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-   -   PNC: how tight is too tight? (http://www.oldrocketforum.com/showthread.php?t=16537)

jeffyjeep 03-16-2017 09:44 PM

PNC: how tight is too tight?
 
4 Attachment(s)
I'm at the finishing stages on a DynaStar Lexxjet.

First of all, I want to thank Shrox for his beautiful and elegant design!

Now for the question: the PNC initially fit very loosely in the main BT so I had to wrap several layers of tape around the PNC's shoulder for a better fit. The fit is good when the model is in launch position, but when the model is in display--or horizontal--position the heavily weighted PNC sags in the BT and creates quite a gap at the top.

Obviously, I can fix this by wrapping more tape around the shoulder of the PNC, but at what point is the fit too tight for a reliable "laundry" deployment at ejection?

Thanks!

neil_w 03-17-2017 09:42 AM

It seems like the more appropriate solution is to do something temporary to improve the display. Can you not just wrap something non-adhesive around the shoulder to tighten it up a bit? Then remove it for launch.

Personally I'd be rather nervous about even asking the question "how tight is too tight" when the motivation is just to improve how it looks on display.

jdbectec 03-17-2017 10:00 AM

I seem to remember reading that you should be able to pick the model up by the nose cone but, be able to shake it loose. Back in the old days we used to blow into the engine mount and see if it would pop the nose cone off.

ghrocketman 03-17-2017 02:26 PM

+1 to what Jeffrey just said above.
Be able to shake it off by holding the cone.
Blowing in the engine mount is a valid test too.
If EITHER of those fails, it is TOO tight.

Jerry Irvine 03-17-2017 05:52 PM

The rule of thumb for low subsonic (<500 mph) is 5 pounds force. No more than 10.

The nose cone has lower drag than the rear body with the fins, despite the frontal area drag. Lovelace effect.

Supersonic rockets use break bolts.

cite:

http://v-serv.com/usr/dd.htm


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