HLS Option B and the Sustaining Lunar Development Phase (Appendix P)

[VSECOTSPE]

It is very surprising to see so much talk about the impending termination of Artemis

No one wrote impending or soon.  Per the text you quoted, I was talking about how the situation would develop 6-8 years out.

What am I missing?

Stuff like this:

So there are no manned Artemis flights to the moon in 2026 and 2027 which is a long gap ( and stupid if you ask me).

You’re only looking at the inputs (political support, funding).  The outputs (missions, crews, lunar infrastructure, advances) are as or more important.

The Artemis budget request for FY23 is $7.6B and rising:

https://spacepolicyonline.com/news/nasa-asking-for-another-steep-increase-in-fy2023/

It won’t be Nelson and probably not this Administration.  But someone in power is eventually going to ask why we’re spending $8B annually on Artemis and only getting a couple or few astronauts ever two the three years in return.  The answer won’t be good and the actions that stem from that conversation won’t be positive for Artemis.

Would an US administration in 2028 or 2032 really decide to cancel it?

Sure.  Despite inheriting Kennedy’s legacy, the Johnson Administration turned off the Apollo production lines while the program was landing on the Moon.  Despite being a different political party, the Nixon Administration agreed and turned off Apollo operations and landings before all the hardware was used.

Artemis is unlikely to deliver even half of what Apollo did.  It could easily happen again.

Seems extremely dubious and a very premature discussion to have.

It’s a half decade or more away.  But to avoid that fate, NASA has to pursue alternatives to Orion/SLS now (as in yesterday).  If NASA waits until 2028 or 2032, it will be too late.  By that point, no one is likely to put up with another half-decade or decade of Artemis while Orion/SLS is phased out.

[yg1968]

[SLS cancellation is] a half decade or more away.  But to avoid that fate, NASA has to pursue alternatives to Orion/SLS now (as in yesterday).  If NASA waits until 2028 or 2032, it will be too late.  By that point, no one is likely to put up with another half-decade or decade of Artemis while Orion/SLS is phased out.

I have replied to this post in the SLS General Discussion thread since SLS cancellation is kind of off-topic in this thread:
https://forum.nasaspaceflight.com/index.php?topic=54967.msg2355304#msg2355304

[yg1968]

One other thing that's lurking in the requirement for either Appendix P or Option B to dock at the Gateway:  Option A LSS must be a passive IDSS implementation, because Orion is active only.  Presumably, Gateway will also be passive only, so the docking ring has to be active.  So LSS has to change its docking architecture to implement Option B.

The obvious thing for LSS to do is to implement the IDSS active-active option (which, AIUI, is really an active soft-capture ring that's latched in the retracted position and has three additional passive capture latches on it), but so far there aren't any active-active implementations with any flight heritage.  Presumably, the Option B/App. P uncrewed test flight would provide a live test for this, but does anybody know how much ground testing and certification hassle is required for such a docking system?

As I understand it, the Option A HLS must implement both the active mode and the passive mode, because it must be able to dock with the Gateway if it is available and it must be able to dock with Orion if Gateway is not available. NASA had already announced that Gateway was optional some time ago.

No, according to the IG, it is SpaceX that has the option to dock with Gateway or not (i.e., it is not NASA that has the option). The chances that Orion and HLS-Starship will dock with Gateway for Artemis III are very slim.

[TheRadicalModerate]

No, according to the IG, it is SpaceX that has the option to dock with Gateway or not (i.e., it is not NASA that has the option). The chances that Orion and HLS-Starship will dock with Gateway for Artemis III are very slim.

But Option B/App. P implementations must be capable of RPODs with Gateway, correct?

[yg1968]

No, according to the IG, it is SpaceX that has the option to dock with Gateway or not (i.e., it is not NASA that has the option). The chances that Orion and HLS-Starship will dock with Gateway for Artemis III are very slim.

But Option B/App. P implementations must be capable of RPODs with Gateway, correct?

Yes.

[VaBlue]

I also posted in the SLS General Discussion thread about this (thanks for the link yg1968!)
https://forum.nasaspaceflight.com/index.php?topic=54967.820

[Robotbeat]

No, according to the IG, it is SpaceX that has the option to dock with Gateway or not (i.e., it is not NASA that has the option). The chances that Orion and HLS-Starship will dock with Gateway for Artemis III are very slim.

But Option B/App. P implementations must be capable of RPODs with Gateway, correct?

Yes.

