Vent holes for donut avbay?

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rocketsam2016

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Hi folks, I’ve designed and put together what I guess is called a donut avbay. As the picture shows (only partially assembled), I have a 1/16” fiberglass board in the space between a 54mm tube and the outer 4” airframe. The whole thing is removable and attaches to the motor mount centering rings with all thread. The center 54mm tube will butt against the 54mm motor mount and serves as either/both a conduit for ejection charge gasses and a continuation of the motor mount around the motor depending on the length of the motor used.


IMG_20160930_074451.jpg


I was concerned that the centering rings on the end wouldn’t be a tight enough fit with the outer airframe to keep ejection gasses from penetrating into the bay, so I built a shield (on the left, sitting upside down) that will sit over the electronics and provide a tight seal all the way around by being pressed against the board by the outer airframe (I’m going to line the periphery with high temp silicone sealant so that the seal is good).


So my question is, how many vent holes should I use and where should I place them? They are forced to all be on one side of the rocket by this design. They will obviously also have to be through both the shield and the outer airframe. There is nothing sticking out of the rocket above any of this area so the airflow should be clean. I was thinking that none should be directly over the air pressure sensor and doing either a triangle pattern of 3 or a zig zag pattern of 4:

———
*........
.........
........*
.........
*........
———


or


———
....*....
.........
........*
*........


———


or


———
.*.......
.......*.
.*.......
.......*.
———


or


———
...*....
......*.
.*......
...*....
———


The egg timer quantum has pretty rock solid safety logic as I understand to prevent getting tricked into firing charges before liftoff, but I figure I can never be too safe. Any particular concerns with how I lay these out or do any of the above seem appropriate?


Thanks!
Sam
 
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Coop

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Given your setup, I'd go with a single static port, sized according to whatever your av-bay volume is. I wouldn't think it'd need to be very big (if I understand your description), as the only thing that needs to be vented is the volume of the shield, as you described. Three or four all on one side seems unnecessary.


Later!

--Coop
 

rocketsam2016

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Thanks! Yeah it's pretty small so the hole(s) can be small, though I'll make them larger than required to hedge against any possible misalignment of the shield with the rocket exterior.

Since posting I also realized the altimeter is on one end of the bay. Better to have the hole(s) biased towards or away from the end with the altimeter or just right in the middle?
 

mpitfield

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Why not consider three holes in the following configuration:

———
........
*.*.*
........
........
———

BTW where will your fins be in relation to these holes?
 

rocketsam2016

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mpitfield: With 3 in line the wind (if any) will just blow right through right? And I worry (hand-waving based on high school physics) about a venturi-like effect that would lower the pressure in the bay with such a wind. The fins are well below towards the aft of the rocket. This bay sits centered right around the neccessary CG point for 1.5 calibers of stability.

soopirV: thanks! I'm actually really pleased with it so far, though ask me again after it sees the air for the first time :). We'll see how well the silicone seal works, but the bay feels like a tank, my batteries and altimeter fit perfectly, the shield actually makes it easier to slide it into the rocket, and I avoid the messiness of the airflow with a nose-count-mounted-altimeter. You can't see in the picture since I just added them, but I've got nuts on the all thread next to both of the centering rings to evenly distribute the pressure of the all thread holding the thing down when I screw it in. The eventual plan is to use a tender descender attached to the top of this. The first launch though will be single deploy with the altimeter firing one charge and the motor supplying a backup charge event.
 

MCriscione

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Unless I misunderstand your design, they are NOT constrained to only being on one side. I would consider sticking with a standard evenly distributed 360 degree hole pattern, and simply connect the interior volumes of the donut into a single volume. Specifically, but putting some holes in the edges of that off-white sheet (of G-12?) to allow flow all the way around the 54mm center tube. A hole (or multiple on a 120 degree portion of the airframe) may possibly provide inaccurate values depending on the rocket's angle of attack.

As I understand it, essentially you want to measure the static pressure surrounding the rocket to gauge altitude, which can be done with equally distributed 'balanced' holes. Holes on only one side are 'unbalanced' and may add some portion of the velocity pressure in varying amounts as the rocket changes speed and AoA.
 

rocketsam2016

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MCriscione - really good point about angle of attack, urgh. I might be able to get away with penetrating the board, but the problem is that I don't really trust the seal between the centering rings and the outer airframe. They are pretty snug, but I had to make them loose enough so that the assembly slides in and out of the rocket and hopefully won't get jammed if I travel somewhere humid, and it looks like there may be variance in the shape of the outer airframe as you go up and down. This is why I built the "shield" you can see in the picture - it means I don't have to worry about perfect seals between the rings and the outer frame.


