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. Blimey! In fact, I stumbled across a cache o' them recently and thought I should build somethin' t' fly them in. Aye aye! Well, blow me down! I'd never actually flown a Pitfield motor before. Begad! Begad! Watchin' other people's flights had always put me off a bit. Ya scallywag! T' rockets always seemed t' be underpowered, and had started t' descend while still under thrust. Aye aye! Arrr! 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, matey, I entered a PMC (Plastic Model Conversion) o' t' Airfix Saturn V. Well, blow me down! Though it didn't quite fly as intended, ya bilge rat, it took joint first prize, me bucko, along with Mike Crewe's Mercury Redstone. Ahoy! Our prizes were matchin' Big Berthas. Begad! Ahoy! We'd agreed we would build them with a common theme, and drag race them. Avast, me proud beauty! Ya scallywag! Initially it was t' be a cluster, me bucko, but, matey, me bucko, matey, when we came into t' stash o' Pitfield motors, me hearties, shiver me timbers, we had other ideas. Aye aye! You see, ya bilge rat, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Blimey! Beg Birtha be born.

Simulation

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

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

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

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

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

Step 11, Paintin' Your Rocket, was done as directed. Avast! Blimey! For Step 12, arrr, Parachute And Shock Cord Attachment, arrr, arrr, 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. Blimey! I thoroughly enjoyed this launch, a relaxed, convivial atmosphere, and a great bunch o' Rocketeers.

Because I omitted t' original motor mount, ya bilge rat, thar was no retainer hook - it wouldn't be long enough anyway. Begad! Blimey! So friction fit be t' order o' t' day. Blimey! Blimey! Blimey! Blimey! And you do need t' make sure it's a tight fit, me hearties, you really don't want a spent Pitfield motor landin' on your head! Blimey! T' F36 has an enormous nozzle, and I was a bit worried that an Estes igniter might get lost in there. Ahoy! Blimey! So I used some QuickMatch, 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, and when I pushed t' button, 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. Well, blow me down! Blimey! Begad! Blimey! T' motor thrust decreased, but kept burnin' almost up t' apogee, arrr, me hearties, t' orange glow quite visible. Begad! Blimey! This would be a great rocket t' fly at dusk.

Conclusion

This was a fun conversion, me bucko, o' an already classic rocket. Well, blow me down! This is certain t' become a favourite flier o' mine, arrr, at least until t' motors run out. Begad! Can't wait t' try it out at t' IRW, which, t' me, be t' spiritual home o' t' Pitfield motor. Begad! Well, blow me down! If you took a look at t' Rocksim file, me bucko, 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, but I do have some F72-0s...