BT 60 Optima downscale

I know, I know. Another Optima. What can I say, I like the design

I wound up with 2 vinyl logos that were perfect size for BT60. Rather than let them go to waste, I HAD to build rockets for them. The best way I figured to get all the parts I needed was to raid the parts box, and grab a Mean Machine kit.

The Optima is 47.5 inches tall, and 2.6 inches wide. Since BT60 is 1.64", I went with a height of about 30". Two optimas therefore only required a total of 3 tubes. I used the 4th to make a "Stumpy Machine"

I made kevlar harnesses and wrapped them around the top of the MMTs, then glued them into place. The kevlar is about an inch shorter than the body, and 48" elastic cords are tied to the ends.


Fins were cut from 1/8 plywood, stacked together and sanded.



This next photo shows the rocket in the fin jig, and the downscale fin conversions. I have a set of Semroc reproduction fins in full size. I measured them, and then converted the fractions to decimal. Then I calculated the downscale. Once I had those numbers I converted back to fractions using this chart: https://www.seoconsultants.com/charts/inches-decimal/ It's not exact, but it's close enough for government work. There's probably an easier way to do this, but it worked for me. Once I had the numbers, I used the full size set of fins to trace the angles.




And all the fins drying up:

Rocksim file. Shows it should Fly on C6-3, C11-3, and D12-5 just fine. E9-6 should push it just over 1,000 feet

View attachment down_estes_optima.rkt

I saw your MM teaser earlier..

So, clockwise from the top of the notepad sketch: 3.23, 3.15, 2.95, 1.57 : 2.64, 0.35, 3.15, 0.709, 2.64. All in decimal inches.
Am I reading that correctly?
Can I also get the space between the upper and lower fins?

And.. what nose cone?

I'm subscribing to this thread; I think I might make one.

Correct on the measurements. The fractions I went with are below.

The nosecones came from the Estes bt60 4 pack. It had two black ones, and two white round ones.

I also picked up the new Estes motor converters for this.

Thanks,
Do you know the length of the NC from the shoulder to tip of either the 60 or the original size?

Original is 8 3/16. It's the same as the Phoenix, shadow, and executioner.

The nc60 is 4 3/4"

5.16" would be scale. It's a touch off but it'll work for me. I may look around and find a proper NC.

Edit: forgot the space between. I haven't measured it, but I put the upper fins right at the body tube joint. Looked good so I went with it.

No new photos, but tonight I got a little work done.

On #1, I added the 48" shock cord, and a 14" LOC nylon chute off my Onyx (she got bigger recently...needed new laundry

on #2, I decided to use the fin jig to pencil on some guide marks for fin placement. I used the base of those marks to make straight lines the length of the tube. This was done with the classic "place the rocket in your doorjam" technique. My girlfriend builds too, so the pencil lines on the doorway are cool

With the lines drawn, it was much easier to line up the fin sets. I also added 1/4" lugs to both rockets. I'll get some more photos up once I get a decent day for painting, and cut up some vinyl for the rings. One is going to be stock colors. The other, nothing is planned other than the "optima" logo is in white. I'm not sure exactly which way to go with it. Many ideas are fighting in my head

On to my least favorite task. Filling.



I don't dilute the wood filler, I just slap it on, and rub it in with my fingers. After everything is covered, including myself, I scrap off as much as I can with a Popsicle stick. Once everything dries, I'll sand it down with the 3M sponges in the photo. Then prime and paint.

Just realized I never posted the finished photos:

Awesome build thread, as usual DizWolf. What camera do you use, the pictures look great!

Alex

qquake2k said:

That turned out awesome! I'm working on a 5.5" Level 2 upscale of the Shadow, I hope it turns out half as nice.

Click to expand...

Thanks guys! My god, a 5.5 is going to be huge. My fast drunk math puts that at about 8 foot tall? Gonna be a fun one

Aksrockets said:

Awesome build thread, as usual DizWolf. What camera do you use, the pictures look great!
Alex

Click to expand...

Canon 7D. Took these in the dark with a flash so the lighting stinks. Gonna do an Optima family photo soon with some proper lighting. Just gotta finish the three I'm working on EDIT: check that... four.... forgot about the semroc mini

some in progress painting shots I also failed to post:



not perfect but workable. I tried to use some testors one coat lacquer on it. Realized it was going to take way too many white coats, so I swapped over to some krylon gloss.

