Question about Human Landing System

[Involute]

Newb here.  As I understand it, the mission profile for an Artemis moon mission involves using SLS to send astronauts to the Gateway in an Orion.  A Human Landing System (modified Starship) will be waiting there, after having been topped off in LEO by multiple Starship refuelings.  The astronauts transfer to the HLS and descend to the moon.  They return in the HLS, transfer to Orion, and return to Earth.

What happens to the HLS?  Even if it arrives at the Gateway with enough fuel for multiple Gateway-moon-Gateway trips, eventually it will run out of gas.  Is there a plan to send one or more Starships from Earth to refuel it?  Or a topped off HLS to replace it (so the first gets abandoned)?  Am I misunderstanding the mission profile?

Thanks for any clarification.

[punder]

I’ve been here a while and would like to hear this explained as well!

But my general impression, as a mere poorly informed enthusiast, is that NASA has no firm ideas past Artemis III for HLS reuse, and that for Artemis III the lander is simply abandoned. But I could be totally wrong.

Welcome!

Edit, or Artemis IV. At the moment I forget which is the first crewed landing.

[DanClemmensen]

Newb here.  As I understand it, the mission profile for an Artemis moon mission involves using SLS to send astronauts to the Gateway in an Orion.  A Human Landing System (modified Starship) will be waiting there, after having been topped off in LEO by multiple Starship refuelings.  The astronauts transfer to the HLS and descend to the moon.  They return in the HLS, transfer to Orion, and return to Earth.

What happens to the HLS?  Even if it arrives at the Gateway with enough fuel for multiple Gateway-moon-Gateway trips, eventually it will run out of gas.  Is there a plan to send one or more Starships from Earth to refuel it?  Or a topped off HLS to replace it (so the first gets abandoned)?  Am I misunderstanding the mission profile?

Thanks for any clarification.

SpaceX is contracted to NASA to provide the HLS service as you described it. the HLS remains SpaceX's property and they can do what they want with it after that: it's not part of the contract. There are actually two HLSs: the uncrewed demo that flies prior to Artemis III, and then the actual Artemis III HLS. (Note, we don't know that depot is in LEO. it might be in a higher EO).

We do not know what SpaceX will do with those HLSs. I personally don't think are will be permitted to simply abandon them in NRHO, so at a minimum they must be disposed of, probably by crashing into the Moon.

Attempting to refuel HLS in NRHO is a major undertaking, requiring a multiple tanker flights. To get a tanker to NRHO, you start by sending multiple tankers to the depot in EO, and then send one tanker (or possibly the depot) to NRHO with enough fuel to get itself plus HLS back to EO.

[DanClemmensen]

I’ve been here a while and would like to hear this explained as well!

But my general impression, as a mere poorly informed enthusiast, is that NASA has no firm ideas past Artemis III for HLS reuse, and that for Artemis III the lander is simply abandoned. But I could be totally wrong.

Welcome!

Edit, or Artemis IV. At the moment I forget which is the first crewed landing.

NASA has fairly firm plans for the follow-on HLS, which is referred to as the "sustainable" HLS. the plan calls for competition, but realistically the only candidate for the second crewed landing is a new versio of Starship HLS. Therefore, NASA will excersize "Option B" of the existing HLS contract with SpaceX to acquire this "sustainable" version of Starship HLS. NASA will also get competitive bids under "Appendix P" of the NextSTEP program from multiple companies for their "sustainable" HLSs. These HLSs will compete with the sustainable Starship HLS for landings after the first two.

I know that NASA has announced these plans. I do not know if Congress will fund them, but Congress pretty much mandated the whole "appendix P" thing.

[StarshipTrooper]

It seems that after completing their contract, either of the HLS vehicles would be a useful resource with all the huge amount of living and storage space. They could have great utility attached to the Gateway, in lunar orbit, or even returned to the surface of the moon.

[yg1968]

Newb here.  As I understand it, the mission profile for an Artemis moon mission involves using SLS to send astronauts to the Gateway in an Orion.  A Human Landing System (modified Starship) will be waiting there, after having been topped off in LEO by multiple Starship refuelings.  The astronauts transfer to the HLS and descend to the moon.  They return in the HLS, transfer to Orion, and return to Earth.

