From my Seminar on Laser Powered Aircraft

High altitude vehicles and nanosattellites are routinely used for reconnaissance and communication applications and the launch of such aerospace vehicles require rockets with advanced propulsion concepts. The cost of launching these spy satellites and airships by conventional rocket technology remains very high.

Technically, despite four decades of propulsion development, there have been no spectacular improvements in the energy density of propellant materials because the Isp* of any chemical propellant is limited by the energy of chemical bonds that are broken. The highest Isp possible (H₂/O₂ reaction) is only 460 seconds. Consequently, the price of launch has hovered around $10,000 per kilogram of payload delivered to low earth orbit. On the economic end, market models predict an essentially flat elasticity of demand for space launch until the payload cost is reduced below $1000 per kilogram[i].

There are two ways to increase the Isp of the propellant past the practical chemical limit of 460 seconds:

• Increase the energy density of the launcher power source.
• Supply energy from outside the rocket, thereby avoiding the chemical energy density limit.

Beamed Energy Propulsion using microwaves or lasers is a promising technology to launch small sized aerospace vehicles. “Lightsail” spacecrafts have captured the imagination of scientists for decades. The earliest recorded description of the idea was published in 1924 by Konstantin Tsiokolvsky and Friedrich Tsander.

Laser-boosted launch system was first suggested by Kantrowitz[2] and Minovich[3] independently. There have been sporadic studies on laser-based propulsion concepts since then. Wang et. al have reported a laser “lightcraft” powered by a 10 kW CO2 laser[4]. A parabolic mirror at the bottom of the craft focuses the laser beam into the engine where the laser heats the air, causing it to break down into plasma. The plasma strongly absorbs the incoming pulse, heating to roughly 18,000 K before exploding from the annular bottom region, generating thrust.

High Power Microwave (HPM) lightcraft development concepts focus on a ground-based microwave transmitter array instead of a laser power to transform the high frequency electromagnetic energy into propulsive power.

The cost benefits of beamed energy propulsion over conventional chemical propulsion concepts cannot be overstated. A small-sized “Lightcraft” sponsored by the USAF Research Laboratory and NASA has already been tested in White Sands, New Mexico desert. The successful tests have helped confirm the promise that useful payloads could be delivered to low-Earth-orbit** using beamed energy propulsion.

Comparison of cost of conventional Vs. Laser propulsion systems[5]

Among the various materials requirements for microwave beamed energy propulsion systems is the need for appropriate window materials. The transfer of radiative energy from one location and its coupling to flowing propellants in a vehicle’s propulsion system at a remote location depends to a large extent on thermally robust, transparent window materials.

Transparent window materials for use in beamed energy propulsion vehicles must satisfy the following conditions[6]-

• Good optical transmittance with low scattering (<1-2%)>
• Absence of micro-level planar strains under conditions of hypervelocity, which will distort transmission.
• High mechanical strength and impact-resistance
• Resistance to thermal shock especially when intense heat loads on window surfaces induced by microwave plasma and laser impingement.
• Low thermal expansion properties
• Robustness to peak pressures in the order of 300 MPa
• Possibility of near-net shape processing - cylindrical shapes
• Low polishing and processing cost.

Diamond windows are best suited for such applications. Transparent ceramics such as aluminum oxynitride (AlON), magnesium aluminate (spinel), and single crystal aluminum oxide (sapphire) also hold promise, not only because of their favorable transparency in the wavelengths of interest, but also due to their robust mechanical properties.

AlON is an isotropic material that can be produced as a transparent polycrystalline compact. The advantage of AlON is that being polycrystalline, it can be shaped in complex geometries using conventional ceramic forming techniques such as pressing and slip casting. Its high cost and the small sizes that can be made limit its application.

