7.5" Modular Booster / upscale Estes SM-3 Seahawk - FINISHED!

[mannyskid]

I have one question Sather, are you going to be using head end ignition of are you just going to run a conduit down from the sustainer Av-bay in order to light the second stage? Great looking build, can't wait to see it all finished!

Manny

 

[SMR]

I have one question Sather, are you going to be using head end ignition or are you just going to run a conduit down from the sustainer Av-bay in order to light the second stage? Great looking build, can't wait to see it all finished!

Manny

Thanks, Manny. I'll be running a 7/32" aluminum conduit down the inside of the sustainer. (Two, actually, for redundancy.) Having a 3" motor in a 5.5" body gives me plenty of room for that. Also, it is my understanding that drilling the forward closure makes it a research motor, and with me being NAR only, not a possibility.

 

[SMR]

Wow, looks like you are erecting some modern looking skyscraper.

Naw, it would need more rebar. And maybe some cranes. I was going for the classic look of the Sulaco from Aliens.

 

[SMR]

I very much like the idea of a one piece motor mount/ AV bay, and I know i will use that in the future. Very nice build... and the size cant hurt either:shock:

Thanks. Only having one payload bay does allow for a shorter rocket, which is important in this case as I am still trying to make it somewhat scale. And I am a huge fan of the Defy Gravity Tether. The downside is you absolutely need a viable drogue to get the main deployment to work.

 

[SMR]

Finishing up the avionics bay of the sustainer. She will have two sleds, one for the tiltometer and timers that manage her airstarted 75mm motor, and one for the universal altimeter skid that will manage her dual deployed recovery. To facilitate the extra sled, I sketched out a pattern for the bulkheads, using four 1/4"-20 stainless all-threads with my standard 2.75" centers for the sled's alignment guides. I transferred the pattern to the bulkheads, drilled them, and installed the hardware, to include a set of Gary T's excellent BlastCaps.

 

[SMR]

One of the (unintended) consequences of my original series of standard av sleds was the requirement to adapt each rocket to the existing sled, however inconvenient that might be. These sleds had the POPO buttons mounted low, as I do a lot of tethered deployments, with the av bay physically mounted to the motor section. I then use the break point (joint) in the body only to disassemble the rocket for movement and to interchange body sections in the event of damage. But for standard dual deploy rockets, I had to drill access holes in the motor section of the rocket, below the av band.

As the diversity and complexity of my fleet have grown, so has the need for different avionics. For airstarts and staging, for example, I need to add timers and a tilt meter. So, I'm in the middle of a big transition to a new generation of avionics sleds. These will have the switches mounted mid-board, for access through the band. To increase available space, I am doing away with the 2nd set of lugs (with 2" centers, which I used only for 4" rockets), and converting all of those bulkheads to 2.75" centers by moving the all-threads further apart and filling in the old holes.

 

[SMR]

Continuing work on the Seahawk, I finally got around to working on the booster's fins. The slugs had been rough cut for quite some time, but I had shelved them while deciding on a final size. (The fins on the Estes version are HUGE, and I feared too heavy to keep scale.) I ran a bunch of simulations with incrementally smaller fins, comparing weight loss with Cp movement. I also evaluated some of the fin lightening construction techniques used in some other projects I have flown recently, and eventually decided to keep scale fins, using larger lightening holes and laminated skin surfaces. (This will hopefully offset with the purists the fact that I am not going to even attempt to emulate the complicated taper of the original fin.) The lightened fin frames weigh 1 pound 2 ounces each, compared to 2 pounds 3 ounces of the slugs.

In the first attached photo, I have started cutting the lightening holes with a jigsaw, but have not yet sanded the edges to final dimensions. Evident in the photo are the various iterations the fins have gone through... there are purple, green, black, and orange (final) versions traced from different templates I have tried.

The 2nd photo shows the first fin frame coming out of the press, with one side of the skin attached. The frame will have the interior holes foamed before adding the second side of skin and adding the leading/trailing edge bevels.

 

[SMR]

Simultaneously with the fins, I continue with the booster's motor section. If I time it correctly, both assembly lines will be ready at the same time, so the fins can be installed in the body without either sequence causing a delay. Here the motor tube, with centering rings #2 and #3 in place, is installed in the glassed 7.5" x 37.0" Polecat body tube. (Ring #1 will be installed after glassing internal fillets at the fin slots, and foaming the cavity.) Screws on the sides are marking the positions of the rail button blocks. In the second attached photo, we are looking down the throat of the body tube at the top of ring #3, with its fillet of West epoxy and chopped carbon fiber. The blue masking tape is to protect the stainless all-thread from epoxy drips. The cardboard cutout serves the same function for the motor tube. Photo #3 has the tape and cardboard mask removed. The four hardwood blocks are stand-offs, to hold the aft bulkhead of the avionics bay 1.5" off of the forward centering ring. The final photo shows the av bay bulkhead installed. It is screwed and glued to the blocks. I still need to run lock washers and nuts down the all-threads, and then install the coupler / av bay.

