Laser Propulsion in Space: Fundamentals, Technology, and Future Missions

Space launches have evoked the same vivid image for decades: bright orange flames exploding beneath a rocket as it lifts off and thunders into the sky. An alternative acceleration system could reshape that vision forever, with rockets leaving their energy source on the ground... or in space. Laser Propulsion in Space: Fundamentals, Technology, and Future Missions takes readers on a comprehensive journey from the theoretical overview of propulsion fundamentals, to reviews of current projects involving high-power CW fiber lasers and energetic mm-wave sources with their ongoing and potential end-use applications in beamed energy propulsion (BEP). Written by experts in the field, this mathematically sound reference text also highlights graphical solutions of equations’ results, as well as case studies with worked-out examples, making this book an invaluable compendium for students, researchers, technology developers and futurists in understanding the promise and challenges of this emerging technology.

• Covers beamed energy propulsion advances
• Highlights state-of-the-art BEP applications of LEO debris removal, suborbital and orbital launch, solar system exploration, and interstellar lightsail probes, as well as advances in related photon source technologies and infrastructures
• Includes opinion sections explaining why we as a technical society should care about each chapter’s topic and the considerably good outcomes that can be achieved with laser engines
• Is accompanied by a website with video clips and other ancillary materials to enhance insight

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 4, 2024
• ISBN: 9780443159022

Book preview

Preface

Claude Phipps     

Overview

Why should we as a technical society care about the topics of each chapter of this book and the good outcomes that can be achieved with high-intensity CW or pulsed laser radiation in space? In this book we have attempted to demonstrate that laser space propulsion is critical for the future of space transport.

Chapter 1 provides an overview of laser space propulsion, giving the basic theory and some tantalizing applications, such as launching satellites into orbit from Earth with 25–40% energetic efficiency, rather than the 4% that can be achieved with chemical rockets. We see how diffraction comes into play in space, to determine the laser focal spot size and maximum fluence on target given a pulse energy. We discuss momentum coupling coefficient and show that there are several different optimum values depending on what we want to achieve. We show that laser launch cost can be as low as $100/kg. We show how the vapor and plasma regime predictions can be combined to give an advance prediction of Cm, depending on the laser parameters of intensity, wavelength, and pulse duration. We also discuss space debris removal by lasers and show that incomplete data is a problem for this field. It is clear that further laser development is the main practical issue for laser propulsion. Megawatt average powers are needed to launch a significant amount of mass into orbit or remove it from orbit. However, such laser powers have been achieved in the past – all that is missing is the will to proceed.

Chapter 2 reviews the challenging and exciting Breakthrough Starshot program, which aims to accelerate a few grams object to 20% of the speed of light in a few minutes, returning a photo of the Alpha Centauri region to us in about 26 years. Through this program, we may learn a lot about the origins of life on Earth. The Breakthrough Initiatives include more aspects (Breakthrough Listen and Breakthrough Watch) than the Starshot element. Again, laser development is the main practical issue, not in the sense of the individual lasers involved, but in solving the problem of phasing the outputs of 100 million such lasers to within a fraction of a wavelength to create one coherent beam.

Chapter 3 gives us the Starshot system model – including, as it must, the effects of relativistic speed of a lightsail – propelled few grams of payload. For example, heating of the sail leads to a blue-shifted forward emission. Necessary developments prior to system operation include a precursor mission, and a beacon satellite to provide phase information to the ground-based laser array, not to mention the 200 GW laser itself. However, the laser is composed of off-the-shelf, 20 kW laser units and the technical problem is to phase them together to form a single beam. Costs are computed for several missions with varying payload, ranging from 10 kg at 0.0001 c to 3.5 g at 0.2 c final velocity, with Alpha Centauri as a goal. For the latter mission, according to Parkin, lasers, optics and storage are estimated to cost 8.8B$. Each Starshot launch costs 6M$. Costs would be $810M/kg for a trip at 0.99 c to Trappist-1 (41 light years distant) [Table 3.12]. Passengers would experience a journey lasting 2.8 years, but it would take 41 years to send a message home, and all your friends would have aged 38 years more than you.

