SPORE Pressure Experiment 1

Version 1.6 February 12, 2020

The first thing I need to determine is how much oxygen will be lost through the exposed Lunar regolith and mega-regolith in the SPORE unit. We need to be able to keep it to some acceptable minimum otherwise SPORE just won’t work.

There are some major challenges though:

The Lunar regolith is poorly defined. Only 382 kilograms of Lunar material, from 9 different sites, have been returned to the Earth for study (1, pg 5). And not all of that material has been exhaustively studied. Additionally, only 24 core samples, taken from a maximum depth of 292 cm, have been returned (1, pg 9). Individual samples may be well defined but the overall regolith is not.
The Lunar regolith is highly variable in density, porosity, particle distribution, and every other parameter. It’s a complex mix of mineral fragments, breccias, crystalline rock fragments, agglutinates, and glass (1, pg 475). There is no “typical” sample of Lunar regolith. The regolith has been shattered, scattered, and fused repeatedly by impacts for eons. The result is a highly heterogeneous mixture that greatly varies between sample sites and between samples within the same site.
The Lunar mega-regolith is largely undefined. None of the Apollo core samples were long enough to reach the mega-regolith so we have no physical samples from this region. We have some ideas on what it might be like but little real data.
Any simulant I made or bought would, at best, only simulate a couple of parameters from a particular sample. There is no way to adequately simulate the vacuum or gravity or all of the physical properties of in-situ Lunar regolith. So these experiments will be rather limited.

The solution to these challenges is to focus on the design and implementation of the SPORE system, not on making it work under an artificially constrained set of parameters (such as a particular regolith sample). A clever design is the key to making SPORE practical.

The data from this, and other, experiments might have limited value for building actual SPORE prototypes. However, the data could be useful for creating a “Proof of Concept” paper (or two). I won’t know until I try.

As a bonus, data from this experiment could also be useful if we tried to dome and pressurize a Lunar crater. I’m not sure WHY anyone would want to do that when subterranean settlements offer more advantages and fewer disadvantages. But we should keep our options open.

Ideally, SPORE will be usable on Mars, asteroids, other moons, and nearly anywhere else we want to start a Homestead. Sure, each environment will be different so we’ll need to adapt SPORE to fit those particular conditions. But the overall concept should work nearly everywhere. Sounds like more experiments are needed. 🙂

Experiment questions

How much pressurized air will pass through a given volume of simulated regolith/mega-regolith mix over a specific period of time?
Does the regolith reach gas equilibrium or saturation?

Experiment design

Simulated regolith

For this experiment I’m not going to use “official” Lunar regolith simulants. They’re expensive, hard to find, and not really close to Lunar regolith anyway. Instead, I’m going to use 3 particle sizes of common terrestrial basalt.

For SPORE Pressure Experiment 1, the following material will be used to “simulate” Lunar mare basalt:

Butler Arts Natural Basalt Decorative Landscaping Gravel from The Home Depot (www.homedepot.com/p/Butler-Arts-0-50-cu-ft-40-lbs-3-4-in-Natural-Basalt-Decorative-Landscaping-Gravel-BST-3-4-40/309543202).
Sure, it’s not the best basalt sample out there but it’s affordable and available locally. And for this experiment those factors win. Plus, this isn’t a chemistry experiment so the chemical composition doesn’t really matter (although Earth and Lunar basalt are almost chemically identical). And Earth basalt is close enough to Lunar basalt for this experiment.
Certain parameters associated with Lunar regolith and mega-regolith are ignored for this experiment:
- Vacuum conditions.
- The angularity of Lunar rocks.
- The presence of agglutinates and glass.
- The presence of micro-fine dust particles.

Mega-regolith simulant

We know next to nothing about the Lunar mega-regolith. And what little we do know comes from remote sensing and modeling.

There is no simulant for the mega-regolith in this experiment. The experiment is too small for the larger rocks thought form the majority of the mega-regolith. Besides, I’m not concerned with pressure loss THROUGH the larger solid rocks that are thought to compose the mega-regolith. This experiment is looking at how the pressurized oxygen would move through the smaller particles between the large rocks.

Air pressure, composition, temperature, and humidity

Since this is the first experiment, I’m going to use pressurized air instead of pressurized oxygen. This will reduce the cost and complexity of the experiment. It will also be less hazardous.

Conducting this experiment in a vacuum (to simulate the Lunar environment) would be extremely expensive and difficult. Instead, I’m going to simulate the pressure difference between the Lunar surface (assuming that the voids in the regolith are also a vacuum) and the inside of the SPORE area by increasing the internal pressure by the same amount. I might use a vacuum pump for experiment 2.

