Beg Birtha

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

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

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

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

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

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

Conclusion

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