Breakthrough Propulsion Study: Assessing Interstellar Flight Challenges and Prospects

Breakthrough Propulsion Study: Assessing Interstellar Flight Challenges and Prospects

NASA Grant No. NNX17AE81G — First Year Report, June 2018

Prepared by Marc G. Millis, Jeff Greason and Rhonda Stevenson, Tau Zero Foundation, Broomfield, CO.

Prepared for NASA Headquarters, Space Technology Mission Directorate (STMD) and NASA Innovative Advanced Concepts (NIAC), Washington, DC.

Abstract

Progress toward developing an evaluation process for interstellar propulsion and power options is described. The goal is to contrast the challenges, mission choices, and emerging prospects for propulsion and power, to identify which prospects might be more advantageous and under what circumstances, and to identify which technology details might have greater impacts. Unlike prior studies, the infrastructure expenses and prospects for breakthrough advances are included. This first year's focus is on determining the key questions to enable the analysis. Accordingly, a work breakdown structure to organize the information and associated list of variables is offered. A flow diagram of the basic analysis is presented, as well as more detailed methods to convert the performance measures of disparate propulsion methods into common measures of energy, mass, time, and power. Other methods for equitable comparisons include evaluating the prospects under the same assumptions of payload, mission trajectory, and available energy. Missions are divided into three eras of readiness (precursors, era of infrastructure, and era of breakthroughs) as a first step before proceeding to include comparisons of technology advancement rates. Final evaluation "figures of merit" are offered. Preliminary lists of mission architectures and propulsion prospects are provided.

Acknowledgments

This work was supported by NASA under grant No. NNX17AE81G, under the direction of Dr. Jay Falker, Jason Derleth, and Dr. Mike LaPointe of NASA Headquarters. The authors would like to acknowledge their encouragement and support for this effort.

Additional assistance came from Jim Gilland of the Ohio Aerospace Institute, Paul Gilster of the Tau Zero Foundation, Les Johnson of NASA, and the staff who helped run the 2017 Tennessee Valley Interstellar Workshop.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Aeronautics and Space Administration.

1. Executive Summary

Dozens of interstellar mission concepts have been published that are based on known physics — and dozens more will be published in the future. All of the concepts require the maturation of one or more technologies or the building of infrastructure — or both. The US is not ready to launch a genuine interstellar mission, but is ready to start making investments to get ready to launch a mission within a few decades. The investment of limited resources will be required to get from where we are today to a future of interstellar travel. Careful selection of where to invest the resources is essential.

Choosing which concepts to fund requires the comparison of very different approaches. A challenge is to compare technologies as disparate as a fusion rocket and a ground-based laser pushing on a sail. In addition, not all of the proposed approaches to interstellar travel rely entirely on known physics. Champions of various technologies have a tendency to focus only on what is good about a given concept, sans implications for a complete system design. Therefore, an additional motivation for our metrics is to help identify “if it is real, would it also be useful?”

Many of the concepts of interest could revolutionize travel within the solar system, in addition to getting us closer to interstellar travel. Thus, investing in mission concepts that can both be employed usefully in near-term missions and be on the roadmap to future interstellar missions is of high interest.

This report derives figures of merit based on the physics of propulsion technologies and other mission factors. The metrics will allow policymakers to make decisions about which technologies would be more valuable, and to identify the subset of technologies that would do double-duty by enabling both longer-term interstellar missions and ambitious nearer-term missions.

At its foundation, these metrics rely on three core parameters: 1) how much energy will be expended, 2) how long does it take to perform a mission, and 3) distance traveled. It's like comparing modes of transportation: an airplane is faster than a car, but travel by car uses less energy and is therefore cheaper. Mission funding can compare the energy it will take versus a scientific objective (such as destination of interest) versus how long it will take to get the data.

1.1. At-a-glance highlights in this report

• Flowchart to guide a user in applying the assessment of a technology: Section 6.2, page 30.
• Sample data plots using the methodology with hypothetical technologies: Section 9, pages 60-61.

1.2. Content by Section

• Section 2 reviews the background and objectives of this study.
• Section 3 provides a technical background about interstellar travel.
• Section 4 identifies problems of interest that will be needed to make interstellar travel attainable, such as propulsion, power, and data transmission.
• Section 5 is a wide-ranging list of technological approaches identified from the literature for how to solve the problems — to be evaluated in Stage II.
• Section 6 organizes the metrics into a work breakdown structure, and provides a flowchart to guide a user in applying the assessment of a technology.
• Section 7 summarizes key mission choices and variables that must be defined in order to assess a mission approach — such as where is the mission going, how long will it take to get there, how much data will be returned, and type of mission: fly-by or orbiting.
• Section 8 lays out the comparison metrics for how dissimilar mission concepts can be quantitatively compared in an equitable manner. The process defines four different propulsion types according to their source of power and reaction mass (internal or external). Each type requires different analyses, where their unique performance measures are converted into the more general measure of energy.
• Section 9 shows examples of data charts based on the methods of Section 8. At this stage the comparisons are only with hypothetical propulsion technologies to test the methods and plotting options. These will be further refined in Stage II.
• Section 10 discusses Stage II of this project.
• Section 11 provides a list of references.

