Introduction

The number of possible fusion fuels is limited: deuterium, tritium, helium3, perhaps lithium or boron. The most abundant of these is deuterium, found in heavy water which can be readily separated from normal water. Tritium is a manufactured, radioactive element with a fairly short life, while Helium 3 is not commonly found on Earth. Extraterrestrial source of He3 are available, however. Lithium and Boron are both accessible on Earth. In this section we will explore the ways these elements could be obtained to create the fuel required for fusion powered interstellar spacecraft Deuterium can be extracted on Earth with conventional ground based infrastructure. It might be advantageously gathered from Mars or Venus, where it is much more concentrated, with an interplanetary infrastructure. It might even be sourced from the gas giants, where it can be separated fairly simply from the hydrogen in the atmosphere, without all the energy required to remove the oxygen atom from a heavy water source. He3, in the quantities required for an interstellar fusion drive, needs to come from the gas giants and requires a robust space infrastructure as well. Tritium could be ‘bred’ on Earth or manufactured en route, although the short life of the element is a serious problem. Lithium and boron are available from minerals and salts.

In this chapter we will be presenting 2 scenarios:

• Energy demand is low, demand barely doubles in the next century and deuterium deuterium is the reaction of choice for fusion propulsion.

• Energy demand increases significantly, tritium fusion is online, but is being superseded by He3 fusion as mining becomes interesting economically.

We will also be looking at tritium breeding, required for some of the fusion processes.

The D2 scenario

R. Freeland, M. Lamontagne

In this scenario, set at about 2100, deuterium is available, not because it is required for energy production, but because it is relatively cheap to produce. Fusion drives using deuterium-deuterium exclusively are possible. Energy demand has doubled from the 2010 levels, following the median prediction of UN studies().

Fusion power requirements

To keep the spacecraft's mass within reasonable bounds, the fusion drive has to be very powerful, transforming about 100 grams of fuel into 100 tonnes of thrust every second. The power required for this rivals the average power use of the entire Earth population, which was 18,000 GW in 2010. Is this reasonable, in any sense of the word? Considering that the drive is supposed to operate for up to ten years continuously at full power output. Using present terrestrial energy costs, this would yield an energy cost, in 2016 dollars, of $8 trillion per year, or over $120 trillion for the whole mission.

These numbers are obviously impractical. It might be supposed that the cost of energy would go down in the future as new power sources become available. But a reduction of about 3 orders of magnitude, bringing the cost down into billions rather than trillions of dollars is far beyond even the most optimistic scenarios(). In such a case, the average per capita cost of energy, that is about $650 per year today, would fall to about fifty cents by the turn of the next century. Can fusion power ever be that cheap? No, But, there is a catch. And it lies in the difference between the cost of producing energy, and the cost of using energy.

“Free” Energy

The cost of energy resides mainly in the required infrastructure. Energy from the sun is free, but the solar cell infrastructure, the power lines and the transformer substations required to distribute and use that energy are expensive. The same will be true for fusion power.

The starships we describe, although large, will mass barely more than a small frigate, or a 10 story building. The dry mass is between 1500 and 5000 tons for our various designs. This is a fraction of the weight of even a single nuclear power plant today, or of a large wind farm. Even if the weight is composed of exotic materials and hyper sophisticated electronics, it’s just not that much material. In a way, a fusion starship is barely more than a big nuclear power core, with practically no infrastructure. So the energy it uses will be, relative to its tremendous quantity, practically free. But it will also be completely unusable for any other purpose than the propulsion of the ship, devoid of any of the infrastructure required to turn it into a valuable product. So it will be both free, and, in a sense, worthless as well.

This raises a new question: does it make sense for the society of 2100 to use up deuterium, or any other possible fusion fuel in a starship, where it produces no monetary value, rather than using it in fusion reactors on Earth, where it would produce usable energy and revenue? Can this future society afford to ‘waste’ all that fuel? What will be the cost for deuterium in 2100 and will there be any available for space exploration?

Energy Demand

To answer this, we can use the median figures from UN projections. We can see that the population has risen by 30%, and the energy demand has more than doubled from the numbers of 2010. So energy demand certainly rises, and producing all that energy will require new infrastructures, but the demand is still in the same order of magnitude as today’s requirements, and there are no changes that make the cost go down significantly, since most of the infrastructure costs remain. We are far from a world where energy costs are a trivial amount of a person’s budget.

Energy Sources

Most of today’s energy comes from the combustion of oil, coal and natural gas. It seems unrealistic, and possibly catastrophic from the environmental point of view, to expect to meet the rising global energy demand using primarily fossil fuels. It's likely that natural gas will remain a practical energy source up to and perhaps beyond the end of the century; however, it would be good, and probably unavoidable, to phase out coal and oil. Large reductions in energy demand will come with the replacement of fossil fuel in transportation, due to increased efficiency, to offset the rise of the energy demands from developing countries.

