MOLECULAR MANUFACTURING FOR SPACE SYSTEMS: AN OVERVIEW

First published in
Journal of The British Interplanetary Society, Vol. 45, pp.401-405,1992

K. Eric Drexler
Institute for Molecular Manufacturing,
555 Bryant Street, Suite 253,
Palo Alto, California 94301, USA.

ABSTRACT

Space flight and space development depend on hardware. The performance, reliability and cost of that hardware depend on manufacturing capabilities. Present manufacturing capabilities have failed to deliver high performance and reliability at a low cost. Molecular manufacturing systems will use high frequency mechanisms to build macroscopic structures from the molecular building blocks. An analysis of these manufacturing systems and their capabilities indicates that they can produce space hardware of high performance and reliability at a cost low enough to reduce the cost of space operations by orders of magnitude. The following discussion summarises some of the basic results from a previous analysis [1], with particular reference to radiation damage models and discusses the implications of these results for space systems, including Earth-to-orbit and interplanetary transportation systems.

Contents

  1. Introduction
  2. What is Molecular Manufacturing
  3. Products Can Have High Performance
    1. Computers Can Be Small and Efficient
    2. Efficient, High-Density Power Conversion is Possible
    3. Materials can be Strong and Stiff
  4. Products Can Be Made at Low Cost
    1. High Frequency Permits High Productivity
    2. Inexpensive Feedstocks Can be Used
    3. Energy Requirements are Moderate
    4. Land, Labour and Maintenance are Inexpensive
    5. Direct Production Costs can Approach Material Costs
  5. Many Products Can Be Highly Reliable
    1. Manufacturing Defects can be Rare
    2. Margins of Safety can be Large
    3. Radiation Damage Presents Special Problems
    4. Extensive Redundancy cna be Provided
    5. Rapid Replacement is Often Feasible
  6. Applications: Earth-to-Orbit Transportation
  7. Applications: Interplanetary Transportation
  8. Conclusions
  9. References

1. INTRODUCTION

Earth-to-orbit transportation systems today rely on chemical rockets. To lessen the mass of launch vehicles and their payloads, many components are designed to operate with only small margins of safety, while the complexity of launch systems demands that most of these numerous components be extremely reliable, to assure that the system as a whole will be reasonably reliable. The combination of numerous components and small margins of safety with grave sensitivity to defects his contributed to the high cost of launch systems and operations. The high cost of launch, in turn, has imposed similar performance and reliability requirements on systems used in free space, raising costs still further and effectively precluding large-scale activities beyond Earth's atmosphere. Although changes in design practice and incremental advances in technology hold considerable promise (we have not yet seen a launch vehicle designed and operated for industrial use) more drastic changes will result from a fundamental change in the manufacturing technology base. Computational experiments and system level analyses indicate that molecular manufacturing will offer fundamental advantages in product performance, reliability and cost. Advances in these areas can combine to make space a readily accessible part of the human world.

2. WHAT IS MOLECULAR MANUFACTURING?

Conventional manufacturing systems use special purpose machines and general-purpose manipulators, such as human and robotic hands, to transform relatively simple parts and materials into more complex products. Typical operations on parts include adding and removing material, transporting them from point to point, e.g. using conveyor belts, and putting them together to form assemblies,

If one omits human hands from the description and recognises molecules and collections of molecules as examples of parts and materials, then the description of conventional manufacturing systems can equally well describe molecular manufacturing systems. The abstract problem, in both cases, is to manipulate pieces of matter so as to build complex structures. Many of the subproblems and required subsystems are entirely parallel.

The differences, of course, are numerous: Molecular manufacturing systems use devices having nanometre-scale rather than millimetre-scale features. Operating frequencies are higher by a factor of ~ 106. The smallest moving parts must be analysed, not in terms of the elastic and surface properties of bulk materials, but in terms of molecular potential energy functions. Typical assembly operations involve the formation of a distinct pattern of chemical bonds via mechanosynthesis, rather than welding, adhesive joining and the like. Unlike conventional systems and their products, molecular manufacturing systems and their products can be eutactic, that is, well-ordered in the molecular sense. Because all products are made with atom-by-atom control, costs are more nearly proportional to mass than to, for example, the number of moving parts. Thermal vibration is a primary source of errors in molecular assembly processes. Radiation damage and photochemical reactions can destroy components. Each of these issues, and many others, must be considered in the design of the products, mechanisms and processes of molecular manufacturing.

