Starship On-orbit refueling - Options and Discussion

[Twark_Main]

15t crew module, 7t deployable payload

That's gonna look really weird with a giant almost-entirely-empty space ship.


Besides the bad optics, the shockingly small dry-mass-to-useful-payload ratio suggests strongly that it will be more efficient (as measured by launch mass per payload mass) to send more payload per vehicle.



To illustrate what I mean, take the extreme example: if only 1% of your landed mass were payload, then you could double your payload for only a ~1% increase in launch mass.

[TheRadicalModerate]

15t crew module, 7t deployable payload

That's gonna look really weird with a giant almost-entirely-empty space ship.

Besides the bad optics, the shockingly small dry-mass-to-useful-payload ratio suggests strongly that it will be more efficient (as measured by launch mass per payload mass) to send more payload per vehicle.

To illustrate what I mean, take the extreme example: if only 1% of your landed mass were payload, then you could double your payload for only a ~1% increase in launch mass.

Four things:

1) It can look however it wants but it more than satisfies the Option A and B requirements, which are what LSS is intended to satisfy.

2) You can make it land heavy stuff easily if you're willing to do more than one refueling.  We've had that discussion up-thread.  I'm still assuming that minimization of conops complexity is a big deal for NASA.

3) I'll note that "a giant almost-entirely-empty space ship" is a pretty good description of any spacecraft that's designed to hold humans for anything other than a few days.  If you have almost the entire ogive for pressurized crew spaces, it's actually easier for them to get down to the cargo deck (where the elevator is) if the cylindrical portion of the payload bay is only a couple of meters high instead of 8m.

4) NASA clearly thinks that heavy cargo gets landed through CLPS, or the App. P Human-Class Delivery Lander (HDL) variant.  Using the same 95t dry and average Isp=368s assumptions, an expendable LSS can land 26t of cargo on the surface for 3 lift tankers, and 175t for 9 lift tankers.  (The latter assumes that a vanilla 120t-dry Starship can lift 150t to LEO.)

[Robotbeat]

Remind me. With all the variations discussed, I've lost track. If they do the big tanks and VLEO, how far can they get? Refuel on lunar orbit before or after landing?  Which lunar orbit?

If I assume a 95t dry LSS, 15t crew module, 7t deployable payload, and Isp=368s (with at least two R2SL's running), a 1500t LSS can just do VLEO-NRHOviaBLT-100dayLoiter-crewXfer-LS-10dayMission-NRHO-crewXfer.

That also assumes 150kg/day boiloff in NRHO, and 500kg/day boiloff on the lunar surface near the poles, and 2% delta-v FPR for all crewed segments.

The mission actually doesn't close if you do fast transit to NRHO.

You can easily do a mission with 1200t tanks if you're willing to refuel both in LEO and something like an LEO+2000m/s HEEO.  But AFAICT, that's not the plan.

AFAICT, your 1500t LSS isn't the plan, either.

[TheRadicalModerate]

AFAICT, your 1500t LSS isn't the plan, either.

True.  So we're not seeing the whole plan yet.

[OTV Booster]

Remind me. With all the variations discussed, I've lost track. If they do the big tanks and VLEO, how far can they get? Refuel on lunar orbit before or after landing?  Which lunar orbit?

If I assume a 95t dry LSS, 15t crew module, 7t deployable payload, and Isp=368s (with at least two R2SL's running), a 1500t LSS can just do VLEO-NRHOviaBLT-100dayLoiter-crewXfer-LS-10dayMission-NRHO-crewXfer.

That also assumes 150kg/day boiloff in NRHO, and 500kg/day boiloff on the lunar surface near the poles, and 2% delta-v FPR for all crewed segments.

The mission actually doesn't close if you do fast transit to NRHO.

You can easily do a mission with 1200t tanks if you're willing to refuel both in LEO and something like an LEO+2000m/s HEEO.  But AFAICT, that's not the plan.

IOW, it works if all the gods smile. Tight.

