Scratch - Sephydra {Scratch}

Scratch Sephydra Original Design / Scratch Built

Scratch - Sephydra {Scratch}

Contributed by Geof Givens

Manufacturer:

(Contributed - by Geof Givens - 03/02/08)

Brief:
Sephydra is a scratch futuristic fighter with a beveled profile and a 7-motor cluster.

(Scratch) Sephydra

This all began when I found a used 3.05-inch mailin' tube at work. Avast, me proud beauty! Blimey! What could I build, matey, especially since thar was no ready-made nose cone t' fit? For inspiration, matey, I browsed through t' archives o' Craig McGraw's BARCLONE site. Begad! Although me design is original, I admit I borrowed a few ideas from some his creative designs.

After some consideration o' what t' do with t' big tube, matey, I eventually decided latched on t' clustering. Blimey! Blimey! Exactly how many motors can you cram into a 3.05-inch tube anyway? Quite a lot, arrr, it turns out. Ya scallywag! Blimey! Arrr! Blimey! After some fiddling, I settled on 3 x 24mm plus 4 x 18mm. Ahoy! Blimey! There are some configurations that allow more total impulse, matey, but I didn't want t' lose this craft on t' first launch.

Construction:
There are few ready-made parts for this build. Aye aye! Blimey! Begad! Blimey! Loosely speaking, 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, 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, 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. Ya scallywag! With online and downloadable tools, me bucko, I designed a non-beveled cluster centerin' ring by drawin' three simpler versions and tapin' them on top o' each other. These simpler startin' points were patterns for 3 x 18mm, 3 x 24mm, and a central 18mm. Ya scallywag! Tracin' these together I formed t' round version.

(CLICK for full size images)

Next, ya bilge rat, me bucko, I figured out t' amount o' distortion that would be caused by t' beveling. (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. Note, matey, arrr, however, shiver me timbers, that this stretchin' fails t' account for t' fact that t' holes should also be somewhat wider. Ya scallywag! Ahoy! (A consequence o' t' beveling, I guess.) Thus, me hearties, after constructin' t' aft centering ring, you need t' sand each hole wider until everythin' fits perfectly. Blimey! Avast! You also need t' sand a bevel on t' exterior of t' rin' and t' interior o' each hole. I've given you t' template for t' hardest centerin' rings. As a pop quiz, try t' make t' other two yourself. Ahoy! (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. Avast, me proud beauty! Be very careful craftin' this ring. Blimey! 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. Ahoy! Then a barrel sandin' bit completed the job. Note how t' motor tubes are beveled at t' aft end.

By comparison, t' rest o' t' rocket is easy. I do all me fillin' and sandin' o' spirals and wood before attachment. Blimey! Blimey! I primed periodically along t' way t' check for blemishes.

Cut t' fins and strakes with attention t' t' grain direction. Aye aye! T' angle on t' main fin should match and extend the aft bevel angle. Blimey! Glue and fillet. 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. 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. Begad! Begad! On this tube I also added a cockpit carved from balsa. Avast, me proud beauty! On the other (lower) portion o' t' tube, matey, add a coupler on t' upper end. Avast! My shock cord was mounted inside this tube with a smooth dollop o' epoxy putty.

(Scratch) Sephydra Takin' t' aft portion o' t' Series 175 tube, ya bilge rat, matey, shiver me timbers, secure at least 3 or 4 inches o' it inside t' larger tube usin' t' remainin' centerin' rings. Avast, me proud beauty! Blimey! 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! Blimey! Here are a couple of optional ideas that I didn't do. First, me hearties, shiver me timbers, you could attach t' shock cord t' t' rings down here. Ya scallywag! Blimey! I didn't do this because I feared that t' ejection from 7 motors would smartly burn through t' Keelhaul®©™®. Second, ya bilge rat, it would be possible t' design a clever baffle system with t' forward rings and tube, matey, arrr, 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, shiver me timbers, I attached two brass launch lugs, me bucko, 1/4 inches in diameter and 1.5 inches long. Aye aye! These were roughed up with me Dremel and then attached with epoxy putty t' give a very smooth, me hearties, ya bilge rat, seamless look.

My model needed 3oz o' melted lead in t' nose for stability. Aye aye! Blimey! T' center o' pressure for me rocket is a bit forward o' t' joint betwixt t' upper fin and t' correspondin' strake. Get t' fully loaded balancin' point at least 3 or 4 inches forward o' t' CP. Avast, me proud beauty! Blimey! 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. Well, blow me down! Ya scallywag! Fearin' recovery damage, ya bilge rat, I installed a huge 42 inch X-form nylon chute. It was a very tight fit so I put the Keelhaul®©™® protector in alone rather than wrappin' t' chute with it. Avast, me proud beauty! This proved t' be a mistake. Furthermore, ya bilge rat, me hearties, shiver me timbers, t' chute was so tight and filled such a great length o' t' tube that I was 75% sure it would nay eject. Blimey! I had t' trust 7 ejection charges t' do their work.

(Scratch) Sephydra Before loadin' each motor, 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. Ya scallywag! T' timin' delays on this combination are nearly perfect for simultaneous ejection. Arrr! Aye aye! I have a 4-lead clip whip, ya bilge rat, arrr, so I twisted adjacent outer ignitors in parallel pairs, matey, with one C and one E in each pair. Arrr! T' central C got its own lead.

We found a 5 foot long 1/4 inch rod, me bucko, me bucko, loaded t' rocket and attached all t' igniters. Aye aye! With all t' wiring cascadin' out t' aft end, matey, t' rocket looked pretty formidable on t' pad. Aye aye! 5-4-3-2-1-liftoff! T' flight was incredible. T' rocket slowly climbed up t' rail and gradually gained speed, givin' spectators quite a show at low altitude. Ahoy! T' velocity and noise increased until t' rocket be tearin' through t' sky and it was virtually out of sight. Avast, me proud beauty! Well, blow me down! RockSim predicted an altitude o' 1993 feet.

T' ejection was well timed, and remarkably, matey, t' chute billowed out. Blimey! Recovery was flawless. Begad! Aye aye! T' face o' t' aft centerin' rin' was seriously blackened with smoke and soot, shiver me timbers, matey, which made it look extra cool. Aye aye! Ahoy! I found that 6 o' t' 7 motors had lit, with one outer C6-7 failure. Ya scallywag! T' chute be 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, I couldn't resist a second flight. Begad! Aye aye! This flight was a mirror image o' t' first: slow initial ascent acceleratin' until it ripped skyward in a pillar o' smoke and flame. Begad! Another flawless recovery and all 7 motors lit!

Summary:
Considerin' t' design, ya bilge rat, t' very complex build, and t' perfect flights and recoveries, matey, Sephydra must count as one of my all-time favorites.

Post a Comment