The physical principles required for this analysis are described in [1], which analyses nanomechanical components, computational systems and manufacturing systems and describes implementation pathways leading from today's technology base in chemistry, biochemistry and proximal probe mechanisms, e.g. scanning tunnelling and atomic force microscopes.

3. PRODUCTS CAN HAVE HIGH PERFORMANCE

3.1 Computers Can Be Small and Efficient

For decades, the size of computers has decreased and their energy efficiency and speed have increased. Molecular manufacturing, by enabling the construction of computer devices with atomic precision in three dimensions, will enable this process to reach its natural conclusion. The range of phenomena that can be used to transmit and switch signals is large and far from completely catalogued. In all likelihood, high-speed computers will exploit electronic phenomena, whether based on small transistors or on less familiar phenomena such as coupled spin systems [2]. The low mass of electrons permits electronic switching to occur at high frequencies and electromagnetic disturbances can propagate at high speeds.

Nanomechanical systems are easy to analyse and have characteristic frequencies high enough to make them surprisingly attractive as computer devices. In such systems, signal transmission occurs by the mechanical displacement of a slim rod. Switching occurs when the displacement of one rod mechanically obstructs the motion of another. A typical switching time is a respectable ~ 10-10s. An analysis of mechanical nanocomputers indicates that a processor capable of executing 109 instructions per second can fit within a volume of ~ 0.1 m3 and consume a power of ~ 10-7 W. An array of 109 such processors in an air-cooled desktop package could deliver more computational capacity than all the computers in the world today combined. A related technology base can provide information storage densities of1010 bits/m3. This analysis takes account of signal propagation speeds, material strength and stiffness, sliding friction, nonisothermal compression losses, entropy generated by register erasure, error rates caused by thermal vibration, power supply, clocking, clock skew, input, output and other issues [1].

The feasibility of constructing parallel computing systems that execute 1018 instructions per second will enable improved computer-aided design and simulation of complex systems. For example, if the time required to model the structural deformations and aerodynamics of a vehicle is reduced to a second or less, simple and powerful iterative optimisation methods can be applied. In a more direct application to space technology, the feasibility of constructing microscopic computer systems supports the feasibility of constructing small automated spacecraft and of providing computers of high capacity on spacecraft of conventional size.

3.2 Efficient, High Density Power Conversion is Possible

Mechanochemical devices can interconvert mechanical and chemical energy in a nearly thermodynamically reversible process. The chief energy dissipation mechanisms result in power dissipation proportional to the square of speed, hence the waste-heat fraction is proportional to the power density. Systems with power conversion densities > 109 W/m3 have estimated efficiencies > 99%. Systems of this sort can store mechanical energy in chemical form and deliver it later at high rates [1].

Scaling laws favour the use of electrostatic mechanisms rather than electromagnet-based mechanisms in small systems and yield high power densities for electrostatic motors of submicron dimensions. A design exercise indicates the feasibility of power conversion densities > 1015 W/m3 with > 99% efficiency for the interconversion of mechanical and electrical energy in direct-current devices; i.e., the same device can serve as a motor or as a generator. Motors providing high power density can facilitate the engineering of a wide range of active structures. As with mechanochemical energy conversion processes, these systems can approach thermodynamic reversibility

These capabilities can work together in energy conversion systems. The ability to convert chemical to mechanical to electrical energy serves the function of a fuel cell. The ability to convert electrical to mechanical to chemical energy, and the reverse, serves the function of a storage battery. The ability to convert electrical to mechanical to electrical energy, but at a different voltage, serves the function of a DC transformer.

For energy collection, molecular manufacturing can be used to make solar photovoltaic cells at least as efficient as those made in the laboratory today. Efficiencies can therefore be > 30% [3]. In space applications, a reflective optical concentrator need consist of little more than a curved aluminium shell < 100 nm thick (photovoltaic cells operate with higher efficiency at high optical power densities). A metal fin with a thickness of 100 nm and a conduction path length of 100 m can radiate thermal energy at a power density as high as 1000 W/m2 with a temperature differential from base to tip of < 1 K.

