NASA Concept to use 100 MW beamed power for ion drive that is 20 times better

John Brophy at NASA Jet propulsion laboratory combines a near term 100 megawatt laser beamed power system to enable an ion drive with 70 megawatts of power and 58000 ISP.

They propose a new power/propulsion architecture to enable missions such as a 12-yr flight time to 500 AU—the distance at which solar gravity lensing can be used to image exoplanets—with a conventional (i.e., New Horizons sized) spacecraft. This architecture would also enable orbiter missions to Pluto with the same sized spacecraft in just 3.6 years. Significantly, this same architecture could deliver an 80-metric-ton payload to Jupiter orbit in one year, opening the possibility of human missions to Jupiter. These are just a few examples of high-impact missions that simply cannot be performed today due to limitations in current technology. Our architecture accomplishes this by combining the following three innovations:

1. A 10-km diameter, 100-MW laser array that beams power across the solar system.
2. A 70% efficient photovoltaic array tuned to the laser frequency producing power at 12 kV.
3. A 70-MW direct-drive, lithium (not xenon)-based ion propulsion system with a specific impulse of 58,000 seconds.

The key to the development of any system for rapid space transportation is the ability to process a lot of power level with little dry mass, combined with the ability to provide a very high total spacecraft velocity change (delta-V) without a lot of propellant. These two requirements translate into the need for a very low specific mass (kg/kW) and a very high specific impulse. A specific mass of 0.25 kg/kW is enabled in our architecture by removing the power source and most of the power conversion hardware from the spacecraft and replacing them with a lightweight, photovoltaic array that outputs electric power at the voltage needed to drive a lithium-fueled, gridded ion thruster system at a specific impulse of 58,000 s. For comparison, the state-of-the-art for specific mass and specific impulse, as represented by the Dawn spacecraft, are 300 kg/kW and 3,000 s, respectively.

The US Military already has 60-100 kilowatt solid state lasers. An array of such lasers could be used to generate the power for this system. Thin film solar power arrays able to generate 100 kilowatts to megawatt power levels are available.

This architecture provides a breakthrough way to take advantage of very high-power lasers, of the type described by Lubin, to provide fast transportation though out the solar system and beyond for conventionally-sized spacecraft. We take as a given the existence the “Orbital filled 10-km array” from, and assume that its output power has been derated by a factor of a thousand from 100 GW down to 100 MW. Our innovation is the recognition that such an array increases the power density of photons available to a spacecraft illuminated by the laser beam by two orders of magnitude relative to solar insolation at all the solar system distances beyond 5 AU, and that this enormous power can then be used to great effect by driving a highly-advanced ion propulsion system.

A high-voltage photovoltaic array tuned to the laser frequency converts the laser power to electric power at an efficiency of 70% and produces an output voltage of 12 kV. The 12-kV output voltage is used directly to provide the net accelerating voltage for the lithium fueled, gridded ion propulsion system eliminating the heavy, inefficient, power processing hardware, and the associated thermal radiators, typically needed to drive ion propulsion systems. The lithium-fueled, gridded ion propulsion system provides a specific impulse of 58,000 s, roughly 20 times the current state of the art. Lithium stores as a solid, is easily ionized, and very difficult to doubly ionize. This allows the thruster to be operated with nearly 100% ionization of the propellant which effectively eliminates neutral gas leakage from the thruster and the production of charge-exchange ions that are responsible for thruster erosion and current collection on the photovoltaic arrays. This key benefit enables very long thruster life and facilitates the development of the 12-kV photovoltaic array.

Lubin at UCSD had detailed proposals for near term beamed laser power

Conceptual design of the deployed spacecraft with two 15 meter PV arrays that produce 50 kW each at the beginning of life for a total of 100 kW electrical, ion engines at the back, and the laser array pointed directly at the viewer. A 2 meter diameter laser phased array is shown with 19 elements, each of which is 1-3 kW optical output. A 2 meter diameter optical system is one of the possibilities for DE-STARLITE. More elements are easily added to allow for scaling to larger power levels. A 1 – 4.5 meter diameter is feasible; no additional deflection comes from the larger optic, just additional range from the target.

The objective of the laser directed energy system is to project a large enough flux onto the surface of a nearEarth asteroid (via a highly focused coherent beam) to heat the surface to a temperature that exceeds the vaporization point of constituent materials, namely rock, as depicted in Fig. 4. This requires temperatures that depend on the material, but are typically around 2000-3000 K, or a flux in excess of 10^7 W/m2. A reactionary thrust due to mass ejection will divert the asteroid’s trajectory (Lubin et al., 2014). To produce a great enough flux, the system must have both adequate beam convergence and sufficient power. From a distance of 10 km, a spot size on the asteroid of 10 cm provides enough flux to vaporize (sublimate) rock (Hughes et al., 2014). Optical aperture size, pointing control and jitter, and efficacy of adaptive optics techniques are several critical factors that affect beam convergence. As mentioned, the optical power output of the laser is projected to be between 35 kW and 70 kW, depending on technological advancements in laser amplifier efficiency in the coming years. Currently the amplifiers are about 35% efficient but it is expected they will exceed 50% within five years. Similar requirements are sought by power beaming systems (Mankins, 1997; Lin 2002). For the optional (non-phase-locked) fiber focal plane array the lasers are even more efficient and already exceed 50%. Any power level in this range will work for the purpose of this mission, but higher efficiency allows for more thrust on the target for a given electrical input as well as for smaller radiators and hence lower mission mass.

The proposed baseline optical system consists of 19 individual optical elements in a phased array.

Single element of laser phased array, showing fiber-tip actuator for mid-level pointing control and
rough phase alignment

Mounted hexagonal laser phased array with a baseline of 19 elements depicted: (a) at 45 degrees, (b)
face on, and (c) from the back

Both the laser array and the PV arrays are easily extended to larger power levels. The mass per unit power of the laser amplifiers is about 5 kg/kW currently with a strong push to bring this down to 1 kg/kW in the next five years. Similarly the PV is about 7 kg/kW, or similar to the laser amplifiers. Interestingly, it is the radiator panels that are the most difficult to scale up, at about 25 kg/kW. This is an area that needs work, though in all simulations for mission masses the assumption is 25 kW (radiated) for the radiator panels.

ATK has to scale their existing 10 meter diameter design to push the PV arrays to 30 meter diameter which will yield about 225 kW per manufactured unit, or 450 kW per pair and still fit in an SLS PF1B 8.4 m diameter fairing. Fig. 9 and Fig. 10 show the scaling and deployment of the PV arrays to larger sizes for various launch vehicles. Even larger sizes into the megawatt range can be anticipated in the future.

 

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