[TheRadicalModerate]

Remind me. With all the variations discussed, I've lost track. If they do the big tanks and VLEO, how far can they get? Refuel on lunar orbit before or after landing?  Which lunar orbit?

If I assume a 95t dry LSS, 15t crew module, 7t deployable payload, and Isp=368s (with at least two R2SL's running), a 1500t LSS can just do VLEO-NRHOviaBLT-100dayLoiter-crewXfer-LS-10dayMission-NRHO-crewXfer.

That also assumes 150kg/day boiloff in NRHO, and 500kg/day boiloff on the lunar surface near the poles, and 2% delta-v FPR for all crewed segments.

The mission actually doesn't close if you do fast transit to NRHO.

You can easily do a mission with 1200t tanks if you're willing to refuel both in LEO and something like an LEO+2000m/s HEEO.  But AFAICT, that's not the plan.

IOW, it works if all the gods smile. Tight.

Yeah, pretty tight.  But there is boil-off and some delta-v margin built in.  And of course we don't know what dry mass and crew module mass will be.  95t and 15t are guesses.  If it's more, then it's two refuelings for sure.  If it's less, things'll be fine.

[Twark_Main]

15t crew module, 7t deployable payload

That's gonna look really weird with a giant almost-entirely-empty space ship.

Besides the bad optics, the shockingly small dry-mass-to-useful-payload ratio suggests strongly that it will be more efficient (as measured by launch mass per payload mass) to send more payload per vehicle.

To illustrate what I mean, take the extreme example: if only 1% of your landed mass were payload, then you could double your payload for only a ~1% increase in launch mass.

Four things:

1) It can look however it wants but it more than satisfies the Option A and B requirements, which are what LSS is intended to satisfy.

2) You can make it land heavy stuff easily if you're willing to do more than one refueling.  We've had that discussion up-thread.  I'm still assuming that minimization of conops complexity is a big deal for NASA.

How silly of me. I forgot this is a flags-and-footprints program, not a cost-per-kilogram program.

3) I'll note that "a giant almost-entirely-empty space ship" is a pretty good description of any spacecraft that's designed to hold humans for anything other than a few days.

Only in movies.

  If you have almost the entire ogive for pressurized crew spaces, it's actually easier for them to get down to the cargo deck (where the elevator is) if the cylindrical portion of the payload bay is only a couple of meters high instead of 8m.

That results from shoehorning in an existing vehicle. It's a historically contingent accident, so we should be cautious generalizing it to "any spacecraft."

[TheRadicalModerate]

Four things:

1) It can look however it wants but it more than satisfies the Option A and B requirements, which are what LSS is intended to satisfy.

2) You can make it land heavy stuff easily if you're willing to do more than one refueling.  We've had that discussion up-thread.  I'm still assuming that minimization of conops complexity is a big deal for NASA.

How silly of me. I forgot this is a flags-and-footprints program, not a cost-per-kilogram program.

So you quote both the point that it satisfies Option A/B and the one that says that you can make it as heavy as you want with a bit of extra complexity,¹ but your response is that it's only good for flags and footprints?

___________
¹Or not:  You can use an expendable LSS and land as much heavy stuff as you want via CLPS or HDL.  And unless tanker flights are priced at well under $10M, it's actually cheaper than trying to reuse a cargo Starship.

[Twark_Main]

Four things:

1) It can look however it wants but it more than satisfies the Option A and B requirements, which are what LSS is intended to satisfy.

2) You can make it land heavy stuff easily if you're willing to do more than one refueling.  We've had that discussion up-thread.  I'm still assuming that minimization of conops complexity is a big deal for NASA.

How silly of me. I forgot this is a flags-and-footprints program, not a cost-per-kilogram program.

So you quote both the point that it satisfies Option A/B and the one that says that you can make it as heavy as you want with a bit of extra complexity,¹ but your response is that it's only good for flags and footprints?

___________
¹Or not:  You can use an expendable LSS and land as much heavy stuff as you want via CLPS or HDL.  And unless tanker flights are priced at well under $10M, it's actually cheaper than trying to reuse a cargo Starship.

