NASA Human Landing System

[tater]

5 minutes ago, sevenperforce said:

Each manned landing adds a new component to the base.

Yeah, this seems like the way to go. Moving the whole habs seems difficult, but I suppose it's not that bad at 1/6g. You'd likely want everything buried, anyway, however, for radiation protection.

The trouble of course is that under the current NASA "plan" these crew missions are part of one lunar day at the site, once a year.

[StrandedonEarth]

The hab area itself shouldn't need a sintered pad to be assembled on. A sintered LZ with a blast-berm around it should be sufficient. The berm could have a movable wall section to cover the  roadway out of the LZ; the wall section would also be useful as an experiment to quantify the effect of exhaust-driven ejecta on the surroundings.  Alternately, simply move habs "around the corner" so they are not in line with the gap in the berm. Use a vehicle similar to the Octagrabber. to move the hab sections (made from descent modules) out of the LZ

Edited February 18, 2019 by StrandedonEarth
syntax error

[sevenperforce]

Just now, StrandedonEarth said:

The hab area itself shouldn't need a sintered pad to be assembled on. A sintered LZ with a blast-berm around it should be sufficient. The berm could have a movable wall section to cover the  roadway out of the LZ; the wall section would also be useful as an experiment to quantify the effect of exhaust-driven ejecta on the surroundings.  Alternately, simply move habs "around the corner" so they are not in line with the gap in the berm. Use a vehicle similar to the Octagrabber. to move the hab sections (made from descent modules) out of the LZ

A treaded octagrabber is actually a ridiculously good idea for this application.

[tater]

7 minutes ago, sevenperforce said:

A treaded octagrabber is actually a ridiculously good idea for this application.

I wonder if the grabber plus a hab would also function as a crusher. Ie: grab hab, and in the process of moving it, you raster around the landing area compressing the regolith (the idea being that you are prepping the surface for later sintering).

[kerbiloid]

I'm afraid, they anyway first need a trench under the landing area made of printed plates. Filled with sorted regolith and maybe containing some hollows as gas trenches.
So, a dozer.

Spoiler

32 minutes ago, tater said:

I wonder if the grabber plus a hab would also function as a crusher

Yes, on every failure. They should keep their eyes up to avoid crushing.

 

 

Edited February 18, 2019 by kerbiloid

[sevenperforce]

21 minutes ago, tater said:

I wonder if the grabber plus a hab would also function as a crusher. Ie: grab hab, and in the process of moving it, you raster around the landing area compressing the regolith (the idea being that you are prepping the surface for later sintering).

With treads, probably yes.

[StrandedonEarth]

25 minutes ago, tater said:

I wonder if the grabber plus a hab would also function as a crusher. Ie: grab hab, and in the process of moving it, you raster around the landing area compressing the regolith (the idea being that you are prepping the surface for later sintering).

Hmmm, that complicates things. It could work as a crusher, depending on how much weight it carries. The crawlerways at LC-39A&B were made of gravel which the Crawler-Transporter crushed to powder under the STS and Saturns. This would be counter to the purpose of LZ prep as it could tear up the sintered surface (tank treads are murder on roads).

That said, if it's not heavy to crush on it's own, it could always carry a crusher unit to crush the 'lith when it would be otherwise idle. Maybe have some mirrors around the berm to focus sunlight for repair sintering

[sevenperforce]

Mockup of what this architecture might look like after several sequential missions but before any octagrabber-related base-connection activity:

Note that each descent module uses a hab module as its core with tanks and engines on the sides, a docking port in the front to mate with a pressed rover, and an airlock in the back. Contingency abort vehicle visible over on the right. 

[Cunjo Carl]

... So hear me out. I have a plan that may sound a little Kerbal, but I swear it's good science. We could actually use the fuel tanks from a spent lander as a very convenient high-energy power source. As a blue-skies guess, 20kW of electricity for a month! And it would let NASA tick off the ISRU box as bonus by using the moon for its Oxygen content.

So there's a famous reaction: Fe₂O₃ + 2 Al → 2 Fe + Al₂O₃ , releasing 10GJ of heat per 1kg Aluminum. It's most famous for making fun chemistry demos...

