The problems with lunar ISRU
by Donald Rapp
|Whereas a stronger case can be made for use of ISRU on human missions to Mars, the case for lunar ISRU in the current ESAS architecture does not stand up to scrutiny. Nevertheless, the belief in the virtues of ISRU has been proclaimed so many times by NASA that, in an Orwellian sense, it is widely accepted.|
In the initial NASA ESAS architecture, the propulsion system for ascent from the Moon was based on methane (CH4) and oxygen (O2) propellants in order that ISRU-generated oxygen from the Moon could be used. Although methane had to be brought from Earth, it provided an implicit connection to Mars ISRU. Later, when the realities of cost and schedule to develop CH4 + O2 propellants became clearer, this ascent propulsion system was dropped in favor of space-storable propellants that are incompatible with lunar ISRU. In the original architecture, the plan was to have two ascents per year from the outpost, each requiring about 4 metric tons (MT) of oxygen, for an annual need of roughly 8 MT.
Since the “gear ratio” (mass in LEO/mass on surface) for polar outposts is about 4:1, the potential mass saving in LEO is about 16 MT per launch. However, because the launch vehicles were designed without ISRU, they will remain unaffected by ISRU. Hence the benefit of ISRU will be an ability to deliver extra infrastructure payload (~4 MT) to the outpost with each launch (but rather late in the campaign, probably beginning in the late 2020s). Even this minor benefit disappears if NASA persists in its present plan to use space storable propellants (NTO/MMH) for ascent, thus eliminating oxygen as an ascent propellant. The “value” of the ~4 MT increase in payload delivery per launch using ISRU can be estimated because over a period of years, with continual infrastructure deliveries to the outpost, a cargo delivery launch might be eliminated once every several years with small incremental increases in each launch.
It is likely that an environmental control and life support system (ECLSS) will be used to recycle air and water resources. JSC has estimated the mass of ECLSS systems, finding the total mass of the system (including backup cache) to be 7,200 kilograms.
Even though ISRU might supply the required amounts of oxygen and water, environmental control will still be required. An oxygen-only ISRU system would save very little mass from the ECLSS and is probably not worth integrating. An ISRU system that produces water and oxygen would provide greater benefits but it is likely that the reduction in ECLSS mass would be only a few metric tons. There might be some mass benefits, but they appear to be modest at best. If the ECLSS works as well as NASA hopes, there may not be any benefit to joining the ISRU and ECLSS systems.
Whereas the amount of oxygen required for ascent from the Moon is a rather puny 4 MT, the amount of oxygen required for descent is over 20 MT. If oxygen (and less importantly hydrogen as well) can be delivered to lunar orbit for fueling Moon-bound descent vehicles, the potential payoff from ISRU would be much higher than if ISRU were used only for ascent propellants. If the gear ratio for delivery to lunar orbit from LEO is roughly 2.5, ISRU generation of descent propellants would save over 50 MT in LEO. The combination of ISRU-provided ascent and descent propellants would save about 70 MT in LEO, and this is likely to increase in the forthcoming revised ESAS architecture.
This system works (at least on paper) after it is established, but how does it get established? If NASA must send crew members to the surface to establish the outpost and set up the tanker/refill system, then we are back to square one because NASA must send the LSAM with full descent and ascent tanks prior to the establishment of the outpost and the tanker/refill system. The potential equivalent mass saving in LEO is over 70 MT per launch. However, as in the case of ISRU providing only ascent propellants, this reduction will not be realized in terms of reduced launch vehicle capability if ISRU is adopted as an afterthought late in the campaign.
|Despite the great challenges involved in extracting oxygen from regolith, documents indicate that JSC remains optimistic that they will succeed. It is difficult not to admire the tenacity of these stalwarts, for whom no engineering challenge is too great or too impractical.|
The main near-term benefit of lunar ISRU according to the current lunar campaign definition is replacement of about 8 MT/yr of oxygen for ascent propulsion beginning rather late in the campaign (late 2020s). It is not immediately obvious how this reduction in mass requirements would be utilized by the exploration enterprise. Since sortie missions will manifestly be designed to function without ISRU, the lunar enterprise will have to develop a launch vehicle and LSAM based on no use of ISRU. When, at a later date, outposts are set up, the same launcher and LSAM will be employed. They will not be reduced in size or capacity.
There are basically three potential resources:
The embedded atoms from the solar wind appear to be far too dilute to be a practical source of resources, although some ISRU enthusiasts conjecture processing on the order of 100,000 tons of regolith to recover 1 ton of product. That leaves regolith silicates and polar ice as the two remaining potential feedstocks for ISRU.
