SSU Documentation

[SiameseCat]

I just tried to compile the Latex files and some of the images seem to be missing. The missing files are SSU_Attachments.jpg, SLWT.jpg and SRB.jpg.

 

[zerofay32]

I'll try to take a look at it. Work has had me on the road a lot (spent over a month flying in New England) And I finally got a new computer. My old one bearly ran SSU, got about 2fps. Runs much better now, and I have a new strategy about how to go about the docs. So, I my be changing it anyway.

 

[DaveS]

Speaking of documentation, could someone write a short tutorial on how to use GNC SPEC 34 ORBIT TGT for rendezvous burns?

 

[Urwumpe]

Speaking of documentation, could someone write a short tutorial on how to use GNC SPEC 34 ORBIT TGT for rendezvous burns?

I could do that, is the SSU implementation equivalent to the FDF?

 

[DaveS]

I could do that, is the SSU implementation equivalent to the FDF?

Based on appearances, yes. Only thing missing is the ORBITER STATE section.

 

[Urwumpe]

Based on appearances, yes. Only thing missing is the ORBITER STATE section.

Well, would be better with it, but I can make you a simple tutorial for the targeting.

 

[SiameseCat]

I could do that, is the SSU implementation equivalent to the FDF?

Mostly. Specifying the elevation angle at TIG (and calculating the TIG based on this) isn't supported. Also, using ITEM 1 to load all the targeting values doesn't work - each item has to be loaded individually.

The last thing to note is that the target vessel has to be specified by name - at the moment, this is done using a line in the scenario file. This is done using

  @BEGINSOFTWARE StateVectorSoftware
  TARGET_VESSEL ISS
  @ENDSOFTWARE

inside the @SUBSYSTEM SimpleGPCSystem block.

 

[Urwumpe]

well, lets start here...

What you first of all need, is the "TARGETING DATA" table from the Rendezvous C/L (page 6-4, 6-5), this contains the reference data for correcting the target data by the target altitude. 13 is for example the targeting data for MC3. This shall fill the input fields with the I-loaded data for the mission.

Delta X, Y, Z describes your relative position on T1 TIG.

If you would have nothing special to do, it would be simply for the MC3 example:

ITEM 1 +13 EXEC "Load MC3 data from GPC memory"

Check and correct target data

ITEM 28 EXEC "Calculate TIG"

Record calculated data in PAD.

Next, you follow the "RCS BURN" or "ORBIT MNVR" Cue card (ORBIT OPS)

If we have no predefined maneuver targets in GPC Memory, which is likely the situation, things are more complex. You have to define all inputs for the burn yourself and then check the solution. This starts with the basetime, which is the time reference for the maneuver. Then you enter EL=0, so the elevation angle search for your non-existing target is prevented. Then you can enter the relative position data and the relative velocity (ITEM 7 - 12). Next, you should set Delta-T to a negative number, so T2 is recomputed when you execute ITEM 28.

Now you can hit ITEM 26 EXEC and load the new data into the selected target slot. Then you can enter ITEM 28 EXEC and compute the solution for both maneuvers. finally you switch to the maneuver software, the CRT timer will be already counting down to the first TIG. The PEG 7 data of the ORBIT MNVR display should already contain the solution data for the first burn.

---------- Post added at 08:20 PM ---------- Previous post was at 08:17 PM ----------

Mostly. Specifying the elevation angle at TIG (and calculating the TIG based on this) isn't supported. Also, using ITEM 1 to load all the targeting values doesn't work - each item has to be loaded individually.

Would be better we could put some general orbit elements into the mission file for the 1-40 target entries. The targeting data is not for specific vessels, but rather for general orbits, that should be reached.

 

[SiameseCat]

Would be better we could put some general orbit elements into the mission file for the 1-40 target entries. The targeting data is not for specific vessels, but rather for general orbits, that should be reached.

From a practical standpoint, it's targeting a specific vessel. In real life Mission Control would provide the state vector (which would be propagated by the shuttle GPCs). Since we don't have simulate Mission Control yet, I think it's better if SSU tracks the target vessel.

 

[DaveS]

So which way is it currently done in SSU? Manual ITEM entries or TGT NO from GPCs?

 

[SiameseCat]

Manual ITEM entries.

 

[Urwumpe]

From a practical standpoint, it's targeting a specific vessel. In real life Mission Control would provide the state vector (which would be propagated by the shuttle GPCs). Since we don't have simulate Mission Control yet, I think it's better if SSU tracks the target vessel.

Actually, the data would come from mission planning and be written on magnetic tape before launch. Updates would be possibly delivered if needed.

It is not just targeting one vessel, but also preliminary orbits to that vessel, like the orbit after NCC, the orbit after MC2, etc...

 

[SiameseCat]

Actually, the data would come from mission planning and be written on magnetic tape before launch. Updates would be possibly delivered if needed.

