Venting 101

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Duncan.Byers

Well-Known Member
TRF Supporter
Joined
Mar 18, 2022
Messages
133
Reaction score
84
So I've gone through various threads looking at the issue of venting a LP 2-stage rocket and can't seem to find a clear answer on this: does it really matter one way or the other if I vent through the BT or through the centering rings of the booster?
 
Just personal preference, but depends on the size differential between the motor and body tubes if you have enough space. Might be a narrow gap between a BT-5 and BT-20, or BT-20 to a BT-50 for example, like if the centering rings are more like thrust rings, that can be notched enough but maybe a little tricky. If you have the space, then venting through the bottom might look cooler, you can even put some dummy fake nozzle bells to masquerade the vent holes on the bottom centering ring. If you go directly outside the rocket, just make the vent holes in a symmetric pattern, at least two holes on opposite sides, or put them in between each fin, so you don't get a brief asymmetric thrust kick out the vents during the staging. Are you gap staging? What does your rocket look like?
 
It's a down-sized Estes Sasha built specifically to fly on 13mm motors. See attached. Note that there are some internal structures that I just haven't removed yet that belong in the full-size version.
 

Attachments

  • Mini Sasha Two Stage 13mm.ork
    86.4 KB · Views: 0
It's a down-sized Estes Sasha built specifically to fly on 13mm motors. See attached. Note that there are some internal structures that I just haven't removed yet that belong in the full-size version.
not able to run OR. can you post a screen capture of the side view?
 
you should have enough room for venting the CR, but you might not be able to reproduce the nozzle at that size, the neck down for that would obstruct the venting path, it would need some notches in it too. Depends on how much gap you would have between the nozzle and the motor tube. You could make a display-only nozzle, that you remove for flight?
 
you should have enough room for venting the CR, but you might not be able to reproduce the nozzle at that size, the neck down for that would obstruct the venting path, it would need some notches in it too. Depends on how much gap you would have between the nozzle and the motor tube. You could make a display-only nozzle, that you remove for flight?
Forgive me - "CR"? Putting that aside, I see what you are saying. I had completely disregarded the decorative nozzle at the aft end of the rocket. Which I've reproduced using a simple transition cone but to your point that would, as it is constructed at the moment, significantly reduce the venting path I think. So I'm guessing vents in the tube is going to be the way to go. On the full-size Sasha it is vented down through the nozzle at the bottom but of course it's a much larger tube.
 
CR=Centering Ring.

The full size model would have a gap between the smallest part of the plastic nozzle and the cardboard motor tube. That might actually look cool during staging since it would have one last blast of smoke as the sustainer lights.
 
CR=Centering Ring.

The full size model would have a gap between the smallest part of the plastic nozzle and the cardboard motor tube. That might actually look cool during staging since it would have one last blast of smoke as the sustainer lights.
You are correct; the full size does have an open area around the motor that is open through the plastic nozzle. Wonder if I have enough room to do the same thing on the 13mm version.... 🤔
 
The question "where to vent" will almost always get a resounding "it depends" answer. I like to go out the back (through the booster centering rings) as long as the back end is clear of any design elements like nozzles.
 
I have done both with success. A converted regular-sized Hi-Flier that I drilled a trio of 1/8 holes in the booster, and a converted Hi-Flier XL with eight small holes in the centering rings, very similar to the SA-2061 Sasha kit. The booster for the XL is pretty long, too, about 230mm (9 inches). A pair of E12s REALLY got it up there.

I generally prefer:

1.). tape staging where possible

2.). centering ring holes where tape staging is impossible

3.). body tube holes where nothing else is possible, like minimum diameter rockets with long fin root chords such as that Hi-Flier.

https://www.rocketryforum.com/threads/super-hi-flier-estes-hi-flier-two-stage-conversion.172611/
https://www.rocketryforum.com/threads/hi-flier-xl-two-stage-conversion.172375/
Why not try venting through the centering rings before messing up its looks with holes in the body tube? I’d bet a rocket this size could survive a “death dive” flight with no sustainer ignition. If it does, then by all means drill that booster.
 
Last edited:
from the side view drawing in your PDF, it almost looks like the two motors are already going to be close enough for direct staging where you might not even need a vent. For direct staging you cellophane tape (cellophane will melt during staging and let the sustainer free when it lights, don't use masking tape) but you might have to use a different kind of retainer for the sustainer if the screw on type gets in the way. Take a look at the Estes Booster units, those don't have vents and are basically a variation of direct staging. You really only need vents when the gap is like more than the diameter of the motor, give or take.
 
from the side view drawing in your PDF, it almost looks like the two motors are already going to be close enough for direct staging where you might not even need a vent. For direct staging you cellophane tape (cellophane will melt during staging and let the sustainer free when it lights, don't use masking tape) but you might have to use a different kind of retainer for the sustainer if the screw on type gets in the way. Take a look at the Estes Booster units, those don't have vents and are basically a variation of direct staging. You really only need vents when the gap is like more than the diameter of the motor, give or take.
the gap is going to end up being 4-5 times the booster diameter. Pretty close but not right on top of each other.
 
the gap is going to end up being 4-5 times the booster diameter. Pretty close but not right on top of each other.
I mean the gap from the top of the booster to the nozzle of the sustainer,
from your PDF drawing:
Screen Shot 2022-06-30 at 8.23.39 AM.png

are your motors going to be farther away than shown here?
 
