RInc correction during ascent.

[Tim13]

Two questions:

--Is it possible to use the RCS in translation mode during the ascent to fine tune the RInc when launching to the ISS? I've been able to get the RInc down to about 0.29 degrees by adjusting the launch inclination in the mission file, but suspect in real ops, someone at MCC would have been fired for that large of a RInc...LOL.

--If it is possible to translate during ascent to fine tune the RInc, what is the proper way to do it, without upsetting the autopilot? How should the DAP(I'm assuming DAP manipulation would be required...) be configured? Would the RCS yaw have to be switched from "auto" to 'css?"

Thanks,

Tim

 

[GLS]

Two questions:

--Is it possible to use the RCS in translation mode during the ascent to fine tune the RInc when launching to the ISS? I've been able to get the RInc down to about 0.29 degrees by adjusting the launch inclination in the mission file, but suspect in real ops, someone at MCC would have been fired for that large of a RInc...LOL.

--If it is possible to translate during ascent to fine tune the RInc, what is the proper way to do it, without upsetting the autopilot? How should the DAP(I'm assuming DAP manipulation would be required...) be configured? Would the RCS yaw have to be switched from "auto" to 'css?"

Thanks,

Tim

The RInc shown in the AlignPlane MFD results from differences in the inclination of the orbital planes and in the difference in the LAN of the orbital planes.

Currently, SSU only targets a specific orbital inclination and doesn't bother with the LAN value. I'd like to work this, but getting SSU to a state where a version can be released for Orbiter 2016 to has priority. :shrug:

Anyway, to get a small error you have 2 options:
1) launch at the "in-plane time", and let it do the ascent in AUTO;
2) launch when it suits you, and go CSS (you have to control all 3 axis... that's the way it is in ascent) when you get into the second-stage and point it in the direction you want.

You can also null the error as you approach the ISS.

 

[Tim13]

I don't want to do a full manual launch, so option two is out for me.

For option one then, launching at "in plane time" means when the ISS passes directly over the launch site? Any deviation east or west during the ISS pass over the launch site, will result in a RInc error, with the error being greater the farther east or west the pass is?

Tim

 

[GLS]

As it takes 8 minutes for the vehicle to reach orbit, launching when the ISS plane is a bit east of the pad is advisable.

 

[Tim13]

As it takes 8 minutes for the vehicle to reach orbit, launching when the ISS plane is a bit east of the pad is advisable.

:facepalm: I never thought of that. That makes sense. Thanks for pointing out what should have been very obvious to me!

Tim

 

[Tim13]

The RInc shown in the AlignPlane MFD results from differences in the inclination of the orbital planes and in the difference in the LAN of the orbital planes.

Currently, SSU only targets a specific orbital inclination and doesn't bother with the LAN value. I'd like to work this, but getting SSU to a state where a version can be released for Orbiter 2016 to has priority. :shrug:

Anyway, to get a small error you have 2 options:
1) launch at the "in-plane time", and let it do the ascent in AUTO;
2) launch when it suits you, and go CSS (you have to control all 3 axis... that's the way it is in ascent) when you get into the second-stage and point it in the direction you want.

You can also null the error as you approach the ISS.

I know this is an oooooold thread, but I thought it better to ask my question, with this back history immediately readable.

Is there an easy way to determine the "in-plane time?" :beathead:

To date, I can only get an autopilot launch with a RInc of 0.31degrees, which of course is huge.

Tim

 

[Thorsten]

--Is it possible to use the RCS in translation mode during the ascent to fine tune the RInc when launching to the ISS?

Given the rather substantial propellant needs of changes of the orbital plane, I suspect using the RCS is two orders of magnitude too weak for anything meaningful.

Is there an easy way to determine the "in-plane time?"

* Make a launch at one specific time into the desired inclination. Get pen and paper and write down the longitude of the ascending node you end up with after MECO.

* Repeat the above step at a different time.

* Do a simple linear interpolation/extrapolation to determine the relationship between launch time and longitude of the ascending node

* Use that relationship to compute the two times during the day you will end up in the right orbital plane

-> That's the planar launch window.

Now, dependent on what time you have, you also want to end up in a position just behind ISS such that a sane phasing program brings you into a rendezvous condition - that's the phasing launch window - basically you want ISS to 'just' pass over you so that you end up 'slightly behind'

You can launch whenever the two overlap... (which is pretty rare). Dependent on your planned propellant consumption, you can possibly use the OMS to null up to 0.5 degrees or so if rel inclination afterwards.

