Need HELP with making a rocket!

[Ayush Chauhan]

When I used the following formula from this website I was getting the following diameter for the Vent hole:

This is too small for me to even cut out on the body tube which is why I went with 2 x 5mm diameter vent holes and a slot which acts as a vent hole.

For my rocket here are the measurements:
D = 4.1 cm
L = 6 inch = 15.24 cm
Number of vent holes = 3

 

[jqavins]

The two 5 mm holes each have an area of about 19½ mm². The slot has an area of about 11½ mm². Is 31 mm² sufficient instead of 38? I would think yes, but wait for others to weigh in. Based on second hand experience, I really would think that a single 5 mm hole would do, and the rest is insurance.

Should you take one of the 5 mm holes out? Is 50 mm² too much? No, not unless they're close enough to compromise the tubes strength.

 

[Ayush Chauhan]

Should you take one of the 5 mm holes out? Is 50 mm2 too much? No, not unless they're close enough to compromise the tubes strength.

What do you mean by this?

 

[jqavins]

I mean something so obvious that you are probably looking for more meaning that's not there. Removing material makes the tube a little weaker, of course. Not an issue with one hole this size. Place 5 mm holes every 7 mm all the way around the tube, or in a line all the way from top to bottom, and you've perforated it like the paper on a legal pad. In between, if these three holes are too close together then you've made a weak area prone to buckling. So don't do that.

Next, I have a feeling you're going to ask "How far about should make them?" I don't know, just use your best judgement. If you think they like kind of maybe too close then move the vent holes away. The slot has to be where you can reach the switch through it of course, but the vent placement is much less important; remember the baromter in the living room.

 

[Ayush Chauhan]

This is the final CAD model of the rocket. This is made to 1 to 1 scale.

 

[Ayush Chauhan]

I mean something so obvious that you are probably looking for more meaning that's not there. Removing material makes the tube a little weaker, of course. Not an issue with one hole this size. Place 5 mm holes every 7 mm all the way around the tube, or in a line all the way from top to bottom, and you've perforated it like the paper on a legal pad. In between, if these three holes are too close together then you've made a weak area prone to buckling. So don't do that.

Next, I have a feeling you're going to ask "How far about should make them?" I don't know, just use your best judgement. If you think they like kind of maybe too close then move the vent holes away. The slot has to be where you can reach the switch through it of course, but the vent placement is much less important; remember the baromter in the living room.

Got it. I did just use my judgement. And put them equidistant above the altimeter.

 

[Ayush Chauhan]

UPDATE!! Hello guys,

This is the progress on the rocket so far!

I had one question with the current design.


I only have 1 hole in the avionics bay. This can serve the purpose of 2 things at the same time:

1. Using it as a vent hole, which will allow air inside the avionics bay so the altimeter can calculate the altitude.
2. This hole will allow me to turn on the switch of my altimeter using a screwdriver, without me taking out the whole avionics bay.

Now my question is:- should I add another 2 x 5mm holes (which I was origninally considering on adding) on the avionics bay? or is this just fine for the altimeter to get enough readings.

Also, please feel free to qestion me about the design or anything. Thanks!

 

[smstachwick]

View attachment 613108
UPDATE!! Hello guys,

This is the progress on the rocket so far!

I had one question with the current design.

View attachment 613110
I only have 1 hole in the avionics bay. This can serve the purpose of 2 things at the same time:

1. Using it as a vent hole, which will allow air inside the avionics bay so the altimeter can calculate the altitude.
2. This hole will allow me to turn on the switch of my altimeter using a screwdriver, without me taking out the whole avionics bay.

Now my question is:- should I add another 2 x 5mm holes (which I was origninally considering on adding) on the avionics bay? or is this just fine for the altimeter to get enough readings.

Also, please feel free to qestion me about the design or anything. Thanks!

I’m impressed with the build!

