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Old 05-16-2018, 11:31 PM   #1
Tim13
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Default Struggling after the OMS-2 burn.

So I'm able launch to the ISS with a RInc of about 0.3 degrees. I can live with that for now. I can do an OMS-2 burn to circularize my orbit.

Now, for the last two days I've been reading many NASA STS docs. They mostly describe OPS 201 and OPS 202, but I'm finding very little in the way of how to execute an orbit adjustment with these MM's.

Does anyone have any pointers on how to execute a burn to correct my RInc? I'd even just take the necessary Item numbers in the order they need to be entered and executed.

In any event, I'll gladly take any, and all help anyone wishes to throw my way.

Regards,

Tim
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Old 05-16-2018, 11:58 PM   #2
GLS
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There is a way to do rendezvous with SPEC 34... but I never got around to understand it.
Search this sub-forum, as this was explained here sometime in the past.

I can tell you about MM201 and 202. The MM201 display is for defining, maneuvering to, and maintaining an attitude. Currently it doesn't do everything, but it is still very useful. The MM202 display is similar to the OMS-1/2 burn displays, and that is the way you can use the OMS engines in orbit.
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Old 05-17-2018, 12:29 AM   #3
Tim13
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OK, I'll search that info out here.

The OMS burns are always programmed, and controlled by the GPC, correct? There is no way to do a manual OMS burn like with the RCS.

Tim
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Old 05-17-2018, 06:09 AM   #4
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Quote:
Does anyone have any pointers on how to execute a burn to correct my RInc? I'd even just take the necessary Item numbers in the order they need to be entered and executed.
Assuming you know the time when the orbits cross and the Delta v you need to correct, you ought to be able to use MM 202 to enter the time (TIG) and the Delta V as PEG-7 target under DVY and that should give you an inclination-changing burn.

Rinc of 0.3 degrees seems a lot to me though, it might cost an unreasonable amount of propellant (for comparison, I've written my ascent logic to acquire an inclination to a precision of 0.01 degrees or better) and you might end up with insufficient propellant to de-orbit.

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The OMS burns are always programmed, and controlled by the GPC, correct?
Correct.

Edit: You might be able to perform the burn with the RCS if you're patient though - last time the question came up this did not need a crossfeed as the RCS propellant needs were automatically satisfied by the OMS tanks, in reality you'd need to set the valves for an OMS to RCS crossfeed to have enough oomph.

Last edited by Thorsten; 05-17-2018 at 06:29 AM.
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Old 05-17-2018, 06:55 AM   #5
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There is also no manual way to open the valves of the OMS engines (for igniting them)

But since landing the Shuttle without at least the single BFS computer is impossible, its maybe better that way. (There is no direct connection between stick and control surface, no manual reversal)
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Old 05-17-2018, 08:17 AM   #6
Thorsten
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There is no direct connection between stick and control surface, no manual reversal
If there were, you still couldn't do it (or at least I can't do it...) - the yaw instability is too hard to manage by hand... (I've tried, but only an augmented manual mode where the computer enforces zero beta is flyable).

Anyway, I agree that it doesn't make much sense to have the OMS burn under manual control - there's too little propellant aboard to just point and burn away when you feel like it.
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Old 05-17-2018, 12:39 PM   #7
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Originally Posted by Thorsten View Post
 Assuming you know the time when the orbits cross and the Delta v you need to correct, you ought to be able to use MM 202 to enter the time (TIG) and the Delta V as PEG-7 target under DVY and that should give you an inclination-changing burn.

Rinc of 0.3 degrees seems a lot to me though, it might cost an unreasonable amount of propellant (for comparison, I've written my ascent logic to acquire an inclination to a precision of 0.01 degrees or better) and you might end up with insufficient propellant to de-orbit.
I tried that last night, but it didn't work out. I need to chew on it a little bit more. I know how the old Shuttle Fleet 4.8 used to orient itself for an inclination correction. When I set up a DVY burn, the shuttle orientation was off. The shuttle was 90 degrees off. It was getting late, so I had to go to bed. I'll try it again today. Maybe I need to be in "free" mode, and manually point the OV in the direction I want, then switch to RLS to hold it there before the burn? Dunno.