Magnesium aluminate spinel (MgAl₂O₄) is a ceramic with a cubic crystal structure and is transparent in its polycrystalline form. Spinel offers some processing advantages over AlON. It is also capable of being processed at much lower temperatures than AlON, and has been shown to possess superior optical properties within the IR region. Spinel shows promise for many applications, but is currently not available commercially in bulk form from any manufacturer although there is considerable ongoing research efforts to make large sized plates that are 12 x 20 inches by TA&T in Maryland.

Single crystal aluminum oxide (Sapphire - Al₂O₃) is a transparent ceramic with rhombohedral crystal structure. Its properties are anisotropic and vary with crystallographic orientation. It is currently the most mature transparent ceramic and is available from several manufacturers, but the cost is high due to the high processing temperature involved and machining costs to fabricate parts from single crystal boules. It has a very high material strength, but the transparency is dependent on the surface finish. Fabrication of complex and large sized objects is difficult and often very expensive.

Infrared-transparent polycrystalline alumina shows significant promise for use in the fabrication of transparent windows. With similar chemical composition and physical properties as sapphire, polycrystalline alumina is expected to be ~1/2 as expensive and can be cast into difficult shapes, such as a cylinder. Alumina is also potentially superior to the competing material, aluminum oxynitride (ALON) since it has approximately 3 times greater thermal shock resistance than ALON.

Despite the above promises, “transparent” alumina ceramics obtained so far exhibit lower mechanical parameters than expected and a low in-line transmission of unscattered light (<> 20 µm) .

Sub-micron grained alumina-rich glasses show potential for fabrication of high-strength, transparent windows with good thermal resistance. Although alumina has been conventionally added in small amounts to silicate glasses as a network-former, this oxide cannot be obtained as a bulk glass by itself. Alumina-based glasses can only be prepared by super fast quenching techniques, which limits their dimensions to a few millimeters [7].

A recent development has been reported by a 3M group involving production of high alumina glass by viscous sintering of glass microbeads prepared by rapid quenching of flame sprayed precursors[8]. This opens the route to other related techniques such as plasma spray-quenching.

Study of these window materials is still in the nascent stages, as there is currently not much information available on the effect of high temperature and plasma on these materials. Development of a suitable window material will bring Star-trek type teleportation a century or so closer to reality.

References

1.NASA (1994). Commercial Space Transportation Study.

2. Kantrowitz, A. Propulsion to orbit by ground-based lasers. Astronautics and Aeronautics 10; 74-76 [1972].

3. Minovich, M.A. California Institute of Technology (1972). The laser rocket - A rocket engine design concept for achieving a high exhaust thrust with high ISP.

4. Wang, T.S., Chen, Y.S., et al. (2002). Advanced performance modeling of experimental laser lightcraft. Journal of Propulsion and Power 18(6): p. 1129-1138.

5. http://www.lightcrafttechnologies.com/applications_math.html#compare

6.Harris, D.C; Durable 3-5mm transmitting IR window materials; Infrared Phys Technol. ; 39(4); 185-201 [1998]

7. Weber, J. K. R., Abadie, J. G., Hixson, A. D., Nordine, P. & Jerman, G. A. Glass formation and polyamorphism in rare-earth oxide-aluminum oxide compositions. J. Am. Ceram. Soc 83, 868-872 (2000).

8. Rosenflanz, A; Frey, M; Endres, B; Anderson, T; Richards, E; Schardt, C; Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides; Nature; 430 [2004]

*Isp is the specific impulse: the ratio of the thrust to the flow rate of the weight ejected. It is expressed in seconds.
Isp = F/qg where F is thrust, q is the rate of mass flow, and g is the acceleration of gravity at ground level.
When the thrust and the flow rate remain constant throughout the burning of the propellant, the specific impulse is the time for which the rocket engine provides a thrust equal to the weight of the propellant consumed. Wikipedia gives a detailed description of Isp here.

**The height to which the craft can be propelled depends upon the power of the laser beam. I read here that a 10-kilowatt laser can carry a vehicle the size and weight of an empty soda can into low earth orbit. We would require 10 times the currently available laser power to fly to the edge of space.