    
Before: Fe₂O₃ + 2 Al. pic by unununium272 on wiki                            After: 2 Fe + Al₂O₃ + 550kJ of heat. pic by CaesiumFluoride on wiki

Thermite! There's enough energy packed in there for all sorts of applications, typically in welding or making large steel castings, but the idea here is to slow the reaction way down, so we can harness the heat to make electric power rather than just melt things. The general concept is surprisingly old, some of the original rocket pioneers had plans to grind up spent fuel tanks and use them as reaction mass in the next stage. Use all parts of the buffalo!

We need a source of finely divided Aluminum, and the lander's spent tanks would be just ideal! As a ballpark guess, the tanks of a Methalox moon lander they're proposing may have 100kg of Aluminum? (Though If someone has a better number, please let me know!) Anyways, for the Thermite reaction (and the one I suggested later), the Aluminum needs to be a very fine powder, but mechanically grinding a tank on the moon would be impossible. Fortunately, if you scratch an Aluminum surface and rub a very small amount of Gallium metal onto it, the Aluminum will fall apart to powder overnight. The Gallium actually soaks inbetween the Aluminum's grains, prying them apart. It can be a little tricky doing this on Earth because the Aluminum scratches heal so quickly with Alumina passivization layers, which provide excellent protection to Ga attack, but in the hard vacuum of the moon it would be a simple task.

Now we need to find our oxidizing agent, Fe₂O₃. According to the internet there's quite a bit of Iron oxides on the moon, so I've checked the kinetics for FeTiO₃, Fe₂O₃ and Fe₃O₄, and they all look ok to use provided the particle size is ~1um. This said, thermodynamically we should be able to use any of the common oxides on the Moon (excluding Na,Mg,Al) so silicaceous soils (SiO₂) would work just as well, though perhaps at a lower rate than the iron oxide bearing soils. Checking out their kinetics would be a project for another day, though.

  
Aluminum Powder. pic by DaveHax on Youtube                                        Lunar Regolith Simulant (fake moon dirt), it's brown from iron! pic by ArnoldReinhold on wiki

We need our thermite to react slowly though, otherwise the heat will be wasted. Conveniently, Hydrogen gas + water vapor can act as a catalyst/moderator for their reaction, so we never need to putting the reactants in direct contact. They can just sit next to eachother and exchange vapors.

2 Al + 3 H₂O  → Al₂O₃ + 3 H₂
Fe₂O₃ + 3 H₂ → 2 Fe + 3 H₂O

As an example of the full device, for 1 day's worth of power, perhaps 3kg of Al and 8kg of moon dirt are put into separate compartments of an evacuated stainless steel box, connected by a pair of tubes. Maybe 20g of water vapor is very slowly injected in, and will react very quickly with the Al powder to create H₂ and our initial heating. (Someone in a nearby lab didn't believe how quickly, or quite how much heat, and nearly took out a wall with the resulting bang! An appendix was added to the local fire code just for them, so in a way they got their work published after all) . The resulting Hydrogen gas can be circulated (or allowed to diffuse) to the lunar soil compartment through the tubes, where it can react with the silicon/iron oxides to become H2O once again. At equilibrium, the amount of water vapor in this system will be extremely small relative to the Hydrogen, so it will mostly be Hydrogen gas (at perhaps .1bar) that circulates the heat. To run the reaction faster, we could raise the heat, circulate faster, or increase the Hydrogen pressure. For a slower reaction, vice-versa.

The reaction can be used to drive the box's temperature up to 300-600C to serve as a heat source, and some Aluminum heat sink rods can be dug into the (~0C?) soil to serve as cold sources. Between them we would put thermoelectric generators, which convert large temperature differences directly into electric power in any of a few different ways (just like on an RTG). The one I'm the most enamored with is the Sterling Radioisotope Generator's Sterling engine. But, I'll assume we'll use something more mundane for the example!

Operating at maybe 5% efficiency, which sounds reasonable in context, from each 1kg of Al we'd get 75KWh. Scaled to the mass of Aluminum in the lander's tanks (again guessing 100kg), this could total to 20kW over a month!  Life support should 100% be run from solar panels of course, but I think this could open up a lot of other avenues of activity on the Moon. It'd be nice to bulldoze some berms and caverns, right? Here's a 20kW power supply that can run a real bulldozer! Also, the waste heat from the reaction could be used to heat an almost unlimited amount of inflatable auxiliary habitats/green-houses.

The resulting 'waste' iron particles could be used to make ISRU metal castings by reversibly liquefying them in carbon monoxide gas at 100-200C, at least according to Zubrin. What you would want to cast out of moon metal is beyond me though.... space swords? My son would sure like that.

