Centering Ring Ventilation for Gap Staging

[lcorinth]

I'm building a gap staged Estes Photon Probe. The booster will be four inches long, so there'll be a gap between motors, and I plan on venting via holes drilled in the centering rings.

I know people have done this before, but I haven't found any guidelines on how much ventilation is appropriate. I was thinking four 1/8 inch holes, drilled through both centering rings.

Question is, is there a minimum amount of venting required? Does it depend on how large the gap is? And is there such a thing as too much ventilation, such that the sustainer motor can't effectively blow off the booster at ignition?

Thanks!

 

[TangoJuliet]

Great question! I hope you get a great answer. op:

 

[tsai]

I made a 2 stage version of the Estes Challenger-II. Booster was ~12" long (9.5-10" between motors). I used a single 1/16" vent hole near the top of the booster, and it worked fine.

Not exactly scientific, but gives you some parameters for something that worked. I've also read in the past about open-air gap staging. I think that would cover any concerns about "too much ventilation."

 

[TangoJuliet]

I think that would cover any concerns about "too much ventilation."

He'll correct me if I'm wrong, but I think Daniel's bigger concern is relieving too much pressure and then not getting the booster stage to separate from the sustainer. Not enough ventilation and one runs the risk of the booster falling away before it ignites the sustainer; too much ventilation and there's not enough pressure to separate the booster.

 

[MALBAR 70]

I built a Dr Zooch Vanguard Eagle it has about a 3.5-4" gap. It is vented through the CR's with three 1/4" holes punched at the top of the booster motor tube and four 3/16" holes through the centering rings out the rear of the booster.

 

[jqavins]

Not enough ventilation and one runs the risk of the booster falling away before it ignites the sustainer; too much ventilation and there's not enough pressure to separate the booster.

So, for what little it's worth, here's the completely unscientific answer from out of my backside. No ventilation (removing no material) would obviously be too little, and suspending the motor tube by thin spokes (removing all the material) would almost certainly be too much. So, shrug your shoulders and remove half the material.

The body tube is BT-55, so 1.325" ID. The motor mount is BT-20, so 0.736" OD, assuming you're not "upgrading" it. That leaves 0.953 sq.in. of centering ring; taking out half means removing 0.477 sq.in.

The maximum hole diameter is something under 0.295" (the difference between the radii). So let's assume 1/4" holes. They have an area of 0.049 sq.in., so to remove 0.477 sq.in. needs about ten of them. The circumference half way between the tubes is 3.237", so ten holes at 1/4" each will fit. That's what I might do. At least, that's what I would probably have done before reading this thread.

Tsai states that he's done this with a single hole at 1/8" and it worked. A single 1/8" hole seems really small to me, but if it worked it worked. So ten 1/4" holes is pretty surely more than you need.

I'd go with two 1/4" holes and expect success. Or you could use four 1/4" holes, if you're thinking that tsai just got lucky. It's probably not critical; after all, it's only rocket science.:wink: And that's my story, for what little it's worth.

 

[BDB]

I vented a 29 mm booster motor with 3 x 1/8" holes, so you should be fine.

 

[tsai]

I guess what I was trying to convey is I think this is an area that has a large range of acceptability. I'm not sure anyone has done exhaustive testing to properly characterize the overall range. Might actually make an interesting study.

Maybe another way to think about it: this is all pressure equalization. Take a drinking straw, and suck air through it. Do you think that the burning propellant generates more gas than you can suck through the straw? It doesn't take a lot of surface area to move a LOT of air to equalize the pressure. Now add in the fact that you don't actually need to equalize. You only need enough to prevent hitting the pressure level needed to separate.

 

[Skp]

I built an ASP WAC Corporal w/Tiny Tim Booster. It has gap staging. The kit comes with a bodytube with a vent hole to slide into the open air gap between stages. I've never used the sleeve and left it wide open. It has flown 5 times with only 1 failure to ignite the upper stage. That failure was when I used an A8-0 as the 1st stage to get a low altitude picture of the event. The photo of the event makes me think that there was enough fire to ignite the upper stage, but perhaps not.