Seems to me the design specifications of Gateway may make that difficult. NASA seemed to set the maximum mass for a lander docked to Gateway to be higher than Starship, higher than ISS which had ability to dock Shuttle which was similar in mass. But that was a while ago. They didn’t expect such a huge lander at the time. I hope they have considered expanding that capability given that they awarded a huge lander the first contract.

[DanClemmensen]

So there are no manned Artemis flights to the moon in 2026 and 2027 which is a long gap ( and stupid if you ask me). SpaceX will already have done a crewed landing of the HLS so what’s to stop them from running their own flights to the surface during this time?

Assuming they can get to/from the moon from LEO , they will have a lander already tested.  Might piss off NASA and some politicians but might be good for further testing Starship in deep space.

Warning: much speculation here.
Not only will they have a tested design, they may also have one or two actual Starship HLS Option A already in space and ready to be reused. the incremental cost to return the Starship HLS from NRHO to depot EO is (guesstimate) three tanker flights. Moving HLS from EO back to an LEO orbit and then back to depot EO is at most one tanker flight. Then you need maybe four tankers flights for the depot EO-lunar surface-NRHO segment.  Thus, an excursion from ISS to the lunar surface and back to ISS would cost less than the Crew Dragon ride to ISS and back. But then reality intrudes: Starship HLS option A is not contractually required to be reusable, so we have no idea if SpaceX will have implemented all of the picky little details that are essential for reusability: specifically all of  the stuff needed for reprovisioning in space. If reprovisioning requires a Cargo Dragon flight, it more than doubles the mission cost. If by some magic SpaceX builds a cargo Starship and a reusable cargo container with Dragon-type docking interfaces, then the cost might be kept down.

[libra]

So there are no manned Artemis flights to the moon in 2026 and 2027 which is a long gap ( and stupid if you ask me). SpaceX will already have done a crewed landing of the HLS so what’s to stop them from running their own flights to the surface during this time?

Assuming they can get to/from the moon from LEO , they will have a lander already tested.  Might piss off NASA and some politicians but might be good for further testing Starship in deep space.

Such absurd situation... two different Starships to the same place (!)
- one for NASA, rammed into an expensive SLS Orion architecture, because Congress
- one SpaceX out of their main Mars effort, fully reusable.
 The only hope is for sanity to prevail and the latter screws the former.

It is absurd but then this is usually the case when it comes to congress funded space programs. I don’t expect sanity to suddenly kick in and plans to change.

I had just hoped then when the U.S. decided to go back to the moon and stay, it really meant “stay”. This Artemis program is nothing close to staying - more of a yearly (if even) short visit and scoot off again and all at a very high cost.

Maybe SpaceX can run a couple of tourist flights to the surface during these years!

By "sanity" I meant: "classic Starship" to the Moon putting Artemis-SLS-Orion-Starship-HLS to shame: delivering more, at lower cost, more often.
Do you think Congress could still justify the enormous expense of Starship-Artemis if Starship "classic" beat it into a pulp, to the Moon ?

And then, this comes on top of the above.

Sure.  Despite inheriting Kennedy’s legacy, the Johnson Administration turned off the Apollo production lines while the program was landing on the Moon.  Despite being a different political party, the Nixon Administration agreed and turned off Apollo operations and landings before all the hardware was used.

Artemis is unlikely to deliver even half of what Apollo did.  It could easily happen again.

The only consolation is that Starship-HLS-Artemis will have helped "classic Starship" - by siphonning out some of NASA / Artemis money into SpaceX.

[yg1968]

No, according to the IG, it is SpaceX that has the option to dock with Gateway or not (i.e., it is not NASA that has the option). The chances that Orion and HLS-Starship will dock with Gateway for Artemis III are very slim.

But Option B/App. P implementations must be capable of RPODs with Gateway, correct?

Yes.

Seems to me the design specifications of Gateway may make that difficult. NASA seemed to set the maximum mass for a lander docked to Gateway to be higher than Starship, higher than ISS which had ability to dock Shuttle which was similar in mass. But that was a while ago. They didn’t expect such a huge lander at the time. I hope they have considered expanding that capability given that they awarded a huge lander the first contract.

