Oberg: "The real significance of the ISS thruster test failure"

they tested the engines since theres a great chance the shuttles will
never be on station again.

its most likely to be fast news, after another shuttle accident or
futher flight delays where the shuttle is permanetely grounded......


as is flight rules are being ignored, managers over ruling engineers to
gt the shuttle flying again. this about the foam loss issue..

Jorge R. Frank wrote:

It doesn't; the resulting torque would be controlled by the RCS thrusters
on Zvezda. Inefficient, but tolerable. The forward of the two nadir
locations is very close to the ISS center of mass for the current config
anyway.


There sure looks like there's a lot of mass ahead of it compared to
behind it.
You've got Zvezda behind it, and four modules as well as the truss
segments, radiator, airlock, and solar arrays ahead of it:
http://spaceflight.nasa.gov/gallery/.../s114e7284.jpg

The loads from the Progress reboost engine are likewise tolerable
even in this off-axis state; remember that this reboost engine is smaller
than even a single shuttle primary RCS thruster.



Do they have some RCS use even in the end docking collar arraignment dye
to the present location of the solar arrays above the ISS' centerline?

Pat

Pat Flannery wrote:


Do they have some RCS use even in the end docking collar arraignment dye

That's "due" BTW.
Colds are such fun.

Pat

Pat Flannery wrote in
:

Jorge R. Frank wrote:

It doesn't; the resulting torque would be controlled by the RCS
thrusters on Zvezda. Inefficient, but tolerable. The forward of the
two nadir locations is very close to the ISS center of mass for the
current config anyway.

There sure looks like there's a lot of mass ahead of it compared to
behind it.
You've got Zvezda behind it, and four modules as well as the truss
segments, radiator, airlock, and solar arrays ahead of it:
http://spaceflight.nasa.gov/gallery/...ly/hires/s114e
7284.jpg

Perhaps I was not clear the first time. The "forward of the two locations"
that I was referring to is Zarya nadir, not Zvezda nadir. You should be
counting Zarya as aft of the CM. The CM of ISS, right now, is at the
Zarya/PMA-1 interface.

The loads from the Progress reboost engine are likewise tolerable
even in this off-axis state; remember that this reboost engine is
smaller than even a single shuttle primary RCS thruster.

Do they have some RCS use even in the end docking collar arraignment
dye to the present location of the solar arrays above the ISS'
centerline?

I'm sure they do. RCS usage would be small, of course, since the CM is only
a few feet above centerline and the moment arm for the Zvezda pitch
thrusters is long.


--
JRF

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

Pat Flannery wrote in
:

Jorge R. Frank wrote:

That plot is not as alarming as it might appear. ISS is now flying
through solar minimum, so it can fly at a lower altitude while
maintaining the same orbital lifetime. And as you pointed out
yourself, Zvezda has plenty of propellant reserves that could be
interconnected to Progress to boost ISS all the way back up to its
maximum altitude in a contingency. Right now the Russians and NASA are
taking advantage of solar minimum to optimize Progress and shuttle
payload.

Thought problem:
Something doesn't go right on the next Shuttle flight and it gets
grounded again.
The Progress reboosts are helping, but how long can they keep kicking
it up on there own once the Zvezda's fuel is expended?
Do they have to wait around for the new ESA Jules Verne module to
raise its orbit in lieu of the Shuttle if that occurs?

Probably not. The answer depends on how long the shuttle is grounded.

Atmospheric density decreases exponentially with altitude, and drag is
proportional to density. Therefore the amount of propellant required for
reboost also decreases with altitude. It takes more prop to maintain a
low orbit than a high one.

So if the Russians knew the shuttle wasn't coming back for a long time or
at all, they'd use as much prop from Zvezda as needed to boost the
station all the way up to the Soyuz/Progress rendezvous ceiling (425 km).
Maintaining ISS at that altitude will require less propellant than
maintaining it at current altitude, and is probably within the capacity
of Progress alone, though ATV will certainly help.

There is one ironic aspect to all this- if the ISS really had been
completed to the point it was supposed to be today on the original
schedule, it would have been a lot more massive than it presently is,
and those Progress reboosts would be having far less effect than they
presently are.

On the other hand, it would require less reboost, since drag deceleration
is inversely proportional to mass.

(ISS will have more frontal area as well, due to the solar arrays, and
that will increase drag. But there are techniques that can be - and are
already - used to minimize area, such as feathering the arrays edge-on to
the velocity vector during orbital night.)

--
JRF

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

In article ,
Pat Flannery wrote:
There is one ironic aspect to all this- if the ISS really had been
completed to the point it was supposed to be today on the original
schedule, it would have been a lot more massive than it presently is,
and those Progress reboosts would be having far less effect than they
presently are.
Having it lag so far behind schedule has kept it light enough for the
Progress' to keep its orbit up...