2. Introduction

Interstellar destinations are about three orders of magnitude farther away than can be reached using current technology. Many advanced propulsion concepts have been conceived, but their performance predictions are not yet certain enough to be ranked reliably using traditional trade studies. Further, the mission architectures in which these concepts were proposed used different assumptions that make equitable comparisons impossible.

There have been several overviews of the challenges and prospects of interstellar flight, most notably starting with the 1976 interstellar exploration program proposed to congress [1], the 1989 Starflight Handbook [2], the 1992 Prospects for Interstellar Travel [3], 2001 "Interstellar Flight Primer" [4], and the 2004 Centauri Dreams [5]. Among the large number of technical papers, several volumes of the Journal of the British Interplanetary Society were devoted to interstellar flight [6-8]. There has not, however, been an impartial, overall evaluation of the prospects and related next research steps.

There is a need for a new assessment method that can compare the uncertain, long-term prospects of emerging technology for interstellar flight. The assessment method must be able to equitably compare concepts that use entirely different propulsion methods (sails, rockets, and others). The comparisons should show which concepts might be the most advantageous and under what conditions, plus identify the most impactive supporting technologies. Further, given the long timescales of interstellar flight, the assessments should have provisions for considering advances that might reach fruition over decades of further advancement. And finally the assessments should suggest a prudent portfolio of next-step research.

2.1. Provocations

The impetus for this study dates back to a 2006 workshop held at Princeton University. As with prior interstellar sessions, various propulsion and mission concepts were presented as if to advocate their selection [9]. In the subsequent discussions, it was agreed that it was not possible to pick a winner. Each concept used different assumptions, plus the performance predictions are unproven. Rather than attempt far-future decisions, the discussion turned to identifying the most critical make-break questions for each approach, and of those, which can be affordably researched next. Pursuit of these questions shaped the goals and strategies of the nonprofit Tau Zero Foundation that was incorporated that same year [10].

Interest in interstellar flight is increasing. The continuing discovery of exoplanets, the announcements of privately funded mission plans, and the inkling that faster-than-light flight is now at least theoretically possible, provokes more interest. The following paragraphs describe some of the more significant provocations, in chronological order.

Theories for faster-than-light (FTL) flight are now part of the scientific literature. The first traversable wormhole article was published in 1988 [11], the first warp drive paper in 1994 [12], and the first scholarly book compiling these challenges along with other breakthrough propulsion pursuits was published in 2009 [13]. Even the hint that FTL might someday be possible, makes interstellar flight more attractive.

In 2014, the first potentially habitable Earth-size exoplanet (Kepler-186f, ≈ 500 ly) was confirmed by the Keck and Gemini Observatories. Now it is certain that potentially habitable planets exist, perhaps even Earth-like planets [14].

In April 2016, an ambitious plan for an interstellar mission was announced, with an offering of $100 Million for its initial research from Russian billionaire, Yuri Milner. The project, called "Breakthrough StarShot," is based on using a powerful laser array to push a small light-sail up to 20% lightspeed to reach Alpha Centauri within a 22 year flight time [15]. This project continues to receive significant media attention, further amplifying public interest in interstellar flight.

And in August of 2016, a potentially habitable exoplanet, Proxima b, was discovered orbiting our nearest neighboring star, Proxima Centauri, at only 4.2 ly distant [16]. Though subsequent analysis casts doubt that the planet could support human life, due to the intense radiation from its sun, the fact that our nearest neighboring star hosts an exoplanet that is roughly similar to Earth's size and temperature spurs further interest in the search for habitable worlds.

Congressional interest followed. An 18 May 2017 report for the fiscal 2017 Appropriations Committee included the following instructions: "The Committee encourages NASA to study and develop propulsion concepts that could enable an interstellar scientific probe with the capability of achieving a cruise velocity of 0.1c. These efforts shall be centered on enabling such a mission to Alpha Centauri, which can be launched by the one-hundredth anniversary, 2069, of the Apollo 11 moon landing." [17].

