Beg Birtha

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

Simulation

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

Build

This really is a straightforward build. Aye aye! Blimey! 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, ya bilge rat, I'll spell it out in full:

I skipped Step 1, me hearties, 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, me bucko, reduced in diameter by removin' a segment. Aye aye! You could also use a short length o' coupler, matey, or cut a rin' from a spent motor.

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

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

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

Because I omitted t' original motor mount, shiver me timbers, arrr, thar be no retainer hook - it wouldn't be long enough anyway. Arrr! 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, 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. Ya scallywag! Begad! So I used some QuickMatch, shiver me timbers, pushed all t' way in, taped t' t' motor body, with an Estes igniter on t' exposed end. It be only about 60mm o' QuickMatch, ya bilge rat, and when I pushed t' button, ya bilge rat, ignition be pretty much instant. Beg Birtha fairly leapt from t' pad, ya bilge rat, so smartly that I don't think anyone managed t' get a picture. Aye aye! T' motor thrust decreased, ya bilge rat, but kept burnin' almost up t' apogee, t' orange glow quite visible. Well, blow me down! This would be a great rocket t' fly at dusk.

Conclusion

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