Of course. That's all Artemis is, and that's what shaped those Option A/B requirements.

If you're doing any real work on the Moon, you "make it as heavy as you want you need to be economical." Using an extremely inefficient payload fraction ain't it.

[Greg Hullender]

Can someone write a succinct summary of Options A and B or else post a link to one? I hunted and hunted, but the most I could find was that Option A is pretty much what I already think of as HLS while option B is "the same thing, but sustainable."

[DanClemmensen]

Can someone write a succinct summary of Options A and B or else post a link to one? I hunted and hunted, but the most I could find was that Option A is pretty much what I already think of as HLS while option B is "the same thing, but sustainable."

It's a bit crazy. Technically, the formal requirements for Option A and Option B are embedded in NASA's RFP for the HLS, which I think is described as "Appendix H of NextSTEP-2". The Option A requirements envisioned a lander something like NASA's three-stage reference design, sort of a glorified Apollo LM, so the requirements are not very ambitious at all. Instead, SpaceX bid a Starship variant, which so grossly exceeded the requirements as to make them seem silly. Starship HLS has so much extra margin that then can add "sustainability" and meet the Option B requirements with (I believe) very little effort. The Option B sustainability requirements are (I think) the same as those for the Appendix P (second source) landers.

(Going from memory here, I may have messed it up.) The sustainability requirements are to support a crew of four, increase the mission duration to a full month, and increase the downmass and upmass from the tiny Option A contract requirements to the slightly less tiny Option B requirements. But "sustainable" is mostly about reduced cost per mission to make the Artemis program more sustainable. Reuse is not a requirement, but was given as an example of a way to reduce cost.

Since Starship HLS Option A will already almost meet Option B, I wonder if it would be cheaper for SpaceX to just build the initial HLS to meet Option B in the first place.

Edited to add:
    The actual reguirements documents are reachable from
   https://sam.gov/opp/d5460a204ab23cc0035c088dcc580d17/view
There is a whole set of PDFs in a zipped folder for "attachment F". Within the folder, the main doc is
     HLS-RQMT-001 SRD Rev R 20190927.pdf
It contains both the Option A and the Option B requirements, which I will now re-read to see how badly I misrepresented them in this post.

[tbellman]

Can someone write a succinct summary of Options A and B or else post a link to one? I hunted and hunted, but the most I could find was that Option A is pretty much what I already think of as HLS while option B is "the same thing, but sustainable."

Attached is the requirements document from the RFP; this is a slightly newer version than the one you find at sam.gov that Dan Clemmensen pointed to.  Chapter 4 is the interresting stuff, with 4.2 containing the higher requirements for the sustained phase (i.e, for Option B).

As Dan mentioned, Option B requires transport of four crew, a bit more mass both up and down, and longer stays on the surface.  But also:
• Be able to dock to the Lunar Gateway (for Option A, providers could chose if they wanted to be able to dock to Orion or to Gateway).
• Be able to reach anywhere on the Moon, not just between 84°S and 90°S (this requires more Δv).
• Survive continous darkness for at least 50 hours (with a goal of up to 191 hours).
• Support longer EVA excursions.

But on the flip side, for the longer stays and when carrying four crew, the provider can assume that there will be a separate habitat waiting on the surface, so the lander does not need to be the habitat for the entire stay.

"Sustainability" here is not just about costs, but also about being more capable.  And also, as the requirements document says, "Beyond the initial HLS missions, the HLS will support sustainable presence on the Moon by providing a regular cadence of reliable transportation services for humans and cargo".

I believe that the Starship provided for Artemis III will not comply with all the requirements for Option B.  I think SpaceX (and NASA) will view that Starship as a kind of prototype to learn what works well and what works less good.  And since they will then plan on throwing away that ship anyway, they will not spend money and effort on outfitting it with features not needed for that first landing.  In particular, I suspect dealing with the lunar night, the extra Δv needed for global access, and procedures for refilling of propellant in NRHO (so the ship can be reused) will not be developed and ready for Artemis III.  (But this depends on how late Artemis III will be compared to how late Starship will be.)

