SpaceX and Replacement of Russian module services on the ISS

[Barley]

The biggest obstacle to using Dragon is that thrusters are in the wrong position.

Could somebody explain this.

For any boost the thrust vector should go through the center of mass of the station.

The Nader and Forward docking ports are at right angles.  The direction of thrust relative to the docked vehicle will also be at a large angle (somewhere between 45 and 90 degrees depending on the exact location of the station CoM).  Therefore at least one of these ports you do not need, and may not be able to use, aft pointing thrusters.  In other words, at one of these ports you need to boost sideways.  Yet it appears that the conventional wisdom is that the thrust direction (relative to the docked vehicle) is the same at all ports.

Alternatively, if the docking ports can only handle axial thrust, how do you boost using the Nader and Zenith ports?

[AS_501]

The biggest obstacle to using Dragon is that thrusters are in the wrong position.

Could somebody explain this.

For any boost the thrust vector should go through the center of mass of the station.

The Nader and Forward docking ports are at right angles.  The direction of thrust relative to the docked vehicle will also be at a large angle (somewhere between 45 and 90 degrees depending on the exact location of the station CoM).  Therefore at least one of these ports you do not need, and may not be able to use, aft pointing thrusters.  In other words, at one of these ports you need to boost sideways.  Yet it appears that the conventional wisdom is that the thrust direction (relative to the docked vehicle) is the same at all ports.

Alternatively, if the docking ports can only handle axial thrust, how do you boost using the Nader and Zenith ports?

Barley:  It may have to do with how the station is oriented relative to its velocity vector.  (I was wondering something similar with the docking of Nauka.  I thought the Russian module would dock at right angles to the VV.  Instead, the entire station was rotated 90 deg. so that Nauka docked along the VV.)  I defer to one of our celestial mechanics on your question.

[Jim]

The biggest obstacle to using Dragon is that thrusters are in the wrong position.

Could somebody explain this.

For any boost the thrust vector should go through the center of mass of the station.

The thrusters for deorbit and primary axial are forward facing.

[vp.]

The plan is to launch the Axiom modules in 2024 to attach them to the ISS. Then the Axiom modules will become an autonomous station.
Therefore, the solution to replace the Russian modules is Axiom.
Well, maybe NASA needs to help accelerate this program.

[Comga]

By now boost servicing shouldn't even be necessary.  The station should have been outfitted with electric thrusters and requisite power sources ages ago.

Wrong for many reasons.
A.  The existing thrusters would still be needed for CMG desat
B.  Electric thrusters still need propellant servicing
C.  Electric thrusters would ruin the micro gravity environment negating the major reason the ISS exists
D.  power sources were not available ages ago
E.  Electric thrusters can't make collision avoidance maneuvers

Rather than poking the bear by responding “Wrong” each of these points can be discussed.

A. CMG desat can be done without thrusters if planned in advance.
Other thrusters could be used.

B. With their order-of-magnitude higher Isp, electric thrusters can get by with an order of magnitude less propellant. However, systems for refueling them are not developed, so the systems might be disposable.

C. Drag compensation would improve the microgravity. Electric thrusters could be modulated to compensate for variations in exoatmospheric density and drag.

D. History says how we got here, but it’s still history. That said, whether there is sufficient electric power available for routine drag compensation is a good question.

E. Not wrong 😊 For example the MEV has what, 0.4 N thrust?  However emergency collision avoidance can be done by thrusting in directions other than prograde with systems not mounted in the rear. It would just not accomplish the dual purpose of reboosting at the same time.

[Lee Jay]

B. With their order-of-magnitude higher Isp, electric thrusters can get by with an order of magnitude less propellant. However, systems for refueling them are not developed, so the systems might be disposable.

I'm dropping way back into my memory now, but I thought ISS produced an excess of hydrogen as a result of the ECLSS.  That hydrogen, if memory serves, is currently dumped overboard using a zero thrust port of some sort.  While hydrogen is far from ideal as an ion thruster propellant, maybe that hydrogen stream could be diverted for that purpose simply because it's available "for free"?

[Comga]

The biggest obstacle to using Dragon is that thrusters are in the wrong position.

Could somebody explain this?

For any boost the thrust vector should go through the center of mass of the station.

The thrusters for deorbit and primary axial are forward facing.

But what about the non-primary axial thrusters in the sidewalls?
If the station was turned around, those would fire away from the ISS while suffering some cosine loss, probably around 40% in terms of propellant usage.
However, with the shared, unexpended abort propellant in a Dragon 2, crew or cargo, there should still be enough for months of reboosting.
(This can be calculated by someone who has more information than I do. Anyone volunteering?)

This thread is predicated on an emergency condition. 
Systems that will be available a year or more from now are not applicable to the question of the survival of the ISS.
Systems, including having available launchers, that are less than optimal, are what you go with in an emergency.

[Asteroza]

B. With their order-of-magnitude higher Isp, electric thrusters can get by with an order of magnitude less propellant. However, systems for refueling them are not developed, so the systems might be disposable.

I'm dropping way back into my memory now, but I thought ISS produced an excess of hydrogen as a result of the ECLSS.  That hydrogen, if memory serves, is currently dumped overboard using a zero thrust port of some sort.  While hydrogen is far from ideal as an ion thruster propellant, maybe that hydrogen stream could be diverted for that purpose simply because it's available "for free"?

one of the early plans for the ISS VASIMR demo was planning on using that hydrogen, but for some reason the VASIMR was going to be pointed sideways, thus being poorly positioned for continuous reboost (but the demo was power constrained, using burst power from an accumulator battery pack, so I guess doesn't qualify as continuous...)

[Asteroza]

It's a shame that some of the ideas around electric thrusters on a pallet held by the existing robot arm aren't applicable here, as the thrust load is small and the arm could adjust the thrust direction for CoG matching. It seems to me that the big issue would be supplying power via a cable, as the arm itself probably can't supply enough natively.