Chapter 4 concerns a different problem: the role of lasers in long-term space operations. The exponential increase in the number of debris leads to a serious problem for future space sustainability. The author states that lasers can be the key to assure long-term space sustainability. The sources of these debris are catalogued, whether from antisatellite tests, collisions, or explosions. Debris can pollute astronomical observations by making streaks across the field of view, although these effects can be mitigated. A number of potential regulations are listed to avoid generation of new debris. The problem here is the lack of enforcement. Given this problem, accurately predicting collisions followed by nudging using lasers or electron beams is a solution. Just-in-time collision avoidance (JCA) is the best solution. Another solution is de-orbiting debris in low Earth orbit using a ground-based high-energy laser. Finally, in geosynchronous orbit, re-orbiting large debris is a solution.

Chapter 5 gives the state of the art in high-power 1-micron wavelength CW lasers, diode-pumped solid-state lasers and high-power fiber lasers. The latter are limited in power by nonlinear optical effects (stimulated Brillouin scattering, stimulated Raman scattering and the optical Kerr effect), transverse mode instabilities and thermal loading. To get around these problems, beams from individual fibers can be combined in various ways. Methods of phasing an array of fibers to obtain a single beam are extensively discussed. The National Ignition Facility, a topic of recent intense interest is discussed.

Chapter 6 discusses high power laser engines in detail. These will have thrust ranging from 1 – 100,000 newtons. These include pulse jets and ramjets operating in the atmosphere. Myrabo's Lightcraft is discussed. Finally, the principles of laser ablation propulsion are outlined. The effect of nozzles is described, as are laser supported detonaton waves and laser sustatined subsonic radiative waves. Methods of suppressing plasma instabilities are described. Various materials (such as POM, polystyrene, carbamoyl hydrazine, metaldehyde and dihydorybenzene) for laser ablative propulsion and their advantages are listed. The aerospace laser-propulsion engine is described.

Chapter 7 discusses the radical transformation in propulsion that will be realized with Photonic Power projection. What does this mean? Professor Lubin's chapter demonstrates many aspects, ranging from flights using beamed power converted to electrical power to drive an engine (indirect drive) to low-mass relativistic missions (direct drive), as described in more mathematical detail in Chapter 3. This chapter includes a very useful roadmap (Fig. 7.2), a plot of kinetic energy vs. mass (Fig. 7.3), and a strategy for beam synthesis (Fig. 7.5) – an all-important aspect of deliverable brightness. Fig. 7.9 gives a calculation of required array diameter for pure photon propulsion of a 1 g and a 1 kg payload. Finally, Fig. 7.12 shows the cost in billions of dollars for various missions. These costs are a critical limit for relativistic missions. Chapter 3 considers much larger values of beta, which have far larger costs.

Chapter 8 shows us how mechanical coupling coefficient and kinetic efficiency of laser propulsion change with single or multiple laser pulses and ablating material. Momentum coupling and kinetic efficiency are the crucial characteristics of any propulsion system. For the working medium, polymers and metals are considered as well as liquids, including liquid hydrogen. Ways of discouraging droplet formation from liquid ablation fuels are described. Energetic ablatants (such as GAP) in which the ablatant contributes additional energy to the ablation process are described. Data provided in support of conclusions of this chapter include momentum coupling coefficient fs. Fluence for titanium bulk and foil and porous targets, the effect of confined layers in increasing Cm, and the variation of Cm with wavelength and pulse duration on aluminum and copper. The electrical efficiencies of various lasers are considered, in order to estimate overall power to thrust efficiency for laser propulsion. Methods of space debris removal using lasers are summarized.

Chapter 9 discusses a truly novel laser engine technology (LITA) in which the laser-powered projectile travels inside a tube. This geometry has been shown to enhance thrust over free space flyers. Detailed geometries of such a flyer which would use an inert driver gas are provided. Also, performance of such a flyer using xenon as the inert gas is presented. A different solution discussed is LITA with an on-board ablator, such as polyacetal, and a flyer using a tube with ablator on the interior wall.

Chapter 10 describes the first laser-powered flight back in 1972, by Prof. Leik Myrabo, which reached 72 meters altitude. It also discusses all aspects of power beaming, including microwaves as well as lasers, going back to the year 1964. A flight powered by a 1.25 kW microwave source in 1992 is discussed. A separate section by Greg Maryniak considers the effect of ideas for competitions (such as a Mars Helicopter Challenge) on developments in laser propulsion. A total of $25M are available over five years, for four prizes in the topic of Flight on a beam of light. Obviously, laser propulsion can approach zero carbon flight, depending on the ablative material, and of course pure photon propulsion contributes no pollution at all, a very attractive feature for its benefits for climate change. The chapter also discusses the ideas of photovoltaic arrays to receive laser energy and convert it to powering atmospheric flight.