What happens to the HLS?  Even if it arrives at the Gateway with enough fuel for multiple Gateway-moon-Gateway trips, eventually it will run out of gas.  Is there a plan to send one or more Starships from Earth to refuel it?  Or a topped off HLS to replace it (so the first gets abandoned)?  Am I misunderstanding the mission profile?

Thanks for any clarification.

SpaceX's hasn't been clear about this. SpaceX has said that Starship would be refueled in Earth orbit. It's not clear what Earth orbit means in the circumstances, it might not be LEO.

[VSECOTSPE]

Everyone else has covered the question.  But I’ll just point out that there’s a Missions to the Moon board (scroll down a bit from this Orion one) where there’s lots more of this lander stuff.

[DanClemmensen]

It seems that after completing their contract, either of the HLS vehicles would be a useful resource with all the huge amount of living and storage space. They could have great utility attached to the Gateway, in lunar orbit, or even returned to the surface of the moon.

Perhaps true in the abstract. However, NASA is paying SpaceX $2.89 million dollars to provide a specific service with specific requirements, and SpaceX as a for-profit commercial company has little incentive to add capabilities to do stuff that is not in the contract, such as landing on the Moon after the end of the mission and sustaining itself in some sort of usable condition for months or years on the lunar surface, or acting as part of Gateway.  The first two HLSs will be in NRHO prior to the arrival of the first two Gateway modules (PPE and HALO) and would need to sustain themselves until its arrival. Gateway probably does not have sufficient docking capacity for those two HLSs in addition to its other customers (Orion, Dragon XL, the next HLS), and HLS is HUGE compared to the rest of Gateway, which may be a technical challenge and is certainly a political embarrassment.

Starship HLS will vastly exceed the HLS contract requirements, but SpaceX did this to save money, not to provide freebies to NASA. The cheapest way to meet NASA's requirements was to adapt Starship, and that resulted in the extra capabilities.

[yg1968]

The post by Woods170 below is relevant to the original poster's question:

https://ntrs.nasa.gov/api/citations/20220003725/downloads/22%203%207%20Kent%20IEEE%20paper.pdf

From that image, it doesn't look like the Propellant Starship is coming back to Earth. I guess that it can stay in Earth orbit and be used more than once.

Depot ship stays in Earth orbit and is to be used for BOTH missions (uncrewed demo mission and the crewed landing).

On a further note: some SpaceX personnel informally refer to the depot ship as "the Shelby", in an obvious stab at a certain senator.

[eric z]

  Since no one else seems to want to, I will volunteer to take the plunge! What have we gotten ourselves into? Am I the only one here who thinks this "Program", and I use that term loosely, is completely ridiculous? Rockets that don't exist.
Rockets that maybe only fly once a year. Years between flights. Massive refueling operations in space - must be simple, they do it in the movies!
  Let's play spin-the-bottle to decide whose space suit to use. Let's junk the "Commercial" capsules we just developed, they don't fit the "architecture". Oh yeah, we need a new "commercial"space station while we're at it, but we will have a baby station in lunar orbit for astronauts to chill and have a beer as they roast in radiation on their way down to the moon base - oh wait, there is no moon base.
 
 I wanted to live at least long enough to see us back on the moon, for good, but I am getting quite pessimistic, sorry!

[yg1968]

  Since no one else seems to want to, I will volunteer to take the plunge! What have we gotten ourselves into? Am I the only one here who thinks this "Program", and I use that term loosely, is completely ridiculous? Rockets that don't exist.
Rockets that maybe only fly once a year. Years between flights. Massive refueling operations in space - must be simple, they do it in the movies!
  Let's play spin-the-bottle to decide whose space suit to use. Let's junk the "Commercial" capsules we just developed, they don't fit the "architecture". Oh yeah, we need a new "commercial"space station while we're at it, but we will have a baby station in lunar orbit for astronauts to chill and have a beer as they roast in radiation on their way down to the moon base - oh wait, there is no moon base.
 
 I wanted to live at least long enough to see us back on the moon, for good, but I am getting quite pessimistic, sorry!

