Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, matey, thought some o' you might. Avast, me bucko, me proud beauty! Well, me bucko, we don't see them around so much as in years gone by, 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. Begad! I'd never actually flown a Pitfield motor before. Watchin' other people's flights had always put me off a bit. T' rockets always seemed t' be underpowered, and had started t' descend while still under thrust. Begad! Who can forget Rick Newlands spendin' almost t' whole o' UKRA 2000 sandin' a nose cone from a balsa tree trunk, me bucko, matey, shiver me timbers, 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. Though it didn't quite fly as intended, it took joint first prize, shiver me timbers, ya bilge rat, along with Mike Crewe's Mercury Redstone. Avast! Avast, me proud beauty! Our prizes were matchin' Big Berthas. Begad! Begad! We'd agreed we would build them with a common theme, and drag race them. Avast, me proud beauty! Initially it was t' be a cluster, shiver me timbers, but, when we came into t' stash o' Pitfield motors, we had other ideas. Avast! Arrr! You see, me hearties, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Well, blow me down! Blimey! Beg Birtha be born.

Simulation

It was easy t' modify a standard Big Bertha Rocksim File. Begad! I just removed t' motor mount and added a new thrust ring. I made some rasp-format engine files for t' motor, ya bilge rat, usin' t' engine editor that come with Rocksim. Blimey! I got t' thrust curve, and data from an old Rocket Services catalogue. Begad! 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 bucko, 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, arrr, strictly speaking, be known as E6-5, and t' "F72-0" is, in reality, an E15-0. Aye aye! Blimey! Havin' said that, shiver me timbers, me bucko, Estes aren't always consistent themselves. Ya scallywag! If you don't believe me, 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, me proud beauty! Blimey! But, because I know our beloved newsletter editor will appreciate t' extra column inches, ya bilge rat, I'll spell it out in full:

I skipped Step 1, arrr, Engine Mount Assembly, me hearties, and put t' parts in me spares box. Instead, shiver me timbers, I made a new thrust rin' from a short section o' toilet roll core, reduced in diameter by removin' a segment. Ya scallywag! Ahoy! 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, matey, as instructed. Well, blow me down! I chose t' do Step 9, me bucko, matey, Shock Cord Mount Attachment, ya bilge rat, at this point, while t' mount was freshly-glued and still pliable.

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

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

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

Because I omitted t' original motor mount, shiver me timbers, thar was no retainer hook - it wouldn't be long enough anyway. Ya scallywag! Begad! So friction fit be t' order o' t' day. Ya scallywag! And you do need t' make sure it's a tight fit, me bucko, you really don't want a spent Pitfield motor landin' on your head! T' F36 has an enormous nozzle, arrr, arrr, and I be a bit worried that an Estes igniter might get lost in there. Avast! So I used some QuickMatch, shiver me timbers, pushed all t' way in, taped t' t' motor body, ya bilge rat, with an Estes igniter on t' exposed end. It was only about 60mm o' QuickMatch, me hearties, me bucko, and when I pushed t' button, ignition be pretty much instant. Avast! Beg Birtha fairly leapt from t' pad, so smartly that I don't think anyone managed t' get a picture. Aye aye! T' motor thrust decreased, arrr, but kept burnin' almost up t' apogee, me hearties, shiver me timbers, t' orange glow quite visible. Aye aye! This would be a great rocket t' fly at dusk.

Conclusion

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