Here’s an old fisherman’s trick to convey scale: get yourself in the photo if at all possible! I’ve been plastering this photo of my L1 project in a bunch of different places. I admit that I am sort of tooting my own horn but I’m also struck by the visual similarity of our airframe designs (aside from the fact that mine is a scale kit and I think yours is an original design).



I believe you decided on a smaller tube though, right? I think it was roughly a BT-60 standard, about an inch and a half diameter?

No comment on the hole, other than that I think it’s pretty neat that you can get a screwdriver in there! That never would have occurred to me.

(Minor edits)

 

[bad_idea]

Now my question is:- should I add another 2 x 5mm holes (which I was origninally considering on adding) on the avionics bay? or is this just fine for the altimeter to get enough readings.

I would add another hole or two, both to null out any anomalies with the localized airflow at the site of your current single vent and also to account for any flow problems around the hardware facing the switch hole.

When I solely relied on wifi-switched altimeters with no external power switch, I would make three identically-sized holes around the circumference of the avbay, choosing the size based on various rules of thumb (but most likely leaning heavily on what @cerving recommended with respect to total area).

When I started using screw switches due to many local clubs in my part of the US requiring them, I figured airflow around the switches and the wiring behind them was probably somewhat restricted, so I continued to use a couple of additional holes, generally sizing them as though whatever size hole I used for the screw switch was actually the same size as that of the additional holes, i.e., if I have a formula saying I need N area in total, I size the additional holes to have an area of N/3, regardless of how large a hole I have to make for the switch.

This may be an overly conservative approach, but not being an aerodynamicist by training, I'm forced to be catious here.

Edit to add: I also try to separate vent holes from direct alignment with the baraometers of my altimeters, both to avoid exposure of the baromters to direct sunlight which could throw off their readings and also - again - to discourage local pressure anomalies from being picked up by those barometers.

 

[jqavins]

View attachment 613108
UPDATE!! Hello guys,

This is the progress on the rocket so far!

I had one question with the current design.

View attachment 613110
I only have 1 hole in the avionics bay. This can serve the purpose of 2 things at the same time:

1. Using it as a vent hole, which will allow air inside the avionics bay so the altimeter can calculate the altitude.
2. This hole will allow me to turn on the switch of my altimeter using a screwdriver, without me taking out the whole avionics bay.

Now my question is:- should I add another 2 x 5mm holes (which I was origninally considering on adding) on the avionics bay? or is this just fine for the altimeter to get enough readings.

Also, please feel free to qestion me about the design or anything. Thanks!

Looking good.

First the caveat: the following responses are based on somewhat limited prior knowledge, but not on first hand experience.

Regarding "1. Using it as a vent hole, which will allow air inside the avionics bay so the altimeter can calculate the altitude", actually, it's letting air out of the avionics bay, but I guess you really know that.

What is your goal for the altimeter? If you're only interested in knowing the peak altitude then I'd bet "credits to navy beans" that one hole is fine and you've got plenty of area. Search this forum and/or the rest of the web* for a vent hole calculator, do what it says, and I'll be really surprised of you don't have enough area there. If you're looking for accurate data while it's on the way up then for reasons like uneven airflow and other aerodynamic acoustic weirdness that I don't understand, another hole or two is probably a good idea.

Why the difference? Well, when you are at or near apogee you're going slowly. The outside air pressure is changing slowly and any aerodynamic weirdness, pressure oscillation, etc. is minimal. A hole like you've got can easily keep up, i.e. let air out fast and smoothly enough. But when you're zipping along at high speed the air needs to get out of the ebay faster and the weirdness is much more likely to be a significant factor.

* When you ask Google for help on this sort of thing, you've got at least a 50:50 chance of ending up right back here anyway.

 

[Ayush Chauhan]

I’m impressed with the build!

Here’s an old fisherman’s trick to convey scale: get yourself in the photo if at all possible! I’ve been plastering this photo of my L1 project in a bunch of different places. I admit that I am sort of rooting my own horn but I’m also struck by the visual similarity of our airframe designs (aside from the fact that mine is a scale kit and I think yours is an original design).

