Pressure relief holes

I have a PML AMRAAM2 underway and I am wondering about the rumoured importance of drilling a pressure relief hole in the airframe to prevent the nose cone from popping off prematurely. Does this actually occur, and if so and if so is it more likely to occur with an H motor than with an F??

If a rocket goes very high, very quickly, the pressure inside the rocket can not equalize with the pressure outside the rocket. This causes the nose cone to be pushed off.

I have seen this happen on larger rockets with large motors.
It would not hurt to drill a small hole in the middle of the airframe.
1/8" of an inch should be fine.

I have vent holes in all of my rockets that will go over 2000'.

Originally posted by mwiggins367
II have seen this happen on larger rockets with large motors.

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It can happen on almost any rocket HPR. I've seen it happen with a PML Explorer on an H motor (on a L1 cert unfortunately).

Like Marty, I drill a pressure relef hole in all rockets as well. I agree on the 1/8" although larger and longer airframes require a slightly larger hole. I prefer drilling my holes a few inches below where the shoulder of the nose cone projects into the body tube.

RocketCal says that atmospheric pressure at 0 feet is 14.7 PSI, 14.17 PSI at 1000', and 12.23 PSI at 5000'. Unless I screwed up the math you get 0.53 PSI differential going from 0 feet to 1000' and 2.47 PSI differential going from 0 feet to 5000'.

A 4" diameter rocket has about 12.6 square inches of bulkhead area. A 10" diameter rocket has about 78.5" of bulkhead area.

If you assume your rocket is airtight then a 4" rocket going from sea level to 1000' will have 6.7 pounds of force trying to eject the nosecone. The 10" rocket will have 41.6 pounds of force pushing on the nosecone.

The 4" rocket going from 0 to 5000' will have a force of 31.1 pounds trying to eject the nosecone. The 10" rocket will have about 194 pounds of force trying to separate the rocket.

Drill those vent holes folks!

Thanks for the breakdown. I don't think many people really realize the importance of this. It is a terrible reason to lose a rocket and an even worse to fail a cert.

So really any rocket that is going over a couple thousand feet should be drilled regardless of the width of the airframe?

That is certainly the safe way to do things. Really if you are clever you can disguise the holes if you are worried about aesthetics otherwise it is a ten second step that could safe your rocket.

disguise the hole.....how??

Why not just drill em through the centering rings and forget about it?

Originally posted by Jeff
So really any rocket that is going over a couple thousand feet should be drilled regardless of the width of the airframe?

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Absolutely. And even less than a 'couple thousand feet'. The Explorer I described was probably only 1000 -1200 ft up when the nose cone blew off due to the pressure in the body tube.

It's silly to even think if it needs a hole or not. Just drill one. It takes 10 seconds.

Originally posted by Scudrunr
Why not just drill em through the centering rings and forget about it?

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Huh? How would that work? The inside of the body tube needs a vent to the outside for the pressure to escape. Drilling through the centering ring doesn't provide the required pressure equilibrium to develop between the outside and the inside of the rocket; pressure would still accumulate inside the tube with the same result....a blown off nose cone.

Actually. if you drilled a hole in ALL of the centering rings, you could vent the airframe. I think this is what scudrunr was refering to.

Personally, I just drill a hole in the tube.


Jim
NAR/TRA L3

Originally posted by fyrechaser
Actually. if you drilled a hole in ALL of the centering rings, you could vent the airframe. I think this is what scudrunr was refering to.

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AAHHH...ok, that makes sense and should work. But seems more trouble than drilling that single hole in the BT.

Aesthics? C'mon guys these rockets are built to FLY and everyone watching them knows that. A hole in the airframe hardly disrupts the aesthics of a FLYING model, regardless if it's scale, it's just plain cool looking, or it's simple 3FNC. Seems to me, in this case, functionality should heavily outweigh any minor (extremely) aesthic impairment.

I agree Mark, I just drill a hole in the BT....can't see it 5 ft away. But hey.....if someone would rather drill the CR's... that's cool too!
If you're flying DD though, don't forget to vent the payload/main tube.

Jim
NAR/TRA L3

From the TRA L2 test (technical section):

25. A small hole is typically recommended near the top, but below the nosecone or payload section, of a high power rocket's booster section. Why?

a. This hole allows excessive ejection charge pressures to vent to reduce shock cord stress.
b. The hole is used to give air pressure readings for on-board altimeters.
c. The hole vents internal air pressure as the rocket gains altitude to prevent premature separation.

