Altimeter switch when static ports are very small.

Up until now, all of my DD rockets have been 3" and 4". I use a rotary switch thru the switch band and turn it on with a screwdriver at the pad.

I just ordered my first 2.6" DD kit (Wildman DarkStar). I am going to use a StratoLoggerCF with it. The CF's instructions give a formula for static ports based on using four of them:

Four Ports, each hole = Diameter * Diameter * Length * 0.0008

The DarkStar2.6 has a 6" coupler, so:

2.6 x 2.6 x 6 x 0.0008 = 0.032448

That is roughly 1/32 of an inch per hole. (1/32 = 0.03125)

This is barely bigger than a pinprick.

Then there is Vern Knowles' formula:

DN = 0.02216*DT*sqrt(L/N)

If I go with a four-hole config for comparison, I get:

DN = 0.02216 * 2.6 * sqrt(6/4)

which gives me 0.0706. That's more than twice the size of the StratoLogger result.

The MissileWorks formula is way more complicated, but for four holes I came up with 0.141. That's twice the size of Vern Knowles!

So, problem #1 -- how big should my holes be?

Problem #2 -- should I use three holes or four?

Problem #3 -- this is the crux. I will not be using a huge rotary switch for this small AV bay, and I am not using a switchband. I was planning to use a screw switch inside the AV bay. But obviously, at least one of my port holes will need to be big enough to accommodate a screwdriver.

I guess the simplest answer I am looking for is something like:

"I have a 2.6" rocket with a six-inch coupler. I use with it with a StratoLoggerCF. I use three port holes sized 0.xxx inches, and I use a screw switch and a ______ screwdriver to activate my switch."

Are you out there? If not, anyone have suggestions?

How about wire, twist and tape? Feed it thru 1 ov the vent holes.

Very nervous about twist and tape going mach+. Also, doesn't that block up one of the holes? They always say to drill clean holes....

If you can have ports alighned on opposite sides of the rocket, a toggle or slide switch can be "poked" from one hole to turn it on, and from the other hole to turn it off. You can even use small pieces of tubing, like a launch lug, to guide a small dowel or screwdriver to the switch, set back a bit from the hole, so it does not interfere with the port.

By my understanding (aero engineer here) port sizes are minimums. Basically, you need enough area to vent air quickly enough that the pressure inside and outside the rocket is roughly equal. You use 3 or 4 holes to prevent angle of attack from throwing off your readings. You don't want them so huge you get a lot of high-velocity flow in the bay, but that's if it's really big.

Even with all that, screwing up the vent holes a little will mostly hurt the accuracy of your altitude reading while moving fast, but you should still detect apogee (when pressure stops going down) pretty well.

I don't think making a hole large enough for a small screwdriver, or a pin to activate a toggle / pushbutton switch would be a problem.

Oberon,

Thanks. Waiting for others to chime in. Everything you read in altimeter manuals tells you to make the holes clean (i.e., not too small due to blockage), and not too big! But how big is too big? And to make them all the same size, of course.

I am seriously thinking about using a magnetic switch and mounting it on 1/4" stand-offs. That gets it w/in an inch of the airframe. Expensive, but maybe the best bet in the long run.

Honestly I suspect the altimeter manuals might be substituting precise equations / textbook answers for an intuitive understanding of what's actually going on / what matters from a practical perspective. But that's just my master's degree in aerodynamics talking. /snark

If you're looking to get a scientifically precise exact altitude, then there probably is a "best" size you should measure carefully (and you probably shouldn't be using an uncalibrated $50 hobby altimeter :wink. And too small is definitely bad, due to measurement lag.

But if all you want to do is make sure your 'chute comes out at the right time, just cut clean holes at least as big as the minimum in the table, don't stick a turbulator in front of them, and don't sweat making a hole a bit big for your micro-screwdriver to fit. You're going to get much bigger errors from atmospheric fluctuation, leakage through the bulkheads, and the altimeter itself than you will from a somewhat big hole.

Bat-mite said:

Oberon,

Thanks. Waiting for others to chime in. Everything you read in altimeter manuals tells you to make the holes clean (i.e., not too small due to blockage), and not too big! But how big is too big? And to make them all the same size, of course.

I am seriously thinking about using a magnetic switch and mounting it on 1/4" stand-offs. That gets it w/in an inch of the airframe. Expensive, but maybe the best bet in the long run.

Click to expand...

Rocket Calculator on my Ipad says three holes 3/32" each. Good idea on the magnetic switch, but I don't understand the need for a standoff. If it's on the edge of your sled, its next to the airframe. Just mark where it is on your switch band. It doesn't have to be near a hole.