I’ll run tests with the following range of pressures:

Minimum test pressure = 127 kPa
- 101.325 kPa (one Earth atmosphere) + 25 kPa (NASA minimum) +0.675 kPa (to round up) = 127 kPa
- The NASA minimum partial pressure for 100% oxygen is 24.82 kPa (4)
  - Keeping the SPORE atmosphere within the breathable range increases safety.
- The Armstrong limit is 6.3 kPa. Building in an extra 19.375 kPa will allow for unexpected pressure drops.
½ Standard Lunar Homestead Atmosphere (SLHA) = 137 kPa
- 101.325 kPa (one Earth atmosphere) + 35.055 kPa (1/2 SLHA) + 0.62 kPa (to round up) = 137 kPa
1 Standard Lunar Homestead Atmosphere (SLHA) test pressure = 172 kPa
- 101.325 kPa (one Earth atmosphere) + 70.11 kPa (1 SLHA) + 0.565 kPa (to round up) = 172 kPa
1 Standard Atmosphere test pressure = 203 kPa
- 101.325 kPa (one Earth atmosphere) + 101.325 kPa (one Earth atmosphere) + 0.35 kPa (to round up) = 203 kPa (total for experiment)
2 Standard Atmosphere test pressure (overpressure) = 304 kPa
- 101.325 kPa (one Earth atmosphere) + 202.65 kPa (two Earth atmospheres) + 0.025 kPa (to round up) = 304 kPa (total for experiment)

For this first experiment, I’m not going to control for temperature or humidity. I’ll document these values though.

Experiment container

The maximum dimensions of the experiment are 210 cm long by 70 cm wide by 230 cm high. That’s so it can fit through the door to the balcony where it will be stored (part of the fun of doing research while living in a small apartment). The entire experiment will be enclosed in plywood to protect it from the elements (outside storage).

The pressurized container holding the regolith simulant will have the following dimensions:

Height = 25 cm
Width = 25 cm
Length = 220 cm (total)
- 15 cm for the simulated SPORE section
  - This section needs enough volume so I’ll have time to correctly set the oxygen flow rate from the pressure tank.
  - I also want enough volume so I can extrapolate the data for a full scale SPORE unit.
  - This section should be small enough that the target pressure can be quickly reached at the start of the experiment.
- 200 cm for the simulated Lunar regolith or mega-regolith section
  - Experiments will be run using 50 cm, 100 cm, and 200 cm of simulant. That way I can see if doubling the thickness makes a measurable difference. The sample chamber however will be a continuous 200 cm.
- 5 cm for the simulated vacuum section
  - This section is open to the atmosphere and is not under pressure.
This is a total of 125,000 cm³ or 0.125 m³ of material for each sample.

I haven’t determined how the simulant will be secured or what material the container will be made of yet.

Instrumentation

Required:

One pressure gauge inside the SPORE unit.
One pressure gauge at the pressure tank
Two thermometers
Two hygrometers (measures relative humidity)

Experiment structure

Concept 1

Concept 1A

Simulated regolith parameters

I’m only focusing on mare parameters for this experiment. Experiment 2 might be for highland parameters.
I’m going to shoot for the following parameters. However, I won’t be able to make the sample to these parameters before the experiment. So, I’ll run the experiment, test the sample after the experiment, and see how close it came to the desired parameters. I’ll then adjust the next run (most likely by either increasing or decreasing the amount of compaction).
Simulate the lowest density with the highest porosity for the mare
- Bulk density = 1450 kg/m³
- Porosity = 54%
Simulate the highest density with the lowest porosity for the mare
- Bulk density = 2000 kg/m³ (soil breccia)
- Porosity = 35% (soil breccia)
Simulate the average density with the average porosity for the mare
- Bulk density = 1725 kg/m³
- Porosity = 44.5%

Simulated mega-regolith parameters (see Lunar Mega-regolith page for sources and more information)

I’m only focusing on mare parameters for this experiment. Experiment 2 might be for highland parameters.
I’m going to shoot for the following parameters. However, I won’t be able to make the sample to these parameters before the experiment. So, I’ll run the experiment, test the sample after the experiment, and see how close it came to the desired parameters. I’ll then adjust the next run (most likely by either increasing or decreasing the amount of compaction).
Simulate the lowest density with the highest porosity for the mare
- Bulk density = 3010 kg/m³
- Porosity = 10%
Simulate the highest density with the lowest porosity for the mare
- Bulk density = 3270 kg/m³
- Porosity = 2%
Simulate the average density with the average porosity for the mare
- Bulk density = 3140 kg/m³
- Porosity = 7%
There is no MR4 sample (simulating maximum compaction for the mare mega-regolith) because the material for the R and MR samples are the same. Maximum compaction would be the same as well.
The mega-regolith consists of particles (rocks, ejecta, breccia, etc.) that are too large for this experiment.
The upper layers of the mega-regolith should contain a large proportion of regolith mixed with larger particles. This experiment can test the pressure loss through the regolith component of the mega-regolith.
This actually makes sense because the pressure loss THROUGH the larger particles themselves should be minimal. It’s the loss through the stuff between the rocks that could be significant.
The deeper Homesteaders go into the mega-regolith, the less regolith they should find. This may help with the SPORE pressure loss. They may also encounter increased fractures and voids which could make the pressure loss worse. We just won’t know until we’re actually doing it.

