Idea for NC AVBay deployment

[Kelly]

I often try to put my AVbay in the nosecone, since it makes sense to have the weight there. This generally means that as the ejection charge blows the NC clear, it is also blowing the 'chute down into the tube. I've been thinking about different options for dealing with this. I've come up with something, not sure if it's been tried, I don't think I've seen it here. Here's what I'm thinking:

On the inside of the sustainer, a few inches below the shoulder of the NC, put a small shelf. (Ignore for the moment that the 'chute could hang up on this; that problem can be solved.) On top of the shelf sits a loose disk, the diameter of the body tube. The shock cord is attached to, and passes through, the disc. The charge well is on the bottom of the nosecone, and when the nosecone is in place, there is a small gap between the NC and the disc. When the ejection charge goes off, the space is pressurized, ejecting the NC forcefully, which pulls out the disc and the laundry.

Advantages of this system:
1) Less BP needed for the charge, as the pressurized volume is smaller
2) Consistent pressurized volume - not dependent on the laundry or motor sizes.
3) Ejection charge does not contact the 'chute nor the majority of harness, eliminating the need for baffles or nomex, not to mention BP residue in tube.
4) Ejection charge cannot push 'chute further down into the body tube

Foreseeable disadvantages:
1) Because of the small pressurization volume, the NC may eject before the charge fully burns, and much of the necessary ejection pressure could be lost.
2) Costs you a few inches of space in body tube (or does it? Seems you could probably have the charge well almost touching the disc.)

Has this been done? Does it make sense?

 

[pcalvin]

I have this same problem on two of my nose AVbay rockets ( a 2.6" small endeavor and a 4" crayon). I noticed it in deployment testing; upping the charge seemed to solve it for the first few flights, but it didn't during later flights. The backup motor charge blew everything out eventually, but there were a few seconds of concern. Perhaps dirty body tubes are adding enough drag to the chute bundle to hang things up again, but the crayon rocket has a ton of clearance. I'm thinking its seating the chute with the nose charge, and the nose inertia isn't great enough to pull it all back out.

I'm going to try your system with one change. I will place the shelf just low enough to place the chute between it and the NC. This way the chute can't be seated any further down the booster by the nose charge, and there isn't a chance for the chute to hang up on the shelf. My other trial is going to be dropping a centrifuge tube charge on a long lead down into the motor area, below the laundry, and let it pressurize behind the chute.

 

[Kelly]

I will place the shelf just low enough to place the chute between it and the NC.

Though, if you're doing that, you may as well glue the disc in, right? When the NC pops it should pull the chute out, so having a loose disc (instead of a bulkhead) doesn't seem to add much?

My other trial is going to be dropping a centrifuge tube charge on a long lead down into the motor area, below the laundry, and let it pressurize behind the chute.

This is how I normally do it.

 

[cerving]

That's really like similar to a piston ejection system, where you epoxy the disk onto a short piece of coupler tubing. The problem with pistons is that you have to clean everything really well afterwards to get the piston to fit back into the tube, but they do (theoretically) prevent you from having to use a chute protector, which may save valuable space on a skinny MD rocket. Also, with your "shelf" system you're not pressurizing the chute compartment, so you're relying on the momentum of the NC ejection to pull the chute out. Ground test it thoroughly...

 

[Kelly]

That's really a piston ejection system

In a typical piston system, the ejection charge and piston are behind the chute, pushing it out. It relies on a good seal between the piston and the body tube.
In the system I sketched, nothing pushes the chute out - the nosecone pulls it out. Also, there is no need for a good fit between the disk and the tube, it can (and should) be quite loose: the 'seal' is between the disk and the shelf, NOT between the disk and the body tube.

 

[cerving]

Yes, I edited my post... however, you really do need a good seal between the disk and the "shelf", otherwise it will leak into the chute compartment and you may not get enough pressure to pop off the NC.

 

[DaHabes]

In a typical piston system, the ejection charge and piston are behind the chute, pushing it out. It relies on a good seal between the piston and the body tube.
In the system I sketched, nothing pushes the chute out - the nosecone pulls it out. Also, there is no need for a good fit between the disk and the tube, it can (and should) be quite loose: the 'seal' is between the disk and the shelf, NOT between the disk and the body tube.

