O3400 Min Diameter L3

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AeroAggie

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I'd just conservatively ignore the effect of the fillet. The tool has several other conservative simplifying assumptions baked in, so taken as a whole, if you can get critical numbers higher than your expected speeds I think you're good to go.
 

airforcetp1991

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I'd just conservatively ignore the effect of the fillet. The tool has several other conservative simplifying assumptions baked in, so taken as a whole, if you can get critical numbers higher than your expected speeds I think you're good to go.
I agree, I did that analysis mainly as a sanity check and out of curiosity for what the approximate difference would be.

-Tony
 

airforcetp1991

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What do most people use for the length of their bevels for high performance fins? I have used 1" in the past but I am curious to see if anyone has tried "longer" bevels. Also, I don't plan to make the fin leading edges too sharp for a couple reasons. 1) safety on ground handling 2) Ablative concerns with heating build up on the very thin CF leading edge 3) weakness of the thin CF at those thicknesses. My current sims have about a 1/16" LE, any BS flags that anyone can see with that plan? Thanks.

-Tony
 

rocketlabdelta

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What do most people use for the length of their bevels for high performance fins?
It’s more common to talk about bevel angle than length. Back in the day, Dynacom used 10° which is a good balance between performance and ease of machining. The Princeston SpaceShot used a 5° bevel on the sustainer fins. Their technical documentation is worth a read—what you’re trying to accomplish is more in line with their work than typical level 3 projects.
 

airforcetp1991

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It’s more common to talk about bevel angle than length. Back in the day, Dynacom used 10° which is a good balance between performance and ease of machining. The Princeston SpaceShot used a 5° bevel on the sustainer fins. Their technical documentation is worth a read—what you’re trying to accomplish is more in line with their work than typical level 3 projects.
I have been looking for a paper like this for a while, thank you. Using degrees as opposed to length makes sense, shows how much machining experience I have… Have you done the beveling yourself for your projects? I was told that MacPerformance Rocketry is a good option for beveling.

-Tony
 

airforcetp1991

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Did some brainstorming over the weekend after being contacted by someone with a new sled prototype that got me thinking about a nosecone based AvBay setup again. Until now I was concerned about fabricating the nosecone sled assembly but the prototype looks like it could really solve that problem in an elegant way. I know there are some clearance issues with the sled CAD, dimensions are approximate and it is meant more as a mock up. This setup would use redundant MAKO line cutters to release a reefed main chute. Double Eagle CO2 for apogee deployment. My thoughts now are to still use a small drogue to minimize flail on the way down. Other than being simpler the 2 biggest advantages I see are

1) Eliminate the separation point between the nosecone and the coupler which is a failure point at high Mach

2) Moves more mass into the nosecone making the SM easier to achieve without ballast. Double Eagle and CO2 cartridges are about 20 oz...

-Tony
 

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airforcetp1991

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If you haven't used these before, be careful, they will bevel a finger far faster than fiberglass.

This is probably the simplest tool for the job. The only reason I don't have one is the need for a router table and the room to keep it.
Agreed. I am no stranger to wood working but a router is something I am still very cautious with. Yes it is in a table mount but 2 hp is going to win the fight with your flinger for sure...
 

cerving

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I would extend that sled and/or rotate the tracker to get the GPS and RF antennas completely clear of the CO2 cylinders. You want to keep them away from any adjacent metal, if possible... since the allthread runs through the length of the NC, the best you can do is to have the GPS antenna point away from it, and live with the (hopefully) minor effect on your RF signal from the allthread.
 

airforcetp1991

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I would extend that sled and/or rotate the tracker to get the GPS and RF antennas completely clear of the CO2 cylinders. You want to keep them away from any adjacent metal, if possible... since the allthread runs through the length of the NC, the best you can do is to have the GPS antenna point away from it, and live with the (hopefully) minor effect on your RF signal from the allthread.
I agree, this mock up is with a 5" sled but I am not sure if that will be the finished length. Just got the Eagle CO2 in the mail and the updated CAD shows better clearance for the components so I think it is definitely a workable concept. This is also modeled with the 35 gram CO2 cartridges which is what I expect to need based on 3x #6 shear pins. RASAero says ~200# peak drag and conservatively assuming all of that will be acting to cause separation. Once I get some more data on internal volume after the hardware arrives I will test the 35g CO2 with the shear pins.

