1/4 or 5/16?

Quick question for the good folks here.
Starting anew build and trying to decide on whether to use 1/4 or 5/16 for the recovery hardware. This will be used in a ~ 25-30 lb. fiberglass rocket. All hardware will be stainless and attachment points will be ubolts.
Right now, I was planning on using (2x) 1/4" ubolts for the booster attachment, 1/4" ubolt for nosecone and 5/16 ubolts and threaded rods for av bay. Weight is not so much an issue based on where it would be distributed.
Build thread will be started in a few days.

richP said:

Quick question for the good folks here.
Starting anew build and trying to decide on whether to use 1/4 or 5/16 for the recovery hardware. This will be used in a ~ 25-30 lb. fiberglass rocket. All hardware will be stainless and attachment points will be ubolts.
Right now, I was planning on using (2x) 1/4" ubolts for the booster attachment, 1/4" ubolt for nosecone and 5/16 ubolts and threaded rods for av bay. Weight is not so much an issue based on where it would be distributed.
Build thread will be started in a few days.

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I have a 39 # (on pad) bird that only uses 1/4" hardware without any problems.

https://dodgeram.org/tech/specs/bolts/SAE_bolt_strength.html

Tensile strength of 1/4x20 (coarse) grade 2 bolt is given above as 2350 lb; 5/16x18 is 3900 lb. Shear strength is much lower, 200 lb and 340 lb respectively, but since the bolts will be under tension, the tensile strength should be more important.

Best -- Terry

M6

If your charges are sized correctly, the only stress on your U-bolts will be the weight of the rocket itself as it descends. Not a fan of the "blow it out or blow it up" method.

5/16....because is 0.0625 betterer!

Bat-mite said:

If your charges are sized correctly, the only stress on your U-bolts will be the weight of the rocket itself as it descends. Not a fan of the "blow it out or blow it up" method.

Click to expand...

I like to use redundant charges; primary charge is "sized correctly", backup charge is "blow it out or blow it up".

For a rocket of that weight..
On a U bolt 1/4"..
On an eye bolt 5/16"..

Teddy

Bat-mite said:

If your charges are sized correctly, the only stress on your U-bolts will be the weight of the rocket itself as it descends. Not a fan of the "blow it out or blow it up" method.

Click to expand...

I'm a little out of my depth on recovery and deployment of this sort, and asking because I need to learn. I may be missing something, and I'd like to know where I go wrong.

From a more basic engineering standpoint that doesn't seem quite right. The charge, sized correctly, needs to take into account the maximum separation force needed and the minimum quantity of BP needed to generate it, after a complete tolerance stack-up. Of course, the tolerance stack-up analysis isn't really done in great detail, but you will size the charge for worst credible case, and then add a little design margin on top of that. (Blow it out or blow it up is a lot of design margin, which I am not suggesting.) So there will be a jolt except in the most extreme, unlikely but credible case.

Also, from an even more basic physics standpoint, it seems inevitable that there will be a momentary load that is more than the weight of the rocket. Before deployment the rocket is descending with one speed, and after it quickly slows to a different speed. That deceleration requires an upward force on the rocket greater than its weight, and since the deceleration is several times one gee the force is several times the rocket's weight.

These two things combined should mean that the hardware sees a momentary load many times the rocket's weight. Have I gone wrong somewhere? And what I don't know is mow many times the weight should be expected.

Based on the figures upthread, 1/4" hardware would provide nearly 80 the rocket's weight (bolt strength ÷ max rocket weight = 2350 lbf ÷ 30 lbf) and that's on a single straight bolt where a U bolt should do better. I would guess that would be adequate, but I'm really guessing out my backside.

jqavins - for the most part you are correct. If everything is working correctly, I have no doubt that 1/4 hardware would be plenty. Now, there is always the possibility of a major failure, in which you try as best as you can (within reason) to keep the rocket "pieces" together.
Ultimately, I went with (2)1/4" ubolts in the fincan, a 1/4" ubolt for the nosecone and 5/16" ubolts and threaded rod for the AV bay.
For me, it was an acceptable compromise between weight and strength. Others may feel differently, opting for bigger or smaller hardware (and that's OK).

I like to see design for at least 20G with a margin thrown in. If it can't do at least that, it probably isn't safe to fly for high power. You can probably compute something out that is much better though. Sim a flight that went off at an angle. Assume deployment is a couple seconds early or late. Find the speed. Take the main chute's drag coefficient, and determine the force generated at that speed using the air pressure at that altitude. You want to be able to handle that force with a fair bit of margin. You might shred, but, the recovery system still has to work.

For high altitude flights, stuff over 100Kft, you'll find the speed can easily exceed Mach 1 and can be quite a bit higher. Air pressure is of course lower. But again, you can compute a force generated by a drogue for non-nominal flight profile. Even for a nominal flight profile it becomes obvious why typical low altitude design methods should not be employed for high altitude flights. And if you go high enough it becomes a reentry problem at multiple Mach, even for a nominal flight.

Gerald