That's a very negative way of seeing things.

Commercial crew capsules will be used for the Commercial LEO Destinations program. NASA is not giving up on LEO.

In terms of SLS, yes once a year is not enough but the development of HLS-Starship and Starship gives me hope that a commercial alternative to SLS will exist in a few years. Hopefully, NASA will take advantage of it once it is operational. 

NASA's plans is to eventually have a commercial lunar destinations program. These plans are for the late 2030s but I suspect that these plans will be accelerated once Starship is operational. NASA's current plan is to have a lunar base camp with only one habitat but eventually in the late 2030s to use commercial lunar habitats.

[punder]

  Since no one else seems to want to, I will volunteer to take the plunge! What have we gotten ourselves into? Am I the only one here who thinks this "Program", and I use that term loosely, is completely ridiculous? Rockets that don't exist.
Rockets that maybe only fly once a year. Years between flights. Massive refueling operations in space - must be simple, they do it in the movies!
  Let's play spin-the-bottle to decide whose space suit to use. Let's junk the "Commercial" capsules we just developed, they don't fit the "architecture". Oh yeah, we need a new "commercial"space station while we're at it, but we will have a baby station in lunar orbit for astronauts to chill and have a beer as they roast in radiation on their way down to the moon base - oh wait, there is no moon base.
 
 I wanted to live at least long enough to see us back on the moon, for good, but I am getting quite pessimistic, sorry!

You are not alone.

[TrevorMonty]

Newb here.  As I understand it, the mission profile for an Artemis moon mission involves using SLS to send astronauts to the Gateway in an Orion.  A Human Landing System (modified Starship) will be waiting there, after having been topped off in LEO by multiple Starship refuelings.  The astronauts transfer to the HLS and descend to the moon.  They return in the HLS, transfer to Orion, and return to Earth.

What happens to the HLS?  Even if it arrives at the Gateway with enough fuel for multiple Gateway-moon-Gateway trips, eventually it will run out of gas.  Is there a plan to send one or more Starships from Earth to refuel it?  Or a topped off HLS to replace it (so the first gets abandoned)?  Am I misunderstanding the mission profile?

Thanks for any clarification.

Attempting to refuel HLS in NRHO is a major undertaking, requiring a multiple tanker flights. To get a tanker to NRHO, you start by sending multiple tankers to the depot in EO, and then send one tanker (or possibly the depot) to NRHO with enough fuel to get itself plus HLS back to EO.

Refuelling HLS in NRHO is relatively straightforward and cheaper than replacing it. Use a disposable tanker that is topped up in EO. Tankers are cheap and light weight as there is no heatshield or need for landing engines.

HLS will need its life support consumerables replaced by next crew. Cargo vehicle or Orion could transport these consumerables to Gateway.

[punder]

Thanks for any clarification.

Ha ha! I think I speak for all of us when I say “You’re welcome.”

/s 

[DanClemmensen]

Newb here.  As I understand it, the mission profile for an Artemis moon mission involves using SLS to send astronauts to the Gateway in an Orion.  A Human Landing System (modified Starship) will be waiting there, after having been topped off in LEO by multiple Starship refuelings.  The astronauts transfer to the HLS and descend to the moon.  They return in the HLS, transfer to Orion, and return to Earth.

What happens to the HLS?  Even if it arrives at the Gateway with enough fuel for multiple Gateway-moon-Gateway trips, eventually it will run out of gas.  Is there a plan to send one or more Starships from Earth to refuel it?  Or a topped off HLS to replace it (so the first gets abandoned)?  Am I misunderstanding the mission profile?

Thanks for any clarification.

Attempting to refuel HLS in NRHO is a major undertaking, requiring a multiple tanker flights. To get a tanker to NRHO, you start by sending multiple tankers to the depot in EO, and then send one tanker (or possibly the depot) to NRHO with enough fuel to get itself plus HLS back to EO.

Refuelling HLS in NRHO is relatively straightforward and cheaper than replacing it. Use a disposable tanker that is topped up in EO. Tankers are cheap and light weight as there is no heatshield or need for landing engines.

