Some questions around the BP calculations

[thehemi]

I have no problem finding the calculators, I won't be asking that question.

#1- Is the BP calculator used for both the apogee and main charges? I assume this is affirmative, and why shear pins are used on the main.

#2- The BP calcs provide me with resulting PSI. The shear pins have their breaking points. To exceed this, am I using PSI x NC surface area?

I think this all sounds logical to me, but wanted to confirm I'm not misunderstanding anything.

Thanks,
Scott

 

[AndrewGil]

To answer your first question, yes. For your second question I am not 100% I just do a few ground tests till I like how it separates with the amount of shear pins I use.

 

[Voyager1]

Yes, the BP calcs are used for both drogue and main. They won’t necessarily be the same amount due to the respective containment volumes.

I generally use shear pins on both. However, if there is a sufficient friction fit for the drogue coupling, it might only be necessary to use shear pins on the main coupling. This would be to prevent a potential premature separation of the main due to the reaction to the drogue ejection event.

 

[Jmhepworth]

I use sheer pins on the main to prevent the main from coming out at apogee. I generally use enough powder to fully extend the shock cord for the drogue, so there is a serious risk that a pop in the shock cord would cause the nose cone to come off and drag the main out. I only use sheer pins for the drouge when the booster bay has so little room that I need something to hold it together.

 

[Handeman]

I only use enough drogue BP to separate the rocket enough to get the drogue chute into the air stream. This reduces the shock on the system when the two halves reach the end of the shock cord, which happens much easier and quicker in the air than in ground tests. 2 or more low flights so you can see the apogee deployment and how the drogue chute is working is highly recommended before flying out of sight. This will allow you to adjust the drogue deployment charge size to reduce a violent deployment and also adjust the size of the drogue to get the controlled drop from apogee to main deployment that you want.

 

[cls]

use pins on the drogue section to prevent drag separation after burnout. but, often just friction is good enough to hold airframe together until the magic moment.

 

[Velocity Aerospace]

Us there a difference between the mjg firewire initiator and the bp rocket starters? I would assume not. I just need them for parachute deployment

 

[cls]

Us there a difference between the mjg firewire initiator and the bp rocket starters? I would assume not. I just need them for parachute deployment

Yes, huge differences! They are not interchangeable.

Ematches like mjg take very low current to fire, and are physically robust (rough ride in a rocket), made to be highly reliable.

BP motor igniters are none of those. High current, fragile, and as you know, not 100% reliable.