Does this leave an opening for fusion? Is fusion an automatic winner in the competition between new energy sources? The answer to these questions will come from the costs of fusion power plants, which are still very much unknown. However, after 40 years of very slow progress, it seems likely that fusion power plants will be large, heavy, expensive, and complex. Terrestrial fusion power is unlikely to come online for several more decades. If ever.

Cheap solar power and good batteries, when they eventually come along, might effectively kill fusion forever as far as consumers are concerned. The development of really cheap solar -- of the “paint it on the wall” type -- seems just as likely, or perhaps even a little more likely, than large fusion power plants. The quantity of available solar energy on Earth is staggering: Over 2000 times the energy requirements projected in the table above for 2100. The quantities available in space are millions of times larger.

Thorium nuclear reactors are also an alternative to fossil fuel and fusion. Potentially less expensive, safer, and easier to build than classical nuclear reactors, they are basically waiting for an increase in the cost of energy to become viable. These may offer a perfect complement to distributed solar power, offering safe base load power for the power grid and industry.

Deuterium Production on Earth

What is the cost of the deuterium fuel for a starship? Compressed deuterium gas is available at about $75 per kilogram (3). So the 20,000 tons of deuterium for Firefly could cost as little as $1 billion. This is misleading, however. Small quantities of deuterium can be extracted fairly cheaply from commercial hydrogen using centrifugal or membrane separation()(). However, large scale production of deuterium from hydrogen, in the range of 2000 tonnes of deuterium per year, would require treating about 20% of the present yearly world production of hydrogen of approximately 50 million tonnes(). Most of which is used for the production of fertilizer by combination with nitrogen to create ammonia using steam reformation of natural gas and the Harber process.

It should be possible to use nuclear reactors to create the steam for steam reformation of natural gas to hydrogen. This would displace a large part of the fossil fuel combustion processes presently used for this purpose, reducing worldwide CO2 production considerably. This would also provide an opportunity to install deuterium separation units on the hydrogen stream. In this arrangement about 50% of the deuterium would come from water, and 50% from natural gas.

Alternatively, it might be possible to create a facility dedicated exclusively to the production of deuterium from heavy water. The concentration of deuterium in the sea is 1 atom of deuterium for every 6400 atoms of hydrogen. 20 000 tonnes of deuterium would require 100 000 tonnes of heavy water, extracted from over 2 billion tonnes of water. If we spread this over a period of 10 years, then our deuterium production facility would need to process about 7 m³ of water per second, or 100 000 gallons per minute.

To compare, this is about the flow of water used today for the cooling of a single mid-sized power plant, be it coal, gas or nuclear. However, the existing heavy water concentration method, the Girdler Sulfide process, require large amounts of heat and chemicals. The heat output of 10 average nuclear reactors would be required to produce the deuterium over 10 years. The production costs might be partly offset by selling most of the electricity. The Gilder Sulfide process might be replaced by nanomembrane separation techniques under development(). This would reduce the environmental impact considerably. If we suppose the new membranes technologies have at least the efficiency of the existing sulfide process, then the cost of the deuterium might be in the range of 40 billion dollars (2001 dollars)() or 2000$/kg. This includes the energy costs for electrolysis of the heavy water, a required step following the separation process.

So the entire production of the deuterium supply for an interstellar probe probe might be the 10-year work of a single installation, encompassing about 10 GWe of reactors and a large separation plant, located somewhere along the seashore. It would make sense to locate the plant close to the spaceport used to deliver the fuel up to space. This power plant would be just one of the 19,000 plants, or part of 100 million or more small power systems, supplying electricity, and perhaps hydrogen, to a world population of 10 billion people, sometime around the year 2100. From that perspective, the fuel costs seems rather insignificant.

The Deuterium from space alternative

Deuterium is concentrated naturally in atmospheres that have lost their water content through interaction with solar radiation. As the heavier deuterium molecule is less sensitive to ultraviolet radiation from the sun than regular water, less deuterium is lost to space than hydrogen. Over time, this has lead to a concentration of deuterium. Earth water has nearly six times the average deuterium to hydrogen ratio found in the universe, while water on Mars is six times Earth’ concentration and Venus’ atmosphere 100 times. Unfortunately, the atmosphere of Venus is also exceptionally dry, and the work required to gather up a sufficient amount of water would offset the fact that the deuterium content is higher. So there is no real gain.

