Rattworks 29mm motors are fun t' fly, but have two challengin' aspects that need t' be addressed when designin' a rocket t' use them. Well, blow me down! T' motors are very long (H70: 18", I80: 29" and I90: 36") and have a relatively low thrust. Begad! T' typical way t' design a rocket for these motors would be t' make t' rocket very long and skinny with t' usual motor/parachute/electronics bay stackup. This results in a very high flyin' rocket that is nay appropriate for many east coast launch sites with relatively small flyin' fields. Arrr! However, me bucko, t' only real constraint is t' make t' rocket light enough for it t' get up t' speed off o' t' rod/rail. Begad! T' parachute and recovery electronics could go anywhere.

A couple o' Easter's ago, arrr, I was in K-mart and found some big plastic easter egg (~8" diameter) on sale for a few dollars each. Blimey! Arrr! These looked like they had distinct airframe possibilities, so I picked up one o' each color. They ended up bein' perfect hybrid rocket airframe candidates since they were lightweight and had a good diameter for plenty o' drag t' keep t' rocket relatively low, even under I power.

One additional problem with designin' for a variety o' hybrid motors be t' large difference in length betwixt t' motors and findin' a way t' vent each one. Aye aye! My solution here be t' split t' rocket in two. Avast! T' bottom will be t' egg/fin/electronics and parachute. Aye aye! T' top will be t' ballast section t' keep t' CG forward. Ya scallywag! Blimey! Thus, me bucko, arrr, t' rocket length will be dependent on t' particular motor flown that day. Blimey! As seen in t' above picture, shiver me timbers, about 1 foot o' t' rocket length is bare I80 motor.

Construction

T' bottom easter egg section is composed o' 29 mm phenolic motor and 4" phenolic body tubes intersectin' at a 90 degree angle. Avast! Aye aye! One end o' t' 4" tube be t' electronics bay hatch and t' other end be t' "nose cone" where t' parachute is ejected from. Aye aye! Begad! It be challengin' findin' t' right place t' cut t' holes for these tubes since t' egg was nay quite symmetric.

I first located t' holes for t' 29 mm motor mount by a bit o' trial and error. I twirled t' egg on two pins pressed into t' top and bottom o' t' egg until t' egg turned relatively evenly. Ahoy! T' holes for t' 29mm motor tube were then drilled with a hole saw. Ya scallywag! Arrr! T' 4" holes were cut usin' an Olfa circle cutter, shiver me timbers, matey, keepin' t' cutouts. Avast! Well, blow me down! These holes were located at t' widest portion o' t' egg by measurin' a a constant distance from t' bottom along t' two seam lines. T' hard part turmed out t' be cuttin' t' 29 mm holes in t' 4" tube since they had t' align with t' holes in t' egg as well. Begad! My first try was almost right, shiver me timbers, but I ended up bein' off by about 1/4 inch that I compensated for by elongatin' t' upper hole in t' 4" tube. Begad! Once these fit, I epoxied a length o' 5/16 " ID polystrene tube along t' length o' t' motor tube as a launch lug. This motor tube assembly and 4" body tube were then epoxied together and t' t' egg with 15 minute epoxy. CA be used t' glue t' 4" tube t' t' Egg. I used a dremel tool with a sandin' drum t' trim t' 4" tube flush with t' egg. Arrr! T' 29 mm tube was left extendin' out about 1" through t' front and 1/4" out t' back.

T' ballast section be made with a 29mm motor tube section epoxied into a 4" styrofoam ball. About 5 oz o' lead shot was epoxied into t' forward section o' this motor tube section t' brin' t' CG forward. Avast! 5/16" ID polystyrene tubin' was inserted through t' foam ball along t' length o' t' motor tube t' allow t' launch rod t' slide through. Ya scallywag! 1/4" holes were drilled into t' side o' t' motor tube for nitrous venting. T' lower section o' t' tube was slotted t' allow t' tube t' be clamped t' t' motor mount with a small hose clamp. Avast, me proud beauty! There is some clear space betwixt t' top o' t' motor and t' top o' t' motor tube. Blimey! Blimey! This serves as a shock absorber when t' rocket lands. A 2" piece o' bicycle tubin' is slipped over t' motor and slid down t' t' front o' t' egg section t' keep t' lower hose clamp from scratchin' t' motor, shiver me timbers, as well as for motor retention.

