Is Hydrolox superior to methalox for large lunar-based craft?

[Skye]

Most of the reason SpaceX chose methane for SS is because it can be made on Mars. So, if you were to make a large rocket intended for landing & returning on & from the moon, why not make it Hydrolox, as it’s easy to make on the Lunar surface?

[Paul451]

Cryo-hydrogen is not exactly going to be easier to handle on the moon.

[spacenut]

Nor is liquid hydrogen easier on Mars.  Boil off is only one problem.  Extreme cold can cause metal to become brittle for long term storage.  Otherwise Mars has water, so making hydrolox for a rocket might be easier than making metholox.  Methane is easier to store and close to the same temperature as liquid oxygen. 

[Robotbeat]

Liquid hydrogen is the lowest density liquid known to mankind, so it’ll punish your mass fraction by quite a bit. Even hydrolox has about half the bulk density compared to methalox. That means your engines produce half as much thrust for a given power and dry mass. And a bunch of other stuff. It’s actually harder to make a SSTO using hydrolox than for methalox.

[Robotbeat]

Oh, and methalox is effectively space storable passively (with the right thermal shielding) whereas hydrogen will boiloff unless you use active cooling. And even if you do decide to do active cooling, it is almost like 100x easier to do active cooling of methane than for hydrogen for a given mass.

[TrevorMonty]

Oh, and methalox is effectively space storable passively (with the right thermal shielding) whereas hydrogen will boiloff unless you use active cooling. And even if you do decide to do active cooling, it is almost like 100x easier to do active cooling of methane than for hydrogen for a given mass.

Compared to HSF and RLV, active cooling of hydrogen isn't that complex. We've been doing it on earth for decades and is already flight demostrated ie James Webb cryocooler. NB JW version is overly complex because low vibration requirements.

https://science.nasa.gov/mission/webb/cryocooler/

[TrevorMonty]

Liquid hydrogen is the lowest density liquid known to mankind, so it’ll punish your mass fraction by quite a bit. Even hydrolox has about half the bulk density compared to methalox. That means your engines produce half as much thrust for a given power and dry mass. And a bunch of other stuff. It’s actually harder to make a SSTO using hydrolox than for methalox.

OTVs an lunar landers don't see same Gs or aerodynamic loads as launch vehicle. Apollo landers 0.8G, Ascent Vehicle  <2G. Because of this tanks can be very light eg Centuar which are SS balloon tanks and flight proven.

NASA has built and tested large composite LH tanks on earth as part of SLS program.

[Twark_Main]

Liquid hydrogen is the lowest density liquid known to mankind, so it’ll punish your mass fraction by quite a bit. Even hydrolox has about half the bulk density compared to methalox. That means your engines produce half as much thrust for a given power and dry mass. And a bunch of other stuff. It’s actually harder to make a SSTO using hydrolox than for methalox.

OTVs an lunar landers don't see same Gs or aerodynamic loads as launch vehicle. Apollo landers 0.8G, Ascent Vehicle  <2G. Because of this tanks can be very light eg Centuar which are SS balloon tanks and flight proven.

NASA has built and tested large composite LH tanks on earth as part of SLS program.

I assume the idea here is that the tanks would launch [partially] empty, or (the expensive option) be built on the surface of the Moon or in space?

[AlphaCoronae]

I think you might end up with methalox being used earliest when there's no ISRU capability on the surface, hydrolox being used in earlier phases of lunar development since lunar water is relatively easy to access, and methalox taking over again further down the line as lunar carbon reserves are accessed. By that point of development rotavators might have mostly replaced landers though, since the low orbital velocity and lack of atmosphere makes them so convenient on the Moon.

Liquid hydrogen is the lowest density liquid known to mankind, so it’ll punish your mass fraction by quite a bit. Even hydrolox has about half the bulk density compared to methalox. That means your engines produce half as much thrust for a given power and dry mass. And a bunch of other stuff. It’s actually harder to make a SSTO using hydrolox than for methalox.

OTVs an lunar landers don't see same Gs or aerodynamic loads as launch vehicle. Apollo landers 0.8G, Ascent Vehicle  <2G. Because of this tanks can be very light eg Centuar which are SS balloon tanks and flight proven.

NASA has built and tested large composite LH tanks on earth as part of SLS program.

I assume the idea here is that the tanks would launch [partially] empty, or (the expensive option) be built on the surface of the Moon or in space?

The fairing can structurally support and shield it during launch, as with Atlas V-Centaur.

[Robotbeat]

Liquid hydrogen is the lowest density liquid known to mankind, so it’ll punish your mass fraction by quite a bit. Even hydrolox has about half the bulk density compared to methalox. That means your engines produce half as much thrust for a given power and dry mass. And a bunch of other stuff. It’s actually harder to make a SSTO using hydrolox than for methalox.

OTVs an lunar landers don't see same Gs or aerodynamic loads as launch vehicle. Apollo landers 0.8G, Ascent Vehicle  <2G. Because of this tanks can be very light eg Centuar which are SS balloon tanks and flight proven.

NASA has built and tested large composite LH tanks on earth as part of SLS program.

Yeah, the centaur tanks are impressive, but they’re still over twice what they would be for the same propellant mass of methalox.

Which doesn’t mean hydrolox loses the trade, but methalox will put up quite a fight because of its far lower dry mass.

And note that lower dry mass also means lower fabrication costs. Also, using methalox means less extreme temperatures, which means fewer materials problems with thermal cycling and easier materials testing compared to hydrogen.

[Robotbeat]

In fact, even for the same starting MASS, a methalox stage can push small payloads to higher delta-v than hydrolox can.