HLS will need its life support consumerables replaced by next crew. Cargo vehicle or Orion could transport these consumerables to Gateway.

The incremental cost of a replacement HLS instead of a disposable tanker is low, and you end up with a fully-provisioned and updated HLS, and no need to figure out how to transfer propellant from a tanker to an HLS. There is no such need in the nominal HLS mission: fuel transfers from tankers to depot and then from depot to HLS, not from tanker to HLS, and we do not know how it will be done.

[TrevorMonty]

Newb here.  As I understand it, the mission profile for an Artemis moon mission involves using SLS to send astronauts to the Gateway in an Orion.  A Human Landing System (modified Starship) will be waiting there, after having been topped off in LEO by multiple Starship refuelings.  The astronauts transfer to the HLS and descend to the moon.  They return in the HLS, transfer to Orion, and return to Earth.

What happens to the HLS?  Even if it arrives at the Gateway with enough fuel for multiple Gateway-moon-Gateway trips, eventually it will run out of gas.  Is there a plan to send one or more Starships from Earth to refuel it?  Or a topped off HLS to replace it (so the first gets abandoned)?  Am I misunderstanding the mission profile?

Thanks for any clarification.

Attempting to refuel HLS in NRHO is a major undertaking, requiring a multiple tanker flights. To get a tanker to NRHO, you start by sending multiple tankers to the depot in EO, and then send one tanker (or possibly the depot) to NRHO with enough fuel to get itself plus HLS back to EO.

Refuelling HLS in NRHO is relatively straightforward and cheaper than replacing it. Use a disposable tanker that is topped up in EO. Tankers are cheap and light weight as there is no heatshield or need for landing engines.

HLS will need its life support consumerables replaced by next crew. Cargo vehicle or Orion could transport these consumerables to Gateway.

The incremental cost of a replacement HLS instead of a disposable tanker is low, and you end up with a fully-provisioned and updated HLS, and no need to figure out how to transfer propellant from a tanker to an HLS. There is no such need in the nominal HLS mission: fuel transfers from tankers to depot and then from depot to HLS, not from tanker to HLS, and we do not know how it will be done.

Tanker, SS, depot or HLS they will all use the fuel transfer system. The HLS has to be top up on its maiden mission so why would it be any different 2nd time round.

[DanClemmensen]

Newb here.  As I understand it, the mission profile for an Artemis moon mission involves using SLS to send astronauts to the Gateway in an Orion.  A Human Landing System (modified Starship) will be waiting there, after having been topped off in LEO by multiple Starship refuelings.  The astronauts transfer to the HLS and descend to the moon.  They return in the HLS, transfer to Orion, and return to Earth.

What happens to the HLS?  Even if it arrives at the Gateway with enough fuel for multiple Gateway-moon-Gateway trips, eventually it will run out of gas.  Is there a plan to send one or more Starships from Earth to refuel it?  Or a topped off HLS to replace it (so the first gets abandoned)?  Am I misunderstanding the mission profile?

Thanks for any clarification.

Attempting to refuel HLS in NRHO is a major undertaking, requiring a multiple tanker flights. To get a tanker to NRHO, you start by sending multiple tankers to the depot in EO, and then send one tanker (or possibly the depot) to NRHO with enough fuel to get itself plus HLS back to EO.

Refuelling HLS in NRHO is relatively straightforward and cheaper than replacing it. Use a disposable tanker that is topped up in EO. Tankers are cheap and light weight as there is no heatshield or need for landing engines.

HLS will need its life support consumerables replaced by next crew. Cargo vehicle or Orion could transport these consumerables to Gateway.

The incremental cost of a replacement HLS instead of a disposable tanker is low, and you end up with a fully-provisioned and updated HLS, and no need to figure out how to transfer propellant from a tanker to an HLS. There is no such need in the nominal HLS mission: fuel transfers from tankers to depot and then from depot to HLS, not from tanker to HLS, and we do not know how it will be done.

Tanker, SS, depot or HLS they will all use the fuel transfer system. The HLS has to be top up on its maiden mission so why would it be any different 2nd time round.

