Photonic laser thruster

The figure illustrates the anatomy of a photonic laser thruster.
The usage of photonic laser thrusters is illustrated here for ultra-precision propellant-free formation flying of spacecraft to form an out-of-plane synthetic aperture on Low Earth Orbit.

A photonic laser thruster[1][2][3][4] is an amplified laser thruster that generates thrust directly from the laser photon momentum, rather than laser-heating propellant. The concept of single-bounce laser-pushed lightsails that utilize the photon momentum was first developed in the 1960s, however, its conversion of laser power to thrust is highly inefficient, thus has been considered impractical. Over 50 years, there had been numerous theoretical and experimental efforts to increase the conversion efficiency by recycling photons, bouncing them repetitively between two reflective mirrors in an empty optical cavity, without success.

In December 2006, Young Bae successfully solved this problem and demonstrated the conversion efficiency enhancement by a factor of 100 and a photon thrust of 35 micronewtons by putting the laser energizing media between the two mirrors as in typical lasers, and the photonic laser thruster was born. In August 2015, the photonic laser thruster was demonstrated to increase the conversion efficiency enhancement by a factor over 1,000 and to achieve a photon thrust of 3.5 millinewtons at Y.K. Bae Corporation. In addition, Propelling, slowing and stopping of a small satellite, 1U CubeSat, in simulated zero-gravity were demonstrated.

The photonic laser thruster was initially developed for use in nanometer precision spacecraft formation,[5] for forming ultralarge space telescopes and radars. The photonic laser thruster is currently developed for high-precision and high-speed maneuver of small spacecraft, such as formation flying, orbit adjustments, drag compensation, and rendezvous and docking. The photonic laser thruster can be used for beaming thrust from a conventional heavy resource vehicle to a more expensive & lightweight mission vehicle, similar to tankers in aerial refueling.[6][7]

The practical usage of the photonic laser thruster for main space propulsion would require extremely high laser powers and overcoming technological challenges in achieving the laser power and fabricating the required optics. Photonic laser thrusters have a very high specific impulse, and can permit spacecraft reach much higher speeds than with conventional rockets, which are limited by the Tsiolkovsky rocket equation. If the photonic laser thruster is scalable for the use in such main space propulsion, multiple photonic laser thrusters can be used to construct a 'photonic railway' that has been proposed as a potential permanent transport infrastructure for interplanetary or interstellar commutes, allowing the transport craft themselves to carry very little fuel.[8]

Background

Photon rocket

The specific thrusts of common conventional thrusters and the photon thruster are plotted as a function of specific impulse (Isp). The relativistic theory (the solid red curve) shows that the specific thrust of matter-based thrusters approaches that of photon thrusters as Isp increases.

The first designs using pure photon thrust for impulse, rather than heat or exhaust from propellant, begins with the photon rocket. This has been researched since the beginning of the 20th century.[9] According to special relativity, the highest velocity of the rocket exhausts can have is the speed of light, c = 3×108 m/s. Therefore, photons are the ultimate rocket fuel that will produce the ultimate specific impulse. Photon propulsion has been widely discussed for decades[10] as a next generation propulsion that can make interstellar flight possible.

The right figure shows the thrust to power ratio, which is defined as specific thrust, of chemical rockets, electric thrusts that include Hall effect thrusters, and Pulsed Plasma Thrusters, in comparison with that of photon thrusters. The specific thrust is the propulsion force divided by the energy used. The specific thrust of the photon thruster is several orders of magnitude smaller than conventional thrusters because it is using the mass of its fuel to gain speed instead of ejecting most of it for thrust. If conventional thrusters can be made to have Isp~107 sec, their specific thrust would be similar to that of photon thrusters. Therefore, in achieving relativistic velocities, the thrust efficiency does not depend on whether the propellant is made of photons or other particles, such as protons or electrons.[8]

Because generation of relativistic ions is highly energy inefficient and technologically challenging with conventional thrusters, for ultra-high Isp flights, photon thrusters become more attractive.[8] More importantly, photon propulsion has a critical advantage over particle-based propulsion: the abilities of beaming and amplifying thrust with laser technologies as presented in the next section. Therefore, it makes much more sense to use photons for relativistic propulsion.[8]

Photonic rockets, such as the nuclear photonic rocket, are technologically feasible, but rather impractical. Regardless of the photon generator characteristics, a unified theory[8] on such onboard photon propulsion can be used for providing insight on the perspective of photon propulsion. In this unified theory, it is assumed that the propulsion system has a single stage. The approximate maximum velocities attainable by various onboard nuclear fission or nuclear fusion powered photon propulsion systems are typically on the order of 0.01% of the light velocity, which are orders of magnitude less than the required 10% of the light velocity for interstellar travel. Therefore, unless extremely complicated and heavy multistage nuclear photonic rocket engines are used, single-stage systems seem to be impractical for interstellar travel.