I think that you meant lower (not higher) but that was later changed. Starship is OK now. See the link below:

https://forum.nasaspaceflight.com/index.php?topic=50467.msg2100085#msg2100085

[tbellman]

Warning: much speculation here.
Not only will they have a tested design, they may also have one or two actual Starship HLS Option A already in space and ready to be reused. the incremental cost to return the Starship HLS from NRHO to depot EO is (guesstimate) three tanker flights. Moving HLS from EO back to an LEO orbit and then back to depot EO is at most one tanker flight. Then you need maybe four tankers flights for the depot EO-lunar surface-NRHO segment.

I'm not sure I understand your conops correctly.  E.g. what orbit is the "depot Earth Orbit", and why is it a different orbit from LEO?  And I don't understand how you have arrived at your guesstimates for the number of tanker flights (by which I assume you mean tanker launches).

But if this involves getting the tanks filled in LEO, and then going from LEO to the surface of the Moon and back to LEO without any further refillings inbetween, then your guesstimates are way off.

The HLS Starship is not practically capable of aerobraking into LEO when returning from the Moon; with no heat-shield it would need so many passes that it would probably take a year before it is down to a circular LEO.  It will thus use some 5.9 km/s delta-v both going from LEO to the Moon, and from the Moon and back to LEO, for a total of 11.8 km/s.  Even with quite optimistic assumptions about inert mass (80 t including passengers, consumables, and other cargo) and average specific impulse (382 s), that would require almost 1800 tonnes of propellant, with no staging or re-filling on the way.  More realistic assumptions give that it would need somewhere between 2500 and 3000 tonnes of propellant, but even 1800 t is more than the capacity of the HLS Starship tanks.

If you absolutely insist on using a Starship without heat-shield, you will need to fill it up once in LEO, and a second time in some later orbit (possibly after having visited the Moon).  And anyway, your guesstimate of

Given the limitations of not having a heat-shield, I am pretty certain that the HLS Starship will only be used for shuttling between NRHO (or maybe EML1 or EML2) and the lunar surface; it will never return to LEO.  For any other lunar missions, I suggest using a Starship with a heat-shield, so it can aerobrake, and even land back on Earth.

[DanClemmensen]

Warning: much speculation here.
Not only will they have a tested design, they may also have one or two actual Starship HLS Option A already in space and ready to be reused. the incremental cost to return the Starship HLS from NRHO to depot EO is (guesstimate) three tanker flights. Moving HLS from EO back to an LEO orbit and then back to depot EO is at most one tanker flight. Then you need maybe four tankers flights for the depot EO-lunar surface-NRHO segment.

I'm not sure I understand your conops correctly.  E.g. what orbit is the "depot Earth Orbit", and why is it a different orbit from LEO?  And I don't understand how you have arrived at your guesstimates for the number of tanker flights (by which I assume you mean tanker launches).

But if this involves getting the tanks filled in LEO, and then going from LEO to the surface of the Moon and back to LEO without any further refillings inbetween, then your guesstimates are way off.

This level of speculative detail should probably be on a different thread, and a real space mission planner can almost certainly do better. However, if a crewed landing that does not need SLS/Orion is possible using only the contracted-for HLS hardware, this affects the feasibility of the landers.

Depot is in an earth orbit (EO). As another poster pointed out, neither NASA nor SpaceX has actually specified this orbit, hence "depot EO".

CONOPS: After the first Starship HLS mission, HLS is in NRHO, so the "reusable" mission analysis starts there.
  -- send retrieval fuel to the depot in EO using three tankers. Two tankers add fuel to depot and return to Earth. The third tanker fills all the way up and transits to NRHO.
  -- tanker transfers fuel to HLS.
  -- tanker returns to earth and aerobrakes to EDL
  --HLS transits to Depot in Depot EO.
  --Yet another tanker flight transfers additional fuel to Depot for the next steps
  --HLS fuels and transits to an LEO near a station (e.g., ISS)
  --HLS crew transfers to HLS, probably by a local taxi run by the crew capsule
  --HLS, with crew, transists back to Depot in Deport EO
  --HLS refuels at Depot and performs EO--> lunar surface-->NRHO
  --repeat get crew back to LEO station.
Note: at most four tanker flights NOT INCLUDING the tanker flights needed for the "nominal" part of the mission.
Note: this assumes EO-->lunar surface-->NRHO needs no more fuel than the Artemis nominal EO-->NRHO-->lunar surface-->NRHO mission.

[clongton]

I am very, very disappointed in HLS Option B, Appendix P. All it does is maintain the least effective, most costly and economically unviable system possible. In my opinion, HLS Option B, Appendix P falls totally flat on it's face and should be scrapped completely. It does nothing at all to create a standard and dependable transportation system to and from the moon, which, if we think about it, it what we all really want. Except, of course, the folks in Congress who don't care about space but like the cash cow that NASA has become for tax money distribution to their own voting districts.