Accordingly, solar collectors can consist of arrays of photovoltaic cells several microns in thickness and diameter, each at the focus of a mirror of ~ 100 m diameter, the back surface of which serves as a ~ 100 m diameter radiator. If the mean thickness of this system is ~ 1 m, the mass is ~ 10-3 kg/m2 and the power per unit mass, at Earth's distance from the Sun, where the solar constant is 1.4 kW/m2 is > 105 W/kg. This calculation assumes no improvements in basic device physics but only in the ability to fabricate small, precise structures.

3.3 Materials can be Strong and Stiff

The performance of structures is often limited by the strength-to-density and stiffness-to-density ratios of their materials. Although composite structures offer superior performance, aluminium structures, e.g. those used in the Shuttle orbiter, are a more familiar standard of comparison.

Molecular manufacturing lends itself to the mechanosynthesis of carbon-rich materials. One such material is diamond, which offers ~ 15 times the stiffness density ratio and ~ 75 times the strength-to-density ratio of a high-strength aluminium alloy [1]. If coated to prevent oxidation, diamond is stable to ~1800 K; aluminium melts at ~ 930 K. Although bulk diamond is brittle, fibrous structures consisting almost entirely of diamond can be tough. If stresses are applied chiefly along a single axis, a common case, division of the material into fibres need not sacrifice much useful strength. With a further modest sacrifice in the strength-to-density ratio, fibres can be interrupted at intervals by mechanisms containing nanomechanical motors, sensors, controllers and actuators, imposing active control of their lengths. An active material of this sort can simulate perfect rigidity so long as stresses remain below the ultimate strength of the structure and so long as fluctuations in stress are slow compared to the characteristic response frequency of the control system, which can, in the present instance, extend to many kilohertz. Active materials can also provide nearly perfect damping of structural vibration.

4. PRODUCTS CAN BE MADE AT LOW COST

4.1 High Frequency Permits High Productivity

A measure of the productivity of a manufacturing system is the time required for it to build a set of systems as complex as itself – roughly speaking, having an equal number of distinct parts. In manufacturing, the number of distinct parts produced by a machine is proportional to the number of motions that the machine performs In printing, stamping, lithography and the like, special-purpose machines produce many parts per motion and the cost, e.g. per transistor or printed character, can be extremely small. In typical machining operations, a more general-purpose machine performs many motions per part produced and the cost of the product objects is relatively large. Machining and subsequent assembly, however, can make complex three-dimensional components and machines.

Molecular manufacturing systems resemble machining systems in requiring many motions per part, and can likewise build machines. They can be more productive than printing, stamping and lithography, however, because the small size of the moving parts in a molecular manufacturing system permits them to perform motions at high frequencies. If millimetre-scale features in conventional manufacturing systems correspond to nanometre-scale features in molecular manufacturing systems (design exercises indicate that this is a reasonable comparison), then the operating frequency of the latter can be greater by a factor of 106. If the number of motions per part were the same (a rougher approximation), then the productivity would likewise be greater by a factor of 106.

More detailed design exercises [1], not based on scaling arguments, indicate that a macroscopic molecular manufacturing system can produce its own mass in comparable product in about an hour. Applying the scaling argument in the reverse direction would suggest that conventional manufacturing systems can produce their own mass in comparable capital goods in about 100 years. Since this is an underestimate of their true productivity, this comparison suggests that the estimated productivity of molecular systems may also be an underestimate. At conventional interest rates, the contribution of the cost of capital goods to the cost of additional capital goods is negligible for systems with productivity of this order.

4.2 Inexpensive Feedstocks Can be Used

Molecular manufacturing systems, of the class that has been analysed, use simple chemical substances of the sort produced today by the petrochemical industry at costs less than $1/kg. With substantially greater input energy and small increases in the mass of the manufacturing system, carbon dioxide and water could provide most of the needed materials, as they do for plants. Raw material costs can accordingly be low.