Chapter 11 treats space policy, which is of growing importance considering the number of nations wishing to use lasers in space.

Companion site/Ancillary materials contains all the Ancillary Materials. In that chapter, and its online adjunct, we provide several worked cases as well as three actual videos of laser propulsion.

Companion Web Site:

https://www.elsevier.com/books-and-journals/book-companion/9780443159039

Synthesis

Laser space propulsion is the most exciting new approach to propelling objects in space. Mass ratios up to 40–50% can be achieved for launch to low Earth orbit using pulsed laser ablation, compared to the 4% possible with chemical rockets. The reason for this is much higher exhaust velocity in laser ablation, which comes from much higher plasma temperatures possible with repetitive-pulse lasers. Also, using just light pressure rather than ablation, relativistic velocities (0.2 c and above) can be achieved. In this approach, CW lasers will play a significant future role. The Breakthrough Starshot initiative will use 200 billion watts of CW power to launch a few gram payload to the Centauri system, using photon pressure rather than ablation. Through this program, we may learn a lot about the origins of life on Earth. After its success, we may well fly through the galaxy at relativistic speeds.

To achieve all this, laser engines in the MW to 200 GW range must be developed, as do the high-power lasers that drive them. It is important to realize that MW-level lasers were developed in the 60's and 70's – all that is needed is the will to proceed. Our chapter on the state-of-the-art in high-power lasers is encouraging in this regard. The book contains three chapters on the design of laser engines.

To be successful, laser space propulsion needs to be a truly international development. Our chapter on space policy (the dual-use dilemma) emphasizes the necessity of confidence-building measures, and how big programs like CERN, ISS and ITER have been developed in this way, and international conflicts avoided. The chapter also points out that massive scientific collaboration can break the gridlock of global governance. The COPUOS process is mentioned as a way to develop guidelines for such cooperation. This chapter claims that the role of large technical systems has been understudied.

I am confident you will find this the most interesting book you have read on future space propulsion! The detailed studies and illustrations provide more information about the future of space propulsion than any other book in this field.

1: Basic theory of laser propulsion⍟

Claude R. Phipps    Photonic Associates, LLC, Santa Fe, NM, United States

Abstract

Laser Space Propulsion (LSP) is the most exciting development in space propulsion since the first launch of Earth satellites. LSP is versatile and more energetically efficient than chemical rockets and can propel objects into space over great distances at the speed of light. Three distinct types of LSP have been reported. In the first, we only use the pressure of light. In the second, we use light with very high intensity to make a high-temperature jet on the propelled object. In beamed energy propulsion, we use a ground-based high-power laser to vaporize and heat a propellant in tanks on a remote spacecraft. Ablation is easy to understand. Momentum is mass times velocity. In high-intensity, repetitively pulsed laser ablation, we may only ablate nanometers of material, but the jet velocity can easily be 40 km/s. If the target is aluminum, 10 nm of material removed at 40 km/s in a 4 cm diameter spot at 11 Hz produces a force of 1.14 N, enough to lift 0.12 kg. Laser ablation propulsion has many applications within the solar system, including orbital debris removal, just-in-time collision avoidance, spacecraft propulsion, and even planetary defense. Much smaller force densities are present with pure photon propulsion, but since no material is used, flights can be infinitely long and, ultimate velocity extremely high. This option has become recently interesting because of the Breakthrough Starshot project, which proposes to explore interstellar space with a first flight that accelerates to 20% of the speed of light and passes by our neighbor star Alpha Centauri in our lifetime. Financial support for the large lasers required for most of these applications is critical. However, at the few-watt level, we describe a laser microthruster for satellites using a small diode laser. The concept of beamed energy propulsion is discussed briefly. Copious illustrations, case studies, and other supplementary materials are provided.

Keywords

Space propulsion; space debris; laser momentum coupling; specific impulse; high power laser; diode pumped solid state laser; laser plasma thruster; just-in-time collision avoidance

Table 1.1 lists all the symbols and abbreviations used in this chapter.