For Mars, if the information gathered in recent years by exploration is correct, there will be large amounts of water available. At six times the concentration, separation will be much more efficient and we might expect to obtain the same amount of deuterium for about one sixth of the cost as on Earth. This gives us about 25 billion dollars to develop the required infrastructure and still save money compared to Earth. This would need to merge with a Mars colonization program to have clients for the excess electrical energy produced by the separation process, or possibly using the hydrogen to create ammonia for fertilizer or to serve as part of large scale energy storage systems. So deuterium from Mars may be a possible source for a 100 years horizon, if Mars colonization takes off. As far as deuterium from the gas giants goes, the next scenario covers that opportunity as well as He3 extraction.

The He3 scenario

This section is based on the IAC 2010 presentation by A. M. Hein, A. C. Tziolas, A. J. Crowl (1) However the scenarios have been changed by Michel Lamontagne and all mistakes are therefore his own.

Introduction

In this scenario, He3 -deuterium fusion is the reaction of choice. A growing interplanetary society is interested in developing fusion propulsion for interplanetary travel, and wants to reach towards the stars within the next few generations.

He3 has a very low abundance on Earth but is available within the atmosphere of outer solar system gas planets. Jupiter was selected as the source for this precious gas(1) in the original Daedalus study. However, due to the enormous resources necessary to establish such a mining architecture and the abundant amount of energy required for that civilization, the Daedalus study group came to the conclusion that this civilization had to be a solar-system wide one.(2)

In order to launch the Daedalus or Icarus type probes, an equivalent to one to three times the current annual world-wide energy production is required(3). It is reasonable to assume that a future civilization will eventually have this amount of energy available, in addition to what is required to sustain itself. This assumption makes the construction and launch of the probe dependent on when this type of civilization might become reality. However, the Project Icarus Terms of Reference (ToR) require the probe “to be designed to be launched as soon as is credibly determined.” (4) Ideally in less than 100 years. In order to fulfill this objective, we have to answer the question if enough He3 can become available in that time frame.

The literature on Helium 3 mining()() gives us a wide range of results and conclusions:

• He3 mining cost range: 1,500,000 – 6000 $/kg

• Economic worth of He3: 15 billion $ per tonne

• Feasibility for Earth-based power generation: ambivalent

• Jupiter or Uranus are possible sources

Before we develop our own architecture, we first develop a scenario in which Helium 3 mining from the outer gas planets is a viable option, assuming an Earth-based civilization.

Scenario development

For a sustainable mining architecture, the needs of potential stakeholders have to be satisfied. The scenario developed in the following will provide a context in which He3 mining from gas planets is a serious option by satisfying the needs of the energy industry and political stakeholders. Two main uses of He3 are assessed here: (a) energy generation within fusion reactors and (b) fusion propulsion for planetary defense.

Use of He3 for future fusion energy production on Earth

Before assessing He3 fusion reactors, we have to ask the question: what role fusion energy may play in the foreseeable future? Various studies have been done to answer this question. Most of them use magnetic confinement Deuterium – Tritium (D-T) reactors as a baseline. [13] and [14] base their analysis on the results from various world energy scenarios, which predict a continuous rise in energy demand until the year 2100.(5)(6) Fusion energy might play an important role in satisfying this demand. Two conditions are given for a certain future market share of fusion energy: Stricter restrictions of greenhouse gas emissions and more stringent safety standards. Depending on the degree of these restrictions and standards, the market share of fusion energy is predicted to be somewhere below 30%. In order to be economically competitive, the cost of electricity (COE) has to be somewhere between 6.5 – 12.5 cents/kWh, the estimated value depending on the prescribed values in the restrictions and standards. The date for the introduction of the first commercial fusion reactors is assumed after 2060, if a 450 – 550 parts per million by volume (ppmv) CO2 limit is introduced. For limits above 650 ppmv or the availability of inexpensive fossil fuels, economical thorium reactors or if cheap Earth based solar or orbital solar power comes on line,(D-T) fusion reactors probably won’t be introduced. (7).

Technical and Socioeconomic Feasibility of D-He3 fusion reactors

After our brief assessment of D-T reactor economics, we turn our attention to the technological readiness of Deuterium – Helium 3 (D-He3) reactors. [16], [17], [18] suggest that commercial D-He3 reactors using three different fusion principles may become feasible: Inertial confinement, electrostatic confinement and magnetic confinement.(8)(9)(10). The energy and temperature requirements for D-He3 fusion are significantly higher than for D-T fusion. Although [19] presents a roadmap for the realization of D-He3 reactors before D-T reactors, we assume that the first reactors of this type will be commercially introduced after D-T fusion has already proved its economic competitiveness.(7) Additionally, research on D-He3 fusion is currently only done by a minority of the fusion research community. In terms of competitiveness the main advantages of D-He3 reactors might be: reduced (but not nil) radiation hazard risk, the use of full lifetime components, low decay heat, no Tritium breeding, no class A waste disposal and a high safety. [19] estimates that D-He3 reactors have a 60% lower COE than D-T reactors without including the cost for the fuel, which is significant. Hence, in order to be economically competitive to D-T reactors, the cost for He3 has to be below the difference of COE between D-T and D-He3 reactors, assuming that the cost for Deuterium production is negligible. A rough estimate shows that the maximum order of magnitude cost of Helium 3 must be between 1 – 10 billion $ per tonne, in order to be competitive. This is a very important result, because it will limit the allowed cost per tonne of He3 obtained in our mining operation, if fusion reactor economics is the dominant factor. This is a different method than to estimate the economic value of He3 itself, like in [11].