4.3 Energy Requirements are Moderate

The basic operations of molecular manufacturing consist of placing reactive molecular structures in a desired location and then permitting (or forcing) a chemical transformation. Calculations indicate that molecules can be moved along controlled paths in nanomechanical systems with little energy dissipation. The energy dissipated in chemical reactions is of the same order, at most, as the heat produced by the combustion of a comparable mass of fuel. Careful design of mechanochemical systems can, in many instances, convert molecular potential energy into mechanical energy (delivered as shaft power), rather than converting it into waste heat. In a process using operations of this kind, the energy required to transform molecular feedstocks into complex products is only moderately greater than the difference in molecular potential energy. Waste heat production is on the order of 106 to 107 J/kg. The conversion of typical organic compounds into predominantly diamond-like products yields surplus hydrogen. If this is combined with atmospheric oxygen by an efficient mechanochemical process, the overall manufacturing process can be a net producer of electric power. In other words, the unwanted components of typical feedstocks, if treated as fuel, yield more than enough energy to run the system.

4.4 Land, Labour and Maintenance are Inexpensive

Molecular manufacturing systems are compact and (internally) wholly automated. They can produce many times their mass in product per day and can operate for years. Accordingly, the costs of land, labour and maintenance can be low by present standards.

4.5 Direct Production Costs can Approach Material Costs

Some costs of production – taxes, licenses, insurance – have no direct relationship to technological capabilities. Since capital costs are low and energy requirements can typically be met from surplus hydrogen, the chief remaining direct cost is that of raw materials. Since these are converted into product structures with little waste, the direct cost of the products can approach that of the raw materials. Accordingly, production costs can be below $1/kg. Among structures of equivalent strength, $1/kg for components made of diamond-fibre material corresponds to ~ $0.01/kg for aerospace aluminium. Since typical space hardware now costs ~ $104/kg, these estimates suggest that manufacturing costs can be reduced by some six orders of magnitude.

This may seem surprising but there is no physical law that demands high costs for manufactured goods. Indeed, agriculture proves that molecular systems can operate on inexpensive feedstock materials to produce material structures of great complexity at costs in the $1/kg range.

5. MANY PRODUCT'S CAN BE HIGHLY RELIABLE

5.1 Manufacturing Defects can be Rare

In some respects, molecular manufacturing resembles digital electronics. In both instances, small devices perform sequences of operations at high frequency and low cost. More fundamentally, however, both digital electronics and molecular manufacturing processes, if well designed, transform discrete states into other discrete states under conditions in which intermediate, ambiguous states are unstable. In digital electronics, each operation produces a pattern of ones and zeros. In molecular manufacturing, each operation produces a pattern of bonded atoms. In these systems, unlike analog electronics and conventional manufacturing, small errors do not occur. A pattern is either correct or incorrect.

Mechanosynthetic operations, like digital switching operations, are subject to thermal and other sources of noise. The probability of an error in a mechanosynthetic operation can be calculated from knowledge of the potential energy surface for the reaction, using transition state theory methods grounded in statistical mechanics. In general, the probability of an error is an exponentially decreasing function of the increment in barrier energy between the trajectory leading to the error state and that of the trajectory leading to the desired state. In a mechanically stiff system, elastic restoring forces with stiffnesses ~ 20 N/m can impose large energy costs (³ 1.5 x 10-19 J) on trajectories that deviate from the desired trajectory by ~ 0.14 nm. At room temperature, an energy difference of this magnitude is usually sufficient to limit the error rate to £ 10-15 per operation [1]. Thus, as in digital logic, errors in molecular manufacturing systems can be made both distinct and extremely rare.

5.2 Margins of Safety can be Large

The ability to make materials 75 times stronger per unit mass than those in common use today can be exploited either to increase performance by reducing mass, or to increase reliability by increasing margins of safety. Where strength is the crucial concern, a factor-of-two margin of safety can be provided while still reducing the mass of a structure to ~ 3% of that of an equivalent aluminium structure.

5.3 Radiation Damage Presents Special Problems

Molecular manufacturing systems can make products that depend on nanometre-scale components for their operation: molecular manufacturing systems are themselves examples of such products; nanocomputers are another. A cubic nanometre of material typically contains only ~ 100 atoms and displacement of a single atom in a nanoscale component commonly will cause failure. Radiation damage creates such defects.

