Boosted Bertha Cruise Missle

[aecky01]

Hello everyone. New to Rocketry Forum and re-found my love for model rockets after having built a number 20 years ago as a kid. Built a couple of Estes systems and I'm starting to move up to Mid Power rockets. I had a rather unsuccessful launch of a Boosted Bertha this weekend that I wanted to get peoples thoughts on to make sure I don't replicate in the mid powered world.

Standard Estes Boosted Bertha, super fun build. No modifications, although took my time with the finishing which meant sealing fins, filler primer and a couple of coats of paint. Flew the "Bertha" rocket without the booster stage first on C5-6. Great flight no issues. Next, I loaded up the booster stage with a C6-0 and a C6-7 in the sustainer. When I put it on the pad my spidey sense started tingling since the rocket seemed pretty unstable with alot of rod flex. The Boosted Bertha uses a 1/8" launch lug, but since the first flight didn't have any issues I want against my better judgement. As soon as I pressed the fire button my brain said "uh-oh". I could instantly tell it was way to slow off the launch pad. The rocket kicked over ( although very little wind on launch day) and shot out sideways at a 45 degree angle. The sustainer ignited and it continued basically parallel to the ground. I was luckily in a pretty remote place and the rocket landed in a famers field a long ways away. The nose cone deployed, but the laundry stayed in the body, I'm assuming due to the high speed it was traveling at ejection. I was able to recovery the rocket and minus some dents and scorch marks it looks in take and able to be flow again with some minor repairs.

My first thought when I got home was to weigh it (mistake #1). I checked CG and it was in front of the rear fins. Given the massive fin section at the back of the rocket I assumed this would be in front of CP (couldn't find a rocksim or open rocket file for this rocket and was lazy). It came in a 160g 140g compared to the estes published 113g. This really surprised me since I didn't modify the kit and thought I was pretty reasonable with glue during assembly, just used titebond II. I probably got a bit heavy with the paint and will do fewer coats in the future. I don't think sealing the fins added too much weight, but their are 8 of them on this rocket? I'm assuming in the future its best practice to weigh the kit prior to assembly and also after build to assess weight compared to the design?

Is there general guidance on max weight for various launch rod diameters? I'm nervous to fly this again as is and was thinking about cutting off the current lug and replacing with a 1/4" lug to use on my larger PVC launch pad with a 6' rod.

Any thing else I should be considering in my post mortem? I remember loving multistage rockets as a kid and was really excited about this one, but now I'm a bit nervous as it could have been a dangerous launch in a less remote area.

 

[Antares JS]

-When you say the rocket "seemed unstable with a lot of rod flex" on the pad, what do you mean? Are you saying the weight of the rocket seemed to be bending the rod?

-When you weighed the rocket, was it just the kit by itself or did it have motors or expended casings in it? I believe the 113g is the "dry weight" with no motors installed.

-I suggest using a smaller (lighter) motor in the sustainer, like a B6-6.

-All said and done, replacing the lug with a 1/4" one couldn't hurt (except it will add drag). You also don't need to cut off the stock lug if you're worried about the rocket's appearance. You can just sand some paint off and glue the 1/4" lug onto the other side.

 

[aecky01]

-I wouldn't say fully bending the rod, but definitely more wiggle and play than the other rockets I've fired recently. I also think the single launch lug probably doesn't help given the size of the stacked rocket.

-Of course I'm a dummy and still had the motors in the rocket. 140g without motors (op editted)

 

[Antares JS]

A single launch lug on a tall rocket like that? That really does seem like a silly thing for Estes to have done. Definitely add another lug at least. I would have cut the stock lug in half and attached both to the body with some distance between them.

Single lugs are really only okay on short rockets maybe a foot tall or less. If your boosted bertha couldn't stand up straight on the pad, your lug friction is going to be worse and will slow you down, and could also have contributed to your rocket flying at an angle when it left the rod.

It's also worth noting that according to the Estes catalog, the estimated maximum takeoff weight of the C6-0 is 113 grams. It kind of raises an eyebrow given that that's the estimated dry weight of the rocket.

If you want to go with the 1/4" lugs for insurance, it couldn't hurt. Again, aside from the added drag from the big lugs.

 

[neil_w]

I have single 2” lugs on most of my rockets (up to about 30” long) and it is absolutely fine.

1/4” lug is excessive for a Boosted Bertha. I would use 3/16”.

 

[Joshua F Thomas]

Long single lugs also have less drag than two lugs spaced apart*.

3/16” is a pretty solid size. 1/4” is really getting large and at that point it may just make sense to go to launch rail, no?

* Research by Tim at Apogee using CFD software

 

[BEC]

I have sixteen flights on my Boosted Bertha, which is also built stock, save for having a baffle in the sustainer and four 3/32 holes for static ports (as I like to fly altimeters in just about everything I fly). The stack weighed 126g with no motors when brand new. I'm sure it's a bit heavier now with the accumulation of ejection residue.

