Straighten out my Bruiser!

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EBurg

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I launched the reborn Bruiser a few weeks ago at Bong, and other than a long walk in the swamp to retrieve it, it was a great flight. It took a sharp turn off the rail for the first hundred feet or so and then straightened out. I am looking at one of two solutions to fix that for next time. It should have been going fast enough off the rail (Openrocket sim'ed 31.7 mph), but it may do better with a more powerful kick. The winds were fairly light (5-7 mph) so I think I should have been fine with the K550.

I'm leaning toward my rail buttons being too close together. I rebuilt this as a dual deploy so kept the upper rail button down low - they are probably 12" apart on an overall length of 84". I liked them low so it could get up to speed on the rail, and also it wouldn't interfere with the laundry coming out. I'm thinking about moving the upper button close to CG but before I do, wanted to get some other opinions.

Video of the flight is here:
https://youtu.be/ZGwhH7f_0ws

Screenshot_20170708-173548.jpg
 

djs

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LOOK BRUISER! YOU'RE GOING TO GO OUT THERE AND FLY STRAIGHT! NONE OF THIS NAMBY PAMBYING AROUND AND FLYING CROOKED! YOU HAVE ONE JOB, AND THAT JOB IS TO FLY STRAIGHT AND COME BACK SAFELY! THAT'S IT! NOW GET OUT THERE AND GIVE ME 50 SMOOTH DEPLOYMENTS! AND NO CHEATING WITH CHUTE RELEASE!

er.. i'm guessing that's not what you meant by "Straighten out my Bruiser".

If this was at the TWA launch, my darkstar did the same thing. Not sure if people didn't tighten down the rails after using them or what. I wouldn't worry too much about it unless it did it on several flights.
 

Rex R

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I have noticed similar flights both in person and in videos, mostly they are somewhere over 5 lb., high thrust/accel flying off 1010 rails. in the slo-mo vids one can watch the rail whip as the rockets clear the pad. if it bothers you, I suppose that you could swap over to 1515 rails.
Rex
 

llickteig1

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Seriously, you're worried about that little bit of movement? I wouldn't be.

1010 rail is not going to flex on a rocket that size on a relatively calm day. A little more spacing on the buttons might help if you're launching off a pretty short rail (6'), which is looked like you might be.

I would say that more likely is that the pad is too light weight and it jumped or shifted a little with the thrust of the K550.

Nice looking flight, IMO.

--Lance.
 

markkoelsch

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31 mph off the rail is pretty slow. Next time at Bong swap to 1515 rail guides and fly off the far pad/tower. It is longer, and I bet that would help.
 

crossfire

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Mark remember Dave Sutton? Almost every launch he flew his Bruiser almost on AT K550s. I can't remember him having problems.
 

Handeman

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I don't have a Bruiser, but I did have a rocket that would make a turn off the pad whenever I flew it on larger motors. Smaller motors were OK. I added some nose weight and all is right with the world.

I little too marginal with the CP/CG and you can get that turn off the pad without being "unstable".
 

markkoelsch

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Mark remember Dave Sutton? Almost every launch he flew his Bruiser almost on AT K550s. I can't remember him having problems.
Of course I remember Dave. Irregardless, that velocity is a bit low off the rail, hence the suggestion to fly off a longer rail. More velocity helps. Using the same hardware you could pop an AT K805 in it, and have a bit more velocity.
 

DAllen

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That really isn't much movement. Personally, I wouldn't worry about it but it would be interesting to see if putting it on a tower makes a difference.
 

EBurg

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Thanks everybody - My Bruiser weighed in at 16# (stock is about 12#) with all the Dual deploy and coupler parts. That explains why a stock one on a K550 flew fine but mine may be slightly underpowered. This flew off the 1515 rail @ TWA - I'll try again with a little higher impulse load before moving rail buttons and all. I'm also just over 1 caliber of stability so some nose weight could definitely help. Seems crazy to need nose weight with a 7' tall rocket, but the back end is pretty heavy.

Just for good measure, I'll try djs' advice also. Can't hurt :)
 

NateLowrie

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Thanks everybody - My Bruiser weighed in at 16# (stock is about 12#) with all the Dual deploy and coupler parts. That explains why a stock one on a K550 flew fine but mine may be slightly underpowered. This flew off the 1515 rail @ TWA - I'll try again with a little higher impulse load before moving rail buttons and all. I'm also just over 1 caliber of stability so some nose weight could definitely help. Seems crazy to need nose weight with a 7' tall rocket, but the back end is pretty heavy.

Just for good measure, I'll try djs' advice also. Can't hurt :)
I don't know that I trust your sims....You said open rocket sim at 46.4 ft/s which should be good enough by itself coming off the rail. But, I think your sim is a little low (unless you are using an insanely short rail).

With a 16 lb rocket (19.2lbs loaded with the motor) on a 8' rail, thrustcurve shows 66 ft/s off the rail. The K550 has a spike of 850N in the initial part of the burn tapering to 600N for the first 2 seconds. If we just use the 600N, your at 7:1 Thrust To Weight. It's plenty of motor even with your heavier rocket.

What rail size were you using? If it's a 6', go to an 8' or 10'.
 

jrkennedy2

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1010 is pretty hard to flex considering that motor's force vector is tangential to the rail. A bit of "sticktion" from a dirty rail may increase this a small bit so maybe keep the rail clean. Also, check the mounting to the tripod (or slab or whatever the rail is mounted to). Any flex or give at the mount point will seem amplified at the end of a 6' rail. The degree of flex applied to a 10' rail may also seem worse. An extra firm mount point is very helpful!

Someone help me with my basic math:

A 3 degree wiggle, at the mount point, becomes a bit over 3" of movement at the end of a 6' rail:

tan(3 degrees)*72" ~= 3.77" of movement at the end of a 6' (72") rail with no rail flex at all.

That 3", applied to the last rail button getting to the end, could cause quite a kick to the side of it rotates about the CG of the Bruiser...

I still like the flight! :)
 
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