Actually, the mass of the station is irrelevant to whether a given reboost
system is adequate. Yes, a more massive station responds less to a
specific rocket burn, but it also responds less to air drag.

Loosely speaking, the requirement is that the thrust of the reboosts,
averaged over the period between them, equal the air drag averaged over
the same period. The mass of the station doesn't figure into it.

What does matter is how much frontal area the station has, that being the
dominant issue in how much drag it incurs.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

Jorge R. Frank wrote:

Perhaps I was not clear the first time. The "forward of the two locations"
that I was referring to is Zarya nadir, not Zvezda nadir. You should be
counting Zarya as aft of the CM. The CM of ISS, right now, is at the
Zarya/PMA-1 interface.




Yeah, that makes a lot more sense from a balance viewpoint.
Can they interconnect the propellant feed from Zvezda through those
docking ports though?
I don't think you really want to string hydrazine and nitrogen tetroxide
plumbing through the inside of the station to connect the Zvezda tanks
to the Progress docked to the Zarya module, do you?
Anyway, using the Zvezda main engines sounds like the simplest solution,
from both the balance and propellant location viewpoints.
Question of the week: Why didn't they use the Zvezda motors and their
propellant before? Say after the loss of Columbia, and the grounding of
the Shuttles?
Were they being kept in reserve for possible use in case something
unexpected arose? They only rarely get the rear docking port cleared,
but it has happened before.


Pat

Jorge R. Frank wrote:

On the other hand, it would require less reboost, since drag deceleration
is inversely proportional to mass.

(ISS will have more frontal area as well, due to the solar arrays, and
that will increase drag. But there are techniques that can be - and are
already - used to minimize area, such as feathering the arrays edge-on to
the velocity vector during orbital night.)


I assumed that was the case; but that truss is also going to kick in drag.
It would be fun to see the mass versus drag ratios on the station as it
was to be today versus what we have now, and then figure in the delta v
that the Progress could provide in relation to its expected orbital
decay rate.
What is obvious from that altitude graph is that all on their own, the
Progress' are fighting a slowly losing battle as far as orbital altitude
goes.
I imagine a dedicated reboost Progress could be launched that would
devote all of its cargo mass to propellants for reboost though.

Pat

Pat Flannery wrote:
Something doesn't go right on the next Shuttle flight and it gets
grounded again.
The Progress reboosts are helping, but how long can they keep kicking it
up on there own once the Zvezda's fuel is expended?


Has the station fallen back to earth in the 3 years since Columbia fell
apart during re-entry ? NO. Except for one token Shuttle flight, it has
been kept up there by Progress vehicles. So as long as Russia and spit
out some Progress vehicles at about the same rate, the station won't
fall back to earth.


Secondly, Zvezda doesn't have the fuel tanks. they are in Zarya (FGB).
Anmd the tanks are refilled by Progress vehicles. In other words, the 3
russian docking ports have plumbing that leads to the Zarya fuel tanks
and pumps to draw fuel from Progress.

(has it been confirmed that they can pump fuel TO a Prgress ship from
the Zarya tanks ?

Do they have to wait around for the new ESA Jules Verne module to raise
its orbit in lieu of the Shuttle if that occurs?

I think you are underestimating Progress. First, there are at least 2
variations of the Progress vehicle. One has more room for goods, one has
larger fuel/water tanks. So if they need to reboost the ISS more, they
will send a progress up configured with more fuel.

The ISS has been up in that config long enough and in a variety of
orientations that the russians and americans have a very good idea of
exactly how much fuel is needed to keep it at the desired orbit.

If you look at the "shopping list", it has probably always exceeded what
Progress has been able to deliver even with only 2 crewmembers. And I
assume that rations for food, water etc have been somewhat more tightly
controlled. And I woudl expect measures such as asking crewmembers to
keep clothes on for longer periods, reducing the amount of new clothes
that need to be shipped in each progress.

But Progress seems to have been perfectly capable of bringing up the
true essentials to keep the station up and running and keep the 2
crewmembers alive.


schedule, it would have been a lot more massive than it presently is,
and those Progress reboosts would be having far less effect than they
presently are.

If ISS had been on-time, the ATVs would have helped reboost the station
too.

Pat Flannery wrote:
There sure looks like there's a lot of mass ahead of it compared to
behind it.
You've got Zvezda behind it, and four modules as well as the truss
segments, radiator, airlock, and solar arrays ahead of it:
http://spaceflight.nasa.gov/gallery/.../s114e7284.jpg


If the progress is at the Zarya nadir port, then it is fairly well
centred since the airlock and Z1/P6 are right next to it, so not that
much torque effect to push those.

Question. can the thrust from the Progress reboost engines be
controlled, or is it an "on or off" type of engine with just one thrust
setting ?

When pushing the station from Zarya-nadir, would the run the engine at a
lower thrust setting for a longer period to reduce the stress on the
station ?