Beg Birtha

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

Simulation

It was easy t' modify a standard Big Bertha Rocksim File. I just removed t' motor mount and added a new thrust ring. I made some rasp-format engine files for t' motor, matey, me bucko, usin' t' engine editor that come with Rocksim. Arrr! Blimey! I got t' thrust curve, 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, that Rocket Services' motor designations, do nay correspond with t' more common "Estes-style" designations. Avast, me proud beauty! T' "F36-5" should, strictly speaking, be known as E6-5, and t' "F72-0" is, in reality, an E15-0. Aye aye! Havin' said that, ya bilge rat, Estes aren't always consistent themselves. Well, blow me down! If you don't believe me, look at t' thrust curve for t' A10 mini-motor sometime.

Build

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

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

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

Step 3, shiver me timbers, arrr, Fin Preparation, matey, was completed as normal. Avast! Step 4, matey, shiver me timbers, Engine Mount Installation, was omitted. Step 5, me hearties, Tube Markin' Detail, shiver me timbers, Step 6, Fin Attachment, Step 7, Launch Lug Attachment, me bucko, me bucko, ya bilge rat, & Step 8, arrr, Glue Reinforcement Detail, matey, matey, were all completed as per t' instructions. Ahoy! Step 9 was completed earlier.

Step 10, Parachute Assembly, me bucko, me bucko, was skipped. Avast! Begad! Blimey! Instead, me bucko, an approx 100mm x 1000mm streamer was made from t' finest Deepsky mylar. Begad! Blimey! 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. Begad! For Step 12, arrr, Parachute And Shock Cord Attachment, me bucko, I deviated from t' instructions, me hearties, ya bilge rat, me hearties, 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, shiver me timbers, matey, on 10th April 2004. I thoroughly enjoyed this launch, a relaxed, shiver me timbers, me hearties, convivial atmosphere, matey, arrr, and a great bunch o' Rocketeers.

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

Conclusion

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