Orbital mechanics folks...why does the ISS reboost matter?

"Lee Jay" wrote:

Craig Fink wrote:
On Fri, 01 Dec 2006 09:29:45 -0800, Lee Jay wrote:
Can someone explain to me why flight day 3 docking opportunities are
available only every other day right now (7th, 9th, ...), but would be
available every day if the planned ISS reboost is completed? Is it
just because the ISS would be a little bit slower after reboost?

If what your saying is true about the every other day, it's probably just
because they want to do it that way. The constraint, if there is one,
isn't Orbital Mechanics.

http://spaceflightnow.com/shuttle/sts116/061201reboost/

"The reboost maneuver is needed to raise the station's orbit enough to
permit Discovery to dock on the third day of its mission, whenever it
gets off the ground...Even with the aborted burn, FD-3 opportunities
are available Dec. 7, 9, 11, 13, 15-22, 24 and 26, the end of
Discovery's launch window...If the burn goes well, FD-3 docking
opportunities will be available every day between Dec. 7 and Dec. 23
and on Christmas day if required."

There is no way, in that list, to differentiate between the limits
caused by orbital mechanics and those due to flight rules.

D.

--
Touch-twice life. Eat. Drink. Laugh.

-Resolved: To be more temperate in my postings.
Oct 5th, 2004 JDL

Derek Lyons wrote:
There is no way, in that list, to differentiate between the limits
caused by orbital mechanics and those due to flight rules.

Okay. Please explain then how the flight rules are affected by whether
or not the ISS gets reboosted a bit. I just don't understand why these
every-other-day windows for an FD3 docking are occuring.

Lee Jay

"Lee Jay" wrote in news:1165034674.007770.3970
@j72g2000cwa.googlegroups.com:

Jim Oberg wrote:
See http://www.jamesoberg.com/books/spt/...TERSw_figs.pdf,
the Appendix 1 to chapter 1,
for generic background, then we can chat.

I think I understand the basics of orbital mechanics as I took it in
engineering school. So hopefully I can follow your explaination,
though the rust in my head might slow things down! ;-)

That will make it easy, then. :-)

The short answer is that ISS' current orbit is close to the right altitude
for a two-day phase-repeating orbit. Every other day, you get a short phase
angle, alternating with a long phase angle about 180 degrees opposite.
Right now the long phase angle is outside the limit for flight day 3
rendezvous. The reboost would lengthen ISS' orbit period just enough to
slide that long phase angle case within the flight day 3 limits.

Jim or I would be happy to elaborate on any of these points - I've noticed
that orb mech classes tend to neglect rendezvous in general, and phase-
repeating orbits specifically.

--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

(Derek Lyons) wrote in
:

"Lee Jay" wrote:

Craig Fink wrote:
On Fri, 01 Dec 2006 09:29:45 -0800, Lee Jay wrote:
Can someone explain to me why flight day 3 docking opportunities
are available only every other day right now (7th, 9th, ...), but
would be available every day if the planned ISS reboost is
completed? Is it just because the ISS would be a little bit
slower after reboost?

If what your saying is true about the every other day, it's probably
just because they want to do it that way. The constraint, if there
is one, isn't Orbital Mechanics.

http://spaceflightnow.com/shuttle/sts116/061201reboost/

"The reboost maneuver is needed to raise the station's orbit enough to
permit Discovery to dock on the third day of its mission, whenever it
gets off the ground...Even with the aborted burn, FD-3 opportunities
are available Dec. 7, 9, 11, 13, 15-22, 24 and 26, the end of
Discovery's launch window...If the burn goes well, FD-3 docking
opportunities will be available every day between Dec. 7 and Dec. 23
and on Christmas day if required."

There is no way, in that list, to differentiate between the limits
caused by orbital mechanics and those due to flight rules.

Right. You need one piece of information beyond that: that ISS is almost,
but not quite, at the right altitude for a two-day phase repeating orbit.
Then it's easy to see that the pattern of flight day 3 opportunities is
entirely a consequence of that. From Dec. 7 to 14, the "short" phase
angles fall on the odd days, and the "long" phase angles on the even days
are above the FD3 phasing limits. If ISS were *exactly* at the right
altitude for a two-day phase repeating orbit, and reboosted frequently to
*stay* there, then the pattern would remain the same. But ISS isn't, so
the pattern shifts a few degrees every day. Eventually the even-day
"long" phase angle "wraps around" 360 and becomes the short case, while
the odd-day short case becomes the long case. At this point both cases
are within the FD3 limits, so from Dec 15-22 there are FD3 opportunities
every day. After that the odd-day long case exceeds the FD3 limits so
that FD3 opportunities are only available on the even days.

