Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, shiver me timbers, me hearties, thought some o' you might. Ahoy! Well, we don't see them around so much as in years gone by, but thar are still a few around. In fact, arrr, I stumbled across a cache o' them recently and thought I should build somethin' t' fly them in. Aye aye! Aye aye! I'd never actually flown a Pitfield motor before. Ya scallywag! Watchin' other people's flights had always put me off a bit. T' rockets always seemed t' be underpowered, me bucko, and had started t' descend while still under thrust. Aye aye! Who can forget Rick Newlands spendin' almost t' whole o' UKRA 2000 sandin' a nose cone from a balsa tree trunk, ya bilge rat, matey, 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. Though it didn't quite fly as intended, it took joint first prize, along with Mike Crewe's Mercury Redstone. Begad! Our prizes were matchin' Big Berthas. We'd agreed we would build them with a common theme, me hearties, and drag race them. Begad! Ya scallywag! Initially it was t' be a cluster, me hearties, but, when we came into t' stash o' Pitfield motors, shiver me timbers, we had other ideas. You see, shiver me timbers, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Arrr! Avast, me proud beauty! Beg Birtha was born.

Simulation

It was easy t' modify a standard Big Bertha Rocksim File. Aye aye! 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. Begad! Avast! I got t' thrust curve, ya bilge rat, and data from an old Rocket Services catalogue. Aye aye! Well, blow me down! 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, shiver me timbers, ya bilge rat, do nay correspond with t' more common "Estes-style" designations. Well, blow me down! Blimey! T' "F36-5" should, me bucko, strictly speaking, shiver me timbers, ya bilge rat, be known as E6-5, matey, and t' "F72-0" is, ya bilge rat, in reality, an E15-0. Avast! Blimey! Havin' said that, Estes aren't always consistent themselves. Aye aye! Blimey! Avast, me proud beauty! Blimey! If you don't believe me, arrr, look at t' thrust curve for t' A10 mini-motor sometime.

Build

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

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

I completed Step 2, Shock Cord Mount Assembly, as instructed. Ya scallywag! Well, me bucko, blow me down! Blimey! I chose t' do Step 9, me hearties, Shock Cord Mount Attachment, me bucko, at this point, ya bilge rat, while t' mount was freshly-glued and still pliable.

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

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

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

Conclusion

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