Solar Electric Propulsion for a Flexible Path of Human Space Exploration

[Robotbeat]

http://multimedia.seti.org/PhD2011/abstracts/PhD2-11-011.pdf
http://multimedia.seti.org/PhD2011/presentations/3-Human%20Exploration%20Session/PhD2-2011-Landau-Talk-[11-011].pdf

We present an architecture for Human missions to Phobos and Deimos in the 2030s using Solar Electric Propulsion (SEP).  These missions would be an extension of near-Earth asteroid missions using similar hardware in the 2020s.  The concept relies on taking existing, flight-proven technologies from unmanned spaceflight and scaling them up to higher power levels for human spaceflight.  When applied to human spaceflight, the robustness of SEP trajectories and the lack of time critical events significantly enhance mission safety for astronauts.  This is accomplished by using SEP boost stages to pre-position a Deep Space Vehicle (DSV), supplies, and chemical boost stages in High Earth Orbit (HEO).  Pre-placing these elements in HEO for later crew rendezvous avoids having the crew onboard the DSV during the 1–2 year long, low-thrust parts of the trajectory, while still taking advantage of the high fuel efficiency of solar electric propulsion systems.

Once these assets are pre-placed in HEO, a lunar flyby is used to drop the perigee of the DSV to the altitude of International Space Station (ISS) orbit.  Astronauts are then launched from the ISS to rendezvous with the DSV in an Orion Crew Module using a chemical boost stage.  Once the crew establishes that the DSV is ready for departure from HEO the DSV performs an Earth escape burn with a chemical boost stage. After Earth departure, the crew uses the SEP stage as part of the DSV to rendezvous with Deimos then Phobos for an accumulated orbit time of four months. Following rendezvous, the DSV returns to Earth using the SEP stage and the astronauts depart in the Orion Crew Module for a direct entry.  After the crew returns, the unmanned DSV uses the SEP stage to return to HEO over the course of a year where it is refurbished for reuse on a subsequent mission.

Observations
• SEP reduces launch mass for missions ranging
from cis-lunar excursions, to NEO encounters, to
Phobos & Deimos rendezvous
• Power levels of 100-600 kW enable SEP missions
with IMLEO comparable to NTR technology with
similar flight times. 
• There are many paths from cis-lunar missions to
Mars through the NEO capability design space
• The evolution of SEP from current levels
introduces flexibility on an exploration path to
Mars.

What is interesting to me is that 100-600kW is about the range of the proposed Solar-electric Propulsion demo, slated at ~400kW. That means the "demo" could easily be used for real manned missions to Mars orbit (and, paired with pre-placed Mars landers, Mars surface), NEOs, etc (besides the obvious cis-lunar applications).

EDIT:Another interesting thing is that it kicks NTR's butt when it comes to delivering cargo to Mars for a given IMLEO, and gets over twice the payload to Mars's atmospheric interface compared to chemical for a given IMLEO. And the assumptions for the array specific power are very conservative. Another interesting thing is that total IMLEO for a Phobos mission can be less than 300mT (with a mission duration of ~2.5 years). That means the average annual per-year IMLEO would be less than 130mT per year for a Phobos mission. HLV optional.

[Hop_David]

And the assumptions for the array specific power are very conservative.

What specific power did they assume? I didn't see that mentioned on the documents you linked to.

[manboy]

Is the pdf using short tons?

[A_M_Swallow]

From page 7 of the talk .pdf

"15% inert/propellant, 30 kg/kW"

For 100 t IMLEO launch using 600kW, 3000 s SEP taking 1.5 years to Mars delivering a mass of 52.8 t to the atmospheric interface.

(Other flight times and mass are possible.)

[Ronsmytheiii]

http://www.parabolicarc.com/2011/09/15/nasa-selects-companies-to-study-solar-electric-propulsion-spacecraft/#more-29549

NASA has selected five companies to develop concepts for demonstrating solar electric propulsion in space. These capabilities are important for the agency’s future human exploration missions to deep space.

– Analytical Mechanics Associates Inc., Hampton, Va.
– Ball Aerospace & Technologies Corp., Boulder, Colo.
– The Boeing Company, Huntington Beach, Calif.
– Lockheed Martin Space Systems Company, Littleton, Colo.
– Northrop Grumman Systems Corp., Redondo Beach, Calif.