That's all pure orbital mechanics; the only flight rules involved are the
ones regarding minimum height of perigee and the propellant priorities
that groundrule out retrograde RNDZ burns, both of which affect the FD3
phasing limit.


--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

Jorge R. Frank wrote:
The short answer is that ISS' current orbit is close to the right altitude
for a two-day phase-repeating orbit. Every other day, you get a short phase
angle, alternating with a long phase angle about 180 degrees opposite.
Right now the long phase angle is outside the limit for flight day 3
rendezvous. The reboost would lengthen ISS' orbit period just enough to
slide that long phase angle case within the flight day 3 limits.

Reading between the lines, I'm guessing this means that a long phase
angle can't be made up in time because of the limits on delta-V placed
on the orbiter by its fuel supply, given a heavy, performance-limited
payload.

I guess I had it in my head that a 180° phase angle was nothing to
make up in 3 days. But it does mean a lower orbit at MECO and after
the OMS burn which, in turn means more delta-V to push it up to the ISS
orbit. You can't get the LOX and LH2 back that you don't burn from the
big tank so you're stuck with the much smaller OMS system.

Am I getting close here?

I appreciate you clearing this up for me.

Lee Jay

There are other ways to do it rather than a highly maneuverable target.
Time compensates for this, so a much less maneuverable ISS could fly
through a plane/phase window some time in the future. Since it's going to
be doing maneuvers anyway to raise it's orbit, just modify the timing.

ISS hitting a plane/phase window has two constraints, or dependent
variables. Therefore, with two appropriate independent variables it should
be able to hit the window at some future date. The date/time of the
maneuver and altitude of the maneuver should do it, or possibly two dates
and altitude maneuvers.

Oh, you want a bunch of days with rendezvous opportunities. Add another
dependent variable, that makes three. So, the two maneuver reboost would
work. Independent variables: Time of burn 1, altitude of burn 1, Time of
burn 2. Dependant variables: Altitude of burn 2 (really could be
considered as altitude at the rendezvous date + delta altitude due to
drag), future rendezvous orbital plane and phase.

Of course, the altitude of the rendezvous would have to be a fixed altitude
where the Orbital phase remains constant from day to day. In other words
the Revolutions per day would have to be an integer.

http://www.heavens-above.com/orbitdi...ST&satid=25544

Currently, ISS is at 15.7853 revolutions per day. So they would have had
to raise it's altitude some time in the past to an intermediate
altitude, then raised it again to get to an even 16 revolutions per day,
with the right plane and phase.

Ahhh, but regression of the plane would still be a problem for multiple
days. Another dependant variable, without a good independant one. Sorry,
you only get one day. Or two? What is the regression rate of ISS at an
integer revolutions per day altitude?

Orbital Science's Pegasus could easily fix the rendezvous plane problem.
http://www.orbital.com/SpaceLaunch/Pegasus/index.html
Or, a winged ascent vehicles from a fixed launch site with a resonable
amount of crossrange. Also, it may not be as much of a problem for the
Russians who launch due east.

It's just a optimization or constraints problem with three independent and
three dependent variables.

NASA wouldn't be interested in this anyway. I think they like having the
three day rendezvous, the Orbiter is pretty roomy and NASA doesn't sell
tickets. But, the Russian might. A couple of extra days at the Space
Station, fewer days in a cramped capsule, overall it would be a much more
pleasant trip for Private Astronauts (Space Tourist). The Russians already
do a same day deorbit/entry.

16 revs per 24 hour day, 2/3, hummm? A phased locked orbit?

--
Craig Fink
Courtesy E-Mail Welcome @
--

On Sat, 02 Dec 2006 16:44:45+0000, Jim Oberg wrote:

FD1 requires a highly maneuverable target that sets up the very tight phase
angle
window for launch, and they rarely repeat more than once every four to six
days,
except for very unique altitudes. Tourist convenience is a very poor driver
for such an operational burden.