 

The main problem with all this is the main problem with literally everything long term on the moon, the gas seals. Charged dust particles love nothing more than to wedge themselves into small spaces, like our gaskets, and wear them out in a hurry. Case and point would be the fate of the samples Apollo returned from the moon! If we could solve this, It'd certainly open up a lot of new avenues. If we ran the box at 300C we could at least use organic o-rings, which I think would be a real benefit.

In conclusion, thermite the moon!
 

Edited February 19, 2019 by Cunjo Carl

[sevenperforce]

13 hours ago, Cunjo Carl said:

... So hear me out. I have a plan that may sound a little Kerbal, but I swear it's good science. We could actually use the fuel tanks from a spent lander as a very convenient high-energy power source. As a blue-skies guess, 20kW of electricity for a month! And it would let NASA tick off the ISRU box as bonus by using the moon for its Oxygen content.

Thermite! There's enough energy packed in there for all sorts of applications, typically in welding or making large steel castings, but the idea here is to slow the reaction way down, so we can harness the heat to make electric power rather than just melt things. The general concept is surprisingly old, some of the original rocket pioneers had plans to grind up spent fuel tanks and use them as reaction mass in the next stage. Use all parts of the buffalo!

We need a source of finely divided Aluminum, and the lander's spent tanks would be just ideal! As a ballpark guess, the tanks of a Methalox moon lander they're proposing may have 100kg of Aluminum? (Though If someone has a better number, please let me know!) Anyways, for the Thermite reaction (and the one I suggested later), the Aluminum needs to be a very fine powder, but mechanically grinding a tank on the moon would be impossible. Fortunately, if you scratch an Aluminum surface and rub a very small amount of Gallium metal onto it, the Aluminum will fall apart to powder overnight. The Gallium actually soaks inbetween the Aluminum's grains, prying them apart. It can be a little tricky doing this on Earth because the Aluminum scratches heal so quickly with Alumina passivization layers, which provide excellent protection to Ga attack, but in the hard vacuum of the moon it would be a simple task.

Now we need to find our oxidizing agent, Fe₂O₃. According to the internet there's quite a bit of Iron oxides on the moon, so I've checked the kinetics for FeTiO₃, Fe₂O₃ and Fe₃O₄, and they all look ok to use provided the particle size is ~1um. This said, thermodynamically we should be able to use any of the common oxides on the Moon (excluding Na,Mg,Al) so silicaceous soils (SiO₂) would work just as well, though perhaps at a lower rate than the iron oxide bearing soils. Checking out their kinetics would be a project for another day, though.

<snip>

 

In conclusion, thermite the moon!

Very Kerbal indeed, but I love it.

The required amount of Ga to pull it off could be prohibitive. Why do you say that mechanically grinding a tank on the moon would be impossible? A robot with a laser cutter could pretty easily slice an aluminum tank into narrow ribbons which could then be fed into an paper-shredder-sized grinder. 

Enrichment of the iron oxide or other soils could be difficult. Regular proton bombardment has resulted in highly reduced soil, with most iron on the moon either being metallic or +2 oxidized instead of +3 oxidized as is standard on Earth. Regular iron oxide is not ferromagnetic, either, so a mechanism for enriching the soil (if its oxygen weight was insufficient to support combustion) would be challenging. That, I think, would be the limiting variable.

[kerbiloid]

Maybe they can use a ball mill to pulverize aluminium.
https://www.quora.com/What-is-the-best-way-to-grind-aluminium-to-a-powder-

 

[DDE]

8 hours ago, sevenperforce said:

A robot with a laser cutter could pretty easily slice an aluminum tank into narrow ribbons which could then be fed into an paper-shredder-sized grinder. 

There’s a significant hazard of the grinding operation consuming a lot of energy.

If not more than is being produced.

[sevenperforce]

1 hour ago, DDE said:

There’s a significant hazard of the grinding operation consuming a lot of energy.

If not more than is being produced.

Seems doubtful to a first order.

[Cunjo Carl]

On 2/19/2019 at 5:10 AM, sevenperforce said:

Very Kerbal indeed, but I love it.

The required amount of Ga to pull it off could be prohibitive. Why do you say that mechanically grinding a tank on the moon would be impossible? A robot with a laser cutter could pretty easily slice an aluminum tank into narrow ribbons which could then be fed into an paper-shredder-sized grinder. 