We do not know that the refuelling connections are symmetrical. We know that a tanker can connect to a depot, and we know an HLS  can connect to a depot. We do not know that a tanker can connect to an HLS. It's possible, and even reasonable that the depot is the only Starship that can connect to other Starships for fuel transfer. There is an extensive thread discussing in-orbit refuelling:
   https://forum.nasaspaceflight.com/index.php?topic=50157.0
If this is true and if you want to refuel HLS in NRHO, then you will need to send Depot to NHRO to do that. Sending Depot on an out-and-back to NRHO is feasible, but the entire mission will take quite a few tanker flights.

[deadman1204]

The "disposable tanker" in lunar orbit will still be a FULL ROCKET getting thrown away. The only difference will be refueling hardware instead of a human compartment.

[TheRadicalModerate]

Refuelling HLS in NRHO is relatively straightforward and cheaper than replacing it. Use a disposable tanker that is topped up in EO. Tankers are cheap and light weight as there is no heatshield or need for landing engines.

You don't need a disposable tanker when you take prop from LEO to NRHO.  Two potential conops:

1) Use a regular lift tanker, filled from the LEO depot, to take the prop to another depot in NRHO.  Then the lift tanker (which is capable of EDL) can return direct to EDL from cislunar space.

2) Use the lift tanker to refuel the Lunar Starship directly, with no depot at all in cislunar, after which it returns to EDL.

Option #1 requires a second depot, but only one depot for an indefinite number of missions.  Option #2 is more parsimonious in terms of hardware, but it requires that the LSS be able to manage boiloff for an extended period of time, until the Orion shows up.  However, I think that's probably a requirement to get Option A to work in the first place.

[TheRadicalModerate]

If this is true and if you want to refuel HLS in NRHO, then you will need to send Depot to NHRO to do that. Sending Depot on an out-and-back to NRHO is feasible, but the entire mission will take quite a few tanker flights.

But once the depot is in NRHO, it can be fueled via lift tankers, which can then return direct to EDL. (I'm assuming that the TPS will be engineered to support EDL from translunar.)  So, even if depots can't do EDL, you only need two, and everything else carrying prop is a lift tanker, which is reusable.

[DanClemmensen]

If this is true and if you want to refuel HLS in NRHO, then you will need to send Depot to NHRO to do that. Sending Depot on an out-and-back to NRHO is feasible, but the entire mission will take quite a few tanker flights.

But once the depot is in NRHO, it can be fueled via lift tankers, which can then return direct to EDL. (I'm assuming that the TPS will be engineered to support EDL from translunar.)  So, even if depots can't do EDL, you only need two, and everything else carrying prop is a lift tanker, which is reusable.

I think it's cheaper to fill Depot in DEO (Depot Earth Orbit) and send to it NRHO (and back) than it is send tankers to fill it in NRHO. Depot is not EDL-capable and therefore has lower dry mass, and is prpbably stretched, which means its dry mass is a lower percentage of wet mass than a tanker.  Yep, it you want to do this as a standard operation, you will probably keep an extra Depot hanging around so you can fill one in DEO while the other one is on an NRHO mission, and then swap.

[deadman1204]

If this is true and if you want to refuel HLS in NRHO, then you will need to send Depot to NHRO to do that. Sending Depot on an out-and-back to NRHO is feasible, but the entire mission will take quite a few tanker flights.

But once the depot is in NRHO, it can be fueled via lift tankers, which can then return direct to EDL. (I'm assuming that the TPS will be engineered to support EDL from translunar.)  So, even if depots can't do EDL, you only need two, and everything else carrying prop is a lift tanker, which is reusable.

I think it's cheaper to fill Depot in DEO (Depot Earth Orbit) and send to it NRHO (and back) than it is send tankers to fill it in NRHO. Depot is not EDL-capable and therefore has lower dry mass, and is prpbably stretched, which means its dry mass is a lower percentage of wet mass than a tanker.  Yep, it you want to do this as a standard operation, you will probably keep an extra Depot hanging around so you can fill one in DEO while the other one is on an NRHO mission, and then swap.

In the HLS selection, NASA said it was much lower risk to do fuel transfers in LEO rather than out at the moon.