Beamed Laser Propulsion

The above theoretical limits posed by photonic propulsion with onboard photon generators can be overcome, if the photon generators and the spacecraft are physically separated. In the Beamed Laser Propulsion (BLP) concept,[10][11] the photons are beamed from the photon source to the spacecraft using lasers. In particular, Robert L. Forward pioneered a wide range of interstellar propulsion concepts including photon propulsion and antimatter rocket propulsion.[10][11] Specifically, Forward proposed for the first time BLP aiming at the goal of achieving roundtrip manned interstellar travel. However, considering the unprecedentedly large world-scale investment required for such interstellar flight, unless there is enormous potential financial return from such endeavors, the chance of sustaining continuous return investment in such programs is dismal. More details on Beamed Laser Propulsion can be found in the Wikipedia articles on Laser propulsion and Beam-powered propulsion.

Recycling Photon Propulsion

An important theoretical understanding and development of terrestrial laser-beam driven rockets was obtained by Marx,[12] Redding,[13] and Simmons and McInnes[14] who calculated that the energy conversion efficiency of terrestrial laser-driven propulsion is approximately proportional to v/c at low speeds (v<0.1c), thus is very small at very low speeds (v<<0.1c). However, once the spacecraft reaches higher speeds (v>0.1c), owing to the favorable Doppler-shift energy transfer, on-board photon propulsion becomes much more energy efficient, thus there is a need to bridge this energy efficiency gap.

Energy transfer efficiency from the photon energy to the spacecraft kinetic energy as a function of β=v/c.

Photons transfer their energy to the spacecraft by redshifting due to the Doppler effect upon reflection, thus the higher the spacecraft speed is, the higher the efficiency is. The right figure schematically shows the energy transfer efficiency from the photon energy to the spacecraft kinetic energy as a function of β=v/c (the spacecraft velocity divided by the light velocity) in photon propulsion. It is interesting that as the spacecraft velocity approaches the light velocity (v ≈ c) the efficiency of photon propulsion becomes 100%, as if the spacecraft acts like a black hole in the moving direction. The lower solid curve in right figure represents the efficiency of conventional photon rocket or sail with photon recycling. The upper solid line represents schematically an example the efficiency of a recycling photon rocket, such as a photonic laser thruster. At low β, the recycling rocket can have a very high thrust amplification factor (in this example, ~3,000), however as β approaches 1, the amplification factor would converge to 1, and the extra overhead of a recycling system would not be needed. Therefore, these rockets are projected to bridge the above-mentioned energy efficiency gap.

Meyer et al.[15] concluded that for interstellar fights, recycling photon propulsion vehicles are much more energy efficient than an onboard photon rocket, such as the nuclear photonic rocket. Possible applications of photon recycling using passive resonant optical cavities (lasers are located outside of the optical cavity), the Laser Elevator, in launching and propelling spacecraft at higher velocities with higher efficiencies than those available by exiting rocket engines, was first proposed and extensively studied by Meyer et al.[15]

Amplification of photon thrust by bouncing photon beams. As the number of bouncing and the distance between the mirrors increase, the beams start to overlap and the cavity changes from the Herriot cell to the Fabry-Perot resonator that is extremely unstable against the mirror movement.

The simplest recycling scheme is a Herriot cell with multi-bouncing laser beams between two high reflectance mirrors without forming a resonant optical cavity as illustrated in the right figure. This Herriot cell type approach was first proposed by Meyer et al.[15] followed by Simmons and McInnes,[14] Their study was in depth analyzed by Mertzger and Landis.[16] This approach requires highly focused laser beam spots on each mirror to avoid the beam interference that may induce optical resonance in the cavity.