Enrichment of the iron oxide or other soils could be difficult. Regular proton bombardment has resulted in highly reduced soil, with most iron on the moon either being metallic or +2 oxidized instead of +3 oxidized as is standard on Earth. Regular iron oxide is not ferromagnetic, either, so a mechanism for enriching the soil (if its oxygen weight was insufficient to support combustion) would be challenging. That, I think, would be the limiting variable.

Agreed about the Gallium. From messing around with it, I think around 1% by weight to the Aluminum would be tenable, but not having a lander handy I might be off by a factor! Still, that represents a few kg, which is certainly non-zero. I was assuming mechanical grinding wouldn't work because it would require an abundance of tools from a cutter, to a shredder and finally a grinder. Every 2 orders of magnitude or so in size would require a different tool, which would get cumbersome.

Also agreed about the enrichment. All this would be dependent on the process working on soil just as it is when dug out of the ground. Thermodynamically it should work on whatever soil's handy (as long as it has Oxygen and not too much Al,Na,Mg in it) and I was able to check the kinetics for iron oxides (in 2+, 2 2/3+, and 3+), but I wasn't able to find good kinetics for Silicon at a glance. Being the most important, it'd be nice to find, I'm sure it's out there. Anyways I feel it would probably work, but I think the only way to know for sure would be to grab some lunar regolith simulant and give it a try.

I had no idea about the soil reduction, if anything I'd assumed the soil would be in a slightly superoxidized state thanks to the solar wind. That's fascinating. I checked a couple papers, and it looks like most (~98% by weight) of the lunar iron is in the form of FeO (the 2+), and conveniently enough this would make for an even more energy-releasing reaction than the higher oxidation states. It's a little backwards, but there you go!

By ratio, the amount of metallic iron is in the lunar soil is apparently in the 0.1 - 1% range across the areas we've landed on already. The iron particles are also all so small (~1-10nm) as to be single-crystal, so they probably have an enormous magnetic-moment to mass ratio. If we dragged a rare-earth magnet across the moon's surface, would we pick up usable amounts of iron? That would be hugely convenient, to put it mildly.

Edit: Apparently the iron nanoparticles are largely caught up within larger (~20um) glassy particles of moon dust, so no free iron. That does mean that moon dust can be moved around with magnets to an extent though, and apparently that they couple extremely well to microwaves, which makes for an easy way to sinter things like @tater was suggesting.

Edited February 20, 2019 by Cunjo Carl

[wumpus]

On 2/17/2019 at 12:30 PM, tater said:

Mars is not a thing, and will not ever be a thing unless commercial vehicles obviate SLS entirely.

It would require catastrophic failure on both SpaceX and Blue Origin (complete inability to launch both Starship/BFR and New Glen (and presumably New Armstrong as well)) to avoid obviating SLS entirely.  SpaceX makes visible progress, Blue Origin is presumably doing something with those billions (at least test fires seem to be happening), but SLS is simply happily consuming the pork.  SLS isn't going anywhere (except the right pockets) [in terms of progressing.  Demonstration/test/make work launches might happen, but the real question is whether or not it will be obsolete before it launches (it might take a few years after launch).

This isn't to say that NASA may yet insist on going to the Moon with SLS.  It just will be even more obvious that it is a make work project for SLS (and presumably the Lunar tollbooth as well).

[kerbiloid]

Spoiler

Then SLS is a bright accretion disk around some invisible matter.

 

[sevenperforce]

A human-sized lunar lander needs to start with an appropriately-sized engine...so, what's out there? I thought I would do a quick roundup of engines that could conceivably be used or adapted for use in cislunar space....

• BE-3U: 530 kN
• RL-10: 110 kN
• SuperDraco: 68-90 kN
• Rutherford Vac: 24 kN*
• AJ-10: 27 kN

Not a big list, and not much to work with. Rutherford is included because although kerolox is problematic for ignition reasons, the cycle is fantastic for cislunar ops.

How many did I miss?

[Ultimate Steve]

40 minutes ago, sevenperforce said:

A human-sized lunar lander needs to start with an appropriately-sized engine...so, what's out there? I thought I would do a quick roundup of engines that could conceivably be used or adapted for use in cislunar space....

• BE-3U: 530 kN
• RL-10: 110 kN
• SuperDraco: 68-90 kN
• Rutherford Vac: 24 kN*
• AJ-10: 27 kN

Not a big list, and not much to work with. Rutherford is included because although kerolox is problematic for ignition reasons, the cycle is fantastic for cislunar ops.

