Author Topic: Which is more practical, lunar hydrolox ISRU or methalox upper stages?  (Read 4900 times)

Offline Pipcard

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New Glenn is using a hydrolox upper stage, and New Armstrong is speculated to do the same thing due to lunar polar ice. SpaceX is using methalox on their Starship instead of hydrolox for the sake of commonality (less manufacturing and operational costs), avoiding hydrogen embrittlement and boil-off when reusing the ship, and because it can be produced on Mars using the Sabatier reaction.

But would a methalox upper stage/spaceship still be better than hydrolox even if lunar industries and O'Neill colonies are the main goal instead of Mars? Or are the statements below reflective of a short-term Mars-focused architecture that is sometimes used for the Moon, and hydrolox does become worth it for Blue Origin's long term goals?

Liquid hydrogen requires more expensive cooling equipment and shading for long term storage or space use.  Methane being close to the same temperature as LOX, is easier for long term storage.  We still really don't know how much hydrogen is actually on the moon also.  LOX is 80% of the weight, so the moon could be used for LOX production while methane could be brought from earth. 

Hydrogen is *also* hard to come by. I think people *vastly* underestimate how hard it's going to be to mine significant amounts of lunar water and turn it into propellant.

Just refuel Starship in orbit. It's going to be cheaper to do that for decades.

Methlox is mostly advantageous on Mars since you can make it from readily available CO2 via the Sabatier reaction.

For the Moon hydrogen is equally valid for ISRU maybe even more ideal since the feed stock would likely be water ice.
Hydrogen still has huge boil-off  and energy density problems, no matter where you find the water, so 'equally valid' is wrong in that sense. 

Hydrogen from the Moon is more likely to be first used for 'domestic consumption' as air and water than it is for fuel on orbit... Deliveries of water to Lunar orbit from Earth will be vastly less expensive for the foreseeable future*.  Methalox propellant will also be delivered from Earth for the foreseeable future. All talk of Lunar ISRU will remain just talk for a very, very long time.  Building the Blue Lander or space tugs is near-future, say next ten years.  Why would anyone do this the hard way because some day there will be Lunar ISRU propellant?

*Today, it might cost $10M/t to deliver water to EML-1/2/HLO from Earth, and it costs 'infinity dollars' to deliver it from the Lunar surface.  With BFR, this cost could drop by a factor of 10-100x, while from the Lunar surface it would still be 'infinity dollars'.  At some point in the future, after a few decades of Lunar development which will cost tens of billions, you might get the price from the Lunar surface to today's price... from Earth.
« Last Edit: 07/05/2020 12:59 am by Pipcard »

Online TrevorMonty

Method maybe better for reuseable 2nd stage that only goes to LEO. BLEO hydrolox is superior.

Offline GreenShrike

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But would a methalox upper stage/spaceship still be better than hydrolox even if lunar industries and O'Neill colonies are the main goal instead of Mars?

Starship's propellant is 78% oxygen. You could get most of the advantage of ISRU by stretching the methane tanks somewhat (or by underfilling your oxygen tanks, I suppose), such that you could land with basically an empty LOX tank but a partially full methane tank. You could then refill the LOX tank with oxygen extracted from Lunar regolith, which is apparently 40-45% oxygen by mass.

Helpfully, this even has the advantage of avoiding the need for a polar base with access to Lunar ice, or otherwise shipping Lunar water across the Moon's surface.

BLEO hydrolox is superior.

If your chief criteria is maximizing ISP to minimize propellant mass, then sure.

If prop on-orbit is cheap, then maybe different criteria rise to the fore.
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Offline Pipcard

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Helpfully, this even has the advantage of avoiding the need for a polar base with access to Lunar ice, or otherwise shipping Lunar water across the Moon's surface.
Would Blue Origin need that, though? That is the real question.