2.2. Objective and Approach

The objective of this study is to create a process for equitably comparing different mission architectures and possible propulsion and power technologies, in order to determine which research paths have the greatest leverage for improving NASA's ability to explore farther, faster, and with more flexibility. The specific goal of achieving 10% lightspeed was included by Congress, and it is likely that this goal will require the combination of a number of different technologies to succeed. The more exact questions that arise from further analysis are: which elements of that goal have more leverage toward success, and what knowledge gaps remain to solve each of those problems?

Toward that end, the process shall establish common performance measures for the disparate propulsion and power approaches, and accommodate the uncertainty in the performance predictions. The comparisons will include the scale of infrastructure needed to build and launch interstellar missions, include consideration of potentially disruptive advancements for spaceflight, and suggest how to plan a research program that systematically seeks the most desirable of such advancements.

In contrast to mission trade studies that seek the best technology to meet a set of well-defined requirements, this study will use topological analyses. Topological methods have been devised recently to compare general goals to broad technology areas to produce "topological maps" that identify research areas of potentially greater impact (instead of creating "road maps" to develop a specific technology) [18, 19]. These tools can determine the sensitivity of mission choices to performance requirements, as well as determining which technologies have greater impact on meeting those requirements (e.g. common elements of more than one subsystems). Figure 1 shows this analysis process in principle, and where the example maps are from Gilland [19].

In support of the general topological comparisons, more deterministic analyses are also developed. This includes the equations to convert the varied propulsion performance measures into common figures of merit.

Since the timescales for interstellar flight are comparable to historic examples of the emergence of breakthrough technologies, it is desired to include promising long-range research whose prospects are still speculative and whose mission impacts cannot yet be quantified. Presently, the practice is to wait until new prospects emerge on their own in a form ready to be evaluated per the familiar mission trade studies. However, this "wait and see" posture lets potentially revolutionary advances languish with little progress.

To search for such latent prospects, this study will include the topic of breakthrough propulsion physics. The term "breakthrough propulsion physics (BPP)" comes from the NASA project by that name which examined non-rocket spacedrives, gravity control, and faster-than-light travel [20]. In contrast to technological advancements rooted in known physics, BPP pursues entirely new technologies from further advances in physics. The grounding reference for this portion of the study is the book, Frontiers of Propulsion Science [13].

The first step of the scientific method is to define the problem. Similarly, this study will begin with an assessment of the challenges and prospects in a manner suitable to the unique situation of interstellar flight. Sponsored by a multiyear NASA grant NNX17AE81G, this "Breakthrough Propulsion Study" is divided into three stages, 1) defining the problem, 2) collecting information, and then 3) analytical testing.

The scope of this report covers the first stage of this study: defining the problem of tracking, assessing, and planning the most effective research paths to reach the stars.

2.2.1. Stage I — Defining the Problem as a Work Breakdown Structure (WBS)

The challenges of interstellar flight and the technological prospects for answering those challenges are examined to determine how to proceed later with a fully-rigorous and impartial assessment. Or, in other words, this stage aims to ask the right questions.

2.2.1.1. Challenges — Top Down Mission Awareness. The prior goals from interstellar studies will be refined to encompass a more complete set of factors — in short to understand the whole problem before suggesting solutions. Recent investigations of interstellar prospects found: 1) mission motivations are often implicit and limited in scope, 2) vehicle concepts often neglect the interplay with the infrastructure needed to build, power, and launch the vehicle, and 3) the major impediments to interstellar flight are less about technological prowess than about limitations of energy, where it appears that roughly two centuries remain before sufficient energy is likely to be available to launch an interstellar mission, regardless of the choice of flight method [21]. Note, however, that the uncertainty bands of those estimates are substantial.

2.2.1.2. Prospects — Bottom Up Technological and Scientific Principles. A wide span of interstellar flight prospects, from the basic solar sail all the way to the speculative FTL flight will be included to 1) provide a cursory understanding of their projected performance, and 2) devise methods to convert their disparate performance measures into common terms. Though the options use a widely varying range of parameters to describe their performance, energy is chosen as the central measure for this commonality. In essence, energy is the fundamental currency of all motion.

With the participation of Tau Zero Personnel, the "Tennessee Valley Interstellar Workshop (TVIW)" was convened in October of 2017 to gain an up-to-date summary of the projections and status of interstellar flight options and issues, discussed in Section 5.4.

In this report, the combination of challenges and prospects are used to create a new WBS to collect the complex information in an organized manner. A set of variables corresponding to that WBS are defined to guide the information collection process of Stage II. This will be expanded to identify which operating parameters of the different propulsion and power methods will need to be collected to continue the analysis.

While the focus of this first stage is the development of the overall structure for the data to be subsequently collected in later phases, some values have been specified as starting estimates, for which more accurate and defensible numbers are sought. If readers have more accurate numbers for any of these values, please contact the authors with that information along with a reference citation for those more accurate values. Further, note that most values herein are specified to only about two significant digits — consistent with the current fidelity of interstellar flight estimates.

