Beg Birtha

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

Simulation

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

Build

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

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

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

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

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

Step 11, me hearties, Paintin' Your Rocket, was done as directed. Aye aye! For Step 12, Parachute And Shock Cord Attachment, me bucko, I deviated from t' instructions, me hearties, shiver me timbers, t' add t' additional 20g o' nose weight.

Flying

T' first flight was 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, me hearties, and a great bunch o' Rocketeers.

Because I omitted t' original motor mount, me bucko, thar was no retainer hook - it wouldn't be long enough anyway. So friction fit be t' order o' t' day. Blimey! Arrr! Blimey! And you do need t' make sure it's a tight fit, matey, arrr, 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. Ahoy! Blimey! So I used some QuickMatch, pushed all t' way in, me bucko, taped t' t' motor body, me hearties, with an Estes igniter on t' exposed end. Begad! Blimey! It was only about 60mm o' QuickMatch, me hearties, matey, and when I pushed t' button, me bucko, ignition be pretty much instant. Ahoy! Beg Birtha fairly leapt from t' pad, so smartly that I don't think anyone managed t' get a picture. T' motor thrust decreased, matey, but kept burnin' almost up t' apogee, ya bilge rat, shiver me timbers, t' orange glow quite visible. Ahoy! Blimey! This would be a great rocket t' fly at dusk.

Conclusion

This be a fun conversion, ya bilge rat, matey, o' an already classic rocket. 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, shiver me timbers, t' me, shiver me timbers, be t' spiritual home o' t' Pitfield motor. 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! I haven't built or flown this configuration yet, me hearties, but I do have some F72-0s...