If not, then Blue Moon, New Glenn, and New Armstrong should all be fully methalox, right?
« Last Edit: 07/04/2020 03:59 am by Pipcard »

Online ThatOldJanxSpirit

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Method maybe better for reuseable 2nd stage that only goes to LEO. BLEO hydrolox is superior.

For a mass optimised system sure, but its not so simple when comparing with a cost optimised system. Starship and NG will do most of their initial work serving LEO and GTO where cost optimisation is key. For BLEO Starship aims to use cheap refuelling to make up for the performance deficit of methalox.

Personal view, practical ISRU on the moon and Mars is sufficiently far in the future that it shouldnít be a design consideration for this generation of launch vehicles.

Offline freddo411

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Method maybe better for reuseable 2nd stage that only goes to LEO. BLEO hydrolox is superior.

For a mass optimised system sure, but its not so simple when comparing with a cost optimised system. Starship and NG will do most of their initial work serving LEO and GTO where cost optimisation is key. For BLEO Starship aims to use cheap refuelling to make up for the performance deficit of methalox.

Personal view, practical ISRU on the moon and Mars is sufficiently far in the future that it shouldnít be a design consideration for this generation of launch vehicles.

Thank you for being very clear and stating the values to optimize.

Online TrevorMonty

Using large RLVs to supply earth fuel maybe good start but its not viable long term if we want to be space faring species. Need to be able to live off land and not be reliant on earth for basics, especially fuel.

Blue is taking long view hence need to develop hydrolox propulsion technology.


Online ThatOldJanxSpirit

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Using large RLVs to supply earth fuel maybe good start but its not viable long term if we want to be space faring species. Need to be able to live off land and not be reliant on earth for basics, especially fuel.

Blue is taking long view hence need to develop hydrolox propulsion technology.

That Ďlong Viewí has significantly increased the complexity of NG with two stages of low commonality. That increases costs and delays fielding an orbital capability. Delays that in this case could lose Blue NSSL.

With my skeptical hat on, I suspect that the switch to hydrolox for the upper stage had more to do with chasing Vulcan for the high energy segment of the market and possibly issues with BE-4, rather than future proofing for a hypothetical long term ISRU economy. Practically I canít see lunar ISRU being a real proposition for 20 years at least. Plenty of time to develop next gen hydrolox vehicles.

Offline GWH

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With my skeptical hat on, I suspect that the switch to hydrolox for the upper stage had more to do with chasing Vulcan for the high energy segment of the market and possibly issues with BE-4, rather than future proofing for a hypothetical long term ISRU economy. Practically I canít see lunar ISRU being a real proposition for 20 years at least. Plenty of time to develop next gen hydrolox vehicles.

Not much of a skeptical view, seems to coincide quite well with timelines. The public switch to hydrolox occurred exactly when they announced intention to compete for NSSL.

A good question is would a high thrust methalox 2nd stage optimized for LEO performance and paired with a hydrolox 3rd stage would offer better performance in the long run for lunar ISRU?

Offline GreenShrike

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Helpfully, this even has the advantage of avoiding the need for a polar base with access to Lunar ice, or otherwise shipping Lunar water across the Moon's surface.
Would Blue Origin need that, though? That is the real question.

If not, then Blue Moon, New Glenn, and New Armstrong should all be fully methalox, right?

If costs are a concern, then methalox is practical for the Moon. If the conops of Lunar Starship results in the lowest costs per tonne on the Moon -- even in the face of hydrolox-equipped competition -- how could it not be?

Essentially, your question is missing a time component, and it's time that's the most important piece of context.


Long-term, hydrolox may gain an edge with Lunar ISRU. Long-term, it may be cheaper to fill a Moon-based hydrolox rocket than refill a Lunar Starship with prop from Earth.

However, the solutions of tomorrow may very well be sub-optimal today. Hydrolox is expensive to develop and unsuited for boosters, so going that route means, at a minimum, two engine development programs -- thus jeopardizing your chances of surviving today to see tomorrow. Whatever the cost of fielding Starship, it is inarguable that saddling it with multiple engine types is going to drive up the cost of both developing and flying it. A single engine type eases development and lowers cost, and a single prop combo eases operations and lowers cost, and essentially duplicating the tankage for both booster and upper stages eases construction and also lowers cost.