Answer is 'C' BTW. Just took the test but haven't gotten to fly yet!

My introduction to vent holes went something like this:

"Hey Warren, you got any vent holes in that?"
"Uh, no. Should I?"
"Warren, remember those mountains you drove through to get out here to the desert? The ones that took you up over 4000 feet?"
"Yeah..."
"How many times did your ears pop on the way up and the way down?"
"Probably about a dozen or so times each way."
"Drill the stupid holes."

WW

Great analogy WW! I really like folks that make us think!

Jim
NAR/TRA L3

I always cover my vent holes with tiny plastic ears.

Originally posted by WillMarchant
RocketCal says that atmospheric pressure at 0 feet is 14.7 PSI, 14.17 PSI at 1000', and 12.23 PSI at 5000'. Unless I screwed up the math you get 0.53 PSI differential going from 0 feet to 1000' and 2.47 PSI differential going from 0 feet to 5000'.

A 4" diameter rocket has about 12.6 square inches of bulkhead area. A 10" diameter rocket has about 78.5" of bulkhead area.

If you assume your rocket is airtight then a 4" rocket going from sea level to 1000' will have 6.7 pounds of force trying to eject the nosecone. The 10" rocket will have 41.6 pounds of force pushing on the nosecone.

The 4" rocket going from 0 to 5000' will have a force of 31.1 pounds trying to eject the nosecone. The 10" rocket will have about 194 pounds of force trying to separate the rocket.

Drill those vent holes folks!

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Outstanding post, Will.

OK OK I'LL DRILL THEM! Thanks for the as always informative replies. I guess I should also go back and drill my already-painted rockets (I hope this doesn't chip and flake the surrounding paint).

Originally posted by mwiggins367
It would not hurt to drill a small hole in the middle of the airframe.

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Since this is a PML kit we are talking about, the middle of the airframe would probably be below the piston. This wouldn’t help relieve pressure between the piston and the NC.
But this brings up the question – does anyone vent the area between the engine and the piston? I haven’t seen the need, but, in theory, couldn’t the same pressure cause the piston to push the NC out?

Originally posted by Rocketeer
Since this is a PML kit we are talking about, the middle of the airframe would probably be below the piston. This wouldn’t help relieve pressure between the piston and the NC.
But this brings up the question – does anyone vent the area between the engine and the piston? I haven’t seen the need, but, in theory, couldn’t the same pressure cause the piston to push the NC out?

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I seriously doubt it, the piston should have a longer distance to travel before it starts really pushing on the chute etc. above it. The nose would pop off first from the same pressurization effect, in my opinion. If there were no vent holes, the piston would feel pretty much the same pressure above it until the nose came off. If there were a vent hole near the top, it would probably be good practice to allow sufficient space between the piston and the nose so that if the piston DOES move before deployment, that it will have some slack before it starts forcing the laundry out the top.

Originally posted by Scudrunr
Why not just drill em through the centering rings and forget about it?

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I'm not an aeronautical engineer by any stretch of the imagination but if you can imagine the strange turbulant flow of air around a rocket at speed you are going to want to avoid the most severe of that. That is why it is recommended to place the hole away from the NC and away from the fin area which in my mind would include aft centering rings.

And as far as disguising goes I try to place them in the black lettering of decals or simply hide them with my paint schemes.

Originally posted by fyrfytr310
And as far as disguising goes I try to place them in the black lettering of decals or simply hide them with my paint schemes.

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I drill my holes just above the upper centering ring on the 'ugly' side of the rocket, usually just above the uppermost rail button.

Originally posted by kaycee
I drill my holes just above the upper centering ring on the 'ugly' side of the rocket, usually just above the uppermost rail button.

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Right, thats the kind of stuff I'm talking about. You can't see them on the pad so you really don't have to try hard.

Correct me if I'm wrong but wouldn't there be a vacum created by the non-boat-tailed aft end of a rocket? Or, does the engine create enough positive pressure to fill the entire butt end and negate a hole drilled through the CRs

Shear pins.

Edward

Originally posted by edwardw
Shear pins.

Edward

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IMO, that's slight overkill on a PML amraam 2.1...

Not really. Gets you used to it and also helps to contain everything you need to get in there.

When using shear pins to prevent premature ejaculation of the nosecone, is it still necessary to drill a pressure relief hole below the collar of the nosecone? Signed, Yours truly frustrated !