Electric Propulsion for ISS

[DaveH62]

 With all of the CRS launch systems now contracted, would it make sense to start launching a SEP system for the ISS? This would reduce reliance on the Russian systems, but more importantly, provide a more stable and strategic solution. I think you would have to have some emergency chemical propulsion system, but for normal altitude control SEP seems superior. This seems like a component you would want for any BEO habitat and good way to test next generation options.

[Jim]

With all of the CRS launch systems now contracted, would it make sense to start launching a SEP system for the ISS? .

What is going to power it and what is going to be the propellant?  How would the CRS vehicles handle the propellant (which they are not on contract to do and would require a new contract)?

[nacnud]

not a great source but...

https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket#Potential_testing_on_the_International_Space_Station

[a_langwich]

With all of the CRS launch systems now contracted, would it make sense to start launching a SEP system for the ISS? This would reduce reliance on the Russian systems, but more importantly, provide a more stable and strategic solution. I think you would have to have some emergency chemical propulsion system, but for normal altitude control SEP seems superior. This seems like a component you would want for any BEO habitat and good way to test next generation options.

I thought the same thing too for a long time, but I've come to realize it's not really a better solution for ISS.

It focuses on the "altitude maintenance" aspect, and ignores other relevant factors.  Many of the experiments on ISS are investigating/using/depending on microgravity, and a constant thrust poisons that.  As a research platform, long periods of "quiet" unpoisoned by even the small accelerations of SEP, punctuated by short, comparatively stronger accelerations that represent holes in the data, are preferred over constant thrust.

Furthermore, there is no set "desired altitude" at which you want to orbit ISS.  It varies as the atmosphere expands and contracts, varies as a function of FOD risk, is constrained as a function of visiting vehicle performance required, and rather than bouncing up and down like an elevator, it is convenient for all these reasons to let it drift at one particular altitude, and then make a minimal number of discrete maneuvers.

An analogy would be drifting on a float in a pool:  the peaceful undisturbed drifting has value, maybe The Value; it's not just a side effect of your propulsion scheme.  If you get close to an edge you can kick off, but that occasional movement is preferred to a constant whirring and churning of a GPS-and-lidar-with-microthrusters-maintained constant positioning system.  Granted, most engineers would get seduced by the gadgety nature of the second solution, but the first solution is better if undisturbed solitude is the desired goal.

[nacnud]

Constant thrust could provide a better Microgravity environment, though I don't know the magnitude of all the perturbations in the system.

http://www.esa.int/Our_Activities/Observing_the_Earth/GOCE/Introducing_GOCE

[arachnitect]

we've had like 4 different variations of this conversation.

Anyone remember the latest thread?

edit: here's a couple examples. I think there was a thread specifically about SEP at some point as well.

http://forum.nasaspaceflight.com/index.php?topic=34342.0
http://forum.nasaspaceflight.com/index.php?topic=37092.0

[Danderman]

Constant thrust could provide a better Microgravity environment, though I don't know the magnitude of all the perturbations in the system.

An appropriately designed system would simply offset drag and thus maintain altitude while decreasing microgravity in the lab.

[Prober]

Constant thrust could provide a better Microgravity environment, though I don't know the magnitude of all the perturbations in the system.

An appropriately designed system would simply offset drag and thus maintain altitude while decreasing microgravity in the lab.

Yes, if that's the only mission for the ISS.


http://forum.nasaspaceflight.com/index.php?topic=36322.0


If however the "mission" for the ISS is to prepare for travel to other worlds there might be conflicts?

[A_M_Swallow]

Currently the ISS is the USA's only spacestation. If Bigelow gets his way there will be spacestations for tourists, other scientific experiments and as interchanges for other worlds. They may not need to be a near zero gravity location and could be interested in electric propulsion for roll control, attitude control and station keeping. Same for the Deep Space Habitat (DSH).

[Bob Shaw]

The time when SEP would start to become an interesting option for the ISS would be when it ceases to be a reliable/affordable asset. You have three choices: abandon in space, and hope for the best; de-orbit; raise altitude significantly.

The first, worst, option would occur only if there was some sort of real crisis, particularly one which left it with no attitude control. It might be a fire, a structural failure or a collision, and would result in a very nasty problem for those of us downstairs when it re-enters. Hopefully, it never comes to that.

The second would be a repeat of the planned Mir disposal; this seems like a waste, but is safe enough and will probably win the day.

