Beg Birtha

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

Simulation

It be easy t' modify a standard Big Bertha Rocksim File. Aye aye! Avast! I just removed t' motor mount and added a new thrust ring. Ya scallywag! I made some rasp-format engine files for t' motor, matey, matey, usin' t' engine editor that come with Rocksim. I got t' thrust curve, ya bilge rat, and data from an old Rocket Services catalogue. Begad! 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. Begad! Arrr! T' "F36-5" should, arrr, me hearties, strictly speaking, ya bilge rat, be known as E6-5, me hearties, and t' "F72-0" is, matey, in reality, me hearties, an E15-0. Avast, me proud beauty! Havin' said that, matey, Estes aren't always consistent themselves. Well, blow me down! If you don't believe me, me hearties, 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". Ahoy! Blimey! But, me bucko, 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, 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, ya bilge rat, shiver me timbers, or cut a rin' from a spent motor.

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

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

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

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

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

Conclusion

This was a fun conversion, o' an already classic rocket. Avast, me proud beauty! 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, which, arrr, ya bilge rat, t' me, me hearties, me hearties, be t' spiritual home o' t' Pitfield motor. Begad! Blimey! If you took a look at t' Rocksim file, arrr, 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, shiver me timbers, but I do have some F72-0s...