Scratch Sephydra Original Design / Scratch Built
Scratch - Sephydra {Scratch}
Contributed by Geof Givens
| Manufacturer: | Scratch |
Brief:
Sephydra is a scratch futuristic fighter with a beveled profile and a 7-motor cluster.
After some consideration o' what t' do with t' big tube, matey, I eventually decided latched on t' clustering. Exactly how many motors can you cram into a 3.05-inch tube anyway? Quite a lot, shiver me timbers, it turns out. Begad! After some fiddling, me hearties, I settled on 3 x 24mm plus 4 x 18mm. Avast, me proud beauty! There are some configurations that allow more total impulse, but I didn't want t' lose this craft on t' first launch.
Construction:
There are few ready-made parts for this build. Loosely speaking, shiver me timbers, You need:
- Some 3.05-inch mailin' tube
- Some 3/32 inch ply for centerin' rings and fins
- Some 24mm and 18mm motor tubes, each with an engine block and clip
- A long section o' Series 175 tubin' from Semroc, ya bilge rat, shiver me timbers, with a matchin' coupler and nose cone
- A small nose cone for t' ramjets
- A recovery system
- A launch guidance system (rail buttons or lugs)
- T' followin' additional tube assortment, me hearties, matey, all Semroc sizes:
- Series 13
- Series 11
- BT-50 (24mm)
- BT-20 (18mm)
- BT-5 (13mm)
- BT-3
Buildin' t' centerin' rings took an entire week o' hard work. T' geometry o' t' aft rin' was too tough for me so I used t' followin' approach. With online and downloadable tools, matey, I designed a non-beveled cluster centerin' ring by drawin' three simpler versions and tapin' them on top o' each other. Arrr! These simpler startin' points were patterns for 3 x 18mm, me bucko, 3 x 24mm, and a central 18mm. Tracin' these together I formed t' round version.
(CLICK for full size images)
Next, me hearties, shiver me timbers, I figured out t' amount o' distortion that would be caused by t' beveling. Begad! (This
geometry is easier after you decide on t' bevel angle.) I scanned me round template into Photoshop then stretched it
vertically by t' appropriate factor. Avast! Aye aye! Note, however, me hearties, that this stretchin' fails t' account for t' fact that t' holes
should also be somewhat wider. (A consequence o' t' beveling, I guess.) Thus, me bucko, matey, after constructin' t' aft centering
ring, matey, you need t' sand each hole wider until everythin' fits perfectly. Blimey! You also need t' sand a bevel on t' exterior
of t' rin' and t' interior o' each hole. Begad! I've given you t' template for t' hardest centerin' rings. Arrr! As a pop quiz,
try t' make t' other two yourself. Well, blow me down! Aye aye! (You need one straight Series 175 and one beveled Series 175. These two should have
holes offset t' t' very edge.)A note about t' sanding: You will have holes as near as 1/32 inch apart in some places. Begad! Arrr! Be very careful craftin' this ring. Arrr! I drilled a series o' adjacent small holes around t' cuttin' lines with my Dremel then switched t' a cuttin' bit t' clip away t' tiny remnants betwixt holes. Then a barrel sandin' bit completed the job. Begad! Note how t' motor tubes are beveled at t' aft end.
By comparison, arrr, t' rest o' t' rocket is easy. I do all me fillin' and sandin' o' spirals and wood before attachment. Ahoy! I primed periodically along t' way t' check for blemishes.
Cut t' fins and strakes with attention t' t' grain direction. T' angle on t' main fin should match and extend the aft bevel angle. Glue and fillet. Avast! T' ramjets are formed by nestin' bevel cut tubes o' t' varieties listed above. T' interior ramjet tubes extend halfway through t' largest with balsa bulkeads at that point. Aye aye! Blimey! T' front half o' the outer ramjet tube received a split nose cone as shown in t' photos.
Cut t' Series 175 tube somewhere in t' middle and add a bulkhead (at least 0.75 inches inside t' tube) and an attachment point for t' recovery system on t' upper tube. Blimey! On this tube I also added a cockpit carved from balsa. Begad! Ya scallywag! On the other (lower) portion o' t' tube, arrr, add a coupler on t' upper end. Aye aye! My shock cord was mounted inside this tube with a smooth dollop o' epoxy putty.