Thirdly, you decide to exploit what you have by lofting the station. Some aspects of the ISS structures have potentially long service lives, others have much less time left. The keel, Canadarm and trolley and the airlock are all valuable commodities, the pressurised tin can parts less so. I could actually see the nodes being re-used, but not the big tin cans, attractive though their volume might be - they're full of *old* stuff. Obviously, you'd need a spacecraft capable of man-tending the ISS while certain elements are removed, and some means of dealing with the excess parts. One simple way to re-use the ISS is entirely passive, where it would be deactivated, arrays retracted where possible and simply used as a counterweight for whatever purpose; another would be to use the lee of it it as a protected harbour for other vehicles if orbital debris becomes more of an issue.

There are major reasons not to choose the third option (not least being that the thing is in the wrong orbit) but I hope that we can make some use of this sturdy and reliable asset, and not let it burn up over the ocean or simply crash.

[Burninate]

we've had like 4 different variations of this conversation.

Anyone remember the latest thread?

edit: here's a couple examples. I think there was a thread specifically about SEP at some point as well.

http://forum.nasaspaceflight.com/index.php?topic=34342.0
http://forum.nasaspaceflight.com/index.php?topic=37092.0

We have indeed.  Here's my contribution.

http://forum.nasaspaceflight.com/index.php?topic=35126

Have fun with all the brevity I failed to include.  Also understand part of the context was people were discussing splitting the ISS at the Iron Curtain during the Ukraine showdown.

* The microgravity concern is bunk - there wouldn't be enough acceleration to matter if it was working orders of magnitude more than required to fight drag, and simultaneously, drag is an acceleration we're trying to fight by nulling it out to zero.  The current strategy of chemical burns does prevent any long-duration experiments that require a high degree of microgravity.  The horizon-following mode the ISS usually uses also keeps such experiments from working well in most of the station, because horizon-following is rotation, and rotation has centripetal acceleration.  There is a tiny locus of microgravity at the center of mass that feels less acceleration from centripetal acceleration than from drag.

* Transitioning the ISS to SEP would require a moderate configuration change to the ISS in order to give freedom from plume impingement.  Jim's constant refrain in these threads seems to suggest he thinks moderate configuration changes are impossible - I think he's used to exploring choices in an environment where he has no power to affect configuration changes.  This tends to collide with the forum's more openminded discussion.  I observe that many changes have been made in the ISS plan over time as things were proposed or cancelled.

* The power concern is probably negligible - The power required to fight drag is a small percentage of the station's power generation, though power is always scarce and some small sacrifice may be needed (as a sacrifice was needed to, for example, power the Russian section without the main Russian solar array).    If they're cut too close, more solar panels are not prohibitive.

* The VASIMR unit Ad Astra was pushing was going to suck down a lot of power, per the plan, without providing any actual propulsion. Its minimum static power is much more than required for SEP stationkeeping, so it was going to fill batteries.  The ISS was just supposed to be a testing platform without getting any benefit - they were performing small, balanced plane changes (like pushing at a wall to see how much force you can exert).

* The best argument against SEP on the ISS is that the ISS is not especially useful when we have developed the capability to build much larger space stations powered by SEP.  What would retrofitting SEP to the ISS and running it for another decade cost, versus designing SEP into a future BA-2100 or MCT-hab?

[Bob Shaw]

What would retrofitting SEP to the ISS and running it for another decade cost, versus designing SEP into a future BA-2100 or MCT-hab?

It would obviously have costs, and implications. But proving that the thing would work for a decade would reduce much of the concern when applying the technology to later stations and vehicles, and that would indeed be a worthwhile outcome.

[Jim]

1.  Transitioning the ISS to SEP would require a moderate configuration change to the ISS in order to give freedom from plume impingement.  Jim's constant refrain in these threads seems to suggest he thinks moderate configuration changes are impossible - I think he's used to exploring choices in an environment where he has no power to affect configuration changes.  This tends to collide with the forum's more openminded discussion.  I observe that many changes have been made in the ISS plan over time as things were proposed or cancelled.

2.   The power concern is probably negligible - The power required to fight drag is a small percentage of the station's power generation, though power is always scarce and some small sacrifice may be needed (as a sacrifice was needed to, for example, power the Russian section without the main Russian solar array).    If they're cut too close, more solar panels are not prohibitive.

3.   What would retrofitting SEP to the ISS and running it for another decade cost, versus designing SEP into a future BA-2100 or MCT-hab?