You also don't want the disk to bind AT ALL when the nose cone pulls it out. I have had plenty of situations where the parachute binds in the airframe (laundry is a wee bit too tight, airframe is dirty, BP charge is too small, shock cord is tangled when stuffed in) and the nose cone stops dead because the pressure is long gone from the BP charge. It helps if the NC is heavy because of the E-Bay, but it is no guarantee.

I agree you would need a good seal between the shelf and the disk. That means the shelf would have to be the entire circumference of the airframe, with a bottom taper so the parachute and/or shock cord can slide out easily. You would likely need to have some sort of gasket to make sure it seats well. That would require a lot of ground testing on a mock up to figure it out.

Candidly, the best option is to use a centrifuge tube, but even there, you want to make sure the centrifuge tube doesn't shatter into a million pieces and shred the laundry. It needs to be contained itself so it doesn't self destruct. If it self-destructs, you lose pressure as well, so the laundry may not get pushed out. Many things to consider...

 

[OzHybrid]

I often try to put my AVbay in the nosecone, since it makes sense to have the weight there. This generally means that as the ejection charge blows the NC clear, it is also blowing the 'chute down into the tube. I've been thinking about different options for dealing with this. I've come up with something, not sure if it's been tried, I don't think I've seen it here. Here's what I'm thinking:

On the inside of the sustainer, a few inches below the shoulder of the NC, put a small shelf. (Ignore for the moment that the 'chute could hang up on this; that problem can be solved.) On top of the shelf sits a loose disk, the diameter of the body tube. The shock cord is attached to, and passes through, the disc. The charge well is on the bottom of the nosecone, and when the nosecone is in place, there is a small gap between the NC and the disc. When the ejection charge goes off, the space is pressurized, ejecting the NC forcefully, which pulls out the disc and the laundry.

Advantages of this system:
1) Less BP needed for the charge, as the pressurized volume is smaller
2) Consistent pressurized volume - not dependent on the laundry or motor sizes.
3) Ejection charge does not contact the 'chute nor the majority of harness, eliminating the need for baffles or nomex, not to mention BP residue in tube.
4) Ejection charge cannot push 'chute further down into the body tube

Foreseeable disadvantages:
1) Because of the small pressurization volume, the NC may eject before the charge fully burns, and much of the necessary ejection pressure could be lost.
2) Costs you a few inches of space in body tube (or does it? Seems you could probably have the charge well almost touching the disc.)

Has this been done? Does it make sense?

View attachment 501988

I use this method on my main. I'll post some photos when I get home.. Your charge well in this position is upside down and will tend to expel unburnt black powder, reducing the charge effective size. Every operation has been successful with NO chute damage. My tapered insert is alloy and secured through the wall by multiple M2 countersunk slotted head screws. Also working on printing it in Carbon Fibre reinforced ABS.
You do need to watch how you pack your chute to ensure all the bits don't meet at the narrowing simultaneously. I'll post a sketch later.

 

[DaHabes]

I also don't like piston systems. I learned the hard way in my rookie MPR days with my PML Bull-Pup that you need to clean the airframe after each launch. I actually got 7 launches out of it before it seized. #8 was an impressive lawn dart. The phenolic airframe was literally in a million pieces. This is all that was left...

 

[Kelly]

I use this method on my main.

Awesome! One question, how much space do you leave between the bottom of the NC shoulder, and the disk?
Agreed on your other points, with the possible exception of charge well orientation... is there any meaning to 'up' when the rocket is at apogee, and free-falling?

 

[OzHybrid]

Up is the direction the BP will get packed in on it's way to apogee under all those positive G's If your BP is already at the mouth of your charge well you'll not get the benefit of a charge well.. That is a small amount of additional compression of hot gasses which promotes the ignition of all of the charge. This is important where you plan to launch above 20kft as the air is then getting thin and you do not have the air molecules available to heat to promote the ignition to the next grain. Ignition of BP in an ejection chargeis a flame front propagation. You can of course initiate it with a detonator and have shock wave propogation. But that would be bad in an AV bay.

 

[Kelly]

Up is the direction the BP will get packed in on it's way to apogee

Ah, of course. Thank you!

Speaking of acceleration, it occurred to me that forces in play during ascent (especially at burnout) might cause the disk to move around and become unseated. I suppose this could be avoided with some tape or to keep it in place, or maybe just make the distance between disk and NC small enough that there is no place to go. Do you do anything to avoid issues with this?

