4 D's in a Estes Fat Boy ???

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Well-Known Member
Jan 19, 2004
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Can it be done ??? I'm not doing it ... but a friend of mine is gonna try ! I've bid on a cluster kit from Thrustline for him !
sure it *can* be done... just check the CG CP relationship, you'll probably have to ad a bunch of nose weight tho... or you could try larger than stock fins, it depends on personal preference:)
get us some pics when its done tho!!
The Fat Boy appears to be only marginally stable with a C6-5. Find out where the CG is with a B6-4 & add nose weight until the 4 D12s balance at the same point. That's my recommendation. It'd be interesting to use a couple of plugged D12-0s & a couple of E9s for power. :) Of course for testing purposes, 4 C11s would be interesting as well.
I never really used my own designs to build a rocket ... I've always just built kits from Estes, Centuri, etc. SOOOOOO ... this might be a dumb question ... BUT ... how do I go about finding the CG CP relationship ?
Originally posted by Aerobee300
how do I go about finding the CG CP relationship ?

You can find the c.g. by loading your rocket for flight, including motor and ejection wadding, and balancing it horizontally on a pivot. Measure and locate the c.g. the best you can (this technique can easily turn into a four-handed operation, if you know what I mean).

You can find the c.p., at least very approximately, by making a cardboard cutout of the profile of your model. If it has four fins, rotate them (mathematically) to the 45 degree position and make your cutout pattern based on the 'shortened' fin span. Balance your cutout horizontally on a pivot, and measure and locate the c.p.

The problem with this c.p. method is that it is conservative. It represents the aerodynamic effects of flying sideways. I have not seen too many rockets actually DO that, but the presumption is that if it is stable sideways, it is probably also stable at small angles of attack.

The actual flight behavior of objects, and their consequent stability limits, is just a bit more complicated. A fella named Barrowman did some R&D work for a NAR contest entry, and wrote out a bunch of equations by which stability limits can be calculated/estimated. This material is usually included as one of the back sections of the 'Handbook of Model Rocketry' (if you can get a copy).

Also, the balancing approach requires you to have a rocket built. This is not too handy if you just want to know whether a design is stable BEFORE you spend the time to build it. The Barrowman eqns will help you in this situation, and will give you a reasonable stability estimate even if there are some quirks and limitations to that approach.

To calculate the c.g. of a 'paper' rocket, try the following thread:


If you don't have Excel software on your computer to be able to see some of the attachments, let me know and I will dig them up and change them to some other format for you.
another way, if you feel so inclined, is to get rocksim (demo should be fine), download the rocket (there's a large library of them at EMRR) and make the mods. You can tweak your fins, add nose weight, cluster to your heart's content, etc, and it will keep updating your CP and CG as fast as you can change components. Pretty handy! Of course, this presupposes you have a computer handy, since you are on one right now...

This may be a silly idea, but I wonder if you balanced a rocket with a string tied around it & got in the passenger side of a car & put the rocket outside the window going about 60mph, if you could get some idea of stability? Of course, one could devise a device from a C clamp with needle points holding the rocket at the balance point & try the same thing. Perhaps a C clamp on a stick to hold it further out from the car. :cool:
Don't forget about the Galejs paper on wind instability - he used the Fat Boy as a primary example of a short, stubby rocket where a stability caliber of 1.0 may not be necessary.

His paper is how the calculated center of pressure moves forward (thus making the rocket more unstable) as the angle of attack increases. (angle away from vertical, vertical being zero angle of attach) This is the main reason for needing a stability caliber, or factor of safety. (a caliber is actually a distance equal to the diameter of the airframe. A standard stability caliber of 1.0 means that the center of gravity is located forward of the center of pressure by a distance equal to 1.0 times the diameter of the airframe.)

Anyway. ON a longs skinny rocket, the true Cp moves forward in huge leaps as the angle of attack increases. On a short stubby rocket (the Fat Boy being his working example) the Cp barely moves at all. In fact, it moves between 0.1 and 0.15 calibers forward as the angle of attack moves from zero the 30 degrees (and I don't know about any of y'all, but I'm not launching any rockets at such an extremely high angle of attack as 30 degrees!!)

