How T' Convert An Estes Big Bertha Into A Payload Rocket

Purpose and Design

I need a series o' payload rockets t' carry experimental flight recorders/dataloggers into t' Troposphere. Blimey! Arrr! T' BBPC is somethin' I came up with after a scratch-build payload rocket project that be enjoyable, but I don't always have t' time for such a grand project. Ahoy! T' BBPC solves that problem.

I am a big fan o' t' Estes Big Bertha rocket. Avast, me proud beauty! Avast! Its BT-60 airframe can handle single or cluster motor mounts o' countless design, kits/repair parts are cheap and plentiful, it has a simple design and it aint too bad on t' eyes, havin' a "retro" profile via its fin pattern and nose cone profile. Ahoy! Blimey! Big Berthas have always flown true for me on any motor up t' an E or even an 18mm tri-cluster. Arrr! My versions are always TTW fin design and are pretty durable, matey, ya bilge rat, me hearties, you can launch them all day long.

In tryin' t o extend t' capabilities o' this excellent airframe, shiver me timbers, this article focuses on convertin' a stock Big Bertha into a Payload Rocket. Avast! My design replaces t' stock Big Bertha nose cone with a payload capsule fashioned from 2 NC-60 nose cones and 6" o' BT-60 tubing. Ya scallywag! T' Big Bertha Payload Capsule or BBPC, can be fashioned smartly and be t' easiest way t' get a payload rocket together if you already have a Big Bertha in your fleet.

Build

T' base o' t' BBPC be t' bottom portion o' a NC-60 nose cone epoxied onto a BT-60 tube. Ahoy! Begad! A 3/16" balsa cargo floor is put in just forward o' t' nose cone base. Blimey! This is t' protect t' cargo from t' recovery blast. A second NC-60 is modified for t' cargo door/nosecone by removin' t' lower portion and epoxyin' two 6-32 aluminum nuts that accept 6-32 nylon bolts onto t' nosecone flange. These nuts/bolts are available at any Home Depot for less than a dollar. T' nylon bolts secure t' cargo door/nosecone t' t' BT-60. Avast, me proud beauty! Well, blow me down! Thats it! Stuff what you can into t' 10+ cubic inches o' cargo space and hope you have enough impulse! Lets go!

T' recovery gear consists o' a Keelhaul®©™ cord t' ¼"elastic cord that terminates at a brass #3 fishin' swivel with clip, shiver me timbers, all extendin' from t' Big Bertha airframe. Aye aye! This clip attaches t' either t' BBPC or t' stock nose cone in t' field dependin' on what I need t' rocket t' do.--Remain a simple bird or t' carry some freight.

Performance

T' BBPC weighs 29 grams with t' paint. This, in addition t' t' cargo weight will drastically change t' SM ratio o' your stock Big Bertha, but it should only shift t' CG further forward in t' airframe than its Big Bertha stock configuration. Aye aye! Blimey! T' extra length added when t' BBPC is in place shifts t' CP forward 1 inch. Avast, me proud beauty! Blimey! T' stock CP position for t' Big Bertha is 24.25" back from t' tip o' t' nosecone. Avast! T' CP for me BBPC rocket is 23.25" from t' tip o' t' nosecone.--But as long as t' CG is one caliber or more forward o' t' CP you should have a good flier in your hands. T' BBPC just decreases t' CGs caliber from t' CP position by an insignificant amount.--One inch, shiver me timbers, which is less than one caliber. Blimey! Begad! Blimey! One caliber on t' Big Bertha is 1.41".

My BBPC rocket weighs 5.25 oz. ready t' launch with no motor. Ya scallywag! An E9 motor weighs 2 oz. Ahoy! This gives you a 7.25 oz. Arrr! Ya scallywag! ready-to-launch platform WITH 10 cubic inches o' cargo space! Future refinements will improve these numbers. Begad! Aye aye! I read somewhere that Estes has produced t' Big Bertha for 30 years.--It is easy t' see why.

I prefer t' stick with t' lower power impulse black powder motors and concentrate on clusters and booster configurations. Begad! Avast! Blimey! Within this low power impulse range, t' BT-60, me hearties, 70 and 80 airframes cant be beat. Avast! Blimey! T' Big Bertha seems t' be t' epitome for t' BT-60 airframe. Aye aye! Blimey! T' BT-70 is runner up in utility, me bucko, arrr, but its availability and price lag FAR behind BT-60 and 80s.

Launch Day 1

T' BBPC has proven, arrr, as much as it could in one day, that it will soldier on as its siblin' t' Big Bertha. On 4/11/09, me hearties, ya bilge rat, a clear day with wind speeds up t' 20 MPH, this Big Bertha airframe conducted 8 flights in its standard and BBPC configurations flawlessly. T' ideal setup for t' Big Bertha is a 15" chute. Blimey! Blimey! T' best recovery for t' BBPC with 62 grams o' ballast is a 30" chute. Blimey! Blimey! I have made numerous successful flights with t' 62 gram ballast, arrr, which be t' size and weight o' me GPSDL, and is what t' BBPC was designed t' haul. Avast, me proud beauty! Blimey! I will launch t' GPSDL and collect data t' week o' 4/13.

I have nay run any altitude calculations yet, matey, but I estimated t' highest altitude achieved that day for t' BBPC was 500', flyin' on 24mm short motors. Well, blow me down! Ahoy! (C & Ds only) This was its altitude at best due t' t' moderate winds out that caused me t' lau nch into t' wind, me bucko, limitin' maximum altitude.--I had no choice after t' Big Bertha got some AWESOME altitude on its first flight, but drifted so far away durin' recovery it crossed a river and took a good 45 minutes t' retrieve t' rocket about ¾ o' a mile away! That caused me t' downgrade t' a 15" chute for t' Big Bertha and a 30" chute for t' BBPC and they work flawlessly with this recovery gear.

T' days launch helped me t' decide that I will build a few more BBPCs; an 18mm tri-cluster and a standard 24mm that takes C, matey, shiver me timbers, D and E motors. Arrr! Ya scallywag! That might nay sound too exciting, but I also plan on addin' tri-boosters t' each airframe. Begad! Ahoy! I have planned a booster mount that is "standardized" so I can choose betwixt 13mm, 18mm or 24mm boosters t' add t' host rocket.--But that is all a dream at this point.

T' swept behind-the-airframe fin configuration on t' Big Bertha is designed t' keep t' CP as far rearward as possible.--An added feature is that t' long, angled and squat fin area is quite durable for rough landings. Ahoy! I plan on tryin' an upscaled Big Bertha fin design on a BT-80 airframe. Avast, me proud beauty! T' logic is that t' closer you keep t' fins t' t' airframe t' stronger they are. Ya scallywag! That means I can spend less time fixin' rockets and more time buildin' embedded flight computers.

My last observation is that t' weight o' recovery gear that runs over a 30" chute size is prohibitive for motors under size F.--You can make it work with clusters/boosters, but then again they are just a "poor mans F" impulse rocket. My other payload rocket, t' TL-96 needs recovery gear o' at least 36" for safe recovery o' electronics. Well, blow me down! T' BBPC recovers electronics just fine on 30" recovery gear. Begad! So once again, me hearties, t' Big Bertha airframe comes out on top! An 18mm tri-cluster variation would be very cost effective t' launch also.

I will update this article after a data collectin' flight or two in t' weeks t' come. Aye aye! Avast, me proud beauty! Blimey! I should be able t' contrast t' data I collect betwixt two different rockets with t' addition o' t' BBPC t' me fleet.