And yes, 0.3 is a lot, but as I understand it, from another question I asked earlier, the ascent AP for SSU targets a specified orbital inclination, but does not correct for a rendezvous targets LAN.

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Old 05-17-2018, 12:45 PM   #8
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Originally Posted by Tim13 View Post
 And yes, 0.3 is a lot, but as I understand it, from another question I asked earlier, the ascent AP for SSU targets a specified orbital inclination, but does not correct for a rendezvous targets LAN.
Yes. Also we lack some mechanisms that the real shuttle had for the later rendezvous missions.

A software update allowed later missions to get an update for the launch trajectory minutes before lift-off, while the earlier missions did not get updates after the software tapes had been final - days before launch.

And we still have to means to insert holds into the countdown to sync with the target, which also adds some LAN errors.
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Old 05-17-2018, 12:49 PM   #9
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About correcting LAN, you ought to be able to do some correction in J3 gravity by a suitable phasing plan - LAN is one of the elements which have a secular drift in J3 gravity that's altitude dependent, so the relative LAN between you and the target changes if you're not orbiting in the same altitude.

(Okay, that's now a bit esoteric and I admit I've never tried how much you can adjust that way... but it seems worth keeping in mind, because if you do not, you end up fighting the effect...)

Quote:
When I set up a DVY burn, the shuttle orientation was off. The shuttle was 90 degrees off. It was getting late, so I had to go to bed. I'll try it again today. Maybe I need to be in "free" mode, and manually point the OV in the direction I want, then switch to RLS to hold it there before the burn?
The SSU folks ought to know whether automatic orientation to burn attitude for a DVY works (that'd actually be the procedure you use in reality with the burn target given to you by MCC, SPEC 34 is only used in the late rendezvous phases), but you sure don't need a free drift DAP, an inertial attitude hold flown manually should also do the trick.

Last edited by Thorsten; 05-17-2018 at 12:53 PM.
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Old 05-17-2018, 12:51 PM   #10
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Quote:
Originally Posted by Thorsten View Post
 About correcting LAN, you ought to be able to do some correction in J3 gravity by a suitable phasing plan - LAN is one of the elements which have a secular drift in J3 gravity that's altitude dependent, so the relative LAN between you and the target changes if you're not orbiting in the same altitude.

(Okay, that's now a bit esoteric and I admit I've never tried how much you can adjust that way... but it seems worth keeping in mind, because if you do not, you end up fighting the effect...)
Or maybe its more effective to use a different bank angle between MECO and OMS-2...
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Old 05-17-2018, 01:00 PM   #11
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Originally Posted by Urwumpe View Post
 And we still have to means to insert holds into the countdown to sync with the target, which also adds some LAN errors.
We can manually stop the clock inside 9 minutes.
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Old 05-17-2018, 01:10 PM   #12
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Originally Posted by GLS View Post
 We can manually stop the clock inside 9 minutes.
Yes, but we can't let an AI mission control ensure a defined L-time during final count.
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Old 05-17-2018, 01:20 PM   #13
Tim13
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 but you sure don't need a free drift DAP, an inertial attitude hold flown manually should also do the trick.
For sure. Point and shoot is not the best option with limited fuel...LOL.

Unless I can figure out if there is a programmable point feature in 201/202, I think the best bet will be the inertial attitude hold.

Tim
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Old 05-17-2018, 03:05 PM   #14
Thorsten
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Or maybe its more effective to use a different bank angle between MECO and OMS-2...
Upon doing some numbers, it does seem kind of smallish... Not sure whether the bank angle effect is larger though...
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Old 05-17-2018, 03:18 PM   #15
Tim13
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Thorsten,

I've been reading a lot of your Flightgear Wiki documents for your shuttle. Very impressive.

As I understand it, any type of rendezvous right not is not possible in Flightgear, outside of a dedicated scenario ie the ISS rendezvous?

Do you think Flightgear will ever support the ability to allow a complete ISS rendezvous mission from launch, to docking, to landing, all in a single scenario?

Tim
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