thinking of adding a third rail guide to Ultimate Darkstar

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glenbo

Rocket Scientist/Fry Cook
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Normally on a manually operated launch pad I wouldn't be concerned but if at LDRS this year I happen to launch on a hydraulic launcher which has alot more whip I'm considering adding a third 1515 rail guide at the switchband.
With shear pins in place connecting the av bay to the booster the area where I would place it is 2 in. directly below a 1/4 in. static porthole (i have three). Being below the porthole would there still be enough turbulence created to interfere with my altimeter readings?
 
I'm starting to see a trend here, thanks guys.
 
If you want a good debate on this topic...
https://www.rocketryforum.com/showt...hread-Performance-Rocketry-Competitor-4/page2

Yes adding 3 will help guidance, but I wouldn't put the 3rd button on the switch band. IMO, putting 3 on your fin section is the best approach for aligning them, plus you won't stress your shear pins. I did this for my L3 and had no issues at all. 3 buttons aren't NECESSARY, but definitely helpful.

What's the deal on "hydraulic launchers?"
 
If you want a good debate on this topic...
https://www.rocketryforum.com/showt...hread-Performance-Rocketry-Competitor-4/page2

Yes adding 3 will help guidance, but I wouldn't put the 3rd button on the switch band. IMO, putting 3 on your fin section is the best approach for aligning them, plus you won't stress your shear pins. I did this for my L3 and had no issues at all. 3 buttons aren't NECESSARY, but definitely helpful.

What's the deal on "hydraulic launchers?"

I am with you.
 
Hydraulic launchers have a tendency to jerk around and be rough especially while stopping.
 
I used 3 on a heavy 5.5" rocket many years ago... a slight mis-alignment, and I could not get the rocket on the rail... Removed one button, and the rocket worked as intended. I recommend 2 buttons securely mounted.
 
This is a rebuild of my Ultimate Darkstar that suffered a crash last year. The booster section could not be salvaged. The original area the rail buttons were installed were just above & below the fore & aft fins. 25in. apart which for a 12ft. long 50 plus pound rocket seemed very close together although liftoff last year was great. I noticed in the rocsim drawing that the top rail guide is 3/4 up the booster section. 42in. from the bottom. So I moved it. Now the top rail guide is within the CG range and with the further space less stress on the guides when installing on the pad. So I have stayed with 2 guides.
 
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I think three, while a little more time consuming to get lined up makes sense. This is not for a smaller rocket where the buttons are a foot apart.

Most rails are 6'. If you have two buttons the rail provides no guidance once the top button leaves the rail. So, if you have buttons far forward you significantly cut down the effective rail length.

Three buttons. One at the bottom, one towards the front of the fins, and third a bit further up helps. On larger rockets the two buttons too close together seems questionable when the rocket is hanging on them.
 
Hydraulic launchers have a tendency to jerk around and be rough especially while stopping.
You mean like this?
[video=youtube;4I7fc15osWg]https://www.youtube.com/watch?v=4I7fc15osWg&index=21&list=PLEISeVEqORmx2VCUFCzOg6gf3nTE7Kl5S[/video]

Seems like you'd torque the hell out of your rail buttons without bungee cords here...

I used 3 on a heavy 5.5" rocket many years ago... a slight mis-alignment, and I could not get the rocket on the rail... Removed one button, and the rocket worked as intended. I recommend 2 buttons securely mounted.
How did you align them? I always use an angle iron and haven't had trouble.

I think three, while a little more time consuming to get lined up makes sense. This is not for a smaller rocket where the buttons are a foot apart.

Most rails are 6'. If you have two buttons the rail provides no guidance once the top button leaves the rail. So, if you have buttons far forward you significantly cut down the effective rail length.

Three buttons. One at the bottom, one towards the front of the fins, and third a bit further up helps. On larger rockets the two buttons too close together seems questionable when the rocket is hanging on them.
Exactly - you lose guidance as soon as you're down to 1 button on the rail. If you have room for a 3rd button some distance above your fins, it will improve guidance AND relieve some of the stress on your aft buttons while the rocket is sitting on the pad.
 
20150228_213029.jpg20150228_213040.jpgDecision has been made. Looks to be alligned perfectly. I have peace of mind now. BTW the fasteners are well nuts that Wildman sells with his 1515 giudes.
 
Decision has been made. Looks to be alligned perfectly. I have peace of mind now. BTW the fasteners are well nuts that Wildman sells with his 1515 giudes.
Well done sir! Happy flying.
 
Glenbo,

Your 3 buttons will help keep the rocket on the rail while loading.
Consider also using bungy cords in 2 locations to hold the rocket while going from horizontal to vertical on the pad.
The hydraulic towers should allow for you to climb and remove the cords before launching.
We have loosened and lost buttons on heavy rockets while loading, especially large diameter builds with transitions.

Tom C.
MARS Geneseo NY
 
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