I have had some flights where it has pitched over some on boost...all that fin area is asking for it, and it does get off pretty slowly with Cs in both stages. But I've not had anything like a cruise missile type flight.

Some observations:

A C6-7 in the sustainer (or a B6-6) is too long a delay flown with either B6-0 or C6-0 in the booster.

The single launch lug is nestled a the fin-to-body junction on the sustainer (one of the few details of the sustainer that are different from a "normal" Big Bertha). It is also as long as the fin root. This is a reasonable placement for the stack. But if you want to try 3/16ths I'd just put a 3/16ths lug on the model in another of the fin/body junctions and see if you have better luck. A four-foot 3/16ths rod would certainly lead to better off-the-rod speed if there is no excess rod drag. Which leads to....

I would suspect paint IN the launch lug and/or dirty launch rod to be contributors to your cruise missile flight. Did the model slide quite freely on the rod? A regular Estes launch lug for 1/8 inch launch rods has quite a bit of slop in the fit....so it should be very free. With the marginal performance on boost this model can't stand any extra rod drag.

Also, as you fly it you will have to rework the booster-to-sustainer fit a few times with some sandpaper as residue from the booster gets into the base of the sustainer. You don't want the fit between the stages to be too tight.

I would love for Estes to release a booster version of the newly released C5-3. Boosted Bertha could really benefit from a little more "kick" off the pad.

 

[aecky01]

BEC thanks for the feedback. Glad to hear others have success with the boosted bertha. Rod drag was something I considered and I'll be sure to whip down the rod well before its next flight. I'm also going to replace the lug with a 3/16ths since I already have the rod (I also see a little paint residue on the inside).

I'm still trying to track down how I turned this rocket into such a pig. Going forward I'll be a lot more diligent about weighing parts before assembly, after assembly, after paint.

It sounds like the full two stage vehicle is pushing the limits of a C6-0 and I agree all that fin area could lead to some windcocking. This is probably one of the limitations of "heavier" 18mm rockets. If you are on the limits of the capabilities of a C6 motor there's not much you can do. At least in a 24mm or 29mm mount you have a lot more flexibility with higher impulse engines if need be.

 

[BABAR]

Issue is not so much as the size of the lug as the size and length of the ROD, with the obvious proviso that if you go up in rod thickness you will need an appropriately larger but not necessarily oversized lug (so 3/16 rod needs appropriate lug, but not quarter inch.). I am not sure an oversized lug is less likely to cause drag on a given rod (better) than an appropriate size lug, unless you happen to have paint or gunk IN the lug. A thicker and longer launch rod is always comforting for multistage rockets. Mini buttons with rails are even better.

I strongly suspect anyone flying a two stage Bertha on a 1/8” three foot rod with a C6 to C6 stack is courting danger with decent probability of rod whip. And if your flight leaves the rod off vertical, it probably will get worse from there.

I also personally never see the point of sticking anything greater than an A8 in the sustainer. The rocket is usually already pretty small visually by the time it lights, going with a higher motor only adds a few seconds to the boost phase (the exciting part) but adds a LOT of time and potentially distance to the RECOVERY phase (which tends to be less exciting and potentially more tedious and frustrating.)

Also, any potential fecal turbine interactions at staging (such as UP but not completely vertical and particularly horizontal or potentially DOWNWARD trajectory) are only exacerbated by a larger sustainer motor. Maybe I am easily amused, I am just happy to watch it launch watch it successfully stage, and then continue up a tiny bit. Then I spend much less time recovering (sometime even FINDING!) the sustainer and more time to set up and fire the next rocket.

 

[aecky01]

Thanks everyone for the feedback. Personally it sounds like their are some flaws in this kit that a less experienced rocket builder might not understand. I assumed Estes meant build and fly with no issues. I'm learning a alot about rocket stability, advanced building techniques and engine sizing with open rocket. I'm getting way more interested in the engineering side of things than I did as a kid (although this is my profession, so not surprising). Given how much heavier I built this kit than Estes spec and max thrust limitations of the C6-0 I'll probably just fly the bertha section for now. Just finished up a mad cow solar express and I'm excited to jump into the mid powered world with alot more information than I had before.

 

[BABAR]

Thanks everyone for the feedback. Personally it sounds like their are some flaws in this kit that a less experienced rocket builder might not understand. I assumed Estes meant build and fly with no issues. I'm learning a alot about rocket stability, advanced building techniques and engine sizing with open rocket. I'm getting way more interested in the engineering side of things than I did as a kid (although this is my profession, so not surprising). Given how much heavier I built this kit than Estes spec and max thrust limitations of the C6-0 I'll probably just fly the bertha section for now. Just finished up a mad cow solar express and I'm excited to jump into the mid powered world with alot more information than I had before.

Hopefully Estes will come out with a C5-0 soon. In fact, could probably completely replace the C6-0.

 

[SCooke123]

I think a C5-0 would be very welcome!!