Lunar Gateway Debate

[AnalogMan]

[...]
Thanks for all that detail, but still, "rectilinear" implies four lines at right angles to one another.  An "elliptical orbit" resembles an ellipse, not a rectangle.  More probably, in my view, the term serves to confuse, not to inform.  Most of us here are broadly familiar with the six orbital elements of, well, elliptical orbits, including those which are "angelic".

If you want to see near 90 degree bends in the NRHO's linked by gently curved sections you need to look at them in an Earth-centered inertial frame.

See this YouTube video from NASA (visualization starts at 1min 7sec):

Near Rectilinear Halo Orbit Explained and Visualized

[DT1]

There are several good papers about NROs. In this one you can read about why NROs are better than other orbits for the gateway:
https://www.google.com/url?sa=t&source=web&rct=j&url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019648.pdf&ved=2ahUKEwis0IOFtsnjAhUFDOwKHWz5BdYQFjAAegQIBRAB&usg=AOvVaw2_fK40EQkLbCw7FldoQV2O

[speedevil]

There are several good papers about NROs. In this one you can read about why NROs are better than other orbits for the gateway:
https://www.google.com/url?sa=t&source=web&rct=j&url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019648.pdf&ved=2ahUKEwis0IOFtsnjAhUFDOwKHWz5BdYQFjAAegQIBRAB&usg=AOvVaw2_fK40EQkLbCw7FldoQV2O

Noting of course 'for the gateway as reached by SLS+Orion' is a metric for this paper.

[jadebenn]

Does anyone know if the ESA is still planning to build the ESPRIT module? I haven't heard anything about it since the move to 'skinny' Gateway.

I understand it's not neccessary for the initial Gateway, but the capability it provides - refueling the PPE - seems pretty important for the long-term.

[whitelancer64]

http://www.esa.int/Our_Activities/Operations/Angelic_halo_orbit_chosen_for_humankind_s_first_lunar_outpost

Excellent graphic showing how lopsided is the expected Gateway orbit.  One of the functions of the Gateway is ostensibly to serve as a rescue habitat should there be problems with the base.  Per Murphy's Law, those problems, should they occur, will occur when the Gateway is furtherest away from the Moon, sebveral days from the lunar base.

Also:  Why they use the word "rectilinear" to describe an ellipse is beyond belief.  There are no angels who would wear a rectilinear halo, for one thing.  For another, calling the orbit "angelic" is worthless pandering.  Do they feel that religious imagery will substitue for a rational celestial mechanics justification of the utility of the orbit?

I mean, really.  How much are halos on Ebay these days?

"Near rectilinear" means that the orbit is so elongated that the path of the orbit approaches being a straight line (older published papers on NRHOs also refer to this as "almost" rectilinear or "nearly" rectilinear). In geometric terms, an ellipse - to clarify, not an elliptical orbit, but an ellipse the geometric shape - with an eccentricity of 1 would be a straight line with the foci at the end points, hence "rectilinear." However, an orbit with an eccentricity of 1 would be a hyperbola.

The "halo" part of the term refers to the gravitational influence of the Earth on the shape of the orbit when it passes beyond the Moon's Hill sphere (which is at roughly 60,000 km). That part of the orbit behaves as a lagrangian halo orbit.

Thanks for all that detail, but still, "rectilinear" implies four lines at right angles to one another.  An "elliptical orbit" resembles an ellipse, not a rectangle.  More probably, in my view, the term serves to confuse, not to inform.  Most of us here are broadly familiar with the six orbital elements of, well, elliptical orbits, including those which are "angelic".

Per the oracle:

https://en.wikipedia.org/wiki/Orbital_elements

The term "rectilinear" has multiple meanings, depending on the context. It can and does mean "lines at right angles" in some cases, i.e. a rectilinear polygon, but that is not the case in this usage. I thought my explanation was very clear, that an ellipse with an eccentricity of 1 is a straight line.

[whitelancer64]

Does anyone know if the ESA is still planning to build the ESPRIT module? I haven't heard anything about it since the move to 'skinny' Gateway.

I understand it's not neccessary for the initial Gateway, but the capability it provides - refueling the PPE - seems pretty important for the long-term.

Yes, my understanding is that it will be delivered to the Gateway around 2023, shortly before the lunar landing.

[JohnFornaro]

I thought my explanation was very clear, that an ellipse with an eccentricity of 1 is a straight line.

Yeah.  And an ellipse with an eccentricity of zero is...

[whitelancer64]

I thought my explanation was very clear, that an ellipse with an eccentricity of 1 is a straight line.

Yeah.  And an ellipse with an eccentricity of zero is...

Is a circle, the foci are the same point.

[Lar]

I think that's his point.

Is this actually all that on topic?