Lunar ISRU based on extraction of oxygen from regolith has two advantages:
Unfortunately, the oxygen in regolith is tied up in silicate bonds that are amongst the strongest chemical bonds that are known, and breaking these bonds inevitably requires very stringent conditions, particularly temperatures well above 1000°C, and for some processes over 1500°C. While JSC continues to remain optimistic about such processes for extracting oxygen from regolith, the fact remains that preliminary testing has not produced any encouraging results. More to the point, the hope to dump regolith into a reactor that functions at >1500°C and remove spent regolith would be an engineering nightmare on Earth, and unimaginable on the Moon.
Despite the great challenges involved in extracting oxygen from regolith, documents indicate that JSC remains optimistic that they will succeed. It is difficult not to admire the tenacity of these stalwarts, for whom no engineering challenge is too great or too impractical, and who are willing to work on technologies requiring reactors at incredibly high temperatures that must take in lunar regolith and discharge spent regolith or slag. However, the probability that a practical process for autonomous lunar operation will come from any of this research appears to be very small.
In the extremely unlikely case that a high-temperature processor for oxygen from regolith on the Moon can be made into a practical unit, one would still be faced with the challenges (and costs) for development and demonstration of autonomous ISRU systems for excavation of regolith, delivery of regolith to the high-temperature processor, operation of the high-temperature processor with free flow of regolith through it (with no caking, agglomeration and “gunking up” of regolith), and removal of spent regolith from the high-temperature processor to a waste dump.
The other alternative is to hope for accessible ground ice in permanently shadowed areas near the poles. This approach has the great advantage that removal of water from regolith is a physical (rather than a chemical) process and requires far less energy and much lower temperatures. However, on the negative side, it will take a considerable investment to locate the best deposits of ground ice (if they are indeed accessible); the percentage of water ice in the regolith is likely to be low, necessitating an extensive prospecting program, ultimately requiring processing a great deal of regolith; excavating ice-filled regolith may prove difficult; the logistics of autonomous regolith delivery, water extraction, and regolith removal from a reactor may prove difficult; and the whole process must be carried out in dark, permanently shadowed craters, necessitating use of nuclear power.
Observations from orbit with a neutron spectrometer will provide a horizontal resolution of many tens of kilometers. Locating the best sites within such regions will require a series of prospecting missions. Initially, long-range rovers equipped with neutron spectrometers would be used to locate the best sites. At the best sites, follow-on missions would take subsurface samples to validate neutron spectrometer indications and make measurements of soil strength. This campaign to locate and validate accessible water ice resources is likely to require at least four and possibly as many as six in-situ landed missions with long-distance mobility, at a probable cost of over $1 billion each. If sorties with human crew are used for the final missions in this series, the cost will go up considerably. The NASA Robotic Lunar Exploration Program (RLEP) seems to have grossly underestimated the requirements and cost of prospecting, the need for mobility on such precursor missions, the requirements for taking subsurface samples with preservation of volatiles, and the extent of the overall campaign.
|Unfortunately, NASA has not adequately defined the campaign for prospecting, demonstrating, and implementing lunar ISRU.|
Development and demonstration of autonomous ISRU systems for excavation of regolith, delivery of regolith to a water extraction unit, operation of the water extraction unit with free flow of regolith through it (with no caking, agglomeration, and “gunking up” of regolith), and removal of spent regolith to a waste dump will require quite a few more billion. It is noteworthy that there is no evidence that NASA is planning to provide funds to develop the nuclear power systems needed for operation in the cold darkness of polar craters.
Overall, the required investment to do prospecting and validation of resources, development and demonstration of regolith excavation and transport, and operation of a water extraction system, appears to be many billions of dollars. The benefit/cost ratio remains uncertain but it may take many years to “break even” on the investment.
Any scenario that we develop for any step (whether that be prospecting or demonstration) should be elements of an overall campaign. A scenario for an individual step only has value as part of that campaign to the degree that it contributes to the campaign because the overall campaign produces the end result.
Unfortunately, NASA has not adequately defined the campaign for prospecting, demonstrating, and implementing lunar ISRU, in the present context of oxygen (and possibly hydrogen) production, mainly for ascent propellants. While “lunar-tics” have plans for manufacturing spare parts on the Moon, producing silicon solar cells on the Moon from regolith, and extracting parts per million of solar-wind deposited atoms, fortunately such work is not yet funded even though it is included in JSC project plans.
Both JSC and ESAS appear to have simplistic notions about what it will take to prospect for polar ice resources and demonstrate ISRU systems that will not hold up to any serious scrutiny. In addition, RLEP is very badly under-funded, under-conceived, and grossly inadequate to do the necessary job.