View attachment 613114

I believe you decided on a smaller tube though, right? I think it was roughly a BT-60 standard, about an inch and a half diameter?

No comment on the hole, other than that I think it’s pretty neat that you can get a screwdriver in there! That never would have occurred to me.

My rocket is wayyy smaller than yours.

Mine is indeed a BT-60 body tube. I will get a photo of my self with the rocket by today and forward it here.

 

[Ayush Chauhan]

I need N area in total, I size the additional holes to have an area of N/3, regardless of how large a hole I have to make for the switch.

What is the N area of ?

Edit to add: I also try to separate vent holes from direct alignment with the baraometers of my altimeters, both to avoid exposure of the baromters to direct sunlight which could throw off their readings and also - again - to discourage local pressure anomalies from being picked up by those barometers.

Ohh! I was going to put the vent holes directly above the altimeter.

Thanks for head's up tho.

But now that makes me wonder where is the barometer located in Easy Mini: Dual Deployment Altimeter. And only then I will be able to make sure that I do not put the vent holes directly above it.

 

[smstachwick]

Looking good.

First the caveat: the following responses are based on somewhat limited prior knowledge, but not on first hand experience.

Regarding "1. Using it as a vent hole, which will allow air inside the avionics bay so the altimeter can calculate the altitude", actually, it's letting air out of the avionics bay, but I guess you really know that.

What is your goal for the altimeter? If you're only interested in knowing the peak altitude then I'd bet "credits to navy beans" that one hole is fine and you've got plenty of area. Search this forum and/or the rest of the web* for a vent hole calculator, do what it says, and I'll be really surprised of you don't have enough area there. If you're looking for accurate data while it's on the way up then for reasons like uneven airflow and other aerodynamic acoustic weirdness that I don't understand, another hole or two is probably a good idea.

Why the difference? Well, when you are at or near apogee you're going slowly. The outside air pressure is changing slowly and any aerodynamic weirdness, pressure oscillation, etc. is minimal. A hole like you've got can easily keep up, i.e. let air out fast and smoothly enough. But when you're zipping along at high speed the air needs to get out of the ebay faster and the weirdness is much more likely to be a significant factor.

* When you ask Google for help on this sort of thing, you've got at least a 50:50 chance of ending up right back here anyway.

I would add that continuously accurate data is more critical for rockets with deployment events triggered by the altimeter.

 

[Ayush Chauhan]

Regarding "1. Using it as a vent hole, which will allow air inside the avionics bay so the altimeter can calculate the altitude", actually, it's letting air out of the avionics bay, but I guess you really know that.

Umm, is it because the air pressure decreases as the altitude of the rocket increases which potentially causes the air to move from a high concentration of air to a lower concentration of air? (This does make sense to my brain now that I think about it)

What is your goal for the altimeter? If you're only interested in knowing the peak altitude then I'd bet "credits to navy beans" that one hole is fine and you've got plenty of area. Search this forum and/or the rest of the web* for a vent hole calculator, do what it says, and I'll be really surprised of you don't have enough area there. If you're looking for accurate data while it's on the way up then for reasons like uneven airflow and other aerodynamic acoustic weirdness that I don't understand, another hole or two is probably a good idea.

Should I just add another 2 holes to be safe?

And in #339 @bad_idea mentioned the following:

Edit to add: I also try to separate vent holes from direct alignment with the baraometers of my altimeters, both to avoid exposure of the baromters to direct sunlight which could throw off their readings and also - again - to discourage local pressure anomalies from being picked up by those barometers.

This makes me wonder, how can sunlight affect the barometer readings?

another hole or two is probably a good idea.

Just for the sake of knowledge and a hypothetical situation, imagine I added like 10-15 holes on the eBay (hypothetical situation) and launched the rocket, would that not make the altimeter readings more accurate? Because more air is flowing out of eBay and almost instantly the pressure inside the eBay and outside the eBay become the same.