2.2.2. Stage II — Comprehensive Update to Interstellar Challenges and Prospects WBS

In Stage II, a web-based system will be created to allow subject matter experts from around the globe and from the span of relevant disciplines, to populate the WBS with their most recent data. Early drafts of technology development roadmaps will begin, with the intent to impose consistent methods of estimating the development durations.

The initial equations and analysis process will be refined, including running test cases with illustrative missions, payloads, and propulsion types. From there, the topological analysis methods will be adapted to this problem.

2.2.3. Stage III — Remaining Analysis and Recommendations

Stage III is where the analyses will be iteratively run and refined to ensure that it is meeting the needs of NASA and the interstellar flight community. The analyses should show which concepts might be the most advantageous, plus identify the most impactive supporting technologies. This includes identifying which knowledge gaps have the highest potential for improving the technology, and then how to solicit research to fill those gaps. This includes prospects for disruptive advancements and ancillary influences. And finally, the assessments will suggest a prudent portfolio of next-step research.

Once completed, technology roadmaps can be devised that are rooted in common standards to allow fair comparison of one roadmap to the other.

2.3. Outside Scope of Study

There are two activities related to interstellar flight whose assessments and recommendations are beyond the scope of study, "interstellar precursor missions," and "world ships."

Interstellar precursor missions are those that can be launched from Earth using foreseeable spacecraft technology and without needing substantial new infrastructure [22-33]. By "foreseeable technology" it is meant those technologies that are mature enough for mission trade studies. This study instead focuses on longer-term and farther-reach technologies whose performance measures are less certain. These precursor missions will, however, be used in this study as performance baselines and scaling examples. Further progress on precursor trade studies are a valuable aid to these longer-range interstellar flight assessments. An example of a precursor mission that would help resolve questions for future interstellar mission planning is the concept of a "Look-Back Mission." A look-back mission would test a suite of exoplanet instruments by looking back toward Earth at various distances to determine the effect of viewing distance and time on target for collecting meaningful information.

Another group of mission and technology concepts which are beyond the scope of this study are "world ships" — concepts for multi-generation, self-sustaining colonies of humans living aboard spacecraft headed toward potentially habitable exoplanets [34]. Even though such goals address the important motive of the sustained survival of humanity, they are out of scope since their major research goals involve sustainable habitats and cultures instead of propulsion.

(Sections 3 and 4 — the primer on the distinctions of interstellar flight, and the top-down technology challenges — are omitted for length; the complete text is at the source.)

5.3. Propulsion and Power Concepts

Table 4 lists concepts for interstellar propulsion and power. The tactic taken here is to first sort the prospects as a one-part or two-part system (just the spacecraft, or the spacecraft plus some base support, like laser systems). Thereafter sorting is by thrusting method, and then by major power source. To reveal systems with multiple stages that are not captured by the concepts' more familiar name, columns will be included to encompass those elements. The final version of this table (spreadsheet) will include:

• Sorting Category
• Concept Name, and Abbreviated Description
• Concept Date
• Reference Citation
• Interstellar Era (see Section 8.1 for definition): 1. Era of Precursors; 2. Era of Infrastructure; 3. Era of Breakthroughs
• Propulsion Type Analysis, IP-OM, RP-OM, RP-XM, IP-XM, see Section 8.4

These remaining columns will be added to the Stage II work:

• Base System, when applicable: source of power (Earth based electrical grid, in-space solar, or nuclear); energy conversion method; output power to spacecraft; thermal radiators
• Intermediate Base System Components (lens, beamed particles)
• Spacecraft System: power receiver (if applicable), with conversion to thrust, conversion to operating power, thermal radiators; onboard power source, being propellant self energy (chemical fission, fusion, antimatter), separate primary power generator (e.g., beamed power receiver, RTG, fission reactor), thermal radiators; thruster type, with energy conversion, key components (e.g., magnetic nozzles), thermal radiators
• Other system performance measures (see variables list, table 6)
• Technology Readiness Levels (TRL) of system elements performance as proposed (list of elements)
• Comparative TRL-6 performance levels of those same elements (if not at TRL-6)

Devising a means of sorting the information was a challenge. All of the following initial sorting methods were attempted with difficulties encountered with each. This final system (one- or two-part system, then thrusting method, then power) is still not free of confusions, but it was the least problematic of the following sorting methods:

