Michael R. Holmes

Solar Thermal Propulsion IHPRPT Demonstration Program

Abstract : Spacecraft powered by solar thermal propulsion engines will be able to provide the velocity change required to economically maneuver large payloads from one orbit to another or to perform interplanetary missions. This innovative concept, when applied, will double the efficiency of currently used LH2 - LO2 chemical upper stages. Solar thermal propulsion uses the suns energy to heat a low molecular weight working fluid such as hydrogen to very high temperatures (3,000 K). The stored thermal energy is then converted to kinetic energy as the working fluid exits a diverging nozzle.

Solar Thermal Propulsion IHPRPT Demonstration Program Results

Abstract : Spacecraft powered by solar thermal propulsion engines will be able to provide the velocity change required to economically maneuver large payloads from one orbit to another or to perform interplanetary missions. This innovative concept, when applied, will double the efficiency of currently used LH2 - L02 chemical upper stages. Solar thermal propulsion uses the suns energy to heat a low molecular weight working fluid such as hydrogen to very high temperatures (3,000K). The stored thermal energy is then converted to kinetic energy as the working fluid exits a diverging nozzle. Under Integrated High Payoff Rocket Propulsion Technology (IHPRPT) funding, the Air Force Research Lab (AFRL) has sponsored the team of Thiokol Propulsion and SRS - Technologies to demonstrate the technological readiness and performance of an inflatable solar thermal propulsion system. This paper will address the results of this program, which includes the fabrication and thermal vacuum testing of a 4 X 6 meter inflatable flight quality solar concentrator. The program culminates in a full-up integrated proof- of-concept ground test of a direct gain solar thermal propulsion system. The results of this test will be reported. These tests will demonstrate that the technology is ready for development of flight hardware for Solar Orbital Transfer Vehicles.

Solar Bi-Modal System Concept: Mission Applications, a Preliminary Assessment

The current fleet of medium/heavy expendable launch vehicles (ELVs) and upper stages are expensive, inflexible, and non‐responsive to the needs of the satellite designer/builder. These transportation systems confine satellite designers to a very narrow operational envelope. If a satellite exceeds its mass budget by even a few percent, mission planners must choose between eliminating instrumentation (reducing the spacecraft’s capabilities) or launching on a larger/more expensive ELV. Many people have suggested the development of a new, less expensive ELV to reduce launch costs. While such a system may eventually repay its development cost, current budgets do not make this approach practical. A new upper stage based on chemical technology is also likely to be expensive, with little performance improvement. In order to significantly improve the cost effectiveness of launch assets, alternate propulsion technologies must be developed. The approach to electrical power system design should also be modified. Curr...

Solar bi‐modal system concept: Common development issues with nuclear systems

Solar thermal propulsion promises great advantages over current chemical upper stages (Meserole 1993). Solar thermal propulsion uses environmentally acceptable and free radiant energy to transfer payloads from LEO to GEO more efficiently than chemical propulsion and more rapidly than electric propulsion. Like nuclear thermal propulsion, solar thermal propulsion can be combined with a power conversion system to form a bi‐modal system capable of providing a spacecraft with both power and propulsion from a single energy source. While the power source for a solar bi‐modal system may be very different from that of a nuclear bi‐modal system, they share a number of common development issues. Both systems must address issues such as: conversion of thermal to electrical energy, waste heat removal, power management and distribution, and the natural tendency to resist change within the spacecraft community. This paper will describe a solar thermal bi‐modal concept and highlight the areas where development work can b...

Optical performance of thin film, inflatable, solar concentrators for illuminating PV arrays

Inflatable technology for antennas and solar concentrators is rapidly maturing. Large inflatable paraboloids, developed for space propulsion systems and antennas are now available for ground test of power systems. Inflatable structures can potentially reduce spacecraft weight and decrease the volume required by a power system. Reflective surfaces do not require much mass so that inflatable antennas or concentrators can be very lightweight. Inflatables can also be packaged more compactly than rigid structures thereby reducing volume constraints in a faring. Therefore, we believe that space power applications could benefit from this technology. This paper includes a basic analysis of a candidate power system using an optical ray-trace code. By using a large area concentrator, it is hoped that the paraboloid concentrators for solar-thermal propulsion. This analysis will show if a uniform distribution of light can be concentrated on a photovoltaic array so that useful power might be generated. In the future, other technical challenges must be addressed. In particular, heat rejection will be a problem since then must be accomplished from the smaller area of the photovoltaic array as opposed to the area of the concentrator.

Solar Thermal Vacuum Testing of an Integrated Membrane Concentrator System at the NASA GRC Tank 6

This paper reports the results of recent highly integrated thermal vacuum testing of key components of a membrane concentrator system. The Dual Use Science and Technology (DUS&T) Electromagnetic Radiation Control Experiment (EMRCE) Team, including SRS Technologies, ATK Thiokol Propulsion, Boeing, Air Force Research Laboratory (AFRL), and National Aeronautics and Space Administration (NASA) is chartered to partner industry with government to develop flight qualified membrane structures. Specifically, DUS&T EMRCE aims to complete development of membrane concentrator technologies to achieve flight experiment readiness in anticipation of a near term flight experiment opportunity. Potential flight experiments are in the areas of solar thermal propulsion, space power, and RF communications.

Wave Front Sensor for Solar Concentrator Control (October 2007)

This article is the culmination of research directed into finding a system to control the position of the focal spot of paraboloid concentrators for use in terrestrial and space solar concentration applications. After a brief introduction into the area of study, the article describes how a normal Shack-Hartmann wave front sensor is modified for use in detecting and tracking the focal spot. The paper details the analysis and development of the algorithms used in locating the focal spot on a thruster absorber utilizing a correlation method and an area centroid method. The article concludes with a paragraph on suitable future work.

Paraboloidal Thin Film Inflatable Concentrators and their Use for Power Applications

Abstract : This paper deals with a proposal to use thin film inflatable concentrators, currently used for propulsion, in other applications, such as power. Technology for precision paraboloidal thin film concentrators is becoming available for use as a byproduct of propulsion technology. The idea is to introduce the possibility of using this formerly strictly propulsion hardware to power photovoltaic (PV) cells. Several intensity profiles will be generated from an optical model and shown for thin film inflatable concentrators.