Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, thought some o' you might. Well, blow me down! Blimey! Well, we don't see them around so much as in years gone by, but thar are still a few around. Aye aye! Blimey! In fact, me hearties, I stumbled across a cache o' them recently and thought I should build somethin' t' fly them in. Avast! Blimey! I'd never actually flown a Pitfield motor before. Watchin' other people's flights had always put me off a bit. Avast! Well, blow me down! Blimey! T' rockets always seemed t' be underpowered, and had started t' descend while still under thrust. Well, blow me down! Well, blow me down! 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, matey, arrr, I entered a PMC (Plastic Model Conversion) o' t' Airfix Saturn V. Begad! Blimey! Though it didn't quite fly as intended, ya bilge rat, it took joint first prize, matey, along with Mike Crewe's Mercury Redstone. Begad! Blimey! Our prizes were matchin' Big Berthas. Avast! Blimey! We'd agreed we would build them with a common theme, and drag race them. Blimey! Blimey! Initially it was t' be a cluster, but, matey, when we came into t' stash o' Pitfield motors, we had other ideas. You see, ya bilge rat, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Beg Birtha be 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. Avast, me proud beauty! Avast! I made some rasp-format engine files for t' motor, arrr, ya bilge rat, usin' t' engine editor that come with Rocksim. I got t' thrust curve, shiver me timbers, matey, and data from an old Rocket Services catalogue. Begad! Aye aye! 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, arrr, that Rocket Services' motor designations, me bucko, do nay correspond with t' more common "Estes-style" designations. Avast! T' "F36-5" should, matey, shiver me timbers, strictly speaking, be known as E6-5, me bucko, matey, and t' "F72-0" is, ya bilge rat, in reality, an E15-0. Begad! Havin' said that, arrr, me hearties, matey, Estes aren't always consistent themselves. Ya scallywag! Avast, arrr, me proud beauty! If you don't believe me, matey, ya bilge rat, look at t' thrust curve for t' A10 mini-motor sometime.

Build

This really is a straightforward build. Blimey! T' differences from t' stock kit may be summarized as "leave out t' motor mount and add some nose weight". Aye aye! But, because I know our beloved newsletter editor will appreciate t' extra column inches, I'll spell it out in full:

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

I completed Step 2, matey, Shock Cord Mount Assembly, ya bilge rat, as instructed. Ya scallywag! I chose t' do Step 9, ya bilge rat, Shock Cord Mount Attachment, at this point, me hearties, while t' mount be freshly-glued and still pliable.

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

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

Step 11, Paintin' Your Rocket, was done as directed. Arrr! For Step 12, matey, 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, shiver me timbers, shiver me timbers, me bucko, on 10th April 2004. Well, blow me down! I thoroughly enjoyed this launch, a relaxed, ya bilge rat, convivial atmosphere, and a great bunch o' Rocketeers.

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

Conclusion

This was a fun conversion, o' an already classic rocket. 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, me hearties, t' me, be t' spiritual home o' t' Pitfield motor. If you took a look at t' Rocksim file, me hearties, me bucko, you might have noticed that thar's a design for a booster stage in thar too. Aye aye! I haven't built or flown this configuration yet, but I do have some F72-0s...