[Norm38]

I'm a bit late to this debate, but my thoughts are that a lunar gateway only makes sense if it improves the economics of spaceflight.  If it reduces launches, costs, improves payload.

If it comes to be that craft like Starship are better off going direct to the surface, then a lunar gateway will be bypassed.  It will then likely wither on the vine due to market forces.  There will be a strong drive to follow the low cost path.

However, there is a good chance that it will make sense to offload cargo and crew to dedicated decent landers.  It takes a lot of fuel to land and re-launch an entire Starship.  And anything from the SLS architecture or international partners likely won't have a direct to surface path.  Thus the Gateway is best constructed as a spacedock / transfer station / fuel depot.  Robotic tankers and cargo ships come in.  Some cargo goes direct, some is offloaded for smaller landers.  Maybe one large cargo ship brings in the mail for multiple surface locations.  By offloading in orbit, that's possible.  Passengers and small cargo would transfer to smaller landers that are more fuel efficient, resulting in fewer tanker flights to support operations.

So that's really all it comes down to.  A Lunar Gateway will pay it's way or not, it will be more efficient or not, it will be successful or not.  I don't see any role for science labs.  Nor any role for habitation or a permanent crew.  Not for a long time.  Should be fully automated, personnel transferring through can handle basic maintenance.

This doesn't really bear on what NASA decides to do short term, except that they should be paying attention to the long term economics and use projections.  It'd be a shame to build a Gateway that no one wants to use, or can't use.  Because I do think there's a place for one.

[Coastal Ron]

I'm a bit late to this debate, but my thoughts are that a lunar gateway only makes sense if it improves the economics of spaceflight.  If it reduces launches, costs, improves payload.

Completely agree.

If it comes to be that craft like Starship are better off going direct to the surface, then a lunar gateway will be bypassed.  It will then likely wither on the vine due to market forces.  There will be a strong drive to follow the low cost path.

Also, if the Lunar Gateway is in the wrong orbit, such as the 7-day orbit currently planned for the Artemis program. More on this below...

However, there is a good chance that it will make sense to offload cargo and crew to dedicated decent landers.  It takes a lot of fuel to land and re-launch an entire Starship.  And anything from the SLS architecture or international partners likely won't have a direct to surface path.  Thus the Gateway is best constructed as a spacedock / transfer station / fuel depot.  Robotic tankers and cargo ships come in.  Some cargo goes direct, some is offloaded for smaller landers.  Maybe one large cargo ship brings in the mail for multiple surface locations.  By offloading in orbit, that's possible.  Passengers and small cargo would transfer to smaller landers that are more fuel efficient, resulting in fewer tanker flights to support operations.

I think this is the future, and it's something I've advocated for. Essentially the SpaceX Starship is a general purpose vehicle, which is fine, but it may be overkill for certain applications.

A clean sheet design would advocate for breaking up the transportation segments into more specific and efficient sections. So if you wanted to get to the surface of our Moon you would 1) take a transport from Earth-to-LEO, 2) transfer to a Earth LEO-to Moon local transit point (LLO, EML, etc.), and then take a local transport down to the surface of the Moon.

Each of those segments can be resupplied separately at their transit locations (a lunar gateway could be such a transit location), and each is optimized for the route it supports.

So that's really all it comes down to.  A Lunar Gateway will pay it's way or not, it will be more efficient or not, it will be successful or not.  I don't see any role for science labs.  Nor any role for habitation or a permanent crew.  Not for a long time.  Should be fully automated, personnel transferring through can handle basic maintenance.

Yes, and I think part of that will boil down to the same three priorities that exist in the real estate market - location, location, location. If the proposed Lunar Gateway 7-day orbit does not meet the needs of the commercial users, then it will be a failure.

This doesn't really bear on what NASA decides to do short term, except that they should be paying attention to the long term economics and use projections.  It'd be a shame to build a Gateway that no one wants to use, or can't use.  Because I do think there's a place for one.

It doesn't matter for NASA getting government employees to the surface of the Moon, but it does matter if Pence/Bridenstine really mean that they want to create a commercial economy on the Moon.

[A_M_Swallow]

By putting the Gateway in a NRHO orbit the space station is not just acting as a way station but a rocket stage in its own right. It moves incoming people from high above the Moon to near the Moon. On the return trip about a week later the people are collected from near the Moon and transported to several thousand kilometres out. During those several days they will be using the facilities of the habitats.

[Norm38]

So a station in an NHRO orbit acts like a lunar cycler?  And it saves fuel?  Vehicles at earth only have to reach the higher orbit and vehicles to the surface only the lower?
But delta-V isn't free.  So does the Moon's gravity provide assist?  Or do transferring vehicles have to provide prop to keep the station in it's orbit?

