Sandworm Design

Humungous Ugly Sandworm

Details of the Sandworm Design

A lunar volatiles harvester will be the key piece of industrial equipment for the first lunar settlement.

One possible lunar volatiles harvester design is described in Sandworms. This article continues with details of that design. A supporting spread sheet is also available at Story Calculations.

You can also find information on the original Mark II design at:

I.N. Sviatoslavsky, "The Challenge of Mining He-3 on the Lunar Surface: How All the Parts Fit Together", University of Wisconsin, November 1993

A general technical specification for Lunar Volatiles Harvesters can be found at Volatiles Harvester Spec.

Systems

Each key system of the miner must be considered in the design and many ideas for each system must be studied and tested. The key systems of the miner are:

Handle the Regolith

Dig it, sieve it, move it into higher pressure, move it hot, move it out of higher pressure, then dump it out the back

Heat to 700 C

Heat it to drive off volatiles. Recycle 85% of this heat.

Collect Volatiles

This is essentially a high vacuum pump.

Recover Iron Fines

Save the fine iron spheres for reuse.

Power System

Provide kilowatts of power and remove the waste heat.

Control System

Control this complex device and communicate with Earth.

Motive

Move the device forward.

Build it

The miner must be easy to construct with lunar material substituted for material from Earth as much as possible.

Maintain it safely

The miner must be easy to repair by human and robot teams.

Sleep at night

It must pass the night and wake up ready for work

Power In

The lunar miner is a large piece of industrial equipment requiring a large amount of power to operate. The majority of this power can be used in the form of heat to drive the volatiles out of the regolith. Additionally, power is needed in the form of electricity to run the electronics, transport the regolith, and drive the minor forward.

Solar energy is the obvious source for the lunar miner's power. It is abundant ½ the time and is well understood. A number of possible ways to harness solar power have been discussed. Clearly a trade study is needed. The underlying physics is uncomplicated enough that such studies can be handled by students through the use of spread sheets (with some programming additions) and with ray tracing.

Sizing Power Need

Schmitt's numbers (pp 119) for the lunar miner are:

Mining power requirements

• lunar process energy (82 Gj/g of solar thermal energy) -- 12.3 MW
• Heat Recovery -- 85%
• Estimate operating electric power -- 200 KW

Mark II solar collector system

• Miner receiver dish (12 meters diameter) -- 112 m2
• Fixed solar reflector (110 meter diameter) -- 9500 m2

Alternative design

• Solar constant -- 1367 W/m2
• Concentration effectiveness -- 75%
• Solar panel efficiency (GaAs) -- 18.5%
• Solar panel output (GaAs) -- 253 W/m2

These numbers will need review as the design progresses. This level of power will require a very large solar collector supplemented with photovoltaic panels. Four physical arrangements for the solar concentrators should be considered in a trade study:

Large Fixed Solar Relay

In this approach a large tracking reflector is placed at fixed location some distance from the lunar miner. The large reflector relays concentrated sun light to a small receiving disk on the miner.

Advantages:

• The receiver disk on miner is of a manageable size.
• The receiver disk has a simplified tracking function.

Disadvantage:

• Inefficiency of the power relay
• Cost of separate installation.
• Cost and difficulty of construction of one large unit

Hillside Multiple Relays

A large number of smaller collectors are built on a hill side near the mining operation. The units are on very short towers and built with very low mass design.

Advantages:

• The receiver disk on miner is of a manageable size.
• The receiver disk has a simplified tracking function.
• Less mass from Earth required to build and operate
• Failure of any single disk does not stop mining

Disadvantage:

• Inefficiency of the power relay
• Cost of separate installation.
• Cost and difficulty of construction of many small units
• Availability of appropriately located hill side

Single Full Disk on Miner

The miner carries one large circular collector dish.

Advantages:

• The power does not have the relay losses.
• The geometry is simple and well understood

Disadvantage:

• The disk is large requiring a large moving base.
• The collector target moves

Single Solar Forge Section on Miner

A single collector consisting of only a section of a turned parabola is mounted on the miner.

Advantages:

• The power does not have the relay losses.
• Tracking the sun is relatively easy.
• The power receiver is in a fixed location.
• The designs supports heat rejection and easy of maintenance.

Disadvantage:

• The miner base is very large.
• The geometry is unusual

Heat Out

One of the most difficult tasks for a high power operation in space is to get rid of waste heat. All power systems operate by heat moving from a high temperature reservoir to a low temperature reservoir. The miner's high temperature reservoir is heated by the solar collector and must be maintained above 700 °C (973 K, 920 °F). The radiator field must keep the low temperature reservoir below 20 °C (293 K, 68 °F, room temperature).