As far as signal issues with the all thread I don't see a way around this totally as I would need to mount the tracker in the nose (FG vs CF) and I will need an all thread in there. Moving away from the CO2 will help for sure though.

-Tony
 

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OverTheTop

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Have you considered using a titanium bike spoke for the metal tie? Very light, around 280kg breaking load, and the lower conductivity would help around your antenna.

Good bike shops will make them to the length you need. Surprisingly well-priced too.
 

airforcetp1991

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Have you considered using a titanium bike spoke for the metal tie? Very light, around 280kg breaking load, and the lower conductivity would help around your antenna.
Funny you should mention that. I have a 1/4x20 titanium all thread from McMaster Carr that threads into my nose cone tip. It is 36" so I will cut it to final length eventually but it is half the weight of the steel all thread. I have their titanium nuts and washers as well.

-Tony
 

airforcetp1991

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Added a bulkhead in the FWD NC section with the intent of reducing the unsupported length of all thread in the NC. My concern is shaking/resonance possibly during the ascent and more so shaking on the long way down. Also helps move more weight FWD and if I do need ballast I can use that to close off the ballast section.

-Tony

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Pic5.JPG
 

airforcetp1991

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Does anyone have actual descent rate data for a high altitude build like this that they would be willing to share? I ask because I am trying to understand the possible benefits to a 'Drogue-less" setup with the nosecone avbay design. I like that is simpler and if I am using a line-cutter why not just use the bound main chute as a drogue right... I have 2 concerns.

1) Rocksim shows a descent rate of ~130fps with a 12" drogue and I would not want higher than this based on opening shock concerns. Would I be correct to assume that a "drogue-less" setup with a tightly wrapped 72" fruity chutes compact chute would fall faster?

2) While a faster descent would be ideal to minimize drift (assuming it is structurally valid as mentioned above) I am unsure how much of a difference it will make at higher altitude. My biggest concern would be the descent up through getting below the jet stream. When we have our flight weather briefs the strongest winds tend to be above 35,000 ft or so, it can be >100 knots above that. The standard atmosphere shows about half the density at 25,000ft and obviously it drops off very quickly as you get up to the 60,000-90,000 range. I bring that up to say I feel like there would be very little difference in high altitude descent rate of a drogue vs drogue-less setup and the real difference would be as it got to around 30K. That is certainly significant but I am not sure if the opening shock risk is worth it. Does that math/logic check?

I would be very interested to see what real world data shows on a descent.

-Tony


Standard-Atmos.JPG
 

Dustin Lobner

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Looks like 50,000 psi tensile strength.

-Tony
Minor diameter of 1/4"-20 is 0.207".
Area = pi * r^2, so .034 sqin.
Area * tensile = 1683 lbs.

That's ultimate tensile strength, so what load does it break at.

Yield strength (where it takes a permanent set) is 40 ksi, so 1346 lbs.


Edit - there's a crapton of assumptions on that. The math is based around a smooth tensile bar the diameter of the minor diameter of the thread...so, not sure how that changes things. Also assumes a straight pull, any sort of bending on it throws the calcs out the window.
 

airforcetp1991

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Minor diameter of 1/4"-20 is 0.207".
Area = pi * r^2, so .034 sqin.
Area * tensile = 1683 lbs.

That's ultimate tensile strength, so what load does it break at.

Yield strength (where it takes a permanent set) is 40 ksi, so 1346 lbs.


Edit - there's a crapton of assumptions on that. The math is based around a smooth tensile bar the diameter of the minor diameter of the thread...so, not sure how that changes things. Also assumes a straight pull, any sort of bending on it throws the calcs out the window.
That checks, been a while since I used those mech terms... 🤦‍♂️ Given a very conservative max NC weight of 5 pounds that would be >300G of acceleration needed to shear the rod which would seem more than sufficient. I agree with the bending considerations but I hope the bulkhead in the FWD section of the NC will help with that. Rocksim shows a descent rate of about 130 fps with the drogue. I would plan to use a forged eyebolt on the end of the rod and have a "locking" mechanism on the end of the rod. Probably either a cotter pin or a tab I can fold over to absolutely minimize the chance the bolt could unscrew. Some loctite couldn't hurt either.

