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MAC Performance Scorpion - a build thread

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EXPjawa

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OK, back at URRF3, I ordered a Scorpion kit from MAC Performance. I probably won't have normally bought a kit of that type or size (or price), as I'm mostly a low/mid power guy despite being L1. But, I had several onsite vendor vouchers for $10 each, which the event staff gave out as a reward for working a shift at the event. So, I had to find a use for them...

Anyway, they were sold out of what they'd brought to the event, but were willing to put my name on one when they got back home. In ensuing conversations with Mike Crupe, I asked him to make a few minor changes. I decided early on that I wanted to build it with an Aeropack boat tail motor retainer. Since this requires the motor mount tube to protrude farther aft than normal, I wanted to extend the mount tube past the standard 12". I had him replace that with an 18" piece, so that it would be more of a stuffer tube. The other change that was needed to accomodate the retainer was the rear centering ring (normally a stepped thrust ring) was replaced with a standard ring that could be recessed into the body tube. So, that sums up the changes. I also bought the Aeropack piece and a fin jig from Mike.

With all that ordered (Mike was kind enough to deliver to the following URRG launch), I set about modifying the Rocksim file downloaded from the MAC site. I wanted to get an idea of what I was dealing with, and of course, it was not too early to start thinking about finishing and color schemes.

Rocksim output:





Once I received everything, the first step was to take it all to work, and put all the parts on the mass comparison scale in our lab. It measures to the 1/1000th of a gram, so I could update the model with accurate values. As you might guess, adding a chunk of aluminum to the tail while pushing the motor farther back didn't do the stability any favors, but I should still be able to have a reasonable margin without ballast - at least withing the range of motors available to an L1.

As far as coloration goes, you can see above that started with all-silver like on MAC's site, but added green to the nose, and threw in some roll pattern for good measure. The green itself is a texture file generated from an image of Ford "Gotta Have it Green", a 2013 Mustang color. While I probably won't use automotive paint, the Testors Lime Ice lacquer seems a near match.
 

EXPjawa

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Here's what I'm starting with:


You can see all the basic bits here, including the 42" Sherichute I ordered for it. That item was a little on the pricey side, but very nice and made to order - quickly. I'd wanted to try one of their chutes on a project at some point, this seemed to be a good opportunity. The chute panels are green, grey & black, to reflect the intended paint scheme. And of course, that was weighed upon receipt, so the only item I'm still missing the correct mass for is the recovery harness. I'll probably order that from One Bad Hawk at some point soon.

Anyway, the first thing I did was to dry fit everything together. As others have noted, MAC's parts are precise enough to do that and have it all free stand:



In fact, the only part with any kind of loose fit is the nose cone, but a little tape will fix that. Shown with my PSII Leviathan for comparison.
 

EXPjawa

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The first "real" step was to build up the motor mount. I test fit parts in several iterations to get the centering rings placed in relation to the motor retainer. So, first things first, I had to sand the OD of the end of the motor tube to allow the retainer's thread ring to be dry fit. That's why the above photo shows a lighter-colored band on one end (and a reference arrow in marker on the side). But once I had that, I could put it all together with a dummy motor in the retainer, and get it so the tail cone threads flush against the body tube so it can act like a thrust plate as well, with a motor in place. I started with the motor tube protruding 1.5" from the body tube, per Aero Pack, but eventually trimmed it inward an 1/8" so everything would line up. The tube all marked out:



You can see where I erased my initial markings and shifted them. The wider band that's blocked out on the right-hand end is that which will protrude from the body tube. Then again with the sub-components fit in place:



Then, after scuffing the tube, the front and center rings were glued in place using BSI 30-minute epoxy. You can see, that included epoxying in the retainer pin for the harness leader, per the MAC instructions.

 
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EXPjawa

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Following with great interest. Chute release?
Absolutely Dave. I didn't mention initially, but this is the single-deploy version of the Scorpion. That's probably apparent from the first picture, with there not being an alt bay and all. But, at this time, I have no interest in old fashioned dual deploy, with their fiddly charges and electronics (simplicity is part of this hobby's appeal for me), and am developing reasonable confidence in the Chute Release. I see that as key to flying higher altitudes (above 2000' or so) on motors bigger than baby-H sized. The Chute Release (and Altimeter2) are included in the above RS file as mass objects, and built into the CG placement already.

On another note regarding recovery, as mentioned, the intention is pop at/near apogee with the chute bound up in the CR, to release around 500'. I'm toying with the idea of adding a long mylar streamer to be deployed at the top, with the sole purpose of increasing visibility as it drops. Thoughts on that, anyone?
 