And *low cost* makes everything easier. That Lunar ISRU industry we so eagerly look forward to is only happening at rock bottom prices -- and preferably accompanied by a clipped coupon or two. ;-)


On the other hand, with Jeff Bezos as a benefactor, Blue can bear the development costs and operational complexity of a mixed fuel vehicle, and can develop hydrolox in anticipation of Lunar ISRU. Blue already has hydrolox engines, so I very much expect New Armstrong to use them -- and, also, that a Blue vehicle will be the first to run Lunar prop through its engines.

On the gripping hand, Raptor's prop choice, and its use on both Starship stages may be an act of current convenience, or fiscal prudence, or due to Martian ISRU. But in any case, today's choices do not preclude a change of direction in the future so, while I would expect carrying excess fuel and just retanking LOX from Lunar ISRU to be SpaceX's conops long before SpaceX chooses to dabble in hydrogen, should future circumstances warrant it, at that point SpaceX is quite free to develop (or buy) a hydrolox engine.

And once the time pressure is off and New Glenn is flying, Blue is free to implement the BE-4U and methalox upper stage originally envisaged, if it ever makes sense.


Ultimately, practicality is whatever is lowest cost. People, however, may disagree whether going for the lowest cost today, or pursuing tomorrow's anticipated lowest costs, is better.
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Offline Lemurion

I donít think the original question can really be answered yet. Both lunar hydrolox ISRU and methalox upper stages have advantages and disadvantages and itís quite likely that each will find a niche in future. Hydrolox offers lower mass and better ISP, but itís also much harder to store on-orbit. In the end itís going to come down to a bunch of factors, many of which we only have a rough idea of at this point.

For one thing, we have no real knowledge  of the cost or difficulty of developing hydrolox ISRU on the Moon. If it proves simple and straight forward to develop, it becomes more likely. If it proves difficult with many setbacks it becomes less likely. Hydrogen storage is hard enough on Earth; we donít know what itís going to really take to set up a tank farm on the Moon.

All we know right now is that people are actively researching lunar ISRU and SpaceX is building a methalox upper stage but donít appear to have finalized the design.

Offline Rocket Science

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Offline Nilof

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It's worth mentioning that lunar methalox ISRU is perfectly viable too. Hydrolox benefits a bit more from LOX-only ISRU due to its higher O/F ratio. Lunar ices are expected to contain CO2 and CO ices, so both hydrolox and methalox should be viable for lunar ice ISRU.

Similar story for captured asteroid ISRU. Carbonaceous chondrites are carbon rich, so if you can extract water from them you can also extract enough carbon for methalox. So Hydrogen's advantage for ISRU isn't actually *that* huge, and cost and commonality/compatibility with other vehicles is likely to be a bigger factor.

Both are definitely viable.
« Last Edit: 09/07/2020 10:04 pm by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline rakaydos

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It's worth mentioning that lunar methalox ISRU is perfectly viable too. Hydrolox benefits a bit more from LOX-only ISRU due to its higher O/F ratio. Lunar ices are expected to contain CO2 and CO ices, so both hydrolox and methalox should be viable for lunar ice ISRU.

Similar story for captured asteroid ISRU. Carbonaceous chondrites are carbon rich, so if you can extract water from them you can also extract enough carbon for methalox. So Hydrogen's advantage for ISRU isn't actually *that* huge, and cost and commonality/compatibility with other vehicles is likely to be a bigger factor.

Both are definitely viable.
Wouldnt lox only ISRU for hydrolox run into tank size issues on the hydrogen tank? The hydrogen tank always massively oversizes the LOX tank, I cant imagine doubling the hydrogen tank (for both down and up, when LOX just has to handle down) would just make the problem worse.

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