Super Big Bertha- Custom build: a question about "Too much" power...

[Michael "lou" Bobb]

I've made the main rocket with 2-29mm motor assembly and I'm wanting to add a 4x24mm booster. Is there any reason this would be too much? This is my 4th rocket & me thinks I may be getting the BIGGER-BUG! LOL

 

[Nytrunner]

Depends on the motors you're putting in it. The tube will most likely be fine, the fins have potential to flutter off at highspeeds.
The best way to find out (short of building it and pushing the button ) Is a simulation with programs like Openrocket (free) or Rocsim ($$)

Which motors are you wanting to put in it, Estes motors, or composite motors? The stresses on the rocket will be very different

Be very mindful of propellant weight. If your rocket contains over 125 grams of propellant, it becomes FAA class 2 (High-power) and requires an airspace waiver to fly legally
Weights of estes motors: (from thrustcurve.org)
24mm D12: 21 grams
24mm E12: 36 grams
29mm E16: 40 grams
29mm F15: 60 grams

2x E16s -> 80gr, good
2x F15s -> 120gr, good
2xE16s + 2 D12s -> 122gr, just under limit
2xE16s + 4 D12s -> 164gr, over the limit

Composite motors come with their own difficulties like pressurization time when lit, and no direct staging ability (requires electric ignition)


Don't get discouraged! Keep building and flying!

 

[Michael "lou" Bobb]

Depends on the motors you're putting in it. The tube will most likely be fine, the fins have potential to flutter off at highspeeds.
The best way to find out (short of building it and pushing the button ) Is a simulation with programs like Openrocket (free) or Rocsim ($$)

Which motors are you wanting to put in it, Estes motors, or composite motors? The stresses on the rocket will be very different

Be very mindful of propellant weight. If your rocket contains over 125 grams of propellant, it becomes FAA class 2 (High-power) and requires an airspace waiver to fly legally
Weights of estes motors: (from thrustcurve.org)
24mm D12: 21 grams
24mm E12: 36 grams
29mm E16: 40 grams
29mm F15: 60 grams

2x E16s -> 80gr, good
2x F15s -> 120gr, good
2xE16s + 2 D12s -> 122gr, just under limit
2xE16s + 4 D12s -> 164gr, over the limit

Composite motors come with their own difficulties like pressurization time when lit, and no direct staging ability (requires electric ignition)


Don't get discouraged! Keep building and flying!

Gosh,
I'm glad I asked! I'm barely under that FAA clearance with the 2 F15s

I do have the option with the composite motors, never staged them though. I bet you'd have the best sites for staging composites on you, eh!? ;-)

Thanks Nytrunner

 

[Nytrunner]

Quite frankly, using a couple single use (or reloads) composite 29s in that sustainer body will give you quite a rush (and make those simulations more important to make sure your fins can take it). They're more energy dense, so less propellant weight for more fwoosh.
But BP staging is a really good skill to learn, and an impressive one to master. (I may be level 2 certified, but BP staging still gives me trouble)

Staging/cluster/airstart forum (vv) Plenty of good talk and build threads by experienced and new fliers alike

https://www.rocketryforum.com/forums/unrestricted-staging-airstarts-clusters.189/

 

[Michael "lou" Bobb]

Thanks

Quite frankly, using a couple single use (or reloads) composite 29s in that sustainer body will give you quite a rush (and make those simulations more important to make sure your fins can take it). They're more energy dense, so less propellant weight for more fwoosh.
But BP staging is a really good skill to learn, and an impressive one to master. (I may be level 2 certified, but BP staging still gives me trouble)

Staging/cluster/airstart forum (vv) Plenty of good talk and build threads by experienced and new fliers alike

https://www.rocketryforum.com/forums/unrestricted-staging-airstarts-clusters.189/

Thanks again, my friend!