WHAT THIS MEANS: On the Fat Boy, you probably don't need a stability caliber of 1.0 to ensure safety. in fact, you could probably get away with a stability caliber as low as 0.3 and still have a stable rocket because of the very low variation in stability with angle of attack (which means much less of a factor of safety is needed. Theoretically, in a controlled and perfect environment, a rocket should fly stable with a stability caliber of 0.0000000000000000001. Theoretically. But perfect never happens, thus the rule of thumb factor of safety of 1.0 calibers.)

There. I'm done rambling.

How do you apply this to your Fat Boy? It means that you won't need as much nose weight to have nice flights as you may have initially thought using the 1.0 caliber rule.
Ok, so I just pulled up the Fat Boy RSim file off the EMRR archive, and modified it to be a 4 x 24mm D engine cluster.

I had to add 3.5 oz of nose weight to acheive a Barrowman stability caliber of 0.36, and an RSim stability caliber of 0.57 (it's a slightly different calculation than Barrowman, but the same idea)

Anyway, as I alluded to above, you can probably get away with a Barrowman caliber of 0.35 on the Fat Boy. Launched with 4 x D12-7's it sims to 2056 ft altitude with an 8.06 optimum delay - so the D12-7 should be the engine of choice in your Fat Boy.

Hope this helps!
That thing oughta rip! The Fat Boy is a cool rocket to modify. Plenty of possibilities with the large airframe. Long motors are a pain though!

With the release of the E6's and F10's, a stock Fat Boy with a 24mm mount would go great on the E, and even better with a 29m mount and an F10...

Or go my route with a 38mm mount...:eek:
Here's a link to my heavily modified Fat Boy that is currently on the bench. It is modified for a 24mm E engine and includes a custom baffle and recovery protection tube. Included are some graphics and Rocksim files. I have a bunch of build pictures, but have not had time to post them yet. I'm really happy with the way the cardstock reinforced balsa fins came out. It's amazing the amount of strenght a couple sheets of cardstock added to the fins.

Fat Boy Mods
Hey ... looks like I may have stumbled into an interesting project ... I know my friend is only planning for a one time launch ... I may have to get my own set of Fat Boy parts and build one right !

The Ol' Swing test is also a great way of determining stability.

Tie a string around the ready to launch model (motors, wadding and all). At the point that it balances, tape the string solidly. Swing the model around and around. If it sails smoothly in the correct direction without a wobble or spinning end over end, it should be stable.

Keep adding nose weight until it performs correctly, though weigh your model after balancing to make sure you haven't gone over the safe 5:1 thrust to weight ratio....
What is the best way to add the 3.5 oz's of weight to the inside of the plastic nose cone ????
Aerobee. Just get some plasti-clay & weigh out 3.5 ounces. Roll it in a slender string & cram it in the opening. You may have to cut a larger hole somewhere in the base. Use a dowel & mash it against the front of the nose. That should secure it. :)
Originally posted by Aerobee300
Can it be done ??? I'm not doing it ... but a friend of mine is gonna try ! I've bid on a cluster kit from Thrustline for him !

This should be awesome!!:D Please take pics!!
Thanks Darian .... that's what I'll do !!!

Oh don't worry John, I'll take pics !!! ....

next question ... for the ejection charge ... should I just run this thing with 3 booster engines and only one with an ejection charge or what ???
All the motors should have ejection charges.

Don't worry about it.

It'll work just fine.


The Fat Boy is awesome clustered, I have a 4x18mm clustered version that rocks on 4 C6-7's.

If you are going to cluster 4 D12-7's, keep in mind you won't be able to use an axial arrangement (i.e. one center, three around the center spaced 120 degrees), they won't fit in a BT-80.

What you'll have to do, is cluster them in a box formation, which means you'll have some creating design issues with the TTW fins. Maybe just going to surface-mount fins, reinforced with fiberglass or paper at the fin joints, would be a good idea.

