Mike, give me a call tomorrow evening whenever works for you, I will be able to talk after 6:30 pm. 970-576-1406

Art

While I haven't done dual chutes on this scale, I've done it a bunch in smaller rockets. I'd think that the basics apply, but there may be differences in application when scaling up that I haven't encountered. In the interest of giving credit where credit is due, most of what I've learned regarding chute clusters was from Model Rocket Design+Constrution (Tim from Apogee's book).

By far, the simplest thing to do would be to put one chute at each end of the recovery harness. As long as the harness length is sufficient, this will get around issues encountered when clustering chutes. What 'sufficient' is, I actually don't know, but at some point, the chutes will start to interfere with each other if they get too close together.

If you're wanting to cluster, you should be aware that multiple chutes do not provide a simple additive effect in terms of increasing drag. In this case, the two 78" chutes have a total of 156" of diameter between them, but will have less drag than a 156" chute will have.

In order to determine the effective chute diameter of a cluster, and ensure that the chutes don't get so close that a collapse occurs, you'll need to calculate the Effective Rigging Line Length, or ERL necessary between the attachment point and the base of the chutes. One formula for calculating this is:

ERL=sqrt(n)*D where n=# of chutes, and D=diameter of chutes

Technically, the D value necessary is the inflated diameter, but the flat diameter is safe to use as well. In this case, the ERL is ~110". So, the effective diameter of this chute cluster is 110". When it comes to line length, the number from the ERL calculation is a minimum. Having a distance between the attachment point and the chute base longer than the ERL is ok. However, adding length to the line won't increase the effective diameter of the cluster. 🙁 Note that the shroud lines of the chute do contribute to the ERL, so you'll probably only need to add a little bit of rigging from them to attachment point, if any at all.

Since 110" is closer to a TAC-9a in diameter than a TAC-9b, it seems that this effective size might be smaller than you may need. Adding a third 78" chute gets you to ~135" of effective diameter, which is pretty close to the TAC-9b's 144" in diameter.

For packing, I've always had good results packing them side by side. You'll probably need to modify how you fold them, so they end up a bit longer and thinner than usual. I also pay more attention to minimizing the amount of shroud line left exposed than with single chutes. I'd test the setup out until you're comfortable that the chutes pack ok, seem to pull out reasonably well manually, and don't seem likely to tangle. I'm not sure what diameter your project is, but dual chutes do take up more room than a single would. If the fit is particularly snug, the usual friction reduction techniques apply.

One folding method mentioned in that book is to S-fold the chutes. Essentially, you stack the unfolded canopies together, and S-fold them until they fit in the rocket. Then, S-fold the shroud lines, and place them on top of the folded canopies, leaving enough length to wrap around the package, and allow for attachment to the recovery harness. Haven't tried it, but seems like it should work.

For attaching, connecting both chutes to a common attachment point is the most straight forward. KISS seems a good rule with these setups, and this is about as simple as it gets. This also minimizes the amount of hardware and harnessing available to cause tangles.

One untried idea here is to tie two water knots close together in the harness, and attach each chute to it's own quick link, and harness loop. That should spread the load out, and provide some redundancy in case something unexpected happens. As long as the knots are kept close together, the amount of harness available for tangling is still at a minimum.

Mike, The 9b is in stock. Kent has set one aside for you. He is sending tubes and Kevlar to me this week. I can send it up if you like. I will have it at the next launch. Sorry it took so long... Tim

No problem. You told Me when I ordered it would be a while.

Please. I was just planning.

I will be at the next launch also (05/02/09)

thanks.

If weather becomes a problem I may have You ship.