This is how transportation to the moon should go:

1. TSTO chemically powered flight from Earth's surface to a transportation facility orbiting in LEO, where passengers and/or cargo are offloaded onto the station. The ship then returns to the Earth's surface, with any passengers that are returning from the moon, where it is refurbished and made ready for its next flight.

2. At the orbiting transportation facility, passengers and/or cargo are transferred to a nuclear-powered NTR translunar spacecraft (think combination of LANTR and Space 2001 lander) that is partially fueled, with enough propellant for its lunar journey only. Once ready for departure, that spacecraft leaves LEO and goes directly to the lunar surface (no Gateway destination), where passengers and/or cargo are offloaded into a lunar surface receiving facility that functions akin to an airport on Earth. The spacecraft's nearly empty propellant tanks are then completely refilled with lunar ISRU propellant. It is enough propellant for a complete round trip, moon to earth and back. Earth bound passengers and/or cargo are boarded at the surface facility and the spacecraft departs the lunar surface and injects directly into a trans earth trajectory.

3. As the spacecraft approaches earth, it power brakes into LEO and arrives at the orbiting LEO transportation facility it left from on its outbound trip. At this point there remains sufficient propellant onboard to make the return trip to the lunar surface. No earth-sourced propellant is required.

4. Earth bound passengers and/or cargo are offloaded to the transportation facility for transfer to the earth surface aboard a similar spacecraft to the one that brought them from earth's surface to the station. The trans-lunar spacecraft is then loaded with moon-bound passengers and/or cargo for the return trip to the lunar surface.

5. Rinse and repeat

Note that except for its very 1st flight from LEO to the moon, all propellant for that trans-lunar ship(s) are sourced exclusively on the lunar surface from lunar ISRU. Climbing up out of earth's gravity well does not need to consider the propellant mass needed for the return trip to earth. The tanks will be filled for the round trip on the lunar surface from lunar surface ISRU facilities, where the gravity well is only 1/6 that of earth's.

Once implemented, this creates a truly sustainable lunar transportation system that can operate as smoothly and economically as a commercial airline. No Gateway or SLS/Orion is required. The LEO station is not a research facility - it is a transportation hub, much like an airport. Earth to LEO and back transportation is a fully reusable system, designed specifically for that role. LEO to lunar surface and back is a fully reusable system, designed specifically for that role. There are no cross-roll compromising designs to impede efficiency. Implementing something like this requires Vision, Drive and Guts. Does anyone know anyone with those 3 qualities?

[Surfdaddy]

It is very surprising to see so much talk about the impending termination of Artemis, I really don't understand.
....
From an outside space fan perspective this looks like the healthiest human exploration program since the ISS. What am I missing?

It certainly seems likely that Orion and Gateway and Starship will actually fly and any decisions regarding the continuation of the program will happen in an environment where the US has regained the capability to land crew on the Moon. Would an US administration in 2028 or 2032 really decide to cancel it? Seems extremely dubious and a very premature discussion to have.

That's what us oldsters thought in the early 1970s, too.

[yg1968]

I am very, very disappointed in HLS Option B. All it does is maintain the least effective, most costly and economically unviable system possible. In my opinion, HLS Option B falls totally flat on it's face and should be scrapped completely. It does nothing at all to create a standard and dependable transportation system to and from the moon, which, if we think about it, it what we all really want. Except, of course, the folks in Congress who don't care about space but like the cash cow that NASA has become for tax money distribution to their own voting districts.

I am guessing that you mean that you are disappointed with Option B and Appendix P. Option B is another HLS-Starship mission with minor upgrades since HLS-Starship likely already meets most of the requirements for a sustainable lander. Appendix P is the second sustainable lander program. Option B and Appendix P are for a total of two new crewed demo landers.

In terms of your criticism of the HLS program, I am not sure that it is warranted. The HLS program is a lander program for services from NRHO to the Moon and back, it was never meant as a replacement for SLS and Orion. Even in a scenario without SLS and Orion, HLS-Starship would still need a regular Starship to carry astronauts from Earth to Earth orbit. So there is some logic in breaking down these services into two: 1- Earth to Earth orbit or NRHO and; 2-NRHO or Earth orbit to the Moon. If you think that SLS and Orion should be replaced with a commercial crew to BLEO program, fine but that is a separate issue from HLS. HLS is the wrong target here.