Helium 3 Demand

Next, we have to make an estimation of the total annual need of Helium3 for fusion reactors. A rough estimate of 10-20 tonnes per year is implied by the considerations above, 20 – 30 years after the introduction of commercial D-He3 fusion reactors. At this time, we are already in the 22^nd century. If environmental and safety considerations become one day the driving factor in energy generation, it is reasonable to assume that D-He3 reactors replace a considerable percentage of D-T reactors. However, even in this case, the total He3 needed will remain in the tens of tonnes.

The economics of different methods to produce He3 on Earth are assessed in [20]. (12) However, they are all considered economically infeasible, even in comparison to the difficulties of a space-based mining approach.

To conclude, we have seen that for a deep market penetration of fusion power plants, stricter environment and security standards are necessary. The comparison of D-T and D-He3 reactors has shown that D-He3 reactors have some advantages, like lower component replacement rates, but the technical and commercial feasibility is still an open question.

The most important results for Helium 3 mining are:

• Allowed He3 cost range: roughly 1-10 billion$/tonne

• Uranus offers a more economical source of He3 because of its lower deltaV cost and easier radiation environment.

• Annual mass to be delivered to Earth, if a large scale commercial use of D-He3 reactors is envisaged: 10-20 tonnes. This is two orders of magnitude less than the production rate required for a vehicle like Daedalus or Icarus, therefore it is extremely unlikely that mining for Earth based fusion power needs will be a sufficient driver for the development of a mining infrastructure capable of supplying Icarus.

Use of He3 for fusion propulsion for planetary defence

If fusion propulsion becomes feasible certain space activities may find stronger political support than others. The list of known Near Earth Objects (NEO) and Potential Hazardous Objects (PHO) is increasing as is public awareness, which suggests one important application of fusion propulsion will be its use within a planetary defense architecture. From [21], [22], [23] planetary defense approaches might directly profit from fusion propulsion by allowing a fast response to threats and the delivery of large masses to the PHOs.(13)(14)(15). Additionally, preventive deflection of PHOs might also become easier than with current propulsion technologies. “Preventive” means that there is no immediate threat from the PHO but the mission has the objective to reduce the risk of an impact. The problem for the use of He3 as a fuel is the existence of alternative propulsion systems based on the D-T or D-D reaction. Although their specific impulse is inferior, they have the significant advantage of better fuel availability. Hence, [23] concludes that for operations in the inner solar system, propulsion systems using He3 are not a feasible option. Leaving this problem aside, regarding the necessary annual He3 mass, we might expect a maximum of one PHO preventive deflection mission per year. From the propellant estimates in [23], an annual need of < 1 tonne of He3 can be estimated. This is an order of magnitude below the fuel need of D-He3 fusion reactors. As the planetary defense architecture probably receives strong political support, a fuel-supplying mining architecture might profit from this as well. However, the fuel alternatives make it doubtful that He3 will be used. Therefore this application is not considered further.

From the results above it appears that to create a situation that requires very large scale helium3 mining within a century we need to create an accelerated development scenario, where humanity spreads out rapidly into the solar system as an interplanetary society, or where an unexpected breakthrough makes the adoption of He3 an economically viable proposition. In other words, He3 mining needs a mr. Fusion, or an Expanse().

Or He3 mining needs to be so cheap that it can be included in the budget of a mildly aggressive Interplanetary space program.

The Cheap He3 mining scenario

Technology scenario

He3 is required not for energy production but for an Interplanetary space program. The Deuterium-He3 fusion process is effective and can be used for a fusion drive. The production methods are so economical that they can be afforded within the parameters of the program.

Policy scenario

A nascent interplanetary society want to pursue a rapid interplanetary space program. To do this requires a cheap source of he3, that fits within the program budget.

General Mining architecture

Primary architecture objectives

• Enable a cost-effective supply of He3 for for interplanetary spaceships and an early Interstellar mission.

