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

John Doe wrote in :

Secondly, Zvezda doesn't have the fuel tanks. they are in Zarya (FGB).

Actually both have tanks. Currently Zarya has 3386.7 kg and Zvezda 764 kg.

A Progress M is capable of transferring 845 kg to ISS from the resupply
system, and 240 kg from the standard prop system.

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

Yes.

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.

Progress M is the variant with the water tanks, while the M1 has the larger
propellant tanks. The Russians only use the M1 when there are shuttles
available to carry water. They've used the M exclusively since the Columbia
accident.

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Pat Flannery wrote in
:

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.

That point isn't reached until an altitude of 278 km (150 nmi), the
internationally agreed "line in the sand" altitude for ISS.

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"Jorge R. Frank" wrote in
:

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.

Oops - got that backward; the final config is more draggy.

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Henry Spencer wrote:


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.



Hold it a second! Are you saying that the total amount of Newtons of
energy to move the ISS from one orbital altitude to another is unrelated
to the total mass of the ISS to be moved? This is a simple matter of
mass to the thrust applied to it to change it's velocity, and therefore
its orbital altitude.
Double the ISS' mass and the same amount of Progress delta v only
affects its orbital velocity by one half as much.

Pat

In article ,
Pat Flannery wrote:

Henry Spencer wrote:


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.



Hold it a second! Are you saying that the total amount of Newtons of
energy to move the ISS from one orbital altitude to another is unrelated
to the total mass of the ISS to be moved? This is a simple matter of
mass to the thrust applied to it to change it's velocity, and therefore
its orbital altitude.
Double the ISS' mass and the same amount of Progress delta v only
affects its orbital velocity by one half as much.

Yes, but if you double its mass then it'll only lose half as much
altitude between reboosts.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

In message
Pat Flannery wrote:

Hold it a second! Are you saying that the total amount of Newtons of
energy

"Newtons of energy"? Tut...

Double the ISS' mass and the same amount of Progress delta v only
affects its orbital velocity by one half as much.

Indeed, But, unless your doubling of the mass has also doubled the
frontal area, the orbital velocity has only dropped by half the amount
since the last reboost.

Anthony

Pat Flannery wrote:

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.



Hold it a second! Are you saying that the total amount of Newtons of
energy to move the ISS from one orbital altitude to another is unrelated
to the total mass of the ISS to be moved?

No, he's not.

This is a simple matter of
mass to the thrust applied to it to change it's velocity, and therefore
its orbital altitude.
Double the ISS' mass and the same amount of Progress delta v only
affects its orbital velocity by one half as much.

But it only has to be done half as often. The total amount of
propellant required for reboost doesn't change over time. That is a
function of drag, not mass.

On Sun, 14 May 2006 04:58:48 -0500, Pat Flannery wrote:

Hold it a second!

But any longer and you're playing with it!

Are you saying that the total amount of Newtons of
energy to move the ISS from one orbital altitude to another is unrelated
to the total mass of the ISS to be moved?

No. Wake up and smell the rhinovirii, Pat!

He's saying that for *this particular situation*
it comes out as a wash. Ypu need to remember that
you have to factor in the *aerodynamics* of LEO.

Thus it turns out that adding mass to ISS, without
greatly increasing its drag...

.... or, in other words, increasing the *cross-
sectional density* of ISS...

(Yes, you recognixe that, don't you? A term often
used in aerodynamic calculations, especially
ballistics.)

.... happens to decrease the effect of LEO air
resistance on the station and thus decreases the
need for reboosts.

In other words to maintain a given orbital height
for ISS you need to counteract the amount of
delta-v lost to drag with an equivalent amount of
delta-v from the engines... and the denser the
station is the *less it's affected by LEO
aerodynamic drag.*

The station is slowed less, so it takes less
acceleration to get it back up to speed. Thus
you can get away with less powerful reboosts.

It's the aerodynamics of LEO, not a violation of
Newton's Third Law

Pat

Those Fire Women are doing you no damn good.
You need Nekomimi!

--
Chuck Stewart
"Anime-style catgirls: Threat? Menace? Or just studying algebra?"

Chuck Stewart wrote:
delta-v from the engines... and the denser the
station is the *less it's affected by LEO
aerodynamic drag.*

I think a better way to say it is that the amount of energy lost due to
drag depends on the aerodynamic properties of the craft, not its mass.

So a reboost to compensate for drag will need to provide the same amount
of energy whether the station is heavy or light.


Having said that, considering that orbital altitude is a function of
speed, not mass, the drag on a lighter station would provide greater
negative delta-V and thus the station would drop to a lower altitude
where drag is stronger, hence faster orbit decay.

The drag on a heavier station may remove the same amount of total
energy, but if the speed itself is reduced by a lesser amount, the orbit
will remain higher and this avoid the higher drag of lower orbits.

In article ,
Pat Flannery wrote:
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.

Hold it a second! Are you saying that the total amount of Newtons of
energy to move the ISS from one orbital altitude to another is unrelated
to the total mass of the ISS to be moved?

Yes... if, and only if, the two altitudes are defined by how far the orbit
decayed due to air drag over a specified period. Greater mass makes it
harder to reboost ISS a specific number of kilometers, but it also reduces
the number of kilometers of reboost required, by pretty much exactly the
same ratio.

By the way, the size of the reboost is not an energy and it's not measured
in newtons. :-) It's an impulse, measured in newton-seconds.
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