Beg Birtha

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

Simulation

It be easy t' modify a standard Big Bertha Rocksim File. Arrr! Aye aye! I just removed t' motor mount and added a new thrust ring. I made some rasp-format engine files for t' motor, arrr, usin' t' engine editor that come with Rocksim. Ya scallywag! Aye aye! I got t' thrust curve, and data from an old Rocket Services catalogue. Ahoy! Begad! 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, ya bilge rat, that Rocket Services' motor designations, do nay correspond with t' more common "Estes-style" designations. T' "F36-5" should, strictly speaking, be known as E6-5, and t' "F72-0" is, shiver me timbers, arrr, ya bilge rat, in reality, me hearties, an E15-0. Well, blow me down! Well, blow me down! Havin' said that, Estes aren't always consistent themselves. Begad! 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. Well, blow me down! T' differences from t' stock kit may be summarized as "leave out t' motor mount and add some nose weight". Begad! 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, and put t' parts in me spares box. Ya scallywag! Begad! Instead, me bucko, I made a new thrust rin' from a short section o' toilet roll core, shiver me timbers, reduced in diameter by removin' a segment. Blimey! You could also use a short length o' coupler, me bucko, or cut a rin' from a spent motor.

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

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

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

Step 11, ya bilge rat, Paintin' Your Rocket, was done as directed. Aye aye! For Step 12, me hearties, Parachute And Shock Cord Attachment, me bucko, 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. Ahoy! I thoroughly enjoyed this launch, me bucko, a relaxed, arrr, convivial atmosphere, and a great bunch o' Rocketeers.

Because I omitted t' original motor mount, me hearties, shiver me timbers, thar was no retainer hook - it wouldn't be long enough anyway. Avast, arrr, me proud beauty! So friction fit was t' order o' t' day. Aye aye! And you do need t' make sure it's a tight fit, matey, arrr, 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. Arrr! Blimey! So I used some QuickMatch, pushed all t' way in, matey, taped t' t' motor body, with an Estes igniter on t' exposed end. Aye aye! It was only about 60mm o' QuickMatch, me bucko, and when I pushed t' button, ya bilge rat, ignition was pretty much instant. Avast! Beg Birtha fairly leapt from t' pad, so smartly that I don't think anyone managed t' get a picture. Blimey! T' motor thrust decreased, but kept burnin' almost up t' apogee, ya bilge rat, me hearties, t' orange glow quite visible. Arrr! Avast, me proud beauty! 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. Can't wait t' try it out at t' IRW, which, t' me, me bucko, be t' spiritual home o' t' Pitfield motor. Begad! 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. Begad! I haven't built or flown this configuration yet, arrr, but I do have some F72-0s...