[DanClemmensen]

I believe that the Starship provided for Artemis III will not comply with all the requirements for Option B.  I think SpaceX (and NASA) will view that Starship as a kind of prototype to learn what works well and what works less good.  And since they will then plan on throwing away that ship anyway, they will not spend money and effort on outfitting it with features not needed for that first landing.  In particular, I suspect dealing with the lunar night, the extra Δv needed for global access, and procedures for refilling of propellant in NRHO (so the ship can be reused) will not be developed and ready for Artemis III.  (But this depends on how late Artemis III will be compared to how late Starship will be.)

Even if SpaceX does not implement all of Option B prior to Artemis III, they may design for it and then not actually install all of the required HW. In particular, they might choose to handle the night by adding a big battery. If so, they will decide prior to Artemis III where that battery will reside within the HLS.

The Δv needed for global access is already accounted for, I think. The difference is the number of tanker flights.

HLS reuse is not a specific requirement. SpaceX already intends to reuse "elements of the HLS system", namely the depot and tankers. This is better reuse than what was contemplated by NASA prior to the award, where they discussed reusing only the ascent module of a three-part HLS.

I think all instances of systems with very slow cadence are prototypes, with lessons learned with each use resulting in changes in the next instance. So sure, the uncrewed demo is a prototype for the Crewed Artemis III HLS, and the Artemis III HLS is a prototype for the Option B HLS. But Option B will be a design tweak, not a redesign and certainly not a new design. Caveat: it might be a big change at the mission level if they decide to put a depot into NRHO.

[TheRadicalModerate]

Can someone write a succinct summary of Options A and B or else post a link to one? I hunted and hunted, but the most I could find was that Option A is pretty much what I already think of as HLS while option B is "the same thing, but sustainable."

Attached is the requirements document from the RFP; this is a slightly newer version than the one you find at sam.gov that Dan Clemmensen pointed to.  Chapter 4 is the interresting stuff, with 4.2 containing the higher requirements for the sustained phase (i.e, for Option B).

As Dan mentioned, Option B requires transport of four crew, a bit more mass both up and down, and longer stays on the surface.  But also:
• Be able to dock to the Lunar Gateway (for Option A, providers could chose if they wanted to be able to dock to Orion or to Gateway).
• Be able to reach anywhere on the Moon, not just between 84°S and 90°S (this requires more Δv).
• Survive continous darkness for at least 50 hours (with a goal of up to 191 hours).
• Support longer EVA excursions.

But on the flip side, for the longer stays and when carrying four crew, the provider can assume that there will be a separate habitat waiting on the surface, so the lander does not need to be the habitat for the entire stay.

"Sustainability" here is not just about costs, but also about being more capable.  And also, as the requirements document says, "Beyond the initial HLS missions, the HLS will support sustainable presence on the Moon by providing a regular cadence of reliable transportation services for humans and cargo".

I believe that the Starship provided for Artemis III will not comply with all the requirements for Option B.  I think SpaceX (and NASA) will view that Starship as a kind of prototype to learn what works well and what works less good.  And since they will then plan on throwing away that ship anyway, they will not spend money and effort on outfitting it with features not needed for that first landing.  In particular, I suspect dealing with the lunar night, the extra Δv needed for global access, and procedures for refilling of propellant in NRHO (so the ship can be reused) will not be developed and ready for Artemis III.  (But this depends on how late Artemis III will be compared to how late Starship will be.)

I agree that there are lots of lessons to be learned in Option A that will necessitate at least some tweaks/redesign before they have an Option B-compliant offering.

One obvious missing capability to turn the Option A LSS into Option B is the ability to refuel in cislunar, which is how the LSS becomes reusable.  That capability actually can't be exercised until the second mission (presumably Arty 6) of the Option B LSS (i.e., the one for Arty 4).