Image url:

http://www.ama-inc.com/images/stories/landscape_large.jpg

[Robotbeat]

Awesome. This will pay dividends in making human exploration architectures a lot more flexible. And lower cost, especially for large missions (like Mars surface) and/or for continual resupply of depots or outposts (if that is ever done).

[Robotbeat]

http://www.parabolicarc.com/2011/09/15/nasa-selects-companies-to-study-solar-electric-propulsion-spacecraft/#more-29549

NASA has selected five companies to develop concepts for demonstrating solar electric propulsion in space. These capabilities are important for the agency’s future human exploration missions to deep space.

– Analytical Mechanics Associates Inc., Hampton, Va.
– Ball Aerospace & Technologies Corp., Boulder, Colo.
– The Boeing Company, Huntington Beach, Calif.
– Lockheed Martin Space Systems Company, Littleton, Colo.
– Northrop Grumman Systems Corp., Redondo Beach, Calif.

Image url:

http://www.ama-inc.com/images/stories/landscape_large.jpg

Is there a NASA.gov link for this?

[Solman]

 Provided a large parabolic concentrator mirror is used to concentrate sunlight onto triple junction or other type of PV to generate the electricity for SEP; the possibility of direct use of the concentrated sunlight for solar thermal rocket propulsion is a side benefit in addition to the 40% or better conversion efficiency of this type of PV. Advantages include higher specific power for the electric generation and the higher thrust for orbit raising from low to highly elliptical orbit of the solar thermal rocket to speed this up vs. electric albeit at lower Isp.
 A mix of thermal and electric can provide a wide range of Isp as well. While hydrogen is best and lithium second best, solar thermal can use almost anything as propellant including direct use of NEO regolith even if that's all that's available. The concentrator mirror can double as a communications or radar antenna.

Steve

[A_M_Swallow]

Providing it uses the same propellant solar thermal may be better for the RCS (Reaction Control System) than ion thrusters.  RCS burns tend to be short where as SEP works better for extra long burns.  I suspect that both can use Argon and nitrogen.

[Solman]

Providing it uses the same propellant solar thermal may be better for the RCS (Reaction Control System) than ion thrusters.  RCS burns tend to be short where as SEP works better for extra long burns.  I suspect that both can use Argon and nitrogen.

 I remember seeing a paper from the Nineties that described a small working model of a solar thermal rocket concentrator that used fiberoptic cables to route the focussed sunlight to the engine.
 This might be a good design for RCS thrusters using solar thermal.

[A_M_Swallow]

Providing it uses the same propellant solar thermal may be better for the RCS (Reaction Control System) than ion thrusters.  RCS burns tend to be short where as SEP works better for extra long burns.  I suspect that both can use Argon and nitrogen.

 I remember seeing a paper from the Nineties that described a small working model of a solar thermal rocket concentrator that used fiberoptic cables to route the focussed sunlight to the engine.
 This might be a good design for RCS thrusters using solar thermal.

Probably this article.
"Solar Thermal Propulsion for Small Spacecraft" PSI-SR-1228
by Takashi Nakamura et al.
http://www.psicorp.com/pdf/library/sr-1228.pdf

[Solman]

Providing it uses the same propellant solar thermal may be better for the RCS (Reaction Control System) than ion thrusters.  RCS burns tend to be short where as SEP works better for extra long burns.  I suspect that both can use Argon and nitrogen.

 I remember seeing a paper from the Nineties that described a small working model of a solar thermal rocket concentrator that used fiberoptic cables to route the focussed sunlight to the engine.
 This might be a good design for RCS thrusters using solar thermal.

Probably this article.
"Solar Thermal Propulsion for Small Spacecraft" PSI-SR-1228
by Takashi Nakamura et al.
http://www.psicorp.com/pdf/library/sr-1228.pdf

 Thanks for the link. Carried to an extreme, fiber optics could be used to distribute heat and light to wherever its needed in a spacecraft. Perhaps the cable could be designed to shift the many frequencies of sunlight into a single frequency as it travels down the fiber optic line and deliver it to a PV element at the point where a device needs electricity. The PV element would be very efficient since it could be designed for a single frequency. The fiber optics could also provide data transfer and control of course. Cutting and welding using concentrated sunlight delivered by fiber optics also a possibility I would think.
 Large concentrators also enable a solar furnace for ISRU and they can double as microwave transmitters for space based power transmission between spacecraft or down to the surface of Mars for instance. They can be formed by a technique I call inflation insituform with degradable elements in which some parts harden when exposed to the Sun's UV light and other parts degrade. The finished product can be optimized for low mass - high specific power and can achieve very high temperatures if adjusted by built-in actuators both as it forms and thereafter. Or at least it seems practical to me.
 Another advantage of PV for concentrated sunlight is that the cells must be actively cooled and the resulting waste heat is useful for keeping spacecraft warm. This means nearly 100% of the intercepted sunlight is used compared to a regular PV array which must use some of its electricity to provide resistance heat and has lower efficiency to begin with. The support structure for the concentrator can be at least partially made of hollow tubes that could double as waste heat radiators perhaps.