As far as enough pressure to drop the booster, I've never had a booster hold on after upper stage ignition. I tend to have my booster barely stay on under it's own weight.
Here is the latest launch of the WAC Corporal.
[YOUTUBE]/ZDWcjTBVi2Y[/YOUTUBE]

 

[MALBAR 70]

I built an ASP WAC Corporal w/Tiny Tim Booster. It has gap staging. The kit comes with a bodytube with a vent hole to slide into the open air gap between stages. I've never used the sleeve and left it wide open. It has flown 5 times with only 1 failure to ignite the upper stage. That failure was when I used an A8-0 as the 1st stage to get a low altitude picture of the event. The photo of the event makes me think that there was enough fire to ignite the upper stage, but perhaps not.
View attachment 319866View attachment 319867


As far as enough pressure to drop the booster, I've never had a booster hold on after upper stage ignition. I tend to have my booster barely stay on under it's own weight.
Here is the latest launch of the WAC Corporal.
[YOUTUBE]/ZDWcjTBVi2Y[/YOUTUBE]

Thanks for posting this. I have one of these on the bench right now and was wondering if the extra tube with the hole was necessary. I guess not.

 

[MALBAR 70]

...

 

[lcorinth]

After a couple replies on the FB NAR page and here, I decided to go with four 1/8 inch holes. Tsai's response about open air staging was reassuring. Now I think of it, I think I have seen a few models like that, and just hadn't thought of them.

The staging coupler will be pretty loose. Not falling off, obviously, but loose enough to come off with little effort, and the ventilation I've settled on seems like it should be adequate, according to what I've seen here and on FB.

Thanks, guys. I'll start a build thread soon.

 

[MALBAR 70]

I'm building a gap staged Estes Photon Probe. The booster will be four inches long, so there'll be a gap between motors, and I plan on venting via holes drilled in the centering rings.

I know people have done this before, but I haven't found any guidelines on how much ventilation is appropriate. I was thinking four 1/8 inch holes, drilled through both centering rings.

Question is, is there a minimum amount of venting required? Does it depend on how large the gap is? And is there such a thing as too much ventilation, such that the sustainer motor can't effectively blow off the booster at ignition?

Thanks!

I'm interested in how you plan to stage the Photon Probe. Where is the separation point going to be?

 

[lcorinth]

...

I bought 31 Photon Probes. The separation point will be at the aft end of the kit. The sustainer will be a complete Photon Probe kit. The booster will be a 4 inch long section cut from a separate kit from the sustainer, and the fins will be from the same kit. The rest of the kit will go into the parts bin.

Most of the other 31 kits will serve as parts. A just over 3 bucks a pop, it seemed a good deal for a bunch of tubes and nose cones I like.

 

[MALBAR 70]

Sounds good Daniel. I thought that's what you might do. I can't wait to see it fly.

Those Estes deals are hard to pass up. I've often thought about doing what you did (a bunch of kits for parts), but with my current backlog of unbuilt kits, I just don't have room to keep them.

 

[Rex R]

I'm thinking that you should be thinking about a 24mm booster motor, the listed weight for a P-Probe is 3.5 oz.
Rex

 

[neil_w]

I bought 31 Photon Probes.

That cracks me up, although I totally don't blame you. I wish I had picked up at least one.

I shudder to think what my wife would say if a box showed up on my doorstep with 31 rocket kits in it. :y:

 

[dhbarr]

That cracks me up, although I totally don't blame you. I wish I had picked up at least one.

I shudder to think what my wife would say if a box showed up on my doorstep with 31 rocket kits in it. :y:

"Again!?" -- Mrs DH

 

[neil_w]

"Again!?" -- Mrs DH

 

[rstaff3]

I'm building a gap staged Estes Photon Probe. The booster will be four inches long, so there'll be a gap between motors, and I plan on venting via holes drilled in the centering rings.

I know people have done this before, but I haven't found any guidelines on how much ventilation is appropriate. I was thinking four 1/8 inch holes, drilled through both centering rings.

Question is, is there a minimum amount of venting required? Does it depend on how large the gap is? And is there such a thing as too much ventilation, such that the sustainer motor can't effectively blow off the booster at ignition?

Thanks!