Table 1.1

1.1 Introduction

Three distinct types of laser propulsion in space have been reported. In the first, we only use the pressure of light. In the second, we use very high intensity light to make a high temperature jet on the propelled object (laser ablation propulsion, Fig. 1.1). In the third type, we use a ground-based high-power laser to vaporize and heat a propellant in tanks on a remote spacecraft.

Figure 1.1 Illustration of the laser ablation propulsion. Courtesy of C. Phipps.

So far, the record for laser propelled flight was that of Myrabo [1] [see Chapter 10], which reached an altitude of 72 m, using a repetitively pulsed CO2 laser to ablate an aluminum flyer. Much more dramatic flights are planned, both with pure photon and laser ablation propulsion.

Ablation is easy to understand. Momentum is mass times velocity. In high-temperature ablation, we may only ablate nanometers of material, but the jet velocity can easily be 40 km/s. If the target is aluminum (density 2700 kg/m³), 10 nm of material removed at 40 km/s in a 4 cm diameter spot gives a momentum of 1.14 mN-s per laser pulse but, if the laser pulse repetition rate is 1 kHz, that amounts to a force of 1.14 N, enough to lift 0.12 kg. In this example, laser pulse energy W is 38 J, laser optical power on target is 38 kW. These numbers are derived in Case Study 2 at the end of this chapter, which is based on equations in this section. Whether we have ablation or just photon pressure, the same basic equations apply.

1.2 Theory of laser ablation propulsion

The impulse coupling coefficient Cm is the ratio of momentum delivered to the target to the incident beam pulse energy W, or of surface pressure to incident intensity, with dimensions of N/W. The advantage is a much higher exhaust velocity than with chemical rockets.

(1.1)

For photon propulsion, in total reflection,

(1.2)

For comparison, laser ablation Cm on aluminum is typically 30 μN/W [2], i.e., about 4,500 times larger. A 10 kW laser perfectly reflected from a solar sail would generate 67 μN of thrust, about 0.00024 ounces of force. The change in target velocity

(1.3)

where the target geometrical efficiency factor takes account of the fact that the ablation thrust vector is the sum of contributions normal to each facet of the target and not necessarily antiparallel to the laser beam. The plasma jet velocity is related to the other parameters by

(1.4)

From the units, you can see that this is a velocity [N-s/J]*J/kg], momentum/mass. Eq. (1.4) is a useful relationship, which allows the estimation of a quantity that is difficult to measure (vE) from the combination of two that are easy to measure. Normally, Cm is measured from the motion of a normal or torsion pendulum in a vacuum to which the target material is mounted. See [3] and [4] for photos of the two types of pendulums. The latter is reproduced in Fig. 1.2.

Figure 1.2 Vacuum thrust stand setup.Power supply is on board the thrust measurement bar with the thruster, and command and data transfer use an IR data link, so that the only mechanical connection with the outside world is the 254-μm diameter steel fiber supporting the bar. An interferometer based on a retroreflecting corner cube is the key to resolving rotation of the bar. Motion damping is provided by a flag immersed in diffusion pump oil. The sensitivity is 25 nN in vacuum. Courtesy of C. Phipps.

Kinetic efficiency is 100% for photon propulsion on a reflective surface:

(1.5)

For ablation, thickness removed is given by

(1.6)

Expressions for ηAB in photoablation are:

(1.7)

The quantity Q is given by

(1.8)

Eq. (1.7) shows that the product CmVE cannot be larger than 2, because ηAB cannot be larger than 100% (chemically energetic targets are the exception). Consequently, CmIsp cannot be larger than 0.20.

Lest all these details seem excessive, each of these relationships is valuable for calculating ablation problems, depending on which variable you know.

It is useful to realize that Eq. (1.7) is an approximation, because vE is actually a velocity distribution:

(1.9)

where the accurate relationship is

(1.10)

We have shown that ψ∼1.15 in high-intensity ablation plumes [8]. For the sake of simplicity, it is usually equated to 1 in this chapter, like , except noted otherwise. Overall electrical efficiency as an electrical device producing kinetic energy for the laser rocket is given by

(1.11)

Ablation fuel usage rate is

(1.12)

so that ablation fuel lifetime

(1.13)

is proportional to Isp². The units of Isp are seconds [cm/s]/[cm/s²]. This is why we use laser plasma propulsion in the first place, because very high values for Isp are possible. The best that a liquid fuel chemical reaction can do is about 550 seconds of Isp [the HF reaction, which gives 15 MJ/kg], whereas with laser plasmas, it is easy to get 4,000 s [Fig. 1.3] [5]. Eq. (1.12) says this gives fuel lifetime 50 times longer than it is possible with chemical rockets.