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Banzai88

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Watching this one, too. Soooooo many rockets, so little time.
 

EXPjawa

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Soooooo many rockets, so little time.
I know what you mean. I've got one of those seemingly 100-kit build piles, but this seemed to be worth pushing to the front of the line. Partly because its size as a kit made it so I couldn't store it with the others well...

Anyway, the motor mount is flipped over and epoxy fillets applied to the other side of the rings:


This brings me to a sort of mini-quandary: the instructions refer to scuffing the inside surface of the body tube where the centering rings will be epoxied. How do I go about getting up into the area of the tube where the front ring will be to do that? I assume the same issue must exist with fiberglass tubes, but I've never worked with those before. Is there better solution than wrapping sandpaper around a stick?
 

TOM C

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Real nice Rick..will read along...thanks...
 

dhkaiser

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Rick,

Subscribed.

We are on similar paths. I have a Scorpion as well. The differences are it is stock except for a Aero Pack retainer and a Mac Nosecone Bay for my BRB900 GPS tracker. I too am enamored with the idea of SD with a Chute Release. Tomorrows Indiana Rocketry launch looks like near perfect conditions and I intend to fly it on a I140 or maybe a I120, we will see.

Even with the additional weight in the nose the I280 stability = 1.21, probably because I foamed the fin can. I will follow yours with interest.

Dan
 

DavidMcCann

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I tape sandpaper to a 1" dowel.


I600R would be a hell of a ride ;)

I'm looking at a CR.... It's evident from my last Optima failure that they fall sideways....so tossing a large motor into one is a real possibility.
 

EXPjawa

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I guess that'll have to do then. That's effective enough to get a pretty even result? It seems like holding the stick at any sort of angle would result in a line-contact between the sandpaper and tube, but maybe that's OK. I wonder if I can do something to let the paper conform more. Ever thought about putting a firm foam under the paper, around the dowel? That might give the paper more compliance to match the tube surface. It also may, I suppose, way complicate an otherwise straight forward thing...:rolleyes:
 

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I guess that'll have to do then. That's effective enough to get a pretty even result? It seems like holding the stick at any sort of angle would result in a line-contact between the sandpaper and tube, but maybe that's OK. I wonder if I can do something to let the paper conform more. Ever thought about putting a firm foam under the paper, around the dowel? That might give the paper more compliance to match the tube surface. It also may, I suppose, way complicate an otherwise straight forward thing...:rolleyes:
Subscribed, love the Mac Scorpion but too many un-built rockets at this stage.

I use the same dowel method as Dave mentioned. Roughly a 1" dowel wrapped with sand paper on the last inch or so. It works very well, is inexpensive and reliable low tech. I also use a snake on my dremmel depending on what I am trying to do, with a flap wheel attached to the end of it. This is also very effective.

re. the Mylar or reflective streamers. I tried this in a few different configurations but ultimately I did not get the configuration to the point where I felt comfortable, only three attempts so take it with a grain. In every case at least part of the streamer became entangled somewhere in the recovery system on the way down. Luckily I never experienced a failure, just a tangled mess to unwind, but I am pretty sure had I done it enough times I may have had a failure, that it unless I figured out how to make it work reliably.

BTW if Dave influences you into the I600, keep in mind that it only comes with a medium 10 second delay and that may be a couple of seconds short of what you need. I found this one out the hard way with my PML Bull pup last year at LDRS when I used engine eject as my backup but it popped about 3 seconds too early at 150'/s...my bad. I am just getting close to paint on the rebuild and will strictly use the altimeter eject, which is retrofitted in the nosecone, along with my new CR, which is replacing my cable cutter.
 

EXPjawa

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I plugged the I600R into the sim and showed a little over 5100', but deployment at 120+ fps. Most other I-class motors show 14-15 seconds as ideal delays, so yeah, 10 sec is a bit short.

What if the streamer was attached to the top of the chute canopy? I have the idea that I've seen something about this in conjunction with the CR being discussed, but I'm not sure where now.
 

mpitfield

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lol, you know I am almost certain that there once was long delay when you ordered the I600, and I do not recall ever ordering a separate long kit...but I would not bet my life on it. I flew that rocket 3 times on the I600, always with engine backup and altimeter primary, and I never had a problem until that last launch. All flights simmed and hit about the 5,300' mark, perfect for mile high competitions, except of course that last launch.

