Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, thought some o' you might. Well, we don't see them around so much as in years gone by, me hearties, arrr, but thar are still a few around. In fact, ya bilge rat, 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. Aye aye! Avast! Watchin' other people's flights had always put me off a bit. T' rockets always seemed t' be underpowered, arrr, arrr, shiver me timbers, and had started t' descend while still under thrust. Arrr! Avast, arrr, me proud beauty! Who can forget Rick Newlands spendin' almost t' whole o' UKRA 2000 sandin' a nose cone from a balsa tree trunk, shiver me timbers, 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, me hearties, it took joint first prize, along with Mike Crewe's Mercury Redstone. Avast! Well, blow me down! Blimey! Our prizes were matchin' Big Berthas. We'd agreed we would build them with a common theme, and drag race them. Ahoy! Initially it be t' be a cluster, but, matey, when we came into t' stash o' Pitfield motors, we had other ideas. Blimey! Arrr! Blimey! You see, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Aye aye! Blimey! Beg Birtha was born.

Simulation

It was easy t' modify a standard Big Bertha Rocksim File. Aye aye! I just removed t' motor mount and added a new thrust ring. I made some rasp-format engine files for t' motor, shiver me timbers, shiver me timbers, usin' t' engine editor that come with Rocksim. Blimey! 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, that Rocket Services' motor designations, matey, me bucko, do nay correspond with t' more common "Estes-style" designations. Blimey! T' "F36-5" should, arrr, strictly speaking, shiver me timbers, be known as E6-5, me bucko, and t' "F72-0" is, in reality, an E15-0. Havin' said that, Estes aren't always consistent themselves. Avast! 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, because I know our beloved newsletter editor will appreciate t' extra column inches, shiver me timbers, I'll spell it out in full:

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

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

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

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

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

Conclusion

This was a fun conversion, matey, arrr, o' an already classic rocket. Avast, me proud beauty! This is certain t' become a favourite flier o' mine, ya bilge rat, at least until t' motors run out. Avast, me proud beauty! Can't wait t' try it out at t' IRW, which, me hearties, t' me, arrr, 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. Blimey! I haven't built or flown this configuration yet, me hearties, but I do have some F72-0s...