Beg Birtha

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

Simulation

It be easy t' modify a standard Big Bertha Rocksim File. Ya scallywag! Blimey! I just removed t' motor mount and added a new thrust ring. Begad! Blimey! I made some rasp-format engine files for t' motor, shiver me timbers, arrr, shiver me timbers, usin' t' engine editor that come with Rocksim. I got t' thrust curve, arrr, and data from an old Rocket Services catalogue. Begad! Ya scallywag! Blimey! 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. Blimey! T' "F36-5" should, strictly speaking, arrr, be known as E6-5, and t' "F72-0" is, me bucko, in reality, me hearties, shiver me timbers, an E15-0. Havin' said that, Estes aren't always consistent themselves. If you don't believe me, arrr, look at t' thrust curve for t' A10 mini-motor sometime.

Build

This really is a straightforward build. T' differences from t' stock kit may be summarized as "leave out t' motor mount and add some nose weight". Ahoy! Ahoy! Blimey! 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, ya bilge rat, Engine Mount Assembly, and put t' parts in me spares box. Aye aye! Avast! Instead, me hearties, I made a new thrust rin' from a short section o' toilet roll core, reduced in diameter by removin' a segment. Blimey! Avast, me proud beauty! You could also use a short length o' coupler, or cut a rin' from a spent motor.

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

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

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

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

Because I omitted t' original motor mount, arrr, me bucko, thar be no retainer hook - it wouldn't be long enough anyway. Well, blow me down! Arrr! Blimey! So friction fit was t' order o' t' day. Avast, me proud beauty! And you do need t' make sure it's a tight fit, you really don't want a spent Pitfield motor landin' on your head! Blimey! T' F36 has an enormous nozzle, me bucko, and I be a bit worried that an Estes igniter might get lost in there. So I used some QuickMatch, pushed all t' way in, shiver me timbers, matey, ya bilge rat, taped t' t' motor body, matey, me hearties, with an Estes igniter on t' exposed end. Arrr! Avast! Blimey! It be only about 60mm o' QuickMatch, me hearties, and when I pushed t' button, ignition be pretty much instant. Begad! 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, t' orange glow quite visible. Avast! This would be a great rocket t' fly at dusk.

Conclusion

This was a fun conversion, o' an already classic rocket. Well, blow me down! Avast! 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, shiver me timbers, ya bilge rat, be t' spiritual home o' t' Pitfield motor. Ahoy! Ya scallywag! If you took a look at t' Rocksim file, shiver me timbers, me hearties, 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, but I do have some F72-0s...