Station Boost With Dragon

[Jcc]

C.  Electric thrusters would ruin the micro gravity environment negating the major reason the ISS exists

I've always been bothered by that stance, since atmospheric drag by definition is messing with the microgravity environment onboard ISS.

Is the claim that continuous variable small thrust by electric propulsion counteracting drag in realtime isn't feasible leading to the thinking that you would then have to have continuous electric thrust at the average drag rate, thus leading to random undesired movement due to imbalance between constant thrust and instantaneous air drag ruining the microgravity environment? I'm having trouble reconciling that with how that would that be qualitatively worse than random air drag also messing with the microgravity environment?

The original phrasing of Ockham's razor was, "entities should not be multiplied beyond necessity".

Jim is still right, but he could have made his point with A and E; the others are superfluous and it's pointless to argue them as long as A and E exist.

Couldn't electric thrusters save fuel of the chemical thrusters, thus reducing the refueling flights and upmass? I don't think they are mutually exclusive.

[baldusi]

C.  Electric thrusters would ruin the micro gravity environment negating the major reason the ISS exists

I've always been bothered by that stance, since atmospheric drag by definition is messing with the microgravity environment onboard ISS.

Is the claim that continuous variable small thrust by electric propulsion counteracting drag in realtime isn't feasible leading to the thinking that you would then have to have continuous electric thrust at the average drag rate, thus leading to random undesired movement due to imbalance between constant thrust and instantaneous air drag ruining the microgravity environment? I'm having trouble reconciling that with how that would that be qualitatively worse than random air drag also messing with the microgravity environment?

The original phrasing of Ockham's razor was, "entities should not be multiplied beyond necessity".

Jim is still right, but he could have made his point with A and E; the others are superfluous and it's pointless to argue them as long as A and E exist.

Couldn't electric thrusters save fuel of the chemical thrusters, thus reducing the refueling flights and upmass? I don't think they are mutually exclusive.

ISS is a microgrativy research lab. Each thruster firing, each translation, each rotation, conspires against that. Being able to time them and be done quickly, helps. When the change is obvious you might be able to filter it. Also, you can generally time your experiments between operations. But if you keep a very subtle thus, you are actually doing a very subtle bias that might invalidate your experiments.
A platform at EML1 in a lassajouss orbit might be better, but in practice ISS is as good as you can get with a crew operated station. Crew tended free flyiers are better, but nobody seems to be willing to pay for.

[Comga]

Wonder if it has been looked at for cargo Dragons?

Their thruster locations are not the best

"best" is  the enemy of good enough.

Are they good enough?
If not, how could they be put in better locations?
What would constitute better locations?

Cosine losses are not all that significant.
Pushing towards the COM would seem to be important.
Being able to activate at a moment's notice, without repointing the whole ISS would seem to be important, particularly for debris avoidance maneuvers.
Minimizing plume impingement and contamination?

[Comga]

Cygnus is getting updated for station boosting.

https://forum.nasaspaceflight.com/index.php?topic=21179.msg2274831#msg2274831

I assume NGIS did this upgrade at NASA's request. Unless there is financial incentive or reliability issue why modify such reliable vehicle.
Sent from my SM-G570Y using Tapatalk

Maybe to test out systems for the HALO module for the Lunar Gateway mention in the tweet by Marcia Smith.

The HALO module has no propulsion or attitude control.  That is what PPE is for.

Let's use the common interpretation of Occam's Razor:
NASA is having NGIS modify Cygnus to boost the ISS because NASA wants the ability to boost the ISS.

[Comga]

(snip)
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

C: Not if they are run at a level of drag compensation.
They would IMPROVE the microgravity environment.
This is what GRACE and GRACE-FO did and do.

D: Even now, there is a question of diminishing returns, where adding solar panels to increase the generation to power ion thrusters would increase the drag, requiring more power and more panels. 
Whether this results in a small or large increase in the required panels could be answered numerically.

E: That's why "electric" thrusters would not be adequate for all purposes.

