Making a better Mega Der Red Max

[paul.nortness]

The recent thread about the Mega Der Red Max and the projected altitude got the wheels spinning. I've been thinking about this for a couple weeks now....how to get better performance out of the Mega Max.

One of the biggest problems was the fins. THEY ARE THICK! With a sheet of 1/8th thick balsa on each side of the 1/8" plywood core, you are looking at a super fat fin....So, I went with a 1/4 inch solid fin cut from birch ply.

The other problem is the power. Being a big 4 inch rocket, it's kind of gutless at 29mm. So....I added a couple of motor tubes, giving it 3 total. They are configured in a straight line rather than a triangle. That way it can be flown with 1, 2 or 3 motors without needing interchangeable mounts and still have balance.

The only downside currently is the clustered motor mount makes through the wall fin mounting troublesome. Some creative thinking should solve that though.

The results are pretty impressive. Granted, it's only in simulation.....but I intend on making this creation a reality once I solve the fin tab issue and doing some real world testing

View attachment Mega Der Red Max with balls.ork

 

[powderburner]

One of the biggest problems was the fins. THEY ARE THICK! With a sheet of 1/8th thick balsa on each side of the 1/8" plywood core, you are looking at a super fat fin....So, I went with a 1/4 inch solid fin cut from birch ply.

That "original" thickness of 3/8 inch would not hurt you so badly if you rounded the leading edge and tapered the balsa portion of the trailing edges to get a TE thickness of 1/8 (or so; 1/16 would be a teeny bit better)

The added weight of your 1/4 inch of plywood will likely hurt considerably more than the 3/8 thickness. Also, if you leave the edges of the plywood square, the increased drag will be worse than a thicker fin with a suitable airfoil (a double whammy in the wrong direction).

If you make any sort of changes to the fins I would suggest a solid core of 3/16 or 1/4 inch medium density balsa, with a round LE shape and a TE tapered down to about 1/16th, then with skins of either 1/64th aircraft plywood bonded on or else a wrap of lightweight (four to six ounce) fiberglass. This gets the material strength and stiffness into the part of the fins where it is needed, still reduces the thickness a bit, and should not add significant weight if you do a clean job of adding those skins.

 

[rocketgeek101]

So....I added a couple of motor tubes, giving it 3 total. They are configured in a straight line rather than a triangle. That way it can be flown with 1, 2 or 3 motors without needing interchangeable mounts and still have balance.

Unless I am mistaken; you will not be able to fly it on 2 motors only, as the thrust will not be centered in relation to the rocket.

 

[paul.nortness]

Unless I am mistaken; you will not be able to fly it on 2 motors only, as the thrust will not be centered in relation to the rocket.

I have a BT-80 based rocket that has 3 18mm mounts in line like this...it flies just fine with an empty motor tube in the center and two outboard motors.

 

[rocketgeek101]

I have a BT-80 based rocket that has 3 18mm mounts in line like this...it flies just fine with an empty motor tube in the center and two outboard motors.

Duh! Brain fart! My bad... sorry.

 

[DizWolf]

make an inner tube to house the 3 29mm and provide a uniform distance for the fin tabs? (a 3.63 ID doesn't leave much room of course)

 

[Devontmp]

Why not go 38 or 54?:confused2:

 

[DizWolf]

no MPR SU in 38 or 54?, clusters are fun, clusters are fun, clusters are fun?

 

[JumpJet]

I'm pretty sure the wood sheeting that goes over the inner ply fin is only 3/32".

John Boren

 

[paul.nortness]

The total thickness of the fins is 3/8"

I didn't go 38 or 54 because that was the obvious solution.....I like to look outside the box...and because I can

 

[JumpJet]

The total thickness of the fins is 3/8"

For those who like to be exact about things.

The plywood center fin is .102” thick while the outer 3/32” sheeting is .09375” thick nominally, which gives you .2895” thick fin. Since there are plus and minus tolerances to the wood you can see it a little thicker or thinner, plus the glue may add some thickness to the stack as well. The Mega Max I have sitting in my office built from kit parts have fins right around .305” thick. So as you can see there is NO way the fins are 3/8” or .375 thick unless you have replaced all the wood that comes in the kit with other wood.

It is possible a kit may contain .125" thick sheeting instead of the .09375" sheeting that I specified for this kit but your fins would still only be .352" thick and not .375" as stated. Now if you decide to cover the fins with paper or thin card stock or fiberglass cloth then your Non-Stock fins can be any thinkness you wish them to be.

So for those who would like to do a flight simulation on this model with stock fins that come in the kit you can use a fin thickness around .310” to .315” for a closer estimate on what a primed and painted fin thickness might be on this model instead of .375” thick.


John Boren

 

[dpower]

This sounds like an interesting mod. I wonder if a 4 cluster would be easier to design? Same central 29, with an additional 29 between each fin tab. This eliminates the 2 cluster, but allows for a 1, 3, or 4.

Of course, a stock build will take up to a 29/360 case, which is the most motor I'd ever want to put in mine!

 

[sodmeister]

Hmmm ,I read this thread and just had to measure mine using two different electronic calipers.

I get the same exact readings from both , 7.90mm / .311" / 5/16"

MAN....I wish it would warm up so I could paint it and fly it !!


Paul t