Steve

[A_M_Swallow]

From the "Re: POLL: Are you supportive of the Prop Depot Concept?" thread reply #121.
http://forum.nasaspaceflight.com/index.php?topic=27297.msg832919#msg832919

Xplor wrote

from: A_M_Swallow on Today at 01:49:08

   

from: Xplor on 24 November 2011, 13:24:01

        For those suggesting starting with SEP, what power level are you proposing?
        1) 10's of Kw
        2) 100's of Kw
        3) 1,000's of Kw

    For a working vehicle - this decade - buy as much as possible off the shelf so 10's of kW.
    Two of the ATK Ultraflex built for the Orion will supply 12 kW and four 24 kW.  There are several Hall Thrusters in that range.
    http://www.aiaa.org/pdf/industry/presentations/AIAA_IECEC_Final_Anderson_7_20_10_Nashville.pdf

My fear of SEP is that NASA will continue down their 100's kw path leading to another multi-billion dollar development/infrastructure demand.   “The flight demonstration mission would test and validate key capabilities and technologies required for future exploration elements such as a 300 kilowatt solar electric transfer vehicle.”

Deriving SEP from today's communication satellites may provide affordable SEP and  benefit America's satellite industry.  This is similar to the arguments in favor of using EELV class launch vehicles.

IMHO.  A large SEP tug will be needed to transport propellant to Mars but a small and cheaper SEP tug can be used as a test bed to develop the tugs systems.

[A_M_Swallow]

NASA has selected five companies to develop concepts for demonstrating solar electric propulsion in space. These capabilities are important for the agency’s future human exploration missions to deep space.

Any information on when these studies are going to be published?

[AnalogMan]

A recent paper which may be of interest to this thread:

Concept Design of High Power Solar Electric Propulsion Vehicles for Human Exploration
62nd International Astronautical Congress NASA/TM—2011-217281 published 1Dec11 16pp

Human exploration beyond low Earth orbit will require enabling capabilities that are efficient, affordable and reliable. Solar electric propulsion (SEP) has been proposed by NASA’s Human Exploration Framework Team as one option to achieve human exploration missions beyond Earth orbit because of its favorable mass efficiency compared to traditional chemical propulsion systems. This paper describes the unique challenges associated with developing a large-scale high-power (300-kWe class) SEP vehicle and design concepts that have potential to meet those challenges. An assessment of factors at the subsystem level that must be considered in developing an SEP vehicle for future exploration missions is presented. Overall concepts, design tradeoffs and pathways to achieve development readiness are discussed.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120000068_2011025608.pdf

[A_M_Swallow]

A reusable SEP tug needs the ability to be refuelled in orbit and to accept new cargoes.  This process can be tested with a 30 kW tug.  The tugs second mission could take a real cargo to GSO or EML-1.

[simonbp]

Transfer of supercritical propellant in-space is non-trivial. It would likely be easier to just have swapable propellant tanks.

[Archibald]

Transfer of supercritical propellant in-space is non-trivial. It would likely be easier to just have swapable propellant tanks.

NASA did a lot of work on superfluid helium transfer in the 80's - when Spitzer was to be refilled to extend its useful life. How hard would it be to transfer helium in zero G ?

[alexw]

Transfer of supercritical propellant in-space is non-trivial. It would likely be easier to just have swapable propellant tanks.

NASA did a lot of work on superfluid helium transfer in the 80's - when Spitzer was to be refilled to extend its useful life. How hard would it be to transfer helium in zero G ?

    Supercritical fluid != Superfluid.

    Also, how far did such plans with Spitzer get?
                                   -Alex

[Robotbeat]

Transfer liquid Xenon instead. As a bonus, the tanks will be smaller and much, much lighter for the same amount of propellant.