The implementation of the Herriot cell concept turned out to be highly complicated. The right figure illustrates that as the cavity length and the number of photon bouncing increase in Herriot cells, the focal spot diameter projected on mirrors increases, requiring extremely large mirrors to avoid the laser beam interference.[3] Once the laser beam starts to interfere, the non-resonant cavity becomes a passive resonant cavity to be impractical for photon propulsion amplification.[1][2] The first experimental attempt on photon thrust amplification in a non-resonant Herriot-cell type optical cavity was performed by Gray et al.[17] who could obtained amplified photon thrust of ~0.4 µN with a 300-W laser and a photon thrust amplification factor of ~2.6.[3]

The passive resonant optical cavity, the resonator in Fabry-Perot interferometer, has been extensively used in high-sensitivity optical detection methods, such as the cavity ring-down spectroscopy, which obtained the photon bounce number of 20,000 with super mirrors with the reflectance of 0.99995.[18] However, the passive resonant optical cavity for photon thrust amplification is unsuitable for propulsion applications, because it is highly unstable against small changes in the distance between the mirrors. Such extreme sensitivity was observed in the gravitational detection system (LIGO) with such high-Q passive optical cavities,[19] in which even one nanometer perturbation in cavity length sets the system out of resonance and nulls the photon thrust. In addition, the injection of laser power into the cavity is extremely challenging.

Photonic laser thrusters

In 2000s, Young Bae began to investigate photon recycling for the use in a nano-meter accuracy formation flight, for a NASA-NIAC project called Photon Tether Formation Flight (PTFF).[5][20] The goal was to sustain fixed-formation flight with a baseline distance between craft of over 10 km, for next generation NASA space missions. In 2006 Bae began to investigate using active resonant optical cavities, in which the optical gain medium is located within the cavity, and coined the term "photonic laser thruster" (PLT) for thrusters that use such optical cavities.[1][2][3][4]

Photonic Laser Thruster mechanism diagram shows its laser-like active resonance cavity with a laser gain medium in the cavity.

Initially PLT was proposed to overcome the difficulties in injecting sufficient laser power in high-Q optical cavities,[5] and it was assumed that the cavity would be very sensitive to the stability of the mirrors and other optical parameters as in the passive cavities. However, during the experiments, Bae accidentally discovered that he could sustain the resonance with the mirror in his hand.[1][2] This would be impossible if the cavity were highly sensitive to the cavity perturbations, such as moving and tilting. They concluded the gain medium in the cavity actively stabilizes the recycling photon beam, through negative feedback. This encouraged the idea that these thrusters could enable a range of applications well beyond PTFF.[2][5][20]

Schematic diagram of the first Photonic Laser Thruster (PLT) demonstration setup in December 2006.

In December 2006, a proof-of-concept of PTFF was demonstrated in a laboratory environment.[1][2][20] The right figure illustrates the first demonstration setup, consisting of a concave High Reflectance (HR) mirror, a Nd:YAG Diode Pumped Solid State laser gain medium and a flat Output Coupler (OC) mirror. The photon thrust was determined by measuring the difference between the weight of the HR mirror with laser on and that with laser off with the use of a digital scale. Below is an infrared picture of the thruster in action.

An infrared picture of the first laboratory demonstration of Photonic Laser Thruster.

The next figure shows the experimental results of the photon thrust measured as a function of the laser power output through the OC mirror, P, with the reflectance greater than 0.997 according to the manufacturer’s specification. The curve fitting of the data resulted in the specific thrust of 20±1 µN/W, resulting in the apparent photon momentum multiplication factor of 2,990±150 and the true OC mirror reflectance used in this demonstration of 0.99967±0.00002. The maximum photon thrust demonstrated was 35 μN. When the demonstration setup was operating at thruster levels near or beyond 35 µN, the setup became unstable and the gain medium started to glow in yellow due to overheating. Therefore, it was realized that the thermal management issues are critical to the scaling such thrusters.[1][2]

The first Photonic Laser Thrust demonstration data obtained with an output coupler mirror with a reflectance of 0.99967.

