how far back from nose cone should altimeter vent holes be?

Hi everyone,
looking for a bit of advice. The best place to put my altimeter is in the nose cone, or close to it. how far back from the shoulder should the vent holes be so as to get a good reading (or conversely, not get a bad reading).

Thanks.

What do you mean by "or close to it?"

If it is a barometric altimeter, then the air that comes in through the holes must be able to pass across the barometer in a steady flow. You don't want stale air trapped inside, and you don't want there to be a lack of air. Does that help?

Most modern altimeters are mach immune, or at least more so than the one's made 10-15 years ago. I mounted my altimeter in a 7.5" nose cone and drilled the holes 1" forward of the shoulder. Just make sure you have 3 or more holes and have plenty of venting for the volume. What size cone are you using and how fast do you expect the rocket to fly?

ChrisAttebery said:

Most modern altimeters are mach immune, or at least more so than the one's made 10-15 years ago. I mounted my altimeter in a 7.5" nose cone and drilled the holes 1" forward of the shoulder. Just make sure you have 3 or more holes and have plenty of venting for the volume. What size cone are you using and how fast do you expect the rocket to fly?

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This is a pretty small rocket that is being used in the TARC competition. It's a BT70 tube. The motor is going to be either an F39 or Cesseroni F51BS, the team hasn't yet determined. It will not be hitting Mach. The altimeter is a Perfectflite Pnut. I don't know alot about altimeters, but I did hear that they shouldn't be mounted in the nose cone because the airflow isn't constant. it is the best place to mount it though, so I just want to advise the TARC team appropriately. Appreciate your input.

when all else fails - read the directions..... (maybe I ought to do that more often)

Static Pressure Sampling Holes
You must drill one or more clean-edged holes in the payload
compartment to allow outside air pressure to be sampled by
the altimeter (see table below for recommended sizes). These
holes should be as far away from the nosecone shoulder and
other body tube irregularities as possible (at least 3 times the
body tube diameter or more) to minimize pressure
disturbances being created by turbulent airflow over the body
tube. Sand the area around the hole as necessary to eliminate
flashing or raised edges.

I was going to suggest that. The current NAR competition rules for static hole placement when using an altimeter in an altitude event read as follows:

2014 NAR competition rules said:

The altimeter must be fully enclosed within the rocket body. The part of the rocket containing the altimeter must be vented to the outside air by at least 3 vent holes evenly spaced around the circumference of the body. There must not be any protrusions or depressions on the body within 1 body diameter of the holes. Any attempt to deliberately produce excessively high altitude readings, such as venturis are specifically prohibited. The ports must be on a section of the model that is an unobstructed cylinder or cone for 1 caliber either side of the ports, and the cone must be no steeper than 1 in 4 taper (.25″ change in diameter per inch of length). In this case, a fin counts as an obstruction, as does a launch lug and would not be allowed within 1 caliber of the ports.

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You'll certainly want to adhere at least to that.

Integrating the altimeter properly with the rest of the payload is one of the fun bits of designing a TARC rocket....

Bat-mite said:

What do you mean by "or close to it?"

If it is a barometric altimeter, then the air that comes in through the holes must be able to pass across the barometer in a steady flow. You don't want stale air trapped inside, and you don't want there to be a lack of air. Does that help?

Click to expand...

Um, actually, no flow is required. All that is required is that there be a path between the vent hole and the barometric sensor so that pressure can be transmitted. Heck, if it didn't hurt the sensor, it could be buried in Vasoline and it would still work the same.

There will be flow in and out of the vent hole, depending on changing relative internal and external pressure, and velocity, and the rocket's attitude. There does not need to be flow across the barometric sensor. There should not be rapid flow across the sensor as it would provide misleading results via artificially dropping the measured pressure a bit more.

Gerald

The rockets I have with altimeters in the nose cone, I put the vent holes on the bottom of the nose cone. That way they don't get and turbulence from the joint between the NC and BT. It's worked for me.

BEC said:

I was going to suggest that. The current NAR competition rules for static hole placement when using an altimeter in an altitude event read as follows:



You'll certainly want to adhere at least to that.

Integrating the altimeter properly with the rest of the payload is one of the fun bits of designing a TARC rocket....

Click to expand...

Yes, the altimeter placement is a pretty important design consideration for the team. it's interesting because the altimeter is an after thought. It took us most of the first year to figure out a good way to seal the altimeter from the ejection gasses. Lots of failed attempts, but ultimately success.

This year, the team wanted to move the center of gravity backwards by flip flopping the egg placement and the altimeter (i.e. putting the altimeter at the front of the rocket and moving the egg back. However, the drawback of this is that they could end up with faulty readings. So, it's back to the drawing board.

This is a good example of what the kids confront in the TARC competition. The real fun is watching the kids work through these problems. You wouldn't believe some of the crazy solutions that they come up with.

Thanks for all of the input.

You can have the altimeter in the nose, and the ports down in the payload bay, as long as you can allow for an unobstructed path between them, like Gerald said.

I've had teams do this in the past and it worked out well for them.

kj

BEC said:

I was going to suggest that. The current NAR competition rules for static hole placement when using an altimeter in an altitude event read as follows:

Originally Posted by 2014 NAR competition rules
The altimeter must be fully enclosed within the rocket body. The part of the rocket containing the altimeter must be vented to the outside air by at least 3 vent holes evenly spaced around the circumference of the body. There must not be any protrusions or depressions on the body within 1 body diameter of the holes. Any attempt to deliberately produce excessively high altitude readings, such as venturis are specifically prohibited. The ports must be on a section of the model that is an unobstructed cylinder or cone for 1 caliber either side of the ports, and the cone must be no steeper than 1 in 4 taper (.25″ change in diameter per inch of length). In this case, a fin counts as an obstruction, as does a launch lug and would not be allowed within 1 caliber of the ports.

Click to expand...

Click to expand...

When I read that bolded part, I'm confused now about how one should place them... I've always thought that the location of the vents was directly into the AV bay. So, presuming an AV Bay's coupler to be 2X the rocket's diameter (+ the length of the ring of body tube to prevent the AV bay from sliding forward or back), my interpretation of the bolded statement would have the vent holes either forward, or behind the AV bay, due to the depression caused by change from the forward segment of body tube, to the ring, to the aft segment of body tube. That interpretation seems really wrong to me.

No - that's exactly what it means - you want at least one caliber between any bumps/rings/protrusions/depressions and the locations of the static ports to get an accurate static pressure value and the resultant calculated altitude. Look at full scale airplanes and how much care is taken in the area around the static ports - smooth flow over them is the requirement.

Now for the purposes of deployment the actual value of the pressure doesn't matter - you just need the minimum in order to time your apogee charge. But to get the actual altitude, you absolutely need smooth airflow over the static ports, so having the vents in the middle of an av bay that is also where the tube joint is would be less than optimum....and not accepted in an NAR competition where altimeters are used to get an altitude score.

And throw in the effort that some rules lawyers devote to gaming the NAR competition rules, so you end up with a rule that tries to nip a theoretical advantage in the bud. There are people still nursing hurt heinies after the first set of altimeter rules were written.

None of that applies to TARC, thank goodness, and as long as the students follow the directions with their altitudes, everything works out ok.

kj