[yg1968]

...That’s pushing 2028-2030.  That’s into the next Administration, which may be too late to help Artemis.

... Any of those singly or in combination would push a service start well into the early or mid-2030s.  I think that definitely risks being too late to help Artemis.

Although I’d love to be surprised, the reality is that none of this is going to get started while Nelson sits in the Administrator’s chair, which pushes it too far into the future to help Artemis at all.  And even if Nelson had to retire tomorrow, I don’t see this Administration grappling with this given everything else they’re juggling, which again pushes this too far into the future to help Artemis at all.  So that’s why I advise terminating Artemis sooner (although I obviously do not expect that to happen) than later.

It is very surprising to see so much talk about the impending termination of Artemis, I really don't understand.

None of the Artemis program hardware has flown, many new contracts are being announced and we are seeing active and successful political pressure to spend more money on it (for the second lander). From an outside space fan perspective this looks like the healthiest human exploration program since the ISS. What am I missing?

It certainly seems likely that Orion and Gateway and Starship will actually fly and any decisions regarding the continuation of the program will happen in an environment where the US has regained the capability to land crew on the Moon. Would an US administration in 2028 or 2032 really decide to cancel it? Seems extremely dubious and a very premature discussion to have.

You haven't missed anything. Some posters on this forum have decided that the entire Artemis program must be cancelled because of the cost of SLS and Orion. A more realistic approach would be to eventually replace SLS and Orion with a commercial crew to BLEO program. I believe that will eventually happen but that will take years and perhaps even a decade or more.

In any event, SLS and Orion aren't getting cancelled any time soon, especially now that they are sitting on the pad. What does this have to with HLS? Absolutely nothing.

[clongton]

I am very, very disappointed in HLS Option B. All it does is maintain the least effective, most costly and economically unviable system possible. In my opinion, HLS Option B falls totally flat on it's face and should be scrapped completely. It does nothing at all to create a standard and dependable transportation system to and from the moon, which, if we think about it, it what we all really want. Except, of course, the folks in Congress who don't care about space but like the cash cow that NASA has become for tax money distribution to their own voting districts.

1. I am guessing that you mean that you are disappointed with Option B and Appendix P. Option B is another HLS-Starship mission with minor upgrades since HLS-Starship likely already meets most of the requirements for a sustainable lander. Appendix P is the second sustainable lander program.

2. If you think that SLS and Orion should be replaced with a commercial crew to BLEO program, fine but that is a separate issue from HLS.

1. Yes; Option B, Appendix P. That was my oversight. I have corrected my post. Thank you.
2. I didn't used to think this but I have come around to this way of thinking more and more lately. It is primarily because of what the NASA funding profile does for HSF. Under the current funding profile it is not possible to develop the kind of system that I described because every part of it is long term thinking, outlasting many congressional elections and several administrations. Both the legislative and executive branches now treat NASA as a political football designed to score points with an ever more fickle and polarized electorate. Every few years it's toss it all and start over because there's a new sheriff in town. There is very little real vision of humanity broadening beyond its home world to other places in the solar system, where business and commerce could flourish, providing real funding for expansion. It's presently impossible to get much done in a meaningful timeframe and in an economically meaningful way. It would not break my heart to see a NGO become a NASA replacement, with a funding profile which was able to find financial support from sources other than people whose main interest was not in the agency's best interest.

Yes, it is a separate issue from HLS so I'll not take this any further OT. Suffice it to say that I'm disappointed that it was really just more of the same, and I'll leave it at that.

[yg1968]

Incidentally, a better place for this discussion would be in the SLS General Discussion thread:
https://forum.nasaspaceflight.com/index.php?topic=54967.820

[tbellman]

Depot is in an earth orbit (EO). As another poster pointed out, neither NASA nor SpaceX has actually specified this orbit, hence "depot EO".

While they might not have specified the exact orbit, it is fairly obvious that it is some kind of LEO.  Remember the fourteen tanker launches mentioned in the GAO decision last summer?  If it had been in high orbit, it would be more like fourty tankers!  (Hint: you want to lift the dry mass of the numerous tankers to as low-energy orbits as possible, so the lower depot orbit, the better (while staying out of the atmosphere).)