Orbital mechanics isn't my strong suit. But if ships come up from Luna and dock at a cycler and hang a bunch of mass on it, it's not going to reach the same apogee as it would unloaded.

[A_M_Swallow]

So a station in an NHRO orbit acts like a lunar cycler?  And it saves fuel?  Vehicles at earth only have to reach the higher orbit and vehicles to the surface only the lower?
But delta-V isn't free.  So does the Moon's gravity provide assist?  Or do transferring vehicles have to provide prop to keep the station in it's orbit?

Orbital mechanics isn't my strong suit. But if ships come up from Luna and dock at a cycler and hang a bunch of mass on it, it's not going to reach the same apogee as it would unloaded.

Possibly why the Gateway needs 3 of NASA's biggest ion thrusters.

[Coastal Ron]

By putting the Gateway in a NRHO orbit the space station is not just acting as a way station but a rocket stage in its own right.

If the Orion had enough maneuvering capabilities I have no doubt NASA would place it in a much lower orbit than the 7-day one currently planned.

It moves incoming people from high above the Moon to near the Moon. On the return trip about a week later the people are collected from near the Moon and transported to several thousand kilometres out. During those several days they will be using the facilities of the habitats.

The Gateway is only in the 7-day orbit because that is the best the Orion can do, it's not because a 7-day orbit is optimal - remember the Apollo missions were able to get between the Earth and Moon in just 3 days, so 7 days to orbit the Moon is a LONG time.

Lots of compromises when you limit your transportation architecture to single-launch systems like the SLS & Orion.

[Steven Pietrobon]

As a comparison on deciding if using NRO is more efficient than using LLO, I plugged in some NRO numbers in my Lunar calculation program that I used to calculate the LM masses and payloads for my "Fly me to the Moon on an SLS Block II" paper. Changes are (see attached program)

dv_loi =     428.5; {m/s, NRO orbit insertion}
dv_pd = 2041.6+731; {m/s, Total powered descent, Apollo 17+NRO}
dv_as = 1890.0+730; {m/s, LM ascent, Apollo 11+NRO}
dv_tei =     411.5; {m/s, Trans Earth injection, NRO}
f_pd =     0.81591; {Proportion of CPS powered descent, 510.4 m/s using LM, same as from LLO}
t_tl     =    5.10; {days, time in TLI transit}
t_llo    = 1.25+0.55; {days, time in NRO and transfer time}
dv_pc  =       0.0; {m/s, Plane change}

The NRO delta-V and travel times are from

"Options for Staging Orbits in Cis-Lunar Space"
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019648.pdf

If using an expendable Lunar Module, staging from LLO gives a cargo mass of 509 kg. However, going through NRO, the cargo mass reduces to -418 kg. A negative cargo mass implies the mission does not close and that using NRO is not as efficient as using LLO. The main reason for the lower performance is the same one why Direct Ascent is not as efficient. That is, the Lunar lander has to be much heavier since it has to carry additional propellant for the additional extra 1461 m/s of added delta-V.

In order to buy back that 509+418 = 927 kg performance loss, we have to increase initial mass in LEO from 142,195 kg to 149,472 kg, an increase of 7,277 kg. That is, every kg on the Moon costs 7.85 kg in LEO.

Of course the question is then how would performance change if using a reusable ascent stage. My expectation is that LLO would still give better performance for the same reason as above. In this case the Gateway can be placed in one of the frozen orbits. The frozen orbit at 86° is attractive since the plane change is only 4° to reach the poles. This requires a delta-V of 114 m/s. If we combine this with the PDI burn of 25 m/s, the PDI burn increases to 116 m/s, meaning that the increase in delta-V is 116-25 = 91 m/s.

Another advantage for LLO is that the abort window to the Gateway from the Lunar pole is every 2 hours, compared to 3.5 days for NRO. However, aborting from LLO back to Earth the window is every 14 days (have to wait for the orbital plane to come into alignment), compared to every 7 days from NRO (have to wait until in correct position in NRO orbit). Another factor is that NRO orbit insertion from Earth or departure to Earth requires two burns each (one near the Moon and the other further away in NRO). LLO only requires one burn for TEI, with LOI typically using two or more burns.

[Joseph Peterson]

So a station in an NHRO orbit acts like a lunar cycler?  And it saves fuel?  Vehicles at earth only have to reach the higher orbit and vehicles to the surface only the lower?
But delta-V isn't free.  So does the Moon's gravity provide assist?  Or do transferring vehicles have to provide prop to keep the station in it's orbit?

Orbital mechanics isn't my strong suit. But if ships come up from Luna and dock at a cycler and hang a bunch of mass on it, it's not going to reach the same apogee as it would unloaded.