• Traditional Concept Discipline: where the breakouts start at the level of sails (solar or beamed), rockets (chemical, electric, nuclear) and propulsion physics (spacedrives, FTL). Though familiar, it only draws attention to one key element, rather than reflecting on the broader functionality.
• Primary Power Source: This gets ambiguous when there are two power conversions (e.g., solar-to-laser, laser-to-sail).
• Primary Power to Spacecraft: This gets ambiguous for concepts whose key elements are a power source in one mode and a reaction mass in another concept (e.g., solar photons).
• Thrusting Method First: Ambiguities encountered with crossover of power source and reaction mass, especially between onboard and externally supplied systems.
• Primary Reaction Mass: Ambiguities encountered between energetic propellants and reaction masses that require a separate source of power to accelerate them.
• Technical Maturity: This is not a constant. This is a factor to track over time with each concept.
• Performance Level: There is no accepted ranking on performance level since those are mission specific as well as being a non-constant discriminator.

Table 4. Interstellar Power and Propulsion Prospects

Columns are: concept name (description) · date · reference · era · type.

I. Independent Spacecraft

I.I. Photon Momentum

• Photon Rocket · 1953 · Sänger · IP-OM
• Dynamical Casimir Effect (vibrating mirror) · 2009 · Maclay and Forward · era 3 · IP-OM

I.II. External Particle and Field Interactions

• Electric Sail and Stellar Winds · 2005 · Pekka Janhunen · era 1 · IP-XM
• Magnetic Sail and Stellar Winds · 2000 · Winglee · era 1 · IP-XM
• Plasma Magnet and Stellar Winds · 2013 · Slough · era 1 · IP-XM
• Alfven-wave plasma propulsion · 1996 · Moore, R. · IP-XM
• Plasma Wave · 2013 · Gilland · IP-XM
• Interstellar Ramjet · 1960 · Bussard · IP-XM

I.III. Propellant With Energy

• Nuclear Fission Pulse Propulsion · 1950 · Teller-Ulam · IP-OM
• Fission Fragment Rocket · 1988 · Chapline · IP-OM
• Pulsed Fission-Fusion (PuFF) Propulsion · 2017 · Adams, R.
• Enzmann (3MT frozen deuterium ball to fusion rocket) · 1964 Enzmann, 1973 Duncan · IP-OM
• BIS Daedalus (pulsed fusion, inertial confinement fusion) · 1978 · Bond · era 2 · IP-OM
• Vista Inertial Confinement Fusion · 1987 · Orth · IP-OM
• Project Longshot (fission reactor power, fusion pulse propulsion) · 1988 · era 2 · IP-OM
• Project Icarus (pulsed fusion, inertial confinement fusion) · 2011 · era 2 · IP-OM
• Continuous Electrode Inertial Electrostatic Confinement Fusion · 2017 · Sedwick · IP-OM
• Fusion Driven Rocket (direct conversion) · 2017 · Slough · IP-OM
• Gradient Field Imploding Linear Fusion Propulsion System · 2017 · LaPointe · IP-OM
• Multi-stage fusion rocket · IP-OM
• ICAN-II, positron catalyzed fission fusion · 1998 · IP-OM
• Antimatter-Catalyzed (pulse) Fusion (AIM star) · IP-OM
• Antimatter-Matter Annihilation Propulsion · Forward · IP-OM

I.IV. Power System to Expel Reaction Mass

• Solar to Electric Ion Propulsion · era 1 · RP-OM
• RTG, Ion · 2011 · era 1 · IP-OM
• Nuclear Electric propulsion · IP-OM
• Nuclear Thermal Propulsion · IP-OM
• Gas Core Nuclear Reactors · Guven · IP-OM
• Tachyon Rocket · 1996 · Cramer · era 3

I.V. Inertia and Inertial Frame (gravitation)

• Negative Mass Propulsion · 1957 · Bondi-Forward · era 3 · IP-OM
• Mach Effect Thruster · 1994 · Woodward · era 3 · IP-XM

I.VI. Spacetime Warping

• Alcubierre, Warp Drive (expansion/contraction) · 1994 · Alcubierre · era 3 · IP-XM
• Warp Tunnel · Krasnikov · era 3 · IP-XM
• Slipping · Natario · era 3 · IP-XM

II. Spacecraft Plus Supporting Base

II.I. Photon Momentum

• Forward's Beamed Energy Sails (including Starwisp) · 1984 · Forward · era 2 · RP-XM
• StarLight · 2016 · Lubin · era 2 · RP-XM
• Breakthrough StarShot · 2016 · era 2 · RP-XM

II.II. External Particle and Field Interactions

• Particle-Beam Pushed Plasma Magnet · Greason · era 2 · IP-XM
• Sailbeam, beam of self-steering impact masses · Greason · era 2 · RP-XM

