Would someone sanity check this OR design?

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neil_w

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This is my first scratch design and I'm trying to do it the right way in OpenRocket. However, there are things like the recovery materials that I really don't know if I'm doing correctly. For this design I put in a wad of dog barf (best as I could estimate it), and then the folded parachute right above it, and the shock cord kind of hear the top end of the parachute (really, where can I assume the shock cord will settle?)

Static margin is coming out between 1 and 2 calibers for the different engines, so I think I'm close, but I fear a GIGO situation. If someone could sanity check this and just confirm that I'm in the neighborhood I'd greatly appreciate it.

Thanks in advance!

diamond_cutter_side_view.jpg

View attachment diamond_cutter_real.ork

(BTW, is it just me or does OR crash All. The. Time.?)
 
This is my first scratch design and I'm trying to do it the right way in OpenRocket. However, there are things like the recovery materials that I really don't know if I'm doing correctly. For this design I put in a wad of dog barf (best as I could estimate it), and then the folded parachute right above it, and the shock cord kind of hear the top end of the parachute (really, where can I assume the shock cord will settle?)

Static margin is coming out between 1 and 2 calibers for the different engines, so I think I'm close, but I fear a GIGO situation. If someone could sanity check this and just confirm that I'm in the neighborhood I'd greatly appreciate it.

Thanks in advance!

View attachment 273554

View attachment 273553

(BTW, is it just me or does OR crash All. The. Time.?)

You always could install a shelf to keep the parachute and the shock cord further forward inside the body tube.

BTW... Here's your rocket design, as we'd see it in a 3D finished view in OR:

 
I think my margin is ok *if* I've realistically located the recovery stuff in the tube. That's really my big question right now. I guess the only problem would be if it pushes back further at launch due to the g's; if it stayed more forward there would be no problem.

I'm also gonna try modeling a baffle in there to see the effects.

Eventually there will be some sort of altimeter up front, which will probably push it towards over stability. Will deal with that later.
 
well Estes body tubes typically have a somewhat higher density (0.511 oz/in^3) than paper so that right there will move your cg forward a smidge, and it looks like you could throw a chute shelf in there(forward surface 7.5" from the end of the tube, need to leave about 6" of space between shelf and aft end of transition shoulder). a shelf need not be complicated a simple Estes centering ring (5060 iirc) would suffice.
Rex
 
Thanks for the density value, I will update my file with that.

The question is, do I really need to move any weight forward? Everywhere I look it says 1-1.5 calibers static margin is a good target, and I'm right there. For the heaviest engines, I can add a bit of nose weight. I don't want to pull the recovery weight too far forward and become overstable. I think.

In the absence of a shelf, does it look like my placement of the chute and shock cord resembles reality? I assume that wherever the frontmost block is (either shelf or baffle), the placement of the wadding (if relevant), chute, and shock cord should be about the same relative to that block.
 
cp moves in relation to angle of attack & airspeed, cg will change as propellant burns. also consider that the 'recommended' cg location is for a 'normal' size rocket (8 - 15 diameters in length), longer rockets ought to have the cg further forward. I personally try for a min of 1.3 cal(which you have). typically the bulk of the shock cord normally rides below the chute but, yes on the whole I think your design ought to work. have fun building it.
Rex
 
Thanks, I'll adjust the shock cord position. Looks like I should be good with the engines I'm considering (E9 is borderline, but it'd also be a bit slow off the rod, so I'll skip that one anyway). A baffle (if I can squeeze one in there) will move the weight up a bit.

One thing I notice is that the speed off the launch rod doesn't change even when I change the length of the launch rod in preferences and then re-run the simulations. That makes no sense, am I missing something?
 
Looks really good to me. Very nice design. But, like you said, you will want to add some nose weight when flying the heavier E engines, just to be on the safe side.

And, yes, OR crashes on me all the time too. But it's free; so I don't complain too much.
 
reasonably sure that you need to use the 'Edit simulation' button to change rod lengths in previously run simulations.
Rex
 
Think about your thrust to weight ratio well before you choose to fly on E9. Low thrust for its weight and loooong burn time means a rocket can veer off real far!

I learned this by losing a rocket ;)

Why not consider an E15 or E30 if your rocket is heavier? The Aerotech motors are cool to fly :)

As a bonus, these motors are lighter than the E9, giving you more stability margin.
 
Yeah, the E9 doesn't look too promising here. Aerotech E15 looks like the best one here although the Estes E12 should do fine as well.

First I gotta build it though. :)
 
Yeah, the E9 doesn't look too promising here. Aerotech E15 looks like the best one here although the Estes E12 should do fine as well.

First I gotta build it though. :)
Sure, but if you sim it like you did, it is easy to see that the Aerotech Es (15 and 30) are much lighter than Estes' ones (9 and 12) and have more punch. It alleviates some restrictions during design.

Have fun with the build!
 
Personally, I would just build it, pack it, and measure the CG position. If you need to move it forward, so be it. Just add some weight to the nose. You will get a better feel for it as you scratch build more designs.
 
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