Launch Rail Button Location On Larger Rocket: Need Advice

[NattyDread]

I am building a 7.5" diameter dual deploy 54mm motor rocket. The total length is 96" and I am putting on 1515 rail buttons. The rocket has a long tail cone (upside down nosecone with the tip cut off) Its a lOC V2. The most secure place to mount them is 22" from the bottom of the rocket and the second one about 6" above that. Any thoughts, problems, guidelines to follow?

 

[rail-buttons]

Sounds a little close together to me. Because of the tailcone, your bottom button has to be up farther than most rockets, but I would mount the top button up a little higher if you can. Any pics?

 

[dixontj93060]

22" between buttons on a 96" rocket? That seems too close together to me but I'm no expert. Contact Loc. They have already made all the mistakes with that rocket and should be able to help you out.

You didn't read the post right--22" from the bottom.

 

[dixontj93060]

The V2 shape is not that different from the PML Bulldog I have. I would have to agree with Randy, for a 96" rocket, buttons 6" apart are too close. You need another farther up the airframe. Having your first button at top of tailcone is fine. Here is my 78" long 6" dia. Bulldog with 1010 buttons ~16" apart.

 

[NattyDread]

. See pictures, I taped the buttons on. It's not the standard LOV kit. I stretched it for DD. Are there any guidelines for rail button placement? I cmt go much lower than the bottom one. A our 22" from the floor (fins extend beyond rocket bottom)

 

[dixontj93060]

Wow, that is a long (non-scale) V2!

Why not move the bottom button down to the tailcone seam? Some say the top button should be approximately at the CG without motor loaded.

 

[rail-buttons]

Wow, that is a long (non-scale) V2!

Why not move the bottom button down to the tailcone seam? Some say the top button should be approximately at the CG without motor loaded.

I agree with Tim. How far will the ebay go into the booster. I would put the top button just below that. It may not pass RSO with the buttons in the taped position, depending on who the RSO is at the time.

 

[terryg]

Is the upper section of the ebay going to be captive? If so you could use a well nut to both secure the ebay to the upper section and hold the rail button. This would give you plenty of length. Just make sure the rail buttons are aligned before taking it to the pad.

 

[NattyDread]

I have added 2 sims for the V2 Stretch that shows the CG with and without motors. The think in the center is a large Altimeter bay. in the modern HP rocketry 2 book, he talks about lugs and says, "the second lug will go forward on the line within a body diameter or two of the center of gravity" Still the question on relationship to CG or something? with or without motors? etc.... Note how much the cg moves when the large 54mm motor is in.

 

[Sailorbill]

View attachment 172144View attachment 172143 I have added 2 sims for the V2 Stretch that shows the CG with and without motors. The think in the center is a large Altimeter bay. in the modern HP rocketry 2 book, he talks about lugs and says, "the second lug will go forward on the line within a body diameter or two of the center of gravity" Still the question on relationship to CG or something? with or without motors? etc.... Note how much the cg moves when the large 54mm motor is in.

Does the motor mount really stick that far out of the rocket? If it was closer to the aft centering ring that would really help the CP/CG Ratio. Longer motors help move the CG forward.

 

[NattyDread]

Yes, see other photos. "Does the motor mount really stick that far out of the rocket? If it was closer to the aft centering ring that would really help the CP/CG Ratio. Longer motors help move the CG forward."
Today, 03:04 PM

 

[JohnCoker]

It looks to me like you can put the aft button at the very aft of the airframe (through the airframe and tail cone). I agree with others that it needs to be as close to the aft end as possible, and the buttons need to be spaced farther apart.

Nice looking fin can by the way.

 

[dixontj93060]

View attachment 172144View attachment 172143 I have added 2 sims for the V2 Stretch that shows the CG with and without motors. The think in the center is a large Altimeter bay. in the modern HP rocketry 2 book, he talks about lugs and says, "the second lug will go forward on the line within a body diameter or two of the center of gravity" Still the question on relationship to CG or something? with or without motors? etc.... Note how much the cg moves when the large 54mm motor is in.

Look at the bold type in post #6--<<without motors>>

 

[NattyDread]

Hi Dixontj: I did see that, but I question it. CG is always calculated using the largest motor you will put in the rocket. So it does not seem reasonable to use a point of reference that will change on the pad. If you weight a rocket to move CG forward, you don't do it without the rocket fully launch pad loaded. I MIGHT BE MISSING SOMETHING HERE?????????????????? Just trying to understand the reasoning.

Look at the bold type in post #6--<<without motors>>

 

[dixontj93060]

Hi Dixontj: I did see that, but I question it. CG is always calculated using the largest motor you will put in the rocket. So it does not seem reasonable to use a point of reference that will change on the pad. If you weight a rocket to move CG forward, you don't do it without the rocket fully launch pad loaded. I MIGHT BE MISSING SOMETHING HERE?????????????????? Just trying to understand the reasoning.

Has nothing to do with the flight. If you place the fore button at approx. CG without the motor, when it gets on the pad your CG moves to the aft (due to loaded motor) and will fall between both rail button locations reducing the leverage point/length and stress on the buttons from wind, tilt, etc.

 

[NattyDread]

Has nothing to do with the flight. If you place the fore button at approx. CG without the motor, when it gets on the pad your CG moves to the aft (due to loaded motor) and will fall between both rail button locations reducing the leverage point/length and stress on the buttons from wind, tilt, etc.

Sorry I am a little daft. I'm not quite sure I understand what you're saying. At launch, w rocket loaded, Are you saying that you want to CG below where the rail button is? Actually, it sounds like you were saying with a loaded rocket, the CG should be between both rail buttons? I think my general problem is, a lack of understanding of the center of gravity and the effects that it has on the Rockets overall. I think I understand the concept of where the center gravity needs to be, in front of the center pressure. But really with the center of gravity IS, I'm not sure I get that.

 

[dixontj93060]

Sorry I am a little daft. I'm not quite sure I understand what you're saying. At launch, w rocket loaded, Are you saying that you want to CG below where the rail button is?

Yes, below/aft of the forward button.

Actually, it sounds like you were saying with a loaded rocket, the CG should be between both rail buttons?

Yes, between both rail buttons.

I think my general problem is, a lack of understanding of the center of gravity and the effects that it has on the Rockets overall. I think I understand the concept of where the center gravity needs to be, in front of the center pressure. But really with the center of gravity IS, I'm not sure I get that.

In simple terms, CG is the balance point of the rocket. A more complete explanation can be found here:
https://exploration.grc.nasa.gov/education/rocket/rktcg.html

 

[dixontj93060]

Yes, below/aft of the forward button.



Yes, between both rail buttons.



In simple terms, CG is the balance point of the rocket. A more complete explanation can be found here:
https://exploration.grc.nasa.gov/education/rocket/rktcg.html

And maybe just to emphasis one more time we are discussing two different things... First there is the CG/CP relationship which work to counterbalance each other during flight which keeps the rocket stable/straight (think two kids on a teeter-totter). And, second, there is the positioning of the rail buttons to provide the rocket an opportunity to get up to speed while positioned to reduce leverage on the button/rail attach points.


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