II.III. Propellant With Energy

• Fusion pellet runway (Bussard Buzz Bomb) · 1997 · Kare · era 2 · RP-XM
• Antimatter ablated Light Sail · 2005 · Jackson · (mix)

II.IV. Power System to Expel Reaction Mass

• Solar Thermal propulsion · RP-OM
• Laser powered ion propulsion · Brophy · era 2 · RP-OM

II.V. Inertia and Inertial Frame (gravitation)

II.VI. Spacetime Warping

• Gravitational Dipole · 1963 · Robert Forward · era 3 · SP-XM
• Traversable Wormholes · 1988 · Thorne, Visser · era 3 · SP-XM

(Sections 5.4 through 7.4 — the 2017 workshop review, the work breakdown structure, and the mission-choice variables — are omitted for length; the complete text is at the source.)

7.5. Figures of Merit

What, ultimately, is most important to mission planners? Presumably, it's having an interesting enough mission that will appeal to a number of stakeholders and be accomplishable within a reasonable time and expense. To make these explicit and measurable so that mission and propulsion options can be compared, the following variables are introduced:

Mission Composite Value, W (#): The value of a mission is defined here as a function of the interest in the destination, the number of motives answered by the mission, and the fidelity of the data that the mission will collect. In principle, this is envisioned as a weighted sum of the "Destination Interest," Di, "Mission Ambition," Wa, and "Sum of Motivations," Wm. The higher the value, the better.

Total Mission Expense, Etm, (J): This is a measure of the resources required to build, launch, and operate the mission. Instead of using financial cost, whose estimations require subjective predictions, the measure will be in terms of the energy, a fundamental, calculable physics parameter shared by all methods. The energy to "build" the mission will be in terms of the required infrastructure, while the energy to launch the mission will be in terms of propulsion energy. Since the expense of operating the mission after launch is assumed to be much smaller than the other factors, it will not be quantified. Specifically, then, "Total Mission Expense," Etm, (J) is the sum of "Total Infrastructure Energy," Eti, (J) and "Total Propulsion Energy," Etp, (J). The lower the value, the better.

Total Project Duration, Ttp, (yr): As mentioned previously, this is a measure of how much time remains between now and the point where all the data has been transmitted back to Earth. The lower the value, the better.

Mission Efficiency, We, (%): The final figure of merit is the efficiency of the mission, which is defined here as the ratio of the kinetic energy imparted to just the payload, Epy, and the Total Mission Expense, Etm. The higher the value, the better.

An alternative definition of Mission Efficiency could be in terms of the Data Volume, Iv, and perhaps Data Fidelity, If, delivered per Total Mission Expense, Etm. In that case, the prior definition of Epy/Etm could be called "Vehicle Efficiency."

It is anticipated that the Stage II and III analyses will reveal which of these factors are more or less impactive of the technology requirements. Thereafter, choosing the relative importance of the options can be informed choice.

8. Methods for Equitable Comparisons

Tied to the mission choices, it is necessary to measure the associated propulsion performance and the expense to deliver that level of performance. To make these calculations equitable across differing missions and differing propulsion methods, the basic strategies are: 1) start with comparing technologies that are at comparable readiness levels before advancing to compare across significantly different readiness levels, 2) compare different propulsion and power concepts using common payload and mission scenarios, 3) devise a common means to measure the expense of building the mission hardware, 4) measure the performance of the disparate propulsion and power approaches using fundamentally common units, and finally 5) devising methods to compare technologies that are at different readiness levels and advancing at different rates.

8.1. Distinct Eras of Interstellar Flight

A starting point is to separate concepts that are at substantially different readiness levels. After reviewing the span of mission concepts and technology prospects, they can be divided into these distinct eras of interstellar flight:

1. Era of Precursors
2. Era of Infrastructure
3. Era of Breakthroughs

The major difference between the first two eras is the degree of infrastructure needed to support the mission. The distinction of the third era is that it requires further advances in physics (whose infrastructure needs are temporarily unknown). Comparisons within these eras are more easily achieved than comparisons across these eras.

8.1.1. Era of Precursors

This era refers to missions that can be launched from Earth with foreseeable technology and without needing substantial new infrastructure. By "foreseeable technology" it is meant those technologies that are already at, or above TRL-6. Examples in this era include:

• Voyager
• Heliopause Interstellar Probe concept of 1999 [22, 24, 25]
• Innovative Interstellar Explorer concept of 2006 [23]
• Interstellar Medium Mission concepts 2015 [29, 31]

For assessment purposes, the performance projections of those technologies are accurate enough to proceed to mission trade studies. Thus, they are not subject to the assessment methods of this report. These concepts are however used as baselines and scaling examples in this study.

