Roll & Attitude control

Discussions on high/extreme altitude and mach busting rockets.

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Re: Roll & Attitude control

Postby kir » Wed Aug 26, 2015 8:58 am

Then, if we go back to your first green/red chart in this thread, it seems that a 12.5kg.cm torque servo can do the job: high angles at low speeds and few degrees at the 1M+. The controller can limit the range depending on speed (or torque/current). There are a few large servos around that can do about 25kg.cm

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Re: Roll & Attitude control

Postby Lister » Wed Aug 26, 2015 12:30 pm

I may be wrong but dont larger servos have major lag issues?

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Re: Roll & Attitude control

Postby OverTheTop » Wed Aug 26, 2015 12:43 pm

I may be wrong but dont larger servos have major lag issues?

Yes and no. If you want the same size servo with larger torque, the penalty is usually slew rate. If you can put up with a larger servo, anything is possible. Larger = higher power. Power = rate of doing work. Higher maneuverability requires more torque to work.

Servo power is one of the major design factors of missiles, and achieving sufficient performance within weight, power and size constraints is always a compromise. Bigger control systems mean smaller warheads, but slow-maneuvering missiles are easy pickings or easily dodged.

I am glad I don't have to design weapons for a living.
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Re: Roll & Attitude control

Postby cryoscum » Wed Aug 26, 2015 7:21 pm

OverTheTop wrote:
I am glad I don't have to design weapons for a living.

You and me both, mate!!
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Re: Roll & Attitude control

Postby High Impulse » Wed Aug 26, 2015 8:27 pm

OverTheTop wrote:
Quote:
I am glad I don't have to design weapons for a living.

You and me both, mate!!


well i guess the only upside would you don't have to worry about using yourisbetterthanminium for the body ;)
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Re: Roll & Attitude control

Postby Cloudbreaker » Wed Aug 26, 2015 11:03 pm

cryoscum wrote:
OverTheTop wrote:
I am glad I don't have to design weapons for a living.

You and me both, mate!!


Hey cryoscum and OTT, you both build some impressive rockets.
Just wondering what is worse... designing them or building them???
Feel free to crap on as long as you want... because I enjoy it.
I excel at equations...I can build structures that have no relationship with reality.

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Re: Roll & Attitude control

Postby OverTheTop » Thu Aug 27, 2015 7:42 am

Just wondering what is worse... designing them or building them???


There is nothing bad about building rockets at all! Probably the only annoying thing is I don't do enough of it due to lack of time :D

I enjoy the mental process of designing, thinking through any ideas and considering repercussions (positive or negative) along the way. Most rockets for me are not quick, throw together, units. The are usually long projects, taking my time to enjoy and think along the way.

One way to think more independently is to work things out yourself before you ask too many other people about your ideas. If someone tells me that something will not work, I consider their opinion, and then think through my design anyway. The number of times I have been told things won't work (from an opinion, rather than a fact base) and then been successful is not insignificant.

As for the building, that has its own challenges with how strong to build without building a concrete donkey, where to put the flight forces. Jigging up to keep things repeatable is one of those sort of problems to consider carefully also.

As for electronics, I do that at work (designing spectrometers) so I just buy other people's gear for the rockets. Gives me more time to build.
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Re: Roll & Attitude control

Postby SpaceManMat » Thu Aug 27, 2015 9:08 am

OverTheTop wrote:
Just wondering what is worse... designing them or building them???


There is nothing bad about building rockets at all! Probably the only annoying thing is I don't do enough of it due to lack of time :D

I enjoy the mental process of designing, thinking through any ideas and considering repercussions (positive or negative) along the way. Most rockets for me are not quick, throw together, units. The are usually long projects, taking my time to enjoy and think along the way.

One way to think more independently is to work things out yourself before you ask too many other people about your ideas. If someone tells me that something will not work, I consider their opinion, and then think through my design anyway. The number of times I have been told things won't work (from an opinion, rather than a fact base) and then been successful is not insignificant.

As for the building, that has its own challenges with how strong to build without building a concrete donkey, where to put the flight forces. Jigging up to keep things repeatable is one of those sort of problems to consider carefully also.

As for electronics, I do that at work (designing spectrometers) so I just buy other people's gear for the rockets. Gives me more time to build.


Total with you there OTT. Even when things go wrong it's usually very exciting. Afterwards you get to learn a whole bunch of stuff about what works and what doesn't.

You have to be willing to fail, pick up the bits and continue. Putting a man on the moon first went to the USA because they were the most determined to do it.
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Re: Roll & Attitude control

Postby citizenspace » Sat Aug 29, 2015 11:13 am

SpaceManMat wrote:You have to be willing to fail, pick up the bits and continue. Putting a man on the moon first went to the USA because they were the most determined to do it.


That and the fact that the Soviets decided 30 engines on the bottom of a seriously large rocket was a good idea :P
The best way to do something well is to make sure you've tried all the ways of making it fail.
NASA does it all the time with simulators :D
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Re: Roll & Attitude control

Postby kir » Mon Aug 31, 2015 4:55 pm

Sorry guys, back to the subject :)

There is a method to reduce torque of plane control surfaces by shifting the axis below the centre of pressure (CoP). Initially, the "negative" torque will be increasing with the speed. But close to M the CoP will start moving down and this will bring the torque back to zero or further to "positive" range.

Looking at Nic's drawings, I can conclude that the CoP is far below the axis. I don't have the equations on hands, but perhaps Nic you can test what the green/red table will be if you shift the axis.

Look at this Russian missile's canards - they seem to use exactly this method:
http://vpk.name/file/img/9m96_140914_1.jpg

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Re: Roll & Attitude control

Postby OverTheTop » Mon Aug 31, 2015 5:52 pm

Nic's fin planform is a little different to mine. I have gone with the double-wedge airfoil because the Cp migration is a bit more predictable. I am not expecting the speeds that Nic is either (in the early days anyway :wink: ) so my moments are a lot less anyway. That's why I can get away with direct drive.

I am taking a bit of a hit in drag also, going for fins that are about 10% thickness ratio. The main reason for that is that I will be 3D printing them and I think 5% (or less, my preferred choice) would be too weak.

You can see from Nic's spreadsheet that it is very easy to exceed the torque of the servo at higher speeds, even with the Cp at near optimum compromise location.

It all comes down to the design of the fin, location on the rocket (the above sims are for clean air), the drive torque available and the flight envelope being designed for.
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Re: Roll & Attitude control

Postby kir » Sat Sep 05, 2015 6:57 pm

The main reason for that is that I will be 3D printing them and I think 5% (or less, my preferred choice) would be too weak.

You could cut the fin's contour from thin carbon-fiber and then glue the halves of your printed 3D airfoil on it.

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Re: Roll & Attitude control

Postby jase » Mon Feb 29, 2016 9:55 am

Didn't know where else to pop this:



8)
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Re: Roll & Attitude control

Postby Lister » Mon Feb 29, 2016 10:02 am

Wow, thats awesome.. do you know how they did it?


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