How many did I miss?

The Lunar Module Descent Engine had a thrust of 45kN maximum, for comparison purposes. So, without hoverslam/suicide burn/extreme throttling/etc. BE-3U is way too big for something LEM sized. I don't see hydrogen being used on a lunar lander, but it could work... I see all of the others working maybe in clusters of 2.

Also AFAIK Rutherford vac is ablatively cooled, which is fine if you don't want to reuse the lander.

[tater]

There is the Masten Katana (18kN). The XEUS concept uses 4 of those.

 

 

The LEM descent stage was 45 kN. This will be a bigger lander, but that gives some idea regarding thrust. Deep throttling is also required.

It sounds like for the stupid Gateway requirement, and the reality that no such lander will ever be sent by SLS, that the 3 pieces all need to be sent via commercial launchers. A tug stage for Gateway<->LLO, then a descent stage attached at Gateway to the ascent stage.

Seems to me, since engines are expensive (particularly RL-10s, lol), the way to go might be to put one engine on the ascent stage and use drop tanks. Legs attach to the drop tanks, so they get left on the surface as the launch pad for the ascent stage, just like Apollo.

I do this with the SSTU lander parts, actually, all the time:

Hard to see, but the decent engine is also the ascent engine, the tanks on the bottom have a large hole clear through.

 

 

Edited February 22, 2019 by tater

[kerbiloid]

Happily we have MonkeyBusiness for that

Spoiler

 

[sevenperforce]

Spent entirely too long digging into the nine companies tapped by NASA for CLPS to expand the roundup of currently-available engines. Here's what I came up with.

• BE-3U (Blue Origin), 530 kN. Hydrolox.
• Broadsword (Masten Space Systems), 160 kN. Methalox.
• Reaver 1 (Firefly Aerospace), 182.2 kN. Kerolox.
• RL-10C (Aerojet Rocketdyne), 110 kN. Hydrolox.
• SuperDraco (SpaceX), 90 kN. Hypergolic.
• Lightning 1 (Firefly Aerospace), 70.1 kN. Kerolox.
• AJ-10 (Aerojet Rocketdyne): 27 kN. Hypergolic.
• HD5 (Intuitive Machines), 24 kN. Methalox.
• Rutherford (Rocket Lab), 24 kN. Kerolox.
• Katana (Masten Space Systems), 18 kN. IPA+LOX.
• PECO (Moon Express), 5.64 kN. RP1+HTP.
• Machete (Masten Space Systems), 4.4 kN. IPA+HTP.
• FME (Team Indus for ORBITBeyond), 0.44 kN. Hypergolic.
• Deep Space Engine (Frontier Aerospace for Astrobotic), 0.44 kN. Hypergolic.
• Draco (SpaceX), 0.4 kN. Hypergolic.

A slightly broader spread, but still not a lot to choose from.

It is a shame that SpaceX's gas-gas methane-oxygen thruster has been put on the back burner. Imagine a SuperDraco with a regenerative cooling loop that pre-vaporizes both LOX and CH4, feeds a small amount back to keep the tanks pressed, and then feeds the rest into the chamber with a simple spark igniter. Gas-gas engines are simple, reliable, and while it wouldn't have the best TWR in history, it would have superb efficiency and relatively decent propellant bulk density.

 

NASA's plan currently requires that all vehicle components be limited to what can be sent to TLI by commercial launch providers AND fit inside a five-meter fairing. It's really rather restrictive. 

Thinking back to Constellation...I would love to see a single-stage transfer, refuel, and descent/surface module which is assembled in LEO via two commercial launches prior to TLI. As it is, the NASA approach requires no less than three commercial launches, plus crew via SLS, for every moon landing, and that's without any supplies being landed on the moon. At least then you would only have a single disposable powerplant braking into cislunar space for each mission instead of literally three.

[tater]

2 hours ago, sevenperforce said:

NASA's plan currently requires that all vehicle components be limited to what can be sent to TLI by commercial launch providers AND fit inside a five-meter fairing. It's really rather restrictive. 

That's pretty much the only available launchers, though. SLS can only fly once a year (someday), and a cargo version is so far in the distance it might as well be Starship.

[sevenperforce]

2 hours ago, sevenperforce said:

NASA's plan currently requires that all vehicle components be limited to what can be sent to TLI by commercial launch providers AND fit inside a five-meter fairing. It's really rather restrictive. 

