How much "pop" when ground testing???

[threelapfancy]

Ground testing some ejection charges on a 5" and 6" diameter rocket. I am trying to figure out how much (or how far) the sections should seperate from each other? I have looked on youtube for some videos and it seems that the majority of them are using way way way too much powder.

For example, on my 6" bird at 6PSI for the main with 3 4-40 shear pins the calculators say I should use about 3 grams BP. So I packed the charge and fired it off. It separated the pieces no problem and shot the nose cone off about 5-6 feet away from the airframe tube.

I am curious since it did not pull the entire shock cord out will the weight of the components along with the moving away from each other at that point pull the rest out?

I have talked to some people that say all you need to do is separate the pieces and let gravity do the rest, and I have talked to (and seen people in my club) damn near destroy the things blowing them apart, sounds like flipping gun shots when the back up charges go off.

As I said above the pieces separated with no trouble at all, just want to see what other opinions are. Also if anyone has any video of known good ground tests for some larger rockets I would love to see them, just let me know where.

Thanks
Steve

 

[Luv2launch]

When I posted questions about ground testing I was told you want to get the nosecone off and the laundry out all done while not blowing your rocket to bits in the process.You gotta figure yes maybe gravity can pull your chute out but then again maybe its packed on the loose side and is stuck in the tube.IMO better to have enough punch to get the laundry out rather then hoping gravity will do the rest.

 

[SpartaChris]

I don't know if there's a rule of thumb or not, but I personally want to see all my laundry come out before I fly. If it can come out on the ground, it can come out in flight, so that's what I shoot for.

 

[Pantherjon]

I don't know if there's a rule of thumb or not, but I personally want to see all my laundry come out before I fly. If it can come out on the ground, it can come out in flight, so that's what I shoot for.

Like the ground tests you guys did last year with the Interceptor-M(then) with the nose cone flying darn near across the lot?:neener:

Kind of what I 'shoot' for as well..But, truth be told, I have only done 2 or 3 total ground tests.

 

[SpartaChris]

Actually, that wasn't my project. Contrary to popular belief, I'm not a member of PRS. More like an honorary hanger on. =)

But yeah- If it won't do it on the ground, you can't reasonably expect it to do it in the air.

In other words- blow it out or blow it up!

 

[Zeus-cat]

In my opinion gravity won't help you pull out a parachute that is still inside the body of the rocket. The nose and the body of the rocket will be pulled downwards by gravity with the same acceleration (32.2 ft/sec-squared (or 9.81 m/sec-squared if you prefer)).

It is likely the nose and booster will fall at different rates if not tied together, but this is due to drag, not gravity. Therefore the drag of the nose will have to greater than the friction that is holding the parachute inside the body of the rocket to pull out the parachute. I am sure most of us have seen parachutes get hung up in the body tube of a small rocket. The ejection charge pops off the nose, but the chute fails to get pushed out. The nose and rocket fall together, but the drag of the nose is too small to overcome the friction needed to pull the parachute out.

I would want to see the ejection charge push the main chute out of the rocket unless I have a drogue that is certain to deploy and can pull out the main chute.

 

[Handeman]

In my opinion gravity won't help you pull out a parachute that is still inside the body of the rocket.
.....
I would want to see the ejection charge push the main chute out of the rocket unless I have a drogue that is certain to deploy and can pull out the main chute.

I agree with what you're saying. Gravity won't pull the chute out of the BT. As far as the ejection charge pushing the main chute out of the BT, that usually doesn't happen either. The drogue is a good idea.

When the ejection charge goes off, it starts creating a high pressure area inside the BT. That pressure is transferred around and through the recovery components to the air inside the BT and the air comes up to the same pressure that is created by the ejection charge. It's this air pressure that then forces the nose cone off the BT. The energy/inertia imparted to the nose cone is what then drags the recovery components out of the BT. If you use a d-bag that is a snug fit in the BT or a piston, then the ejection charge will actually push the recovery components out because the pressure created can't be transferred to the air in the other end of the BT. The d-bag or piston moves because of the difference in pressure from the ejection charge side and the air in the rest of the BT. If the nose cone isn't physically pushed off by the piston or d-bag, then it will come off as the pressure on the two sides of the piston equalizes to the high pressure created by the ejection charge.

The short answer is that the ejection charge does not push the chute out of the BT. It only pops the nose cone off. If you want to make sure the chute comes out, attach the chute close to the nose cone and pop the nose cone off the BT hard enough to drag the chute out too, or use a drogue that will pull the chute out.