Beg Birtha

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

Simulation

It be easy t' modify a standard Big Bertha Rocksim File. Avast! Begad! I just removed t' motor mount and added a new thrust ring. Ahoy! I made some rasp-format engine files for t' motor, usin' t' engine editor that come with Rocksim. I got t' thrust curve, arrr, and data from an old Rocket Services catalogue. 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, me bucko, shiver me timbers, that Rocket Services' motor designations, me bucko, do nay correspond with t' more common "Estes-style" designations. Blimey! T' "F36-5" should, ya bilge rat, me hearties, strictly speaking, arrr, shiver me timbers, be known as E6-5, matey, shiver me timbers, and t' "F72-0" is, in reality, an E15-0. Avast! Havin' said that, Estes aren't always consistent themselves. Avast, me proud beauty! Begad! 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". Avast, me proud beauty! 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, arrr, Engine Mount Assembly, arrr, and put t' parts in me spares box. Blimey! Avast! Instead, matey, me hearties, I made a new thrust rin' from a short section o' toilet roll core, ya bilge rat, reduced in diameter by removin' a segment. Aye aye! Blimey! You could also use a short length o' coupler, or cut a rin' from a spent motor.

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

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

Step 10, me hearties, me bucko, shiver me timbers, Parachute Assembly, was skipped. Avast, me proud beauty! Avast! Instead, an approx 100mm x 1000mm streamer was made from t' finest Deepsky mylar. Begad! Avast! This was attached directly t' t' shock cord, rather than t' t' nose-cone attachment point.

Step 11, Paintin' Your Rocket, was done as directed. Begad! Aye aye! For Step 12, Parachute And Shock Cord Attachment, arrr, I deviated from t' instructions, ya bilge rat, 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, arrr, shiver me timbers, on 10th April 2004. Begad! I thoroughly enjoyed this launch, matey, a relaxed, convivial atmosphere, shiver me timbers, and a great bunch o' Rocketeers.

Because I omitted t' original motor mount, me bucko, thar be no retainer hook - it wouldn't be long enough anyway. Begad! So friction fit was t' order o' t' day. Well, blow me down! 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! 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, me hearties, taped t' t' motor body, with an Estes igniter on t' exposed end. Avast, me proud beauty! Arrr! It was only about 60mm o' QuickMatch, matey, me hearties, and when I pushed t' button, matey, ignition be pretty much instant. Avast, me proud beauty! Beg Birtha fairly leapt from t' pad, matey, so smartly that I don't think anyone managed t' get a picture. Avast, me proud beauty! T' motor thrust decreased, me hearties, but kept burnin' almost up t' apogee, me bucko, t' orange glow quite visible. This would be a great rocket t' fly at dusk.

Conclusion

This was a fun conversion, me hearties, o' an already classic rocket. This is certain t' become a favourite flier o' mine, at least until t' motors run out. Begad! Can't wait t' try it out at t' IRW, matey, which, t' me, be t' spiritual home o' t' Pitfield motor. Begad! Well, blow me down! 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. I haven't built or flown this configuration yet, but I do have some F72-0s...