On a similar note I am going back and forth with regard to screw vs. rotary switches. I have used both before and I like the smaller size of the crew switches but I feel there is a limited amount of torque you can put on them. I like that the rotary switches have a positive detent and they can be mounted such that Z axis acceleration of launch/coast is inline with the armed position.

-Tony
 

AeroAggie

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For most aerospace structures, you size to criteria of something like:
  1. No detrimental deformation (yield) at 115% of Design Limit Load (let's call that your operating load here)
  2. Safety Factor = 1.5 at Ultimate Load (civil engineering applications like factories may have SF=3 or 5, if lifting loads are applied)
  3. For a threaded rod or bolt, you would also check shear strength of the threads. McMaster doesn't publish that for the threaded rod but you can google it
So for a 1683lb Ultimate capability, you would have a max allowable load of 1122 lb, if you're using conventional aerospace structures analysis methods.
 

airforcetp1991

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For most aerospace structures, you size to criteria of something like:
  1. No detrimental deformation (yield) at 115% of Design Limit Load (let's call that your operating load here)
  2. Safety Factor = 1.5 at Ultimate Load (civil engineering applications like factories may have SF=3 or 5, if lifting loads are applied)
  3. For a threaded rod or bolt, you would also check shear strength of the threads. McMaster doesn't publish that for the threaded rod but you can google it
So for a 1683lb Ultimate capability, you would have a max allowable load of 1122 lb, if you're using conventional aerospace structures analysis methods.
That makes sense, I have always used SF 1.5 for manned aircraft design and I think 1.15 minimum makes sense for a project like this.

With that math a 5 lb nosecone would have to experience over 200G to exceed 1000lb of force through the rod.

For the threads here is how I did the math for a nominal eyenut seen here

6C0E3408-22D8-45DB-8F0D-9F0E28547917.jpeg

5/16 threaded length x 20 threads per inch x 0.031 in2 per thread = 0.1937 in2

That is significantly more than the cross sectional area and so I would take that to mean the threads would not be the break point? Does that check?

-Tony
 

AeroAggie

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It is more area, but the shear strength is generally a lot lower than ultimate tensile strength, so you'd need to know that number. Generally around 50%-60% of Ftu is a decent place to start.
 

AeroAggie

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The 1.15 criteria would apply to the Yield strength, which actually gives you a higher load than Ftu/1.5. You'd size to the lesser of the two, which in this case is the ultimate strength.

Different materials have different ratios between yield and ultimate strength but for heat treated materials it's generally going to be around that 1.5 relationship.

That McMaster Ti rod is commercially pure, very soft, so yield and ultimate are very close together. It needs to be alloyed to get real strength. As it is, it's marginally better than the shittiest aluminum, which makes me question why you're spending the money on it if your goal is simply reduce weight in the nose. Use an appropriately sized aluminum rod for 1/10 the cost and 2/3 the weight.
 

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I've had really great luck making my own custom eye bolts and all thread using mcmaster 7k series aluminum rod. Best of all I can adapt it to be multifunctional in the electronics bay by having the eyebolt machined directly onto the threaded rod I use to hold everything together. The only reason I prefer Ti in something like this is it's lower interference compared to Al with electronics.
 

AeroAggie

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The only reason I prefer Ti in something like this is it's lower interference compared to Al with electronics.
Do you have a reference that documents this somewhere? I'm woefully ignorant of RF interference characteristics, except for the gross generalizations like don't put an antenna inside a carbon shell, and such.
 

airforcetp1991

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The 1.15 criteria would apply to the Yield strength, which actually gives you a higher load than Ftu/1.5. You'd size to the lesser of the two, which in this case is the ultimate strength.

Different materials have different ratios between yield and ultimate strength but for heat treated materials it's generally going to be around that 1.5 relationship.

That McMaster Ti rod is commercially pure, very soft, so yield and ultimate are very close together. It needs to be alloyed to get real strength. As it is, it's marginally better than the shittiest aluminum, which makes me question why you're spending the money on it if your goal is simply reduce weight in the nose. Use an appropriately sized aluminum rod for 1/10 the cost and 2/3 the weight.
Fair enough, when I bought the rod it was for a previous build that would have used 2 all threads in a more traditional AvBay setup and so I wasn't as concerned with the strength and more with the weight and trying something new. Given I may need ballast anyway I may just use a cheaper steel rod...

-Tony
 

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