Secondary architecture objectives

• Serve as a technology development program for the participating nations

• Enable the financing of spin-offs which may serve security purposes

• The architecture shall be scalable, in order to offer flexibility to changing He3 needs

• Enable science opportunities

Requirements

• Set up a permanent mining infrastructure, between 10-20t of He3 in the atmosphere of a gas giant, scalable up to much larger volumes for the Interplanetary vessel

• Deliver some of the He3 to the Earth’s surface. Store most of the He3 on site.

• Make a maximum use of in-situ ressources.

Description

The basic resource acquisition sequence is composed in the following manner:

• Earth departure

• Transit to the gas giant

• Orbit injection

Arriving at the gas giant in a parabolic or hyperbolic trajectory, the spacecraft may either perform an impulsive breaking at perigee or spiral down continuously. The impulsive breaking can be performed by propulsive or aerodynamic means.

• Perform atmospheric operations

• Performs in-situ propellant acquisition as part of the mining operations

After descending into the atmosphere from orbit and reaching a specific attitude, the mining operation starts. He3 and deuterium are delivered back to orbit in either a batch or continuous stream. Only He3 will be returned to Earth.

• Departure of a He3 payload

• Transit to Earth

Mining Architecture options

In a first step, key architecture options are generated and traded. These options belong to the following areas:

• Planet

• Atmospheric segment

• Transfer vehicle propulsion

• Mission sequence

Planet- Selection of Uranus as source of He3

In order to determine essential mission parameters, a target gas planet has to be selected. The four planets are Jupiter, Saturn, Uranus, Neptune. Each gas planet poses different engineering advantages and disadvantages with respect to the mission objectives. (16)(17)(18) The assessment of the different planets was already done in [7], [11], [12]. Here, we summarize the key results. Jupiter, which was originally chosen by the Daedalus study team, was eliminated due to its very strong radiation belt and the high orbital velocity (42.1 km/s), even with exploiting the natural rotation (12.7 km/s). Additionally, atmospheric operations are extremely demanding due to huge convective currents. This aspect was already identified in [3] as a problem but at that time there was much less information about the atmospheric conditions available. Saturn was eliminated due to its radiation belt and atmospheric conditions as well. Additionally, the ring system makes proximity operations difficult and the velocity requirements are also high. A further aspect is the low abundance of Helium and hence of He3 within its atmosphere. Neptune was basically eliminated due to its large distance from Earth and offering in principle the same conditions as Uranus. Therefore, as confirmed in [7], [9], [10], Uranus is chosen as the target gas planet of choice, with a total 'rising' deltaV of 15 km/s.

Atmospheric segment

The atmospheric segment is the most relevant mission driver. It determines the payload mass to be delivered from Earth. Additionally, the feasibility of the overall concept of gas planet mining depends on it. This is due to the huge technical difficulties of atmospheric mining. These difficulties can be seen by looking at the three high-level functional requirements:

• Deceleration and descent from orbit into the atmosphere to processing height

• Enable stable processing of the atmosphere until the required amount of He3 and other propellant is obtained

• Returning the He3 and propellant to orbit.

Each of these functions poses significant technical challenges. However, returning the mined He3 into orbit is the biggest challenge, due to the high gravity and velocity requirements.

In the following, options to satisfy the requirements are presented and an option for the baseline design selected.

Deceleration and descent

Due to the high DeltaV required for breaking and descent into the atmosphere, the aerodynamic option is the most suited one in comparison to propulsive breaking. There are several options to perform aerodynamic breaking:

• Aeroshell

• Lifting-body(19)

• Balloon(20)

• Hypersonic waveglider(21)

• Cylinder with thermal shield, control surfaces and a propulsive component()

The balloon and mining facility is to be delivered using an aeroshell, as the balloon and propellant factory only need to enter the atmosphere once.

The shuttle will be a cylinder with thermal shield. This is the method proposed for future Mars landings as well, and has been demonstrated on the SpaceX first stage return flights. The shuttle goes down with a full oxygen and partial hydrogen load,uses aerodynamic braking and a final vertical burn rather like the SpaceX Falcon 9. It hooks onto the balloon from below.

At the production plant, it can add the return hydrogen propellant (and possibly oxygen as well) from the atmosphere. It loads He3, and deuterium, and then powers up back to orbit and over to an orbital station where it unloads the He3 and D2 and reloads with H2 and O2 for its return to the production plant

Atmospheric processing plant

In order to support the processing plant in the atmosphere, several options are available:

• Balloon

• Airship

• Airplane (propeller, jet, ornithopter, RAM, SCRAM)

[30] gives a good introduction into the selection of aerobots in planetary atmospheres.(22) For our application, airplanes are excluded first due to the current lack of knowledge of the atmospheric wind conditions. Due to the high velocities relative to the wind, airplanes might be vulnerable. Additionally, a complex control system has to be implemented. A RAM or SCRAM jet would be nuclear due to the infeasibility to transport large liquid oxygen masses from Earth. An open nuclear reactor would heat the air up and propel the plane.(23) However, the long mining duration of many months and the necessary operation in denser atmospheric layers makes RAM and SCRAM jets infeasible.

