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

Pat Flannery wrote in
:

Can they interconnect the propellant feed from Zvezda through those
docking ports though?

Yes. The Zarya-Zvezda and Zvezda-Pirs interfaces all have interconnects.

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?

The interconnects don't go through the pressure hull, as I understand it.

--
JRF

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In article ,
Pat Flannery wrote:
(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.

Less than you would think. To a good first approximation, it's only the
actual frontal area of the truss struts that matters. This is molecular
flow, which follows different rules from the more familiar flow of denser
air -- a molecule which doesn't actually hit a strut zips straight through
the truss and creates no drag.
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John Doe wrote:


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.

It is steadily losing altitude since the end of the planned Shuttle
reboosts:
http://www.hq.nasa.gov/osf/station/images/issalt.gif
As of today, it's at 359.1 km average altitude, which is getting fairly
near its lowest altitude ever (around 330 km)
As Jorge pointed out, the deeper it sinks into Earth's gravity well, the
more the air drag increases, the more the rate of descent increases, and
the more energy is required to raise it back to the desired altitude.
At some point the situation arises where it hits the point where you
can't get its orbit raised enough even with the Shuttle's boost
capacity, and it just spirals in.
The Zvezda engine burn will be a big help, but on their own the Progress
spacecraft haven't raised its orbit as far as it sinks between their
visits with the present launch rate and with their present fuel capacity.
If they do need more Progress' built I'm sure the Russians will be happy
to build them.
And I'm sure the bill to us for doing that will be something to see.

Pat

Jorge R. Frank wrote:



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?



The interconnects don't go through the pressure hull, as I understand it.




I knew that was the way it was done on the Salyuts and Mir for safety's
sake (remember when they blew the propellant line on the exterior of
Salyut 7?) But that's going to be one mighty long trip for the
propellants to take to reach the Progress from Zvezda. I wonder if they
vacuumate the plumbing to keep bubbles out during the transfer process?

Pat

John Doe wrote:


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 ?



I'm fairly certain it's fixed thrust and only has an "on/off" capability.

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 ?



The thrust is so low (400 kgf) that it doesn't hardly stress the station
at all.

Pat

Pat Flannery wrote:

I knew that was the way it was done on the Salyuts and Mir for safety's
sake (remember when they blew the propellant line on the exterior of
Salyut 7?) But that's going to be one mighty long trip for the
propellants to take to reach the Progress from Zvezda. I wonder if they
vacuumate the plumbing to keep bubbles out during the transfer process?

Zarya isn't just fitted with fuel lines, it can store a significant
amount of fuel in its own tanks. I'm not sure where they are located on
the module (probably in the 'flared' section), but they are a lot closer
to the docking port in question than the aft end of Zvezda.

--Chris

Chris Bennetts wrote in news:44664d2b$0$16026
:

Pat Flannery wrote:

I knew that was the way it was done on the Salyuts and Mir for safety's
sake (remember when they blew the propellant line on the exterior of
Salyut 7?) But that's going to be one mighty long trip for the
propellants to take to reach the Progress from Zvezda. I wonder if they
vacuumate the plumbing to keep bubbles out during the transfer process?

Zarya isn't just fitted with fuel lines, it can store a significant
amount of fuel in its own tanks. I'm not sure where they are located on
the module (probably in the 'flared' section), but they are a lot closer
to the docking port in question than the aft end of Zvezda.

Zarya's tanks are on the exterior, hidden under the pairs of radiators on
the zenith and nadir sides.


--
JRF

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John Doe wrote in :

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 ?

On/off.

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 ?

As Pat wrote, that's not necessary. But if it were, Progress could also
perform a reboost with its aft RCS thrusters. This is normally not done
because these thrusters have lower Isp and are canted 15 degrees off the
main axis, making them considerably less efficient.

--
JRF

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(Henry Spencer) wrote in
:

In article ,
Pat Flannery wrote:
(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.

Less than you would think. To a good first approximation, it's only
the actual frontal area of the truss struts that matters. This is
molecular flow, which follows different rules from the more familiar
flow of denser air -- a molecule which doesn't actually hit a strut
zips straight through the truss and creates no drag.

True, though it's worth pointing out that much of the truss is covered with
thermal blankets, thus increasing the frontal area.

--
JRF

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Pat Flannery wrote in
:

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.

The ballistic number of the current config is 166.0 kg/m^2 while flying
in XVV (normal low-beta) attitude. That's a mass/area ratio; the higher
the number, the *less* draggy the station is.

The ballistic number of the station at assembly complete will be 70.02
kg/m^2, which is considerably less draggy than the current config.

Both of those numbers are average values to account for the changing
frontal area of the rotating solar arrays.

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.

Different things will be "obvious" from that altitude graph to different
observers, depending on how much contextual information they know. Your
interpretation is only "obvious" if you assume that each Progress is
devoting all of its propellant to reboost, rather than diverting some of
it to build up ISS reserves.

What's obvious to *me* looking at the same graph is that the ISS altitude
curve is nicely matched to the F10.7 solar flux curve as it drops toward
solar minimum. It's also obvious to me that the Russians feel no urgency
for a drastic ISS reboost. They are maintaining ISS at around 340 km
because that's a convenient two-day phase-repeating orbit that optimizes
Soyuz/Progress launch opportunities and rendezvous phasing, while
maintaining a nice safe 320-day predicted orbital lifetime. They also
know that some of the upcoming shuttle assembly flights are heavily
loaded and require low rendezvous altitudes (as low as 335 km, for STS-
117), so they want to keep ISS at a point where they can just let it
decay to those altitudes rather than waste prop on an orbit lowering
burn.

It's also obvious to me that the Russians are sitting on a nice fat
propellant reserve of over 4000 kg, and that if they really felt the
need, they could boost ISS up to a 425 km circular orbit any time they
wanted to and still have nearly 1000 kg of propellant left over for
attitude control.


--
JRF

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