"Special" Altimeter advice

Okay- everybody knows by now I can't post pics from the Fischer-Price PC (stands for Pre Columbian) so here goes:
I have a 5.5. scratch w/3 38mm tubes in it. Motor deploy will be the backup (hopefully). I have an altimeter housed very snuggly into a sealed carbon fibre tube (Thanks Alex!) INSIDE the bodytube at the end of the MMT's, mounted against one wall in between two motor tube outlets. This protects it from the heat and smoke. It's got a switch access hole to the outside and my question is: Since there is very little volume left over in the tube do I need more that one hole? Should I sample the interior of the chute bay (I think it's a bad idea) or do you think I should induce a little spin so there is no aerodynamic 'shadow' effect at play if a less than perfect AOA occurs? I have had perfect success ground testing deployment (this thing masses at 9.9 lbs) with the av bay in place. The whole mess is removable from the bird for prepping. What say the crowd?

Could you drill a larger hole through the bottom of your ebay compartment into the space between the CRs, and then put vent holes around the fin area? Basically tie the ebay into the volume inside your fin can, and then vent the fin can. With 3 MM in there it isn't actually a huge volume anymore.

I suggest this, but if it were me, I would set the arm altitude for the altimeter to about 400 ft to give some extra safety against it being tricked on the pad by a gust, drill that one access hole plenty big, and go with it as is.

Cl(VII) said:

Could you drill a larger hole through the bottom of your ebay compartment into the space between the CRs, and then put vent holes around the fin area? Basically tie the ebay into the volume inside your fin can, and then vent the fin can. With 3 MM in there it isn't actually a huge volume anymore.

I suggest this, but if it were me, I would set the arm altitude for the altimeter to about 400 ft to give some extra safety against it being tricked on the pad by a gust, drill that one access hole plenty big, and go with it as is.

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Thanks- beautiful solution! I am a bit worried as the wierd 'av bay' is now a chute cannon, but a little more charge should dump the laundry. I do have it arming at 300' This pig will never see mach!

fyrwrxz said:

Motor deploy will be the backup (hopefully). ....... Should I sample the interior of the chute bay (I think it's a bad idea)

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I'm not sure I fallow everything, but if the motor ejection fire, the altimeter will fire the main and drogue at the same time if you sample the air in the same tube the motor ejection fire in. The pressure of the motor ejection will make the altimeter believe is way lower if not under launch site level. But at 300' it may be actually a good thing.

Gerard- Have no fear- 1) the altimeter is cpmpletely enclosed in a carbon fibre tube, sealed off from the open motor tubes at the bottom of the 'chute cannon' 2) This is in turn epoxied to a coupler that slides down and rests on the upper centering ring of the fin can/motor bay. That assembly is secured with screws thru the outside to nutplates mounted on the coupler. There is a grommet around the access hole to the Schluter switch inside the altimeter bay to compete the sealing when everything is tightened. Only one hole to the outside. It is impossible for the altimeter bay (right now) to see any pressure fluctuations from motor deploy or barometric changes in the body tube it's self. There is no drogue. Think of any short fat stubby rocket you like and instead of the avbay in the nose cone, it's actually off to one side (leaving room for the parachute and harness) I was worried weather I needed to induce a little spin so I didn't 'shadow' the pressure port if it arced a little.

Area66 said:

I'm not sure I fallow everything, but if the motor ejection fire, the altimeter will fire the main and drogue at the same time if you sample the air in the same tube the motor ejection fire in. The pressure of the motor ejection will make the altimeter believe is way lower if not under launch site level. But at 300' it may be actually a good thing.

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Is a momentary pressure spike really sufficient to fool modern altimeters which include code to ignore such things through mach transition (for example)?

Zebedee said:

Is a momentary pressure spike really sufficient to fool modern altimeters which include code to ignore such things through mach transition (for example)?

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yes it is, I experienced it twice, the last time was 2 weeks ago.

Area66 said:

Zebedee said:

Is a momentary pressure spike really sufficient to fool modern altimeters which include code to ignore such things through mach transition (for example)?

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yes it is, I experienced it twice, the last time was 2 weeks ago.

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I would expect that is because a wind gust that causes a pressure "spike" inside the av-bay is a long duration event compared to the spike from mach transitions. Depending on the wind, the pressure changes created could last several seconds. The software filters may work perfectly for the mach spikes, but be completely fooled by the wind pressure changes.