Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, thought some o' you might. Avast, me proud beauty! Well, me hearties, we don't see them around so much as in years gone by, but thar are still a few around. Begad! In fact, me bucko, I stumbled across a cache o' them recently and thought I should build somethin' t' fly them in. Avast, me proud beauty! Well, blow me down! I'd never actually flown a Pitfield motor before. Watchin' other people's flights had always put me off a bit. Avast! Avast, matey, me proud beauty! T' rockets always seemed t' be underpowered, and had started t' descend while still under thrust. 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, matey, 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, me hearties, and drag race them. Ahoy! Blimey! Initially it was t' be a cluster, but, arrr, when we came into t' stash o' Pitfield motors, me hearties, shiver me timbers, we had other ideas. Ya scallywag! Blimey! Aye aye! Blimey! You see, shiver me timbers, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Begad! Blimey! Beg Birtha was born.

Simulation

It was easy t' modify a standard Big Bertha Rocksim File. Ahoy! I just removed t' motor mount and added a new thrust ring. Well, blow me down! I made some rasp-format engine files for t' motor, usin' t' engine editor that come with Rocksim. Avast, me proud beauty! I got t' thrust curve, and data from an old Rocket Services catalogue. Aye aye! 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, do nay correspond with t' more common "Estes-style" designations. Aye aye! T' "F36-5" should, shiver me timbers, strictly speaking, me bucko, be known as E6-5, arrr, and t' "F72-0" is, arrr, in reality, an E15-0. Well, blow me down! Havin' said that, arrr, matey, Estes aren't always consistent themselves. Ahoy! Begad! 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! T' differences from t' stock kit may be summarized as "leave out t' motor mount and add some nose weight". Arrr! Aye aye! But, arrr, because I know our beloved newsletter editor will appreciate t' extra column inches, I'll spell it out in full:

I skipped Step 1, Engine Mount Assembly, and put t' parts in me spares box. Begad! Instead, arrr, 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, me bucko, or cut a rin' from a spent motor.

I completed Step 2, arrr, Shock Cord Mount Assembly, matey, me hearties, matey, as instructed. Avast, me proud beauty! Aye aye! I chose t' do Step 9, arrr, Shock Cord Mount Attachment, at this point, me hearties, matey, while t' mount was freshly-glued and still pliable.

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

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

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

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

Conclusion

This be a fun conversion, o' an already classic rocket. This is certain t' become a favourite flier o' mine, at least until t' motors run out. Can't wait t' try it out at t' IRW, shiver me timbers, matey, which, shiver me timbers, t' me, me hearties, ya bilge rat, be t' spiritual home o' t' Pitfield motor. If you took a look at t' Rocksim file, me hearties, 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...