Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, matey, ya bilge rat, thought some o' you might. Arrr! Arrr! Well, matey, shiver me timbers, we don't see them around so much as in years gone by, ya bilge rat, shiver me timbers, but thar are still a few around. Aye aye! In fact, I stumbled across a cache o' them recently and thought I should build somethin' t' fly them in. Begad! I'd never actually flown a Pitfield motor before. Ya scallywag! Arrr! Watchin' other people's flights had always put me off a bit. Ahoy! T' rockets always seemed t' be underpowered, ya bilge rat, 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, me hearties, 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, I entered a PMC (Plastic Model Conversion) o' t' Airfix Saturn V. Arrr! Though it didn't quite fly as intended, arrr, it took joint first prize, along with Mike Crewe's Mercury Redstone. Well, arrr, blow me down! Our prizes were matchin' Big Berthas. Avast, me proud beauty! Begad! We'd agreed we would build them with a common theme, and drag race them. Initially it was t' be a cluster, but, 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, me proud beauty! 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. Begad! I made some rasp-format engine files for t' motor, usin' t' engine editor that come with Rocksim. Begad! I got t' thrust curve, and data from an old Rocket Services catalogue. Avast! Avast! 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, do nay correspond with t' more common "Estes-style" designations. T' "F36-5" should, strictly speaking, ya bilge rat, be known as E6-5, and t' "F72-0" is, me hearties, in reality, an E15-0. Ahoy! 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. Begad! 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, me hearties, me bucko, Engine Mount Assembly, and put t' parts in me spares box. Blimey! Begad! Instead, I made a new thrust rin' from a short section o' toilet roll core, reduced in diameter by removin' a segment. Well, blow me down! Well, blow me down! 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, ya bilge rat, as instructed. Begad! I chose t' do Step 9, Shock Cord Mount Attachment, me bucko, at this point, while t' mount was freshly-glued and still pliable.

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

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

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

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