One of the factors that limit the maximum obtainable velocity of the accelerating mirror and its accommodating spacecraft is limited by the Doppler shift of the bouncing photons. Doppler shift effect on the active resonant cavity behavior is an extremely complicated issue.[1][2] Optical gain in the laser cavity can only occur for a finite range of optical frequencies. The gain bandwidth is basically the width of this frequency range. For example, the gain bandwidth of the YAG laser system with the laser wavelength in the order of 1,000 nm is in the order of 0.6 nm,[21] which is ~ 0.06% of the wavelength. For an order of magnitude estimation, it can be assumed that a thruster utilizing the YAG laser system will be limited by the gain bandwidth to the first order, then, theoretical maximum spacecraft velocity is ~1.8×105 m/s (180 km/s) that is 0.06% of the light velocity, c=3x108 m/s. To overcome this redshift limitation, at high operation velocities, wide bandwidth lasers should be used. Faster speeds would require further technological developments.[8]

Providing greater thrust would need higher power lasers.[22] Lasers are often designed to maximize laser power outside the laser cavity, however the PLT calls for maximizing laser power inside the cavity.[1][2] The laser operation parameters to maximize intracavity power (circulating power) in laser cavities is still poorly understood.

A Photonic Laser Thruster based on thin disk laser (TDL) technology.

Bae and colleagues concluded that a thin disk laser is one of the best laser systems for these drives, and began to develop one using such lasers in 2013.[22] Typically, a thin disk laser with a 0.2 mm disk thickness can be operated with a 99% output coupler, and provide an intracavity power 100 times larger than extracavity power.[23] A schematic diagram of the PLT system based on this design is shown in the right figure. A thin disk gain medium is coated with a High Reflectance (HR) with a reflectivity of up to 99.999% and attached to a heat sink. The recycling photons between the gain medium and the HR mirror located in the mission platform deliver amplified thrust beaming from the resource platform.

In 2014, Bae's group, working through NIAC, reported intracavity power of 154 kW with a 0.6 cm diameter thin disk laser, which can be translated into a photon thrust of 1.03 mN.[24] In April 2015, the group successfully measured photon thrust up to 1.1 mN with a digital scale. They were able to accelerate, slow and stop a 0.45 kg spacecraft-simulating platform along a 2m frictionless air track in a Class-1,000 cleanroom.[25] In August 2015, the group successfully demonstrated and measured photon thrust up to 3.5 mN with the use of the newly developed NIST/Scientech radiation pressure sensor. An intracavity power over 500 kW was also demonstrated. In addition, they were able to accelerate, slow and stop a 1U CubeSat along the 2m frictionless air track in the Class-1,000 cleanroom.

Open problems

The maximum range of operation for a photonic laser thruster is uncertain. Bohn[26] of the German Aerospace Center (DLR) reported the Rheinmetall Defense demonstrated a 1 km-long laser resonator similar to the PLT cavity in 1995, and proposed that such resonators could scale to 100 km . Further studies need to be performed to determine whether the same design could be used for interplanetary or interstellar distances.[8]

Another technological issue is intracavity laser beam aiming, aligning, and tracking over long distance.[8] Owing in part to the rapid advancement in directed energy weapons, technologies required for or aiming, alignment, and tracking of laser beams over large distances are rapidly evolving. However the speed of light limits how quickly information can travel between the two ends of the cavity over very long distances.

Applications

Near Term Applications

Examples of satellite formation for forming synthetic apertures using PLT pushing-out force and counterbalancing "virtual tug" generated by differential gravity.

Using thrust beaming for spacecraft propulsion has been investigated for decades, as an alternative to the "all-in-one" single-spacecraft approach.[4][6][7]

Photonic laser thrusters have primarily been studied for maneuvering spacecraft in near earth orbit, propellantless operation, thrust and power beaming for "perpetual" stationkeeping, and ultra-precision spacecraft formation flying, with or without tethers.[5][6]

Norman and Peck analyzed[6][7] a group of spacecraft that can exploit relative positions and velocities so that differential gravity provides an a force opposite the photon thrust from a photonic laser thruster. In such a scheme, with two orbiting platforms moving in formation, when the photon thrust changes, their positions relative to the center of the mass would change as well, until the "virtual gravitational tug" counterbalances it again.[4][7] Existing formation concepts with conventional thrusters are not persistent, are mechanically constrained (e.g. with tethers), or lie within the orbit plane, severely limiting their aperture for earth-observation missions.[7] This system in contrast could control of out-of-plane motions. The figure to the right shows examples of "virtual tug" satellite formations for forming large synthetic apertures in Low Earth Orbit.

