New project - BT70 upscale Advanced Target Drone

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EXPjawa

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After building my clone of the Estes Advanced Target Drone and then finding that Excelsior prints decals for a BT70 upscale version, I decided to add that to my list of projects. That's now cycled round to the top of the list. So, first things first, I made an upscale design using RockSim (View attachment ATD70-1.pdf) to get the bugs worked out, make sure CP/CG was suitable, etc. The regular ATD uses cardstock that's about .040" thick for the bodywork, but for this I elected to use 1/16" aircraft plywood. My intent is to make seperate panels for the horizontal and vertical surfaces, then glue them together. Fin templates were made from the RockSim patterns:
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Then, all the basic parts were gathered up:
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The parts list as such is as follows:
  • 2 18" BT70 tubes
  • 1 segment of BT50 to make the motor mounts from (a scrap that I had)
  • 1 BNC70G nose cone
  • 3 CR5070W centering rings
  • 1 BT70 baffle kit to serve as tube coupler
  • 1 30" Top Flite parachute
  • a sheet of 1/8" balsa
  • a sheet of 1/16" light ply
  • a partial sheet of 1/8" light ply
  • 1/4" launch lug
  • 3/8" dowel for the nose cone
  • there will also be some misc hardware that hasn't been gathered up yet

The BNC70G isn't quite the right shape; ideally it would be more parabolic rather than ogive, but its pretty close. The dorsal & ventral fins will be made of balsa, the wings themselves will be plywood. Note that I said motor mounts - the design I came up with uses triple 24mm mounts in a linear cluster. There is an E-sized mount in the main tube and will have a D-length mount in each body pod. From the RockSim simulations, it should fly well enough on 3 D12s, or combinations of an E9/12 with 2 C11s or D12s. The RockSim file is attached. One catch for the short term is regarding the decals - as long as Excelsior's printer is down, I'll have to hold off on finishing. But that's down the road, I'm not going to worry about it right now...
 
Well, the fins have all been marked out:
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Cutting out the balsa fins was easy enough with an Xacto knife, but plywood parts less so. I cut out the body work pieces from the 1/16" sheet that way, which took a lot of effort:
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However, I think I'll take the 1/8" piece over to my folks' house and use my father's bandsaw for that. In the meantime, I decided that the balsa fins had more flex to them than I liked, so I papered them:
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It always impresses me how much of difference that actually makes. The next step (aside from cutting out the wings), I suppose is to trim the forward tube to length and slot the aft tube for the fins. Then I can start dry fitting parts together...
 
Over the weekend, I got a few things done. First off, I went into Michael's on friday, since I had a coupon and it was next door to Trader Joes (where I was going anyway). They have a disappointing selection of engines these days, just A8-3, B6-4 & C6-5, all of which I have plenty of. They used to have 24mm motors as recently as Christmas time, but it didn't look like they had spots for them anymore. Anyway, I did buy some balsa, some basswood sheets, and a razor saw (finally). The later turned out to be a much tool for cutting the wings out of the 1/8" ply sheet:
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If the tabs look odd, there is a reason. I trimmed and slotted the body tubes:
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And then moved on to the tubes for the motor mounts:
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I found that the centering rings were a little loose, so I used small bits of masking tape to shim the fit. I kept a good amount of the base tube exposed, though, for gluing purposes. The centering rings and mount tube were marked out for notching and alignment for the TTW fin tabs, and the mount was assembled:
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This was test fit as-shown, the notches were cut out before final gluing. Then corresponding notches were marked and cut on the fins. That got me to a point were I could dry fit everything together:
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And then with an assist from the masking tape:
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It starts to take shape! I'm always surprised by how big upscales seem (even minor ones) at first, because I'm used to the normal version. To see it all together initially is always impressive, even though I've been playing with the individual parts for a while and know how big the bits are. The whole is still sort of more than the sum of the parts. Anyway, it was then broken back down and back in its box so I can clean the mess I made... I think next is installing the main mount and assembling the baffle. Then I'll be able to tube sections together.
 
I recently made a bit of progress with this project. The fins and motor mounts have all been installed, as has the baffle that doubles as a tube joiner. The forward section of body tube has not yet, however, been attached. Since the outboard motor mounts are set into notches in the wings, the wing bisects the forward opening and would be exposed to the ejection charge blast. So, I layered aluminum foil tape in the exposed area to act as a heat shield. It seems like I ought to also add some to the sides of the main body tube and the inside surfaces of the pod structure around the blast area as well. The intent is that the charges will simple blow forward through the simulated air ducts and into open air, with only the charge from the center mount being used for ejection. That way, it can be flown on a single center engine, or with all three. I plan to epoxy fillet the outboard mounts to further reinforce them before closing up the pod structure.