That refueling capability also makes it easy for the LSS to have access to all lunar locations, and may even provide a cheap way to survive darkness, by using an APU and however much methalox you need to provide the needed power.

One of the things we haven't discussed a lot is that the existence of both cislunar and LEO refueling not only enables Option B, but it also dramatically reduces the cost for other providers to provide Appendix P / SLT services.  This obviously requires other providers to accept SpaceX/Starship methalox refueling as a first option, probably with some kind of backup system (FH, Vulcan, New Glenn...) in case Starship goes down for a while.  However, in general, App. P bidders can provide much more capable offerings if they can buy methalox from SpaceX in NRHO.

This does require that SpaceX be willing to offer their refueling interconnects and any docking/stabilization mechanisms as an open system, and be willing to contract with third parties to prove that depot-to-3rd-party refueling works cleanly.  But the upside of doing so is massive for the App. P providers and NASA.

The question is how aggressively SpaceX might want to go into the open refueling business.  But my guess is that it's a very nice, very profitable business, with close to double the number of tanker launches they'd get if they're only responsible for their own HLS/HDL birds, which will presumably get only half the missions, per NASA policy.  And everybody wins.

[Greg Hullender]

One obvious missing capability to turn the Option A LSS into Option B is the ability to refuel in cislunar, which is how the LSS becomes reusable. 

Don't they also need some way to inspect/test the vehicle before reuse? That seems to me to be the biggest missing piece--although it provides a great reason to have a crew at Gateway. :-)

[DanClemmensen]

One obvious missing capability to turn the Option A LSS into Option B is the ability to refuel in cislunar, which is how the LSS becomes reusable. 

Don't they also need some way to inspect/test the vehicle before reuse? That seems to me to be the biggest missing piece--although it provides a great reason to have a crew at Gateway. :-)

There is no hard requirement in Option B for reuse of the HLS lander. The requirement is for "sustainable" access to the moon, meaning (more or less) one or more times a year for not too much money. The original NASA reference lander has three elements: transit, descent, and ascent, and only the ascent element was reusable in its Option B. The Starship HLS system has three elements: Depot, Tanker, and HLS. The tanker and Depot are reusable and the HLS is not very expensive, so even if it is expended  the system still meets the sustainability requirement. It is probably cheaper to expend it than to refuel it, but the big problems for reuse are reprovisioning and loading new large cargo. Provisioning and cargo are easy to load on a new HLS on Earth, much harder to transfer in space (cislunar, LEO, or other).

HLS is "not very expensive" because much of it is built from standard Starship parts in a high-production SpaceX factory.

Starship HLS also increases sustainability by eliminating most separate cargo missions. Cargo missions have generally been assumed to be one-way anyway.

It might make sense to reuse Starship HLS if SLS/Orion is retired and HLS conveys the crew from LEO to the Lunar surface and back.

[oldAtlas_Eguy]

One obvious missing capability to turn the Option A LSS into Option B is the ability to refuel in cislunar, which is how the LSS becomes reusable. 

Don't they also need some way to inspect/test the vehicle before reuse? That seems to me to be the biggest missing piece--although it provides a great reason to have a crew at Gateway. :-)

There is no hard requirement in Option B for reuse of the HLS lander. The requirement is for "sustainable" access to the moon, meaning (more or less) one or more times a year for not too much money. The original NASA reference lander has three elements: transit, descent, and ascent, and only the ascent element was reusable in its Option B. The Starship HLS system has three elements: Depot, Tanker, and HLS. The tanker and Depot are reusable and the HLS is not very expensive, so even if it is expended  the system still meets the sustainability requirement. It is probably cheaper to expend it than to refuel it, but the big problems for reuse are reprovisioning and loading new large cargo. Provisioning and cargo are easy to load on a new HLS on Earth, much harder to transfer in space (cislunar, LEO, or other).

HLS is "not very expensive" because much of it is built from standard Starship parts in a high-production SpaceX factory.