"Craig Fink" wrote in message
news
Targeting the station's altitude increase would give a Flight Day 1
Rendezvous opportunity for an on-time on-day launch. Maybe they shouldn't
waste the Space Station's Rendezvous Targeting Capabilities on the Space
Shuttle, because NASA likes to have a few days On-Orbit before rendezvous.
Soyuz could launch, rendezvous and dock before bedtime. An added bonus
would be an extra couple of days at the Space Station.

I would think Private Astronauts (Space Tourists) would much rather get
out of the cramped ascent/entry vehicle as soon as possible. I mean
really, even if you don't get space sick, do you really want to watch,
smell, and hear the fellow who does, vomiting for a day or two?

--
Craig Fink
Courtesy E-Mail Welcome @
--


On Sat, 02 Dec 2006 07:19:43-0500, John Doe wrote:

Isn't the position of the space station more or less random (in practical
terms) along the orbital path at the time the plane crosses over KSC ?
(aka: launch time for Shuttle)

Does increasing the station's altitude (and incresing the time needed for
a
full orbit) really make such a big change to the relative position of ISS
to KSC at the time the orbital plane passes over KSC ?

"Lee Jay" wrote in
ups.com:

Jorge R. Frank wrote:
The short answer is that ISS' current orbit is close to the right
altitude for a two-day phase-repeating orbit. Every other day, you
get a short pha
se
angle, alternating with a long phase angle about 180 degrees
opposite. Right now the long phase angle is outside the limit for
flight day 3 rendezvous. The reboost would lengthen ISS' orbit period
just enough to slide that long phase angle case within the flight day
3 limits.

Reading between the lines, I'm guessing this means that a long phase
angle can't be made up in time because of the limits on delta-V placed
on the orbiter by its fuel supply, given a heavy, performance-limited
payload.

In one sense, it's purely geometrical. Phasing rate is proportional to
the delta-H between the orbits. The ISS altitude and the orbiter minimum
height-of-perigee flight rules combine to limit the delta-H, and
therefore the phasing rate.

Now, you *could* blame it on prop supply by allowing the orbiter to boost
*above* the target and phase backwards, turning a large positive phase
angle into a small negative one. That approach wastes prop since you have
to spend more to get above ISS and then again to drop back down to ISS
altitude. It is possible but it severely limits your options in
contingency cases, and denies you the opportunity to use that prop for
other purposes, such as reboosting ISS or performing mated attitude
maneuvers (which conserves ISS prop and ISS CMG lifetime).

I guess I had it in my head that a 180° phase angle was nothing to
make up in 3 days.

And you're right - 180 is well within the FD3 phasing limits. But since
the shuttle program has groundruled out rendezvous from above, the phase
angle is always positive by convention, so past 180 the range is 181-359
rather than going negative. At ISS current altitude, there isn't enough
delta-H wiggle room to provide FD3 rendezvous for a 360-degree phase
angle. Once the heavyweight assembly flights are over, ISS will start
boosting higher which will open up the FD3 phasing limit.

There are other things to consider as well. For one thing, "flight day
3" doesn't imply you've got 3 full days available to complete the
rendezvous. Flight days are numbered by crew sleep cycles, not by mission
elapsed time (MET), and launch is typically near the middle-to-end of
flight day 1. The crew launches, performs post-insertion, usually
performs one phasing burn, then goes straight to bed around 4 hours after
launch. So Flight Day 2 begins around an MET of 12 hours, and Flight Day
3 around 36 hours, with the rendezvous complete around 44 hours. For
example, STS-116 launch is scheduled for 12/7 2035 CST, with docking on
12/9 1655 CST (MET 1/20:20).

For another, the "day of rendezvous" profile is standardized to minimize
flight-specific crew training, and that profile starts with a certain
relative perigee to the target. That implies a minimum phasing rate, and
therefore a minimum phase angle for FD3 rendezvous if you want to
preclude retrograde burns. I forget the exact number but it's in the 20-
30 degree range. So if you launch into a 10-degree phase angle, the true
phase angle is really 370 degrees since you can't make FD3 without a
retrograde burn - you need to insert low and "lap" the target.

But it does mean a lower orbit at MECO and after
the OMS burn which, in turn means more delta-V to push it up to the
ISS orbit. You can't get the LOX and LH2 back that you don't burn
from the big tank so you're stuck with the much smaller OMS system.