It is not just targeting one vessel, but also preliminary orbits to that vessel, like the orbit after NCC, the orbit after MC2, etc...

The way SPEC 34 is used, it really targets a position instead of an orbit (the T2 burn is never performed - the point is to target a particular position relative to the rendezvous target).

SPEC 34 always targets the same state, but the offset changes between burns. The TI burn is supposed to occur at a particular position relative to the ISS, so the NCC burn is set up so the shuttle will be in the correct position at TI. For the TI burn, the offsets are changed to set up the MC4 burn.

 

[Urwumpe]

The way SPEC 34 is used, it really targets a position instead of an orbit (the T2 burn is never performed - the point is to target a particular position relative to the rendezvous target).

SPEC 34 always targets the same state, but the offset changes between burns. The TI burn is supposed to occur at a particular position relative to the ISS, so the NCC burn is set up so the shuttle will be in the correct position at TI. For the TI burn, the offsets are changed to set up the MC4 burn.

Does not fit to the description of the display: Yes, you can omit the second maneuver, but the target is the position of a virtual satellite in a target orbit, all seven elements described. If you would use it without the second maneuver, you would reach the target without matching velocity.

Also, better read the Rendezvous C/L again, the burn targets are called by different target IDs, but have otherwise very similar data entries, only varying by orbit altitude.

 

[SiameseCat]

Does not fit to the description of the display: Yes, you can omit the second maneuver, but the target is the position of a virtual satellite in a target orbit, all seven elements described. If you would use it without the second maneuver, you would reach the target without matching velocity.

Also, better read the Rendezvous C/L again, the burn targets are called by different target IDs, but have otherwise very similar data entries, only varying by orbit altitude.

Matching velocity is only done at then end of the rendezvous and is done manually using the RCS thrusters (this prevents problems due to nav error, and the relative velocity should be small at this point). If you look at the Rendezvous C/L, the T2 burn is never performed.

For any single altitude, the NCC, TI and MC4 burns all have different targets. MCs 1-3 have the same targets (except for the time) as TI, which is because they're midcourse correction burns. Each burn has identical entries (except for the timing) regardless of target altitude, which I think comes from NASA trying to standardize the rendezvous profile as much as possible.

 

[Urwumpe]

Matching velocity is only done at then end of the rendezvous...

yes, but still the targeting software uses Kepler elements or something comparable, even if the second burn is omitted in the rendezvous process. Otherwise the entries would not make sense at all. Look at the table data and propose a better explanation what the targets must be like.

 

[SiameseCat]

The T1 burn targets a specific position relative to the target object (the DX,DY,DZ values) at a specific time (the DT value). The T2 burn then nulls the relative velocity between the shuttle and the target.

The targeting software needs to know what orbit the target object is in, but this is only so it can calculate the target's position/velocity at T2.

 

[Urwumpe]

The T1 burn targets a specific position relative to the target object (the DX,DY,DZ values) at a specific time (the DT value). The T2 burn then nulls the relative velocity between the shuttle and the target.

The targeting software needs to know what orbit the target object is in, but this is only so it can calculate the target's position/velocity at T2.

Yes, exactly that. But not just nulling the velocity, it also permits a bias. And the target is not always a spacecraft. You can see that by looking at the table in the rendezvous C/L: different targets and positions, but similar offsets and elevation angles.

 

[SiameseCat]

Yes, exactly that. But not just nulling the velocity, it also permits a bias. And the target is not always a spacecraft. You can see that by looking at the table in the rendezvous C/L: different targets and positions, but similar offsets and elevation angles.

I don't think there's any way to specify a velocity bias for the T2 burn. The elevation angle is used to determine the T1 TIG; it doesn't affect the target position. If the elevation angle is specified, the T1 burn will occur when the target is at a particular elevation above the shuttle.

I don't understand why you would use SPEC 34 (and then perform a NCC burn, TI burn, 3 midcourse corrections and the MC4 burn) unless you're targeting a spacecraft. If you just want to change orbits, a Hohmann transfer is a lot simpler.

 

[Urwumpe]

I don't think there's any way to specify a velocity bias for the T2 burn. The elevation angle is used to determine the T1 TIG; it doesn't affect the target position. If the elevation angle is specified, the T1 burn will occur when the target is at a particular elevation above the shuttle.

See above, TIG T1 is calculated only when EL != 0 - otherwise it is free.

I don't understand why you would use SPEC 34 (and then perform a NCC burn, TI burn, 3 midcourse corrections and the MC4 burn) unless you're targeting a spacecraft. If you just want to change orbits, a Hohmann transfer is a lot simpler.

... you are aware that

a) any rendezvous is a matching of orbits?
b) you only have spec 34 to calculate a hohmann maneuver?