Large volume rocket ejection charge?

[Samuron]

I'm building a 5.5" rocket, based on a LOC Big Nuke, and I'd like to use the anti-zipper design and motor ejection.

If I do, that would mean 38" of open airframe. Is a standard (i.e. full AT)charge enerjetic enough to acheive ejection pressure for a space that large?

 

[Pantherjon]

What size motor? 38mm? Should work as the 38's and above come with 1.7g of BP, IIRC...I have used the stock ejection charge successfully on a few flights with a rocket that is 4"x48"..

 

[Samuron]

What size motor? 38mm? Should work as the 38's and above come with 1.7g of BP, IIRC...I have used the stock ejection charge successfully on a few flights with a rocket that is 4"x48"..

I'll be using a 54mm J315 or therabouts.

It looks like the volume of mine is about half again large as yours.

 

[ttabbal]

Personally, I would use more BP. But it's easy enough to ground test to find out how much you really need. Put the case + forward closure in there with the charge on the closure. I just run the ignition wire down the hole in the ejection well. Then apply power with a battery or launch controller. I've used the Estes E controller for smaller rockets and a HP controller for bigger ones I didn't want to be that close to.

I have a number of e-matches and ignitors I keep around to ground test with. I just bought a pyrogen kit and built a bunch of them for this use. With BP you could probably use use nichrome wire without the pyrogen for a ground test.

 

[Samuron]

Personally, I would use more BP. But it's easy enough to ground test to find out how much you really need. Put the case + forward closure in there with the charge on the closure. I just run the ignition wire down the hole in the ejection well. Then apply power with a battery or launch controller. I've used the Estes E controller for smaller rockets and a HP controller for bigger ones I didn't want to be that close to.

I have a number of e-matches and ignitors I keep around to ground test with. I just bought a pyrogen kit and built a bunch of them for this use. With BP you could probably use use nichrome wire without the pyrogen for a ground test.

Unfortunately, I was hoping to figure out if the configuration would work before assembly.

My guess is now that it won't be near enough., so I'll probably go with a standard configuration and risk zipper. It should be a fairly slow rocket, so hopefully the stress won't be too much.

 

[terryg]

You can easily add black powder to the case by wrapping tape around the well to get whatever height is needed and folding it over to contain the charge.

 

[JDcluster]

I have successfully deployed from a 5.5" rocket many times using no more than 2 grams for apogee deployment. I doubt that you'll need more than 3 grams max. If you want to use even less you could invest into a CO2 system, but it gets pricey for the system starting around $150.00.

Another idea would be to add a removable stuffer tube to take up some volume?


JD

 

[Samuron]

I have successfully deployed from a 5.5" rocket many times using no more than 2 grams for apogee deployment. I doubt that you'll need more than 3 grams max. If you want to use even less you could invest into a CO2 system, but it gets pricey for the system starting around $150.00.

Another idea would be to add a removable stuffer tube to take up some volume?


JD

I thought about that, but this is my Level 2 attempt, and I'm really trying to follow KISS.

With a standard (non-zipper) configuration, it looks like the standard ejection charge is just right.

I appreciate all the feedback; I think, at this point, I'm going to trust the stock LOC setup.

 

[Handeman]

I'm not sure what the difference is between your zipperless design and the "standard" design would be.

My L2 was done zipperless. The lower BT stays with the av-bay and seperates from the fin can. A standard design as I understand it, the lower BT would stay with the fin can and seperated from the av-bay. There's no difference in internal volume, only what part of the rocket the lower BT is attached to.

What ever amount of BP is needed for one design, it should be the same for the other.

Either way, use this calulator. 5 pounds of pressure should be great for 5.5 inch diameter rocket. You have about 24 sq in of nose cone surface so with 120 lbs of force, the nose cone should pop off quite nice.

If you look at the calculator, he used the diameter squared for area which actual is too large. I've found the amounts to be a little high. The formula is there if you want to use Pi r squared and recalculate.

Do a search here and I'm sure you'll find more equations for calculating the amounts.

 

[Samuron]

I'm not sure what the difference is between your zipperless design and the "standard" design would be.

My L2 was done zipperless. The lower BT stays with the av-bay and seperates from the fin can. A standard design as I understand it, the lower BT would stay with the fin can and seperated from the av-bay. There's no difference in internal volume, only what part of the rocket the lower BT is attached to.