It was not until that last launch when I destroyed my Bull Puppy that I looked into the root cause and was surprised to see the medium 10 second delay, when I had it in my head that it was 14 seconds. I also checked to see if there was an I600 with a long delay and I could not find one, so possibly AT changed the way they provide the long delay option?
 

mpitfield

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What if the streamer was attached to the top of the chute canopy? I have the idea that I've seen something about this in conjunction with the CR being discussed, but I'm not sure where now.
That may have been my post

I will sell you a custom 12" fruity drogue with a "pig tail" connector sewn right into the shroud lines, extending through the spill hole. This was one of the methods I tested, and it resulted in the chute, streamer and part of the one bad hawk Kevlar harness becoming a twisted mess. Now to be fair I never witnessed that particular descent as it was a mile away and I only picked it up after going to the GPS coordinates. So it is possible that it descended fine, from 15,000, and did not wrap until the main popped at 800'AGL...I really do not know.

Regardless I purchased 3 of these 12" classic elliptical chutes so if you want one I will sell the one unused one to you.
 

DavidMcCann

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lol, you know I am almost certain that there once was long delay when you ordered the I600, and I do not recall ever ordering a separate long kit...but I would not bet my life on it. I flew that rocket 3 times on the I600, always with engine backup and altimeter primary, and I never had a problem until that last launch. All flights simmed and hit about the 5,300' mark, perfect for mile high competitions, except of course that last launch.

It was not until that last launch when I destroyed my Bull Puppy that I looked into the root cause and was surprised to see the medium 10 second delay, when I had it in my head that it was 14 seconds. I also checked to see if there was an I600 with a long delay and I could not find one, so possibly AT changed the way they provide the long delay option?
I'm pretty sure they used to sell them with longer delays, then chopped everything to 10's at some point.

nerp..... checked the highly updated AT catalog.... I600's were only M delay. weird.
 
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mpitfield

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I'm pretty sure they used to sell them with longer delays, then chopped everything to 10's at some point.

nerp..... checked the highly updated AT catalog.... I600's were only M delay. weird.

I really must be losing it as I do not recall ever ordering those delays, it is possible though that I had too much red wine when I ordered it. Speaking of ordering it, I just checked my motor inventory and I have an I600 in stock so I ordered some of those longer delays and think the best way to christen my newly rebuilt Bull Puppy is to shove an I600 in her.


I guess that'll have to do then. That's effective enough to get a pretty even result? It seems like holding the stick at any sort of angle would result in a line-contact between the sandpaper and tube, but maybe that's OK. I wonder if I can do something to let the paper conform more. Ever thought about putting a firm foam under the paper, around the dowel? That might give the paper more compliance to match the tube surface. It also may, I suppose, way complicate an otherwise straight forward thing...:rolleyes:
Rick one thing I wanted to say in regards to a uniform sanding result. Intuitively I have always tried to get a non-uniform result when it comes to roughing up the tube, fins, MMT, BT, etc., when it comes to gluing them together. My rationale for this is that it provides a better bond, especially in the CR to MMT to BT, that is if you sand in the same horizontal plain of the CR. I could be wrong but it just seems to make sense to me.
 

DavidMcCann

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yea... i leave the sandpaper over the edge of the stick, and basically just sand with the edge. It's ugly, but works.

that puppy loves I600's.... I've got a photo of yours somewhere here in this pile of 150,000 photos...
 

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I've been on the rocket down low for the past year but this is the path I want to get back on. Dig the chute release idea and single deploy Mac kit. I'm watching for sure.
 

EXPjawa

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Well, I didn't have a 1" dowel, but I did have a section of 1" PVC pipe leftover from the mid-power pad build (a spare leg). Having something hollow allowed the sand paper to be folded over the edge into the middle:



So, with that, I scuffed up the inside the best I could in the centering ring areas. I don't really have a way to check how rough the surface is, but it is visually different:



even if the camera had a hard time focusing in the tube... The bigger issue I had was actually getting the epoxy applied sufficiently way up inside the body tube. I used the above PVC piece for that task, but was still difficult to tell if I had it spread around well. However, once I pushed the motor mount up in, I saw that I had a pretty good bead being pushed up by the centering ring:



Of course, its still hard to see when the camera won't focus on the right spot. Anyway, it is somewhat odd to have the body tube be translucent; I had previously marked the outside of the tube where the centering ring should be for later rail button installation. I could put my finger on that mark and hold the tube up near a light, and then see up inside that the epoxy was in the right place. You can't do that with cardboard. But, once the forward CR was clear of the fin slots, I reached up in with a stick and put down more epoxy for the mid CR. Once the mount was pushed up in place, I rotated it within the body tube a few times to further spread the epoxy around and get a good film. The mount is now installed:



So, I guess the next step is to install the fins.
 