PLT stationkeeping for north/south orbit maintenance for a GEO spacecraft originally envisioned by Mason Peck. Additionally illustrated is multi-vehicular orbit drag compensation with PLT by a resource vehicle on Low Earth Orbit.

Peck also proposed[27] persistent configurations of multiple spacecraft can be realized with active feedback, using nonlinearities and periodic behaviors. This would allow for propellant-free stationkeeping, including north/south orbit maintenance for a GEO spacecraft as illustrated in the right figure.

Similarly, small orbital craft that need to correct for orbital drag could do this with significantly less propellant, by moving a larger resource vehicle into a similar orbit with low inter-vehicle velocity are used for making the mission much more economical and reliable.[27] The replacement of such a resource vehicle could be faster and more economical.

Far Term Applications

Specific energy as a function of spacecraft velocity relevant to Mars missions. The flight time to Mars is for flyby missions, and rendezvous missions would take more than twice longer.

It is still uncertain whether this design could work at large distances and large thrusts, and many evolutionary, technological and financial challenges would need to be resolved. However Bae has proposed a development path towards powering interstellar flight.[8] This would involve an interplanetary Photonic Railway, a permanent energy-efficient transportation infrastructure based on Forward's model of beamed-laser propulsion[10][11] and the photonic laser thruster techniques (a PLT-BLP hybrid).[1][2][3][4] This would cut down on the cost and duration of interstellar commutes via proposed spacetrains.[8]

The Interplanetary Photonic Railway,[8] was investigated for a range of future space applications involving planets and asteroids, such as mining and setting up permanent habitations, with regular travel between planets.[10][11]

Platforms could be built in Earth orbit and then used for constructing further platforms at either one of the Lagrange points of a planet of interest to structure an Interplanetary Photonic Railway.[8] For Mars, the solar power is still strong, thus solar pumped platforms could be operated near Mars without too much disadvantages. However, planets farther away from the sun might not support solar pumping, and the railway would involve two PLT-BLP systems near the Earth.

Bae compared the specific energy needed to bring spacecraft up to certain speeds, for conventional rockets and for a PLT-BLP system with photon amplification of 1,000.[8][15] The figure to the right estimates the specific energy (J/kg) as a function of spacecraft velocity (km/s) for four systems: a photonic railway using only BLP (no photon reuse), a railway using a combination of PLT and BLP, and conventional rockets with specific impulse of 500 s and 3,000 s.

According to his calculations, BLP alone would be more energy efficient than a rocket with Isp=500 s, if travel time needs to be shorter than 1 month (implying a spacecraft speed of ~65 km/s).

A simple PLT system would provide continuous and constant thrust in a straight line. However, travel around the solar system involves interacting with planets and the sun, so trajectories and travel time are more complex. Fu-Yuen Hsiao has investigated the trajectories of spacecraft relying entirely on a photonic laser thruster, since 2011.[28]

Finally, Bae concluded that developing such a propulsion system beyond the solar system would require many magnitudes of improvement in x-ray lasers and materials design. It might be possible within a century, but can't be evaluated properly now.