Starship HLS also increases sustainability by eliminating most separate cargo missions. Cargo missions have generally been assumed to be one-way anyway.

It might make sense to reuse Starship HLS if SLS/Orion is retired and HLS conveys the crew from LEO to the Lunar surface and back.

The cost of the LSS is the crew habitat and other LSS crew related addons to the basic SS. A basic SS hardware may be easily as low as $50M. But an LSS hardware is likely to run at greater than $150M. NOTE here is that a depot and a BEO tanker would be at the close to a basic SS costs. Thus such to use an LSS at least 3 times will gain significant savings overall for surface missions of $50 to >$100M on each mission. Which can say a lot about a stripped down oneway cargo hardware to cost ~$75M for the cargo SS that delivers about 75 to 100t of cargo to the surface.

Even using a throw away tanker vs a Lunar Depot you would still save money by just reusing the LSS 3 to 5 times. You could even refuel the LSS for return to LEO so it could be loaded for next mission and still save money. Such that initially only Tankers are thrown away until its possible to return EDL the Tanker from Lunar orbit. Once that happens an additional ~$50M is saved per mission.

[OTV Booster]

One obvious missing capability to turn the Option A LSS into Option B is the ability to refuel in cislunar, which is how the LSS becomes reusable. 

Don't they also need some way to inspect/test the vehicle before reuse? That seems to me to be the biggest missing piece--although it provides a great reason to have a crew at Gateway. :-)

There is no hard requirement in Option B for reuse of the HLS lander. The requirement is for "sustainable" access to the moon, meaning (more or less) one or more times a year for not too much money. The original NASA reference lander has three elements: transit, descent, and ascent, and only the ascent element was reusable in its Option B. The Starship HLS system has three elements: Depot, Tanker, and HLS. The tanker and Depot are reusable and the HLS is not very expensive, so even if it is expended  the system still meets the sustainability requirement. It is probably cheaper to expend it than to refuel it, but the big problems for reuse are reprovisioning and loading new large cargo. Provisioning and cargo are easy to load on a new HLS on Earth, much harder to transfer in space (cislunar, LEO, or other).

HLS is "not very expensive" because much of it is built from standard Starship parts in a high-production SpaceX factory.

Starship HLS also increases sustainability by eliminating most separate cargo missions. Cargo missions have generally been assumed to be one-way anyway.

It might make sense to reuse Starship HLS if SLS/Orion is retired and HLS conveys the crew from LEO to the Lunar surface and back.

The cost of the LSS is the crew habitat and other LSS crew related addons to the basic SS. A basic SS hardware may be easily as low as $50M. But an LSS hardware is likely to run at greater than $150M. NOTE here is that a depot and a BEO tanker would be at the close to a basic SS costs. Thus such to use an LSS at least 3 times will gain significant savings overall for surface missions of $50 to >$100M on each mission. Which can say a lot about a stripped down oneway cargo hardware to cost ~$75M for the cargo SS that delivers about 75 to 100t of cargo to the surface.

Even using a throw away tanker vs a Lunar Depot you would still save money by just reusing the LSS 3 to 5 times. You could even refuel the LSS for return to LEO so it could be loaded for next mission and still save money. Such that initially only Tankers are thrown away until its possible to return EDL the Tanker from Lunar orbit. Once that happens an additional ~$50M is saved per mission.

I question the $50m price tag on SS. An F9 full stack has been estimated at $30m and never hit the production numbers that SS is being designed for. Maybe something in the $10-25m range? A cost that low might skew the numbers against reuse although the landing engines would bump the costs.

A point to remember is the need for later landings to host a crew of four and stay alive through extended darkness. If folding this into the first build bumps the cost enough to justify reuse, so be it, but is does crimp redesign from 'lessons learned'.

In principal I am in favor of as much reuse as possible but I just can't make the case close in this instance. The build rate, the build cost, the expanding mission requirements, the inability to redesign for lessons learned, all seem to work against it.

OTOH, when it time for the 4 crew
overnight version, reuse might make sense. 

