Looking for Advice on Final Steps of MPR Scratch Build

[AndyJ]

Hi, I'm a long time reader, but I don't post often. I have been working on this custom built rocket for approximately a year, and would say that I'm 90% finished with the build including finishing and painting. The main step remaining is that I'm undecided on a nose cone, and had a small question on over-stability. I'm considering using a Premade Loc-Precision 3.90 inch nose cone or Vaughn Brothers nose cone of the same size (the main body tube is a Loc 3.90 Airframe). Ideally though, I would prefer a perfectly conical Nose Cone at a 1:1 Length to Diameter Ratio, but I'm not sure if the expense of having a custom made one can be justified.

The other question I had was that RockSim predicts my rocket to be overstable by 3.55 Calibers, though I suspect it will be less as the true center of mass is farther back. I was wondering how adverse an effect this over stability would have in mild conditions, say 10 mph winds.

Below are pictures of the current build, though I have extended the top portion by another 6 inches using a coupler. The fins are attached by punching small holes in the body tube and fins to create a better bond, and by using an epoxy mixed with chopped carbon fipers. The shock cord is a 1 inch elastic and I will be using a 36 inch military flare parachute. It is designed with a 29mm motor mount, though I regret not using a 38 and downsizing, though that can be for another project. I would appreciate any advice any of you rocketry veterans can offer, I'm all ears.

https://www.flickr.com/photos/40706270@N02/3748457128/
https://www.flickr.com/photos/40706270@N02/3747667965/

 

[n5wd]

...I would prefer a perfectly conical Nose Cone at a 1:1 Length to Diameter Ratio, but I'm not sure if the expense of having a custom made one can be justified.

The gent who mentors our school's TARC and SLI teams found some conical plastic material at Hobby Lobby, a local craft store, and used it as the base for a home-made approx 1.5:1 nose cone that looked pretty sharp. A little material such as fibreglass over one of those and...

The other question I had was that RockSim predicts my rocket to be overstable by 3.55 Calibers, though I suspect it will be less as the true center of mass is farther back. I was wondering how adverse an effect this over stability would have in mild conditions, say 10 mph winds.

Just checking, but you did remember to put a motor in it before checking the stability, yes? Well, IMHO with your design, 3.5 calibers will probably weathercock pretty well in a 10mph wind. Those are some pretty big honkin fins you got there. Have you strengthened them at all? I'd be worried about snapping one off the first time it hit anything hard (like packed dirt or sidewalk) on the way down.

The shock cord is a 1 inch elastic and I will be using a 36 inch military flare parachute. It is designed with a 29mm motor mount, though I regret not using a 38 and downsizing, though that can be for another project. I would appreciate any advice any of you rocketry veterans can offer, I'm all ears.

Can you swap out the elastic for some Kevlar cord? A 29mm motor is gonna put out a good deal of hot ejection gas along with particles of unburned black powder. If for any reason the elastic gets laid over the ejection port, you're looking at a nosecone separation (he says, the voice of experience here). Many folks will chant "elastic is for undershorts" and I've joined that club as well.

If you can't swap it out, now, because of it being inaccessible - just use a long drill to drill through the centering rings from the bottom up through the body tube, and poke a straightened clothes hanger through to use as a pilot to pull a Kevlar cord through and epoxy it to the bottom centering ring.

By the way - what year are you in at CM-Engineering? One of my former students (and a national finalist at the TARC rocketry competition) is an incoming freshman at CMU and will be spending a lot of time in the engineering buildings.

 

[AndyJ]

Thanks for the reply, I'll be sure to check out my local hobby shop (Michael's is the only one close to me, they don't have a Hobby Lobby on this side of the state).

As far as oberstability is concerned, I did simulate it with the motor in, but I'm fairly certain because of the amount of epoxy I used in the back end for the fins (which are reinforced with chopped carbon mixed with epoxy and a technique I think is called fin riveting) the center of gravity will be a few inches backwards. Purely from estimates, especially if I have a very lightweight nose cone, the CG will be ~2.4 calibers forward of the CP. I'll still be wary of moderate winds, especially on the first launch.

As for the shock cord, it is easily removable (simply tied to an eye-bolt at the moment). I figured being 1" thick would avoid any problems,and I am probably going to put in a custom baffle, but I have had the experience of a burnt through shock cord on a few rockets and I agree, it isn't fun. I'll look into getting some kevlar cord.

I'm going in to my Junior Year at Carnegie Mellon. Do you know what field of engineering the incoming freshman is looking at? Also, you should let him know if you keep in contact with him that we have a model rocketry club run by myself and another experienced rocket builder, we'll be at the club fair at the beginning of the fall semester if he wants to join. Again, thanks for the help.

 

[n5wd]

Sounds like you've got a good handle on things, Andy. Be sure and let us know how it flies (pix are good, too! :clap: )

Do you know what field of engineering the incoming freshman is looking at? Also, you should let him know if you keep in contact with him that we have a model rocketry club run by myself and another experienced rocket builder, we'll be at the club fair at the beginning of the fall semester if he wants to join.

Well, to be honest, I don't remember which discipline she was thinking about - something in bio-engineering, I think. If you'll PM me with your contact info, I'll forward it to Courtney so she'll have your info when she gets up there next month (Gadz, is it already the end of July?).

 

[AndyJ]

PM sent. It's amazing how fast the summer slips by. I'll post to the MPR section of the forum once I add the finishing touches. Thanks again for the help.