 

[SMR]

Continuing progress on the booster's fins. Here we have the full set. All four have had the lightening holes cut, the backside skin installed, and the interior cavities foamed. The two on the left have had the frontside skin installed and routed to final shape, with routing of the leading and trailing edges to go.

Also of note, Tim D. has started an excellent discussion thread on the pros and cons of lightened fins, here. Relevant factors include whether to lighten the tang (structural considerations), orientation of the spar (aerodynamic forces vs landing energy dissipation), and performance expectations of the rocket (flutter resistance).

 

[patelldp]

Continuing progress on the booster's fins. Here we have the full set. All four have had the lightening holes cut, the backside skin installed, and the interior cavities foamed. The two on the left have had the frontside skin installed and routed to final shape, with routing of the leading and trailing edges to go.

Also of note, Tim D. has started an excellent discussion thread on the pros and cons of lightened fins, here. Relevant factors include whether to lighten the tang (structural considerations), orientation of the spar (aerodynamic forces vs landing energy dissipation), and performance expectations of the rocket (flutter resistance).

By skinning these fins, foaming, etc...did you really save that much weight vs. the difficulty added to fabrication?

 

[SMR]

By skinning these fins, foaming, etc...did you really save that much weight vs. the difficulty added to fabrication?

Probably not enough to justify the time spent doing it. On this rocket, anyway, since the fins are relatively small. The lightened frames saved a little over a pound apiece - 4 pounds, 4 ounces total. I use 2 pound foam, which doesn't add much weight back. But any weight saved has an added bonus, especially on anything (Jayhawks, for example) which requires nose weight for stability. Every pound saved aft of the Cg saves at least another pound in the nose. But the trade-off is that the lightening cutouts do increase the chance of breakage on landing.

 

[SMR]

Simultaneously with the fins, I continue with the booster's motor section. If I time it correctly, both assembly lines will be ready at the same time, so the fins can be installed in the body without either sequence causing a delay.

Well, I came pretty close. The first attached photo shows the body tube marked, and the fin slots being cut by hand with a Dremel tool. In the second photo, I had originally beveled only the leading and trailing edges of the booster's fins. My daughter, in charge of aesthetics, suggested I do the outer edge as well. IMHO, I do think that does make them look better. Thanks, Kelly! The third photo shows the first fin installed.

 

[SMR]

Continuing progress on the booster. All four fins have now been installed. Next steps are to glass tape the internal fillets, and then foam the fin can before permanently installing the aft centering ring. Also, just about ready to start on the interstage coupler.

 

[dixontj93060]

My daughter, in charge of aesthetics, suggested I do the outer edge as well. IMHO, I do think that does make them look better. Thanks, Kelly!

I have seen your brood. If I remember right females outnumber males. Better keep the majority happy (and continuing to attend launches ).

 

[SMR]

I’ll be the first to admit I’m not the most focused builder in the room. I do most of my construction in brief spurts of energy, with lots of “thinkin’ time” between. Falling slightly behind my timeline to be ready to fly at MidWest Power X in November. I have finished the 75mm motor mount of the sustainer (photo 1). I added Kevlar straps (ala Wildman kits) in addition to U-bolts. Twin aluminum tubing runs down from the top ring thru the aft ring, for redundant igniters to airstart the motor. Wooden blocks are attachment points for rail buttons when flown as a single stage. And the 6” length of 98mm G-10 covers the 1/2” thick centering ring, which is the attachment point for the 75mm Aero Pack body.

The second thru fourth pictures show progress on the booster. Leaving the aft ring off allowed me room to work inside the fin can, where I glass reinforced all interior fillets (8 x fin to body and 8 x fin to motor tube). After the last one was done, the entire cavity was foamed prior to installation of the aft ring, which is epoxied to the tubes, and screwed into the fin tangs. The final photo shows the excess motor tube cut off (dremel) and sanded smooth, ready for installation of the 98mm Aero Pack body. The body is also marked for installation of the aft rail buttons (1515 on one side, unistrut on the other.)

