Shear pins in the booster?

[smoon]

Really don't remember where I found it but it was authored by Tim Dixon!

View attachment 312036

Tim - from the way that is calculated, do you know if positive or negative Fsep numbers mean there will be separation force at motor burnout? Dennis's link indicates that it could be either way, depending on how things are calculated, and the sheet does not say.

Steve

 

[tim cubbedge]

Tim - from the way that is calculated, do you know if positive or negative Fsep numbers mean there will be separation force at motor burnout? Dennis's link indicates that it could be either way, depending on how things are calculated, and the sheet does not say.

Steve

Your decelleration at burnout will be negative. Think that spreadsheet had a positive number. Don't think I have played with the sheet that much to be competant to answer that question.

 

[djkingsley]

Attached is from my L3 documentation, if Fsep is negative, the force will separate the components. This was developed from "jderimig's" post I gave you the link to.

 

[jderimig]

Tim - from the way that is calculated, do you know if positive or negative Fsep numbers mean there will be separation force at motor burnout? Dennis's link indicates that it could be either way, depending on how things are calculated, and the sheet does not say.

Steve

Sorry about the sign conventions, I was tired when I re-derived that formula for the old thread.

In the spread sheet the + direction is assumed down (again sorry). So deceleration would be entered as a positive number. If you getting this number from a sim don't forget to subtract 1g from the earth frame deceleration because gravity doesn't contribute to drag separation. For example if you sim says you will have -5g's of acceleration at burnout, enter +4g into the spreadsheet (or whatever units using are using for the calculation).

Fsep is calculated as a compression force on the coupler (sorry again). So a positive Fsep means there are no forces separating the section.

 

[jderimig]

Another observation is that the calculation of the drag ratio, R, is overestimated by many I think. When I perform the calculation I just take the ratio of drag coefficients (Cd) and neglect the fin area.

The reason for this is that "base drag" is a large component of the lower section drag coefficient. Multiplying this base drag component by the area including the fins may be incorrectly overestimating the drag force on the lower section. For this reason and it makes the calculation easier I just take the Cd ratio's. YMMV.

 

[soopirV]

I just did this after the discussion here on the thread- my new Rayzor has a silky smooth fit. I had a bad experience on my adventurer 3 that had a similar fit- I used a bit of white masking tape, but it must've rolled just a touch, and caused a failed separation. With the pins my ground test with 3 2-56 pins was a complete success! Was going to add picture, but can't seem to from phone app after the fact.

 

[soopirV]

Further question on the topic- if booster and nose are both pinned, are vent holes still required/beneficial?

 

[Bat-mite]

Further question on the topic- if booster and nose are both pinned, are vent holes still required/beneficial?

Beneficial. it is way easier to slide a coupler into a tube when there is adequate room for the air inside to escape. Sliding an AV bay into an unvented booster is not easy. Only place to let the air out in the micro-fraction of an inch of gap between the tube and the coupler.