Experiment protocols

Experimental samples

Three particle sizes
- Gravel – approximately 3/4″ (19,050 microns) (straight from the bag) (the bag says 3/4″ but they range from 1/2″ to 1″)
- Mid – #10 sieve (2000 microns)
- Fine – #230 sieve (63 microns)
Three levels of compaction
- No compaction
- Mild compaction (TBD)
- Maximum compaction
Number of runs per sample
- 5 runs at 50 cm
- 5 runs at 100 cm
- 5 runs at 200 cm
Samples
1. 100% Gravel/No Compaction
2. 100% Gravel/Mild Compaction
3. 100% Gravel/Max Compaction
4. 100% Mid/No Compaction
5. 100% Mid/Mild Compaction
6. 100% Mid/Max Compaction
7. 100% Fine/No Compaction
8. 100% Fine/Mild Compaction
9. 100% Fine/Max Compaction
10. 33% Gravel/33% Mid/34% Fine/No Compaction
11. 33% Gravel/33% Mid/34% Fine/Mild Compaction
12. 33% Gravel/33% Mid/34% Fine/Max Compaction
13. 50% Mid/50% Fine/No Compaction
14. 50% Mid/50% Fine/Mild Compaction
15. 50% Mid/50% Fine/Max Compaction
16. 50% Gravel/25% Mid/25% Fine/No Compaction
17. 50% Gravel/25% Mid/25% Fine/Mild Compaction
18. 50% Gravel/25% Mid/25% Fine/Max Compaction
19. 25% Gravel/50% Mid/25% Fine/No Compaction
20. 25% Gravel/50% Mid/25% Fine/Mild Compaction
21. 25% Gravel/50% Mid/25% Fine/Max Compaction
22. 25% Gravel/25% Mid/50% Fine/No Compaction
23. 25% Gravel/25% Mid/50% Fine/Mild Compaction
24. 25% Gravel/25% Mid/50% Fine/Max Compaction
25. 70% Gravel/15% Mid/15% Fine/No Compaction
26. 70% Gravel/15% Mid/15% Fine/Mild Compaction
27. 70% Gravel/15% Mid/15% Fine/Max Compaction
28. 15% Gravel/70% Mid/15% Fine/No Compaction
29. 15% Gravel/70% Mid/15% Fine/Mild Compaction
30. 15% Gravel/70% Mid/15% Fine/Max Compaction
31. 15% Gravel/15% Mid/70% Fine/No Compaction
32. 15% Gravel/15% Mid/70% Fine/Mild Compaction
33. 15% Gravel/15% Mid/70% Fine/Max Compaction

Sample preparation

Sample preparation tools
- Still being worked on
- Geology screen sieves
  - #5 (4000 micron)
  - #230 (63 micron)
Sample analysis tools
Steps

Experiment steps

Experiment

The experiment will be set up and unpressurized.
The clock will start and the SPORE section will be brought up to the target pressure as quickly as possible.
Additional pressurized air will be added to the SPORE section to maintain the target pressure.
The experiment will continue until a known amount of air (TBD) has been released into the SPORE section.
The clock will stop.
Calculate the volume of air lost (fixed) over time (variable).
Extrapolate that value for a hypothetical full-scale SPORE unit.
Remove samples to calculate density and porosity.

Sample analysis

Remove small samples from large experiment sample
1. Density
2. Porosity

Resources

Lunar Sourcebook (www.lpi.usra.edu/publications/books/lunar_sourcebook)
Center for Lunar and Asteroid Surface Science – Planetary Simulant Database (sciences.ucf.edu/class/planetary-simulant-database/)
Permeability of JSC-1A: A lunar soil simulant (www.sciencedirect.com/science/article/pii/S0019103510004835)
Man-Systems Integration Standards – Natural and Induced Environments (msis.jsc.nasa.gov/sections/section05.htm#_5.1_ATMOSPHERE)
Lunar Regolith Simulant Materials: Recommendations for Standardization, Production, and Usage (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060051776.pdf)