Estes Magnum BT-70 upscale

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Oct 15, 2015
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Magnum time again! I wanted to build another Magnum but this time a upscale. The hard part of upscaling this model is finding a 5:1 ogive nose cone. I found a BT-80 and a BT-70 one. If I went with a 2.6" airframe then it would have been heavy enough to need composites and staging composites is something I don't want to get into yet. For this one, I thought if I went with a 2.2" BT-70, then it would still be light enough to use a 29mm E16 or F15 in the booster with a 24mm E12 or D12 in the sustainer. That would also keep with the original concept of staging to a smaller dia motor.

For the fins, I decided to use balsa, but maybe just a bit more firm. I took two sheets of 1/16 balsa and sandwiched some 2oz. fiberglass between. I also papered the outside using thinned white glue. I think if I did this again, I would have used maybe 6oz glass between. The 2oz didn't add as much strength as I had hoped, combined with the papering, they were firmed up enough though. I tested a bit of extra material and one thing to note with the 2oz fiberglass layer in between is that if a fin snaps, it will stay attached and maybe flop around rather than fly off somewhere. Much like windshield glass, it will stay intact if broken.

Then I decided to go ahead and assemble all of the parts that I would need so that I could progress as if it were a kit.

So for the nose block, I 3D printed a BT-70 upscaled version of the original part in blue. This is how it looked just after printing.

The blue color wasn't a match for my paint and the surface was a bit rough as it was a low-res 0.3mm print. After sand/prime/sand.../paint I got a smooth enough finish and then ran a loop of kevlar cord around as the attachment point since the original 3D printed loop was super sketchy.

Then I measured and drew lines for the fins and cut out fin tab slots. Double-checked the alignment after cutting since when cutting by hand, being a bit off a bit can cant the fin.

Here's a quick dry fit of the fins to check spacing. Doesn't get much closer than that...

Next I'll be epoxying in the front motor mount with 3" of spare BT-70 coupler ahead of it to help strengthen the airframe. IIRC, the point just forward of the fins is the most stressed due to torque from the fins as well as ejection charges.
Looks great!

Just be careful to vent the 29 mm booster. I had issues with 29-->24 mm staging, even though it wasn't gap-staged) until I figured out that I needed to vent it.
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1/4" should be fine. I just cut grooves in to the 29 mm casing to make a vent. My other suggestion would be to use a 1/4" launch lug or rail buttons. I used the stock launch lugs on mine and realized that E16 + 3/16" launch lug = rod whip.

I like your idea for papered balsa-FG-balsa fin material. What adhesive did you use for the FG in the middle?
Staging a 29mm to a 24mm means you can't tape them together or recess one into the other like you can with 24 to 18mm.

So, as discovered in this thread: Magnum II There may be a need for a small gap between the two or at least a way to vent the air.

Considering that one small goal for this build was to not need friction-fit(the one thing I hate about staging), I tried to think of a way to have a small(3/16" or so) gap between the engines and keep it easy to load.

After a few hours of dry-fitting and measuring profusely, I came up with something that I think might work. The 24mm sustainer will be held in with a motor hook, now that there's room for that. The 29mm booster will be held in from behind(nozzle end) by a motor block, the same way it's done on an Estes Air Commander. To keep the 29mm booster from thrusting forward and closing the gap, I thought of placing three small dowels around the sustainer MMT to hold it in place. This way, they can have a slight gap while being super easy to load.

Here are the parts for both MMTs:

Here is how the sustainer will look. The dowels will keep about a 3/16" or 5mm gap between the two.

And here is a close-up using empty casings to demonstrate how it all lines up.

Will this work? I'm mostly confident it will but still going to be a "heads up!" on the first launch.
Chris, I'll be following your progress on this, and anxious to hear how your maiden flight goes. I bought some 29mm Estes motors when they first came out, with the intention of eventually trying staging. My club location does not have the area I think is needed for using a 29 mm E or F upper stage, so I'm thinking a 24mm upper stage is the way to go for me also.

I'll be especially interested to see how your staging methods work.

Wishing you much luck!
I like what I've seen so far... :grin:

I'll be watching this one with interest. :pop:
3/16 gap should be fine.

I am assuming there will be some "nesting" or other coupling of the outer shell body tubes of both booster and sustainer. As others have posted, make sure you have a VENT (or vents) that will depressurize the space between the booster and sustainer so they don't "blow" apart prior to sustainer ignition.
I am not aware of any "science" on size and positions of the holes, but on something like this I might use 3 "paper punch" size holes equally spaced around the circumference

Options include holes in the sides of joined section (if nested, make sure the holes mate up between segments). Another method, which is both more aerodynamic AND cosmetic if your booster is NOT minimum diameter (i.e., booster has motor mount with centering rings) is to either build it with holes in the centering rings, or drill holes in the centering rings after the build, to vent the gases backward out the tail end of the sustainer.
Great idea! My only concern is to make sure the booster motor's thrust is directly transferred to the MMT of the sustainer via the dowels.
The main fins are on with internal epoxy fillets as well as the rear CR. This part went well, everything lined up great. 15-minute epoxy is the sweet-spot for this part. I'll do the external fillets later on once I have the booster built.

