Recently I built a model that called for 5ozs. to be added to the nose. That put the distance between the Center of Gravity and the Center of Pressure. I think I understand that all models are different but what equation is used to figure this out.?? Do you like a close distance between the 2, CP/CG or a longer distance.?? Is there a golden rule other than the CG needing to be forward of the CP.??
Golden Rule for CP/CG
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This explains the basics:
Newsletter462.pdf (apogeerockets.com)
The distance between CP and CG you are referring to is called the Stability Margin in this article.
Most of us call it calibers of stability.
I hesitate to state a golden rule because that will depend on the rocket design.
i.e. there will always be exceptions.
But generally I try for 1.5 or more.
Newsletter462.pdf (apogeerockets.com)
The distance between CP and CG you are referring to is called the Stability Margin in this article.
Most of us call it calibers of stability.
I hesitate to state a golden rule because that will depend on the rocket design.
i.e. there will always be exceptions.
But generally I try for 1.5 or more.
PS a software program like Open Rocket or Rocksim will calculate CP, CG, and calibers of stability for you with different amounts of nose weight and different motors.
Open Rocket is free.
Open Rocket is free.
Thanks for the reply. Its a good one.
I've asked this because on the Rising Star I recently built the CP is at 33". I put an E30-7 in it and the CG was at 18". In other words, there is a large distance between the 2 numbers. Does it sometimes happen this way.?? This model is 39" in length and weighs 7.4ozs.
I've asked this because on the Rising Star I recently built the CP is at 33". I put an E30-7 in it and the CG was at 18". In other words, there is a large distance between the 2 numbers. Does it sometimes happen this way.?? This model is 39" in length and weighs 7.4ozs.
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I've downloaded both.... OpenRocket and Rocsim. My computer doesn't like either one. Tells me I have to switch out of 'S' mode. I'm going to have Apogee walk me through it Monday. We'll see how that goes. I think I can get there, just don't know how at the moment.
I think kits tend to have a larger distance betweeen the two so that they can handle a number of motors at varying weight and to compensate for heavy building on the fin can but that’s uneducated speculation. If they design a kit to handle an E with a margin of 1 then the rocket could be unstable if you put an F or G in it. If that makes sense.
An overstable rocket is not necessarily bad. But be cautious on breezy days, it might windcock.
"S" mode in Windows provides security exclusively for apps from the Microsoft Store. If you want to run other apps you need to turn off "S" mode. However if you turn it off you cannot go back. So a decision has to be made. I would wait until Monday and have Apogee walk you through it.
"S" mode in Windows provides security exclusively for apps from the Microsoft Store. If you want to run other apps you need to turn off "S" mode. However if you turn it off you cannot go back. So a decision has to be made. I would wait until Monday and have Apogee walk you through it.
Perfect stability is one caliber, i.e., the distance between the points is the same as the diameter of the booster. If the CG is too far aft, you get a spinning, skywriting rocket. Too far forward, and you get weathercocking. Weathercocking can be alleviated by more thrust, but nothing can fix instability.
If you set a long block on end on a table, and tipped it a tiny bit, its CG would pull it back to upright. You would have to tip it more and more to get it to eventually give in and fall over. This is what CG does for a rocket. The CP will try to spin the rocket, because the surface area of the fins is larger than the nose cone. As air tries to spin the rocket, the CG tips the rocket back to a straight trajectory.
How the exact measurement of one caliber of stability was arrived it, I don't know. But it works.
If you set a long block on end on a table, and tipped it a tiny bit, its CG would pull it back to upright. You would have to tip it more and more to get it to eventually give in and fall over. This is what CG does for a rocket. The CP will try to spin the rocket, because the surface area of the fins is larger than the nose cone. As air tries to spin the rocket, the CG tips the rocket back to a straight trajectory.
How the exact measurement of one caliber of stability was arrived it, I don't know. But it works.
Did you already put weight in the nosecone?
Here's the RockSim configuration with that motor;
Also, it's advisable to check the actual CG to determine your static margin.
Shows a straight flight profile with moderate winds. May weathervane with stronger winds.
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Good info all around on CG to CP stats.
To add to what kuririn said here:
Switching from S mode can have some huge drawbacks. Windows will say it's security related but in my experience this isn't really the case.
Unfortunately a lot of 'S' computers have tiny hard drives, they can be as small as 32gb. If you switch out of S mode with one of these tiny hard drives your computer will try and download huge updates, not finish the installation and then those files just sit there taking up space as the updates stick as 'pending'. This can slow your computer to a crawl or just not allow anything else to be installed anyway due to a full drive.
If your computer has a 64gb drive or bigger you will likely be fine, as long as it is also relatively empty.
Cautionary tale from years in IT, I'd hate to see you make your computer unusable because of upgrading. Apogee may have a workaround for their software and render this unneeded. If you have questions though there's usually enough of us computer nerds here to help.
To add to what kuririn said here:
Switching from S mode can have some huge drawbacks. Windows will say it's security related but in my experience this isn't really the case.
Unfortunately a lot of 'S' computers have tiny hard drives, they can be as small as 32gb. If you switch out of S mode with one of these tiny hard drives your computer will try and download huge updates, not finish the installation and then those files just sit there taking up space as the updates stick as 'pending'. This can slow your computer to a crawl or just not allow anything else to be installed anyway due to a full drive.
If your computer has a 64gb drive or bigger you will likely be fine, as long as it is also relatively empty.
Cautionary tale from years in IT, I'd hate to see you make your computer unusable because of upgrading. Apogee may have a workaround for their software and render this unneeded. If you have questions though there's usually enough of us computer nerds here to help.