Classical radiation target theory, validated by studies of damage to proteins in vacuum [4], indicates that the probability of a damaging event is proportional to the radiation dose D (rad) and the component mass m (kg), and experimental data indicates that the probability that a component remains operational [1] is

P ~ exp(-1015 Dm)

A typical terrestrial dose rate is < 0.5 rad/year. Space environments vary widely with location, time and shielding. In a typical spacecraft interior, outside Earth's magnetosphere but shielded by several millimetres of aluminium, a single solar flare can deliver ~ 100 rad spread over tens of hours. Without shielding, dose rates are higher. A dose of ~ 10-4 rad, which would be received in several thousand seconds of terrestrial exposure, causes a density of defects on the same order as the ~ 10-15 per atom suggested by the previous description of errors in manufacturing.

5.4 Extensive Redundancy can be Provided

In computer systems, physical scaling laws encourage the extreme miniaturisation of components. Small size decreases the frequency of damaging events per unit time but increases the probability that a damaging event will destroy a component In computer technology, techniques for building redundant systems are well developed. Building with smaller components permits increased redundancy in a system of a given mass. Efforts to mitigate the effects of radiation damage will encourage the design of computational systems in which redundancy is provided at the level of small modules, rather than at the level of large subsystems. Computer CPUs of considerable capability have been built using 105 transistors and are complex enough to serve as modules in redundant systems. Comparable CPUs implemented using nanomechanical interlocks can have a mass of £ 10-17 kg. An object of this mass has a 0.1 probability of experiencing a damaging event after an accumulated dose of ~ 10 rad. Since a dose of ~ 100 rad can be delivered by an individual solar flare, achieving adequate radiation resistance is not a trivial problem, even with the use of extensive redundancy.

Potential solutions to this problem include a shift to still smaller modules (the feasibility and performance costs of doing so will require a detailed logic system design), or a shift to components large enough to tolerate radiation damage while remaining functional (this strategy entails substantial performance penalties). Minimising size while ensuring damage tolerance is not a simple problem. To design a system in which all atoms occupy defined sites, only one structure need be considered, but to design a damage-tolerant system, all possible radiation-induced defects must somehow be taken into account. This becomes simple only when all components are large compared to the expected defects.

Manufacturing systems can be designed to tolerate an accumulated dose of 10 rad by exploiting small module size and extensive redundancy in some subsystems and larger damage-resistant structures in other subsystems. Information storage systems can use small modules, moderate redundancy and error correcting codes to provide essentially effort-free records despite extensive radiation damage.

5.5 Rapid Replacement is Often Feasible

Accumulated radiation doses can be limited by replacing devices after suitable intervals. The productivity of molecular manufacturing systems is large enough to per mitt a manufacturing system to build its own replacement in an hour, while also producing several times its mass of other products. The turnover time can be short enough to reduce by a large factor the dose accumulated by any one device during the course of a solar flare. A mixture of strategies including redundancy, replacement and intrinsic damage

tolerance can be used to design systems suited to the space radiation environment. Special problems arise only when molecular manufacturing has been used to make systems that depend on the operation of numerous nanoscale parts. Typical aerospace systems – power conversion equipment, structures (even active structures), and so forth – can benefit greatly from molecular manufacturing without incorporating such sensitive parts.

6. APPLICATIONS: EARTH ORBIT TRANSPORTATION

Molecular manufacturing can produce the hardware needed to implement any of the diverse systems that have been proposed for transporting cargo from Earth to orbit. In each instance, it can be expected to, yield improvements in performance, reliability and cost relative to systems constructed using present manufacturing technologies. Several systems that are impossible to build with present materials, e.g. tapered tethers reaching from geosynchronous orbit to the ground, will become possible. Systems that are difficult to build with present materials, e.g. single stage to orbit vehicles using liquid-fuel rockets, will become relatively easy to build.