I mean the gap from the top of the booster to the nozzle of the sustainer,
from your PDF drawing:
View attachment 524957

are your motors going to be farther away than shown here?
Yes - I haven't gotten around to modifying the OR file to reflect the change to the motor mounts from the full size. But when I get a chance to I'll repost.
 
I mean the gap from the top of the booster to the nozzle of the sustainer,
from your PDF drawing:
View attachment 524957

are your motors going to be farther away than shown here?
So the top of the booster motor is going to be about 3cm away from the bottom of the sustainer.
 

Attachments

  • Mini Sasha Two Stage 13mm.image.pdf
    59.7 KB · Views: 0
  • Mini Sasha Two Stage 13mm.ork
    135.4 KB · Views: 0
That is about the length when you probably need some venting. I would run the booster motor mount tube all the way up to the nozzle of the sustainer, maybe leave a little gap there, or run it over the exposed portion of the sustainer motor and add vent holes to that motor tube just below the sustainer nozzle. This creates a venting path forwards to the sustainer nozzle before the centering ring vents out the back. But, again this is not a large gap, so your layout may be totally fine and work without any problems.


Screen Shot 2022-06-30 at 4.56.46 PM.png
 
That is about the length when you probably need some venting. I would run the booster motor mount tube all the way up to the nozzle of the sustainer, maybe leave a little gap there, or run it over the exposed portion of the sustainer motor and add vent holes to that motor tube just below the sustainer nozzle. This creates a venting path forwards to the sustainer nozzle before the centering ring vents out the back. But, again this is not a large gap, so your layout may be totally fine and work without any problems.


View attachment 525072
Not many folks mess around with 13mm staging for some reason...... 🤔 I think I'm going to vent down through the CRs. I have room. I'll let you all know how badly it goes wrong.
 
That is about the length when you probably need some venting. I would run the booster motor mount tube all the way up to the nozzle of the sustainer, maybe leave a little gap there, or run it over the exposed portion of the sustainer motor and add vent holes to that motor tube just below the sustainer nozzle. This creates a venting path forwards to the sustainer nozzle before the centering ring vents out the back. But, again this is not a large gap, so your layout may be totally fine and work without any problems.


View attachment 525072
This IMO extending the booster motor mount is excellent advice. ”Guiding” the booster motor burn-through gases as close to the nozzle of the sustainer really helps increase reliability of next stage ignition. A gap of about 1/8 inch between the forward end of the motor mount extension and the nozzle end of the next stage (in this case the sustainer) is what I routinely and generally successfully use, with gaps up to 54 inches.

hope you get two straight trails and two short walks!
 
Not many folks mess around with 13mm staging for some reason...... 🤔
I’ve built two airframes for this purpose. A Gnome and a Luna Bug, each with matching boosters.

I found it was a good way to nail down the basics with less expense and hazard than going for standard motors. I can also fly them at my normal LPR site and not bust the low ceiling. Maybe one of these days I’ll get around to doing the rear motor block thing, but for now a friction fit has worked fine.
 
This IMO extending the booster motor mount is excellent advice. ”Guiding” the booster motor burn-through gases as close to the nozzle of the sustainer really helps increase reliability of next stage ignition. A gap of about 1/8 inch between the forward end of the motor mount extension and the nozzle end of the next stage (in this case the sustainer) is what I routinely and generally successfully use, with gaps up to 54 inches.

hope you get two straight trails and two short walks!
That's sort of what I was thinking. I haven't attached the booster engine mount yet so I'm probably going to make that change to it before I do so.
 
So I closed the gap between the booster and sustainer to about 7mm. ORK file is attached if you're interested. The motor mount has a paper cone (transition) for that detail Finicky little bugger but once I saturated it with ultra-thin CA it sands down to somewhat circular.... The vents are around the base of the cone rather than inside of it. And I've started adding a simple wooden post for taping the motor to rather than trying to tape to the motor mount tube which gets to be a pain as you know especially in flush mounted tubes. I use dowels or, in this case because of the size, just a cut-down toothpick. Easier than the metal motor retainer and won't bend of course. As I finish and launch more photos of course. BTW - it's not a beautiful build. My eyes aren't what they were and at this size it's all a little finicky. 20220702_104957.jpg20220702_105315.jpg20220702_110205.jpg20220702_114534.jpg
 

Attachments

  • Mini Sasha Two Stage 13mm.ork
    128.5 KB · Views: 0
The mini motors are overlooked by a lot of people. Staging 13 mm works as well as 18 mm does.
I also have 1) a limited space back yard launch area and 2) a 3-year old (well, 3 1/2) who has limited patience but loves to chase rockets so the backyard launch area is ideal. It's big enough for well-thought-out 18mm motor launches but everything is pretty much limited to <1000ft. And anything pushing 600ft is starting to get landing in retention ponds....and the 300-500ft mark is really the sweet spot. Hence the reason I'm exploring doing some (what I think is) cool stuff in 13mm.
 
Back
Top