Question about Human Landing System

[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.