 

[qquake2k]

Very nice!

 

[daveyfire]

I had originally beveled only the leading and trailing edges of the booster's fins. My daughter, in charge of aesthetics, suggested I do the outer edge as well. IMHO, I do think that does make them look better. Thanks, Kelly!

Hey Sather - how'd you do the bevels?

 

[SMR]

Very nice!

Thank you. It is getting there, albeit slowly.

 

[SMR]

Hey Sather - how'd you do the bevels?

Just running the fins (vertically) across a router table. I use a straight Carbide bit with a 1/2" shank. My router bolts under the table saw with the bit facing up, and I use a fixture attached to the rip fence to guide the fin stock. A 15° bit gives a nice long bevel on fins up to 1/4" thick, but takes off more material, so I have to use a 22.5° bit on thicker fins like these. (Else the length of the beveled edge would exceed the length of the bit.)

Note - I took a good slice out of my thumb before realizing that you need to leave a little flat edge around the perimeter of the fin, particularly with glass or carbon fiber. You can make a very sharp knife edge if the bevels touch.

 

[SMR]

Been kept busy doing all the little unseen administrative tasks and details that will hopefully come together at the end. Added a final fillet of JB Weld around the perimeter of the aft centering ring, and installed the aft rail buttons on opposite sides.

There is a lot of rocket forward of the mid-seam on the booster section, so to strengthen the coupler, I applied two layers of 4 ounce fiberglass cloth using a technique shown to me by Tim D, cjl, and 5x7. (posts #72 thru #94 during the Jayhawk build). I had some 36" balloons left over after that procedure, purchased from the Balloon Place... https://www.balloonplace.com/catalog/Qualatex_latex.html. After pressing the glass cloth into the epoxy, I inflated one inside the coupler (with an air compressor, or I'd still be there puffing!). After the epoxy cured, the balloon released s-l-o-w-l-y, leaving a nice smooth surface inside the coupler. (note - if I had been more patient, I could probably have re-used the balloon. Instead, I fiddled with it and it tore. Thankfully, the 3' balloons are only $2.89 each, while 4' ones are $20.95 each! That would warrant more care.)

 

[SMR]

Once the interior of the coupler was done, I skim-coated the exterior with epoxy and sanded it smooth. Partly to protect the cardboard, and partly to cover the mess I made of it while doing the interior. The coupler was then epoxied into the motor section of the booster, sitting on the top bulkhead of the motor mount. The payload bay’s tube was placed over the coupler, marked, and drilled for payload bay pressure venting (3/16”), four altimeter bay static ports (11/32”), and the eight 1/4”-20 bolts and tee nuts that hold the two body tubes together (19/64”). All-threads are spaced to hold 2 altimeter sleds inside the coupler / avionics bay, with POPO switch access through the side of the body tube below the joint line. As mentioned previously, only one payload bay was planned, to shorten the rocket’s length and keep it as close to scale as possible. Dual deployment of the booster will be by Defy Gravity “Tether”, available here… https://defyg.com/tether.html

 

[SMR]

Last weekend at the joint Tripoli Wisconsin / WOOSH launch, I picked up the sustainer’s lower body tube (motor section) from Wildman’s (photo 1). It has unusual fin slots due to the short tangs, which are only on the front half of the fins, since the collar of the interstage coupler must slip-fit inside the last 6” of the sustainer’s body tube. The back half of the fins are surface mounted like a minimum diameter rocket. The motor tube was slid into the sustainer’s body tube without epoxy, since it is critical that NO epoxy gets dripped in the aft portion of the 5.5" body tube, which would be next to impossible to sand off with the 4” section of body tube at the end of the 3” motor tube in the way. This also allowed me to finess the location of the motor tube to ensure the button blocks were in proper alignment, centered inline between the fin slots. The rail buttons were then installed, (photo 2) to “thru-bolt” the tubes together until injection fillets are added after fin installation. Photos 3 and 4 show an Aero Pack motor retainer temporarily installed on the 75mm to 98mm ring at the end of the motor tube, and the core of the interstage coupler. (Which is finally placed in position in photo 5.) It gets a 5.5" to 7.5" fiberglass transition, formerly a Polecat Bullpup tailcone. Next on the agenda is making the custom rings needed to assemble the ISC. Running slightly behind schedule for a MWPX launch, so looking more like a Thunderstruck 4 (Apr 2013) maiden voyage "L" to "L", with a follow-up (if possible) "M" to "M" at MWP 11 (Oct 2013).