Airships would enable certain mobility in the atmosphere. However, there doesn’t seem to be a need to move within the atmosphere, at least not in a way which wouldn’t be possible by using balloons.

Hence, balloons were chosen to support the processing unit stable in the atmosphere. [3], [30], [32] present trade-offs for different balloon systems.(24) The most suitable option seems to be a double-walled hot air balloon as in [3]. Within an atmosphere, which mainly consists of hydrogen, a hot air balloon is the best option to generate buoyant force. The buoyant force is limited by the maximum temperature difference between the air in the balloon and the environment. The double-wall enables a higher temperature difference than a single walled balloon due to its better insulation. This is the same solution that was adopted for Daedalus.

Return He3 to orbit

The biggest challenge is posed by the ascent into orbit. In order to reduce the payload mass from Earth and the overall operation cost, we must reduce the necessary mass at Uranus as much as possible. Therefore, in-situ resource utilization (ISRU) must be used where possible. ISRU might be most attractive regarding Hydrogen, which comprises the majority of the Uranus atmosphere. Furthermore, cryogenic hydrogen is left as a byproduct of atmospheric processing. Therefore, it wouldn’t be far-fetched to use Hydrogen as a propellant. However, the possibilities are quite limited. Chemical propulsion can’t be used due to its low performance. Therefore, the only remaining options are nuclear propulsion systems. A combined nuclear RAM/SCRAM-rocket might be possible in theory but has several disadvantages. Due to its long acceleration time within the atmosphere, drag losses are significantly higher than for rockets. Additionally, the thermal loads also become an issue, enforcing adequate protection of the system, hence increasing its mass.

Due to its “simplicity”, a one stage nuclear thermal rocket was chosen as the best solution. The illustrations in this chapter show a Liquid oxygen Augmented Nuclear Thermal Rocket (LANTR)(). This type of engine is compatible with ISRU oxygen and hydrogen extracted from one of the moons of Uranus and hydrogen from the atmosphere of Uranus itself.

Alternative atmospheric segment concepts

Are there radically different options, which do not need a breaking and descend into the atmosphere? One attractive possibility is to use systems with tethers.

Aerodynamic tether

The principle of an aerodynamic tether for mining operations is to have a spacecraft in orbit with a tether hanging down into denser regions of the atmosphere.(25)(26) The tip of the tether is equipped with a scooping and processing unit, which collects the atmospheric gasses and does the processing. The concept of atmospheric scooping without a tether was already explored e.g. in the LACE and PHARO studies.(27)(28)(29) At the end of the collection process, the tether is spooled back into the spacecraft in order to retrieve the He3.

For reasonable collection times (up to several years) the drag of the system is very high. In order to be sustained by the thrust of a space propulsion system over this time period, the required propellant masses are prohibitively large, unless propellant was made available from a second In-Situ propellant processing system from one of Uranus’ moons. Therefore this option is considered unlikely and overly complex.

Rotovator

The rotovator skyhook system uses a rotating tether, fixed to a spacecraft, which dips its tip into the atmosphere and hook-up a payload. The payload is then accelerated up from the atmosphere and launched at the appropriate point in the upswing towards the waiting return vehicle. An advantage in comparison to the aerodynamic tether is the low relative velocity to the atmosphere, reducing the aerodynamic drag and thermal loads on the system. The high complexity of the system, the difficulty in testing it and the many risks associated with its operation also makes this solution rather unlikely.

Uranus moon space elevator

[38] proposes a tether suspended from a tidally locked moon of Jupiter into its atmosphere.(30) A processing unit is mounted onto the tip of the tether. However, this option doesn’t seem to be viable for Uranus due to the non-existence of appropriate moons. The innermost moon Cordelia is followed by several ring systems, which block the line of sight from the moon to the upper atmosphere. A deployed tether would be immediately hit by particles from the ring. However, there is the possibility to use several tethers, each held at a certain distance by a truss structure. Even a single tether, deployed from a long truss erected on the moon perpendicular to the orbit plane would be possible. The single tether concept is depicted in Fig. 1.

A rough estimate of the required mass can be obtained by using [39] and some advanced material like Colossal Carbon Tubes (CCT). (31)(32) For a tip loading of 20 tonnes, an approximate tether mass of 440 tonnes is required. The relative velocity to the atmosphere is 2.57 km/s, inducing significant drag and thermal loads on the tether. However, the drag is compensated by a lowering of the moon’s orbital radius, which is very low due to its large mass. The mined He3 is transported to Cordelia via elevator climbers.