 

[OzHybrid]

I’ll post some pictures first as they’re coming from my phone. Then edit it in a few minutes. If you’re just up to here, I’m not finished. Done...
The top of the charge container sits on top of and nearly touches the sealing aluminium plate.


Tapered seating ring inside.

Shackle gets attached with about 3 ft of Kevlar rope to aluminium top. It's a tightish pack.

The top of the plate seals on top of the taper. The reduced section is a good but loose fit into the hole through the taper. It isitself tapered to avoid any jamming. Any sticking is bad sticking. There is no "it's just sticking a little bit"

The 2 U Bolts are offset to ensure top is pulled out at a consistent angle. I can't see a way to rotate a photo. The kevlar rope is not that stiff....


Center of the Iris is pulled slightly forward Note loops in centre pull cord. This prevents jamming as the chute lumps and bumps do not all have to go through the tapered ring at the same time.


This shows the difference between a 60" Eliptical and a 48"Iris with associated hardware. Deployment bag for 60"and alloy top for 48 "Iris in orange and black. Both have the same descent rate.

That's tight!!!!!! 2.5"tube

That fits nicely. Same tube..... Same descent rate.


This is only what I did. YMMV...... Good luck.

 

[teepot]

I was thinking of a gasket as well. Maybe soft rubber or foam. And one time I blew up a rocket by putting the charge down by the motor. Maybe to much BP or to confined a space.

 

[OzHybrid]

I was thinking of a gasket as well. Maybe soft rubber or foam. And one time I blew up a rocket by putting the charge down by the motor. Maybe to much BP or to confined a space.

I don't think it's necessary provided your mating parts are clean and flat. I have one top in aluminium and another in 3/16" G10 fiberglass. Both work with no issues I've seen.

I've said before, and you cannot say this too often however annoying it might get, "test what you're going to do and do what you test. And. Making changes on the day is always a recipe for introducing a bad variable you've not had the time to think of."

YMMV..

 

[OzHybrid]

I also don't like piston systems. I learned the hard way in my rookie MPR days with my PML Bull-Pup that you need to clean the airframe after each launch. I actually got 7 launches out of it before it seized. #8 was an impressive lawn dart. The phenolic airframe was literally in a million pieces. This is all that was left...

View attachment 502058

The piston ejection systems are very efficient in terms of the amount of BP required, but yes, the pistons get gummed up. Also I had a blue tube piston that was a bit variable in it's operation due to what seemed like the humidity on the day. I gave up on them.
Nice monument. What depth was it at?

Norm

 

[teepot]

I don't think it's necessary provided your mating parts are clean and flat. I have one top in aluminium and another in 3/16" G10 fiberglass. Both work with no issues I've seen.

I've said before, and you cannot say this too often however annoying it might get, "test what you're going to do and do what you test. And. Making changes on the day is always a recipe for introducing a bad variable you've not had the time to think of."

YMMV..

It blew up during a ground test. Have the video too.

 

[OzHybrid]

It blew up during a ground test. Have the video too.

Sorry I'm not understanding what you are saying. Sketch?

 

[Kelly]

Thanks, Norman, that's very helpful. I just did a ground test with some mocked up parts (section of 3" tube, 3D printed shelf, plywood disk, and a nosecone) and it seemed to work pretty well. I'm definitely going to build this into the rocket I'm building now.

 

[OzHybrid]

Thanks, Norman, that's very helpful. I just did a ground test with some mocked up parts (section of 3" tube, 3D printed shelf, plywood disk, and a nosecone) and it seemed to work pretty well. I'm definitely going to build this into the rocket I'm building now.

I would go with aluminium or fiberglass G10 for the disk. It will never break and it's the junction point for your hot section through to the parachute. Happy to do a skype some time and have a chat.

 

[OzHybrid]

Thanks, Norman, that's very helpful. I just did a ground test with some mocked up parts (section of 3" tube, 3D printed shelf, plywood disk, and a nosecone) and it seemed to work pretty well. I'm definitely going to build this into the rocket I'm building now.

Here's my SCAD file. Counterbore section isn't functional, but should give you a good idea.

Regards
Norm

 

[teepot]

Sorry I'm not understanding what you are saying. Sketch?

Just above the mmt a section of body tube shredded when I set off the charge. To muck BP to tightly confined. I was getting ready for first dual deploy and wasn't very knowledgeable. Now I just blow in av bays. Blow it out or blow it up.