Madcow Twitch and a couple questions

[CoachSteve]

So my wife got me my first fiberglass kit for Christmas (yes, she is awesome)- a Madcow Twitch.
fantastic 3" kit, Mine came with a black nose cone, Black G10 fins and a Green body tube - way cooler than the stock photo on the Madcow website. Not sure if all kits come with colored body tubes now and black fins but since mine did I will doing a no paint build for sure.
I will post build picts and such but this is a pretty standard single deploy build.
Here are a few questions I have though:

The fin slots in the BT are about a centimeter longer than the fins. This leaves a open slot either behind or in front of the fins. I would like to have the fins as far back in the slots as possible as the Stability is already marginal on this short rocket. If I do this the open gaps are in front of the forward centering ring. I assume I need to fill these in some way?

Since I will be building this kit with an AeroPack tailcone - do I need an aft centering ring? the fins will be internal and external filleted.

 

[tim cubbedge]

I have one and placed fins as far aft as possible. Don't really have to fill the gaps but can't hurt. When I did the external fillets, the rocketpoxy was thick enough to fill the space at forward section of fins.
Don't think I have ever built a rocket without aft CR but someone else have and can state their views.

 

[jsargevt]

When you do your external fillets, just fill in the gaps with epoxy. If you mask off your fillets correctly, you can integrate them right into the overall fillets for the fins. If you are going unpainted, this is a good opportunity to tint your epoxy black and that gives a nice look with the fins on the unimproved body tube.

You do not NEED an aft CR with the tailcone but I think that would make the build a little more complicated in my opinion. I can see why you'd want to leave the rear CR off to make the internal fillets easier but I'd recommend using an injected internal fillet method which requires a rear CR to work properly. That is a simple method and results in a rock solid fincan.

 

[CoachSteve]

I will post up a pict this evening - the gap I will have is longer than a fillet cover - I think

Also - the stability of this kit - according to Open Rocket seems marginal - Seems like significant nose weight required to get to even 1 stability (and that's before adding a motor)?
thoughts here?
No motor

With motor

 

[tim cubbedge]

I upgraded mine with a fiberglass NC and still had to add 6.0 oz. With no motor loaded, stability is now 2.1 calibers.

 

[CoachSteve]

Here is the gap I have:

 

[crossfire]

The gap won't be a problem. Your fin fillets will cover the slots. If not just mask off the extra slot and fill gap with expoy. A thick glue like JB Weld or Rocketpoxy will work fine.

 

[tim cubbedge]

Yea don’t sweat the gap.

 

[CoachSteve]

AeroPack tail cone retainer arrived - work of art

 

[CoachSteve]

AeroPack retainer JB welded on

Forward centering ring placement

 

[CoachSteve]

Nose cone with lower eye bolt, then carbon fiber rod holding second bolt with nuts and washers at the tip for nose weight.

 

[Nytrunner]

Yep, noseweight is in the cards here.

Your rocket is jsut over 10:1 L/D so you definitely need at least 3" of static margin

 

[CoachSteve]

3 inches with motor loaded would be a ton of nose weight

 

[tim cubbedge]

6 oz is all I added. It flies fine.

 

[CoachSteve]

6oz? - that's over a 1/3 of a pound - seems like quite a bit?

 

[Nytrunner]

Some designs require noseweight to make them stable

 

[CoachSteve]

The sim above with stability of 1.0 would require 190g (6.8 oz) of nose weight. man that seems like a lot

 

[Nytrunner]

The motor is hanging back which moves cg aft, a tailcone reduces base drag which moves CP forward. Thick fillets get heavy too.

All that together requires some noseweight. Fortunatelt your nose is already set up for adding and removing as needed. No red flags from my position

 

[tim cubbedge]

6oz? - that's over a 1/3 of a pound - seems like quite a bit?

I get a stability about 1.4 with just about any motor. You'll just have to see where CG is once you get it fully assembled and adjust to your needs.

 

[CoachSteve]

Timbuck - how do you get 1.4? I have adjusted the files above with my actual weights so far and get no where near that (I don't even get 1.0 without a motor, with the current 60g in the tip). I have even drilled out the fin tabs with 5 - 5/8 holes, reducing the overall fin weight down to 150g from 212g

All thanks for the advice so far. I am no stranger to adding nose weight for sure - this just seemed like a lot
I built the nose this way with the intention of being adjusted once I was finished.
The plan will be small fillets as well.
Here are the fins getting tacked into place.
The forward ring is epoxied in place as well
Next I will do black external fillets then remove the tail cone and do internal.

 

[tim cubbedge]

Timbuck - how do you get 1.4? I have adjusted the files above with my actual weights so far and get no where near that (I don't even get 1.0 without a motor, with the current 60g in the tip). I have even drilled out the fin tabs with 5 - 5/8 holes, reducing the overall fin weight down to 150g from 212g

CP = 26.23"
CG = 19.75"
That CG was measured with recovery & nose weight sans any motor.

 

[CoachSteve]

How are you measuring actual CP?

 

[tim cubbedge]

I just messed around with my file. By adding a tailcone, the CP moved forward ~1.5"

 

[tim cubbedge]

Open rocket calculated CP. I have no control other than changing design.

 

[Tyler P]

The colour is wicked on that tube!

 

[CoachSteve]

ok - I am using Open as well - good to know I am using the proper tools

 

[CoachSteve]

I agree - I was so surprised to get the kit with a colored tube - couldn't be more pleased

 

[CoachSteve]

Here is a shot of the fin weight loss"treatment:

 

[crossfire]

Looks like a fun build. Are you not going to use the bottom CR?

 

[tim cubbedge]

Looks like a fun build. Are you not going to use the bottom CR?

See post #1