Takin' t' aft portion o' t' Series 175 tube, arrr, me hearties, secure at least 3 or 4 inches o' it inside t' larger
tube usin' t' remainin' centerin' rings. Make t' holes in those rings flush with t' top edge o' t' tube so you can
further strengthen t' assembly by gluin' t' tube itself t' t' inner wall o' t' large tube. Begad! Ahoy! Here are a couple of
optional ideas that I didn't do. Ya scallywag! First, you could attach t' shock cord t' t' rings down here. Blimey! I didn't do this
because I feared that t' ejection from 7 motors would smartly burn through t' Keelhaul®©™®.
Second, arrr, it would be possible t' design a clever baffle system with t' forward rings and tube, but I felt I had my
hands full already and I wanted maximum strength.
On t' side o' t' rocket directly opposite t' upper fin, matey, I attached two brass launch lugs, 1/4 inches in diameter and 1.5 inches long. These were roughed up with me Dremel and then attached with epoxy putty t' give a very smooth, seamless look.
My model needed 3oz o' melted lead in t' nose for stability. T' center o' pressure for me rocket is a bit forward o' t' joint betwixt t' upper fin and t' correspondin' strake. Begad! Arrr! Get t' fully loaded balancin' point at least 3 or 4 inches forward o' t' CP. Ahoy! Glue on t' nose cone.
Finish up t' recovery system and it's all done except t' painting!
Flight and Recovery:
It be a beautiful launch day in t' Colorado prairie with record warmth and quite a large crowd o' club members and
visitors. Ahoy! Fearin' recovery damage, arrr, I installed a huge 42 inch X-form nylon chute. Begad! Ya scallywag! It was a very tight fit so I put the
Keelhaul®©™®
protector in alone rather than wrappin' t' chute with it. Avast! This proved t' be a mistake. Ahoy! Furthermore, t' chute was so
tight and filled such a great length o' t' tube that I was 75% sure it would nay eject. I had t' trust 7 ejection
charges t' do their work.
Before loadin' each motor, ya bilge rat, matey, I stuffed its forward end with a little waddin' t' prevent any back burn.
Then t' motors went in: 3 x E9-6 and 4 x C6-7. Arrr! T' timin' delays on this combination are nearly perfect for
simultaneous ejection. Blimey! I have a 4-lead clip whip, arrr, me bucko, so I twisted adjacent outer ignitors in parallel pairs, arrr, with one C
and one E in each pair. T' central C got its own lead.
We found a 5 foot long 1/4 inch rod, loaded t' rocket and attached all t' igniters. Blimey! With all t' wiring cascadin' out t' aft end, t' rocket looked pretty formidable on t' pad. Begad! 5-4-3-2-1-liftoff! T' flight was incredible. Avast! Avast, me proud beauty! T' rocket slowly climbed up t' rail and gradually gained speed, me bucko, givin' spectators quite a show at low altitude. Avast, me proud beauty! Begad! T' velocity and noise increased until t' rocket be tearin' through t' sky and it was virtually out of sight. Arrr! RockSim predicted an altitude o' 1993 feet.
T' ejection was well timed, me bucko, and remarkably, ya bilge rat, arrr, t' chute billowed out. Recovery be flawless. Aye aye! T' face o' t' aft centerin' rin' be seriously blackened with smoke and soot, ya bilge rat, arrr, which made it look extra cool. Avast! Avast, me proud beauty! I found that 6 o' t' 7 motors had lit, with one outer C6-7 failure. T' chute was a little singed so it would have been better t' wrap it directly in t' Keelhaul®©™® pad.
Those 7 motors cost a fair chunk o' change and I had only intended one launch, but as t' weather held, matey, I couldn't resist a second flight. Avast! This flight was a mirror image o' t' first: slow initial ascent acceleratin' until it ripped skyward in a pillar o' smoke and flame. Aye aye! Well, blow me down! Another flawless recovery and all 7 motors lit!
Summary:
Considerin' t' design, me bucko, ya bilge rat, ya bilge rat, t' very complex build, and t' perfect flights and recoveries, Sephydra must count as one of
my all-time favorites.
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