Wayco said:

Rocket Calculator on my Ipad says three holes 3/32" each. Good idea on the magnetic switch, but I don't understand the need for a standoff. If it's on the edge of your sled, its next to the airframe. Just mark where it is on your switch band. It doesn't have to be near a hole.

Click to expand...

If the bay is 2.5 inches wide, and the sled is 1/8" thick, and the switch is 1/16" thick, and the magnet has to get within 1" of the switch, then it would need to stand a little higher than the sled to get within range.

I won't have a switch band, but I will clearly mark where the switch is.

This thread was a good read: https://www.rocketryforum.com/showthread.php?64891-Static-Port-hole-question. The original poster there had a Darkstar Jr. and a stratologger, and the same question.

In particular, the replies from guys at Missile Works and Eggtimer were enlightening.

Not quite 2.6, but in a 54mm airframe rocket, I have a ~4.75 coupler, minus about .187 for the endcap portion, twice.... blah blah blah. I fly an rrc2+, use a missleworks screw switch. The switch sits on the board under a hole that is 9/64. There are two other holes, both 1/8" left over from another design. One of them is sort of opposite (but lower/higher) the switch hole and the third one is in line with the switch hole by about 1.5".

Too many, too big, and my radial alignment is off..... rocket flies great, altimeter works!

N

I haven't used the formulas (head in sand) but got alot of mileage out of Tommys Adept report which basically said 1,3 or 4 holes --never 2 .


Most of my DD rockets have flown with 1 larger hole from 1/8th up to 1/4 inch , on my DarkStar3 I used 3 smaller holes and prefer the one large hole if doing an internal switch like a rotary or the small screw switch.

The BlueByYou rocket had a 2.56" coupler 6 inches long and a 1/8th inch hole and a rotary switch center mounted on the sled . If I was doing over would try to fit 2 CF size on sled and maybe drop to 2 screw switches.

WarnerR and I built a eGGtimer remote switch and bought the fob transmitter , but I have not integrated on a sled yet - but has proven very handy on the bench .

Kenny

Bat-mite said:

If the bay is 2.5 inches wide, and the sled is 1/8" thick, and the switch is 1/16" thick, and the magnet has to get within 1" of the switch, then it would need to stand a little higher than the sled to get within range.

I won't have a switch band, but I will clearly mark where the switch is.

Click to expand...

But if you mounted it to the sled towards the side, would it not be within 1" of that side of the rocket. No reason that I can think of for it to have to be perpendicular to the sled to get within 1" of the airframe?

Greg

Bat-mite said:

Oberon,

Thanks. Waiting for others to chime in. Everything you read in altimeter manuals tells you to make the holes clean (i.e., not too small due to blockage), and not too big! But how big is too big? And to make them all the same size, of course.

I am seriously thinking about using a magnetic switch and mounting it on 1/4" stand-offs. That gets it w/in an inch of the airframe. Expensive, but maybe the best bet in the long run.

Click to expand...

I pretty much use the featherweight magnetic switch exclusively now, although it has not been with out some trepidation. I use the same method you are considering, nylon stand-offs to mount it closer to the outside of the airframe. Although you can get a lot more reach with the larger magnet, which is what I use. Not to claim they are perfect but I have yet to suffer any issues with them, brownout, blackout, running the battery down due to a continuous draw, none of it. I have used them up to Mach 1.3 and not sure about the G's but I have launched quite a few Vmax motors with them in the AV bay.

One thing I would recommend is to use something on the external airframe to mark where they are in the AV bay, and key the AV bay so it cannot rotate. This will ensure that the mag switch is where it is supposed to be on the outside of the airframe. It can be frustrating standing on a ladder waiving your magnet around your rocket trying to listen for the altimeter beeps.

My one and only complaint is that I have scrambled two credit cards by forgetting the magnet is in my pocket when I throw my credit card in my pocket.

Some AV bays I have used them in, all with stand-offs.

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Very helpful pics and comments. Thanks!

This is going to be a fun challenge.

Oberon said:

.....If you're looking to get a scientifically precise exact altitude, then there probably is a "best" size you should measure carefully (and you probably shouldn't be using an uncalibrated $50 hobby altimeter :wink. And too small is definitely bad, due to measurement lag.......

Click to expand...

