Reducing Orbital "tumble"

[tblaxland]

I don't think the ISS does this, though, because it uses gyros to store angular momentum, which from gravity gradient should average to zero over an orbit, someone correct me if I'm wrong. In Orbiter, as far as I know, there is no way to model gyroscopes yet, at least not without a way-cool custom dll.

I'm not quite sure what you mean by "average out". If your vessel starts out with zero angular momentum, it will have a small non-zero angular momentum after one orbit, so they don't average out in that way. This is because over the course of the orbit a torque will be generated on the vessel as the axis with the highest PMI rotates away from the local horizontal plane.

The ISS also has to deal with some aerodynamic forces from the thermosphere. This is why it flies in the torque equilibrium attitude I mentioned earlier so that the aerodynamic and gravity gradient forces almost cancel out, so the CMGs don't have too much work to do, even over the course of a single orbit.

And yeah, am I currently wading through a document titled "A Geometric Study of Single Gimbal Control Moment Gyros - Singularity Problems and Steering Law" as I plan to shortly start work on a CMG solution for ISS Ultra. This document is mostly about single gimbal CMGs but it also contains some useful info on dual gimbal CMGs as used on the ISS. If anyone can point me in the direction of some reference info on dual gimbal CMGs, particularly their steering law, it would be most appreciated.

And no matter how finessed, your gonna need to killrot sometimes no matter what as you suggested.

Just to clarify, you shouldn't be aiming to "killrot", ie, kill all rotation, since that will be too unstable. What you should be aiming for is to tidally lock the rotation of your station to the earth. In other words, based on the PMIs you posted earlier, you want to have the Z-axis always pointing parallel to the R-vector. This means you should have the Y axis perpendicular to the orbit plane and hence a rotation around that axis of 360/(orbit period) °/s.

 

[Andy44]

Your rotation rate about the earth in LEO is about 0.06 deg/sec, so if you open up Attitude MFD and very carefully pulse your thrusters until your rotation matches that, you should keep one side of your spacecraft pointed at the earth the entire orbit. If that axis is the one with the smallest PMI, you are in a gravity gradient attitude, although you will still rock back and forth a bit. And in time accel, you will eventually rock too much and have to adjust.

Example: while flying the space shuttle orbiter, place yourself in an attitude with the tail pointing at earth center and the right wingtip pointing prograde. Then kick the yaw pedals slightly to the right until your yaw rate is 0.06 deg/sec to starboard. Now your orbiter is roughly gravity gradient stabilized.

 

[XonE32]

Good discussion guys, I love listening to people smarter than myself on subjects I know nothing about... and no that wasn't sarcasm.

Just to clarify, you shouldn't be aiming to "killrot", ie, kill all rotation, since that will be too unstable.

Agreed Tblax. I usually only want to killrot when I come back to the station from a flight and wish to dock (knowing I'll have to re-orient the station properly afterward). You can imagine a flight to Jupiter and back or some such, by the time you return my station's will be all over the place and not condusive to docking with larger vessels like the Deepstar.

Your rotation rate about the earth in LEO is about 0.06 deg/sec, so if you open up Attitude MFD and very carefully pulse your thrusters until your rotation matches that, you should keep one side of your spacecraft pointed at the earth the entire orbit. If that axis is the one with the smallest PMI, you are in a gravity gradient attitude, although you will still rock back and forth a bit.

I think I've done that, but I did not know about the Attitude MFD or the .06deg/sec. I will attempt to finesse it a bit more.

With regard to Orbiter's vessel .cfg's and .ini's: Using the example of the Airdock2 above [PMI=(58.2,62.1,7.7)], If I change the mass by adding more propellent as a resource for refueling (e.g. increase the Fuel Mass line from the original 20000 to say 1,000,000). The mass of the ship would change from:
empty weight=40000 + fuel Mass=20000 = 60,000kg to 40000 + 1,000,000 = 1,040,000kg.

Would I not have to change the PMI values to represent the increased mass?

As mentioned above, I've never created my own vessel, but I've tinkered with other's add-ons, I'm just curious that perhaps my tikering has made it impossible to achieve a proper gravity gradient orbit that's somewaht stable.

Thanks again for all your responses. I will go and grab the Attitude MFD if still avialble and see If I can finesse the orientation a bit more.

