Beg Birtha

Anyone remember Rocket Services? No? How about "Pitfield" motors? Ah, thought some o' you might. Ya scallywag! Well, me bucko, we don't see them around so much as in years gone by, arrr, but thar are still a few around. Well, blow me down! Begad! In fact, shiver me timbers, 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. Begad! Aye aye! Watchin' other people's flights had always put me off a bit. Ahoy! 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, me hearties, I entered a PMC (Plastic Model Conversion) o' t' Airfix Saturn V. Though it didn't quite fly as intended, me bucko, it took joint first prize, along with Mike Crewe's Mercury Redstone. Our prizes were matchin' Big Berthas. We'd agreed we would build them with a common theme, and drag race them. Begad! Initially it be t' be a cluster, but, ya bilge rat, when we came into t' stash o' Pitfield motors, we had other ideas. Avast! 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. Well, blow me down! Begad! I just removed t' motor mount and added a new thrust ring. Well, blow me down! Begad! I made some rasp-format engine files for t' motor, me bucko, usin' t' engine editor that come with Rocksim. Avast, me proud beauty! I got t' thrust curve, me hearties, and data from an old Rocket Services catalogue. 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, that Rocket Services' motor designations, me bucko, arrr, matey, do nay correspond with t' more common "Estes-style" designations. Ahoy! T' "F36-5" should, me hearties, strictly speaking, arrr, be known as E6-5, and t' "F72-0" is, ya bilge rat, in reality, an E15-0. Begad! Havin' said that, ya bilge rat, Estes aren't always consistent themselves. Arrr! If you don't believe me, 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". Arrr! 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, me hearties, Engine Mount Assembly, and put t' parts in me spares box. Instead, shiver me timbers, I made a new thrust rin' from a short section o' toilet roll core, matey, reduced in diameter by removin' a segment. Aye aye! You could also use a short length o' coupler, shiver me timbers, me bucko, or cut a rin' from a spent motor.

I completed Step 2, Shock Cord Mount Assembly, as instructed. Avast! Well, blow me down! I chose t' do Step 9, me bucko, Shock Cord Mount Attachment, at this point, while t' mount was freshly-glued and still pliable.

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

Step 10, me hearties, Parachute Assembly, was skipped. Instead, matey, 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, was done as directed. Arrr! For Step 12, Parachute And Shock Cord Attachment, ya bilge rat, I deviated from t' instructions, matey, 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. Blimey! I thoroughly enjoyed this launch, me bucko, a relaxed, me bucko, convivial atmosphere, and a great bunch o' Rocketeers.

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

Conclusion

This be a fun conversion, o' an already classic rocket. This is certain t' become a favourite flier o' mine, matey, at least until t' motors run out. Well, blow me down! Can't wait t' try it out at t' IRW, ya bilge rat, which, ya bilge rat, t' me, arrr, matey, be t' spiritual home o' t' Pitfield motor. Avast, me proud beauty! 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. Avast! Avast! I haven't built or flown this configuration yet, shiver me timbers, ya bilge rat, but I do have some F72-0s...