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Senior Space Cadet

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As soon as I think I have things figured out, some new piece of information gets me all confused again.
For a free CAD program, Open Rocket is phenomenal. Whoever is responsible should get a congressional medal or something. But having said that, Open Rocket is one source of misleading information. It is invaluable for getting a good approximation of CG and CP, beyond that, I'd take a lot of the information you get from it with a grain of salt. I'm pretty sure NASA doesn't use Open Rocket.
I keep seeing photos of people, out on some dry lake, launching mid and high power rockets. I'm always struck by the fact that their rockets look nothing like what I thought they should look like, from what I thought I knew about rocket design. Namely, they were really long and skinny with itsy bitsy fins.
Either they were idiots, or I was missing something. I was pretty sure they were idiots, but I thought I should look into it ;) .
First off, the design of a subsonic rocket is going to be somewhat different from a trans-sonic or supersonic rocket.
Here is what I currently think is correct information about the design of subsonic rockets for those of you that are just as confused as I was.
Best nose cone is Ellipsoid. Ogive is only slightly better than hemispherical. Length should be 2 to 3 time base diameter. Since a longer nose cone allows you to shorten the body tube, I think you can fudge on that.
Body tube should be ten to sixteen times the diameter. There are two types of stability, dynamic and static. Dynamic stability favors long body tubes, static favors big fins. Go for dynamic.
The best fin shapes are Elliptical or clipped delta. Tip chord should be about half of root chord (talking trapezoidal here) and sanding the tip to a sharp edge is important. All edges should be rounded, at the very least. I'm a big airfoil fan.
I'm not sure about height of the fin. I'm going for around 1.5 times the diameter. They should have a lot less sweep than I've been giving my fins.
Make the outside really smoooooooth. Dance around it while playing Paranoid, by Black Sabbath. Finnish by meditating to Misty Mountain Hop.
 

Antares JS

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I think you need to define what you mean by "should be" and "best." As you've found, it's possible to safely fly rockets far outside these dimensions you cite. Other people's objectives with their rockets may not be the same as yours.

NASA doesn't use Open Rocket because they don't fly model rockets at NASA. Open Rocket is specifically for hobbyists and has been made to serve the needs of hobbyists, not professional space launch. Different things for different needs.
 

Joshua F Thomas

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Can I suggest you apply your skills to the communication aspect of the hobby? It would be good for you - and everyone else - if you are able to write in a concise manner. The "wall of text" format you currently have is difficult to read and off-putting.
 

Jay Rairigh

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Cadet... a lot Rockets we see are people designing stuff to see how FAR from the 'perfect' design they can get and yet still get it to fly. See the flying spools and such.

Mostly, people design their rockets just for the look they are after. A large, long, skinny rocket with small fins, sitting on a dry lake bed in the desert, (as you described), about to be fired off looks pretty damned cool!
 

rklapp

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I’m pretty sure the Italians didn’t use OR when designing the Vega. Maybe woulda saved them from the failure to light 2nd stage problem last year...

 

dr wogz

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Doesn't NASA use raw math? [F=MA at the very least!!] And really really smart people, those who can solve really long & complex equations?

Don't they also use advanced CAD, Modelling, and mathematical programs to prove their work? As well as using stress simulation software to also help define & refine their designs? (I've dabbled with FEM / FEA here at work. definitely a bit of 'black magic'!)

Don't forget that some of those long & skinny rockets you see in the desert are designed around the motor & recover. The motor itself can be over 4' tall, yet only ~3" in diameter.. (Loki anyone?) Make it a 2-stager, with each stage having it's own recovery.. Potentially a 10' x 4" rocket.. and easily 25K - 75K feet!

Remember, the design is a function of it's purpose; it's intent. A paper & wood rocket, barely 2' long & 2" in diameter, hitting 650'.. what's the goal? purpose? other than bragging rights & something 'cool to do'?
 

n27sb

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Seriously,
For me form follows function.
Some day I will go back to building Funky Rockets. Hmmm but then Funky will be the function and form will still follow.
 

Jay Rairigh

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...on another note. In regard to what space cadet has been trying to do here with his rockets/frustrations, I would enjoy trying different parameters, testing them on flights to see what flies highest, but you should start with the basics from what other people doing the same thing have learned.
 

gna

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It seems you are spending an inordinate amount of time with OR. Use OR to check your designs, and get an idea of performance. Build and fly some rockets.
 

SCooke123

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Worrying that much and trying to get the optimal design, etc would suck all the fun out of rocketry for me. When a hobby for me quits being fun it turns into work. I change my interest and look for something fun again to occupy my free-time!
 

dr wogz

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You may also want to brush up on your physics..
Inertia, momentum, force / impulse. and know about 'drag': Cd

Remember as well, that these re all designed to meet a specific challenge:
NASA put hundreds of pounds to 18K mph, and hundreds of miles 'up' (Way outside our atmosphere)
NASA & other research & testing people put their rockets up tens of miles into the thinnest parts of our atmosphere, and at MACH+ speeds.
We put our models up hundreds of feet, in the thickest parts of our atmosphere.. at speeds of a rail dragster (Yes, with notable exceptions!!)
 
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