General Question Space Shuttle Orbital/OMS procedures.

[Slaintemaith]

Mostly I was going for a 'close enough' answer so that I'd at least have a clue as to 'normal' OMS-2 procedures.

I'm going to go with "OMS-2 puts you in an orbit that's high enough to occlude re-entry, and low enough from the ISS to facilitate chasing it."

This could possibly be completely wrong, but it makes sense to me.

 

[David413]

Mostly I was going for a 'close enough' answer so that I'd at least have a clue as to 'normal' OMS-2 procedures.

I'm going to go with "OMS-2 puts you in an orbit that's high enough to occlude re-entry, and low enough from the ISS to facilitate chasing it."

This could possibly be completely wrong, but it makes sense to me.

If you look at the text file I attached, the OMS2 burn resulted in an initial orbit of 124 x 85 nautical miles. NC1 then raised the orbit to 175 x 121 nm, and so on.

Dave

---------- Post added at 15:46 ---------- Previous post was at 15:44 ----------

I always thought the OMS gimbal was to point the resulting force at the CoG to avoid torque. Isn't that the case?

Yes, which is pretty much what the original question was asking about the shuttle's attitude when burning OMS2. At least, that's how I took the question. I was just adding that attitude changes (in particular yaw) are also performed for significant attitudes. The gimbals are used mainly for minor adjusts such as burning thru the CoG, etc.

 

[Slaintemaith]

Respectfully, I'm reasonably certain at least one of the uses of 'attitude' there means 'altitude?' (I only care about spelling when it matters)

David: I'm not so ungrateful that I didn't -look- at the file you provided; I'm just not entirely certain I understood it. =)

 

[garyw]

Respectfully, I'm reasonably certain at least one of the uses of 'attitude' there means 'altitude?' (I only care about spelling when it matters)

And in this case attitude is correct.

Attitude refers to the Orbiters position in space.

As an example, during some deorbit burns the Orbiter will burn with most of the burn component out of plane. This puts some thrust into the actual deorbit and the rest into a minor plane change to bring it closer to its landing site. The position of the orbiter is the attitude and thus correct in the earlier post.

 

[FordPrefect]

And in this case attitude is correct.

Attitude refers to the Orbiters position in space.

...The position of the orbiter is the attitude and thus correct in the earlier post.

Gary, I believe a better word for position would be orientation here, no?

:hello:

 

[garyw]

Gary, I believe a better word for position would be orientation here, no?

:hello:

Yup! Much simpler than my explanation. Cheers! :thumbup:

 

[Slaintemaith]

I thought "Attitude" merely represented the states of roll/pitch/yaw for the orbiter--as it would for any other craft--air, space, or other.

Altitude refers to the orbiter's height. In fact my original question had everything to do with -altitude- and not attitude at all, since I was wondering if the shuttle burned to a shallower-than-ISS apogee at the end of OMS-2.

Maybe you can see my confusion with the whole attitude/altitude thing now, since the only attitude I'd be concerned with (at least as far as Orbiter is concerned) is prograde. =)

 

[garyw]

Altitude refers to the orbiter's height. In fact my original question had everything to do with -altitude- and not attitude at all, since I was wondering if the shuttle burned to a shallower-than-ISS apogee at the end of OMS-2.

At the end of the OMS-2 burn the orbiter is lower than the ISS. If you grab the STS-135 FD2 Execute package You'll see that the altitude is 169 x 121 nautical miles.

The FD03 Execute package has the altitude at 173 x 122 Nautical miles.

So you can see how the shuttle started out below the ISS but then was slowly raised in altitude. There are several burns on FD03 which tune this further.

FD04's execute package (post docking) lists the altitude as 212 x 207 Nautical miles - a huge increase on the start of FD03.

 

[David413]

Respectfully, I'm reasonably certain at least one of the uses of 'attitude' there means 'altitude?' (I only care about spelling when it matters)

David: I'm not so ungrateful that I didn't -look- at the file you provided; I'm just not entirely certain I understood it. =)

"Attitude" means "attitude" or if you prefer, "orientation".

Regardless, let's take a moment and look at one entry of the text file I posted, and see if we can make it a little more useful (my apologies, I sometimes assume too much, I make the same mistake with my students...).

[SIZE=1]Maneuvers contained within the current ephemeris are as follows:[/SIZE]
 
 
[SIZE=1]  IMPULSIVE TIG (GMT)   M50 DVx(FPS)      LVLH DVx(FPS)      DVmag(FPS) [/SIZE]
[SIZE=1]  IMPULSIVE TIG (MET)   M50 DVy(FPS)      LVLH DVy(FPS)      Invar Sph HA[/SIZE]
[SIZE=1]  DT                    M50 DVz(FPS)      LVLH DVz(FPS)      Invar Sph HP [/SIZE]
[SIZE=1]  ------------------------------------------------------------------------[/SIZE]
[SIZE=1]  189/16:05:07.738          39.9              98.1              98.1   [/SIZE]
[SIZE=1]  000/00:38:21.738         -46.6              -0.0              123.6  [/SIZE]
[SIZE=1]  000/00:01:06.417         -76.6               0.1              84.9[/SIZE]

The first column has three rows of entries, the time of ignition ("TIG") in both the GMT time, the mission elapsed time (MET) and finally the length of the burn (in this case, OMS2 was predicted to occur on the 189th day of the year (7/8/11) at 16:05:07.738 GMT (the launch was originally predicted for 15:26:46 GMT), that would have been an MET of 38m:21.738s, and the total burn duration was predicted to be 1 minute, 6.417 seconds.

The second column shows the velocity changes (DV) in each of the principle orbiter axis (x,y,z) in feet per second (FPS) in the M50 frame of reference (an 'inertial frame'). For OMS2 then, it was predicted that the x component would be changed by 39.9 FPS, the y component would be reduced by -46.6 FPS, and the z component would be reduced by -76.6 FPS in that frame of reference.

The third column shows the velocity changes in the relative frame of reference of the local-vertical, local-horizonal (LVLH); this is probably a little easier to understand frame as it is referenced directly to the principle orbiter (shuttle orbiter, not our favorite simulation) where the x axis runs from the tail through the nose, the y axis runs from wing to wing, and the z axis runs upwards and downwards through the body of the orbiter. Again the units are feet per second, and now we can see an almost complete velocity increase prograde of around 98.1 feet per second and a very slight increase of the vehicle along the z axis of 0.1 FPS.

The fourth and final column shows the absolute magnitude of the OMS2 burn in FPS, again referenced to the LVLH frame of reference, in this case the 98.1 FPS, then the new orbit's apogee (HA) in nautical miles of 123.6 nm and the new orbit's perigee (HP) in nautical miles of 84.9 nm.

These state vectors are frequently updated for both the shuttle (when one is flying, sadly too soon this will end), and periodically for the ISS as well. They can be found at:

http://spaceflight.nasa.gov/realdata/elements/

Hopefully, this helps make the file a little more useful, or at least informative.

 

[Slaintemaith]

David, you never need to apologize for that--it means you assume everyone is as smart as you are. =)

I've always been more on the 'fuzzy' side.