Let’s pick rl-10c-x isp of 461s vs Helios’ 390s isp. Bulk density of hydrolox of 358kg/m3 vs 801 methalox. Due to scaling following the pressure vessel relationship, dry mass is proportional to volume.

If the centaur has a 10:1 wet:dry mass ratio, so dry is 1t and wet is 10t, prop is 9t, and each m^3 of propellant needs about 39.8kg of dry mass. the methalox is 10t wet and… 474kg dry mass. 9526kg prop. So… the methalox stage will be able to push a 500kg payload to 9.1km/s, whereas the hydrolox stage of the same wet mass can only push it to 8.8km/s. For the same starting wet mass! And the methalox stage will be less than a half the dry mass (less than half the total volume and size) and thus probably about a half the cost to build.

[Robotbeat]

In fact, for a wet mass 10 ton stage, any payload 950kg or less will get higher delta-v with methalox than hydrolox.

Contrary to almost everyone’s assumptions, hydrolox (and nuclear thermal, for that matter) really needs more staging than methalox or kerolox does. Easier to do SSTO with methalox or kerolox than with hydrolox or nuclear thermal (hydrogen).

[Vultur]

Yeah, hydrogen is probably too much of a pain to mess with. It's one thing for upper stages that are fueled on Earth and used quickly, but for in space operations...

[Robotbeat]

The point is actually for pure performance reasons, it’s worse if your stage needs very high delta-v. The whole messiness argument is just icing on the cake.

[TrevorMonty]

Thread title was never about using hydrolox for earth LVs. For lunar landers and transporters that are using lunar fuel then they make lot sense. DVs are typically <4km/s between refuelling depots. LEO to EML1 being longest haul.
For landers hydrolox is way to go. Methalox between LEO and EML1 would require producing Methalox at EML1 from lunar water and earth carbon.

[Joris]

The balance between fuel types shifts depending on what state your ISRU is in. The major difference with the two is that there is ofcourse no carbon on the moon. Then again, oxygen is the major mass component and hydrogen on the moon isn't as common as oxygen from regolith, althoug processes for getting water out of ice-regolith mixtures might be easier.

I think you'll end up in a situation where methalox makes sense purely becuase you don't need the dV, and even with lunar oxygen importing methane from earth will be what happens. Only after a very large scale infrastructure where water is made in large quantities does hydrolox possibly make sense and its drawbacks can be overcome.

This is qutie far into the future and we're talking about a permanent presence on the moon, both because of the ISRU infrastructure and the requirement for regular cislunar launches. Perhaps Aluminium-Oyxgen hybrid engines will be able to be made there, or someone makes engines run on silane, that might be efficient fuels for lunar ops, but it's all far into the future.

[redneck]

The balance between fuel types shifts depending on what state your ISRU is in. The major difference with the two is that there is ofcourse no carbon on the moon. Then again, oxygen is the major mass component and hydrogen on the moon isn't as common as oxygen from regolith, althoug processes for getting water out of ice-regolith mixtures might be easier.

I think you'll end up in a situation where methalox makes sense purely becuase you don't need the dV, and even with lunar oxygen importing methane from earth will be what happens. Only after a very large scale infrastructure where water is made in large quantities does hydrolox possibly make sense and its drawbacks can be overcome.

This is qutie far into the future and we're talking about a permanent presence on the moon, both because of the ISRU infrastructure and the requirement for regular cislunar launches. Perhaps Aluminium-Oyxgen hybrid engines will be able to be made there, or someone makes engines run on silane, that might be efficient fuels for lunar ops, but it's all far into the future.

Mine impactors for carbon and there is a lot of carbon on the moon.

[Skye]

The balance between fuel types shifts depending on what state your ISRU is in. The major difference with the two is that there is of course no carbon on the moon.

That’s exactly why I started this thread. Would it be worth, for a lunar-optimised craft, using ISRU HLox because carbon is much harder to get? 

“Carbon is present in the lunar regolith (soil) in trace amounts, estimated at 82 parts per million (ppm). This carbon is primarily introduced by the solar wind and micrometeorite impacts.”

[InterestedEngineer]

The balance between fuel types shifts depending on what state your ISRU is in. The major difference with the two is that there is ofcourse no carbon on the moon. Then again, oxygen is the major mass component and hydrogen on the moon isn't as common as oxygen from regolith, althoug processes for getting water out of ice-regolith mixtures might be easier.

I think you'll end up in a situation where methalox makes sense purely becuase you don't need the dV, and even with lunar oxygen importing methane from earth will be what happens. Only after a very large scale infrastructure where water is made in large quantities does hydrolox possibly make sense and its drawbacks can be overcome.

This is qutie far into the future and we're talking about a permanent presence on the moon, both because of the ISRU infrastructure and the requirement for regular cislunar launches. Perhaps Aluminium-Oyxgen hybrid engines will be able to be made there, or someone makes engines run on silane, that might be efficient fuels for lunar ops, but it's all far into the future.

Mine impactors for carbon and there is a lot of carbon on the moon.

Kind of like earlly iron workers mined impacts of high iron impactors on earth.

Can one find such with remote detection from a lunar orbiter?

[Paul451]

Mine impactors for carbon and there is a lot of carbon on the moon.

If they have the infrastructure required to mine buried carbonate impactors on the moon, it implies they've already solved the reusable-lander problem you are trying to solve by mining buried carbonate impactors.

It's the Catch-22 with a lot of higher-end ISRU proposals. They typically aren't viable until they aren't necessary.

Can one find such with remote detection from a lunar orbiter?

Possibly for metallics, but I doubt you could detect buried carbonate impactors. (Other than optically: "Crater, crater, crater...")