I'd say it would make the readings more accurate.

But there is always a point where you reach where adding more holes does not make a difference to the actual readings.

 

[bad_idea]

What is the N area of ?

N is a stand-in for whatever value the guide, calculator, or rule of thumb you use recommends for the total area of your vent or vents.

But now that makes me wonder where is the barometer located in Easy Mini: Dual Deployment Altimeter.

This makes me wonder, how can sunlight affect the barometer readings?

I've been referring to "barometers" in shorthand, but we're really talking about a barometric sensor that needs code in the altimeter to make sense of the sensor readings. Adrian Adamson of Featherweight Altimeters started a thread here a while back regarding the light sensitivity of barometric sensors which should help with both your questions:

Light sensitivity of barometric sensors

In his first post, he shows a photo of the most common sensor in hobby rocketry altimeters, one which is also used by Altus Metrum. If you can't identify it on your board, Altus' documentation or an appeal to their support email should help you find it.

I would add that continuously accurate data is more critical for rockets with deployment events triggered by the altimeter.

Very good point. The use of a screw switch put me in the mode of thinking about a deployment altimeter. If the altimeter is only logging - especially if it's only logging apogee - then the sizing of the hole is a lot less critical and can be adjusted by trial and error if need be.

Just for the sake of knowledge and a hypothetical situation, imagine I added like 10-15 holes on the eBay (hypothetical situation) and launched the rocket, would that not make the altimeter readings more accurate? Because more air is flowing out of eBay and almost instantly the pressure inside the eBay and outside the eBay become the same.

I'd say it would make the readings more accurate.

But there is always a point where you reach where adding more holes does not make a difference to the actual readings.

At some point I believe you would open up enough area to the outside that turbulence in the airflow along the surface of the rocket would introduce sufficient noise to degrade the readings instead of improving them. I'm not however an expert in any way and have done no experimentation on this subject.

Bearing in mind that all my thoughts on the matter are derived from other people's experience, my impression from what I've read and people I've talked to is that the idea behind using multiple vents is to keep the altimeter from being confused by a localized pressure disturbance at the site of any given vent. Many rules of thumb in rocketry are merely tribal sayings that may or may not be correct (like I'm repeating others' words and thoughts here), but actual altimeter designers typically have well-informed opinions on this subject. It's probably a good idea to consult the documentation for your altimeter and follow their recommendation as closely as is feasible for your rocket, including their recommendation as to the number of vents in addition to the total area.

 

[rcktnut]

That hole is big enough. I just measured my 4 in dia. rockets and I have a single 8mm. dia. hole. Just face the altimeter away from the hole, mounting board facing the hole. Just like bad_idea mentioned: I also try to separate vent holes from direct alignment with the baraometers of my altimeters, both to avoid exposure of the baromters to direct sunlight which could throw off their readings and also - again - to discourage local pressure anomalies from being picked up by those barometers. Edit: Keep forgetting your not even using it for DD. That hole is good either way.
Nice job on the rocket!

 

[NTP2]

View attachment 607372
This is the final CAD model of the rocket. This is made to 1 to 1 scale.

That is some serious talent in CAD!

 

[jqavins]

Umm, is it because the air pressure decreases as the altitude of the rocket increases which potentially causes the air to move from a high concentration of air to a lower concentration of air? (This does make sense to my brain now that I think about it)

Yes, exactly, and it can be stated more simply: you need free enough air flow for the inside pressure and the outside pressure to continuously be the same, within some very small tolerance. (And it's "pressure", not "concentration". "Concentration" in this context doesn't have a really specific meaning, and could reasonably be interpreted as a number of things that are not what you care about.)

Drive up the side of a mountain and the car's passenger area is leaky enough to keep the inside and outside pressures essentially the same, and a slightly cracked window is more than sufficient to keep them dead on the same for all intents and purposes. Launch a rocket through the same altitude increase in a few seconds, a really big rocket with the same internal volume as your car, and you definitely need a window wide open.

