Well, if it makes you feel better your far cheaper than my L3 by over $600
But yours is at least $1200 prettier than anything I could ever hope to build.
Very kind, thank you. I had big help from many in the rocket hobby industry that's for sure.
Really enjoying your build and the behind the scene work your doing. Any thoughts on where / when / what your going to launch it?
Thanks! I love the science and experimentation part of it. It appeals to my scientist background.
I'm targeting MWP in November for the cert launch; that's the closest event that would likely have a TAP present to witness it. I'm also eyeing Walnut Grove in the summer as a possibility. Or maybe Mini MWP in May....
I might do a shakedown launch on a CTI L2375 in the summer. Not sure about that since I don't have long-term plans for a 4 grain 75 mm CTI case, so I'm not too motivated to purchase that.
You've certainly started some great technical threads. v/
Well darn. When L3 time comes, I'm in almost the opposite situation. I think there are 3 TAPs in my local club, but the nearest two L3CC members are 3.5-4 hours away (one in another state)
Do you have a longer case that can adapt down with a spacer? Hmmm.....does CTI do spacers that big?
Mark,
I have a AT 75/5120 case that I've used the L2375 motor in with no problems. You're welcome to borrow it at Mini MWP if you want. (standard borrow rules apply :wink: ). Just let me know.
You may want to review your calculation. You used the equation for energy, which has units of Newton meters, not Newtons.
A better calculation which would give you Newtons is to calculate F = mass * acceleration, where you substitute for acceleration by solving for the change in velocity over the distance you expect the fin to move the rock when it strikes it, which is just going to be a guess. I would expect may a centimeter or about 0.01 meter.
Steve Shannon
Yeah. I took the slow down distance to be 1 to oversimplify the equation which isn't right.
F s = E
F = E / s
The point of impact would be up to 360 lbs then.
No, E= 158.7 Joules, s = 0.01 meter, therefore F = 158.7/0.01 = 15,870 N, or 3,566 lbf. for the force of impact.
In order to have a force of 360 lb. you would need to move the rock 10 centimeters or 4 inches.
Moral: don't hit a rock.
Moral 2: always do dimensional analysis.
Don't let this demoralize you.
Steve Shannon
Also, my calculations are incorrect in that they assume the rocket is perfectly rigid. The fin and the rocket would flex, bringing the impact force down.
Steve Shannon
Change the fins so the butt end of the motor hits first.
Nothing should be coming down sideways.
Thanks! I got confused with Joules = Newtonmeters. I'm a molecular biologist, numbers are hard.
Assuming I avoid rocks and put a 4 inch dent into the dirt, I'm still looking at approximately
158 J / 0.1016 m = 1555 N (~350 lbs).
I think a 1/8" tip of plywood won't take that impact unreinforced.
Fred is right. I think that your rocket will hit on the retainer first. When you hang it from the shock cord what does it look like?
I fly on a rocky field. It's amazing what one plate sized rock can do to a fin, but it's just as amazing how dirt parts to allow penetration. If your rocket does hit fin first, it won't travel four inches before something else hits and the force distributes itself.
Still, 1/8 inch is very thin plywood for that size rocket. I used 3/16 inch G10 for my L3.
Steve Shannon
It hangs straight down off the shock cord and kevlar bridle, so I'm probably ok. I'm probably overthinking out of an unnecessary abundance of caution. I've had tons of MPR to HPR launches and never broken a fin.
The 1/8" is for the very tip of the fin. It's tapered from 1/8" up to 1/2" in a Nike Smoke fin profile.
I think you'll be just fine.
Better to over analyze than screw the pooch.
Steve Shannon
Were there any tricks to slotting the tube? I'm really enjoying this thread.
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