A simple calculation indicates the minimum performance to be expected from a single stage to orbit vehicle constructed using a molecular manufacturing technology base. In present aerospace practice, launch vehicles constructed chiefly of aluminium have a dry mass that is a small fraction of their fuel mass. A hypothetical vehicle in which the dry mass equalled the fuel mass would include enough structural material to make wings, landing gear, additional engines and so forth, all with large safety margins in their strength and stiffness, but such a vehicle would be too massive to reach orbit. Yet if one were to modify a conservative design of this sort by substituting diamond-based structures for aluminium structures, the dry mass could be reduced to < 0.02 that of the fuel while retaining both the capabilities and the safety margins. (Reductions in the engine mass, etc., can roughly parallel reductions in the general structural mass). The ideal mass-ratio for a rocket using liquid oxygen and liquid hydrogen is ~ 5 for a launch to low Earth orbit. Since wings can both reduce gravity losses during takeoff and enable transit of the dense lower atmosphere at low speed and, hence, low drag, this ideal mass ratio can be approximated in practice and without using air-breathing engines beyond subsonic speeds. With this mass ratio, ~ 0.2 of the gross lift-off mass can be delivered to orbit. With a dry mass fraction of ~ 0.02, ~ 0.18 of this mass can be payload. To be more concrete: if four passengers with luggage, air, seating, and so forth have a mass of 500 kg, the gross lift-off mass of the vehicle can be ~ 3 tons. The dry mass of the vehicle is only ~ 60 kg (the strength-equivalent of ~ 3 tons of aerospace aluminium).

7. APPLICATIONS: INTERPLANETARY TRANSPORTATION

A 20 nm thickness of aluminium has a reflectivity approaching that of the bulk material (~ 0.9). Lightsails constructed on the multikilometre scale can have structural masses that are small compared to the reflector mass, if a suitable pure-tension structure is employed to transmit forces from the sail to the payload. At Earth's distance from the Sun, the outward acceleration of an unloaded sail using 20 nm aluminium reflectors is ~0.16 m/s2, or ~ 14 km/s per day.

For a given charge density, fields caused by space-charge effects decline as devices are reduced in size. This enables arrays of small ion thrusters produced by molecular manufacturing to have high thrust-to-mass ratios. If the total mass of a solar-electric vehicle is ~ 10-5 kg/W then a system operating with an exhaust velocity of ~ 250 m/s can accelerate at ~ 0.8 m/s2, or ~ 70 km/s per day. After escape from a planetary orbit, a vehicle capable of this exhaust velocity and acceleration can follow what are essentially two-impulse interplanetary trajectories, travelling at a mean velocity of 100 km/s (the total DV of 200 km/s requires a mass ration of ~ 2.2, implying a somewhat lower initial acceleration). This speed covers one astronomical unit in ~ 17 days. In the inner Solar System, solar-electric propulsion systems in this class can outperform proposed gas-core nuclear rockets [5].

8. CONCLUSIONS

Molecular manufacturing will provide the ability to design and build engineering systems with control over the placement of the fundamental atomic building blocks of matter. This can greatly improve the performance and reliability of products and is compatible with surprising reductions in cost. Radiation damage presents special difficulties for the operation of nanomechanical systems in poorly shielded space environments but these can be mitigated by several strategies and avoided by the use of larger, radiation-tolerant components.

Molecular manufacturing can produce materials and energy conversion systems of sufficiently high performance that single stage to orbit vehicles become easy to implement. They can be used to construct superior solar sails, as well as solar electric propulsion systems capable of crossing distances in the inner Solar System with travel times on the order of a month. Molecular manufacturing and its products can make space flight inexpensive, fast and practical. The challenge is to develop the required tools and to learn to use them to build the proper products [1].

REFERENCES

  1. K. E. Drexler, "Nanosystems: Molecular Machinery, Manufacturing and Computation", John Wiley & Sons, New York, 1992.
  2. R. P. Feynman, "Quantum Mechanical Computers". Optics News, 11:1120(1985).
  3. H. M. Hubbard, "Photovoltaics Today and Tomorrow", Science, 244:297-304(1989).
  4. G. Beauregard and M. Potier, "Temperature Dependence of the Radiation Inactivation of Proteins", Anal. Biochem., 150:117-120 (1985).
  5. M.W. Hunter II, "Thrust into Space", Holt, Rinehart and Winston, Inc., New York, 1966.

Created: May 27, 1998
Last Modified: August 22, 1999
HTML Editor: Robert J. Bradbury