Implications of Molecular Nanotechnology Technical Performance Parameters on Previously Defined Space System Architectures

Molecular nanotechnology offers the prospect for significant increases in various technical performance parameters, such as material strength and density. This performance would enhance the capabilities of many classes of space systems. To begin analyzing this effect, a first step is to take previously defined space system architectures, not originally intended to use molecular nanotechnology, and calculate how well those systems would perform when simply using the MNT technical performance parameters. This paper discusses chemical rockets for putting payloads into Earth orbit, both single stage and two stage architectures, synchronous and rotating skyhooks, solar sails and solar electric ion engines, and large, inhabited space colonies. In all cases, MNT offers the possibility of significant system improvements.

Introduction

Molecular nanotechnology (MNT) has been laid out as a vast, long-term project , ultimately leading to a general ability to design and build products to atomic precision, thus providing general applications (figure 1). Research along the development pathways are crucial to any realization of this vision. That realization, however, will have dramatic consequences, requiring significant foresight to properly handle. Developing the appropriate foresight rests upon developing a sufficiently accurate and detailed understanding of the actual capabilities MNT will provide.

Figure 1. Simplified roadmap of the effort to develop and apply molecular nanotechnology (MNT). From [2], used with permission.

Scope and Objectives

This paper develops early explorations of the capabilities MNT will provide in the area of space operations. It is an example of theoretical applied science , examining what sorts of applied capabilities are and are not feasible within our current understanding of physical law. The best example of theoretical applied science in the area of molecular nanotechnology is , which developed estimated technical performance parameters for molecular nanotechnology (table 1). This paper takes those parameters, and applies them to models of previously defined space system architectures. It also discusses a methodology for extending, deepening and refining out understanding of the actual capabilities molecular nanotechnology will provide, applicable to many fields.

Table 1. Estimates used in this research for the technical performance parameters of MNT are taken from .

Parameter	Value	Unit
Product Tensile Strength	5 x 10^10	Pa
Corresponding Material Density	3,510	kg/m3
Mechanosynthetic Device Operating Rate	10^6	Hz
Marginal Manufacturing Costs	0.1 - 0.5	$/kg

Main Advances

Most of the space system architectures considered here were not developed to particularly exploit the capabilities of MNT. In particular, one of the major benefits of MNT would be the novel function of a general purpose material processing capability. Since this is a novel function, however, it is not included in previously defined space system architectures.

The capability of MNT which previously defined space system architectures do take particular advantage of is the increased strength-to-density afforded by MNT structural components. The material properties assumed in the prior analyses of space system architectures, and the comparable MNT values, are shown in table 2 below.

Table 2. Values for material properties used in this research.

Material	Strength (Pa)	Density (kg/m3)
Aluminum (cold formed)	3.52 x 10^8	2,650
Steel (cold drawn)	1.240 x 10^9	7,800
Titanium (cold formed)	9.31 x 10^8	4,540
Graphite crystals	2.1 x 10^10	2,200
MNT Structural Material (Diamondoid)	5 x 10^10	3,510

The main conclusions of this research are that rockets can be made significantly more efficient, and vastly more cost efficient, the feasibility of orbital skyhooks is improved, interplanetary propulsion could have high performance, and very large inhabited space colonies could be structurally sound.

Methodology and Larger Goals

This research is meant to embed in a larger methodology, shown in figure 2 below. That methodology has the larger goals of progressively developing a better understanding of the future capabilities for space operations that will be provided by MNT, at the system and mission level, and of progressively developing an understanding of feasible and appropriate strategies for operating with this unfolding capability. The methodology can also help focus ongoing efforts in the theoretical applied science of MNT, by defining which uncertainties about MNT are operationally most significant.

While this larger methodology is expressed in terms of a focus on space operations, it could be useful in all application areas for MNT. The methodology is based on a hierarchy of technical capabilities, system architectures, operational concepts, and grand strategies.

MNT will make dramatically improved systems possible, and it thus is important to develop and analyze system concepts using MNT [4]. Such systems must be evaluated in their operating context. MNT will so change the total environment of capabilities and actions, however, that it will transform the operating environment context for doing the analysis. This "chicken-and-egg" problem requires an iterative analysis methodology to resolve.

Figure 2 describes such an iterative process, built around the hierarchy of technical capabilities, system architectures, operational concepts, and grand strategies. Each level provides building blocks for assembling concepts at the level above. Thus, one can think synthetically, combining building blocks into concepts at the next higher level. Also, one can think holistically, using an intuition or theory about what MNT can achieve at a particular level to develop a concept at that level. In this latter case, one must then do a top down analysis to make sure that the lower levels can actually support this new concept, resulting in either a refutation or added confidence in the concept. Another result of this top-down analysis likely to be additions to the menu of building blocks at the levels below.

Figure 2. An iterative methodology for assessing system architectures using MNT, likely to continue for years or decades by many researchers across many research projects.