BTW, TVM's rocket book says one 1/4 inch hole is what you need for gap staging, smaller if you use multiple holes. It seems like more can't hurt too much as the mass ejected from the motor burn would also be a factor towards pushing the lower stage off. That seems to be confirmed by earlier comments.

 

[neil_w]

I never thought of this before but it is an interesting concept. If I have this right, you are venting into the gap around the motor mount? I bet more is better as I doubt the pressure can dissipate before the hot particles from the motor reach the upper stage. I'm just thinking out loud. Kinda of like a pressure capacitor.

I assumed Daniel meant venting both centering rings, so the pressure is dissipated all the way out the back of the motor mount into the outside world. This seems like a very good approach, although if the booster is BT55 and the motor mount is 24mm then the centering rings are only about 3/16" tall, so the actual process of drilling or venting them would seem to require some care. An 18mm booster would probably make it easy, although I agree with Rex that a 24mm booster would seem to be more appropriate for a model of this size and weight.

 

[rstaff3]

I assumed Daniel meant venting both centering rings, so the pressure is dissipated all the way out the back of the motor mount into the outside world. This seems like a very good approach, although if the booster is BT55 and the motor mount is 24mm then the centering rings are only about 3/16" tall, so the actual process of drilling or venting them would seem to require some care. An 18mm booster would probably make it easy, although I agree with Rex that a 24mm booster would seem to be more appropriate for a model of this size and weight.

Well, I'm a doof...I was venting the wrong direction...ill edit my post...thanks for point this out. (well, I guess half my post was not totally insane :eyeroll

 

[Rex R]

it takes 4, 1/8" holes to equal 1, 1/4" holes. a paper punch makes short work of venting the motor tube. an 1/8" bit + dremel( used carefully) will do the same for the rings...if you have a 6" long bit one could drill them after they are installed.
Rex

 

[lcorinth]

That cracks me up, although I totally don't blame you. I wish I had picked up at least one.

I shudder to think what my wife would say if a box showed up on my doorstep with 31 rocket kits in it. :y:

Yeah, the Rocket Room looks a bit like a hobby shop right now. I realized after one of the end of year sales was over, that if I'd bought more than one Monarch (a rocket I kind of like but don't love), I'd have a ton of quality, cheap, BT-55 parts.

And I'm planning a big experimental project for the blog, so I need lots of the same parts. But while I have them, I might as well build a Photon Probe.

I'm considering doing another 24mm booster. I just kept it simple this time. But I have a lot of fin stock that may not all get used, so I can make extras.

 

[Micromeister]

For general use when venting motors I use the Inside area measurement of the motor to determine the number of 1/8" holes needed to vent the ejection gas.

 

[hcmbanjo]

While Estes had you Scotch tape the two motors together,
Centuri was the first to introduce "Pass-Port" or vented staging.

Check out the tube coupler on page 5 from the old Black Widow kit:
https://www.spacemodeling.org/jimz/kb-6.htm



The HTC-70 coupler was 1" long. It had two vent holes on opposite sides of the coupler.
It looks like those holes were about 3/16" diameter.
The old Estes Scout had 1/4" holes punched in the body tube sides. If I remember correctly,
the Centuri vent holes on the staging coupler were a bit smaller.

This certainly isn't the only way to do it, just food for thought.

 

[BABAR]

Daniel
Probably I am just slow, but when you say you are venting through the centering rings I want to make sure I understand.
Are you back venting through your centering rings out the back of the rocket? So the hot gas from the booster goes up the motor mount, to a short gap between mount and sustainer engine, then BACKWARD out the back of the rocket? If so, fine, I don't think you can make the gap too big, a couple of quarter inch holes probably enough. Main thing is, however you do it, need a clearly define path the takes the gas by the sustainer nozzle. Radiant heat from the hot gas (not particles) will go up the nozzle and light your propellant.

I usually just do two 1/4 inch holes on each side, just below the sustainer engine, with a chimney going up from the motor mount to about 1/4" below the sustainer. I am successful thus far with gaps up to thirty six inches (D12-0 to D12-7) although I have yet to be successful at seventy two inches.

Looking forward to your thread.
Tom