N241-015 TITLE: Enhanced Emissivity in High-Speed Window Materials

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Hypersonics; Space Technology

OBJECTIVE: Identify and develop methods to enhance the emissivity of sensor window materials capable of surviving high-speed flight environments.

DESCRIPTION: Weapons technology advancement has been driving missile systems to strive for greater speeds, ranges, and accelerations; all of which put much greater thermal and mechanical stresses on the system. Often the weak link in these designs is the sensing aperture, whether it be an Electro-Optic/Infrared (EO/IR) window or a Radio Frequency (RF) radome. In addition to being a structural material, these must also be transparent within the relevant wavebands, and maintain this transmission throughout the flight. This limits material choice significantly, and forces performance trade-offs to survive the high enthalpies.

Over the duration of a flight profile, transient heating of a window typically involves an energy balance of convective heat transfer, radiative heat transfer and conduction between the window, the external environment the weapon is flying through, the internal environment of the weapon, the weapon structure the window is attached to and the radiative exchange between the window and both the external and internal environments. The majority of the flight profile results in a strong convective aeroheating input into the window with some periods of aerocooling where the window is hotter than the surrounding recovery temperatures. As the window rises in temperature, radiative heat loss to both the external and internal environments also occurs. As the window temperature gets hotter, the magnitude of that radiative heat loss increases by a factor of temperature to the fourth power and at higher temperatures can result in equilibrium temperatures hundreds of degrees cooler than without the presence of radiative heat loss. Heat removal due to convection is limited in effectiveness due to the flight speeds involved and the negative effects of relying on internal convection to sink heat to the interior of the weapon. Enhancing the conduction of windows is limited in effectiveness without unintentionally altering the electromagnetic properties; plus the structural attachment location of the window is at similar temperatures, preventing the needed thermal gradient to conduct heat away into those surrounding structures. Radiation, however, is different; while the distribution of energy "available" to be radiated is simply a function of temperature (known as the blackbody curve), each material has its own emission spectrum, which describes at what wavelengths energy can be emitted. There is often, and in the case of IR windows necessarily, a gap in which the material cannot emit radiation. If part of the blackbody curve lies within this gap, that energy cannot be radiated, and as such can’t contribute to cooling the window.

This is particularly a problem for IR windows, which rely on this gap in order to function. The typical mid-wave IR (MWIR) band is from 3–5 µm, meaning there can be little to no emission within these wavelengths, and typically not much below this as well. Even at the relatively low temperature of 500 °C, nearly 50% of the available energy lies between 1 and 5 µm, where essentially no emissions are expected to occur. This only gets more significant as temperatures climb and the blackbody curve shifts to shorter wavelengths.

If even narrow emission peaks could be engineered at shorter wavelengths, without interfering with the desired transmission window, it could increase energy dissipated through radiation drastically. At 1000 °C, a half micron band centered around 2.25 µm contains 15% of the energy available in the entire spectrum, roughly the same as all emission above 6 µm (which is about where state-of-the-art MWIR windows begin emitting). If this unused energy can be taken advantage of, the range of environments a window could operate in could be greatly expanded, and with it the mission space and possibly the performance of the system as a whole. A similar approach was used on the space shuttle, with the black tiles on the bottom used to maximize emissivity; it just didn’t have the complication of needing to be a functioning aperture.

A successful project would produce a set of test articles demonstrating a significant increase in emissivity while maintaining transmission characteristics at high temperatures (> 1000 °C for IR materials, > 1250 °C for RF materials) within the chosen waveband (MWIR, X-Band, Ka-Band, etc.). The test articles must also demonstrate resiliency to stresses which would be encountered in high-speed flight. Testing for this may vary depending on the proposed solution, but may include: high high-temperature mechanical tests, thermal shock tests, electrical tests, arcjet/plasma torch testing, and microstructural examinations.

PHASE I: Develop a process/material that demonstrates significantly increased emissivity of the chosen window material without degradation of transmission in the relevant waveband. Show that the concept can feasibly meet the requirements of high-speed flight through analysis, modeling, and/or characterization of materials of interest. The Phase I effort will include prototype plans to be developed under Phase II.

PHASE II: Develop and deliver notional full-scale prototypes (minimum of two) that demonstrate functionality under the required service conditions, including thermal and mechanical stresses. Produce sufficient test samples for material characterization efforts to show viability for high-speed flight as described in the Description section.

PHASE III DUAL USE APPLICATIONS: Work with a program office to produce a system-applicable window. Participate in qualification testing equivalent to the system, including environmental and hypersonic wind tunnel testing.

There have been some recent efforts looking into controlling emissivity to provide efficiency increases in thermophotovoltaic power generation, which this could possibly feed into. Space applications are also possible, as the only way to dump heat in a vacuum is through radiation. There could be some niche private sector applications, which utilize high-temperature windows as well, but unless commercial high-speed travel grows, this market is limited.

REFERENCES:

1. Harris, D. C. (1999). Materials for infrared windows and domes: properties and performance (Vol. 158). SPIE press. https://doi.org/10.1117/3.349896
2. Walton, J. D. (Ed.). (1970). Radome engineering handbook: design and principles (Vol. 1). M. Dekker. https://www.worldcat.org/title/1314367401

KEYWORDS: Window; Aperture; Hypersonic; Emissivity; Infrared; Radome

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