Following along also. Great stuff so far, and providing good ideas for an Estes Guardian upscale with a booster. :pop:
While not being 100% satisfied with the dowel supports being the only mechanism for holding the booster motor in place, I started to think of alternatives and thought: Why not add a "reverse" motor hook?

The dowel supports should be fine for the low thrust of the Estes 29mm BP motors. But what about after a few flights? They might get a bit cooked and weakened, being wood right next to the flame and all. With a reverse motor hook at the front, the motor will be held in place should the dowels fail.

I bent one end of a motor hook around so that it would lay flat and hook around a band of fiberglass. Two wraps of 5oz and it won't go anywhere. I'll just have to sand the root of the fin tabs on one side to adjust for the height of the fiberglass band.

I also replaced the motor block on the bottom with a low-profile one to make it easier to fit those Estes starters properly.

I suppose with the motor hook, I could omit the dowel supports. I would however prefer to have the upward thrust transferred to the upper stage directly through the CR rather than through the airframe, which would be the case with a hook only or friction fit setup. Either way, once this has a couple flights on it, it will be more apparent what is needed and what is not.
Getting there... Booster has MMT in place with fins. Using forward fins for alignment. Once internal mini-fillets are in then just the rear two CRs.

Here's the original BT-60 on the right for size comparison.

Lots of progress, nearly complete!

Here is a closeup of the clear payload bay(BT-70 clear tubing from Apogee) where I tied short section of 220# kevlar between the nose cone and nose block, just as extra insurance that the nose cone and payload contents don't go flying off in case of a hard eject. I did the same thing with my original Magnum.

I had the bright idea of using a few wraps of double-sided tape for attaching the clear payload bay to the nose block and thought it would look clean. It didn't. I'll try another way next time. Good thing the lower part of the payload bay gets painted over.

The recovery system was done with stock PS-II parts(shock cord, teabag mount, parachute) as this will be sufficient for this size rocket and the motors that will be in it.

1/4" launch lugs were the last part to go on. As soon as I lay down some modest fillets on the fins, it will be physically ready to fly! I'll probably hold off painting it until it gets at least one successful flight. Here it is along side my well-travelled original size Magnum.

Once it earns it's paint, it will be painted with the stock color scheme. Why not different color this time? Well, I have some background plans for a red-and-black version dubbed "Magnus".

Good chance I'll be able to launch this on an E16/D12 combo this Saturday. If conditions are unfavorable for staged rockets though, it might have to wait until May. If that's the case then I'll go ahead and paint it anyway.
I love this. The BT70 size is just perfect for this rocket! (Until I get around to a 3" version)
A worthy upgrade I'd say. :wink: I look forward to a flight report and then some paint.
So last Saturday I had it ready to go however it was way too windy to try. One rocket nearly went cruise missile from weathercocking so bad. The next, my Cherokee D clone, caught a thermal and was blown up up and away out of sight, like a lost balloon. No way I'm launching a low thrust 2-stager.

So now its in primer, waiting to dry fully so I can start sanding away before the white base coat goes on. Patiently waiting for the next launch window.
Paint on the top part is complete and the rest is in it's final coat of white. Next is masking the fins for the blue/silver parts. I'm going to mask and paint the upper decal same as I did with my regular Magnum. For the fin decals, I'm going to do those as printed paper glued on and clear coated over.

Finished painting the booster. The silver came out nice this time. Also painted the remaining blue parts including half the upper "decal". Next is to mask off the silver lines and then do the fin decals.

Aaaaand complete. Hard to spend much time working on rockets since I've been working 55-hour weeks lately but I managed to finish this off last night. The bottom decals turned out OK. The blue was a bit lighter than the paint, but I'm not redoing it to get it perfect. The blue is close enough. I might give it a coat of clear as long as I know it's not going to eat the paint.

47 inches tall. It's a beauty!

Maiden launch might be next week if the weather stays dry enough for our club's field to drain.
Fail. Tried on an E16/C11 combo today. Great boost but 2nd stage failed to light. Lawn dart! Upper section survived but the body tube is in a million pieces. Back to the drawing board?
That sucks! Did the stages push apart but the sustainer not light? That's what happened to me.
Man, I'm sorry to hear what happened to it. I hope you can work out what went wrong and get it going again?

Did you forget to vent the booster? By allowing gasses to vent at the staging event, you reduce the likelihood that the booster will try popping off prematurely.