Optimal Delay Times

Any thoughts on ball park velocity figures for a safe deployment with out the risk of a zipper. Obviously with Rocksim you get an optimal delay, and a corresponding velocity at deplyment with the value you chose to use. What would be acceptable?

The speed at which you'll see a zipper is going to vary drastically from one material to the next and one deployment to the next.

If you could clarify what your BT material will be, what type/width shock cord you'll use and whether or not the BT will be reinforced, I think you'll get much better input. More info = better responses.

For instance: If I have a 1"-wide shock cord on a 19"-long x 3"-diameter mailing tube with 2 layers of 6-oz. glass (and I do... it's my first scratch-built and I really over-built it), it's far harder to zipper than having a 1/2"-wide shock cord on an 6"-diameter unreinforced tube.

I did actually manage to slightly zipper that short, 3"-diameter bird, too... hard deployment about 5 seconds past apogee on its third I600R flight. I cut it from 20.5 inches to its present 19 inches. Now I may need the extra delay time.

The kit is a THOY Falcon. A 4" diameter body made of glassine, 67inches in length with piston ejection and total weight around 60 oz. As of now the body has not been reinforced. Chute will probably be a SkyAngle 44 and shock cord will either be 1 in. tubular nylon or kevlar cord.

People tend to make mistakes on delay times, even Rocksim will not give you accurate numbers on delay times, just an estimate.
Other factors like windy conditions affects delay times also. Best way of not getting a zipper is to use the zipperless design where you have a bulkhead on the fin can tube.
Other choice is to use an altimeter that will eject the drogue at apogee. It will deploy once it senses drop in altitude(air pressure) and less chance of a zipper and use the motor ejection set long for backup.
Glassing your tubes are better than naked cardboard. Hope this helps.
Mark

Oh c'mon folks - he wanted numbers.

This is probably pretty conservative, but if Rocksim tells me it's under 10 MPH, I don't even think about it a second time - that's just fine. 10-20 MPH: Sure, why not? 20-30 MPH? Here I'd start looking at alternatives. More than 30 MPH? I think not.

Is any of this based on anything other than gut feelings? Nope. The delay times aren't that accurate, the conditions make a big difference (esp. wind, of course), and no matter how much you tailor your design file, it's not the same as the rocket itself. If the "apogee" and "ejection" lines on the graph are near each other, and you've built well, go ahead and launch it.

At the very minimum, you should be building your rocket to survive a deployment undamaged if the ejection charge is within +/- 2 seconds of apogee. Since g is 32 ft/s/s, that 64 ft/s or ~ 41 mph. That's also within the velocity of a main deployment in a dual deployment rocket.

A design goal of +/- 3 seconds or 96 ft/s = 62 mph is even better and not all that hard to accomplish by using tubular nylon shock cord.

Bob

would that 41mph vary with the aerodynamics of the rocket? or only with the terminal velocity?

41 mph assumes no drag, only gravity, so it will only be get slower, no matter what shape the rocket is, it shouldn't be faster...

HOWEVER!

Don't forget horizontal velocity...if your rocket weathercocks its going to have a not-insubstantial velocity that gravity won't slow down. I've seen rockets deploy at apogee and still rip the recovery system out of the rocket because of weathercocking (It was an underpowered TARC flight).