Am I getting close here?

That's roughly it. The shuttle program doesn't vary the MECO altitude
much, but it does vary the post-MECO Direct Insertion (DI) altitude
(apogee). For ISS flights, the program has already decreased DI from 173
nmi to 122 nmi, and relaxed the minimum Height of Perigee (HP) from 105
nmi to 85 nmi (at least on FD1). That did two things: it increased the
phasing capability for FD3 by giving more delta-H wiggle room, and it
made more MPS propellant available to increase payload performance. Of
course, the flip side is that it makes the OMS margins tighter.

--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

"Jorge R. Frank" wrote in message
...
(Derek Lyons) wrote in
:

"Lee Jay" wrote:

Craig Fink wrote:
On Fri, 01 Dec 2006 09:29:45 -0800, Lee Jay wrote:
Can someone explain to me why flight day 3 docking opportunities
are available only every other day right now (7th, 9th, ...), but
would be available every day if the planned ISS reboost is
completed? Is it just because the ISS would be a little bit
slower after reboost?

If what your saying is true about the every other day, it's probably
just because they want to do it that way. The constraint, if there
is one, isn't Orbital Mechanics.

http://spaceflightnow.com/shuttle/sts116/061201reboost/

"The reboost maneuver is needed to raise the station's orbit enough to
permit Discovery to dock on the third day of its mission, whenever it
gets off the ground...Even with the aborted burn, FD-3 opportunities
are available Dec. 7, 9, 11, 13, 15-22, 24 and 26, the end of
Discovery's launch window...If the burn goes well, FD-3 docking
opportunities will be available every day between Dec. 7 and Dec. 23
and on Christmas day if required."

There is no way, in that list, to differentiate between the limits
caused by orbital mechanics and those due to flight rules.

Right. You need one piece of information beyond that: that ISS is almost,
but not quite, at the right altitude for a two-day phase repeating orbit.
Then it's easy to see that the pattern of flight day 3 opportunities is
entirely a consequence of that. From Dec. 7 to 14, the "short" phase
angles fall on the odd days, and the "long" phase angles on the even days
are above the FD3 phasing limits. If ISS were *exactly* at the right
altitude for a two-day phase repeating orbit, and reboosted frequently to
*stay* there, then the pattern would remain the same. But ISS isn't, so
the pattern shifts a few degrees every day. Eventually the even-day
"long" phase angle "wraps around" 360 and becomes the short case, while
the odd-day short case becomes the long case. At this point both cases
are within the FD3 limits, so from Dec 15-22 there are FD3 opportunities
every day. After that the odd-day long case exceeds the FD3 limits so
that FD3 opportunities are only available on the even days.

That's all pure orbital mechanics; the only flight rules involved are the
ones regarding minimum height of perigee and the propellant priorities
that groundrule out retrograde RNDZ burns, both of which affect the FD3
phasing limit.



Thanks, this write up made a lot of sense to me.

Danny Dot

Okay, so to list the reasons I was confused:

- I forgot that you can only go forward if you are below (duh!). That
means it's not +/- 180°!

- I didn't think about the minimum altitude requirement as a limitation
on catch up rate.

- I didn't realize there was a minimum approach phase for training
purposes.

- I didn't realize that "FD3" docking could be T+44 hours. I was
thinking more like 64 hours.

All these consipire to place a limit that's more than three times as
tight as I was thinking it was.

Thanks for clearing that up for me.

Lee Jay

On Sat, 02 Dec 2006 12:31:47 -0600, Jorge R. Frank wrote:

That did two things: it increased the
phasing capability for FD3 by giving more delta-H wiggle room, and it
made more MPS propellant available to increase payload performance. Of
course, the flip side is that it makes the OMS margins tighter.

But overall performance is more with the Space Station in a lower orbit,
just because the Shuttle doesn't have to go up as high and return from
there. I would think that the OMS assist burn would be varied to get the
OMS margins right. The OMS assist burn could even be varied for the
planned launch time in the window to eliminate excess margin and increase
Station boost capability on the Day of Launch. Essentially shifting excess
margin from the ET into the OMS tanks. It would even work a lot better if
the On-Board Software varied the assist burn with the actual launch time.

Are they doing that yet?

--
Craig Fink
Courtesy E-Mail Welcome @