Keep the CG at or fore of the CG measured on a standard Fat Boy loaded with a C6-3, and you'll be fine. BUT do NOT use the standard chute! You *MUST* go nylon, here - the stock chute will not hold up. Coat the inside of the BT-80 with epoxy, up to where the nose cone shoulder sits (but no further), so the heat from 4 D ejections doesn't char the tube. And use lots of wadding. ;)

I am contemplating a core "E" + 3 outboard 18mm motors, for my next Fat Boy project.
For the fin attachment issue:

I attach all of my fins using thick CA all along the root edge for primary attachment, then I use Fix-It epoxy clay (available from Apogee www.apogeerockets.com) rolled into thin "snakes" and pressed into the joint for fillets. Works fantastic! I've done a 4 x D12-5 launch on a BT80 rocket with the fins attached this way, with abolutely no sign at all of flexing, cracking, or whatever.

Heck, in my other regular 18mm rockets with the fins attached this way, I've experience a few failed recoveries :)o ) where the rocket came crashing back to earth. Not a single cracked fin. In fact, one rocket (my Teros clone) has had this happen *twice* and no damage, other than a couple of paint chips on the body tube *between* the fins. I can only attribute this to the fins flexing the body tube on impact and chipping the paint that way (you'd have to see the chip to understand fully.)

In my opinion, this fin attachment method would sooner cause the body tube itself to fail than the fin joint itself. And the fix-it clay is extremely easy to work with - no fumes, cleans up with plain tap water (until it sets hard as rock, that is.)

Good luck!
OK OK .... I didn't win the 4 D setup from Thrustline ... soooooo ... I'm gonna chicken out a bit .... I'm just gonna buy a 3 D cluster setup from Thrustline ... should be just about as awesome and will make attaching the TTW fins much easier (and sturdier) ! Plus I just got back from the hobby shop .... dang those 24 mm engines are expensive !!!
Originally posted by Aerobee300
OK OK .... I didn't win the 4 D setup from Thrustline ... soooooo ... I'm gonna chicken out a bit .... I'm just gonna buy a 3 D cluster setup from Thrustline ... should be just about as awesome and will make attaching the TTW fins much easier (and sturdier) ! Plus I just got back from the hobby shop .... dang those 24 mm engines are expensive !!!

A "3-D" Fat Boy will still be one great performer.

p.s. Make up some 18mm - 24mm motor mount adapters, for when you're feeling frugal or the field's just too small!
Why chicken out with 3 D12 when 4 fit perfectly in that ol'e BT-80.. Yeah I know ya gotta break into another pack of motors but man would that thing HONK!
Here's a neat, clean & easy way to retain the four motors in a BT-80. Hope this helps with whichever way you decide to go:D
If you still want to TTW the fins, you'll have to switch to a 4-fin configuration. Of course, then it's no longer a true "Fat Boy", it's some kind of mutated "Fat Boy". The 3 motor config would work well for the 3-finned rocket if you absolutely must keep TTW fins.

As for retention, I'm a big fan of using tape to friction fit the motors. If you can only just barely twist the motor with your fingers when you have it inserted and you are using all the strength you can muster, then you stand a pretty good chance of not kicking the motor upon ejection charge firing.

3 D's it is !!! Just placed my order with Thrustline ... If I get this one to fly right maybe the 4 D job will be a future project !

I should tell Ya all .... this will be my very first cluster engine rocket ... lol ... we'llllllll seeeeeeeeeeeeeeeeeee
OK guys .... I have the Fat Boy 3 X D Motor mount done and have started mocking up the rocket itself ..... Fore Check .... could you redo your figures for nose wieght for me ???? You have it at 3.5oz's for the 4 D's .... how much less for the 3 D's ?

This thing is gonna rock !!!!

At what point do we break out of low-power and can call a rocket mid-power ?
Another note .... what does Estes use to cut fin stock with ??? a club !!! I wish they would either go back to templates and let us do the cutting (like Thrustline) or lazor cut 'em (like Semroc) !!!