1.  Major change. Where is it going to be mounted?  The original propulsion trusses were eliminated.  It would have to be either on the Service module or the forward node.  Both are prime docking locations.  Moderate changes are not impossible just that there is no chance in hell of them happening.   It has nothing to do with my power, it is acknowledgment of the existing and future environment.  It isn't open mindness, it is willful ignorance of reality and engineering practices.

2.  Power is not negligible and there is little reserve power.  Where would additional arrays be mounted?  There was no sacrifice to power the Russians.


3.  There is no need to "test" SEP.  It has already flown operationally many times

[redliox]

* The VASIMR unit Ad Astra was pushing was going to suck down a lot of power, per the plan, without providing any actual propulsion. Its minimum static power is much more than required for SEP stationkeeping, so it was going to fill batteries.  The ISS was just supposed to be a testing platform without getting any benefit - they were performing small, balanced plane changes (like pushing at a wall to see how much force you can exert).

I wondered whatever happened to that VASMIR proposal.  So even with the substantial arrays in place it's not enough to balance between normal station needs versus stations keep; SEP isn't worth it when the power to keep the crew from burning up is keeping them from breathing.

* The best argument against SEP on the ISS is that the ISS is not especially useful when we have developed the capability to build much larger space stations powered by SEP.  What would retrofitting SEP to the ISS and running it for another decade cost, versus designing SEP into a future BA-2100 or MCT-hab?

Right, since apparently the ISS wasn't originally conceived to support SEP in the first place nor run it with its current power supply.  So if anyone wants to see a SEP space station, the best bet would be looking toward the Deep Space Habitat?  I wonder how much thought has been given toward SEP in that avenue, whether it be station keeping or boosting the whole thing to Lunar Lagrange/Lunar Orbit?

[A_M_Swallow]

Some figures.
F = m a
v = u + a t so a = delta-v / t

setting t = half a year
ISS station keeping delta-n to 100 m/s (from Wikipedia)
ISS mass to 419,455 kg

F = m * delta-v / t = 419,455 * 100 / (0.5 * 365.25 * 24 * 60 * 60) = 2.658 N

The off the shelf NEXT ion thruster, which produces 0.236 N at 6.9 kW and Isp 4190 s

so 2.658 / 0.236 = 11.26 thrusters are needed

Power = 11 * 6.9 kW = 75.9 kW

By increasing the burn time fewer thrusters and small solar panels can be used but the same quantity of propellant will be needed.

[QuantumG]

Now do polar coordinates, go!

[A_M_Swallow]

Now do polar coordinates, go!

I will leave that to the polar bears.

[AlesH]

ISS station keeping delta-v to 100 m/s (from Wikipedia)
... Power = = 75.9 kW

I don't know, what is included in 100 m/s "station keeping" (and in what time), but for merely maintaining the orbital altitude of ISS is sufficient much less delta-v. ISS on average drops in two months less than 5 km ( http://www.heavens-above.com/IssHeight.aspx ), so sufficient delta-v is less then 3 m/s for two months or less then 20 m/s for a year (or less then 10 m/s for half a year). So then could be sufficient one NEXT ion thruster and less then 8 kW of power. Xenon consumption would be around 200 kg per year. Or am I wrong somewhere?

[A_M_Swallow]

ISS station keeping delta-v to 100 m/s (from Wikipedia)
... Power = = 75.9 kW

I don't know, what is included in 100 m/s "station keeping" (and in what time), but for merely maintaining the orbital altitude of ISS is sufficient much less delta-v. ISS on average drops in two months less than 5 km ( http://www.heavens-above.com/IssHeight.aspx ), so sufficient delta-v is less then 3 m/s for two months or less then 20 m/s for a year (or less then 10 m/s for half a year). So then could be sufficient one NEXT ion thruster and less then 8 kW of power. Xenon consumption would be around 200 kg per year. Or am I wrong somewhere?

100 m/s per year is the worst case station keeping for 400-500 km LEO. The table shows an average of 25 m/s per year.

Reducing the burn time from 1 year to 6 months gives a good safety margin. The thrusters would probably be switched off when the ISS is docking. We do not have to worry about batteries failing, however sunlight only burns may put the ISS into an elliptical orbit.

[baldusi]

AIUI, most of those things are CGM desat and DAM. Those simply can't be made with SEP. I remember that we had calculated the drag and it was around 0.25N/s