 

[Rob40]

I'm planning a build of one as well, dual 29 mount in in my parts pile, but hesitant over the thin wall bt80 tubing. Because the potential exists to run a dual g40....and thats over 2,000+ feet and chute release territory. Besides, setting them both off and what not experimenting... I think some durability is needed. So I think I'm needing to buy several couplers and build up the body. Figuring out the slotted fins is another issue since theres not a center 29 tube they were designed for. I may just cut the same shape out of ply and go that route if I'm doing this durability thing.

Now, boosting. The boosting stage will get heavy enough to need some recovery stuff if you have enough engines. My plan to stay 125gram compliant was a normal sBBertha 29 mount over a trhiple 18-24-18 stage and a nomex wrapped chute around the 24 tube. The boost stage is near 200grams in planning, and on a normal bertha, would add several hundred feet over the f15 rated altitude. Now, launching two 18mm composites next to the e12 center for the ultimate......

Im needing a slight gap stage for this too. I successfully ran a gap stage booster inside a larger tube with a vent tube porting outside. Worked. Its just a big flame coming up.

 

[Rex R]

I flew a Big Bertha look a like on a 38mm g67, the fins stayed on(1/8" balsa). hard part was finding it on landing.
Rex

 

[NOLA_BAR]

On the main rocket you might want to armor the interior of the thin wall of the BT80 with a coupler just above the motor mount. I have a optima clone that uses 3x24mm flown successfully several times, but the tube has gotten noticeably weak from ejection charges.

 

[jqavins]

I think some durability is needed. So I think I'm needing to buy several couplers and build up the body.

The forces on the body tube are mostly straight along its length and those cardboard tubes are stronger in that direction than you think. Protection from ejection gas as NOLA_BAR mentioned is one thing, but for strength you probably don't need it. And there are other ways to accomplish protection from the heat and flame, not that I'm knocking the use of a coupler.

All that said, if you still want to thicken the tube with an internal sleeve, you need to consider:

• The added weight's effect on performance: rod speed, altitude, optimum delay, etc.
• The resultant CG movement and its consequences for stability. It's probably be good for stability but be sure.
• Interference with the nose cone shoulder. Be sure to stop the sleeve short of the top of the tube.
• Interference with the motor mount. Either stop the sleeve short of the forward centering ring or modify the centering rings accordingly.

Finally, Balsa Machining Service sells coupler stock in 34 inch lengths, which will save you a bunch of headache compared to lining the body tube four inches at a time.

 

[Michael "lou" Bobb]

thanks everyone... keep them coming

 

[Rex R]

Loc 38 mm engine mount tubes are just about the same O.D. as bt-60, you will need a balsa cone though.
Rex

 

[BABAR]

Whatever system you go with (composite with electric staging or BP to BP) I always recommend going with the lowest newton motor or motors on the sustainer for the first flight, even consider using an adapter to downsize the motor mount to a lower newton motor.

Reasons:

1. Safety. Until your bird is proven flight worthy, unless your have fancy electronics to make sure at point of staging your bird is both aimed pointy end up and has sufficient velocity for sustainer to be stable, if sustainer does light catywampus I want as small a motor pushing it as possible.

2. Most people want to fly their rocket more than once. Assuming it stages above 100 feet, even the smallest motor is likely to take it near the limit of visual range or further. If you REALLY want altitude, build a minimum diameter single stage with a huge motor in it. The fun in staging is to make it work and keep as much of the flight in visual range as possible so you can see it worked. Sustainers that light even a few degrees off vertical due to weather cocking or other untoward events can go a long way laterally as well as vertically. So save yourself a long walk or a lost rocket, at least for the first flight. The worst that can happen is maybe you will be underwhelmed by the sustainer flight (but I doubt that will happen, it is just supercool just to see it stage successfully!). Assuming you recover the rocket, you can put bigger motors in next time. If you START with the bigger motors and lose the rocket, you don’t get the chance to down size the motor cuz there is no next flight.

For BOOSTERS, first priority is enough oomph to get off the pad. Especially if you are clustering, nice to have enough oomph that even if one motor doesn’t light you still will be safe off the rail or rod. Beyond that, again for fist flights keep the Newtons low, the excitement is seeing the staging

Hope you get at least two straight trails!