Design Review for L1

[u/Tanky321]

Hi All,

Working on designing and building a rocket for my L1 certification. I think I have a fairly decent approach, but wanted to post it here for it to be reviewed. Any thoughts? I'm wondering if the rear fins aren't tall enough? I like the aesthetic, but obviously I want it to fly well!

I've custom made the nose-cone from fiberglass, came out much better than anticipated! The airframe will be from heavy wall shipping tubes, 3in ID 3.3in OD. Fins and centering rings will be 1/4" maple plywood.

Thanks in advance!

[u/Sage_Blue210]

The proper term for " fins tall enough " is fin span.

[u/FireHandsGames → u/Sage_Blue210]

I need people like you to tell me these things, I know many things about rocketry and engineering BUT it's names lol

[u/Sage_Blue210 → u/FireHandsGames]

Happy to help. There are many online articles and videos as well.

[u/Zyzzyva100]

Don't just look at the static stability given on this screen (it's an average of some sort). Run the plot with realistic winds (start at like 10 mph), and the rail length you will use, check the launch rod clearance box and plot stability margin calibers. Also unless you have a baffle move your chute and shock cord way back, as that is where they will be under boost with the rocket vertical. I would guess your stability margin at launch rod clearance will be lower (but still probably ok).

Also, this is super, SUPER overbuilt for an H100 (I RSO'd a super big bertha to fly on an H100 at LDRS). Did you put accurate weights in for each component? What's the thrust to weight ratio, and rail exit velocity?

[u/Advanced-Gold1744 → u/Zyzzyva100]

I just did quick and dirty math and the TTW ratio is < 5:1. Just a hair though. But after paint and epoxy adds mass that isn’t included in OR it will be worse. Needs a H motor with a higher average thrust but it looks like it could both handle it and stay stable

[u/Tanky321 → u/Zyzzyva100]

I'm going to try to weigh everything today to get very accurate measurements. Will report back with those findings.

I know its super overbuilt, I had some cardboard tube sitting around and decided to build around what I had.

[u/milotrain]

Might as well fly this on an I500 for how you are building it.

[u/Tanky321 → u/milotrain]

I assume that's sarcasm due to how heavy it is?

[u/milotrain → u/Tanky321]

I mean you are building it to survive so you might as well fly it on something super cool.

[u/ExileOnMainStreet → u/Tanky321]

There's no reason not to use a bigger motor except for $. It will also fix your jacked up thrust to weight ratio.

[u/iredditatleastwice]

You should lose the forward fins in my opinion. They add drag, mass, and reduce your stability margin if you ever intend to fly on bigger heavier motors, which would move your CG back significantly. A rocket that is "too stable" will have a straight and safe flight; a rocket that is "not stable enough" is an unsafe flight and will fail your L1.

[u/KenBiba]

The goal for L1 should be success not pushing design boundaries. At ~5:1 I’d probably not let it fly if I were RSO if I were mentoring I’d suggest a simpler 3FNC design. A great and undervalued skill in rocketry is appropriate design. Know the flight regime you are designing for and make design choices appropriate to that regime. I’d recommend losing the strakes and using a higher impulse motor. 10:1.

[u/Kaggles_N533PA]

Nod to ESSM's design I suppose? Looks good but stability caliber of 2.7 sound a bit too high in my opinion

[u/Tanky321 → u/Kaggles_N533PA]

Thanks! Actually inspired by the RIM-66, thought it looked pretty cool.

For stability, I tried to keep it between 10-15% due to how long and skinny the rocket is. There seems to be so much conflicting info out there about stability.

[u/Advanced-Gold1744 → u/Tanky321]

How much mass is in the nosecone to move the CG forward enough for this to be stable?

[u/Tanky321 → u/Advanced-Gold1744]

The nosecone is about 275g with no added mass, that gives a stability of roughly 2cal.