Booster Separation

[Hal8472]

Hi

I have been thinking of the problems of booster stage separation. I know the simple solution is to put in nylon screws to prevent it and then ensure sufficient pressure to force separation at the right time.

When are the screws required? I have been looking online regarding drag force and the booster stage seems to be without the frontal coefficient of drag. There is still significant surface area, particularly on the fins. Granted that the forward section is generally heavier, and thus retains more mass to overcome the drag.

Where I am going with this is changing the idea of the design to require the drag separation in flight. But now I need to calculate it to ensure that it happens instead of just throwing some screws on in case.

Anyone have a good comparison for two objects, one without frontal drag?

Andrew

 

[bobkrech]

How about telling us a bit more about the rocket. Is it a model rocket or a high power rocket? Black power or APCP? The requirements for staging depends a lot on what you are trying to accomplish.

Bob

 

[crossfire]

From model to L-3 HP rocketry I have never used shear pins in the booster. Friction fit works very well. I really don't see a need for it.

 

[Hal8472]

Well, it really is a physics question that applies to each type of rocket. It does not appear to be a significant issue in low power due to friction being a function of velocity.

Crossfire, your point gets to the heart of the issue. When is friction fit sufficient, and when is it not? When the designer is in doubt then they put them in, but this is not scientific. The whole point of modeling ahead of time is to know if it is going to work prior to launch.

For half of the calculation I would use a pull scale on the pieces and convert to newtons.

The second half is calculating the force of the drag acting on the two sections independently. The nose cone has the frontal section area while the back has the fins. The surface area of the body tube depends on the design of your rocket. If the front has more drag then the two sections will stay together regardless. If the back has more drag then the question is whether the difference between the two pieces is sufficient to overcome the resistance (such as friction fit or even sheer pins).

So, I would like to know ahead of time specifically how much force is needed. Anyway, if people are using Friction Fit successfully or sheer pins to be cautious then it would probably take an unusual design for it to be a significant issue.

Thanks for your help

 

[Steve Shannon]

Is there any reason to prevent drag separation? Under thrust the booster won't drag separate. Once the booster motor is done firing you want it to separate as soon as possible, otherwise it's holding the sustainer back. Drag separation is beneficial at that time. That may not be the optimum time to ignite the sustainer though, so don't put the sustainer ignition electronics in the booster.
In my limited experience with building two stage rockets the interstage coupler was an unrestricted slip fit into the sustainer. I didn't add anything to prevent drag separation. If I picked up the sustainer the booster would slide out.

 

[Bat-mite]

Steve, I didn't see anything in either of his posts to indicate he was staging. I think he is just concerned about drag separation at motor burnout. If there is a sudden stoppage of acceleration at burnout, then the lighter payload/NC half will continue forward as the heavier and more draggy booster will want to fall.

Here is what the smart folks here at TRF taught me about calculating shear pins for booster retention until apogee: https://www.rocketryforum.com/showthread.php?132695-sheer-pins-101&p=1559229#post1559229

And here is a more comprehensive thread: https://www.rocketryforum.com/showt...-APPROACH&highlight=shear+pins+drag+calculate

 

[bobkrech]

Well, it really is a physics question that applies to each type of rocket. It does not appear to be a significant issue in low power due to friction being a function of velocity.

Crossfire, your point gets to the heart of the issue. When is friction fit sufficient, and when is it not? When the designer is in doubt then they put them in, but this is not scientific. The whole point of modeling ahead of time is to know if it is going to work prior to launch.

For half of the calculation I would use a pull scale on the pieces and convert to newtons.

The second half is calculating the force of the drag acting on the two sections independently. The nose cone has the frontal section area while the back has the fins. The surface area of the body tube depends on the design of your rocket. If the front has more drag then the two sections will stay together regardless. If the back has more drag then the question is whether the difference between the two pieces is sufficient to overcome the resistance (such as friction fit or even sheer pins).

So, I would like to know ahead of time specifically how much force is needed. Anyway, if people are using Friction Fit successfully or sheer pins to be cautious then it would probably take an unusual design for it to be a significant issue.

Thanks for your help

I'm not sure you asked the right question.

1.) Are you planning to launch a single stage rocket and concerned about drag separation on ascent?

If this is your concern size matters. For light weight model rockets that don't go very high, friction fit is fine. If you can hang the rocket from you hand and shake it slightly and it does not separate, your are ok.

If you have a high power rocket that will apogee at several thousand feet or higher, and/or has a relatively large diameter with a light nosecone, or has a very heavy nose and a light aft section, friction fitting may not be enough. In the latter case you can have drag separation, and in the former cases, unless you have a small hole in the airframe to allow the internal pressure to equilibrate with the outside pressure on ascent, the internal trap gas will pressurize the inside of the rocket and can act like an ejection charge to push the rocket apart. The way to avoid this in addition to a small vent hole is to use shear pins to keep the rocket together until you want it to come apart by firing an ejection charge.

2.) Or are you planning to launch a 2 stage rocket and concerned about the booster separating from the second stage before the motor ignites?

If you are concerned about a booster separating from an upper stage before ignition you have a different situation. BP motor staging to BP motors ignite promptly after burnout with little change of drag separation, and the sustainer ignition will push the stages apart.

In high power rocket, you need electronic ignition to ignite the upper stage and you want the booster to drag separate before the sustainer ignites, so you typically want coast for a second or two before you ignite the second stage so you don't cook the booster. In some situations it might be advantageous to keep the booster attached to the sustainer during most of the coast phase. In this situation you may want to pin the booster to the sustainer and use an ejection charge or release mechanism to separate the booster from the sustainer just before ignition of the sustainer.

Different solutions for different situations.

Bob

 

[Steve Shannon]

Steve, I didn't see anything in either of his posts to indicate he was staging.
Snip...

You're absolutely right. Oh, no! I've become "that guy!"
I guess I got it from the title: Booster separation and then the OP's reference to the "booster stage".
I don't consider it a booster unless there's staging. I believe that's technically correct.

 

[Hal8472]

John, perfect. That helps a huge amount. Glad to know that others are thinking about it. Interesting that it led to such a passionate discussion but nice to know that we have passionate engineers in our midst willing to simplify a complex problem for us. My key word searching on the forum did not pick up the thread (as I was focusing on the calculation of drag and not the required pins to overcome it), so thanks for providing the link.

 

[Bat-mite]

John, perfect. That helps a huge amount. Glad to know that others are thinking about it. Interesting that it led to such a passionate discussion but nice to know that we have passionate engineers in our midst willing to simplify a complex problem for us. My key word searching on the forum did not pick up the thread (as I was focusing on the calculation of drag and not the required pins to overcome it), so thanks for providing the link.

A pleasure. Just paying it forward.