Regards

XonE 32

 

[Andy44]

The question about the propellant mass is a good one. I can't say for sure, but I think that simple .cfg vessels do not model changing PMI with changing propellant masses. It treats the propellant mass as a point mass at the center of the vessel, as far as I know, and does not recalculate PMIs on the fly. Perhaps one of the more high-speed add-on developers will know.

 

[XonE32]

The question about the propellant mass is a good one. I can't say for sure, but I think that simple .cfg vessels do not model changing PMI with changing propellant masses. It treats the propellant mass as a point mass at the center of the vessel, as far as I know, and does not recalculate PMIs on the fly. Perhaps one of the more high-speed add-on developers will know.

Thanks Andy. I think you're right. I actually looked (for the first time, so colour me embarassed) at the .PDFs in the Orbiter SDK and I think the answer is that I maybe should adjust em for realism sake, but don't have to because your assessment is correct. At least I hope that's right 'cause I couldn't make heads nor tail of the shipedit utility lol.

Regards

XonE 32

 

[tblaxland]

I think I've done that, but I did not know about the Attitude MFD or the .06deg/sec. I will attempt to finesse it a bit more.

The 0.06°/s comes from completing one full rotation (360°) over the course of one orbit. So for LEO your orbit period will be about 5400s: 360/5400 = 0.067°/s.

Thanks again for all your responses. I will go and grab the Attitude MFD if still avialble and see If I can finesse the orientation a bit more.

In case you haven't found it yet:
[ame="http://www.orbithangar.com/searchid.php?ID=3165"]Attitude MFD V3.2[/ame]

The question about the propellant mass is a good one. I can't say for sure, but I think that simple .cfg vessels do not model changing PMI with changing propellant masses. It treats the propellant mass as a point mass at the center of the vessel, as far as I know, and does not recalculate PMIs on the fly. Perhaps one of the more high-speed add-on developers will know.

The PMIs in the config file (and DLLs) for Orbiter are mass-normalised so Orbiter calculates the actual PMIs by multiplying them by the total vessel mass. As you add fuel to the vessel, its mass increases and therefore so does the PMIs. In this sense, fuel mass in Orbiter can be considered to be distributed in the same way as the dry mass, which can be a little unrealistic. For example, if you had a vessel with a large fuel tank near the centre of mass, adding fuel would not increase the PMIs as much as Orbiter would assume. This effect can be coded out in a DLL but I don't think most developer's worry about it. My Ananke code has dynamic centre of mass and PMI code in it but its purpose is to do deal correctly with the tether reeling. I haven't bothered about the fuel because it is a pretty small proportion of the overall mass in that case.

 

[XonE32]

The 0.06°/s comes from completing one full rotation (360°) over the course of one orbit. So for LEO your orbit period will be about 5400s: 360/5400 = 0.067°/s.

Aaah, that's interesting and a little embarrassing (that I don't know it lol). It's interesting in of itself, but also because my orbit for this station is a bit higher so I'll have to check the T interval and do the adjustment. Ta for that!


In case you haven't found it yet:
http://www.orbithangar.com/searchid.php?ID=3165

Got it installed and was just about to use it when I forgot what was said above (D'oh), so I had to come out of Orbiter before making the adjustments. Glad I did too. And thanks for making the MFD BTW. From what I've read it gets high praise.


The PMIs in the config file (and DLLs) for Orbiter are mass-normalised so Orbiter calculates the actual PMIs by multiplying them by the total vessel mass. As you add fuel to the vessel, its mass increases and therefore so does the PMIs. In this sense, fuel mass in Orbiter can be considered to be distributed in the same way as the dry mass, which can be a little unrealistic. For example, if you had a vessel with a large fuel tank near the centre of mass, adding fuel would not increase the PMIs as much as Orbiter would assume. This effect can be coded out in a DLL but I don't think most developer's worry about it. My Ananke code has dynamic centre of mass and PMI code in it but its purpose is to do deal correctly with the tether reeling. I haven't bothered about the fuel because it is a pretty small proportion of the overall mass in that case.

Good to know, I'd hate to go buggering around with Greg's PMI's.

Thanks Tblaxland

Regards

XonE 32