8.1.2. Era of Infrastructure

The era of infrastructure refers to propulsion and power concepts that are rooted in the established laws of physics and are a matter of further engineering. This is where the bulk of interstellar propulsion concepts reside. The reason this is called the era of infrastructure is because even the smallest payload example from this group (1 g) requires substantial new infrastructure, specifically a 100 GW laser array spanning 1 square km. Examples of concepts in this era include:

• Project Daedalus, 1978 [49]
• Forward's Microwave Staged Lightsails, 1984 [48]
• Project Icarus (started 2009) [79]
• Breakthrough StarShot, 2016 [7, 40, 57-62]

The performance projections of these concepts are ambitious and still unproven, making the use of traditional trade studies unreliable. The other unknown for each concept is the remaining time required to mature its suite of technologies to mission readiness. And lastly, these concepts assume that the required infrastructure already exists — but there are no roadmaps yet to develop that infrastructure. The process for estimating the dependency of the mission architectures on infrastructure is explained in Section 8.3.

8.1.3. Era of Breakthroughs

The era of breakthroughs refers to concepts aimed at the highest impact, revolutionary performance gains that go beyond extrapolation of existing technology. This requires further advances in physics. A starting reference for the span of these concepts and the next-step research required to further assess them, is the book, Frontiers of Propulsion Science [13]. Examples of concepts in this era include:

• Negative mass propulsion, 1957 [80, 81]
• Propellantless thrust via inertial fluctuations (1994), now called "Mach Effect Thruster" [82-86]
• Spacedrives, in general [87]
• Dynamical Casimir Effect, 2004 [88]
• Traversable wormholes, 1988 [11, 13 ch.15]
• Warp drive, 1994 [12, 13 ch.15]
• Faster than light communication [13 ch.16, 89-91]

In addition to propulsion and power breakthroughs, breakthroughs in communication can also play a powerful role in enhancing the mission — especially considering the impact on total mission duration. If FTL communication was possible, then the mission duration would be shortened up to a year for each light-year distance. The possibilities of FTL communication are discussed in the literature, including some quantum and other communication systems [89-91].

For assessment purposes, some of the breakthrough concepts have matured to the point where their propulsive energy can be calculated. For others, hypothetical analogs will need to be specified. In the case of generic spacedrives, for example, the propulsive energy can be modeled with basic kinetic energy and an efficiency factor for energy conversion. Section 8.4.4 describes the initial attempts for making estimates of this group.

Regarding their infrastructure dependence, this cannot be accurately determined until after they have been sufficiently advanced to TRL 3.

(Sections 8.2 through 8.4.3 — the baseline mission scenarios and the analysis methods for the three other propulsion types — are omitted for length; the complete text is at the source.)

8.4.4. Type IP-XM: Internal Power and External Reaction Mass

This type refers to systems which receive reaction mass from external sources but carry their own energy. This is analogous to aircraft engines, where air is the reaction mass, and the energy source is the fuel. In terms of interstellar flight, this includes, for example, Bussard ramjets that gather protons and accelerate them with the help of onboard energy [101], and spacedrives that convert some form of stored energy into propulsive motion (kinetic energy) using as-yet-unconfirmed physics [82-87].

This category also includes "drag devices" such as magnetic sails (magsails) or plasma magnet sails. These devices are possible solutions to the challenge of braking at the destination. Without braking, the flyby time is very short. See table 8 in Section 7.2.1 for examples. After a flight time of decades, a flyby time of just hours seems disproportionate and would limit the fidelity of observations.

Therefore, the problem is not only one of how to get up to speed, but how to get rid of the speed. Drag devices — devices that serve the same purpose in interstellar flight as do aerobrakes and parachutes in planetary exploration — in principle dissipate the kinetic energy of the spacecraft against something else (usually, the ionized gases in the interstellar medium).

Finally, there is a class of potential "plasma wave" drive concepts in which traveling waves are launched into the interstellar medium at a velocity far below the speed of light, and the resulting reaction force propels the spacecraft [102]. These are usually low specific power (low thrust) drives but they use the surrounding medium as reaction mass. Such an ability is similar to the goal of a spacedrive, but using existing physics.

Depending on the nature of the reaction mass, the analysis methods vary. For things like plasma, interstellar protons, or stellar wind, analogies to aircraft propellers can be used. For new breakthrough propulsion physics (devices that can be viewed as using inertial frames or the properties of spacetime as an effective reaction mass), then other techniques are suitable.