Thanks SkyFire.!! I gave the wrong number in my original post. My mistake. With the E30 installed I am right at 23.75" on the CG. So I'm right in the ballpark according to the diagram you posted. I'm thrilled with that.!! The biggest motor I'll fly will most likely be an F32-8T. Apogee shows about 1900ft. with that motor.
I can't wait to get going with a sim. So valuable.
I can't wait to get going with a sim. So valuable.
Thanks a bunch mooffle. I think I ran into an option to download a free 'switch.' I nearly clicked it but caution took over. I'd rather put $600 into rocket motors than that amount into a new computer. So thanks a lot for your reply.
Are you familiar with such an option.??
Are you familiar with such an option.??
It isn't that it's in any way perfect, it's a good rule of thumb for a "normal" rocket. Something short and stubby might be able to get away with less, something like a superroc might need more. I tend to like a bit more (like 2 or so), but don't have any empirical data backing me up.
Good points. Short, fat rockets can get away with far less margin due to the base drag. Supersonic flights experience a sudden shift in CP at the Mach transition, and a 2:1 margin is recommended for those flights.
you can buy a modern computer with a 32GB hard drive? Wow. That’s incredible. i think my last computer with a drive that small was in 2004.
s mode is only for programs thru ms store just like apple store. yes get out of it. go to ms store and search s mode. I just did it for my new laptop otherwise you can not just download any sw.
let me add the only reason ms states s mode is more secure is that the vendor or avendor has tested the sw with windows and used any design constraints in coding the sw. Unless you have a very small hardrive, 256 gig is my minimum. if you run s mode you might as well but an Apple . The argument for Windows is you can download and use any SW including all the freeware out there. See if RocSim is available thru MS store
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It comes down to almost predatory marketing. Someone at Microsoft decided these are the minimum specs for windows 10:
- Processor: 1 gigahertz (GHz) or faster processor or SoC
- RAM: 1 gigabyte (GB) for 32-bit or 2 GB for 64-bit
- Hard disk space: 16 GB for 32-bit OS or 20 GB for 64-bit OS
- Graphics card: DirectX 9 or later with WDDM 1.0 driver
- Display: 800 x 600
These are absolute bare minimum. But someone who doesn't know anything about computers walks into a best buy they can buy a new laptop for $300 and unfortunately they've pretty much been swindled.
Those specs really are from 2002-2005 era.
Many of these machines then have S on them so updates don't have to happen, because they literally can't happen without adding a flash drive to the computer just so it has space to download them.
OTH the windows surface book was shipping with windows 10 S and it actually sports pretty good hardware. I have no idea why it comes with S, besides the security thing
Further reading . . .
Dave F.
Dave F.
As far as I've been able to tell, the 1 caliber stability rule doesn't have any firm theoretical basis. It's just a rule someone made up that happens to work OK for some kinds of rockets.
If you're looking for a simple rule, 10% of rocket length is actually better than 1 caliber. It doesn't break down as badly for long or short rockets, and it has a clearer meaning for rockets with complicated shapes.
If you're looking for a simple rule, 10% of rocket length is actually better than 1 caliber. It doesn't break down as badly for long or short rockets, and it has a clearer meaning for rockets with complicated shapes.
If you all could help me out I'd be forever grateful.!! I am very green on computer stuff. In fact, I hate it. That said, I should open my mind up to take advantage of a computer and what it offers. In other words, I need to learn the benefits of using a computer in ways I never have before.
Does this help.....
Ideapad S145
Running MicroSoft Edge.
Ryzen 3
Radeon Vega Graphics
Network and Discovery Sharing is turned off.
It is worth the effort.
For now, we need to know disk size and DRAM quantity.
To get disk size:
1) Open a Windows Explorer window (yellow folder icon in your task bar.)
2) On the left, scroll down until you find OS (C
.3) Right click on that, and select "Properties".
4) Tell us what it says for Used Space, Free Space, and Capacity.
For the rest:
1) Open Settings (Start Menu -> Gear icon in lower left)
2) In the settings window, select "System", in the upper left
3) On the left, go all the way to the bottom (might need to scroll) and click "About"
4) Under "Device Specifications", tell us what it says next to "Processor" and also "Installed RAM".
Thanks, that's the stuff. Your machine is totally satisfactory to run normal Windows.
In the future maybe we'll talk about taking screenshots.
In the future maybe we'll talk about taking screenshots.
Okay Neil..... Thank you. So where do I go from here.?? What are you suggesting.??
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The pop up window that scared me was the one that said S-Mode is safer, faster yada yada yada...... Will I maintain 'fast' and 'safe' if I switch out of S-Mode.?? Should I exit MicroSoft Edge and go to a Windows operating system.?? Once again, I'm green. Don't really know what I'm doing. I can surf for hours BUT...... lol.
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The pop up window that scared me was the one that said S-Mode is safer, faster yada yada yada...... Will I maintain 'fast' and 'safe' if I switch out of S-Mode.?? Should I exit MicroSoft Edge and go to a Windows operating system.?? Once again, I'm green. Don't really know what I'm doing. I can surf for hours BUT...... lol.
Having not actually gone through this process before, I'm going to hang back for a bit to see if anyone else with more direct knowledge of this can chime in.
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Here's a how to directly from Microsoft. It's not super detailed so PM or reply here for some more help. Hopefully that's a good start.
If you decide to switch this how to makes it sound like you hit the button and you're done. However there are probably updates it will have to do that may take quite some time.
If you decide to switch this how to makes it sound like you hit the button and you're done. However there are probably updates it will have to do that may take quite some time.
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