Airframe Shred Prevention Calculator

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mccordmw

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EDIT:

Calculator is up. Here is the link. https://docs.google.com/spreadsheet...FxY3m08tIn-kY2F8sNV7NQd8k/edit#gid=1801452371


After a ton of material science and physics research, I've saved all of my formulae in a google sheet to auto calculate the drag + inertia forces the rocket experiences to see if your frame can stand the stress. Any interest in having this cleaned up as a tool? It will tell you if your frame needs reinforcement to prevent a shred at the speed you input. I'd have to clean it up to make it more user friendly.

1. It will show you how much pounds of force the airframe can take before crushing axially.
2. It will show you how much pounds of force the airframe can take before it will buckle.
3. It will show you how much drag + inertia force you will experience on your airframe depending on your anticipated acceleration and max speed (supports subsonic and mach formulae).

If #3 exceeds either #1 or #2, you will potentially have issues.

I have axial and buckle data for paper tubes.
I only have axial data for LOC, glassine, and fiberglass.
I'd love for people who know more than me to enhance it for other airfrmes.

ScreenShot011.jpg
 
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mccordmw

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Also has thrust to weight check to make sure my motor is in the safe range.
Not seen, but also has speed off 6' and 8' rods with the acceptable safe wind speeds (usually speed off rod should be 4x wind speed).
 

BDB

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This sounds great! Please share.
 

Nytrunner

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Are you making an assumption that the Drag acts entirely on the nosecone (neglecting fins)?

Kind of like treating the airframe as a 2-force member between the thrust and the drag on the cone?
 

mccordmw

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Yeah. Following Nakka's modeling assumption. I would assume that drag on a thin fin is negligible compared to the cone anyway.
 

Nytrunner

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That's reasonable. I just wanted to see if I could follow your logic in the spreadsheet.
This could be a really good resource.
 

mccordmw

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Ok everyone. I cleaned up my sheets and made it easy to follow. Here's the sheet. The green cells are unlocked so you can input values (so is the blue material selection drop down). Note that there are wood selections in the materials. Those are in there since I sometimes include wood as internal structural reinforcement - like what I'm considering for my sonotube rocket.

Ideally, someone can pick up this tool and publish it as an interactive calculator on their website.

I think I have all the calculations down ok. Everything is documented on separate tabs. The biggest area of uncertainty are the reference values. I mined those from several sources (seen on the reference tabs). Those very well could need correction. I'd love for someone to provide real testing data on compression strength and tensile modulus.

Keep in mind that I'm not a materials scientist or a mechanical engineer. I'm a lowly molecular biologist, so this physical stuff is hard. :p.

Anyway, here is the link to my safety checks calculator. It makes sure you can get off the pad fast enough for the wind, has enough thrust safety margin, and makes sure you aren't going to crumple the frame and have a shred.

Be gentle! :pop:

Rocket Performance Safety Calculator
 
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mccordmw

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You might want to make a copy if you're going to play with it a lot. While the important stuff is protected, other people might pop in and edit your values. Such is the downfall of sharing google sheets. :p
 

timbucktoo

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Any reason why you are mixing engineering units? Can you standardize, either keep it all MKS or Imperial or give choice?
 

mccordmw

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Any reason why you are mixing engineering units? Can you standardize, either keep it all MKS or Imperial or give choice?
Mainly because I buy components based on imperial (like a 4" airframe), and the default in OpenRocket is metric for flight profiles. I got used to looking a m/s and m for flights, so I made my calculations like that. A choice would be a nice enhancement...maybe some time. I'm most concerned about the material data accuracy. Is it realistic?
 

timbucktoo

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I appreciate your effort but you do know that you can change your units in OR?
I was playing with the spread sheet but too much converting units back & forth for me. My units in OR are Imperial. Too easy to make mistakes. Isn't that how they crashed a rocket on Mars?
 

mccordmw

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Shouldn't be too hard to accommodate. I can bind units to a validation list and normalize behind the scenes for the formulae so I don't have to modify those.
 

Nytrunner

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Isn't that how they crashed a rocket on Mars?
Quite the opposite. They missed orbital insertion by a couple hundred miles (or was it kilometers....).

The unfortunate thing with these calcs is that mass and accelerations are much easier in metric, but we're most familiar with English dimensions.
When I've made quick/dirty calculation spreadsheets, I usually just have a column next to it in the other unit system.
 

fyrwrxz

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Quite the opposite. They missed orbital insertion by a couple hundred miles (or was it kilometers....).

The unfortunate thing with these calcs is that mass and accelerations are much easier in metric, but we're most familiar with English dimensions.
When I've made quick/dirty calculation spreadsheets, I usually just have a column next to it in the other unit system.
He's referencing the one that core sampled. They figure the retro's went off underground.....
 

fyrfytr310

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I like it! Just curious, why 6:1 on the thrust:weight? I've always used 5:1 and thought that was pretty standard.
 

mccordmw

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I like it! Just curious, why 6:1 on the thrust:weight? I've always used 5:1 and thought that was pretty standard.
Tripoli has the 3:1 minimum safe rule.

G. Weight Limits
1. The maximum lift-off weight of a rocket shall not exceed one-third (1/3) of the average thrust on the motor(s) intended to be ignited at launch

I've seen people say 5:1 is better.

I'm risk-averse, so I doubled the Tripoli ratio. :D

I updated it to 5:1 since most go by that and might use the tool.
 

mccordmw

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I appreciate your effort but you do know that you can change your units in OR?
I was playing with the spread sheet but too much converting units back & forth for me. My units in OR are Imperial. Too easy to make mistakes. Isn't that how they crashed a rocket on Mars?
Easy to update. I added in a reference table to a bunch of units. Those calculate to a normalized value used in the formulae. See the calculator v2 tab. I can't update the main one since people are currently using it. Which is a good thing. :D
 

NateLowrie

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Tripoli has the 3:1 minimum safe rule.

G. Weight Limits
1. The maximum lift-off weight of a rocket shall not exceed one-third (1/3) of the average thrust on the motor(s) intended to be ignited at launch

I've seen people say 5:1 is better.

I'm risk-averse, so I doubled the Tripoli ratio. :D

I updated it to 5:1 since most go by that and might use the tool.
5:1 is really conservative. I try to stay above 4:1.
 

Nytrunner

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For a general calculator, the 5:1 thrust/weight is a safe standard.

For my flights, I evaluate the thrust curve pattern (regressive, flat, bell/mountain shaped) and predicted rail exit for the motor and rocket combo. Haven't had an RSO reject a flight when presented with that data.
 

mccordmw

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I've found that below 5:1, unless the initial thrust is very high, the speed off the rail is generally too slow. It gives a flight that really weathercocks in moderate winds and is too horizontal.
 
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