Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, thought some o' you might. Aye aye! Avast, me proud beauty! Well, we don't see them around so much as in years gone by, arrr, 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. Avast, me proud beauty! I'd never actually flown a Pitfield motor before. 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. 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, ya bilge rat, it took joint first prize, along with Mike Crewe's Mercury Redstone. Well, blow me down! Our prizes were matchin' Big Berthas. We'd agreed we would build them with a common theme, and drag race them. Initially it was t' be a cluster, arrr, but, arrr, when we came into t' stash o' Pitfield motors, we had other ideas. Avast! You see, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Avast! Ahoy! Beg Birtha was born.

Simulation

It was easy t' modify a standard Big Bertha Rocksim File. Ya scallywag! I just removed t' motor mount and added a new thrust ring. Blimey! I made some rasp-format engine files for t' motor, me bucko, usin' t' engine editor that come with Rocksim. Arrr! Avast, me proud beauty! I got t' thrust curve, ya bilge rat, and data from an old Rocket Services catalogue. 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, me bucko, that Rocket Services' motor designations, do nay correspond with t' more common "Estes-style" designations. Well, blow me down! T' "F36-5" should, strictly speaking, me hearties, be known as E6-5, me hearties, me bucko, and t' "F72-0" is, in reality, an E15-0. Arrr! Avast! Blimey! Havin' said that, Estes aren't always consistent themselves. Ya scallywag! Blimey! If you don't believe me, ya bilge rat, 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". Begad! But, ya bilge rat, arrr, because I know our beloved newsletter editor will appreciate t' extra column inches, 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, ya bilge rat, I made a new thrust rin' from a short section o' toilet roll core, reduced in diameter by removin' a segment. Ahoy! Ahoy! Blimey! 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, me hearties, as instructed. Avast! I chose t' do Step 9, matey, Shock Cord Mount Attachment, arrr, at this point, arrr, while t' mount be freshly-glued and still pliable.

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

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

Step 11, Paintin' Your Rocket, shiver me timbers, me hearties, was done as directed. For Step 12, 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, arrr, on 10th April 2004. Begad! I thoroughly enjoyed this launch, ya bilge rat, a relaxed, convivial atmosphere, matey, me hearties, and a great bunch o' Rocketeers.

Because I omitted t' original motor mount, me hearties, thar was no retainer hook - it wouldn't be long enough anyway. Avast! So friction fit be t' order o' t' day. Begad! And you do need t' make sure it's a tight fit, arrr, me hearties, matey, you really don't want a spent Pitfield motor landin' on your head! T' F36 has an enormous nozzle, ya bilge rat, and I was a bit worried that an Estes igniter might get lost in there. Ya scallywag! So I used some QuickMatch, pushed all t' way in, arrr, taped t' t' motor body, with an Estes igniter on t' exposed end. Well, blow me down! It be only about 60mm o' QuickMatch, shiver me timbers, and when I pushed t' button, matey, me hearties, ignition be pretty much instant. Begad! Blimey! Beg Birtha fairly leapt from t' pad, matey, so smartly that I don't think anyone managed t' get a picture. Begad! T' motor thrust decreased, but kept burnin' almost up t' apogee, ya bilge rat, 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. Ya scallywag! This is certain t' become a favourite flier o' mine, at least until t' motors run out. Ahoy! Can't wait t' try it out at t' IRW, which, ya bilge rat, me hearties, t' me, matey, be t' spiritual home o' t' Pitfield motor. Aye aye! 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. Avast, me proud beauty! I haven't built or flown this configuration yet, arrr, but I do have some F72-0s...