View Full Version : CG and the Merc Redstone K-41

05-07-2009, 01:40 AM
Pardon my ignorance here, but I am at a quandary. I just restored Redstone K-41 from days back. The first and only launch sent it went head over heels about 20 years ago. I believe the CG was off, even though I thought I followed the directions explicitly. . I have read that placing some cross members half way down the tube helps keep the insulation and chutes from changing the CG too far aft. I tried balancing it with the two nose weights as in original build and noted its nose heavy even with a C6-3. My question is should I remove the nose weight(s) or add some around the engine area?? Or am I barking up the wrong tree and need to address something else? What is the measurement point for CG from the tail anyway if that is easily known? :confused:


05-07-2009, 08:45 AM
The original Estes K-41 Mercury redstone requires two of the standard NC-1 leadweights to be on the screw eye that is screwed into the capsule base for a proper CG.
Without them it will NOT fly in a stable manner.
I'm pretty sure these weights are required due to the heavy, complex, and relatively small fins at the aft end of the tube.
Replacements can be obtained from Semroc.
For "safety's" sake I use 3 of these on mine and it flies great.

See original instructions here:

05-07-2009, 09:19 AM
Yes, you need those nose weights to keep it stable. Mine (nearly 40 years old) still flies
great!! :D

05-07-2009, 11:02 AM
On mine, I added a piston, made of a cardboard disc and 3 small diameter dowels, to keep the chutes up near the front of the airframe. I also reinforced the capsule with Aeropoxy Lite epoxy filler and to help secure a small brass tube. It was a good thing I did because once (I forgot how) the capsule chute got separated and Freedom 7 came in ballistic. The capsule was fine but a little dirty (it actually came in blunt nose, just like reentry!) and the LES only required minor repair.


05-07-2009, 11:49 AM
If I am reading from what you are all saying the nose is supposed to be heavy?? It is still not been made clear as to where the supposed correct CG is on the K-41 or if I need to be concerned about checking CG with or without an engine. When I built the original one I did use both nose weights and it went up and nosed in. I used the Semroc fin kit this go around, so I feel I was on the right track using the oversized set. To get this right I really need to know how many cms from the tail I should find the CG if I am using two lead disks and an engine if necessary to balance it. Can anyone check this out with their K-41s? If some of you could get this measurement to me so I have a few data points it might really help me nail this issue down.

Thanks again,

05-07-2009, 12:12 PM
Now this is a guess, so take it for what it is worth. I estimate that the CP is about 5 inches (give or take, so somebody with RockSim or VCP jump in) from the bottom. For the MR, you want the CG to be about 2 airframe diameters in front, so about 9 inches. My Estes kit was 2167 with the airframe OD at 2.047 inch. It is best to determine your CG in its launch configuration, which means the motor and chutes are in.


05-07-2009, 06:30 PM
I nearly had a similar situation with my original Centuri vintage Mercury Redstone (a superb
kit, I might add). Built it stock as per instructions, and the first two flights showed quite a bit
of wobble, especially after burnout. After the second flight, I added a nickle (you know, the
coin you can get out of your pocket) to the Mercury capsule (using CA) and flew it again, and
this time it flew perfectly with no wobble during flight. John Pursley had told me (after I mentioned the first couple of launches) that the Mercury Redstone is marginally stable and
does need corrective action on the part of the modeller.

05-08-2009, 08:22 AM
That "oversized" fin set as you call it from Semroc is NOT oversized if you use it for the right scale rocket.
The Semroc fin set was NOT for the BT-60 1.637" diameter based Estes K-41 Redstone, it is for the Centuri Redstone that has a 2" tube !!!!

05-08-2009, 07:35 PM
Not to split hairs here, but if that is the case, then the Semroc fins are oversized for my MR and that re-enforces what was already stated myslef and others. I have the Estes body (1.627"dia) and not the Centuri version. The logic would infer if the body was larger then the fins would follow suite I would think if kept to scale. I compared the fin cut outs for the K-41 to the ones I am using now and they are about twice as large in size and extend more into the air stream. I find that to be a benefit and am satisfied I chose to go this direction. I attached 2 lead disks to the capsule as per the original specs.

I want to thank you all again for the input. I post how the launch goes. I plan to make the attempt either 5/13 or 5/17. :)

05-10-2009, 02:49 PM
I want to give you all a follow-up. I launched the MR on 5/09/09 using a B6-2. It went about as straight up as I could have ever hoped or imagined possible. Both chutes deployed and we caught both before landing on the same soccer field. Looks like all the input was helpful gents. Thank You.

05-10-2009, 03:15 PM
Nice build you have there; great job on that hard to assemble tower and fins!