To conclude, a Uranus space elevator might be worth considering for the very long run, if very large masses have to be mined continuously from Uranus. It is excluded from our further analysis due to many open engineering issues.

Transfer vehicle

A transfer vehicle is required between Earth and Uranus.

Propulsion and design

The most important aspect to consider for the Earth to Uranus transportation system is the selection of the propulsion system. The following options are considered here:

• Nuclear thermal propulsion (NTP)

• Nuclear electric propulsion (NEP)

• Fusion propulsion

• Aerodynamic breaking/Aerocapture (AD)

• Solar/Plasma sail

A Uranus orbit injection with NTP is not feasible due to the large amounts of cryogenic hydrogen which have to be carried all along the way. As can be seen from Table 2 the NEP covers almost all mission phases and seems very appropriate as the main propulsion system. Fusion would also be an adequate option but is left here as an advanced option. Aerodynamic Drag (AD) is particularly helpful for breaking at Uranus and holds a large propellant mass reduction potential. It might therefore be the method of choice for Uranus orbit injection. The atmospheric section is released during this breaking operation and continues to slow down until it descends into the atmosphere whereas the transportation section increases its orbital height via NEP, to remain in orbit waiting for the rendez-vous. A solar sail option for outer planet sample return was considered in [41] and is a very interesting option due to the low mass arriving at Earth/L2.(33) According to [41], it holds the potential to reduce travel times for the inbound leg to Earth by 1/3 or more.

The option which satisfies the high power requirements for cooling the He3 on its way back to Earth and enables reasonable trip-times with available launchers is the NEP option. Its feasibility for the near future was shown during Project Prometheus.(34) Additionally, aerodynamic breaking might be used during Uranus orbit injection. The heat shield would be similar to the one used for the atmospheric section. However, this was not considered in the present scenario.

Preliminary systems design

In this section, the results of a rough numerical analysis of the different systems presented. As one of the objectives was scalability, we first present the design for a mining mission capable of returning 500 kg of He3 in order to assess its initial feasibility. We also derive estimates for a 10,000kg mining mission. 500kg and 10,000kg are chosen in order to enable a comparison between the numbers from this paper and [11]. Fig. 4 gives an overview of the various mission phases.

Atmospheric section mass budget

The primary driver for the overall mass is the atmospheric section. Therefore, the estimated mass for it is presented here first in Table 4. As all components have to be developed from scratch, a mass margin according to [43] of 30% is added.(35).

For a first order estimate for the transfer vehicle mass budget, we used the approach presented in [41]. A total DeltaV for the mission is estimated by referring to [41] and taking the lower velocity requirement for a Neptune sample return mission of 70 km/s. The on-board nuclear reactor has a specific mass of 50 kg/kWe and 30kg/kWjet accordingly. With this specific mass, the ion propulsion system has an estimated Isp of 12,000s. The total power generated by the reactor is estimated as 300kW. The resulting mass budget is presented in Table 5.

Cost estimates 500 and 10 000 kg of He3

Finally, we would like to check the economic feasibility of the mission. As mentioned before, the cost per kg He3 should be in the range of 1 – 10 billion $.

We make the following assumptions:

• The development cost are publicly funded and therefore excluded, which is not unreasonable for a strategic infrastructure project.

• The specific transportation cost to LEO are assumed as 50 $/kg.

• The specific cost for the spacecraft is assumed as 100 000$/kg, which is representative for geostationary communication satellites in serial production.

Hence, the specific cost of He3 exceeds the economically acceptable range for the 500 kg proof of concept mission, but is acceptable for the 10 000 kg per flight mining architecture.

The use of advanced electric propulsion, fusion propulsion and advanced atmospheric mining techniques holds the potential to reduce the system mass significantly and should be considered in future analysis.

⭐ He3 extraction example

Discussion

It must be noted that this chapter only gives a very rough sketch of how a mining architecture might look like. In the following, we would like to discuss the basic assumptions and results in the order they are presented in this paper.

One argument which might be put forward is that the energy situation beyond the year 2100 might require a need of He3 orders of magnitude higher than projected here, if an exponential growth is assumed. With a much higher demand for He3, economy of scale might reduce the cost for mining significantly. However, we might object that there is no “natural” law for an exponential growth of energy need and we doubt that this type of growth can be sustained over centuries. Referring to a prediction up to the year 2400 and beyond in [14], the total energy need is steadily increasing. After the shortage of fossil reserves, it is rather a linear or logarithmic increase than exponential. We might further repeat that only strict environmental and security standards will foster the introduction of fusion energy. There are alternative energy generation methods like fission reactors, which may rely on virtually inexhaustible Uranium resources extracted from seawater, or renewable energies such as solar and wind coupled with energy storage. It is reasonable to assume that fusion energy will play a role in a mix of different power generation methods, but not be the main one .