[DanClemmensen]

If this is true and if you want to refuel HLS in NRHO, then you will need to send Depot to NHRO to do that. Sending Depot on an out-and-back to NRHO is feasible, but the entire mission will take quite a few tanker flights.

But once the depot is in NRHO, it can be fueled via lift tankers, which can then return direct to EDL. (I'm assuming that the TPS will be engineered to support EDL from translunar.)  So, even if depots can't do EDL, you only need two, and everything else carrying prop is a lift tanker, which is reusable.

I think it's cheaper to fill Depot in DEO (Depot Earth Orbit) and send to it NRHO (and back) than it is send tankers to fill it in NRHO. Depot is not EDL-capable and therefore has lower dry mass, and is prpbably stretched, which means its dry mass is a lower percentage of wet mass than a tanker.  Yep, it you want to do this as a standard operation, you will probably keep an extra Depot hanging around so you can fill one in DEO while the other one is on an NRHO mission, and then swap.

In the HLS selection, NASA said it was much lower risk to do fuel transfers in LEO rather than out at the moon.

The HLS selection is based on the reference Artemis architecture, which assumes that an HLS is used once and then thrown away. Here, we are discussing a way to retrieve HLS. If the crew has already returned to Earth on Orion, then refuelling HLS in NRHO does not risk a loss of crew or loss of mission, and a refuelling failure is just an economic risk.

By comparison to the Artemis reference architecture, Starship HLS has a much simpler CONOPS even if you add this option to return HLS to LEO.

[TheRadicalModerate]

If this is true and if you want to refuel HLS in NRHO, then you will need to send Depot to NHRO to do that. Sending Depot on an out-and-back to NRHO is feasible, but the entire mission will take quite a few tanker flights.

But once the depot is in NRHO, it can be fueled via lift tankers, which can then return direct to EDL. (I'm assuming that the TPS will be engineered to support EDL from translunar.)  So, even if depots can't do EDL, you only need two, and everything else carrying prop is a lift tanker, which is reusable.

I think it's cheaper to fill Depot in DEO (Depot Earth Orbit) and send to it NRHO (and back) than it is send tankers to fill it in NRHO. Depot is not EDL-capable and therefore has lower dry mass, and is prpbably stretched, which means its dry mass is a lower percentage of wet mass than a tanker.  Yep, it you want to do this as a standard operation, you will probably keep an extra Depot hanging around so you can fill one in DEO while the other one is on an NRHO mission, and then swap.

Absolutely not, unless your depot has a heat shield and can be aerocaptured.  Even returning an empty 95t Starship (my SWAG for what the dry mass for a non-EDL system will be) to LEO, propulsively, via BLT, requires 140t of prop, and delivering 140t of prop to NRHO costs something like 600t of prop to LEO.  That's four tankers, on top of the ones needed to fill the depot for the Option B mission.

BTW:  Low structural mass fraction for a full, stretched depot isn't a useful figure of merit, because you want to return it as empty as possible.  The dry mass is the dry mass.

Look this isn't... well, it is rocket science, but a particularly basic form of it.  The big advantage of using lift tankers to haul stuff to cislunar¹ is not that they can return for as little as 80m/s, plus landing delta-v.  A depot, even with all the BLT magic that you can conjure, requires 3300m/s.

_____________
¹An obvious exception is the first time you fly a depot out to NRHO, when you fill it to the gills.  But for subsequent missions, the intermediary should be a lift tanker, not the depot.

PS:  You said "send tankers [plural] to NRHO".  No.  You fill a single tanker from the LEO depot, then send it to NRHO.  When it's transferred the prop to the cislunar depot, it goes home.

I'll spare you the "we really need a 1500t LSS and tankers to make this work" speech, since you've heard it.  It still applies, though.  And it would definitely require that the lift tankers have room for 1500t as well.  But even if you were to use two 1200t tankers, it would still be cheaper (a lot cheaper, in this case) to use the tankers than it would be to bring the depot back propulsively.

[oldAtlas_Eguy]

If this is true and if you want to refuel HLS in NRHO, then you will need to send Depot to NHRO to do that. Sending Depot on an out-and-back to NRHO is feasible, but the entire mission will take quite a few tanker flights.