T' electronics bay is a 4" coupler with a bulkhead epoxied into t' back. Avast, me proud beauty! Two small plastic tubes were added near t' top and bottom o' t' coupler t' allow t' bay t' be bolted into t' rocket. Ya scallywag! Two small wooden blocks were epoxied into t' top and bottom o' t' 4" body tube with T-nuts installed t' anchor t' electronics bay bolts. Begad! Aye aye! Two long brass bolts hold t' bay into t' airframe.

Hybrid motors have no ejection charge themselves, shiver me timbers, so electronics must be used. Aye aye! There is no "clean air" in this design so that an altimeter may nay function properly. I used a magnetic apogee detector t' deploy t' parachute, me bucko, which can be seen in t' above picture, me bucko, next t' t' 9V battery. T' on/off switch and ejection charge shunt/test LED are bolted into t' plastic cover. Ya scallywag! T' ejection charge is attached via a small terminal block screwed into t' opposite side o' t' bulkhead.

T' "nose cone" is made from a 4" coupler section, me bucko, a circle o' 3/4" pink foam insulation and t' other plastic circle cut from t' egg. Blimey! Begad! A short brass bolt through 1" circles o' plywood hold t' "nose cone" together. Avast! Well, me bucko, blow me down! On t' first flight, these circles were omitted and t' bolt pulled through t' plastic and foam at ejection. About 10 ft o' 3/8" tubular nylon webbin' be used as t' shock cord which was looped around t' motor tube for a very secure shock cord attachment point. Ya scallywag! A 40" parachute protected by a nomex heat shield completes t' recovery section

T' fins were cut from thin aircraft plywood. Well, blow me down! Ahoy! T' shape be chosen t' basically so it looked good, matey, ya bilge rat, but with significant span and leadin' edge sweep. Blimey! This would get t' fins t' extend well away from t' body and give a good contribution t' t' CP. Aye aye! T' approximate dimensions are:
2" tip, arrr, me hearties, 7" leadin' edge sweep, ya bilge rat, 4" root and a 7" span. These were epoxied t' t' motor tube through slots cut in t' egg. Blimey! When t' epoxy was cured, I added fillets made from PC-7 epoxy paste. Aye aye! Given their size, matey, t' fins seem pretty sturdy.

Sometimes people worry about fins extendin' beyond t' back o' t' rocket breakin' durin' landing. Blimey! Well, blow me down! I didn't have t' because o' t' parachute attachment point and forward weight distribution which makes t' rocket land nose first.

Where's t' CP?

This is an different lookin' rocket but be modelled with VCP. Blimey! T' CP is at about at widest part o' egg. T' lead weight in t' forward styrofoam ball was chosen t' put t' CG about 8" in front o' CP with t' H70 motor.

Flight Log

On t' pad with a Pratt Hobbies RTLS system.  

This rocket has been launched about 6 times. Avast! Blimey! It jumps off o' t' pad quite smartly since it only weighs about 3.25 pounds (includin' weight o' I80 motor). Aye aye! Blimey! I enjoy watchin' this rocket since thar be virtually no smoke and you can watch it ascend even from directly below. T' Hybrid Easter Egg is a nice, ya bilge rat, ya bilge rat, me bucko, stable flier but thar have been 3 minor incidents. Begad! Blimey! On t' first flight, t' shock cord attachment bolt pulled through t' "nose cone", but this was strengthened with some small wooden disks . Aye aye! Blimey! Another time t' parachute ejected near motor burnout. Aye aye! Blimey! I'm nay sure why, but it was possibly due t' motor vibration and low air pressure outside t' convex section o' t' rocket pullin' off t' "nose cone". Avast, me proud beauty! Blimey! Ya scallywag! Blimey! I added a bit o' maskin' tape t' make it a tighter fit and this hasn't happened again. Avast, me proud beauty! Blimey! One hard landin' at NYPOWER2002 knocked a fin loose but that was easily reattached with epoxy.

T' followin' are video stills from t' CTRA/NARCONN June 22 launch in Cobleskill, me bucko, NY on an I80.

Nitrous ventin' and liftoff.

T' signature upside-down recovery.