What generation rocket altimeters are you using? The current crop of hobby rocket altimeters using self-contained digital pressure sensors are extremely accurate. Theses MEMS barometric sensors include the NTIS Standard Atmosphere and measure the ambient temperature in their hardware and are 100% factory calibrated. The Bosch BMP180 digital barometer, typical of this generations sensors, is extremely accurate. We use it as this barometric altimeter sensor in our InstantEye MK2 Gen3 sUAS. In FAA altimeter calibration testing required for FAA N-number registration, the standard deviation in the measure altitude versus the FAA standard was +/-11' from sea level to 20 kft. That an order of magnitude better than the FAA requirement, and more accurate than any mechanical altimeter, which is phenomenal for a $3 MEMS sensor using a factor calibration! This is the same sensor used in the current PerfectFlite altimeters and several others.

There can be pressure measurement errors in rocket applications near mach where transient shocks alter the sampling port pressure and under high acceleration when the sampling port areas are too small, however at apogee, and during descent where the velocity is low, there is little if any lag in the static port time response and the calculated AGL altitude.

Bob

So Bob, what size (and how many) holes would you use in a 2.6" rocket with a 6" AV bay going mach+?

Thanks for this thread.
Exactly the question I was about to ask and now I have all the answers. Great information and as ever, obvious when you know why.
I have some other questions regarding altimeter settings which I'll post on a new thread.

SO.

PS.

Oberon: "I aim at the stars. But sometimes I hit London".

Please would you aim your rockets toward the east as the originator of your quote did. They might then, land in the Pacific and keep London free from your hobby. If you can't then maybe slightly south so that Paris or Barcelona can become recipients instead of us. Thank you. :wink:

Three 1/16" or 3/32" holes will be fine for such a small AV bay. <plug> You might want to take a look at the Eggtimer Remote Switch, you can turn it on or off from about 30' away with your AV bay all closed up. It fits fine on a 2.6" x 6" sled. </plug>

Have you considered drilling one larger hole instead of 3 or 4 small ones? That should give you enough clearance to access your switch, if you're enamored with reaching through one of the ports to do this.

You could consider mounting your switch on the av-bay coupler, or switch band if you use one, thus eliminating this problem...


Later!

--Coop

Maybe try something like this:




Later!

--Coop

If you do twist and tape (which will survive mach+ flights), you can just drill another small hole to pass the wires out, then seal from the inside with epoxy or similar so you don't mess with any airflow through the actual vent holes.

That size bay I might do 3 .25" holes. Recess a Missileworks or Featherweight screw switch inside the hole a bit so that when the screw is tightened to turn the switch on there is about a 1/16th or 1/8 clearance around between the screw and the hole in the airframe. I think this will work fine.

I recently built an AV bay using BT-55 tube (34mm OD). This AV bay is 4" long. I too needed small static ports. I decided to use a snap switch that allows me to pull the wood dowel to arm the altimeter. I made a much large hole for the dowel but it lines up directly with the launch lug guide on top of the snap switch. You can see the small static ports (3 total) above the larger hole. I bought the switches at Fry's for $1.49 each. Just another idea.

I have always been told not to use a even number of vent holes.

If you build your own screw switches using a #2 screw with a dome head and recessed hex drive you can go to a very small hole. Ace hardware sells a nice #4-40 with a reduced hex head that gets you down to under a 3/32 hole.

I've done many minimum diameter, Mach+ flights. Use just a single hole and mount a miniature latching push button on the board below it (https://www.ebay.com/itm/like/251998385473)

AlfaBrewer said:

If you do twist and tape (which will survive mach+ flights), you can just drill another small hole to pass the wires out, then seal from the inside with epoxy or similar so you don't mess with any airflow through the actual vent holes.

Click to expand...

I use a shorting plug to enable the altimeter on some of my birds. I make them out of this https://www.micromark.com/mini-connector-kit-pkg-of-10,8839.html. This has worked very well for me so far. I connect the shorting plug on the pad just before launch.

I use a 1/8" headphone jack as my reset switch for my Eggtimers. Put the 1/8" cotter pin in and it holds the altimeters inactive. I don't know if you have similar capabilities with a Stratologger.

Wow, thanks, everyone, for all the cool pics and ideas. It is awesome to see how many people come up with different ways of doing things. It is what makes the hobby so interesting.

I have decided to go with an Aerocon magnetic switch, and use three 1/16" holes for ports. I am also going without a switch band. I am going to epoxy the coupler into the payload bay, and slide in the sled/allthread assembly Giant Leap-style.

Next up: I have a 2.6" x 7" FG sled that needs to fit into a 2.56" x 5.8" coupler. Any ideas on how to cut it down? I don't have a chop saw.