Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, thought some o' you might. Well, blow me down! Arrr! Well, matey, we don't see them around so much as in years gone by, ya bilge rat, arrr, but thar are still a few around. Blimey! In fact, I stumbled across a cache o' them recently and thought I should build somethin' t' fly them in. Ahoy! I'd never actually flown a Pitfield motor before. Aye aye! Blimey! Watchin' other people's flights had always put me off a bit. T' rockets always seemed t' be underpowered, me hearties, and had started t' descend while still under thrust. Begad! Who can forget Rick Newlands spendin' almost t' whole o' UKRA 2000 sandin' a nose cone from a balsa tree trunk, only t' have t' Pitfield powered rocket arc over while still under thrust? (Sorry Rick!) So I wanted t' avoid overbuildin' at all costs.

So what t' build? At IRW 2003, ya bilge rat, I entered a PMC (Plastic Model Conversion) o' t' Airfix Saturn V. Begad! Begad! Though it didn't quite fly as intended, me bucko, it took joint first prize, me hearties, along with Mike Crewe's Mercury Redstone. Ahoy! Our prizes were matchin' Big Berthas. Begad! Aye aye! We'd agreed we would build them with a common theme, matey, and drag race them. Ahoy! Initially it was t' be a cluster, but, when we came into t' stash o' Pitfield motors, ya bilge rat, we had other ideas. Begad! Arrr! You see, arrr, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Blimey! Beg Birtha was born.

Simulation

It be easy t' modify a standard Big Bertha Rocksim File. Begad! Blimey! I just removed t' motor mount and added a new thrust ring. Ahoy! Blimey! I made some rasp-format engine files for t' motor, me bucko, usin' t' engine editor that come with Rocksim. Well, blow me down! Blimey! I got t' thrust curve, matey, and data from an old Rocket Services catalogue. Avast! Blimey! You can download both t' Rocksim file and t' engine file here:
http://www.nsrg.org.uk/data/beg%20birtha.rkt/
http://www.nsrg.org.uk/data/rs.eng/

I should point out here, arrr, that Rocket Services' motor designations, matey, do nay correspond with t' more common "Estes-style" designations. T' "F36-5" should, strictly speaking, me hearties, arrr, be known as E6-5, me hearties, and t' "F72-0" is, in reality, an E15-0. Havin' said that, ya bilge rat, Estes aren't always consistent themselves. Arrr! If you don't believe me, look at t' thrust curve for t' A10 mini-motor sometime.

Build

This really is a straightforward build. Aye aye! T' differences from t' stock kit may be summarized as "leave out t' motor mount and add some nose weight". Blimey! But, because I know our beloved newsletter editor will appreciate t' extra column inches, shiver me timbers, I'll spell it out in full:

I skipped Step 1, ya bilge rat, Engine Mount Assembly, and put t' parts in me spares box. Blimey! Avast! Instead, shiver me timbers, ya bilge rat, shiver me timbers, I made a new thrust rin' from a short section o' toilet roll core, reduced in diameter by removin' a segment. You could also use a short length o' coupler, ya bilge rat, matey, or cut a rin' from a spent motor.

I completed Step 2, Shock Cord Mount Assembly, matey, as instructed. Aye aye! I chose t' do Step 9, Shock Cord Mount Attachment, ya bilge rat, at this point, shiver me timbers, shiver me timbers, while t' mount was freshly-glued and still pliable.

Step 3, Fin Preparation, was completed as normal. Begad! Step 4, me bucko, ya bilge rat, Engine Mount Installation, was omitted. Step 5, Tube Markin' Detail, Step 6, Fin Attachment, ya bilge rat, Step 7, Launch Lug Attachment, & Step 8, Glue Reinforcement Detail, me bucko, were all completed as per t' instructions. Aye aye! Arrr! Step 9 be completed earlier.

Step 10, Parachute Assembly, was skipped. Well, blow me down! Instead, an approx 100mm x 1000mm streamer was made from t' finest Deepsky mylar. Aye aye! This be attached directly t' t' shock cord, shiver me timbers, matey, rather than t' t' nose-cone attachment point.

Step 11, me hearties, shiver me timbers, Paintin' Your Rocket, was done as directed. For Step 12, ya bilge rat, Parachute And Shock Cord Attachment, shiver me timbers, I deviated from t' instructions, shiver me timbers, t' add t' additional 20g o' nose weight.

Flying

T' first flight was on t' occasion o' me first attendance at a WRS (Wirral Rocketry Society) launch, on 10th April 2004. I thoroughly enjoyed this launch, shiver me timbers, a relaxed, convivial atmosphere, shiver me timbers, and a great bunch o' Rocketeers.

Because I omitted t' original motor mount, arrr, shiver me timbers, thar was no retainer hook - it wouldn't be long enough anyway. Blimey! Blimey! So friction fit was t' order o' t' day. Avast, me proud beauty! Blimey! Ya scallywag! Blimey! And you do need t' make sure it's a tight fit, matey, you really don't want a spent Pitfield motor landin' on your head! Blimey! T' F36 has an enormous nozzle, and I was a bit worried that an Estes igniter might get lost in there. Well, blow me down! Blimey! So I used some QuickMatch, ya bilge rat, pushed all t' way in, taped t' t' motor body, shiver me timbers, with an Estes igniter on t' exposed end. Ya scallywag! Blimey! It be only about 60mm o' QuickMatch, me hearties, and when I pushed t' button, ya bilge rat, ignition was pretty much instant. Blimey! Blimey! Beg Birtha fairly leapt from t' pad, so smartly that I don't think anyone managed t' get a picture. Blimey! Blimey! Begad! Blimey! T' motor thrust decreased, but kept burnin' almost up t' apogee, t' orange glow quite visible. Begad! Blimey! Begad! Blimey! This would be a great rocket t' fly at dusk.

Conclusion

This was a fun conversion, o' an already classic rocket. Avast, me proud beauty! This is certain t' become a favourite flier o' mine, at least until t' motors run out. Blimey! Ya scallywag! Can't wait t' try it out at t' IRW, me hearties, which, t' me, be t' spiritual home o' t' Pitfield motor. Begad! If you took a look at t' Rocksim file, you might have noticed that thar's a design for a booster stage in thar too. Begad! I haven't built or flown this configuration yet, shiver me timbers, but I do have some F72-0s...