DizWolf said:

Thanks guys! My god, a 5.5 is going to be huge. My fast drunk math puts that at about 8 foot tall? Gonna be a fun one

Click to expand...

You are correct, sir! I'm actually going to make it 95" tall, so it'll stand up inside the house.

I've never seen plywood fins on something so small.

The way I measure for rescales is to take an actual size scan/printout of the fins (black lines), draw in square boundries around the fin (red lines) then measure how far each corner is in terms of XY with my caliper (blue lines). It's then really easy to punch those numbers into openrocket, go into the rescale menu to resize it, then print out templates for the new size. I did do the angle method once, but only because I was out of printer ink.

I'm in the planning stages of a 4 inch optima. 73 inches long. Everything programmed into openrocket, still looking at engine choices and deciding on deployment options.

Sparkytfl said:

I've never seen plywood fins on something so small.

The way I measure for rescales is to take an actual size scan/printout of the fins (black lines), draw in square boundries around the fin (red lines) then measure how far each corner is in terms of XY with my caliper (blue lines). It's then really easy to punch those numbers into openrocket, go into the rescale menu to resize it, then print out templates for the new size. I did do the angle method once, but only because I was out of printer ink.

I'm in the planning stages of a 4 inch optima. 73 inches long. Everything programmed into openrocket, still looking at engine choices and deciding on deployment options.

Click to expand...

I've used 1/8" plywood for fins on pretty small rockets. If I'm worried about weight, I'll use basswood. Open Rocket has a feature to scale fins up or down for you. In the fin configuration window, click on the Shape tab, then click on Scale Fin. Sure has made my life a lot easier. The attached file are the fins sized for 4" tube.

View attachment Optima_4in_fins.pdf

Another tool, together with graphics to view what's happening

https://www.rocketryforumarchive.com/showthread.php?t=2175

BTW, when I downscaled the Estes bird I called it the 'Sub-Optimum'

qquake2k said:

The attached file are the fins sized for 4" tube.

Click to expand...

Looks very close to what I have, but your front fin seems a bit more angled at the leading edge than mine, and your side edge of the main fin is a bit more or a bit less (dont remember which) parallel to the body than mine. My original measurements in terms of (x, y) coordinates from fin scans were (0,0) (5.085, 0) (1.140, 5.005) (3.580, 4.475) for the main fins and (0, 0) (5.070, 0) (.290, .520) (4.495, .975) for the front fins. I put those into openrocket and scaled the way you said. But I dont intend to derail this thread, you can count on there being a build thread when the time comes, probably this winter.

View attachment optima4in.ork

Sparkytfl said:

Looks very close to what I have, but your front fin seems a bit more angled at the leading edge than mine, and your side edge of the main fin is a bit more or a bit less (dont remember which) parallel to the body than mine. My original measurements in terms of (x, y) coordinates from fin scans were (0,0) (5.085, 0) (1.140, 5.005) (3.580, 4.475) for the main fins and (0, 0) (5.070, 0) (.290, .520) (4.495, .975) for the front fins. I put those into openrocket and scaled the way you said. But I dont intend to derail this thread, you can count on there being a build thread when the time comes, probably this winter.

Click to expand...

The fin shape inconsistency probably has to do with the Open Rocket file I used. I don't remember where I got it, probably from Rocket Reviews.

I calculated the new measurements, then used a full size fin from Semroc to set the angles.

DizWolf said:

I calculated the new measurements, then used a full size fin from Semroc to set the angles.

Click to expand...

I should compare yours to the fins on my 5.5" upscale. I bet yours are more accurate.

sorry I didnt setup for scale shots, but here's some comparisons. It's pretty close. To be honest, I've built 6 optimas, and I've got kits/parts for three more. I don't think any two of my forward strakes match after sanding. If you're interested in running the numbers, my notepad is in a photo in the OP with all the side lengths. (or bigger here, easier to read: https://farm7.staticflickr.com/6051/6320614786_a4709937ac_b.jpg) I can grab angles with a cheap protractor if you like.

I love seeing classic designs re-scaled. Your rockets really turned out nice, good job man.

Not sure how accurate I made that file. I may have gone "close enough for govt work" I drew out the fins themselves by hand and using the full size set.