What are the effects of greater than calculated static port holes?

[N0RTtheEngineer]

Hello,

I am wondering about static port holes. I currently have a 6 inch Diameter rocket with a 12 inch AV bay. The current simulation is that it will be flying at Mach 0.53 as the highest velocity with an apogee of 4823ft (This is with an Aerotech L1520T motor). Now my question here is that the static port holes that I drilled were 4x 0.43 inch diameter holes. When going back through the calculations for the static port holes, it was noted that a math error occurred. The true value of the holes should be 0.23. What are the effects of having a larger static port hole on this launch vehicle? I have looked through the internet to find some knowledge and currently can see that there is a potential threat of having an inaccurate apogee reading. Now since the launch vehicle is not going above or near Mach 1 will this effect be very profound or slightly negligible?

 

[cerving]

The difference for that flight and size rocket will be irrelevant. Don't lose any sleep over this.

 

[SolarYellow]

I'd probably OCD about the external aerodynamics and make some reducer bushings to bring the holes down to the 0.23 diameter. But I'm kinda OCD.

 

[Steve Shannon]

Hello,

I am wondering about static port holes. I currently have a 6 inch Diameter rocket with a 12 inch AV bay. The current simulation is that it will be flying at Mach 0.53 as the highest velocity with an apogee of 4823ft (This is with an Aerotech L1520T motor). Now my question here is that the static port holes that I drilled were 4x 0.43 inch diameter holes. When going back through the calculations for the static port holes, it was noted that a math error occurred. The true value of the holes should be 0.23. What are the effects of having a larger static port hole on this launch vehicle? I have looked through the internet to find some knowledge and currently can see that there is a potential threat of having an inaccurate apogee reading. Now since the launch vehicle is not going above or near Mach 1 will this effect be very profound or slightly negligible?

That’s probably nothing to worry about. The calculation is an estimate anyway. I just drill three holes using a drill bit that’s convenient, usually about 5/32” because I keep losing the 1/8” bits.
If the holes are way too large, they can admit wind gusts, causing problems while the rocket sits on the pad and in flight. Also, don’t line up any of the holes and the port on the barometric sensor.
If your holes are just a little too large you’ll get noisier reads, but it should not hurt anything. Your altimeter might even compensate for that. Similar effects can happen in rockets approaching mach.
If the holes are too small there’s a time lag between apogee and detected minimum pressure because air is slow to flow out of the av-bay.
Today’s altimeter manufacturers are more sophisticated than ever. I would not worry about it.

 

[sharkbait]

Hello,

I am wondering about static port holes. I currently have a 6 inch Diameter rocket with a 12 inch AV bay. The current simulation is that it will be flying at Mach 0.53 as the highest velocity with an apogee of 4823ft (This is with an Aerotech L1520T motor). Now my question here is that the static port holes that I drilled were 4x 0.43 inch diameter holes. When going back through the calculations for the static port holes, it was noted that a math error occurred. The true value of the holes should be 0.23. What are the effects of having a larger static port hole on this launch vehicle? I have looked through the internet to find some knowledge and currently can see that there is a potential threat of having an inaccurate apogee reading. Now since the launch vehicle is not going above or near Mach 1 will this effect be very profound or slightly negligible?

If you’re married, this is a great excuse to buy another rocket. Honey, I messed up the static port holes on this one so I’m going to have to order another rocket ASAP

 

[cls]

I'm aware of 3 incidents of premature apogee detection, I believe caused by waaay too large ports. 1/2"!?

And an old altimeter, software didn't do any smoothing... Open ports with some pressure resonance in the ebay, maybe?

So my feeling is it's better to have more restricted ports that let out pressure gradually and monotonically descending...

 

[SolarYellow]

I imagine that more, smaller holes, not all at the same point lengthwise on the rocket, would help eliminate any tendency to have resonant oscillations.

 

[BEC]

As Cris and Steve have suggested - just go fly.

 

[jahall4]

Directly from TIm at Apogee in 2016, all truth, largely corroborated via personnel experience

"Most people want to put the smallest vent holes in as possible. So from what I understand, the sizes that they recommend are the smallest you’d want to go.

I personally like big holes, because they are harder to clog (with padding and such).

There might be two issues with big holes. The pressure sensors are light sensitive. The sunlight heats them up (pressure increases with temperature), and they can give you false readings. That happened to me this summer with a different altimeter. As long as the sun doesn’t hit them directly, you should be fine. I’ve never had a problem with the Stratologger. (I have experienced this problem with a StratoLogger, but NOT because if hole size, but because of the translucency of an unpainted airframe in FG rocket..

The other issue is swirling air inside the rocket. Large holes “might" cause an issue, because more air can get in. However, I’ve never experienced that issue. The swirling air is really only a problem if you are trying to measure speed, because the pressure sensor is taking readings at 20 times per second (I made that number up, but it is high). So you “could" have fluctuating speed readings. But it doesn’t really affect the altitude measurement. Why? Because the rocket is traveling so slow near apogee, that the air has time to calm down inside the rocket and make a good altitude measurement. So if you’re using the altimeter for measuring peak altitude, I wouldn’t be concerned about the hole size.

If you’re using the stratologger for dual-deploy, then you’ll be ejecting at peak altitude. And once the chute is out, the rocket is traveling slow, so it can also get a good pressure reading. Then you can look at the altitude plot after downloading the data and see on the chart where peak altitude is. My experience is that they reading is really close to what you’ll see on the graph. See Peak-of-Flight Newsletter #208 at: http://apogeerockets.com/index.php?main_page=peak_of_flight&pof_list=archives&pg=education"

 

[Buster]

I found this report “HPR Research-Static Port Holes from Nescience to Science” by Gary Stroick, Off we go Rocketry. He also had a spreadsheet for use.