A first step in this methodology is to take a broad range of previously defined system architectures, and apply the Technical Performance Parameters of MNT to those architectures. This will provide an initial set of MNT-based system architectures from which to launch further analysis. Such system architectures are likely to be unbalanced, however, and thus will probably perform significantly less well than system architectures designed to exploit the particular capabilities of MNT. The research is this paper is part of that first step.

Earth to Earth-Orbit Transportation

The standard concept for launching payload into Earth orbit is rockets, although different staging architectures are often considered. An alternative approach which might better take advantage of the material strength-to-density ratios offered by MNT are very long orbital cables, also called skyhooks and orbital towers. Both concepts are examined here.

Chemical Rockets

Typical top-level comparison of six different architectures for transporting payloads from Earth using chemical rockets. This paper assumes LH/LOx engines with a 462 second specific impulse, and a 200 km circular reference orbit inclined 51deg.. The six architectures examined are:

1. Single Stage to Orbit (SSTO) with Horizontal Take Off (HTO) using an undercarriage [SSTO(H)];
2. SSTO with HTO using a sled [SSTO-SL];
3. SSTO Air Launched from an aircraft at 8 km altitude, traveling at 180 m/s [SSTO-AL];
4. SSTO with ground-launched Vertical Take Off (VTO) [SSTO(V)];
5. Two Stage To Orbit (TSTO) using an undercarriage [TSTO(H)]; and,
6. TSTO with ground-launched VTO [TSTO(V)].

All stages in all architectures use horizontal landing, and thus have wings and undercarriages. The wings and undercarriages are sized for the vehicle's return, unless they are used in take off, as for example by 1) SSTO(H). All architectures are sized to a common take-off mass (GLOW) of 500 tonnes, but their different architectures result in different payloads delivered to the reference orbit (Table 3).

The primary means of taking advantage of MNT without changing these architectures is to use diamondoid materials with much higher strength-to-density ratios. This will reduce the mass of the vehicles' fuselages, wings and tails, undercarriages, and propulsion systems, roughly in proportion to the relative strength-to-density of diamondoid to the materials used. (Many parts of the propulsion system, however, would require appropriately coated surfaces, and external surfaces will require thermal protection for re-entry.) This research assumes that the systems were all be built of titanium in the original paper. The nature of the "on-board equipment" is not clear described in [5]. It probably includes mass for life support for pilots of the vehicles. Since the exact nature of this mass is unclear, the conservative assumption made here is that this mass could not be reduced at all with the use of MNT.

By flying the same trajectories, the mass savings from applying MNT to the vehicle's structure can be applied to the payload. This occurs directly for the SSTO vehicles and the second stages of the TSTO vehicles. Mass savings on the first stage of the TSTO vehicles are added proportionately to all components of those vehicles second stages, including the payload.

The resulting vehicle and payload masses, and mass ratios, are shown in table 3 below. The "Titanium" based numbers (first entry) are from [5]. The "Diamondoid" based numbers are assuming MNT materials. These results are consistent with [6], which describes an MNT-based SSTO with a payload to GLOW ratio of 1/6 and a ratio of payload mass to dry, empty vehicle mass of ~8.

Table 3. Values for material properties used in this research. The difference from old to new is the use of MNT materials in the place of titanium.

Architecture Titanium/ Diamondoid	"Dry, Empty Vehicle Mass"	Payload Mass	Mass Ratio	"Payload to Dry, Empty Mass Ratio"	Cost per kg to Reference orbit
1) SSTO(H)	92.5 / 16.5	-32 / 44	-6.4% / 8.79%	-35% / 267%	NA / $5.19 / $375
2) SSTO-SL	57.3 / 10.5	9.8 / 56.6	1.96% / 11.33%	17% / 541%	$29k / $3.91 / $185
3) SSTO-AL	54.3 / 9.5	17.0 / 61.8	3.40% / 12.36%	31% / 653%	$16k / $3.54 / $153
4) SSTO(V)	57.4 / 9.4	4.8 / 52.5	0.96% / 10.50%	8% / 540%	$59k / $4.26 / $185
5) TSTO(H)	107.8 / 20.4	17.0 / 49.5	3.40% / 9.90%	16% / 243%	$31k / $4.55 / $412
6) TSTO(V)	70.4 / 14.5	25.0 / 53.5	5.00% / 10.70%	36% / 368%	$14k / $4.17 / $272

The costs for the titanium systems were not in [5], but are based on a very simple minded per mission cost estimate of $1000 per kg of dry, empty launch vehicle mass, averaged over the life of the program. This may be extremely optimistic; using traditional aerospace approaches it would require a tremendous number of fleet missions conducted with low overhead. The cost for the diamondoid, MNT-based systems is based on the high end of the MNT manufacturing cost estimate from , applied to producing the vehicle and the fuel mass, and assuming the vehicle is used once. The third cost entry applies the per mission cost estimate of $1000 per kg of dry, empty launch vehicle mass to the diamondoid system masses.