What ever amount of BP is needed for one design, it should be the same for the other.

Either way, use this calulator. 5 pounds of pressure should be great for 5.5 inch diameter rocket. You have about 24 sq in of nose cone surface so with 120 lbs of force, the nose cone should pop off quite nice.

This is a single-deploy rocket, so there is no AV bay.

With a "standard" design, the upper section is sealed off by the bulkhead at the aft end of the coupler; the chute is in the lower section, in a 17" space, for a volume of 386 cu". This seems to be about right for a 1.2 g charge, at 6 psi, which I figured made sense using the InfoCentral chart.

With a "zipperless", as shown on InfoCentral, the parachute is in the upper section, and the bulkhead is vented to allow the ejection to pass through; the internal volume is thus 864 cu".

Using Chuck Pierce's calculator, that would suggest a charge of about 2.7 g; I'll see how much that would overload the well on the motor.

 

[Handeman]

This is a single-deploy rocket, so there is no AV bay.

With a "standard" design, the upper section is sealed off by the bulkhead at the aft end of the coupler; the chute is in the lower section, in a 17" space, for a volume of 386 cu". This seems to be about right for a 1.2 g charge, at 6 psi, which I figured made sense using the InfoCentral chart.

With a "zipperless", as shown on InfoCentral, the parachute is in the upper section, and the bulkhead is vented to allow the ejection to pass through; the internal volume is thus 864 cu".

Using Chuck Pierce's calculator, that would suggest a charge of about 2.7 g; I'll see how much that would overload the well on the motor.

Couldn't you use the standard design and just attach the lower BT to the bulkhead coupler and keep the chute in the lower half? You would have to get another coupler for the fin can and use a smaller diameter centering ring for the top ring to make it zipperless, but that shouldn't preclude using the central bulkhead to reduce the volume.

If you do make the fin can zipperless and you need a more space, put the bulkhead at the forward end of the mid body coupler.

You might want to consider using an empty av-bay as the central coupler. Then you'll have it ready to go when or if you decide to go with electronics an try dual deploy.

 

[new2hpr]

Err, you're close. In the zipperless configuration (single deploy), your chute is still in the lower compartment. IF you use an altimeter, it goes in the ebay (inside the coupler), with the charge below it, and the chute below that. The zipperless coupler goes above the motor, with a perforated bulkhead to allow the ejection charge to pass through. The split in the rocket is right above the motor, not up at the normal top ebay coupler location.

Just make sure the chute is a very loose fit in the tube, and attached about 1/3 of the length of the shock cord, closer to the motor end of the cord.

Good luck!

-Ken

 

[Handeman]

Just make sure the chute is a very loose fit in the tube, and attached about 1/3 of the length of the shock cord, closer to the motor end of the cord.

Good luck!

-Ken

I would do that part differently. I'd put the chute close to the upper part of the rocket. That way, the fin can touches down first, hopefully on the motor and it doesn't break a fin, the weight of the fin can comes off the chute and it's rate of decent slows by almost half, it then gently lowers the upper part of the rocket to the ground.

 

[madmax]

Have you already started construction? If not, increasing the length of the motor mount tube up to the bottom of that coupler you can decrease the volume needed.

Could you give some more information regarding the length of the different sections?

By the way, AT 54mm motors do not come with ejection charges like their 38mm counterparts. However I do know that 2 grams of BP will fit fine in the ejection well.

Personally I like the zipperless design better. Be sure to reinforce the ends of the upper airframe to help with landing damage. Doesn't have to be extensive, you can either coat the inside few inches of it with thin epoxy or just run thin ca on the ends of the tube. I prefer the first approach since then you can sand the inside of the tube for a perfect fit and it seals it against small amounts of water. Also if you coat the coupler with either ca or epoxy you can sand that super smooth as well plus it hardens it up a bit.

 

[Samuron]

Have you already started construction? If not, increasing the length of the motor mount tube up to the bottom of that coupler you can decrease the volume needed.

Could you give some more information regarding the length of the different sections?

Sure; lower section is 42", the upper is 24". The forward centering ring is 19.5" from the front of the lower section, and the coupler is 11".

That means I actually miscalcualted somewhere, because the parachute space is 14", not 17".