Also, drive at highway speed with one window open and you will sometimes fell a thrumming due to an acoustic effect as air rushes past the open window. At low speed that doesn't happen. At high speed you can (usually) kill the effect by opening a second window.

All of this is why I presented different answers depending on whether you want to know the apogee or track altitude on the way up. The low speed of the rocket near apogee is like tooling around town in your car at low speed, and just cracking open a window is good enough. If you want to know how it was doing the whole time, you need more total vent area, and at least two would be a good idea so you don't measure the thrumming.

How important it is to keep holes away from the sensor is beyond me. In my experience, that thrumming in the car tends to happen when someone in the back has a window down, not when I (the driver) do, and I actually kill it by opening my own window, which is analogous to a vent directly over the sensor and feeling it help. (My ears are the sensors.) You've got a good reason to have a hole directly over the board, reaching the board with a screwdriver, so I wouldn't move it away.

As I recall, your official goal is only about the altitude at apogee, and at apogee is the moment when all of this matters the least, because the speed is lowest. My advice, after all this discussion, is to put a second hole somewhere around 90° away from the first, and don't sweat the subtleties. Your apogee reading will be good, and you can see afterward how much or how little the rest mattered. Come to think of it, it's a better experiment regarding the subtleties if you don't add the second hole, and yes, your apogee reading will still be good.

 

[jqavins]

I would add that continuously accurate data is more critical for rockets with deployment events triggered by the altimeter.

I would think that for apogee detection it shouldn't matter at all, and for mains deployment, if you're still going fast enough to worry about this stuff, then your drogue is way too small.

 

[smstachwick]

I would think that for apogee detection it shouldn't matter at all, and for mains deployment, if you're still going fast enough to worry about this stuff, then your drogue is way too small.

I was thinking more in terms of breaking Mach in high-power rockets, but you’re correct.

 

[Ayush Chauhan]

That hole is big enough. I just measured my 4 in dia. rockets and I have a single 8mm. dia. hole. Just face the altimeter away from the hole, mounting board facing the hole. Just like bad_idea mentioned: I also try to separate vent holes from direct alignment with the baraometers of my altimeters, both to avoid exposure of the baromters to direct sunlight which could throw off their readings and also - again - to discourage local pressure anomalies from being picked up by those barometers. Edit: Keep forgetting your not even using it for DD. That hole is good either way.
Nice job on the rocket!

hmm... a lot of mixed opinions here.

So a simple question... should I add another 1 or 2 holes to be safe? or is just that one hole big enough?

@everyone

The dimensions of the the vent hole that I currently have are:
1.3cm in length
0.4cm in width

 

[smstachwick]

hmm... a lot of mixed opinions here.

So a simple question... should I add another 1 or 2 holes to be safe? or is just that one hole big enough?

@everyone

The dimensions of the the vent hole that I currently have are:
1.3cm in length
0.4cm in width

If you’re getting mixed results through an opinion poll, that means it’s time to send it.

Since you’re only getting an apogee reading from it and not controlling deployment, inaccurate readings should be obvious but not disastrous. It should come close to an accurate simulation file and be in the same neighborhood as any trigonometrically-derived result.

 

[bad_idea]

You'll find in rocketry that for every question there are a dozen different opinions as to the right answer. For a deployment altimeter, see what its documentation says and proceed from there. Unless the documentation said otherwise, I personally would add a pair of small holes each 1/3 of the way around the body tube from your switch hole if using the altimeter for deployment, simply based on what I've absorbed from other flyers. If you're just using the altimeter for logging, I'd fly it as you have it now and only change things if you find the logs vary suspiciously from your simulations.

 

[Ayush Chauhan]

Drive up the side of a mountain and the car's passenger area is leaky enough to keep the inside and outside pressures essentially the same, and a slightly cracked window is more than sufficient to keep them dead on the same for all intents and purposes. Launch a rocket through the same altitude increase in a few seconds, a really big rocket with the same internal volume as your car, and you definitely need a window wide open.

