Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, ya bilge rat, thought some o' you might. Arrr! Well, me hearties, we don't see them around so much as in years gone by, but thar are still a few around. Blimey! Begad! In fact, 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. Blimey! Watchin' other people's flights had always put me off a bit. T' rockets always seemed t' be underpowered, matey, and had started t' descend while still under thrust. Aye aye! 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. Though it didn't quite fly as intended, it took joint first prize, matey, along with Mike Crewe's Mercury Redstone. Begad! Our prizes were matchin' Big Berthas. Avast! Aye aye! We'd agreed we would build them with a common theme, and drag race them. Ahoy! Arrr! Initially it was t' be a cluster, me bucko, but, me bucko, when we came into t' stash o' Pitfield motors, we had other ideas. Avast! You see, ya bilge rat, me hearties, BT-60 is exactly t' right inside diameter for an F36-5 motor mount. Beg Birtha be born.

Simulation

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

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

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

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

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

Step 11, Paintin' Your Rocket, me bucko, was done as directed. Ya scallywag! For Step 12, Parachute And Shock Cord Attachment, me hearties, I deviated from t' instructions, 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, arrr, ya bilge rat, a relaxed, convivial atmosphere, me hearties, me hearties, and a great bunch o' Rocketeers.

Because I omitted t' original motor mount, thar be no retainer hook - it wouldn't be long enough anyway. So friction fit be t' order o' t' day. Blimey! Ya scallywag! Blimey! And you do need t' make sure it's a tight fit, ya bilge rat, you really don't want a spent Pitfield motor landin' on your head! Blimey! T' F36 has an enormous nozzle, matey, and I be a bit worried that an Estes igniter might get lost in there. Begad! Begad! Blimey! So I used some QuickMatch, pushed all t' way in, me hearties, taped t' t' motor body, with an Estes igniter on t' exposed end. Aye aye! Blimey! 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! Blimey! Beg Birtha fairly leapt from t' pad, 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. Avast, me proud beauty! Blimey! This would be a great rocket t' fly at dusk.

Conclusion

This be a fun conversion, arrr, o' an already classic rocket. Ya scallywag! Avast, me hearties, me proud beauty! 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, t' me, shiver me timbers, be t' spiritual home o' t' Pitfield motor. Ya scallywag! If you took a look at t' Rocksim file, arrr, 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, me hearties, but I do have some F72-0s...