Altimeter Pressure Sensing Holes

I've heard several different opinions on this but, how many body diameters below a transition should an altimeter's pressure sensing holes be drilled to avoid the interference caused by turbulent flow over the transition? I've heard as little as 1 and as much as 5.

Wish I could help, the typical answer I've seen is 'several'. I don't think I've ever heard anybody offer any hard facts to pin this number down. Would an altimeter manufacturer would have this info?

I'll be the first to admit that I have not had overwhelming success using altimeters in the past 3 years when I got started with a Missile Works and then a G-Wiz altimeter. My successes have greatly improved just by being careful and following a checklist, however, I have never heard this about transitions.

But before I make a fool of myself, are you saying a "transition" meaning from one body tube size to another, verses a "coupling" from the same body tube size to another.

This is an interesting question and if nothing else my post will help clarify "transitiion" verses "coupling" for other readers.

Thanks,
Nick

Yes, in this case a transition is a true transition from one BT size to another, not a coupling on the same sized tube. The turbulence caused by a transition (or the nose cone) can affect barometric altimeters. The questions are basically how much? and how far away does the vent have to be, to effectively be out of that turbulence?

Note that the G-Wiz launch, burnout, and apogee detection are all performed with the accelerometer. It will work fine in turbulence, Mach transitions, totally enclosed compartments, etc. The only functions affected are its apogee recording and, if totally enclosed, the main firing channel (400/800 ft.) The latter may not be affected too much since you are almost back where you started when this is activated? I dunno about the latter, just my ranting.

Yes. I'm building a scratch built rocket that will have a 4" main body tube section on the bottom that transitions to a 2.5" body tube section at the top. I had originally designed the rocket to house the altimeters in the transition section. Afterwards, I found out about the turbulent flow problem over nosecones and transitions. Rather than redesign the rocket to relocate the altimeters (a big problem), I've decided that I can build a small, sealed 4" diameter bay below the transition that I can drill the pressure sensing holes into. Below this bay will be the main payload bay. The question is, how long will this "pressure bay" need to be in order to allow the holes to be drilled low enough to avoid the effects of the turbulent airflow?

Originally posted by rstaff3
Note that the G-Wiz launch, burnout, and apogee detection are all performed with the accelerometer. It will work fine in turbulence, Mach transitions, totally enclosed compartments, etc. The only functions affected are its apogee recording and, if totally enclosed, the main firing channel (400/800 ft.) The latter may not be affected too much since you are almost back where you started when this is activated? I dunno about the latter, just my ranting.

Click to expand...

I'm using a G-Wiz MC and a MissleWorks RRC2X for a backup altimeter. My concern is mainly for the MissleWorks altimeter. The G-Wiz should register apogee fine but, the pressure problems might screw up the altitude measurements and I wouldn't get a proper low altitude ejection. The MissleWorks, on the other hand, could eject pre-maturely and ruin my whole day.

Just thinking out loud....

Might the Apogee-distributed AeroCFD program program be used to determine the answer? I haven't looked into what it provides exactly, but I thought it showed the speed of the airflow around the rocket. Could you visualize the turbulance?

Also, it there a way for you to mount the altimeters where you want (in the transition) and extend the vent to a point that is considered 'safe' using a smaller vent tube (above or below the transition)?

Just a thought....

The idea of a transition messing with the altimeter (etc) only applies in the pre apogee phase. On the decent, in dual deployment, the transition is either pointing horizontally or backwards. (I hope that makes sense.) So, theoretically, the low altitude ejection shouldn't be affected by the transition cause the forces on the ascent just aren't there during the descent. Does that make sense? ...or... am I wrong?

I tried AeroCFD when it first came out to see what it was like. My first impression was that it was very basic. You couldn't model rockets with transitions. It could only handle 3FNC rockets. It sure would be nice if it would allow you to import Rocksim files to get the profile.

Originally posted by rstaff3
Also, it there a way for you to mount the altimeters where you want (in the transition) and extend the vent to a point that is considered 'safe' using a smaller vent tube (above or below the transition)?

Click to expand...

Basically, that's what I'm doing. I'm just not using the tubes. I'm trying to create a section below the transition that will be at the current atmospheric pressure and still be low enough to avoid the turbulence. The problem with using tubes is that it poses a problem with connections in the payload bay and may present dangers of "hanging up" my main chute.

Originally posted by PGerringer
Just a thought....

The idea of a transition messing with the altimeter (etc) only applies in the pre apogee phase. On the decent, in dual deployment, the transition is either pointing horizontally or backwards. (I hope that makes sense.) So, theoretically, the low altitude ejection shouldn't be affected by the transition cause the forces on the ascent just aren't there during the descent. Does that make sense? ...or... am I wrong?

Click to expand...

You're right about the altimeter's main job comes after the apogee event. However, if the turbulent flow causes the altimeter to get it's calibration screwed up, it may not know where it is and not fire at the correct time.

You are correct! There will be no turbulance on the way down. So, the G-Wiz should be safe without worrying about turbulence. I'm not sure I'd put the vent on a surface that can scoop up air (ie on the face of the transition). I don't know if you might get funny altitude readings this way?

I don't know how the Missleworks altimeter works, though.

Originally posted by rstaff3
You are correct! There will be no turbulance on the way down. So, the G-Wiz should be safe without worrying about turbulence. I'm not sure I'd put the vent on a surface that can scoop up air (ie on the face of the transition). I don't know if you might get funny altitude readings this way?

I don't know how the Missleworks altimeter works, though.

Click to expand...

My concern with the G-Wiz is that the barometric calibration would get screwed up if the turbulence created a vacuum over the pressure sensing holes. If the computer forgets the altitude that it started from, it won't know how high above the ground it is.

The MissleWorks altimeter is 100% barometric based. It won't even arm until the rocket has ascended 300+ feet.

The only problem would be if the turbulance caused the altimeter to get confused, thinking it had apogee when it really didn't. This would goive a bad apogee reading. IF this happened before you got to 400ft (or 800 depending on the model), then it might think its time to fire the mains. Unless of course it it smart enough to only fire these if the general trend is downward, which is likely.

The GWiz Yahoo group might be a good place to take this conversation. The designer monitors that group.

I doubt that it will forget where it started from. This is most likely tied to the launch detection.

Its easy to join the GWiz group. I could also post the questions and then cut the answer in here.

First, I apologize about the synchronization of my messages. I should figure out how to quote things

The GWiz is known to be Mach safe. Thus, you can probably assume other distrubances on the way up won't give it fits either, and both the apogee reading and main channel will be OK.

I believe I've found the answer here: Adept Rocketry.

Basically, it says that I should have at least 4 calibers below the transition before drilling my static ports.

I think I found an alternative solution to my problem. Instead of placing my static ports below the transition, I'll add a few more inches to the forward body tube in front of the transition. The static ports can be drilled in front of the transition, where the airflow is still laminar, and behind where the front body tube will separate to release the drogue chute. This will allow me to seal the transition altimeter section from the forward parachute compartment and the ejection gases but, still let the transition altimeter section be open to the outside air.

What does everyone think?

uh...you KNOW my answer bro!

That sounds like a great solution! I need to check out that site just for GPs.