Also, drive at highway speed with one window open and you will sometimes fell a thrumming due to an acoustic effect as air rushes past the open window. At low speed that doesn't happen. At high speed you can (usually) kill the effect by opening a second window.

All of this is why I presented different answers depending on whether you want to know the apogee or track altitude on the way up. The low speed of the rocket near apogee is like tooling around town in your car at low speed, and just cracking open a window is good enough. If you want to know how it was doing the whole time, you need more total vent area, and at least two would be a good idea so you don't measure the thrumming.

Man that was a beautiful explanation.

Makes sense now! I want the altitude to be tracked all the way up, just to see if I will be able to do anything else with the data after the rocket launch like write a python program to make a 3D trajectory of the rocket or something (not sure if I can do that yet with the information the Easy mini altimeter provides) But now I know I need to put another 2 vent holes in the rocket.

How important it is to keep holes away from the sensor is beyond me. In my experience, that thrumming in the car tends to happen when someone in the back has a window down, not when I (the driver) do, and I actually kill it by opening my own window, which is analogous to a vent directly over the sensor and feeling it help. (My ears are the sensors.) You've got a good reason to have a hole directly over the board, reaching the board with a screwdriver, so I wouldn't move it away.

This is how the avionics bay will go. So the vent hole that I made will be above the battery and not the altimeter.

I was thinking of adding two holes of diameter 5mm (is 5mm diameter plenty?) directly above the altimeter as shown below:-

This is what my plan is.

As I recall, your official goal is only about the altitude at apogee, and at apogee is the moment when all of this matters the least, because the speed is lowest. My advice, after all this discussion, is to put a second hole somewhere around 90° away from the first, and don't sweat the subtleties. Your apogee reading will be good, and you can see afterward how much or how little the rest mattered. Come to think of it, it's a better experiment regarding the subtleties if you don't add the second hole, and yes, your apogee reading will still be good.

I mean I could try that. But if I will be able to do something with the data of altitude at every point etc.. I might be able to do something with the data afterwords like make a trajectory of the rocket in python or something as mentioned early on.

 

[Ayush Chauhan]

get yourself in the photo if at all possible

Here it is.

The rocket is about 49 inches in height.

 

[smstachwick]

View attachment 613191

Here it is.

The rocket is about 49 inches in height.

Oooo I like it! Looks like it’ll be a bit of a hod rod on G power, you’ll hit 1500ft no problem.

 

[Ayush Chauhan]

I found the barometric pressure sensor. I belive this is it.

This is located at the bottom of the chip anyway because I have the following orientation of the chip:

Now does it matter if I place the vent holes directly above the altimeter because the pressure senor is on the other side of the chip anyways.

In my opinon I should be able to to do this.

 

[Ayush Chauhan]

Oooo I like it! Looks like it’ll be a bit of a hod rod on G power, you’ll hit 1500ft no problem.

but it has a 24mm motor mount and I believe G motors require a 29mm motor mount.


Edit:


Nope I was totally wrong. I am going to be using a G-class in that case.

 

[smstachwick]

but it has a 24mm motor mount and I believe G motors require a 29mm motor mount.

I see. F motors shouldn’t be that hard to come by in that size, there are a bunch in both single-use and reload form. An AeroTech F32T single-use might be simplest.

CTI manufactures 24mm G reloads motors, but some are designated as high-power, some propellants are not currently being manufactured, and all are subject to pretty long wait times now. I’m not sure if they’re even going to the UK at this time. But if you’re doing that maybe look at the CTI G65.

 

[bad_idea]

I found the barometric pressure sensor. I belive this is it.

This is located at the bottom of the chip anyway because I have the following orientation of the chip:

Now does it matter if I place the vent holes directly above the altimeter because the pressure senor is on the other side of the chip anyways.

In my opinon I should be able to to do this.

Yes, that is it, and you're correct, direct light won't be a problem for you in this case.