Figure 1.3 Experimental data for Au target vs. incident intensity with 4.5 ns, 1064 nm wavelength laser.Ablation efficiency approaches 100% near 1 MJ/m ² . C m ranged between 10 and 100 N/W. Specific impulse based on measured mass loss (solid red line) was as large as 3,700 seconds, and could be varied between 0.2 and 3.7 ks by varying incident intensity. I sp based on ion TOF measurements agrees roughly with I sp from mass loss up to 150 GW/m ² , but the two trends diverge at higher intensity, as ablation efficiency drops to the 1% level and I sp based on ion time of flight goes to values as high as 8 ks. This plot also shows why optimum fluence is important. C m and I sp can drop very fast on either side of optimum. Note that 100% η AB was obtained within a factor of 2 range in fluence (dashed pink line). Courtesy of C. Phipps.

Much higher values yet are routinely obtained in inertially confined (laser) fusion work.

1.3 Fluence (energy density on the target)

Fluence is not a free variable. The optimum fluence is a little more than that required to ignite a plasma on the target [Fig. 1.3], varies roughly according to the square root of pulse duration [2],[6] for pulses with duration in the range of 100 ps–10 ms. Beam diffraction controls how much fluence you can deliver to a target at a certain range.

1.4 Pulse duration

Because optimum fluence decreases roughly with the square root of pulse duration, and high fluence demands expensive high-energy pulses from lasers, we want short (∼100 ps) pulses. Femtosecond pulses are not desired for several reasons. First, fluence for plasma ignition is constant below 100 ps, so pulse energy is not saved by shorter pulses. Also, energy is wasted by mechanical friction caused by shockwaves inside the target material with fs pulses. Finally, fs pulses cause spallation, in which the back side of the target is blow off by the intense shock wave created in the material.

1.5 Diffraction limits

According to an often-used rule of thumb [6], the product of pulse energy W and system aperture diameter squared (Deff²) required to deliver a fluence Φ at range z is

(1.14)

However, this is only accurate for focal spots that are much smaller than the aperture diameter. In general, one must start at the target with a desired beam waist diameter 2wo, and work back toward the laser source using the expressions

(1.15)

and

(1.16)

where z′ is measured backwards from the target in the direction of the laser source. When 2w=Deff, you have the accurate maximum range to your target, and you are done. It does not get any better than this, unless you use a shorter wavelength or a better beam quality factor M². This calculation can be done easily with an Excel spreadsheet [Fig. 1.4, Case Study 3].

Figure 1.4 Illustration of the difference between geometrical [Eq. (1.14)] and Gaussian focus [Eq. (1.15)]. Courtesy of C. Phipps.

1.6 There are several optima for Cm

This is not obvious [Fig. 1.5]! Cm can be chosen [2] by appropriately mixing ablation materials with different measured maximum Cm values. But what to choose, at what performance cost? A good reference for optimization is [7]. In designing a laser rocket to go from Earth to LEO, do you really want the highest possible Cm? No, because Eq. (1.7) says Isp will be very small and Eq. (1.13) says your fuel will be gone before you get there.

Figure 1.5 Optima for LEO launch. 10 dyn/W = 100 N/MW. Shown are the results of many computer simulations with a real atmosphere and gravity variations showing that mass, mass ratio and cost are optimal at different values of C m . In particular, m/M maximizes at C m  = 0 (infinite I sp ), mass delivered to Leo at 200 N/MW and cost at 300–400 N/MW. Here we assumed unit ablation efficiency. Used with permission from J. Laser and Particle Beams , Cambridge University Press.