But once the depot is in NRHO, it can be fueled via lift tankers, which can then return direct to EDL. (I'm assuming that the TPS will be engineered to support EDL from translunar.)  So, even if depots can't do EDL, you only need two, and everything else carrying prop is a lift tanker, which is reusable.

I think it's cheaper to fill Depot in DEO (Depot Earth Orbit) and send to it NRHO (and back) than it is send tankers to fill it in NRHO. Depot is not EDL-capable and therefore has lower dry mass, and is prpbably stretched, which means its dry mass is a lower percentage of wet mass than a tanker.  Yep, it you want to do this as a standard operation, you will probably keep an extra Depot hanging around so you can fill one in DEO while the other one is on an NRHO mission, and then swap.

Absolutely not, unless your depot has a heat shield and can be aerocaptured.  Even returning an empty 95t Starship (my SWAG for what the dry mass for a non-EDL system will be) to LEO, propulsively, via BLT, requires 140t of prop, and delivering 140t of prop to NRHO costs something like 600t of prop to LEO.  That's four tankers, on top of the ones needed to fill the depot for the Option B mission.

BTW:  Low structural mass fraction for a full, stretched depot isn't a useful figure of merit, because you want to return it as empty as possible.  The dry mass is the dry mass.

Look this isn't... well, it is rocket science, but a particularly basic form of it.  The big advantage of using lift tankers to haul stuff to cislunar¹ is not that they can return for as little as 80m/s, plus landing delta-v.  A depot, even with all the BLT magic that you can conjure, requires 3300m/s.

_____________
¹An obvious exception is the first time you fly a depot out to NRHO, when you fill it to the gills.  But for subsequent missions, the intermediary should be a lift tanker, not the depot.

PS:  You said "send tankers [plural] to NRHO".  No.  You fill a single tanker from the LEO depot, then send it to NRHO.  When it's transferred the prop to the cislunar depot, it goes home.

I'll spare you the "we really need a 1500t LSS and tankers to make this work" speech, since you've heard it.  It still applies, though.  And it would definitely require that the lift tankers have room for 1500t as well.  But even if you were to use two 1200t tankers, it would still be cheaper (a lot cheaper, in this case) to use the tankers than it would be to bring the depot back propulsively.

My calculation for a Depot that goes out to NRHO and returns would only have 360t of prop available to use by LSS which is insufficient for a HLS mission from NRHO and back from the Lunar surface. You need ~450t of prop for a surface and back mission that also delivers 30t of cargo.

A 1500t lift tanker that can EDL would deliver to  a permannet Depot at NRHO 500t of prop which will enable a mission that reuses the LSS that is at NRHO.

My additional items is that 8 tanker launches with the last continuing on to NRHO and then a 1200t prop Cargo that returns EDL from NRHO filled with 200t of cargo and at LEO takes on 800t of prop (4 lift tanker launches) which is enough cargo to support 6 HLS surface missions and an additional 20t of cargo for Gateway as well.

Also a crew transfer in space only that travels between LEO and NRHO would need 4 tanker loads of prop to make the round trip.

Summary is that with these items which is just a repurposing of the existing available Starships plus some number of Dragon launches could accomplish a complete from Earth surface to Lunar surface and back for <$500M.

[TheRadicalModerate]

My calculation for a Depot that goes out to NRHO and returns would only have 360t of prop available to use by LSS which is insufficient for a HLS mission from NRHO and back from the Lunar surface. You need ~450t of prop for a surface and back mission that also delivers 30t of cargo.

A 1500t lift tanker that can EDL would deliver to  a permannet Depot at NRHO 500t of prop which will enable a mission that reuses the LSS that is at NRHO.

I get about 440t usable prop with a 1500t EDL-capable tanker, but that assumes fast NRHO, 250m/s of landing delta-v, and 0.5% FPR.  If you can figure out how to make an EDL tanker sufficiently boiloff-resistant, you can use BLT to NRHO and get 510t of usable prop.