Plasma Reaction Mass. For concepts that interact with plasmas or interstellar protons, the fundamentals of conservation of energy and momentum result in the "propeller equations" familiar for propeller and air-breathing jet operation within an atmosphere can be used:

P = ½ · (dm/dt) · ((Vsm + ΔVt)² − (Vsm)²) (Eq. 22)

where dm/dt is the mass flow rate of media through the thruster (kg/s), Vsm is the velocity of the spacecraft through the media (m/s), and ΔVt is the delta V of the media by thrusting effect (m/s). And where:

F = (dm/dt) · ΔVt (Eq. 23)

In the case of interacting with the interstellar medium, during the dominant part of the trajectory, the speed of the spacecraft through the media, Vsm, is much higher than the delta V that the thrusting effect can impart to a portion of that media, ΔVt. In that case, the power equation can be approximated by the much simpler form which illuminates the fundamental truth of all such propulsion — that the faster the spacecraft, the higher the power requirements:

P ≈ F × Vsm (Eq. 24)

For acceleration using plasma reaction, Propulsion System Specific Power (Psp) of these propulsion systems is still very important; usually this is dominated by the power supply carried with the spacecraft. Because Type IP-XM systems are not limited by their stored reaction mass, the energy content of the power supply is what ultimately controls the ΔV available.

ΔV = (F / Msl) · Ta (Eq. 25)

where ΔV is the change in velocity imparted to the spacecraft (m/s), Msl is the spacecraft launch mass, being payload plus spacecraft empty mass (kg), and Ta is the acceleration duration (s).

This applies both to thrust and drag devices — however, while available power supplies tend to have low Psp and hence low accelerations, some drag devices offer high decelerations because they are dissipating power into the interstellar plasma rather than consuming it. Drag devices may be power-limited (in which case they can be modeled as thrust devices), or they may have a constant "ballistic coefficient" like a parachute, in which case their drag varies with the square of the velocity through the medium, Vsm.

In the case where Vsm is high, one also must check whether thrust, F, needs to be replaced by a "net thrust" or thrust minus drag. It may seem counterintuitive that one would be concerned with "drag" in the thin interstellar medium, but devices to collect reaction mass, almost by definition, have some way of interacting with the interstellar medium and so do offer drag. Neglecting this led to some early over-estimates of performance of some types of Bussard ramjets, for example. While the means of estimating drag is rather specific to the particular device in question, it is usually sufficient for comparison purposes to check if it is significant at the speeds in question (it often is not), and to account for it only in cases where this is not so.

Spacetime and Inertial Frames as Effective Reaction Mass. Concepts like negative mass propulsion, the Mach Effect Thruster, and the warp drive all face the challenge of an ambiguous reaction mass. Regardless of those specifics, the techniques to estimate their performance can be crudely estimated in terms of converting stored energy into kinetic energy of the spacecraft, with some conversion efficiency, η, and a Propulsion System Specific Power, Psp, (W/kg). Absent of better values, comparable efficiencies and specific powers from other concepts can be considered as a starting point.

For the special case of the warp drive, energy conversion equations exist [13 p.491, 103], but there are no equations yet that remotely resemble Propulsion System Specific Power.

8.5. Estimating Comparative Rates of Advancement

The timescales for interstellar missions are comparable to prior technological revolutions (figure 3). Thus it is entirely possible that a revolutionary technology will emerge and surpass the performance of a more evolutionary technology already in development. But how does one predict if, and when, that might happen and what to do about it?

Recall that the objective of this study is not just to reveal which propulsion concepts might be the most advantageous (and under which circumstances), but also to identify the most impactive supporting technologies to guide the selection of a prudent portfolio of next-step research. This requires developing methods to estimate, not only the impact of a particular technology, but also estimate when that technology might be ready for mission commitment.

While it is not possible to predict the future, technology developments do follow patterns that can be used as a guide. First, there are the "Technology Readiness Levels (TRL)," that are both a way to assess the readiness of a given technology as well as identifying what further steps are needed to advance that technology to mission readiness. Second, there is the "S-curve" pattern to technological improvements and revolutions that suggest how to model that process [41]. Another source under consideration is the "Technology Forecasting and Readiness Assessment" methods of Darryl Web, et al [104].

(Sections 8.5.1 through 10.3 — the technology advancement models, sample data plots, and the Stage II portal plans — are omitted for length; the complete text is at the source.)

10.4. Concluding Remarks

Any challenges to the assumptions and initial estimates in the report are welcome. While the focus of the first stage work was the development of the overall structure for the data to be subsequently collected in later phases, some values have been specified as starting estimates, for which more accurate and defensible numbers are sought. The online repository is the envisioned mechanism to keep these values up to date. Prior to that system being available, readers who have more accurate numbers for any of these values, please contact the authors with that information along with a reference citation for those more accurate values. Further note that most values herein are specified to only about two significant digits — consistent with the current fidelity of interstellar flight estimates.