The term cycler only applies in the sense that NRHO is a class of orbits that "cycles" between using the Circular Restricted Three Body Problem(CR3BP) and Clohessy-Wiltshire depending on where in the orbit Gateway is.

Gateway won't provide significant delta v.  It can provide a staging point in an orbit Orion can use.  The advantage is Orion only needs 900 m/s for a round trip, 900 m/s less than needed when staging in LLO(Low Lunar Orbit).  The cost is needing a lander with 5500 m/s round trip, 1500 m/s more than a lander staging in LLO.  Assuming we use Lockheed Martin's fully reusable hydrolox lander concept(assuming no change in dry mass for the different size tanks) this translates into needing 54.5 tonnes of hydrolox when staging in NRHO and 32.5 tonnes when staging in LLO.

There is a Lunar gravity assist involved, but this is to lower Orion's one way transit time to ~6 days.  This savings is already factored into the 900 m/s round trip saving Orion gains above.

NRHO is stable with a bit of station keeping propellant.  Visiting vehicles will need to deliver propellant to Gateway, but this mass is insignificant compared to the extra propellant the lander needs. 

The is no good way to discuss NRHO without getting into the orbital mechanics.  Here are the two most useful papers that I've found in case you want to learn more:

https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Conferences/2017_IAA_ZimHowDav.pdf
https://www.frontiersin.org/articles/10.3389/fspas.2018.00045/full

Here is a visualization I find useful.



Delta v figures are from in Crusan 2018, page 8:

https://www.nasa.gov/sites/default/files/atoms/files/20181206-crusan-gateway-reduced-v4.pdf

Whitepaper for Lockheed Martin's fully reusable 4600 m/s hydrolox lander proposal:

https://www.lockheedmartin.com/content/dam/lockheed-martin/space/documents/ahead/LM-Crewed-Lunar-Lander-from-Gateway-IAC-2018-Rev1.pdf?_ga=2.116053317.291590913.1538592165-885407726.1538592165

[yg1968]

Debate between Greg Autry and Bob Zubrin on NASA's Proposed Gateway - 22nd Annual International Mars Society Convention:

[Eric Hedman]

By putting the Gateway in a NRHO orbit the space station is not just acting as a way station but a rocket stage in its own right.

If the Orion had enough maneuvering capabilities I have no doubt NASA would place it in a much lower orbit than the 7-day one currently planned.

It moves incoming people from high above the Moon to near the Moon. On the return trip about a week later the people are collected from near the Moon and transported to several thousand kilometres out. During those several days they will be using the facilities of the habitats.

The Gateway is only in the 7-day orbit because that is the best the Orion can do, it's not because a 7-day orbit is optimal - remember the Apollo missions were able to get between the Earth and Moon in just 3 days, so 7 days to orbit the Moon is a LONG time.

Lots of compromises when you limit your transportation architecture to single-launch systems like the SLS & Orion.

If Something changes in the use of Orion, such as a more capable service module, or replaced by another vehicle for crew the Gateway can be moved to a better orbit.

[Hog]

By putting the Gateway in a NRHO orbit the space station is not just acting as a way station but a rocket stage in its own right.

If the Orion had enough maneuvering capabilities I have no doubt NASA would place it in a much lower orbit than the 7-day one currently planned.

It moves incoming people from high above the Moon to near the Moon. On the return trip about a week later the people are collected from near the Moon and transported to several thousand kilometres out. During those several days they will be using the facilities of the habitats.

The Gateway is only in the 7-day orbit because that is the best the Orion can do, it's not because a 7-day orbit is optimal - remember the Apollo missions were able to get between the Earth and Moon in just 3 days, so 7 days to orbit the Moon is a LONG time.

Lots of compromises when you limit your transportation architecture to single-launch systems like the SLS & Orion.

If Something changes in the use of Orion, such as a more capable service module, or replaced by another vehicle for crew the Gateway can be moved to a better orbit.

Here is a direct snip from Wiki that I found interesting(esp. if accurate) It's about the "Orion Service Module" compared to the "Apollo Command and Service Modules".  I just found it interesting is all.

" In comparison with the Apollo command and service module, which previously took astronauts to the Moon, the Orion service module generates approximately twice as much electricity (11.2 kW vs 6.3 kW), weighs nearly 40% less fuelled (15,461 kg, vs 24,520 kg) and is 12% smaller (208.66 m2vs 238 m2 volume) supporting the environment for a slightly (45%) larger habitable volume on the crew module (8.95 m3 vs 6.17 m3) though it will carry 50% less propellant for orbital maneuvers (8,600 kg usable propellant vs 18,584 kg).
The Orion service module will be able to support a crew of four for 21 days against the 14 day endurance for the three-man Apollo."

I always thought that the Orion SM could/should be upgraded depending on mission requirements.