Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, arrr, thought some o' you might. Well, ya bilge rat, we don't see them around so much as in years gone by, ya bilge rat, but thar are still a few around. In fact, arrr, matey, I stumbled across a cache o' them recently and thought I should build somethin' t' fly them in. Well, blow me down! I'd never actually flown a Pitfield motor before. Blimey! Watchin' other people's flights had always put me off a bit. Begad! Aye aye! T' rockets always seemed t' be underpowered, and had started t' descend while still under thrust. Avast! 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, shiver me timbers, 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, along with Mike Crewe's Mercury Redstone. Blimey! Arrr! Blimey! Our prizes were matchin' Big Berthas. Avast! Avast, me proud beauty! Blimey! We'd agreed we would build them with a common theme, matey, and drag race them. Ya scallywag! Blimey! Initially it be t' be a cluster, shiver me timbers, but, me bucko, when we came into t' stash o' Pitfield motors, we had other ideas. You see, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Avast! Blimey! Beg Birtha was 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. Begad! I made some rasp-format engine files for t' motor, usin' t' engine editor that come with Rocksim. Arrr! I got t' thrust curve, shiver me timbers, and data from an old Rocket Services catalogue. Aye aye! 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, me hearties, me bucko, do nay correspond with t' more common "Estes-style" designations. Avast, me proud beauty! Arrr! T' "F36-5" should, strictly speaking, matey, arrr, be known as E6-5, and t' "F72-0" is, in reality, an E15-0. Begad! Arrr! Havin' said that, arrr, Estes aren't always consistent themselves. Well, blow me down! If you don't believe me, ya bilge rat, 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". But, me hearties, because I know our beloved newsletter editor will appreciate t' extra column inches, I'll spell it out in full:

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

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

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

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

Step 11, ya bilge rat, Paintin' Your Rocket, ya bilge rat, was done as directed. Avast, me proud beauty! For Step 12, Parachute And Shock Cord Attachment, matey, matey, 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, a relaxed, matey, ya bilge rat, 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. So friction fit was t' order o' t' day. Well, blow me down! 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 was a bit worried that an Estes igniter might get lost in there. Ya scallywag! Begad! So I used some QuickMatch, pushed all t' way in, me bucko, me hearties, taped t' t' motor body, arrr, with an Estes igniter on t' exposed end. Begad! Begad! It be only about 60mm o' QuickMatch, me bucko, and when I pushed t' button, ya bilge rat, ignition was 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. T' motor thrust decreased, shiver me timbers, but kept burnin' almost up t' apogee, t' orange glow quite visible. This would be a great rocket t' fly at dusk.

Conclusion

This was a fun conversion, ya bilge rat, o' an already classic rocket. Blimey! Blimey! This is certain t' become a favourite flier o' mine, at least until t' motors run out. Ahoy! Blimey! Can't wait t' try it out at t' IRW, me hearties, matey, ya bilge rat, which, t' me, be t' spiritual home o' t' Pitfield motor. Begad! 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. Arrr! Blimey! I haven't built or flown this configuration yet, matey, but I do have some F72-0s...