How concerning do you think the angle of attack issue is? Enough to be worth building a periscope like thing to bridge to the far side while still not relying on the centering rings sealing perfectly to the outer frame? Or should I just bite the bullet and use greased o-rings on the outside of the centering rings and make that seal really good?

The good news is that the quantum has safety measures to only unlock the ejection charge circuit (not to mention actually firing it) when the rocket has gotten high enough and has slowed down.
 

mpitfield

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mpitfield: With 3 in line the wind (if any) will just blow right through right? And I worry (hand-waving based on high school physics) about a venturi-like effect that would lower the pressure in the bay with such a wind. The fins are well below towards the aft of the rocket. This bay sits centered right around the necessary CG point for 1.5 calibers of stability.
Angle of attack issues aside, while I do not recall my high school physics, you may have a point. However I thought that was the point of having more than one sampling hole, to cancel out pressure differentials. I recall reading another thread where the discussion was about the importance of having the sampling holes at the same level plane to mitigate any differences in pressure samplings but that was not with this unique AV design. And of course there is always the possibility that I am misinterpreting this or that it was incorrect, more likely the latter.

FWIW I do have an AV bay with this design but it is not for ejection/recovery it is for a baro altimeter simply to record whatever flight data I can get. I have launched this particular MPR rocket about 3 dozen times now and have always been able to capture the data. Of course the obvious is I have no way of knowing if the sampling holes are effecting this data and without having something like an ejection charge on the line it may not be so obvious. So it may be a case of ignorance is bliss.

Regarding the seal of the AV bay. From what I understand having a seal on the AV bulkheads is important so the pressure from the gasses of the ejection charge does not interfere with the baro sensor. Beyond that I would be surprised that some reasonable ambient leaking in the AV bay will not have a negative effect, otherwise I imagine that there would be many more failures being reported.
 

rocketsam2016

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Pondering your reply on the walk home from work I think I have two candidate solutions that don't require perfect centering ring sealing but have symmetric vents.

Option 1
The plan:
- 4 holes symmetrically around the donut as if it was a normal bay. The holes are sized for the whole donut. This hole size is about the same as the size needed for a single hole for the actual av bay (volume of the avbay is ~20% of the donut).
- 1 hole through the board connecting the avbay to the rest of the donut. This hole is the same size as the others.

Results:
- In the absence of wind, the single hole into the avbay from the exterior is easily enough on it's own and we can ignore the rest of the system (though the rest improves the ventilation)
- In a wind or non-zero angle of attack, air is still able to pass through the system. Perhaps not perfectly but well enough to improve things in the steady state.
- If the ejection charge leaks, it isn't going to fully pressurize the donut since the leak gap is small and by far the path of least resistance is up through the nose. Whatever does get through will dissipate faster through the 3 holes on the non-av-bay side than they will penetrate into the avbay. I'll look at the graph from the first flight and if I see a meaningful jump in pressure when the charge fires, I'll add in a discharge hole into the lower centering ring with a corresponding vent to the outside.


Option 2
- Assume that the butting of the central 54mm tube to the motor mount tube is a perfect seal (bad assumption?)
- Vent that ~1" cavity aft of the lower avbay centering ring with 3 or 4 symmetric vents.
- Vent the avbay through the lower centering ring into that space

This works beautifully as long as I don't get a leak from junction with the motor mount tube. I do have a thin layer of silicone sealant on that edge and I can/will tighten the avbay down with pretty substantial force, but it does make me a little nervous.

Thoughts? I'm actually starting to lean towards Option 2. It also has the fantastic benefit of removing any concerns about aligning holes in the shield with holes in the outside.
Sam
 

Coop

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Sounds like the first order is to determine the volume you are actually trying to sample from. My previous post was assuming the rest of the bay did not communicate to the sled--that it was sealed off. If this is not the case, and the rest of the bay does communicate, I'd put 3 holes of appropriate size 120 degrees to each other. If it is sealed off, and only the section above the sled is the sample area, then I stand by my first post.


Later!

