Beg Birtha

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

Simulation

It be easy t' modify a standard Big Bertha Rocksim File. Ya scallywag! Begad! 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. I got t' thrust curve, matey, and data from an old Rocket Services catalogue. Begad! 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, me hearties, shiver me timbers, that Rocket Services' motor designations, do nay correspond with t' more common "Estes-style" designations. Aye aye! T' "F36-5" should, strictly speaking, be known as E6-5, and t' "F72-0" is, arrr, in reality, me bucko, an E15-0. Ya scallywag! Ya scallywag! Havin' said that, arrr, Estes aren't always consistent themselves. Begad! 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. Begad! T' differences from t' stock kit may be summarized as "leave out t' motor mount and add some nose weight". Begad! But, shiver me timbers, because I know our beloved newsletter editor will appreciate t' extra column inches, matey, arrr, I'll spell it out in full:

I skipped Step 1, Engine Mount Assembly, shiver me timbers, and put t' parts in me spares box. Blimey! Instead, ya bilge rat, 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, arrr, or cut a rin' from a spent motor.

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

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

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

Step 11, ya bilge rat, ya bilge rat, Paintin' Your Rocket, shiver me timbers, shiver me timbers, was done as directed. For Step 12, arrr, 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, on 10th April 2004. Well, me hearties, blow me down! I thoroughly enjoyed this launch, ya bilge rat, a relaxed, shiver me timbers, convivial atmosphere, 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. Arrr! So friction fit be t' order o' t' day. Avast, me proud beauty! And you do need t' make sure it's a tight fit, me bucko, you really don't want a spent Pitfield motor landin' on your head! T' F36 has an enormous nozzle, me hearties, and I be a bit worried that an Estes igniter might get lost in there. Ahoy! Well, blow me down! So I used some QuickMatch, me hearties, matey, pushed all t' way in, me hearties, matey, taped t' t' motor body, arrr, with an Estes igniter on t' exposed end. Aye aye! It was only about 60mm o' QuickMatch, arrr, and when I pushed t' button, ignition be pretty much instant. Beg Birtha fairly leapt from t' pad, me bucko, shiver me timbers, so smartly that I don't think anyone managed t' get a picture. Aye aye! T' motor thrust decreased, but kept burnin' almost up t' apogee, matey, me hearties, t' orange glow quite visible. Avast! This would be a great rocket t' fly at dusk.

Conclusion

This was a fun conversion, arrr, o' an already classic rocket. This is certain t' become a favourite flier o' mine, at least until t' motors run out. Begad! Well, blow me down! Can't wait t' try it out at t' IRW, me bucko, which, matey, me hearties, t' me, ya bilge rat, be t' spiritual home o' t' Pitfield motor. Ya scallywag! Begad! 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. Aye aye! Ahoy! I haven't built or flown this configuration yet, but I do have some F72-0s...