(Even if the AP could target the full orbital plane rather than just inclination, it still would not help you much because at most you're able to get a degree or so extra by flying a dog-leg, so all you really do is widen the launch window, but even then it's still pretty narrow - there's no way a Shuttle can launch 'when it suits you' and make a rendezvous).

 

[Gingin]

Thorsten sum it up well.

@Tim: To fine tune the Launch Window along the SSU scenario, open the Align Plane MFD once on the PAD.
What works well I found it is to launch around 300 seconds before the next node ( descending one for us to have a North Eastward launch azimuth)

Note the MJD when you are 300 seconds from the next node (2:30 mn before the In Plane Time), then edit the Launch MJD in the scenario with that one.

Even without LAN guidance, I almost always end up at less than 0.1 degrees of RInc after MECO

 

[Tim13]

Oh, for pete's sake. I haven't flown the old shuttle fleet in version 2010P in so long, that I completely forgot about the whole 300 seconds before a node trick.

Thanks for that reminder. I can't believe I wasted so much time yesterday on this....LOL.

Tim

 

[GLS]

(Even if the AP could target the full orbital plane rather than just inclination, it still would not help you much because at most you're able to get a degree or so extra by flying a dog-leg, so all you really do is widen the launch window, but even then it's still pretty narrow - there's no way a Shuttle can launch 'when it suits you' and make a rendezvous).

This.
In the future I hope to make it go after a target orbital plane, like the real one did, but even with that the vehicle only has a 10 minute window for a 51.6º orbit.

 

[Tim13]

I think my words may have been slightly misinterpreted. I wasn't trying to launch at my convenience, and get on plane. I know that the launch window is narrow just to have a chance of being on plane, and that virtually every launch time in the window will result in being slightly off plane without some sort of correction on ascent. Also, most of the launch windows were not usable once a day 3, late morning post meal, with the sun in the correct position criteria was factored in.

One simulator question: I copied one of the included missions, and am using it to learn with. I noticed that the targeted inclination in the mission is 51.4º, yet GLS says the launch should be for a 51.6º inclination. I don't think I ever altered the targeted inclination in the mission file. Which is correct?

Lastly, a real world question if someone knows off the top of their head. On a typical mission to ISS, after MECO, what was the shuttles RInc with regards to ISS? I'm assuming it was only hundredths or even thousandths of a degree off, but really don't know.

Tim

 

[GLS]

I think my words may have been slightly misinterpreted. I wasn't trying to launch at my convenience, and get on plane. I know that the launch window is narrow just to have a chance of being on plane, and that virtually every launch time in the window will result in being slightly off plane without some sort of correction on ascent. Also, most of the launch windows were not usable once a day 3, late morning post meal, with the sun in the correct position criteria was factored in.

That is an issue of the current ascent guidance: it only targets inclination, and LAN depends only on launch time. The result is that there is exactly one launch time to match the ISS orbit. The real shuttle "went after" the orbital plane of its target, thus allowing some margin in the launch time.

One simulator question: I copied one of the included missions, and am using it to learn with. I noticed that the targeted inclination in the mission is 51.4º, yet GLS says the launch should be for a 51.6º inclination. I don't think I ever altered the targeted inclination in the mission file. Which is correct?

51.6º is the "mean" ISS inclination, but it varies with time. The shuttle should launch to whatever inclination the ISS is at the time.

Lastly, a real world question if someone knows off the top of their head. On a typical mission to ISS, after MECO, what was the shuttles RInc with regards to ISS? I'm assuming it was only hundredths or even thousandths of a degree off, but really don't know.

I have no idea. You could search the first TLEs for a shuttle mission and compare with the ISS.

 

[Gingin]

@Tim: In reality, looking at the different Plane change burn that rarely go above 50 ft/ish,
I would say for sure Rinc of less than 0.1 degrees, most probably around 0.05 ish ° ( easily doable in Orbiter)

 

[GLS]

BTW: the launch time can be delayed by inserting a hold in the final countdown via the LCC MFD in the LCC vessel. Currently there is no way to launch earlier than the time specified in the scenario file.

 

[Tim13]

Update. So, after slapping my forehead for not remembering the 300 second rule of thumb, and then slapping my forehead again, for not remembering that 300 seconds of real time, does not equal 300 seconds Tn in the align planes MFD.....LOL, I was able to set up a scenario that resulted in a RInc of 0.7 degrees. Much better than the 0.38 degrees I was getting earlier.

Thanks for the help once again. It's greatly appreciated!

Tim