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Wouldn't it be easier to remove the ejection charges from the external motors? All motors except black powder can.
Scott
 
Guess it would help if I read your full thread, looks like the external motors are black powder. My bad.
Aerotech does make a 18 mm reload case that you could remove the charge from, in my defense, if it helps :)
 
It would also be easier if I could get plugged 24mm motors. In this case, all three mounts are 24mm, and all would be black powder if flown as a cluster. The local club I belong to doesn't allow clustering with composite engines, so to fly with all three burning, they'd have to be BP. I'm picturing flying with one E9 and two C11s or somesuch. I expect I'd be tempted to try three E9s together sooner or later, though the outboard mounts are sized for 2.75" long casings, and E engines would stick back pretty far. I think the first flight will probably be on a single E30-4, however. I don't think that a single E9 would have the punch to get it off the pad, and an E12 would be borderline.
 
Its a little too soon to say with certainty. The RockSim model makes it just shy of 10 ounces. I'd expect that to be more like 11 and change with glue & paint, maybe 12. According to the model, it would need an exceedingly long rail to get to stable speed on a single E9. I didn't model it on an E12, though, as I don't have the file for that. An E30 lifts it readily, though.

Anyway, last night, I started attaching the engine housing body panels. Just the top and sides so far, I dry fit them into place and marked where the side panels aligned on the wings, then tacked them on with CA glue. Then I could set the top panels in place with Titebond. I should be able to attach the lower panels tonight.
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I haven't updated in a while, but since the last one, I've installed the remaining body shroud panels. It turned out to be a little trickier than expected to get them all aligned, but they've been joined, filleted, and sanded. Here's a (crappy) photo of it posing with the standard-sized Advanced Target Drone:
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I wound up using the Dremel with a drum sander to smooth out one of the joints, where there was a bit of overlap. But now its all blended and filled; yesterday I sprayed a coat of primer.
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The other thing did yesterday was drill the cone for the nose weights needed. As you might guess, three largish BP engines in the rear makes stability a potential challenge. This design requires 3/4" once of ballast in the middle of the cone. I also tucked 1/4" lugs into one of the body shrouds, much like the original design did. But I'd really like to use mini buttons on this, I'm just not sure where I can get away with putting them that will work but won't look funny. TBD. I think I'd like to actually get the airframe buttoned up before I do more priming and sanding. Right now, the forward section hasn't been attached so that I have access to attach the recovery harness. I have yet to order shock cord materials needed, so that more or less puts this on pause for the moment.
 
This is looking real good. Any concern that your side motors are slightly off center (they rest on top of the "wings")?
 
They don't rest on top of the wings, they're on centerline:
Advanced Target Drone BT70_cluster2.jpg

The actual concern is that the wings bisect the burn through / ejection path of the outboard motors. If I could get plugged 24mm motors, it would be perfect. But short of that, well, that's why I lined the wing edge and surfaces with foil tape and a coat of epoxy, to form a heat shield. I'll have to pay attention to how it holds up. I'm hoping that burning -0 motors in those positions will be easier on it than motors with ejection charges. The design of the Advanced Target Drone seems to suggest that some sort of motor ought to be in those shrouds, that's why I went that way. But it does lead to some complications.

To the original question - the asymmetric thrust concern was why I've elected not to launch my BT60 Hawkeye in a cluster configuration. I built the turbines to accommodate 13mm engines - more for effect than thrust - but I did not build any angle of incidence into them to counter the asymmetry. A pair of 1/2As that are 3/4" off center might not do much to upset the flight path, given that model's large wing/fin area, but it might be enough to arc it over into the corn field. Rocksim doesn't seem capable of knowing, but logic dictates that there must be at some influence on flight path. I put enough time and detail into the build that I don't want to risk it unnecessarily. So, I didn't make the same mistake in designing this model. I'm sure I made new and unique mistakes instead...
 
Well, a little more progress: I drilled the nose cone to install the nose weight ballast. I put a 1/2" bore down 3.5" deep, which let me push 3/8oz weights into where Rocksim says the weight needs to be. I wanted to push them as far forward as practical so that the amount of weight was minimized. The fit was snug, but the epoxy acted as a lubricant when it was still fresh. I then capped the hole with a 2" long 1/2" dowel to install the eye screw into:
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The result seems quite stout, and cone on the rocket (with the parachute in place) seems to be about right by eyeball; I haven't measured it yet. I still don't have the shock cord in yet, so that'll pull the CG a little farther forward yet.
 
It would also be easier if I could get plugged 24mm motors. In this case, all three mounts are 24mm, and all would be black powder if flown as a cluster. The local club I belong to doesn't allow clustering with composite engines, so to fly with all three burning, they'd have to be BP. I'm picturing flying with one E9 and two C11s or somesuch. I expect I'd be tempted to try three E9s together sooner or later, though the outboard mounts are sized for 2.75" long casings, and E engines would stick back pretty far. I think the first flight will probably be on a single E30-4, however. I don't think that a single E9 would have the punch to get it off the pad, and an E12 would be borderline.

