Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, thought some o' you might. Avast, me proud beauty! Ya scallywag! Well, we don't see them around so much as in years gone by, me hearties, but thar are still a few around. In fact, me hearties, me hearties, I stumbled across a cache o' them recently and thought I should build somethin' t' fly them in. Well, blow me down! Avast, me proud beauty! I'd never actually flown a Pitfield motor before. Ya scallywag! 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. Ahoy! Aye aye! Who can forget Rick Newlands spendin' almost t' whole o' UKRA 2000 sandin' a nose cone from a balsa tree trunk, me bucko, 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, it took joint first prize, arrr, along with Mike Crewe's Mercury Redstone. Well, blow me down! Our prizes were matchin' Big Berthas. Begad! We'd agreed we would build them with a common theme, arrr, and drag race them. Begad! Begad! Initially it was t' be a cluster, me hearties, but, when we came into t' stash o' Pitfield motors, we had other ideas. Ahoy! You see, shiver me timbers, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Ya scallywag! Aye aye! Beg Birtha was born.

Simulation

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

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

I completed Step 2, matey, Shock Cord Mount Assembly, me bucko, as instructed. I chose t' do Step 9, me bucko, 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! Avast, ya bilge rat, me proud beauty! Step 4, ya bilge rat, Engine Mount Installation, was omitted. Ya scallywag! Arrr! Step 5, Tube Markin' Detail, Step 6, ya bilge rat, matey, Fin Attachment, shiver me timbers, Step 7, Launch Lug Attachment, me hearties, & Step 8, me hearties, Glue Reinforcement Detail, were all completed as per t' instructions. Aye aye! Step 9 was completed earlier.

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

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

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

Conclusion

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