[A_M_Swallow]

Transfer liquid Xenon instead. As a bonus, the tanks will be smaller and much, much lighter for the same amount of propellant.

First check to see if you can use the much cheaper Argon.

[Archibald]

Transfer of supercritical propellant in-space is non-trivial. It would likely be easier to just have swapable propellant tanks.

NASA did a lot of work on superfluid helium transfer in the 80's - when Spitzer was to be refilled to extend its useful life. How hard would it be to transfer helium in zero G ?

    Supercritical fluid != Superfluid.

    Also, how far did such plans with Spitzer get?
                                   -Alex

I have difficulties understanding the difference between the two !

About Spitzer (formerly SIRTF, for Shuttle / Spacelab / Space InfraRed Telescope Facility):
It started as a Spacelab telescope, then moved to a polar orbit free flyer. The manned servicing capability was lost after 1990 in the multiple cuts that shrunk Spitzer from shuttle / Titan IV to Atlas II then to Delta II.
Long after the dust settled in 1993 a shuttle mission STS-57 carried a SuperFluid Helium On Orbit Transfer experiment - SHOOT.
http://cryo.gsfc.nasa.gov/SHOOT/STS57.html

[Robotbeat]

Supercritical just means the pressure and temperature are both high enough that there's no longer a solid line between gas and liquid... Supercritical is an in-between phase.

Superfluid means there's no viscosity at all (though with enough effort, you can form these weird quantized vortex things). It's a quantum mechanical effect that's really weird. Superconductivity happens when the electrons in a material enter a superfluid state. In regular matter (and other than electrons), it pretty much only happens with liquid helium.

[alexw]

Long after the dust settled in 1993 a shuttle mission STS-57 carried a SuperFluid Helium On Orbit Transfer experiment - SHOOT.
http://cryo.gsfc.nasa.gov/SHOOT/STS57.html

Thanks for that reference.
"The porous plugs used in SHOOT were manufactured by Coors, a company which has been making porous filters for many years. Lately, they have branched out as a manufacturer of dilute ethanol solutions, which they filter by pumping them through their own porous filters."
       -Alex

[yg1968]

A couple of articles on the topic:
http://www.aviationweek.com/Article.aspx?id=/article-xml/AW_05_28_2012_p20-460440.xml&p=1
http://aerospace.engin.umich.edu/newsevents/northrop-grumman-tapped-by-nasa-to-develop-solar-electric-propulsion-flight-concepts-for-future-space-missions

[A_M_Swallow]

They will have to change the name of the arrays, MegaFlex is already the name of a UK company.

[Prober]

Transfer liquid Xenon instead. As a bonus, the tanks will be smaller and much, much lighter for the same amount of propellant.

just started looking into an upgrade for the ISS stationkeeping.  Wonder if Xenon based thrusers could do the job?
 
Could the 702SP satellites, or 702HP be the starting point?
http://www.boeing.com/defense-space/space/bss/factsheets/702/702fleet.html
 
Further efficiency derives from the 702's advanced xenon ion propulsion system (XIPS), which was pioneered by Boeing.  XIPS is 10 times more efficient than conventional liquid fuel systems. Four 25-cm thrusters provide economical stationkeeping, needing only 5 kg of fuel per year - a fraction of what bipropellant or arcjet systems consume.

[pathfinder_01]

 
just started looking into an upgrade for the ISS stationkeeping.  Wonder if Xenon based thrusers could do the job?
 

Bad idea. The thrusters could cancel out the microgravity the station wants. They would induce a constant or near constant acceleration. There are plans to test VASIMR at the station and it would cancel out drag a bit but it would only run for short burst both due to power issuses and microgravity issues.

[Robotbeat]

 
just started looking into an upgrade for the ISS stationkeeping.  Wonder if Xenon based thrusers could do the job?
 

Bad idea. The thrusters could cancel out the microgravity the station wants. They would induce a constant or near constant acceleration. There are plans to test VASIMR at the station and it would cancel out drag a bit but it would only run for short burst both due to power issuses and microgravity issues.

If you did it just right, it's possible to get /better/ microgravity on ISS using electric thrusters.