Ref. [7] points out that there are actually three optima. These are: maximum mass mo delivered to LEO, maximum mass ratio mo/MT delivered to orbit [oddly, this is not the same thing] and minimum cost in J/kg in orbit. My apologies that in the year 2000 we were still using cgs units – suffice it to say that 1 dyn/W = 10 N/MW, units we like to use now. Optimum Cm for getting to LEO with minimum cost was about 300 N/MW [7] (fascinating to me then, and now). Note that energy expended by a frictionless elevator taking our target to 150 km is only 1.5 kJ/g [8], but accelerating to orbital velocity requires 32 kJ/g, for comparison with the red triangles in the figure. The energy required to bore through the atmosphere is about twice that.

What if ηAB is uncertain, but you still want to estimate delivered mass ratio? If we normalize Cm and laser power P to ηAB using

(1.17)

so that thrust F=CmP is independent of ηAB and laser power varies inversely as ηAB, then, in a force-free environment (no gravity or drag), we have

(1.18)

for the delivered mass ratio [8]. This is a really useful way of looking at laser-driven space travel, showing that if Cmo is small enough (or Isp high enough), and laser power Po is large enough, almost any mission is possible. This applies both for separated laser and target as well as for a system that carries the laser. The key is Isp.

1.7 Calculating coupling coefficients for a flight

What Cm values are desirable? It depends entirely on the mission, as we showed in Fig. 1.5. Warning: by far the best thing is to measure Cm and Isp for a given circumstance, but if no measured values are available and you do not have a National Lab behind you, there are ways to make pretty good estimates.

To calculate Cm for a laser ablation plasma, it is critical to consider both the vapor and the plasma regimes, otherwise you will get a wrong answer. How do you do this and how do you combine results involving such different physical domains?

To think about this problem, Fig. 1.6 helps [9].

Figure 1.6 Three regimes of laser interaction: heating, vapor ejection, and plasma ejection. Courtesy of C. Phipps.

1.7.1 Plasma regime

The pure plasma regime, which occurs at high laser intensity, has been treated precisely 10, based on a modified hydrodynamic theory for inertial confinement fusion.

(1.19)

(1.20)

and electron temperature

(1.21)

The ablation rate is

(1.22)

In Eq. (1.22),

(1.23)

The approximate ionic charge Z is determined from the Saha equation [11],

(1.24)

with Z = ne/ni and

(1.25)

In the limits of this theory, the product CmpIspp = 0.08 and ηAB = 0.40. Eq. (1.19) describes many very high-intensity experimental results well (Fig. 1.7, from [10]), for pulse durations from ms to ns and ranging from the UV to 10.6 μm. Bumps in the plot are from changing ionic charge state Z as intensity varies. Note, however, that as intensity goes to zero in Eq. (1.19), Cmp goes to infinity. This can not be physical. A separate vapor regime theory is needed, and a way to transition from one to the other smoothly.

Figure 1.7 Plasma regime experimental results on Al match Eq. (1.19) theory quite well. Letters on the graph are data sources listed in [10]. We added 80 ps and 400 fs data from [2]. This shows that 80 ps data fits, but 400 fs does not. This is not surprising. Underlying figure for aluminum reproduced from Fig. 13 of [10], with the permission of AIP Publishing. Overlying data (color) courtesy of C. Phipps.

1.7.2 Vapor regime

Calculating Cmv in the vapor regime is based on two different models, which depend on the data available for a material. For polymers for which an ablation threshold fluence has been

(1.26)

found with / and C is an adjustable free variable to match measured behavior [12],

(1.27)

Alternatively, for elemental materials like aluminum, for which tables of vapor pressure vs. temperature exist [the SESAME tables at Los Alamos or QEOS or PURGATORIO-based equation of state models], we can work backward from hydrodynamic variables to the incident intensity that must exist to balance these variables to obtain [13]

(1.28)

where

(1.29)

and we extract p as a function of I and simply use Cm=p/I.

1.7.3 Combining the vapor and plasma regimes

This is tricky. If you don't do it right, the most important part (Cm at the peak in Fig. 1.6) will be missed. The method we chose [14] uses the ionization fraction as a proportionality variable

(1.30)

to combine coupling coefficients from the two regimes,

(1.31)

This approach worked very well [Fig. 1.8] for aluminum at three different wavelengths and several pulse durations [15–17]. Not much data exists for the vapor regime, so we were fortunate to have the aluminum data from Rosen at 350 nm (blue curve in Fig. 1.8). Determining requires a computer and is discussed in [14].

Figure 1.8 The vapor and plasma regime data fit using our combined vapor/plasma regime theory. Courtesy of C. Phipps.

