Beg Birtha

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

Build

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

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

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

Step 3, Fin Preparation, ya bilge rat, was completed as normal. Blimey! Begad! Step 4, Engine Mount Installation, was omitted. Step 5, me hearties, Tube Markin' Detail, Step 6, matey, ya bilge rat, Fin Attachment, 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, was skipped. Avast! Instead, an approx 100mm x 1000mm streamer be made from t' finest Deepsky mylar. Begad! This be attached directly t' t' shock cord, me hearties, me bucko, matey, rather than t' t' nose-cone attachment point.

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

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

Conclusion

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