Jerry Irvine was making a point in another thread about lightweight building techniques for mid and high power. He mentioned using an Aerotech G25 in an Estes Big Bertha and I was wondering if anyone else had tried it, or if Jerry would like to talk a bit more about whether he did anything special with the Bertha, other than adding a 29mm mount.

I've gone as big as an F39 in a BB, it held up fine to that built stock (structuraly).

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The pentultimate example I saw was Ray Goodson, the pioneer of the slotburner which is known in the market as an Aerotech C-slot, flew a BT-55 rocket similar to a Comet with white glue construction and balsa fins with no fillets with a 29mm full H slow propellant slotburner.

The key feature of a slotburner is a totally regressive thrust trace resulting in a more nearly flat velocity vs time curve. It went incredibly high and did not shred.

The G25, which started out life as the Composite Distribution - Toy Rockets G30, was originally a (Wood) moonburner. I suspect the current variant is a nearly square side slot for production purposes.

Here is a USR G008 in a 1.3" rocket on a somewhat windy San Diego day. It went nearly straight up.



Tech History Jerry

I have only went as large as an F24 in a BB, but that G25 must be one heck of a flight.

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When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
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Well, I have no actual experience to share on this topic, but since when did I ever let that stop me from talking about something?

Seriously, my engineering instincts are to focus on stiffening the fins a bit and preparing for a hard deployment - ie, the jerk due to early or late ejection. With that much motor burn and a rocket known for weathercocking, the rocket could be well away from top-dead-center when the chute pops. And putting out the laundry horizontally in a 100mph slipstream is asking for shreds, zippers, etc.

I'm thinking a long, mylar streamer will help both with minimizing any jerk on the recovery harness as well as making it more trackable for recovery.

The fins, extending aft of the airframe, will be in turbulent air, I think, and thus be more susceptible to vibration, so stiffening them a bit seems like a good idea. Card stock lamination would be my suggestion. Or maybe some light glass.

IOW, I think you can make one of these G25-ready without overbuilding it, the common sin of our hobby. Glassing the entire airframe, adding plywood fins (thru the wall) and foaming the fin can are all unnecessary, IMNSHO

Doug

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YORF member #11

Why all the hatin' on OVERbuilding ?
I would much rather have a flying MANHOLE COVER that is "STRONG LIKE OX" rather than something that is easily damaged/beat up/shredded EVERY TIME.
To each their own....

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When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, and HAVOC !

The entire point is that most rockets are way overbuilt. Now for a sport rocket where ruggedness is more important than absolute performance , that's fine. When it becomes an issue is when the rocket is over 50% more massive than necessary and you now need double the motor to fly it. This is VERY common BTW.

The motor makers are certainly in favor of "motor wasters" and some folks with motor size envy simply want to fly the highest letter motor possible in their field. That also contributes to the popularity of cheater power motors like 1706 K casings with loads that have under 1600ns when a J is 1280 and a K is 2560.

If efficiency or performance are AT ALL a factor in your flying decisions, it is a simple matter indeed to not grossly overbuild at least some of your rockets. The best brands for that are Estes with wood fins and U.S. Rockets.

Needless to say, efficiency of motor usage has been a primary factor in my own rocket and motor design over the years to facilitate those users who simply want to fly more, whatever the power or altitude they prefer.

Jerry

GH,

I dislike overbuilding for a variety of reasons. Don't get me wrong, I truly appreciate American excess. Lord knows nobody likes a hemi in a go-kart better than I do

But in this hobby, we should all practice good engineering, as best we can. Everyone's abilities will vary in that regard, but most of us can use our TLAR skills to realize that not everything needs to be solid G-10.

In my (personal) experiences, beefing up the recovery harness a little is a good thing. Beefing up the fins a little is, too. And, since these things don't get handled by highly trained NASA techs wearing white gloves, they need some extra toughness to survive the ride to the launch site.

But all that can be achieved - at least aimed at if not hit - without buying all G-10 components from Performance Rocketry.

The key message is beefing up "a little". Card stock lamination or 3/4 oz glass is a little. G-10 is a lot

Or, coming down off my high horse , in more practical terms, glassing the airframe takes an hour or two of work and adds a day to the production cycle. That's extra labor and schedule, speaking in our manufacturing frame of reference

So, while none of my birds are light enough to win the D-eggloft-duration event at NARAM, they won't come thru the roof of my SS either, in the case of a failed ejection event, for example

In short, the engineering challenge is to find that delicate balance between frangibility and durability. Mix in a little practicality as well. That is, for this hypiothetical G25-powered Bertha, if it's easier to cut some 1/8" ply fins than it is to laminate the 1/8" balsa, then that's a reasonable tradeoff of performance for time. And I doubt it will make the rocket a WMD should it lawn dart; it won't make it too heavy.

So I'm not altogether snooty (snotty?) about this. But, when I'm building a rocket, I usually start out seeking an elegant solution, then back off some as reality dictates

Doug

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My point is a stock Estes Big Bertha can fly with amazing power totally stock if only you tailor the thrust programming toward regressive.

If on the other hand you have a Wildman CF 24 I strongly suggest a progressive 24mm H200 so the final thrust is the highest, which it can totally handle, and both the peak velocity (M2.5) and the peak altitude (>9k) will simply astound.

As an example I show a comparable and superior USR kit offering: Sonic 100F along with associated performance reports: Link

Jerry

I personally saw Mary Roberts fly a Big Bertha at Estes in Penrose on a B6-2. Then I saw a USR G60-8 fly in a USR Banshee in Ed Brown's back yard. Hi Mary! Hi Ed. Thanks Vern.

Back in the late 90's when SU were still quite prevalent I built an Estes Maniac and did fly it twice on G 25's. It was simmed to go to about 5k. I never saw it deploy but was found by some else at the launch . There were no added mods on the rocket other than the 29mm motor mount upgrade to accept the G25's.


JD