Jozef

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Subscribed... Interesting build, Rick. I'm a fan of MAC Performance. Got my L2 on a MAC Peformance Rayzor at URRG recently. I used a 42 inch Spherachute on my Rayzor, so you will be good there. I found the chrome vinyl I got from Mark at Stickershock provided some good reflection as the airframe decended from 4,000 feet. You may want to give that try if concerned with streamer entanglement. The Scorpion looks like a good HPR candidate for a chute release.
 

EXPjawa

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OK, going back to Thursday, I set about installing the fins. The thickness of the fin material actually measured about .005" more than the width of the tube slots, so although I could get them to go in, it was difficult. I sanded the fin sides and the slots a little bit, though it didn't remove alot of material from the fins. But it seemed to help. Once there was some epoxy in place, it went a little easier. Before that, though, I nipped the corners off of the leading base else of the fins so as to clear the epoxy fillet on the centering ring.



From there, I used the Dave McCann method, described by him elsewhere, of putting epoxy over the slot and pushing the fin through it. A couple of applications of this, and had sufficient adhesive at the root of the fin and between the fin and tube. I use BSI 30-minute epoxy for this, as I did with the motor mount. It was thin enough that it was a little tricky to keep in place and a bit of challenge to keep neat. I had to wipe drops and excess off of the body tube a few times to make sure that the fin jig wouldn't become part of the fin structure:



Retrospectively, I'm not sure if it was best to install all four fins at one time, and then jig them for curing, but that's what I did. The epoxy was slow-curing enough to get away with it. I indexed how it rested 90 degrees every few minutes to try to prevent the epoxy from running to one side. Seemed to work. Fins all in place:



All up, the rocket is as tall as my nephew was when he was 13. Anyway, I started laying in the (external) epoxy fillets on Friday, after scuffing up the fin & tube surfaces with course paper:



I did two fillets Friday evening, two more Saturday morning, two Saturday evening, and the final two Sunday morning. I put the rocket in front of a heater to help the cure, though I don't know what impact that really had. I used Loctite 5-minute epoxy for this, as I've had good luck with how it self levels. I am not, however, going to start internal fillets at this time. Last week, I ordered some epoxy syringes and 6" mixing tips, which should arrive tomorrow. So I'll be patient and wait for those, which should make doing the internal fillets easier.

External fillets all cured:



They came out pretty well. Only one of the eight was a little uneven, the result of not having the epoxy sufficiently smoothed before it started to harden. So, for that one, I've put a little filler in. Otherwise, last night I started sanding the fillets to get rid of any small bubbles or unevenness at the edges where it blends onto the mating surfaces.



I've decided that even though the fin can isn't closed up, I can start to prep the rest to prime. Hopefully, I can shoot that within the next day or so, but I'll have to do a little filling on the nose cone seams first...
 

EXPjawa

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Last night, I scuff sanded the fins so as to better hold the primer. This was more involved than I'd thought; the finish on the fin material is pretty hard. I also scuffed up the rest of the body tube, then moved on to sanding/filling the seam on the nose cone. After that is finish sanded, I think that I can shoot a coat of primer...
 

DavidMcCann

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Looking good. I like the looks of this rocket.... and your planned setup is perfect for G/H
 

EXPjawa

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It looks like (according to Rocksim) that it'll be a touch heavy for a lot of G-motors, though a G80/88/125 should work. The lift off would be fairly slow. But H & I motors no problem.

A little filler on the nose cone:


And a coat of primer:


In other news, the company I ordered the epoxy syringes & tips from forgot to include the sealing pistons in the order, so I can't readily do the internal fillets until they arrive. Hopefully that's by the end of the week...
 

EXPjawa

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Last night, I sanded down the first primer coat, which smoothed out a lot of areas, then shot a second primer coat. With luck, I can scuff sand that coat tonight and shoot the base color coat.

 

EXPjawa

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With silver base coat:



I used two full cans of Testors lacquer silver in three coats. Of course, they're little cans. Hopefully, I can mask and add the green paint tomorrow. The decals from Stickershock showed up today. It looks the half-sized logo will be about perfect for what I want. The other thing of note - I received the recovery harness from Teddy a few days ago and took it to work to weigh it on the scale in the test lab. It is substantially lighter than what I'd entered in Rocksim previously, so I may have to add some ballast to the nose cone to fly most any I-class 38 mm motor. It should be fine for now just using a 3 or 6-grain 29mm with an adapter.
 
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