Nose Cone Shoulders -- How big?

[BigMacDaddy]

Why are most nose cone shoulders so large / deep? How big should a shoulder be? What does it really accomplish besides keeping the nose cone in the body tube during launch and making sure it smoothly / firmly ejects. Does it need to stick a bit to build up pressure or anything?

I have been 3D printing nose cones and usually make my shoulders like 10mm or so. I have never had an issue with the nose cone coming off during a launch. I am doing all low powered stuff and have not launched hundreds of scratch builds so just wondering if I am missing something or setting myself up for future failures.

Thanks!

 

[prfesser]

Depends in part on the diameter of the airframe. A 10 mm long shoulder in a 38 mm or larger tube could cock a bit and jam in the tube. My rule of thumb when turning a nose cone is a shoulder at least 3/4 as long as the tube diameter.

For higher-altitude flights, the internal (sea level) pressure could pop out the nose cone, depending on friction and external pressure.

Best -- Terry

 

[BigMacDaddy]

Depends in part on the diameter of the airframe. A 10 mm long shoulder in a 38 mm or larger tube could cock a bit and jam in the tube. My rule of thumb when turning a nose cone is a shoulder at least 3/4 as long as the tube diameter.

For higher-altitude flights, the internal (sea level) pressure could pop out the nose cone, depending on friction and external pressure.

Best -- Terry

Thanks very much -- that is interesting about the higher altitude flights and I can see how having a bit of length on shoulder would allow for expansion of internal gas volume. I had not thought of that since I do not have fields large enough to launch rockets very high. Good to know if I design a rocket that might go higher...

 

[jqavins]

A common rule of thumb is that you should be able to hold most LP rockets by their nose cones and not have them fall off, but they should drop off if you shake them. Friction will be greater with a longer shoulder, all else being equal, so there's a range of lengths that will let it pass the hang-and-shake test. That's not an exact science, but the shoulder lengths on typical off the shelf nose cones seem to work well, and it may well be that this is part of why they are the length they are.

If you make the shoulder a little bit wider, you both increase the friction for equal length and decrease the ability to become cocked. So a snugger fit should make a shorter shoulder viable, but there's a knife edge threshold where suddenly it's gone from snug to tight. For mass production you want to stay away from knife edge thresholds, so you'd be better off using added length in fractions of an inch than added diameter in fractions of a millimeter.

After a while, the shoulder lengths on the market have just come to look "right". Which is because they're what we're used to seeing. Just like rocket heights in the ballpark of 10 to 15 diameters look right, and there's a reason we've seen that so often that we're used to it; it works very well for flight dynamics. I can only assume that nose cones have the shoulder lengths we're used to because that, too, works very well.

Keep questioning everything. I'm not saying "That's what works, so use it!" Indeed, the opposite. I bet the reason that what we're used to is what it is is, at least in part, that it works for mass production. For turning and printing your own, a different choice (e.g. a fraction of an inch shorter and a fraction of a millimeter wider) might make great sense.

 

[lakeroadster]

Why are most nose cone shoulders so large / deep? How big should a shoulder be? What does it really accomplish besides keeping the nose cone in the body tube during launch and making sure it smoothly / firmly ejects. Does it need to stick a bit to build up pressure or anything?

I have been 3D printing nose cones and usually make my shoulders like 10mm or so. I have never had an issue with the nose cone coming off during a launch. I am doing all low powered stuff and have not launched hundreds of scratch builds so just wondering if I am missing something or setting myself up for future failures.

Thanks!

View attachment 480663

FWIW: I typically design the length of the shoulder so it is at least as long as the body tube inside diameter. But the fit is important, so the nose cone doesn't cock over sideways during flight. I typically make the nose cone shoulder a bit loose in the BT... and then wrap the shoulder with blue painters tape to get a tight slip fit.