[deltaV]

According to the Keck study on asteroid moving (http://www.kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf) (italics added):

Near-Term Application of SEP Technology for Human Missions to NEAs
The development of a 40 kW-class SEP system would provide the valuable capability of being able to pre-deploy several tons of destination elements, logistics, and payloads. Initial estimates identify that approximately 3,100 kg of elements and logistics, along with approximately 500 kg of destination payload, could be pre-deployed in support of a human NEA mission, rather than carried with the crew.
This approach would reduce the requirements for the launch vehicles and in-space propulsive elements required to conduct a human mission. The amount of mass that could be pre-deployed along with the SEP system is primarily a function of the launch vehicle utilized, the orbital energy requirements of the NEA target, the efficiency of the SEP system, and the desired amount of returned mass. Although a SEP system and associated cargo could be delivered to low-Earth orbit (LEO) by the launch vehicle and spiraled out to escape the Earth’s gravity, the time required to perform this operation along with the radiation and micrometeoroid and orbital debris (MMOD) exposure resulting from the spiral from LEO would make it desirable for the launch vehicle to be able to propel the SEP system and payload to an escape C3. Additionally, since the departure windows for accessible NEAs could be short and since it is likely that pre-deployed assets would be required to be at NEA prior to crew departure from Earth, the duration of the pre-deploy mission would be a critical factor.

The italicized text contrasts with other proposals that use SEP to bring cargo from LEO to EML-2, e.g. http://www.nasaspaceflight.com/2013/01/boeing-outlines-technology-crewed-mars-missions/. The concern about launch windows seems strange since the SEP tug could be launched and sent spiraling out well before the departure window and timed so that C3=0 is reached at the appropriate time. Is the radiation and MMOD concern a big deal? Is NASA likely to use SEP to spiral HSF components out from LEO?

Developing both large-scale SEP technology and an SLS upper stage sounds difficult given expected budgets. Wouldn't it be better to just develop one of the two? If you have SEP that works from LEO you could probably get by without an SLS upper stage except for a small existing one (e.g. ICPS) to send MPCV towards EML-2. Alternatively skip the SEP, crank up the IMLEO, and build an upper stage/EDS to throw stuff towards EML-2. Thoughts?

Related thread on SEP: http://forum.nasaspaceflight.com/index.php?topic=28115.0 .

[sdsds]

I'm not clear whether everyone discussing the use of electric propulsion to spiral out of LEO means the same thing. In the simplest case of starting in a circular LEO and applying continuous thrust, each successive orbit in the spiral gets larger but they are all circular. Marco Christov though discusses thrusting only at perigee and thus the orbits in his spirals are successively more eccentric ellipses.

These two approaches might be substantially different in relation to passing through the Van Allen belts and thus the question of whether radiation is a big deal or not. Also, the Van Allen belts do vary quite a bit. There's a great video at:
http://www.nature.com/news/ephemeral-third-ring-of-radiation-makes-appearance-around-earth-1.12529

[Robotbeat]

http://spirit.as.utexas.edu/~fiso/telecon/Oleson_3-6-13/Oleson_3-6-13.pdf
http://spirit.as.utexas.edu/~fiso/telecon/Oleson_3-6-13/Oleson.mp3
"Combined SEP-Chemical for Piloted Mars Missions"

(Has anyone posted this yet?)

BTW, this guy is the lead at Glenn for the group who thought up how to bag an asteroid. COMPASS is a pretty sweet group. They have a bunch of people in a room, one person per subsystem (one comms, one for power, one for structures, one for cost, one driving CAD, etc...) sitting at a computer station and all working together, with Steve at the middle leading everyone. There are projectors displaying the lead's display on three walls, plus a couple of rapid prototyping machines for getting an idea of the spacecraft structures. One of the coolest places I've been at Glenn, it looks like exactly where spaceships ought to be born.

EDIT:Yes, this has been posted before:
http://forum.nasaspaceflight.com/index.php?topic=29918.msg1024104#msg1024104

[deltaV]

Studies comparing mass efficiency (IMLEO) of solar electric and chemical in-space propulsion (for human missions) are dime a dozen. Are there any studies that compare cost and schedule of SEP vs. chemical? Basically I'm looking for a study like the Augustine Report but focusing on in-space propulsion instead of launch vehicles.

[Hop_David]

Studies comparing mass efficiency (IMLEO)

What's IMLEO stand for? I recall NSF used to have a list of abbreviations. For me that was very helpful in making sense of the alphabet soup. But I can't find it at the moment.

[Lee Jay]

Studies comparing mass efficiency (IMLEO)

What's IMLEO stand for? I recall NSF used to have a list of abbreviations. For me that was very helpful in making sense of the alphabet soup. But I can't find it at the moment.