Thinking back to Constellation...I would love to see a single-stage transfer, refuel, and descent/surface module which is assembled in LEO via two commercial launches prior to TLI. As it is, the NASA approach requires no less than three commercial launches, plus crew via SLS, for every moon landing, and that's without any supplies being landed on the moon. At least then you would only have a single disposable powerplant braking into cislunar space for each mission instead of literally three.

Logistically speaking...

There is value in having hypergolics on the ascent stage, so let's assume an ascent stage powered by either a single SuperDraco or clusters of smaller hypergols. It will need to dock to the lander stage with capacity for propellant transfer. The smaller the better, for pretty much every reason. This also means the lander will need to carry hypergols for refueling the ascent vehicle, though not necessarily for itself.

For your mass ratio, you want to shed spent mass whenever possible, and for complexity/reliability you want to drop stages as infrequently as possible. The ideal scenario is to have your ascent module refuel tanks bolted to the same jettisonable module as your lander's lunar orbit insertion tanks (since it's braking itself into cislunar space) so you only have one separation event. For dV reasons, let's assume cryos for the lander.

Scenario:

1. Crew and unfueled ascent module are already at LOP-G.
2. Commercial launch 1 places lander module in LEO with just enough propellant for lunar deorbit and landing.
3. Commercial launch 2 takes refueling module into LEO but does not separate.
4. Lander executes rendezvous and docking with refueling module, then Commercial launch 2 restarts its engine for TLI burn.
5. Lander completes TLI burn (if necessary), coasts, and then brakes into cislunar orbit at LOP-G, drawing from cryo tanks in the refueling module.
6. Crew enter ascent module and dock to lander.
7. Ascent module is refueled from refueling module.
8. Refueling module is jettisoned either at the LOP-G or once in LLO.
9. Descent stage lands and the surface mission is conducted.
10. Ascent module returns to LOP-G to repeat the cycle.

This expends only one BLEO engine assembly and achieves a full lunar landing in only two commercial launches rather than three. Because the lander is sent to LEO, it can be dramatically larger than otherwise possible. With Atlas V 551 it could go up to 19 tonnes; with Delta IV Heavy, 28 tonnes; with Falcon Heavy reusable it could go up to 30 tonnes or more. New Glenn, Vulcan, or Delta IV Heavy would be ideal for launching the refueling module because the residual hydrolox will give better TLI performance.

[tater]

The nice thing about the XEUS concept is that it uses the RL-10 where possible, and the Masten engines are simply for the terminal landing burn. So most of the work gets done at very high I_sp.

It;s also important to look at subsequent flights. The Gateway LLO tug needs refilling, as does the ascent stage. Descent is a throw away.

Also, and this is true of all the extant LVs with a 5m fairing, the mass to LEO is mostly propellants left over in S2. If the entire fairing of FH was in fact remade as a tank (with a docking port on top covered by a disposable cone), there would still be props left in S2 one it got to LEO. As a result, none of those LVs are going to deliver the stated mass to LEO of useful cargo.

[sevenperforce]

17 minutes ago, tater said:

The nice thing about the XEUS concept is that it uses the RL-10 where possible, and the Masten engines are simply for the terminal landing burn. So most of the work gets done at very high I_sp.

How far along are the XEUS plans for handling boiloff? That's always a challenge, but particularly with hydrolox. And a dual-thrust-axis lander makes the addition of a crew capsule rather challenging, particularly if the crew capsule is sized for independent abort.

17 minutes ago, tater said:

It;s also important to look at subsequent flights. The Gateway LLO tug needs refilling, as does the ascent stage. Descent is a throw away.

That's why I suggested putting a refuel module up on a separate LEO launch in order to rendezvous with the lander. Its propellants would be used both to refuel the ascent stage and to act as a drop tank for the descent stage, such that the descent stage can fill the role of tug. Plus, it will be light, meaning that there will be plenty of residuals in the launch vehicle upper stage to push both it and the lander onto TLI.

17 minutes ago, tater said:

Also, and this is true of all the extant LVs with a 5m fairing, the mass to LEO is mostly propellants left over in S2. If the entire fairing of FH was in fact remade as a tank (with a docking port on top covered by a disposable cone), there would still be props left in S2 one it got to LEO. As a result, none of those LVs are going to deliver the stated mass to LEO of useful cargo.

Does anyone know the upper limit of discrete payloads for LEO by most commercial vehicles? Because if we know that number then we can start sizing accordingly.