--Coop
 

rocketsam2016

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The original design didn't communicate between the larger donut and the sled (the shield on top of the solid fiberglass board ensured that). The concern raised earlier in the thread is that asymmetric positioning of the vents around the rocket (namely them being concentrated on one side) was problematic in the face of wind or non-zero angles of attack once launched in the same way that having a single vent hole for a traditional avbay is problematic.
 

cbrarick

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I have a rocket with a off center ebay by necessity (had to leave room for the motor to go into the nosecone). I did a single port and used some tubing to run from the ebay to where I could port it off. I made it about 25% bigger then need be so that I could compensate for any frictional issues. works well! your mileage may vary, especially if you stay at a Holiday Inn Express....
 

MCriscione

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Sorry to comment and run yesterday. I didn't have much time, and even less brainpower to think about your issue. First, so long as the fit between the bulkheads/coupler/cover and the airframe are a reasonably close fit, i.e., no significant 'slop', the vast majority of your ejection charge pressure wave is going to be going up your motor mount.

Taking some time this morning, I'm beginning to think more about your sled design. Most that I've seen are built with stepped bulkheads so that a coupler covers the sensitive electronics, forming an enclosed volume. It looks like yours is designed such that the electronics, etc. are exposed to the airstream after ejection. Your partial cover addresses that for the most part although it's not clear how it stays attached. I'd suggest using the other portion of the coupler you sliced up to make the cover to make a similar cover on the back half. Permanently attach it to one of the bulkheads, probably whichever one you have the 54mm tube mounted to. Then proceed as I previously suggested.

If you don't like or can't add the coupling to isolate the bay, consider adding an external coupler to the 54mm tube to 'seal' the gap there, and then you'd only be dealing with infiltration from above. I don't know what your setup looks like. It's hard to tell how the AV-bay is secured in the rocket, but if it's like most, the AV bay is pinned (somehow) to your upper payload section of airframe. If so, you can tighten that joint up as much as you like, as it doesn't need to be a true slip fit, a little interference won't hurt so long as you can manually remove it on the ground. If you can achieve even a light seal there, perhaps with tape, although I can also think of an interesting arrangement using 1/16" craft foam cut just slightly over-sized and glued to the bulkhead, then you will limit (or eliminate) infiltration from above, and you can proceed as I previously suggested.

I can see ways of achieving what you're trying to attempt while still maintaining a traditional static port arrangement. I'll be interested in seeing what you end up coming up with.
 

rocketsam2016

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Mark - thanks for the thoughts and I'm sorry I think I've been a bit unclear in my design. This assembly is attached for the whole flight to the motor mount centering ring with the all thread you can see in the picture. It might have been interesting to try and turn it into a more traditional DD design with fore and aft compartments for separate parachutes but I didn't really want to lengthen the rocket and there isn't much space.


I've attached a rendering I did a while back (so measurements are no longer accurate) of what the final layout in the rocket will be:

LittleJohn2.jpg


The red donut is the assembly shown in the picture above, the yellow box is the altimeter and the white boxes are batteries. This assembly sits right above the motor mount, and above it I’m left with a compartment about 7” long before the nose cone. The diagram shows how the eventual plan is to divide that fore compartment in two. One side will have a tender descender (purple box), the other side will have a drogue and a main parachute (orange cylinders, where the main is in a deployment bag).




Dual deployment would then go as follows:
Drogue: both the motor ejection charge and the altimeter have a shot at this (for redundancy). They push the nose cone and drogue out, but the tender descender keeps the main chute from being pulled out.


Main: The tender descender fires, letting the drogue pull the main deployment bag out.




For the first flights though I won’t mess with all this. There will be no tender descender - instead it will be single apogee deploy (altimeter with motor ejection charge backup) of a single parachute. I may even just leave the altimeter in recording mode for the first flight, we’ll see.


So, back to the original question, I’m pretty convinced that the best option for venting is as follows: there is a compartment created between my donut assembly and the motor mount centering ring. I will vent that with 4 vents, and then drill a single hole that vents the avbay electronics compartment into that. This means I have symmetrical vents to the outside and I’m still well protected against ejection pressure/gasses coming from above. I’m pretty confident in the seal between the donut assembly and the motor mount tube, and I’ll beef that seal up a bit.


Finally, I see you belong to CMASS. I’m hoping to L1 cert with this rocket (single deploy setup!) at MMMSC on Oct 22. If you’re around I’d be happy to show you!
 
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