CONOPS: After the first Starship HLS mission, HLS is in NRHO, so the "reusable" mission analysis starts there.
  -- send retrieval fuel to the depot in EO using three tankers. Two tankers add fuel to depot and return to Earth. The third tanker fills all the way up and transits to NRHO.
  -- tanker transfers fuel to HLS.
  -- tanker returns to earth and aerobrakes to EDL
  --HLS transits to Depot in Depot EO.
  --Yet another tanker flight transfers additional fuel to Depot for the next steps
  --HLS fuels and transits to an LEO near a station (e.g., ISS)
  --HLS crew transfers to HLS, probably by a local taxi run by the crew capsule
  --HLS, with crew, transists back to Depot in Deport EO
  --HLS refuels at Depot and performs EO--> lunar surface-->NRHO
  --repeat get crew back to LEO station.
Note: at most four tanker flights NOT INCLUDING the tanker flights needed for the "nominal" part of the mission.
Note: this assumes EO-->lunar surface-->NRHO needs no more fuel than the Artemis nominal EO-->NRHO-->lunar surface-->NRHO mission.

How did you arrive at the various number of tanker flights needed?  If you don't know what orbit the depot is in, and the delta-v needed to go between your different orbits, you can be off by a factor ten; or your mission may not close at all regardless of the number of tankers if the required propellant doesn't fit on the ship.

Here's one example:
• The empty HLS Starship masses 80t (optimistic) and tankers 120t.
• Passengers, their consumables and other cargo, are massless, so the HLS Starship with passengers is still 80t (extremely optimistic ).
• Depot orbit is LEO.  Tankers can bring 250t to the depot orbit (optimistic).  Δv to NRHO is 3.65 km/s, and effective I_sp is 380 seconds (optimistic).
• The third tanker in your first point will thus have 750t propellant on board before departing for NRHO.  It will use up 540t to reach NRHO; it will need 15t to return from NRHO and crash-land on Earth, so it can give at most 195t to the HLS Starship.
• HLS Starship needs 133t propellant to reach LEO, so that's OK.
• Δv LEO -> lunar surface is 5.95 km/s, and then maybe 2.5 km/s from lunar surface to NRHO.  It needs 700t propellant for that trip; but since it had some propellant left over from the first refilling, you only need 2.5 additional tankers for that.
• So this is actually pretty close to your four tankers, but it is under quite optimistic assumptions.  And the passengers are now in NRHO, not in LEO.  To bring them down to LEO, you need another 2.5 tankers or so.

Another example: same masses and specific impulse, but as high depot orbit as you can:
• Depot orbit is a high elliptic orbit with perigee in LEO and apogee somewhere around the Moon.  3.2 km/s Δv to reach from LEO, and 430 m/s Δv from NRHO.
• The tankers that have 250t propellant when they reach LEO, will arrive at the depot orbit with only 37t left each, or 111t for three tankers.
• The tanker going to NRHO will need 25t for that, and 15t to get back and crash-land on Earth, leaving 71t for the HLS Starship.  The other tankers need approximately 0t to aerobrake and crash-land on Earth.
• The HLS Starship needs 10t to go to the depot orbit.  The depot however had another tanker reach it, giving it 37t, so the HLS Starship now has 71t-10t+37t=98t propellant in its tanks.
• But it needs 108t just to reach LEO, so obviously the mission fails.
• But you planned to go depot orbit -> LEO -> depot orbit, and that requires 365t propellant, or ten tanker flights, not one.
• And then it needs to receive another 250t propellant (7 tankers) from the depot to go from depot orbit -> lunar surface -> NRHO.

So we are at twenty tanker flights.  Not four.  And this is assuming the massless passengers are OK with ending up in NRHO.

Depot orbits that are somewhere inbetween LEO and the above high elliptic orbit, will need intermediate number of tankers.

[yg1968]

Depot is in an earth orbit (EO). As another poster pointed out, neither NASA nor SpaceX has actually specified this orbit, hence "depot EO".

While they might not have specified the exact orbit, it is fairly obvious that it is some kind of LEO.  Remember the fourteen tanker launches mentioned in the GAO decision last summer?  If it had been in high orbit, it would be more like fourty tankers!  (Hint: you want to lift the dry mass of the numerous tankers to as low-energy orbits as possible, so the lower depot orbit, the better (while staying out of the atmosphere).)

In an interview, Jim Bridenstine had said that SpaceX's would refuel/refill in LEO (see the link below) but most presentations only say Earth orbit.
https://forum.nasaspaceflight.com/index.php?topic=48676.msg2143546#msg2143546