Effort chronicled of converting an Estes Baby Bertha rocket kit into a semi-scale representation of a V2 rocket. Semi-scale because I’m not ready to construct the boat-tail rocket body just yet.
Estes Baby Bertha
Original Kit Estes Baby Bertha is not really close to a V-2 rocket in look, beyond the stubby body and a faint similarity in the fins, oh and the number of fins (4).
German V-2 Vergeltungswaffe
As you can see from this diagram the V2 Rocket has a much longer, oglive type of nose section and much shorter fins:
Starting the Modifications
The first order of business is to get as close a representation as possible of the V2 fins. Do do this I’m simply going to scale up a planform drawing of the V-2, to match that of the kit. I measured out overall body length of the modified rocket (with new BT-60 Estes plastic nose cone) to 310mm.
My printer is not quite large enough to print the full 310mm, so I have cut the image at the CL of the rocket body mass (at 157mm). But first I had to calculate how much additional length to the overall rocket was required for the section of fun extending below the fuselage. It turned out to be 3%. So the total overall plan image is is 322mm. However the total image size my printer will handle is 251mm. The simplest thing to do was to cut off the top 71mm from the image and print it.
This up-scaled view was compared to the target size, which matched, and the final fin design was based on this image.
As it turned out, the fin is very close in size and style between the two rockets. In fact the root length is identical.
The new fin plan was laid out on fresh 3/32″ balsa wood ($3.00 at the local hobby shop), and 4 fins were constructed from the pattern (seen here).
Rule: When laying out fins, never run the root of the fin parallel to the grain of the wood. Doing so will seriously compromise the strength of the resulting fin, and possibly cause in flight detachment. It’s not only important to NOT have the grain parallel to the fin root, but every effort should be made to make the parallel to the fin’s leading edge! A recent construction error on my part had the grain running parallel on 2 fins, and perpendicular to leading edge on two others. The two with the perpendicular orientation are VERY FLEXIBLE and might sheer off in flight. So, really, there is only one ‘right’ way to orient the grain of your fins.
Following the cut-out of fins, they were sanded on all sides to equalize size, and straighten cut edges. These are no laser cut fins, this is old school fin construction circa 1970’s.
Once glued to the body, and fin joints suitable filleted, rocket is left to dry for 8 hours.
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Next order of business is construction of the engine mount. The rocket body is much larger than the engine mount itself, so it requires the user of centering rings. These are stamped out of fiber board at the factory. What they charge for a motor mount kit is more than an entire rocket kit (crazy). Going along with the experimental theme of this rocket I have taken it upon my self to partially fabricate the motor mount myself.
The most challenging part is getting just the right diameters on the centering rings. So I have taken a Sharpy and darkened the die-punched rings in original kit, photocopied them, and glued the resulting pattern to some 3/32″ balsa (first photo). The original die-cut rings were marked with the body size (BT-60) and filed for future use. Cutting them out from the balsa, I affixed them to the main motor mount tube (blue) and sanded the edges smooth. Once the glue had setup on the mount, the assembly was installed in the rocket body according to the original specifications (last photo). Inside of the body tube is coated with glue.
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The recovery system was also modified, based on the parachute I found in one of the vintage rockets I have from yesteryear. I recall ‘back in the day’ that a lot of the parachute recovery systems for these rockets had an opening in the top to accelerate decent and to keep drift manageable. I was happy to find that the oldest rocket I have still has it’s parachute and that chute has the original markings for the fast-descent configuration!
I took it’s pattern, transferred it to the new chute and replicated the modification. On a side note, the fast-descent chute I pulled from the old Bull-Pup 12D rocket was installed in my modified payload carrying Taser and it worked great! So I’m confident this will not result in too rapid a descent.
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Application of ‘ Dope‘ sanding sealer on the balsa parts, letting that dry and priming the body in white paint was next.
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It’s first test flight was planned for the 4th, but wind was just too high. Here is the new Rocket next to the rest of the current flying fleet for a little size comparison.
While I was prepping the rocket, painting, sanding what have you, there was nagging issue. Well, two nagging issues:
- The fins looked too large
- Two of the 4 fins seemed excessively flexible.
I pulled out my scale renderings of the rocket, and had a major DOH! moment. While working up the drawings, there was an over sized and properly scaled diagram printed on opposite sides of a sheet of paper. I must have been interrupted because despite all my efforts to create the 2nd, properly scaled layout, I used the older OVER SIZED layout for the rocket fins. The V2 looked more like something Jules Verne drawn, to get to the moon.
Combined with what I deduced was a major construction flaw (the wrong grain direction on the fins), I really had not choice but to CUT off the fins and start that part of the project over. So, I did just that.
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A couple of hours of work went into removing the old fins, creating the new fins, cutting off all the old material, filing, sanding and installing the new fins. Almost back to square one, but I have the right size fins and the right grain direction on all the fins. I feel much better about flying this rocket in the next few days, than the previous incarnation.
After hours of additional filling, sanding and repair the fist of the yellow base coats were applied. Again the Valspar pain (about $4.00 a can) resulted in a very nice smooth color. This photo was after only a single coat. the final coat really looked great.
Video of first test launch. You can see the nice slow liftoff. This was with a small A-class engine so altitude is not very high.
A couple of days later the final filling and sanding was completed, yellow base coat applied and ready for application of the test pattern masks.
And here is the final result. After a day of the paint drying, mask removed and we have the DD05 V2 semi-scale rocket model.
It was modeled after this actual rocket, seen here during testing at White Sands New Mexico.
Here is another on display at the Museum in White Sands.
Construction History
29-Sep-2009
Construction started. Fin design finalized. Fins cut. BT-60 body tube marked for fins.
30-Sep-2009
Fins sanded to size. Fin markings on rocket body compared to V-2 side elevation for positioning. Fins installed and filleted.
5-Oct-2009
Primed and ready for a test flight. Final color schemed undecided. However a potential construction error on my part might ground the rocket due to incorrect balsa grain direction on 2 of the 4 fins.
6-Oct-2009
REBUILDING FINS: As much as I did not want too, I cut off the incorrectly built fins. I took photos that show how I botched the first ones. Body needs to be sanded, new fins made and attached.
7-Oct-2009
Priming, sanding, priming, sanding. Repairing damage to the main body tube from removal of the larger fins. Also used a commercial sanding sealer (really nasty stuff) on the balsa fins. It adds weight but it provides a nicer finish in return.
8-Oct-2009
More sanding and priming of the body. It’s just about ready for it’s final paint job, and I’ve finally decided on the scheme. A post WWII test pattern used on captured V2 rockets tested at the White Sands facility. I’m still tempted to paint in the ragged desert camo, but I also want to be able to find the thing when it lands!
10-Oct-2009
First coats of the base body color applied. First 2 test flights performed with great success. It’s a very nice flying rocket. It’s additional weight gives it a slightly slower and more realistic takeoff. The size of the rocket body also makes it easier to track in the sky.
13-Oct-2009
Project Completed! Final sanding, base and test pattern applied.