competition rocket. Do some testing work and see if a lighter and simpler (and less draggy?) approach will work for you, **before** you build 'flying iron bridgeworks'
My competition rockets usually were built fresh anyway for each contest, and only were expected to last a few flights (if they didn't float away on the *first* flight).