Initial (or insertion?) Mass to Low Earth Orbit.

[savuporo]

Sandia and NG were working on Brayton cycle power conversion
http://energy.sandia.gov/?page_id=14240
Whatever happened to that ?

The grant was here http://www.spaceref.com/news/viewpr.html?pid=35890

[Robotbeat]

Sandia and NG were working on Brayton cycle power conversion
http://energy.sandia.gov/?page_id=14240
Whatever happened to that ?

The grant was here http://www.spaceref.com/news/viewpr.html?pid=35890

They're probably still working on it, back-burner-ish. Stuff like that tends to sit around in a warehouse until someone gets another grant to continue work.

[rusty]

** Updates **

Jan 17th - http://www.nasa.gov/press/2014/january/nasa-administrator-surveys-agencys-work-in-advanced-propulsion-technologies/
"Glenn Center Director Jim Free guided Bolden, U.S. Senator Sherrod Brown and U.S. Rep. Marcia Kaptur, both of Ohio, on a tour of Glenn’s Electric Propulsion Laboratory, which houses a large space environment simulation chamber. The chamber is being enhanced for future testing of solar electric propulsion technologies ...
Later this year, NASA and its commercial partners will mark another milestone in industry partnerships when it tests at Glenn the large solar array system to demonstrate the structural integrity of large array designs that one day will support advanced SEP."

Jan 22nd - http://www.satellitetoday.com/technology/2014/01/22/atk-demonstrates-megaflex-solar-array-for-nasa/
"ATK has deployed its Solar Electric Propulsion (SEP) MegaFlex solar array under a NASA contract to further the development of a high-power system for future manned and robotic exploration missions.
The MegaFlex solar array can generate 40kW of power when fully populated with solar cells, and ... is based on ATK’s UltraFlex platform that powered NASA’s Mars Phoenix Lander in 2008."

Mar 6th - http://www.sacbee.com/2014/03/06/6215607/nasa-officials-to-view-new-solar.html
"NASA Administrator Charles Bolden and Associate Administrator for Space Technology Michael Gazarik will visit the agency's Glenn Research Center's Plum Brook Station Wednesday, March 12, to see an advanced solar array system being tested at the facility. ...
NASA selected ATK and Deployable Space Systems (DSS), both of Goleta, Calif., in 2012 to develop advanced solar systems to support advanced solar electric propulsion. DSS expects to test its system later this year."
--------------------------------------------------------------------------------

What ever happened with the contracts originally proposed by this thread? Are these three stories related as I've shown, or separate? Was this a different contract award than the OP's, or were those slimmed down until only this outsider was approved? Are there still other ongoing contracts?

[Robotbeat]

Cool, didn't know they were testing the array at Plum Brook already. Neat facility. So are the other vacuum chambers at Glenn (they've a vacuum chamber at Glenn that's been being used for years on end to endurance test the NEXT thruster... I have a picture somewhere).

[rusty]

Anyone familiar with or can provide studies on Lunar SEP?
Everything I've read focuses on either deep-space missions or LEO to EML/LLO, but I haven't seen anything on using SEP for only Lunar orbit maneuvers or just EML to LLO. In these instances, cargo would be launched to or through EML where SEP would direct it into its final Lunar orbit, then return for the next. This severally cuts down transit time, avoids Van Allens, reduces thrust requirements and I think is the best use for tugs. Thank You.

[A_M_Swallow]

Anyone familiar with or can provide studies on Lunar SEP?
Everything I've read focuses on either deep-space missions or LEO to EML/LLO, but I haven't seen anything on using SEP for only Lunar orbit maneuvers or just EML to LLO. In these instances, cargo would be launched to or through EML where SEP would direct it into its final Lunar orbit, then return for the next. This severally cuts down transit time, avoids Van Allens, reduces thrust requirements and I think is the best use for tugs. Thank You.

LEO to LLO is one of the standard examples where a SEP tug can be used.  VASIMR is a suggested engine for the tug, so are Hall thrusters.

This Wiki article can point you are some references.
https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket

[rusty]

LEO to LLO is one of the standard examples where a SEP tug can be used.
VASIMR is a suggested engine for the tug, so are Hall thrusters.
This Wiki article can point you are some references.

Thank you for completely ignoring my question, answering what I explicitly said didn't need to be and providing the most unscientific resource short of some John Doe's blog. Can anyone else address the question? Thank you.