May not be elegent, but could glue a bulkhead (or nosecone) in the front of the outboard mounts and use motor eject for black powder non-plugged engines.

For Cs or Ds falling casing shouldn't be an issue.

Doubt you'd hurt anyone with a conk on the head from an E casing but that would be pushing it.

Could even attach a teflon plumbers tape streamer (doesn't need wadding) with kevlar thread (carefully drill a tiny hole at front of casing for attachment---- obviously avoid the powder) to the engines so they would come down with a streamer and still eject out the back, but suspect that would "stabilize" the engine (fall oriented vertically) and may actually fall FASTER with the streamer than tumbling without it.
 
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For the life of me I can't understand why plugging a black powder booster motor for the purposes being described here is such a big deal. I think a much riskier scenario is mixing black powder motors with composite motors because of the difference in ignition reliability between these two types of motors. The fact that Estes has successfully made motors plugged with a solid clay bulkhead demonstrates to me that any non-flammable material such as epoxy or water putty can be used successfully to produce a safe aftermarket plugged motor.

https://www.rocketryforum.com/showthread.php?123625-Plugged-motors-question&p=1436810#post1436810
 
The motors usually eject inside the rocket. Except for some carbon and within 1", I wouldn't expect any problem from a booster or regular motor ejecting into external surfaces in front of the motor, in the face of 200 MPH of wind. If you wanted to put nose cones over them to reduce drag, that would complicate things.
 
I thought about adding bulkheads - they could could be pushed in against the motor blocks - so that the engines rear eject. But I know that 1) some clubs have issues with dropping casings on the launch area, and 2) I don't really want the aerodynamic penalty when I fly a single motor in the center mount. Granted, air flow through the body shrouds might be messy anyway, but right now if no motors are installed in the outboard mounts, the whole thing sort of acts like a funky tube fin. Plugging most of that area up with a disc has to make a difference; at least when a motor is in there, there is thrust to counter the drag. If I were going to do that, I'd probably section a nose cone to go above and below the wing.

I think, though, that Bill is right. Motors normally eject into the body tube. Normally, the cardboard and wood internal components are exposed on some level to the charge or flame out. If I make sure that the areas immediately in front of the motor casings are shielded, it should work OK - though I guess we could debate whether a -0 motor at burn through or a motor with a charge is "safer". If the -0 motor burns through and there's nothing to hold pressure in front of it, it'll probably quickly flare out with minimal damage to the surrounding areas.

That brings me to a tangent question - if two motors burn through or blow an ejection charge forward into the air as the rocket is still accending (assuming the outboard motors have shorter delay timing or none at all), then does that act as retro jet/thrust? Is there enough force to slow the rocket down early, or would the air flow at speed blow it back through the motor casing? My concern is how that may (or may not) play into predicting delay timing for the actual chute ejection .
 
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On Saturday, the shock cords I'd ordered arrived, so i was able to put together the recovery harness, which allowed me to button up the forward airframe. Now with that in place and the parachute stuffed in the tube, this is basically ready to fly once the weather allows it. I just need to put the final paint on it and decals...
 
I brought the model into work today so as to put it on the postage scale: 13.7 oz. A little heavier than I hoped, but not really unrealistic either.
 
After a little more sanding and repriming, I started laying down the paint. As with my standard size ATD, I sprayed this with Rustoleum 2X Paprika satin. I used this same color on my D-Region Tomahawk nose section as well. Its not quite bright red, almost a tomato soup color, but it seems about right for a target drone to my eye. Anyway, I painted the top half yesterday:
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I prepainted the insides of the duct areas with a zinc chromate color, figuring that it would be a likely plating on internal aircraft components. I'm not quite sure how I'll resolve the paint transition though.
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That oughta' look mighty impressive during liftoff!
Nice work!
Thanks. I hope so - it'll require all three engines burning to lift it with black powder. I should be able to make single engine flights with single-use composites though. In retrospect, I probably should have built it so that the center engine was 29mm, so that I'd have a wider range of choices. Then I could use an E16 in the center with D11/12s in the pods, or adapt down for a 3XD12s. Lesson learned...
 
Finally done with this one - I put the decals all on last night. Waterslide decals this large a little tricky, but it all came out pretty well. As seen with its junior side kick:

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Here's a few photos taken outdoors under more natural light. I guess I'm still waiting for bright & sunny though...
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First flight over the weekend, on an Econojet E20-4W. I'm not going to try a cluster launch until this has proven itself in single-engine flights. Anyway, the flight was good, despite a shroud line causing mischief.

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