Mount Altimeter Bay to Hatch Door??

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konoctipirate

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On my scratch built 4" Batman rocket, I'm doing an e-bay with hatch on the main airframe for the first time.

The airframe has 2 layers of CF on it. The coupler that I'll be using with hole cut in it to screw the hatch to will have several layers of glass on the inside to make it a solid base for the hatch cover to get bolted to. I could even put plywood supports between 2 centering rings with brass inserts and have the hatch bolts screw into them.

Here's my question. I'm thinking about building a completely closed altimeter bay that's mounted directly to the hatch cover. That would make access very very easy and all I'd have to do is wire the e-matches to connectors on the outside. Obviously, I can vent the bay through the hatch cover. And I could have bolts from outside the airframe that secure the altimeter bay to the inside of the hatch.

As long as I make sure I've got a great anchor system for the altimeter bay to hatch and hatch to airframe, can anyone see any downsides to this approach?
 
Originally posted by konoctipirate
As long as I make sure I've got a great anchor system for the altimeter bay to hatch and hatch to airframe, can anyone see any downsides to this approach?

I'm not sure I'd feel comfortable with all of my vent holes clustered together on a hatch. I think my E-bay may only have 2 holes in it (it's been a long time since I flew it, but I need to pull it out and put the Sandhawk back together, especially now that there's a 120ns 38mm case), but they're on opposite sides of the tube. Wind gusts can do funny things to the pressure at a single hole -- I have no scientific evidence for wanting holes on multiple sides of the tube, just a gut feeling that you're likely to get better readings that way. Of course, it's possible that the pressure inside the bay responds slow enough to changes outside the bay that it's moot, but I personally wouldn't feel real comfortable about it.

-Rick
 
Originally posted by Rick Lindsey
I'm not sure I'd feel comfortable with all of my vent holes clustered together on a hatch. I think my E-bay may only have 2 holes in it (it's been a long time since I flew it, but I need to pull it out and put the Sandhawk back together, especially now that there's a 120ns 38mm case), but they're on opposite sides of the tube. Wind gusts can do funny things to the pressure at a single hole -- I have no scientific evidence for wanting holes on multiple sides of the tube, just a gut feeling that you're likely to get better readings that way. Of course, it's possible that the pressure inside the bay responds slow enough to changes outside the bay that it's moot, but I personally wouldn't feel real comfortable about it.

-Rick
One of our club members and I were looking at the altimeter data log from his L3 flight, and during the descent before his main deployed, the recorded altitude was jumping around by maybe ten's of feet. Our best theory was that because the vent holes for that e-bay were on the same side in a straight line, not spread around the rocket's circumference, as the rocket tumbled "under drogue" the direction of the wind relative to the holes kept changing. That's why multiple identical holes spread around the circumference gives better results.

I've heard that TWO holes opposite each other is a bad idea. I'm going to guess that you can get some sort of odd resonance effect that way which doesn't happen with an odd number of holes, or with even numbers > 2. Just a theory.
 
Thanks for flagging this issue on the number of vent holes and squirrely altimeter readings.

So what about a single vent hole?

The volume of the altimeter chamber attached to the back of the hatch door would be really small. Some of the design inspiration for this rocket comes from Andy/Polecat Aerospace's 10" Fatman in which the altimeter chamber, according to the instructions, has only a single 3/16" vent hole.
 
I've used electronic bays with single holes with success before. I've limited them to my smaller, <= 3" diameter, airframes however. I'd say that if the volume you are venting is small enough to vent with a 1/4" or smaller hole you should be OK. See the manual for the MissileWorks RRC2 altimeter which is available at www.missileworks.com for the method of calculating a pressure relief port.

Ken Holloway
NAR #78336, L-II
 
Mounting your altimeter *on* the hatch shouldn't pose a problem. You just have to remember to make your opening big enough to fit the altimeter you plan to use. Since you're using multiple layers of CF on the airframe, and fiberglassing the inside of the coupler, you should have a pretty sturdy setup.

I would wire the pyro leads into a permanent harness, with a 'Quick-Release' connector to let you easily remove the altimeter from the bay.

Also, I tend to distrust avionics bays with a single vent-hole. I prefer three vent-holes in my bays... less chance of wind affecting the readings.

The only concern I would have is - how are you planning to power-up and arm your altimeter? My worry, if you're going to apply power and arm your system before closing the hatch, is that random accelerations from the act of securing the hatch into place may be enough to trick your altimeter into thinking that it's in flight. For that reason, I have nothing on my own hatch-covers... all of the electronics, including the power and arming switches, are contained within the bay itself.
 
So, I'm getting towards decision time on this and I'm definitely going to mount the electronics on the inside of the hatch. The answer on the switch is that I'm using one of those 110/220v switches that will get mounted into the hatch door itself so that the screw slot to arm it is on the the outside of the AF.

Since there is going to be a little more weight (and value with the altimeter there) built into the hatch door, I'm going to pay careful attention to the anchoring of the door to the coupler-flange and of the coupler-flange to the AF itself. I think both call for bolt use.

For anyone interested, I did start a build thread for this rocket. I'll be posting text and pics there as I implement this design.
 
Originally posted by konoctipirate
Obviously, I can vent the bay through the hatch cover.

Could you install a couple of small tubes running from the other sides of the body tube into the hatch? I was thinking about the little tubes that iginitors come in. (with G or H motors)
 
Thanks for the suggestion. I could do that but I've figured out the problem I was trying to solve when I was thinking about totally sealing the altimeter etc on the hatch. That problem was how to stop ejection gases from coming back through the ematch holes that I couldn't permanently seal because of lack of access on the igniter/charge side of the hole. My solution is to use tire stem valves and caps with a hole in the cap for the ematch wires to pass through and modeling clay within the stem. I attached these to the inside of the ebay yesterday having cleared out the middle with a 3/16" metal drill bit.

The electronics will still be attached to the hatch door but I'll do 3 vent holes: one on the door itself and 2 others around the airframe.

And for anyone interested, I'm keeping a construction blog for this scratch build.

droppedImage.jpg
 
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