18mm Estes Mini Fat Boy??h

[Bill Gibson]

Has anyone tried this foolishness?? Im about to..... I just built one for 18mm motors.
What could possibly go wrong??!!??

[dhbarr]

Paper the fins and drop in a d21.

[rstaff3]

Has anyone tried this foolishness?? Im about to..... I just built one for 18mm motors.
What could possibly go wrong??!!??

What's foolish is not to put in an 18mm mount...or a 24?

[samb]

Well the Mini FB is close to the size of a Baby Bertha so probably not too outrageous an upgrade. The challenges will be space for the recovery device and getting the right balance. The Mini FB airframe is about an inch shorter than the Baby. The nose cone has a conical projection that extends at least a half inch below the shoulder. The 18mm motor mount is one inch longer than the 13mm. That's going to squeeze the parachute compartment a bit. I think added nose weight is probably a good idea; I'd start with an amount equal to the difference in weight between the two motor mounts plus the difference between a C6 and an A10 motor.

Some have gone before:

http://www.rocketryforum.com/showthr...-and-sprint-xl

http://www.rocketryforum.com/showthr...de-CG-Question


Always nice to check out a SCIGS30 build of a model you're considering, even if it's stock:

http://www.rocketryforum.com/showthr...-Fat-Boy-Build

[rstaff3]

Sounds like plenty of room to me. I stuffed a 29mm for the E16/F15 into my Baby Bertha so the the 18mm and short 24s should fit in the FB just fine.

[samb]

Sounds like plenty of room to me. I stuffed a 29mm for the E16/F15 into my Baby Bertha so the the 18mm and short 24s should fit in the FB just fine.

Wow! Did you recover it ?

Yeah I think enough of these have been done to show it's not a problem. Just something to consider.

[rstaff3]

Yeah I think enough of these have been done to show it's not a problem. Just something to consider.

If you plan to fly one on a D, I recommend buying at least two. And yes, nose weight needs to be looked at. The Bertha fins help in that regards.

[Bill Gibson]

WOW! Thanks for the GREAT positive feedback and information everyone!!!!
Samb, I did the same differential deal on my 18mmEstes Mercury Redstone that's D powered and it flies great, so I will be sure to do the same for my MFB! Thanks again!!

[rstaff3]

Wow! Did you recover it ?

Yeah I think enough of these have been done to show it's not a problem. Just something to consider.

My Sacrificial Bertha has flown three times...two E16s and one F15.

[Rex R]

with mine I added 0.5oz. to the nose and added 1" to the airframe to regain recovery room. flies great on a 'Big' B6-4 motor .
Rex

[samb]

My Sacrificial Bertha has flown three times...two E16s and one F15.

Well alright ! Any knucklehead can stick a big motor in a small rocket... steely-eyed missile men get 'um back.

[rstaff3]

Well alright ! Any knucklehead can stick a big motor in a small rocket... steely-eyed missile men get 'um back.

Well, the last recovery relied on one observer who decided not to look in the expected direction and saw it come down behind some trees. He was the steely eyed one that day

[hcmbanjo]

Hi Bill!
One way to get the nose weight right is to not glue in the assembled 13mm mount - yet.
Build both the kit 13mm mount and your new 18mm mount.

Finish the model, slide in the 13mm mount with an A10-3t engine in it and find the C/G.
Slide in an 18mm mount with a C engine inside, or largest engine you'll probably fly.
Add clay to the front end of the nose cone until it matches the C/G when the 13mm mount was in place.
Add your clay and glue in the 18mm mount. You should be good to go!

Here's how I did an 18mm mount in the Curvilinear:
http://modelrocketbuilding.blogspot....d-parts-9.html

[samb]

Hi Bill!
One way to get the nose weight right is to not glue in the assembled 13mm mount - yet. ...

That is a great explanation of the technique from someone who's built a model rocket or two and is good at 'splaining.

[mrwalsh85]

only 18mm? I put a 24mm G55 in mine.... I got it back too.

see: http://www.rocketryforum.com/showthr...classic-motors

[Bill Gibson]

This feedback is amazing!!!! Chris, that's a great idea for the balance! Thanks so much!!
Unfortunately it's already built!! Rex, I came up with .6oz which is mighty close to your model, so I thought I'd give that a try....!

[Rex R]

not going to quibble about 3g, I may be mis-remembering. a 12" thin mil chute works for recovery
Rex

[RodRocket]

I have one in the works also. Plan to launch it with the 18mm AT D's. Will cut taper from nose cone base. Made a motor mount tube long enough that only nozzle of motor is below rocket. Ply rings and a kevlar cord. Once fins are on, will get CG right. Fins will be thinner ply than stock balsa but remain stock size. Should be a little screamer!

Lots of success with your build!

[mrwalsh85]

Surely someone else has some G55's laying around. I even have some old Aerotech F55's from back in the day.... But that's a waste in a Mini Fat boy! Minimum diameter, baby!

[mrwalsh85]

For what it's worth, I used a reflective mylar streamer from Apogee Rockets and carefully wrapped it around the motor. Unfortunately some of the streamer fused to the motor, so next time I'll insulate the mylar from the motor with some estes paper wadding. But the motor is about 1" away from the tip of the nose cone, but the CP/CG relationship with the G55 worked very well - it was VERY stable. I cracked a fin upon landing so I'll have to either fix it or build a new one, as it separated the layers of glassine. Maybe I'll take a look at it tonight.

-Mike