 

[mannyskid]

Just running the fins (vertically) across a router table. I use a straight Carbide bit with a 1/2" shank. My router bolts under the table saw with the bit facing up, and I use a fixture attached to the rip fence to guide the fin stock. A 15° bit gives a nice long bevel on fins up to 1/4" thick, but takes off more material, so I have to use a 22.5° bit on thicker fins like these. (Else the length of the beveled edge would exceed the length of the bit.)

Note - I took a good slice out of my thumb before realizing that you need to leave a little flat edge around the perimeter of the fin, particularly with glass or carbon fiber. You can make a very sharp knife edge if the bevels touch.

Did you learn that little bit from Justin? That's how we beveled the fins for "The rocket that shall not be named" Really great to see that this build is progressing nicely So what's you plan for motors? M3400-M3700? That would be AWESOME! What about an M840 on top? Can't wait to see this fly!

 

[SMR]

Did you learn that little bit from Justin? That's how we beveled the fins for "The rocket that shall not be named" Really great to see that this build is progressing nicely So what's you plan for motors? M3400-M3700? That would be AWESOME! What about an M840 on top? Can't wait to see this fly!

Thanks, Manny. It's a pretty standard beveling technique... most router tables have the blade spinning vertically, which defines the board (fin) placement. I'm planning an L3150 to L850 for her first flight, and an M3400 to an M1315 for her second. Gotta stay under the waivers.

Hey, have fun and good luck at AIRFest. Looking forward to seeing / hearing about your certification flights.

 

[mannyskid]

Thanks, Manny. It's a pretty standard beveling technique... most router tables have the blade spinning vertically, which defines the board (fin) placement. I'm planning an L3150 to L850 for her first flight, and an M3400 to an M1315 for her second. Gotta stay under the waivers.

Hey, have fun and good luck at AIRFest. Looking forward to seeing / hearing about your certification flights.

I'm looking forward to seeing her go at Thunderstruck. I'll make sure that you guys are getting a real time, play-by-play summary of my cert flights

 

[SMR]

I started cutting the fins and strakes for the sustainer on Saturday. Sunday it rained (finally!!). Monday I finished the rough cuts. Cutting G-10 is extremely slow work. The strakes are a combination of 1.5" and 7/8" strips. Four fins for the sustainer, a few sets of my favorite fin pattern for future 5.5" projects, and a couple of bulkheads to use up the remaining sheet of 3/16" G-10. While I had the saws set up, I made a few blanks for avionics sleds. I'll need to drill holes in the aluminum angle bracket at 2.75" centers for all-threads in my bays. The Seahawk will need four...

(a) in booster - recovery sequencing (2 x MAWD)
(b) in ISC - staging (2 x miniTimer 3G)
(c) in sustainer - airstart motor (2 x miniTimer 3G plus 1 x RockeTiltometer 2)
(d) in sustainer - recovery sequencing (2 x StratoLogger 100)

Now to sand the fins' edges to make matched sets, and then add bevels.

 

[SMR]

Now to sand the fins' edges to make matched sets, and then add bevels.

Later that same day...

fins were all sanded in sets to clean up the edges from the jigsaw, and then run across the router to set the 15° bevel. Cleanup of mass quantities of dust took another few hours...

 

[SMR]

Slow but steady progress on sustainer fin installation. The fins, when all installed and cured, will be the reference point / plane for the strake installation. The strakes are two parts each, to allow for the step in span, so another two days per set, 8 days total. I have a rule of thumb I call "glue events". No matter how efficiently you create the parts, building a rocket still takes a finite amount of time for the epoxy to set. Since I use slow epoxy, I allow a day per glue event. Some things you can do in pairs (fillets, etc.), but it is still a day per side, then roll the rocket and do it again. Primer, Super Fil, top coat, all still a minimum of a day per. When you get close to a launch date, you can kind of get a feel for whether or not it will be ready by counting days available and glue events required. This one is not gonna make MWP, but a definite for Thunderstruck in the spring.

 

[Rex R]

be careful around those beveled fins now, you wouldn't like having them turn red....
rex

 

[SMR]

be careful around those beveled fins now, you wouldn't like having them turn red....
rex

Thanks for the advice, Rex. I took extra care to leave a little flat spot around the perimeter to avoid a sharp edge. Centering rings have historically been my nemesis. I got complacent (and rushed) a while back, not clamping one down when drilling out the 98mm center hole. It got caught in the drill press, spinning and whacking my thumb several times before taking off across the garage and putting a serious dent on the inside of the door. Live and learn.

:surprised: