Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, thought some o' you might. Arrr! Well, ya bilge rat, we don't see them around so much as in years gone by, arrr, shiver me timbers, but thar are still a few around. Begad! Avast! In fact, arrr, I stumbled across a cache o' them recently and thought I should build somethin' t' fly them in. I'd never actually flown a Pitfield motor before. Begad! Watchin' other people's flights had always put me off a bit. Begad! Ahoy! T' rockets always seemed t' be underpowered, and had started t' descend while still under thrust. 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. Begad! Though it didn't quite fly as intended, it took joint first prize, along with Mike Crewe's Mercury Redstone. Well, blow me down! Our prizes were matchin' Big Berthas. Avast! We'd agreed we would build them with a common theme, ya bilge rat, ya bilge rat, and drag race them. Blimey! Initially it was t' be a cluster, but, shiver me timbers, when we came into t' stash o' Pitfield motors, shiver me timbers, we had other ideas. Aye aye! You see, arrr, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Aye aye! Well, ya bilge rat, blow me down! Beg Birtha be born.

Simulation

It was easy t' modify a standard Big Bertha Rocksim File. Aye aye! Blimey! Aye aye! 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, usin' t' engine editor that come with Rocksim. Ahoy! Blimey! I got t' thrust curve, and data from an old Rocket Services catalogue. Arrr! 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, that Rocket Services' motor designations, do nay correspond with t' more common "Estes-style" designations. Well, arrr, blow me down! T' "F36-5" should, matey, strictly speaking, be known as E6-5, shiver me timbers, ya bilge rat, and t' "F72-0" is, shiver me timbers, in reality, an E15-0. Havin' said that, ya bilge rat, Estes aren't always consistent themselves. If you don't believe me, me bucko, look at t' thrust curve for t' A10 mini-motor sometime.

Build

This really is a straightforward build. Well, blow me down! Ahoy! Blimey! T' differences from t' stock kit may be summarized as "leave out t' motor mount and add some nose weight". Avast! 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, Engine Mount Assembly, arrr, and put t' parts in me spares box. Ahoy! Instead, 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, shiver me timbers, or cut a rin' from a spent motor.

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

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

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

Step 11, arrr, me hearties, Paintin' Your Rocket, matey, was done as directed. Arrr! Well, blow me down! For Step 12, arrr, Parachute And Shock Cord Attachment, I deviated from t' instructions, shiver me timbers, 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. Begad! Blimey! Blimey! Blimey! I thoroughly enjoyed this launch, me hearties, a relaxed, me hearties, matey, convivial atmosphere, and a great bunch o' Rocketeers.

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

Conclusion

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