That extra 70t of prop makes a big difference to an LSS mission that stages out of NRHO.  Of course, if you stage out of LEO (which is almost certainly what's going to happen eventually, whether the SLS folks like it or not), then you need a full 1500t+ in the LSS, but only about 180t of prop to refuel in NRHO post-lunar-ascent.

My additional items is that 8 tanker launches with the last continuing on to NRHO and then a 1200t prop Cargo that returns EDL from NRHO filled with 200t of cargo and at LEO takes on 800t of prop (4 lift tanker launches) which is enough cargo to support 6 HLS surface missions and an additional 20t of cargo for Gateway as well.

You're omitting a big piece of tech, though:  How do you transfer unrpressurized cargo from one Starship to another in NRHO?  I think it's reasonable to assume that the Gateway will eventually grow an arm that's capable of doing this, but it's not gonna be before Arty 6 or 7--and by that time, we may already be staging out of LEO.

Also a crew transfer in space only that travels between LEO and NRHO would need 4 tanker loads of prop to make the round trip.

Summary is that with these items which is just a repurposing of the existing available Starships plus some number of Dragon launches could accomplish a complete from Earth surface to Lunar surface and back for <$500M.

These are more optimistic than the numbers I get, although I tend to use about $55M per launch, once SpaceX marks things up for profit. I get 14 total lift tankers for an LEO-staged LSS and NRHO refueling.  Figure $300M for the LSS itself, $770M for the tankers, and $250M for a D2 to haul the crew up and down, and you're at $1.32B per mission.

Still beats the hell out of $4.1B for SLS/Orion, plus a ~$500M marginal cost for the LSS.

On the Starship-based LEO-NRHO-LEO transit option, with a transfer to the LSS in NRHO:  That makes a lot of sense if you're leaving your LSS in NRHO between missions.  However, I'm skeptical that's going to happen in the medium term.

I wouldn't be incredibly surprised to see an LSS that becomes EDL-capable, even if you're still using a D2 to get the crew to and from it in LEO for safety reasons.  I suspect that regolith dust contamination is going to be a big fat hairy deal, which will be much more easily refurbishable on the ground.  Add to that the fact that you can integrate your cargoes using well-understood payload processing, and the extra tanker or two that's required to carry the extra dry mass may be quite a deal--especially if it extends the LSS life from 2-3 missions with no dust mitigation to 10+ with it.

[sdsds]

[snip]

Basic agreement with all you write; thanks for doing the calculations!

If you can figure out how to make an EDL tanker sufficiently boiloff-resistant

Then haven't you essentially made an EDL depot?

[TheRadicalModerate]

If you can figure out how to make an EDL tanker sufficiently boiloff-resistant

Then haven't you essentially made an EDL depot?

Yeah, which is why I think it's more likely that you use fast transit to NRHO (which costs an extra 350m/s).

In the best of all possible worlds, the depot "kit" that's added to a tanker is easily deployable on all tankers, in which case it could be deployed while the tanker is in the ballistic lunar trajectory, then stowed before interacting with the depot.  Of course, if you have this ability on a tanker, then you don't need the depot; you just send the tanker to NRHO, let it refuel a lunar mission, and then stow everything before it hits EDL.

[sdsds]

Since we're not in the SpaceX section of the forum is seems reasonable wonder:

Are there numerical estimates of how much more difficult low boil off technology is for hydrolox compared with methalox? Blue seems to think an HLS lander could use propulsion from a BE-3U engine....

[oldAtlas_Eguy]

Since we're not in the SpaceX section of the forum is seems reasonable wonder:

Are there numerical estimates of how much more difficult low boil off technology is for hydrolox compared with methalox? Blue seems to think an HLS lander could use propulsion from a BE-3U engine....

I suggest looking through the ACES white papers by Boeing and ULA. They contained the methodologies and boiloff rates for different mitigation against boiloff. They specifically used the LH boiloff to keep the LOX from boiling off. Then the result after the H heated up more to then use it to do orbit maintenance as a cold gas thruster prop. Deep space duration was in the months time frame for a manageable boiloff amount. But it also required a lot of heat insulation methodologies which would not work very well on the Moon surface. Boiloff rates are significantly one sided with a hydrolox system.