If we take the underlying assumptions for the energy and political scenario we created here, does the proposed architecture contradict one or more of them? One argument we must take seriously is the following: If there are much stricter environmental and safety standards in the future, how will the public tolerate the launch of spacecraft including NEP and NTP? This is a weak spot of the concept presented here. We may only comment that the use of NEP and NTP is an option for mining “low” quantities of He3. If it is required to mine large quantities of He3 in the far future, technologies like fusion propulsion and a Uranus space elevator might replace the use of potentially hazardous technologies.

The budgets for the atmospheric section and the transfer spacecraft are first estimates and should not be treated as definitive results. Especially the cost estimate might be a matter of debate. It must be added that the assumptions made for the cost analysis were rather optimistic and it is not unreasonable to assume that the real cost will be several times higher. The 8.6-10 billion $ per tonne figure was in the upper end of specific fuel price economically acceptable.

Finally, we have to consider the consequences of the results for space exploration and an interstellar probe using D-He3 propulsion. For the annual rate of 1500 tonnes of He3 required for the Daedalus probe, it is reasonable to assume a civilization which has a much higher need for He3 for energy generation or interplanetary transportation. A rough estimate shows that assuming a civilization which is willing to spend 3% of its energy production for an interstellar probe would have an energy output eight times higher than the current annual world energy production. Taking the long-term energy growth estimates from [14], this will take many centuries (after year 2400). A possible solution to this problem would be a drastic reduction of the payload mass of the probe. E.g. a reduction by two orders of magnitudes would require an annual mining rate of 15 tonnes. If 10% of He3 are bypassed from the mining architecture, an annual mining rate of 150 tonnes is required, which may be reasonable for the 23^rd - 24^th century. However, the Icarus study has shown that this kind of reduction of the probe size is unlikely for a fusion probe.

Deuterium production at the gas giants

Recent work on deuterium/hydrogen separation membranes has shown that using monoatomic membranes of hBN on Nafion might allow separation at an energy cost of about 0,2 GJ/kg(bb). So a production rate of 2000 tonnes per year, or 0,06 kg/s, would require about 13 MW of power, that might be supplied by two 10 MW production units.TBC

Propellant production from Uranus rings

The NEP vehicles might produce their own propellant from the ice rings of Uranus(), This might reduce the size of the mission considerably and make a regular production facility easier to build and operate. In fact it might make the production of He3 cheap enough that it could be operated solely as a propellant source for fusion vehicle in the solar system and Interstellar probes.

The H3 (tritium) scenario

Tritium (H3)may be required to provide the initial ‘spark’ to power an Inertial Confinement Fusion drive. Daedalus required 2% of its fuel as tritium, or about 1000 tonnes. An ICF Icarus with the same ignition scheme would require about 500 tonnes (for an overall fuel supply of 20 000 tonnes), plus a breeding scheme capable of producing the required 50 tonnes of tritium for the stop burn. It is not practical to try to provide an original oversupply of tritium to decay into He3, since the 12,4 half life means that an initial mass of 14 000 tonnes of tritium would be required, and that at the end 13950 tonnes of he3 would be left, or about an order of magnitude too much propellant.

Tritium can be created from lithium by the reaction Li6 + n= He4+Tr. He4 is not usable as a fuel and would be discarded, or used to resupply ship stores. If we suppose a conversion rate of 10%, then 1100 tonnes of lithium would be required to breed 50 tonnes of tritium, and otherwise serve as coolant for the drive. 17 tonnes of neutrons would be required from the neutron source, supposing perfect capture.

The propulsion pellets would need to be at least partly manufactured ‘en route’, to insert the tritium at the required time.

The current world supply of tritium is about 20 kg_(a).

If the entire world supply of energy in 2100 (38 000 GW) came from fusion reactors operating in an ICF configuration at 50% efficiency, the world would use 900 tonnes per year of tritium,

18 000 tonnes per year of deuterium and 27 000 tonnes per year of helium 3. That is close to the fuel supply for Daedalus, or about twice the fuel supply of most Icarus ICF designs.

Tritium

Half Life: 12.4 years

Decay Heat: 500 W/kg

Tritium breeding

dV= Delta V

dV=Ve x ln(Mo/Mf) or Mo/Mf= e(dV/Ve)

Ve = Exhaust velocity

Mf = Final mass = payload+structure

🛠 Metrics

Secondary propellant

Required power

References

The D2 scenario

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The He3 scenario

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a) https://www.iter.org/mach/tritiumbreeding

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