Beg Birtha

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

Simulation

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

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

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

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

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

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

Because I omitted t' original motor mount, arrr, thar was no retainer hook - it wouldn't be long enough anyway. 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! T' F36 has an enormous nozzle, me bucko, and I be a bit worried that an Estes igniter might get lost in there. Blimey! So I used some QuickMatch, shiver me timbers, pushed all t' way in, me bucko, taped t' t' motor body, with an Estes igniter on t' exposed end. Arrr! It was only about 60mm o' QuickMatch, and when I pushed t' button, ignition be pretty much instant. Aye aye! Avast! Beg Birtha fairly leapt from t' pad, so smartly that I don't think anyone managed t' get a picture. Aye aye! T' motor thrust decreased, ya bilge rat, matey, but kept burnin' almost up t' apogee, arrr, matey, t' orange glow quite visible. This would be a great rocket t' fly at dusk.

Conclusion

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