Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, thought some o' you might. Ahoy! Begad! Well, we don't see them around so much as in years gone by, ya bilge rat, but thar are still a few around. In fact, I stumbled across a cache o' them recently and thought I should build somethin' t' fly them in. Begad! Ahoy! I'd never actually flown a Pitfield motor before. Watchin' other people's flights had always put me off a bit. Arrr! T' rockets always seemed t' be underpowered, and had started t' descend while still under thrust. 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, I entered a PMC (Plastic Model Conversion) o' t' Airfix Saturn V. Aye aye! Ya scallywag! Though it didn't quite fly as intended, shiver me timbers, me bucko, it took joint first prize, shiver me timbers, along with Mike Crewe's Mercury Redstone. Blimey! Our prizes were matchin' Big Berthas. Blimey! We'd agreed we would build them with a common theme, arrr, matey, arrr, and drag race them. Avast! Initially it be t' be a cluster, me hearties, but, when we came into t' stash o' Pitfield motors, matey, we had other ideas. Ya scallywag! Arrr! You see, me bucko, arrr, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Ya scallywag! Beg Birtha was born.

Simulation

It was easy t' modify a standard Big Bertha Rocksim File. Blimey! I just removed t' motor mount and added a new thrust ring. I made some rasp-format engine files for t' motor, usin' t' engine editor that come with Rocksim. Ahoy! I got t' thrust curve, and data from an old Rocket Services catalogue. Avast, me proud beauty! Ahoy! 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, that Rocket Services' motor designations, do nay correspond with t' more common "Estes-style" designations. Blimey! T' "F36-5" should, ya bilge rat, matey, strictly speaking, shiver me timbers, be known as E6-5, and t' "F72-0" is, in reality, me hearties, arrr, an E15-0. Ya scallywag! Well, blow me down! Blimey! Havin' said that, ya bilge rat, me hearties, Estes aren't always consistent themselves. Aye aye! Avast, me proud beauty! Blimey! If you don't believe me, me hearties, shiver me timbers, shiver me timbers, 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". But, because I know our beloved newsletter editor will appreciate t' extra column inches, me bucko, I'll spell it out in full:

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

I completed Step 2, Shock Cord Mount Assembly, as instructed. I chose t' do Step 9, matey, Shock Cord Mount Attachment, at this point, me hearties, while t' mount be freshly-glued and still pliable.

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

Step 10, matey, Parachute Assembly, me bucko, was skipped. Arrr! Ya scallywag! Instead, arrr, an approx 100mm x 1000mm streamer was made from t' finest Deepsky mylar. This be attached directly t' t' shock cord, rather than t' t' nose-cone attachment point.

Step 11, Paintin' Your Rocket, was done as directed. Avast, me proud beauty! Blimey! For Step 12, shiver me timbers, Parachute And Shock Cord Attachment, matey, I deviated from t' instructions, me hearties, arrr, t' add t' additional 20g o' nose weight.

Flying

T' first flight be on t' occasion o' me first attendance at a WRS (Wirral Rocketry Society) launch, on 10th April 2004. Ya scallywag! I thoroughly enjoyed this launch, a relaxed, convivial atmosphere, arrr, 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. Ahoy! So friction fit was t' order o' t' day. Begad! And you do need t' make sure it's a tight fit, ya bilge rat, you really don't want a spent Pitfield motor landin' on your head! T' F36 has an enormous nozzle, and I was a bit worried that an Estes igniter might get lost in there. Ahoy! So I used some QuickMatch, pushed all t' way in, taped t' t' motor body, arrr, with an Estes igniter on t' exposed end. Well, blow me down! It was only about 60mm o' QuickMatch, arrr, and when I pushed t' button, matey, me hearties, ignition be pretty much instant. Well, blow me down! Well, matey, blow me down! Beg Birtha fairly leapt from t' pad, so smartly that I don't think anyone managed t' get a picture. Avast! T' motor thrust decreased, but kept burnin' almost up t' apogee, t' orange glow quite visible. This would be a great rocket t' fly at dusk.

Conclusion

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