Struggling after the OMS-2 burn.

[Tim13]

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.:tiphat:

Regards,

Tim

 

[GLS]

There is a way to do rendezvous with SPEC 34... but I never got around to understand it. :facepalm:
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.

 

[Tim13]

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

 

[Thorsten]

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.

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.

 

[Urwumpe]

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)

 

[Thorsten]

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.

 

[Tim13]

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.

Tim

 

[Urwumpe]

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.

 

[Thorsten]

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...)

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.

 

[Urwumpe]

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... :lol:

 

[GLS]

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.

 

[Urwumpe]

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.

 

[Tim13]

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

 

[Thorsten]

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... :lol:

 

[Tim13]

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

 

[Thorsten]

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?

Yes - I hope some time this year. The pre-requisite I needed was a fast way to reproduce the orbital dynamics of JSBSim - that now exists with the LEO targeting code - so I 'just' need to code the numerics for ISS and then work on the targeting code (SPEC 34) and that should be it.

It's not conceptually complicated, just *very* tedious because FG comes with zero helpers for orbital dynamics, so I have to write them all from scratch.

 

[Tim13]

Nice! I only heard of Flightgear recently. I think I need to download it, and check it out. Since my interest only lies in the STS stuff in Orbiter, when all the parts are in place, it will be another great space diversion.

Tim

 

[Gingin]

Hey Tim, it’s actually quite easy with SSU to do orbit inclination adjustements.
First, you have to open Align Planes MFD in a remote MFD to have an idea of Ascending or Descending nodes.

Second, for burn programmation , as you want to change plane only, all the velocity will be added in Y components in OPS 201, item 20.
If you are at a descending node ( DN) , you will have to add a négative Y speed ( minus sign before speed, like -100 ft/ s for example)

If you are at an ascending node, the other way round , positive Y speed

Then for the duration , I don’t remember exactly at how much feet per second is corresponding 0,1 degree of Rinc ( you can try it to have some value in mind)
I think to change 0.1 degree of Rinc, you need 100 ft/s, but to be confirmed

So to sum up, MET is like 00 20 00 00 ( 20 hours in the mission )
You want to change 0.1 degree of Rinc at the next ascending node with ISS in 30 min

Ideally , you would put the burn time to start at half of the burn duration before the node .
You want to do a 100 ft per second burn, oms accélération value is 2 ft per square second, à burn of 100 feet sec is around 50 sec, lets say one minute.
So you want to start the brun 30 sec before the node.

So first, you program in ops 201 with item 10 time of the burn Here it will be at 00 20 29 30 which is at 20 hours 29 mn and 30 seconds into the mission .

Then the value , ascending node so item 20 + 100 to add 100 ft/ sec on y axis.
Then the standard item 22 to load data , item 23 for timer and item 27 with dap in auto to manœuvrer the shuttle in correct burn position .


Then you monitor during the burn the align plane mfd.
It works well , just need to confirm that 100 ft sec is ok for 0.1 degrees ( you can test that by seeing how much Rinc will have change after such a burn and update your tactic )



Concerning flight gear, you will have a lot to learn just for a normal mission. By the time you master it, full rendez might be there :tiphat::tiphat:

 

[Tim13]

Hey Tim, it’s actually quite easy with SSU to do orbit inclination adjustements.
First, you have to open Align Planes MFD in a remote MFD to have an idea of Ascending or Descending nodes.

Second, for burn programmation , as you want to change plane only, all the velocity will be added in Y components in OPS 201, item 20.
If you are at a descending node ( DN) , you will have to add a négative Y speed ( minus sign before speed, like -100 ft/ s for example)

If you are at an ascending node, the other way round , positive Y speed

Then for the duration , I don’t remember exactly at how much feet per second is corresponding 0,1 degree of Rinc ( you can try it to have some value in mind)
I think to change 0.1 degree of Rinc, you need 100 ft/s, but to be confirmed

So to sum up, MET is like 00 20 00 00 ( 20 hours in the mission )
You want to change 0.1 degree of Rinc at the next ascending node with ISS in 30 min

Ideally , you would put the burn time to start at half of the burn duration before the node .
You want to do a 100 ft per second burn, oms accélération value is 2 ft per square second, à burn of 100 feet sec is around 50 sec, lets say one minute.
So you want to start the brun 30 sec before the node.

So first, you program in ops 201 with item 10 time of the burn Here it will be at 00 20 29 30 which is at 20 hours 29 mn and 30 seconds into the mission .

Then the value , ascending node so item 20 + 100 to add 100 ft/ sec on y axis.
Then the standard item 22 to load data , item 23 for timer and item 27 with dap in auto to manœuvrer the shuttle in correct burn position .


Then you monitor during the burn the align plane mfd.
It works well , just need to confirm that 100 ft sec is ok for 0.1 degrees ( you can test that by seeing how much Rinc will have change after such a burn and update your tactic )



Concerning flight gear, you will have a lot to learn just for a normal mission. By the time you master it, full rendez might be there :tiphat::tiphat:

Gingin,

That an excellent response! It confirmed just about everything I was going to try tonight, or tomorrow, and is a great walk through.

I've been using Orbiter for about 15 years now, on and off. I'm surprised I've never heard of using a "remote MFD." I need to search that out!

It's funny you mention how long the FG shuttle might take to learn. I've been flight simming for over 30 years. First with MSFS, and now with X-Plane. I've always flown Boeings in the flight sims. This spring, I bought my first Airbus addon aircraft. I'm still trying to figure out the "Airbus way" when it comes to avionics. :rofl:

Thanks again, it's appreciated!

Tim

 

[Thorsten]

It's funny you mention how long the FG shuttle might take to learn.

One has to mention that GinGin actually is into multiple failure scenarios like a launch abort caused by simultaneous failure of two electrical buses. You need an order of magnitude more knowledge about how things work to deal with such scenarios than you need for a nominal mission.

And things like GPC reconfiguration or nav data incorporation I've kept optional because they're very inaccessible - so I hope it's not as bad has he mentions :lol: