Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, thought some o' you might. Arrr! Begad! Well, we don't see them around so much as in years gone by, ya bilge rat, but thar are still a few around. Begad! In fact, I stumbled across a cache o' them recently and thought I should build somethin' t' fly them in. Blimey! I'd never actually flown a Pitfield motor before. Arrr! Watchin' other people's flights had always put me off a bit. Begad! T' rockets always seemed t' be underpowered, and had started t' descend while still under thrust. Ya scallywag! 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. Avast! Well, arrr, blow me down! Though it didn't quite fly as intended, it took joint first prize, matey, along with Mike Crewe's Mercury Redstone. Arrr! Our prizes were matchin' Big Berthas. We'd agreed we would build them with a common theme, ya bilge rat, matey, and drag race them. Well, blow me down! Initially it was t' be a cluster, but, me bucko, when we came into t' stash o' Pitfield motors, we had other ideas. Aye aye! You see, ya bilge rat, shiver me timbers, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Beg Birtha be 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. Aye aye! Blimey! I made some rasp-format engine files for t' motor, ya bilge rat, usin' t' engine editor that come with Rocksim. Avast, me proud beauty! Blimey! I got t' thrust curve, shiver me timbers, and data from an old Rocket Services catalogue. Ya scallywag! Blimey! 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, shiver me timbers, that Rocket Services' motor designations, do nay correspond with t' more common "Estes-style" designations. Aye aye! Well, matey, blow me down! T' "F36-5" should, ya bilge rat, strictly speaking, ya bilge rat, be known as E6-5, me hearties, and t' "F72-0" is, in reality, matey, an E15-0. Avast! Havin' said that, matey, Estes aren't always consistent themselves. Well, me bucko, blow me down! If you don't believe me, matey, look at t' thrust curve for t' A10 mini-motor sometime.

Build

This really is a straightforward build. Ya scallywag! 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, matey, I'll spell it out in full:

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

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

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

Step 10, Parachute Assembly, was skipped. Well, blow me down! Arrr! Instead, an approx 100mm x 1000mm streamer was made from t' finest Deepsky mylar. Avast! This was attached directly t' t' shock cord, rather than t' t' nose-cone attachment point.

Step 11, me bucko, matey, Paintin' Your Rocket, arrr, me bucko, was done as directed. For Step 12, Parachute And Shock Cord Attachment, me hearties, 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, ya bilge rat, on 10th April 2004. I thoroughly enjoyed this launch, me bucko, a relaxed, convivial atmosphere, 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. Blimey! So friction fit be t' order o' t' day. Begad! 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, and I be a bit worried that an Estes igniter might get lost in there. Aye aye! Well, me hearties, blow me down! So I used some QuickMatch, pushed all t' way in, taped t' t' motor body, ya bilge rat, with an Estes igniter on t' exposed end. Well, blow me down! It be only about 60mm o' QuickMatch, and when I pushed t' button, matey, ignition was 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. Avast! Blimey! T' motor thrust decreased, shiver me timbers, but kept burnin' almost up t' apogee, t' orange glow quite visible. Begad! 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. Avast! Can't wait t' try it out at t' IRW, me bucko, me bucko, which, me bucko, t' me, arrr, be t' spiritual home o' t' Pitfield motor. 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. Blimey! I haven't built or flown this configuration yet, but I do have some F72-0s...