The only reasonable process for the lunar miner is to dump the heat to deep space using thermal radiators. This is made more difficult because the lunar surface heats up during the day to over 200 C. Not only must the radiators not see the sun, they should not see the hot lunar surface.

This design problem is considerably easier for polar locations that for equatorial ones. The ground is not nearly so hot, around 0,0 °C (273 K, 32 °F). Also the nights are not nearly so long and the sun stays near the horizon.

A fluid (liquid or gas) is circulated through serpentine plumbing in metal panels which face the cold of deep space. Mechanical means will be needed to keep a view of the sun and of the hot lunar surface away from the thermal panels. This will include mounting panels on the back of the solar collector so they always face away from the sun and mounting metal louvers.

Even with a system that recovers 85% of the processing heat, the processed regolith fines ejected out the back of the miner will tale a significant amount of heat energy with them.

The equipment bays will also probably need smaller independent radiator panels to maintain the temperature of electronic equipment. These may need shades or louvers.

We also want to avoid projecting heat in front of the miner where it might drive volatiles off the unprocessed regolith.

Sizing Power Rejection Need

Major thermodynamic study is needed. This is a significant omission from the Mark II design. Such studies cost tens of thousands of dollars, but are the only way to reliably size the thermal radiators.

Heat rejection from small space systems where the direction of the sun is fixed is not hard. Small radiator panels driven by heat pipes work very well. The task only becomes difficult for high power systems which is the very case we have here. As general rule the radiators need to be comparable in size to the solar collector. In this case their total area must be inconveniently big.

The lunar miner operating environment makes this problem particularly difficult:

• There is direct sun on most possible radiator surfaces at some time of the day
• The hot lunar surface takes up much of the field of view in most area
• The solar collector restricts view of sky from the body of the sandworm
• The weight of thermal panels would require a much stronger collector structure
• We must avoid heating unprocessed regolith in front of the miner
• The system must survive the night and restart at dawn

Design Studies

Two types of panels could be analyzed to determine relative advantages in this difficult application:

• Fixed Panels - The panels do not move but have shields and shutters that open and close.
• Tracking Panels - Panels are mounted on the back of the collector or on purpose build structures that track deep space. These must be connected through flexible tubes.

Regolith Throughput

Regolith is a very gritty material. Handling volumes of such material on Earth often results in difficult maintenance problems. The material continually sand blasts the inside of your processing equipment. Maintenance is a particularly difficult and expensive problem for space equipment.

Steps in processing lunar regolith for He-3

The front end of the regolith processor must dig material from the trench wall, screen out material larger than 100 um (1.0E-4 meters, course sand), and move it into the processing section. Material larger than this size contains only a very small portion of the volatiles and is directed to the bottom of the trench. After this screening the passed material is referred to as "fines".

The fines, and any volatiles released by the initial handling, must be move into a higher pressure area. This is necessary to prevent the escape of volatiles during the heating process and is a very difficult design problem. A "high pressure" area on the Moon would still be a very good vacuum on Earth.

The fines are then preheated with energy recovered from the exhaust material to create an efficient process. The warm fines are then moved to the main heating area and heated to 700 ° C. This drives off the volatiles. This heating process will involve some combination of contact with hot surfaces, infrared heating, microwave heating, electromagnetic induced heating, and the injection of hot gas.

The volatiles are then captured with a process resembling a high vacuum pump. Separation of the volatiles from fine dust will be a significant problem.

Then fines are move out of the high pressure area without significant lose of gaseous material. Most of the heat is recovered in this section. The warm fines are then moved to a lower pressure section.

The fines contain 1% to 2% ferromagnetic material made up of microscopic spheres of iron in dust particle sized glass spheres. These particles may be useful in processing the regolith because they are subject to being moved by electromagnetic force and heated by microwave energy. They also are a good quality iron ore. If ferromagnetic fines are needed for this process or are considered to be a variable resource, they need to be separated and handled in this last section. This sorting can be done only with the iron fines below the iron Curie temperature of 1053 K.

The remaining fines must then be ejected out the back of the miner in such a way as to fill the trench. Fine dust working back to cover the solar collector, thermal panels and solar panels may be a real problem. Mounting panels on the moving parts helps as the dust can be tipped off.

Sizing Requirement

The Schmitt (pp 119) design requires processing 556 tones (1,225,424 pounds if on Earth) per hour of lunar regolith fines. With the trench 3 meters by 11 meters in cross-section, this requires a forward speed of 23 meters per hour. This would be a significant industrial operation even on Earth.