[Anguy]

I'd like to ask how effective are current TPS tiles as long time insulation for supply depot? Most proposals suggest stripping the depot bare, but wouldn't it be more effective to do the opposite a cover it all around with tiles? They could be dual used as simple omnidirectional insulation (so Earth radiation would be blocked too) instead of some deployable sunshield and the depot can also be used for Lunar missions and return to LEO by aerobraking, saving a lot of propellant.
Adding 20+tons to ship dry mass is not that big problem, if they can be used this way...

[TheRadicalModerate]

One obvious missing capability to turn the Option A LSS into Option B is the ability to refuel in cislunar, which is how the LSS becomes reusable. 

Don't they also need some way to inspect/test the vehicle before reuse? That seems to me to be the biggest missing piece--although it provides a great reason to have a crew at Gateway. :-)

There is no hard requirement in Option B for reuse of the HLS lander. The requirement is for "sustainable" access to the moon, meaning (more or less) one or more times a year for not too much money. The original NASA reference lander has three elements: transit, descent, and ascent, and only the ascent element was reusable in its Option B. The Starship HLS system has three elements: Depot, Tanker, and HLS. The tanker and Depot are reusable and the HLS is not very expensive, so even if it is expended  the system still meets the sustainability requirement.

Mostly agree.  I assume that by "expendable HLS" you mean one that returns to NRHO but isn't reused.  Leaving it on the surface is obviously hard on the crew...

HLS is "not very expensive" because much of it is built from standard Starship parts in a high-production SpaceX factory.

Probably true from a bill-of-materials standpoint, but almost certainly not true from an assembly and certification standpoint.  Crew-rated ships are harder simply because the quality control and documentation requirements are massive.

Starship HLS also increases sustainability by eliminating most separate cargo missions. Cargo missions have generally been assumed to be one-way anyway.

I think it's highly unlikely that most crewed missions will carry heavy unpressurized cargo, for the following reasons:

1) For new HLS ships, the LEO-NRHO-LS-NRHO conops is sufficiently tight that heavy cargoes often will blow your delta-v budget.  This can be avoided if you're willing to have a crew do a refuel in an HEEO but you don't want that.  It's operationally complex, risks the crew during refueling ops, and the crew gets exposed to more radiation than necessary.

2) Doing on-orbit transfer of heavy cargo through the HLS hatch is an unsolved problem, which is likely to be really hard.  The cargo you'd transfer is incredibly expensive, and subjecting it to the risks of unstowing it from the supply ship, getting a robot arm to precision-position it in the target ship, batten it down for transport, and then essentially re-integrate it, simply subjects it to too much risk.

3) Heavy cargo launches on a dumb, expendable (left on the surface) LSS are cheap.  No crew involved, so manufacturing costs are low, and you can get almost 40t of cargo on an LSS CLPS or HDL mission for a measly 3 tankers of prop.

4) There won't be any such thing as extremely heavy cargo on an HLS mission, because any payload constructed will likely have to be compatible with whatever the other Appendix P offering is--and it won't be capable of handling heavy cargo.  AIUI, the requirement is only for pressurized cargo that can be passed through an IDA tunnel.

It might make sense to reuse Starship HLS if SLS/Orion is retired and HLS conveys the crew from LEO to the Lunar surface and back.

It makes all kinds of sense.  But it doesn't help the cargo situation.  microgravity is microgravity, and vacuum integration is vacuum integration.

Side note (and pet peeve):  One of the other App P vendors can make an absolute killing if they design their ship to be transported to NRHO on a vanilla Starship, then recaptured and returned to EDL on the same Starship.  Not only does this allow heavy cargo to be integrated in a regular payload processing facility, but it also allows the vendor to thoroughly clean and refurbish the HLS, which will almost certainly extend the number of missions it can perform.  They'd obviously need a Plan B in case Starship dropped out of service for a while, but the advantages (and cost savings) are huge.  I doubt Blue would do this, but Dynetics might.  And so might Lockmart.