Kevlar or nylon?

What are the benefits to using either kevlar or nylon? I know kevlar is flame resistance, but is it necessary? My next build will be a scratch 38mm minimum diameter and I wanted to know if I should go one way or the other. Thank you.

Is the flame resistance "necessary"? Well no, not if you pack smart and protect the nylon inside a nomex blanket or sleeve, or make sure your blanketed parachute is between your ejection charge and the shock cord, or some other method. Nylon can certainly be used, and has been on many rockets.

The other major benefit of kevlar though is size/weight. For a given length of shock cord, at a given strength, you can pack it MUCH smaller if you use kevlar rather than nylon. This might very much come into play with a 38mm MD rocket. For my money, with an MD rocket, kevlar is absolutely the way to go. But nylon web can definitely be used if you wish and you have the packing space - plus it will likely be a fair bit cheaper.

s6

1/4" 750lb test Kevlar is the way to go.

The age old question: My answer is kevlar.

Stealth6 explained it very well.

I have two rockets that use 38mm motors. Both are 4 inch diameter, and they share the same 30ft long 9/16" nylon shock cord. I wouldn't want to try packing that into a minimum diameter rocket.

I have a good supply of the #750 Kevlar, and use it all the time. For minimum diameter, I think Kevlar is a better choice, especially because it will take much less room.

For most HP rockets I use both, a Kevlar leader long enough to get out of the tube (for flame resistance) and a longer nylon shock to absorb deployment shock (kevlar does not have much stretch to it).