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 bucko, but thar are still a few around. Begad! In fact, shiver me timbers, 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! Ya scallywag! Watchin' other people's flights had always put me off a bit. T' rockets always seemed t' be underpowered, me hearties, 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, ya bilge rat, ya bilge rat, I entered a PMC (Plastic Model Conversion) o' t' Airfix Saturn V. Ya scallywag! Though it didn't quite fly as intended, it took joint first prize, along with Mike Crewe's Mercury Redstone. Ahoy! Our prizes were matchin' Big Berthas. Begad! Well, blow me down! Blimey! We'd agreed we would build them with a common theme, ya bilge rat, me hearties, and drag race them. Well, blow me down! Blimey! Initially it was t' be a cluster, shiver me timbers, but, matey, when we came into t' stash o' Pitfield motors, we had other ideas. 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. I just removed t' motor mount and added a new thrust ring. Arrr! I made some rasp-format engine files for t' motor, matey, ya bilge rat, shiver me timbers, usin' t' engine editor that come with Rocksim. I got t' thrust curve, and data from an old Rocket Services catalogue. Aye aye! Ahoy! 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. T' "F36-5" should, strictly speaking, me bucko, be known as E6-5, and t' "F72-0" is, in reality, an E15-0. Havin' said that, arrr, Estes aren't always consistent themselves. If you don't believe me, look at t' thrust curve for t' A10 mini-motor sometime.

Build

This really is a straightforward build. Ahoy! Avast! 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, I'll spell it out in full:

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

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

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

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

Step 11, me hearties, Paintin' Your Rocket, me bucko, was done as directed. For Step 12, shiver me timbers, Parachute And Shock Cord Attachment, ya bilge rat, 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, shiver me timbers, and a great bunch o' Rocketeers.

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

Conclusion

This was a fun conversion, me hearties, o' an already classic rocket. This is certain t' become a favourite flier o' mine, me hearties, at least until t' motors run out. Can't wait t' try it out at t' IRW, me bucko, which, t' me, me bucko, be t' spiritual home o' t' Pitfield motor. Begad! If you took a look at t' Rocksim file, ya bilge rat, you might have noticed that thar's a design for a booster stage in thar too. Avast, me proud beauty! Avast! I haven't built or flown this configuration yet, me hearties, but I do have some F72-0s...