References

  1. 1 2 3 4 5 6 7 8 9 Bae, Y.K. (2007). "Photonic Laser Thruster (PLT): Experimental Prototype Development and Demonstration". AIAA Space 2007 Conference Proceedings: 2007-6156-318.
  2. 1 2 3 4 5 6 7 8 9 10 Bae, Y.K. (2008). "Photonic Laser Propulsion: Proof-of-Concept Demonstration". AIAA Journal of Spacecraft and Rockets 45: 153–155. doi:10.2514/1.32284.
  3. 1 2 3 4 5 Bae, Y.K. (2007). "Photonic Laser Propulsion (PLP):Photon Propulsion Using an Active Resonant Optical Cavity". AIAA Space 2007 Conference Proceedings: 2007-6131-818.
  4. 1 2 3 4 5 Bae, Y.K. (2007). "Photon Tether Formation Flight for Distributed and Fractionated Architectures". AIAA Space 2007 Conference Proceedings: 2007-6084-275.
  5. 1 2 3 4 5 Bae, Y.K. (2006). "A Contamination-Free Ultrahigh Precision Formation Flying Method for Micro-, Nano, and Pico-Satellites with Nanometer Accuracy". AIP Conference Proceedings. AP813: 1213.
  6. 1 2 3 4 Bae, Y.K. (2012). "DE Momentum Beaming (DEMB) for Innovative Spacecraft Maneuvering" (PDF). DEPS Conference Proceedings.
  7. 1 2 3 4 5 Norman, M.C.; Peck, M.A. (2009). "Orbit Maneuvers Through Inter-Satellite Forcing". AIAA Conference proceedings: 2009-6097.
  8. 1 2 3 4 5 6 7 8 9 10 11 12 13 Bae, Y.K. (2012). "Prospective of photon propulsion for interstellar flight". Physics Procedia 38: 253–279. doi:10.1016/j.phpro.2012.08.026.
  9. ↑ Tsander, F. (1967). "From a Scientific Heritage". NASA TTF-541.
  10. 1 2 3 4 5 Humble, R.W.; Henry, G.N.; Larson (1995). Space Propulsion Analysis and Design. McGraw-Hill Co., New York. pp. 631–683.
  11. 1 2 3 4 Forward, R.L. (1987). "Advanced Space Propulsion Study". AFRL Final Report: F04611-86-C-0039.
  12. ↑ Marx, G. (1966). "Interstellar vehicle propelled by terrestrial laser beam". Nature 211: 22–23. doi:10.1038/211022a0.
  13. ↑ Redding, J.L. (1967). "Interstellar vehicle propelled by terrestrial laser beam". Nature 213: 588–589. doi:10.1038/213588a0.
  14. 1 2 Simmons, J.F.L.; McInnes, C.R. (1993). "Was Marx right? Or how efficient are laser driven interstellar spacecraft?". American Journal of Physics 61: 205. doi:10.1119/1.17291.
  15. 1 2 3 4 Meyer, T.R.; et al. (1985). "Rapid Delivery of Small Payloads to Mars". American Astronautical Society Science and Technology Series 62: 419.
  16. ↑ Mertzger, R.A.; Landis, G. (2001). "Multi-Bounce Laser-Based Sails". AIP Conference Proceedings 555: 397.
  17. ↑ Gray, P.A.; et al. (2002). "Photon Flux Amplification for Enhancing Photonic Laser Propulsive Fo". AIAA 33 rd. Plasmadynamics and Lasers Conference Proceedings.
  18. ↑ Romanini, D.; et al. (1997). "CW Cavity Ring Down Spectroscopy". Chem. Phy. Lett 264: 316–322. doi:10.1016/s0009-2614(96)01351-6.
  19. ↑ Sheard, B.S.; et al. (2004). "Observation and Characterization of an Optical Spring". Phys. Rev. A 69: 051801(R). doi:10.1103/physreva.69.051801.
  20. 1 2 3 Bae, Y.K. (2006). "A Contamination-Free Ultrahigh Precision Formation Flight Method Based on Intracavity Photon Thrusters and Tethers". NASA Institute for Advanced Concepts (NIAC).
  21. ↑ Yariv, A. (1975). Quantum Electronics. John Wiley & Sons.
  22. 1 2 Injeyan, Hagop; Goodno, G. (2011). High Power Laser Handbook. McGrow-Hill.
  23. ↑ Latham, W.P.; et al. (2011). "6.5 kW, Yb:YAG Ceramic Thin Disk Laser". AFRL-RD-PS-TN-2011-0002.
  24. ↑ Bae, Y.K. (2013). "Propellantless Spacecraft Formation-Flying and Maneuvering with Photonic Laser Thruster". NASA Innovative Advanced Concepts (NIAC).
  25. ↑ Y.K. Bae Corporation (12 May 2015). "Photonic Laser Thruster Propels Simulated Spacecraft". Youtube.
  26. ↑ Bohn, W.L. (2008). "Novel Aspect of Laser Propulsion". Proc. of SPIE: 7005: 70081C.
  27. 1 2 Peck, M.A., private communication (2012).
  28. ↑ Hsiao, F.Y.; et al. (2012). "Trajectory of spacecraft with photonic laser propulsion in the two-body problem". Acta Astronautica 84: 215–226. doi:10.1016/j.actaastro.2012.11.006.
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