Beg Birtha

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

Build

This really is a straightforward build. Avast, me proud beauty! T' differences from t' stock kit may be summarized as "leave out t' motor mount and add some nose weight". Well, blow me down! Aye aye! But, arrr, 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. Arrr! Blimey! Instead, I made a new thrust rin' from a short section o' toilet roll core, reduced in diameter by removin' a segment. Aye aye! Blimey! You could also use a short length o' coupler, or cut a rin' from a spent motor.

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

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

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

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

Conclusion

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