1.8 Photon propulsion

Because this is the subject of Chapter 2, we refer you to details on very large projects in that chapter. However, we will criticize photon propulsion vs. ablation propulsion for small payloads in this chapter. The Breakthrough Starshot concept involves a 200 GW CW laser system driving a 4.1-m diameter reflective sail and payload with 3.6 g mass to 20% of the speed of light in 9 minutes, at which time it will be 20 Mkm from Earth, about 1/3 of the minimum distance to Mars. The transmitting mirror is 2.7 km diameter. To achieve this, we use a phased array of many 10–20 kW lasers. From Eq. (1.2), maximum force on the sail will be 1200 N, producing an acceleration of 34,000 G. Thermal limitations reduce these numbers by about a factor of two. The device will send back photos of Proxima Centauri and its planets on its arrival in 22 years. This project is a dream, but dreams of the future are critically important to civilization.

Pure photon propulsion is the only way to reach relativistic speeds.

1.9 Beamed energy propulsion

In the introduction, we referred to two other types of propulsion. One of these, Jordin Kare's idea [18] for beamed energy propulsion, is not covered in detail here [Fig. 1.9]. Suffice it to say that Parkin has claimed [19] that the availability of high-power low-cost microwave sources such as gyrotrons makes microwave-heated fluid propulsion systems less expensive than laser-heated ones, notwithstanding the much larger transmitting apertures required to focus on the target by the longer wavelength. A transmitting array diameter of about 170 m and 490 MW of transmitted power would be required. With microwaves, a beam range of only 200 km would be possible.

Figure 1.9 The Beamed Energy Propulsion Idea. [News release July 21, 2015, courtesy of Escape Dynamics, Inc.]

1.10 Comparing laser-driven and electric thrusters

Table 1.2 shows the main characteristics of heavy electric thrusters which can propel large spacecraft. We assume that the engine itself is 10% of total station mass. Considering that together with the mass of their batteries, solar arrays, heat dissipation and guidance and control systems, an electrically propelled station of >50 kg total mass would be very inefficient for chasing and moving 10-kg satellites, considering the typical 15 km/s Δv required to move from one chased object to another.

Table 1.2

This is not an unfair disadvantage for electric propulsion. We note that the two 10 W ion thrusters such as offered in Table 1.8 are equivalent to a 4 kW laser unit given 100 N/MW and 25% electrical efficiency.

1.10.1 Electric thrusters

Electric thrusters have several categories. Hall effect thrusters (HET in Tables 1.2 and 1.3) use combined crossed current and magnetic fields (JxB) in an annulus to accelerate plasma [Fig. 1.10]. Ion thrusters use an electric field to accelerate ions, which are then neutralized at the thruster exit. Other types are arcjets and magneto-plasmadynamic thrusters (MPDs). Performance of several of these is compared in Tables 1.2 and 1.3.

Figure 1.10 Hall Effect Thruster using Xe gas. Public Domain Courtesy of NASA/JPL-Caltech.

1.10.2 Other thruster types

FEEP thrusters are low-thrust (μN) devices that use field emission of a liquid metal with acceleration of the metal ions by a strong electric field. Heavy metals such as Cs, In or Hg have been used. Yet another type with nN levels of thrust are electrospray thrusters, which electrostatically accelerate colloid droplets.

The water cannon concept [30] uses light from a fiber laser to cause ejection of water droplets in a specially designed target structure. This concept is characterized by very high Cm but such low Isp that range is not useful [31]. Layered targets in Fig. 1.11 refers to performance of a glass sandwich covering a black energetic surface. Please note the spot for pure photon propulsion in Fig. 1.11.

Figure 1.11 Plot of C me vs I sp for laser, ion and electric thrusters. The figure compares the available range of I sp with chemical, electrical and laser ablation thrusters. The label Earth to Pluto in 8 mos. points to the minimum I sp required to do that voyage. The purple triangle at far right in the Figure shows the fixed operating point of photon propulsion. Lines represent constant electrical efficiency. The pink square labeled 2 ms diode laser on polymer refers to Photonic Associates' laser plasma thruster. It can have an ablation efficiency greater than 100% because of the contribution of the energetic polymer ablator when it is heated, and the definition of η AB , which only considers laser and not laser plus chemical energy. Courtesy of C.