Available Resources

Acceptable designs may recycle materials from earlier separation for use in the process. The available materials include:

• Gases - Hydrogen, neon, argon, krypton, and xenon
• Volatiles - Water, carbon dioxide, carbon monoxide, and methane
• Solid - Iron fines consisting of microscopic iron particles embedded in glass beads and Ilmenite (FeTiO3).

The problem is to get the regolith fines to act as if they were a fluid. This might be done by mechanical vibration, inducing gas, electrostatics, and / or magnetically agitating the iron fines.

The most difficult design challenge is to set up a confined heating area. Volatiles must be blocked from escaping both out the front and out the back. This requires the maintenance of a "high pressure" area for the heating.

Finding low-maintenance materials for this environment will also be difficult. We are dealing with tones of pure grit.

Five means of working the regolith and establishing the high pressure area should be compared in a trade study:

Cutting Head

One possible design for the cutting head is shown above and is based on Earth machines for similar materials. It features a slowly rotating head in the shape of a large acorn. It has curved slot openings each with a line of heavy carbide teeth. As the head turns the teeth scrap a layer of regolith into the interior.

On the Moon, a good head design would not only loosen the regolith but would screen the material. The material with particles smaller than about 0.1 mm, called fines, will be moved into the processor. Larger material ranging from boulders to course sand must be rejected, not processed, and moved out of the way.

The volatiles are predominately adsorbed on the surface of crystals. Since the fines have a much higher surface area than the larger material, the fines contain most of the volatiles.

Mechanical Valves

This process defines the high pressure area with mechanical valves. The regolith fines are then handled in batches with two or three batch processors being in operation at any time. This process was described in the Mark II design 2, but not chosen.

Strengths:

• Simple
• Well understood
• Supports high pressures

Weaknesses:

• Batch processing requiring switching between paths
• Grit wear on valve seats

Mechanical Screws

This process defines the high pressure area with screw feeds. The regolith fines are then handled continuously with a high power, low speed screw mechanism feeding the material into the high pressure chamber and a second one moving it out. This device was described in detail in the Mark II design.

Strengths:

• Standard industrial device
• Well understood
• Supports high pressures

Weaknesses:

• Grit wear on all moving parts
• Works best for semi-liquids and will have trouble with pure grit.

Funnel seal

Fines Drop

The fines must be acting very much like a fluid. At four points where a pressure step is needed, the fines fall through a funnel designed so that the weight of the fines carries them down while they block volatile flow back.

Strengths:

• Very simple
• Uses lunar resource

Weaknesses:

• Works best when the gas is moving with the grit
• Lunar gravity is weak
• Only low pressure possible

Gear pump to regolith seal

Gear Pump

At the points where a pressure step is needed, the fines encounter a lobbed gear device that compresses the fines while moving them. The compression squeezes out the volatiles. Above the gear is a space that tends to pump volatiles backward.

Strengths:

• Very simple
• A common industrial design, but for liquids

Weaknesses:

• Works best when the gas needs to move against the grit
• Subject to significant wear

Fluid Standing Waves

This is the approach intended for the HUS Model 1.0. It requires that the regolith fines act very like a liquid. This is done by the introduction of previously separated gases and iron fines. The regolith fines are moved by some combination of mechanical vibration, hot gas jets, electrostatics, and electromagnetic force on the iron fines. These actions induce two separate a standing waves. There is one standing wavy for the volatiles and one for the fines. The fines wave had four peaks that block volatile flow. The volatile standing wavy has a peak in the heating area to aide its recovery. This design is a tall order.

Strengths:

• Minimum wear inducing contact with regolith
• Makes good use of lunar resources

Weaknesses:

• Poorly understood
• Low 'high pressure"
• Major research effort required

More Work Needed

Much more work is needed on the Sandworm design. The entire lunar settlement idea hinges on their success. We need to work out these concepts:

• Mirror Segments

One big mirror of flexible material that unfolds will probably not concentrate the light well enough to do the difficult industrial work we need. A more likely arrangement would be to build the mirror in hexagon segments and assemble them. We need to work out the quality of the optics we need to do this job. We need to work out a control system for the mirror surface. The Sandworm needs a lot of work.

• Polar Location

All the designs in this paper were done for an equatorial location. It now looks like we will be in polar locations for the foreseeable future. We need to adjust this design for the poles.

• In the Cold and the Dark

All the designs in this paper are for fully lighted areas. If there is really a lot of volatiles in the permanently shaded regions near the poles, then we need to figure out a completely different way to harvest it. Definitive information should be available by 2010. ____