[John Santos]

Before anyone goes off on a SuperDraco tangent, they would be unsuitable for ISS boosts...
1) too much thrust, even throttled back as far as possible and as few (2?) engines running.
2) Horrible ISP.  They have partial, sea-level nozzles.  It would be far more efficient to use regular Dracos, which have full, vacuum nozzles, much higher ISP, and lower thrust.  Running them longer would produce the same delta-v for much less fuel use.  Regular Dracos are on both the crew and cargo Dragons, whereas the SuperDracos are only on the crew version.
3) After the unfortunate static fire incident (Rapid Unplanned Deflagration), SpaceX added burst disks to the fuel lines for the SDs, meaning they can only be fired once in a given flight, and there is risk of fuel and oxidizer mixing in the pipes if they attempt to fire them a second time.  This could be fixed by redesigning and replacing the plumbing and valves, and maybe adding a post-burn purging system.  (Many hypergolic engines and thrusters fire multiple to hundreds of times in the course of a given mission, so it isn't impossible to fix.  I'm looking at you, Agena and Titan Transtage and Apollo service module and Shuttle and Soyuz and Fregat...)  But it would cost a lot of money for little benefit on a system that SpaceX isn't interested in updating.

It would be theoretically possible to mount a SuperDraco with a full, vacuum nozzle inside the trunk, but it would need a separate fuel supply (don't want to run fuel lines through the heat shield!), would present difficulties for crewed Dragon (not sure they have the mass budget to carry ANY cargo in the trunk on crewed flights, and it would totally disrupt the aerodynamics and separation characteristics during an abort) and it would take up precious unpressurized cargo space and mass on cargo Dragons.  And even one SuperDraco might be too much thrust for ISS orbit boosting.

(A pair of fuel tanks and full-vacuum SuperDraco might make a nice 3rd stage for Falcon and particularly FH for high-velocity missions, but I don't think SpaceX has any interest in doing this on their own and I don't think any customers have offered to pay for it.  This idea is also totally off-topic for this thread.)

[Jcc]

(snip)
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

C: Not if they are run at a level of drag compensation.
They would IMPROVE the microgravity environment.
This is what GRACE and GRACE-FO did and do.

D: Even now, there is a question of diminishing returns, where adding solar panels to increase the generation to power ion thrusters would increase the drag, requiring more power and more panels. 
Whether this results in a small or large increase in the required panels could be answered numerically.

E: That's why "electric" thrusters would not be adequate for all purposes.

This makes sense to me. But for D: I don’t think having enough electric power is an issue, if the electric thrusters are very small and only provide enough thrust to compensate for atmospheric drag in real time. In other words making them small enough solves the issue of microgravity and also solves the issue of enough power, and reduces somewhat upmass requirements to refuel the chemical thrusters, but doesn’t remove the need for chemical thrusters.

[baldusi]

(snip)
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

C: Not if they are run at a level of drag compensation.
They would IMPROVE the microgravity environment.
This is what GRACE and GRACE-FO did and do.

D: Even now, there is a question of diminishing returns, where adding solar panels to increase the generation to power ion thrusters would increase the drag, requiring more power and more panels. 
Whether this results in a small or large increase in the required panels could be answered numerically.

E: That's why "electric" thrusters would not be adequate for all purposes.

If you want to use electric thrusters compensate for drag without compromising the microgravity engironment, you must vector them through the CoG of the ISS as a whole and run them continuously at exactly the drag. Where drag is a function of local gas density, temperature and station attitude. ISS already has a lot of different consideration on which attitude it must take, plus all the VV, solar panes gimbal and so many other variables. Would it be possible? I guess. Is it simple and reasonable given the expected life of the ISS? I don't think so. In fact, I would haphazard a guess and the next generation commercial station will have some sort of freeflyer that will offer a better environment for those that really need a better environment.
As I said before, the thrusters that could do reboost on Dragon 2 are on the side of the docking port pointing towards the station. Is not that Dragon 2 needs a few mods. It would need to redesign the whole RCS system, plus the deorbit attitude and conops. Cygnus just needed some beefed up thrusters in its thrust pod.

[Barley]

Back of the envelope

The ISS masses about 400,000kg.
At 400km orbit needed boost is about 2km per year, corresponding to about 1.2 m/s delta V.
Draco thrust is 400 N.

A single Draco can accelerate the ISS at 1 mm /s^2
So a 1200s (20 minute)  should do it using about 150kg of fuel.
Cosine loss should not be worst than root 2 so 30 minutes if everything aligns wrong.
This is the same order of magnitude as Dragons deorbit burn.

Questions
0) Are any of my facts or calculations off by more than a factor of 2?
1) Is this inherently implausible for any thruster of Draco's size?
2) Why is it implausible for Dragon using Draco thrusters?