Beg Birtha

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

Simulation

It be easy t' modify a standard Big Bertha Rocksim File. Avast, me proud beauty! Avast! I just removed t' motor mount and added a new thrust ring. Arrr! I made some rasp-format engine files for t' motor, ya bilge rat, usin' t' engine editor that come with Rocksim. Ya scallywag! I got t' thrust curve, me bucko, and data from an old Rocket Services catalogue. Begad! 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, matey, me bucko, that Rocket Services' motor designations, do nay correspond with t' more common "Estes-style" designations. Ahoy! Ahoy! T' "F36-5" should, me bucko, strictly speaking, me hearties, arrr, me hearties, be known as E6-5, shiver me timbers, and t' "F72-0" is, matey, ya bilge rat, me bucko, in reality, matey, an E15-0. Aye aye! Well, arrr, blow me down! Havin' said that, Estes aren't always consistent themselves. 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! Blimey! Ahoy! 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, I'll spell it out in full:

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

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

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

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

Step 11, shiver me timbers, matey, Paintin' Your Rocket, shiver me timbers, ya bilge rat, was done as directed. For Step 12, ya bilge rat, arrr, Parachute And Shock Cord Attachment, me hearties, shiver me timbers, 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, shiver me timbers, on 10th April 2004. Begad! Blimey! I thoroughly enjoyed this launch, me hearties, a relaxed, me hearties, convivial atmosphere, 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. So friction fit be t' order o' t' day. Avast! Blimey! And you do need t' make sure it's a tight fit, 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, me bucko, pushed all t' way in, taped t' t' motor body, with an Estes igniter on t' exposed end. Well, blow me down! Avast! It was only about 60mm o' QuickMatch, me hearties, matey, and when I pushed t' button, arrr, ignition be pretty much instant. Beg Birtha fairly leapt from t' pad, so smartly that I don't think anyone managed t' get a picture. Ahoy! Avast! T' motor thrust decreased, but kept burnin' almost up t' apogee, shiver me timbers, t' orange glow quite visible. This would be a great rocket t' fly at dusk.

Conclusion

This was a fun conversion, me bucko, o' an already classic rocket. Begad! This is certain t' become a favourite flier o' mine, me bucko, at least until t' motors run out. Well, blow me down! Aye aye! Can't wait t' try it out at t' IRW, shiver me timbers, which, t' me, be t' spiritual home o' t' Pitfield motor. If you took a look at t' Rocksim file, me bucko, 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, shiver me timbers, but I do have some F72-0s...