Volume 8, Number 2     March/April 2000

Liquid Rocket Engine Performance Improved

Liquid rocket engine performance has historically been constrained by the limitations of materials and processes used for component fabrication. Typically, the thrust chamber, in which the propellants are combusted, must be cooled to avoid materials degradation or failure during operation. Cooling can effectively be achieved by various means, including the use of boundary layer cooling, but the requirement for cooling results in an efficiency penalty.

Rocket engine efficiency is generally defined by a parameter identified as specific impulse, Isp, which is effectively a measure of pounds of thrust per pound of propellant per second consumed. Isp is measured in units of seconds and is more or less analogous to miles per gallon‚ with higher values of Isp indicating higher operating efficiency. When considering the high costs of sending a single pound of payload to orbit, the value of a modest gain in Isp becomes evident. For selected rocket engine operating cycles, increases in Isp can be gained by increasing the allowable operating temperature of the thrust chamber, thus decreasing the coolant flow requirements. However, such increases in operating temperature must not be made at the expense of reliability.

Applying this concept of higher efficiency to satellite apogee insertion is particularly beneficial for both commercial and government, including military, users. As satellite complexity and power requirements have grown with demands such as those for enhanced communications capabilities, satellite weight has also increased. Achieving a balance between launcher capability and the ability to insert these heavier satellites into geosynchronous orbit from their initial low-Earth orbit has led to a need for more efficient satellite insertion engines.

Such efficiency has been achieved by the use of the high-temperature oxidation-resistant thruster materials that were developed in part as a result of NASA Small Business Innovation Research (SBIR) contracts with Ultramet. The resultant materials and processes have led to capability for reliable thruster operation at temperatures greater than 2,000 degrees Celsius while relying only on radiative cooling. This capability has limited the need for boundary layer cooling of apogee insertion engines, leading to several seconds gain in Isp, thus allowing for the insertion of heavier satellites into geosynchronous orbit. This technology was successfully flown in space on the Hughes Orion 3 in May 1999.

The approach used for the development of such high-performance materials is simple in concept but innovative in its execution. Using chemical vapor deposition (CVD) processing, Ultramet produces these high-temperature capability thrusters from the inside out. First, a mandrel is fabricated with an outer diameter conformed to the desired thruster inner diameter. This mandrel is then fitted within a CVD reactor. A thin layer of high-melting-point iridium metal is deposited by CVD onto the mandrel. The thin, inner iridium layer provides high-temperature oxidation protection for the remainder of the materials system. A thicker, structural layer of rhenium metal with a melting point of 3,180 degrees Celsius is then deposited by CVD over the iridium. Additional outer layers to facilitate joining of the structure to other components and to provide desired high-temperature emissivity characteristics are finally deposited onto the outer surface. The mandrel is removed using a proprietary process that does not adversely affect the iridium liner. The overall structure has a net-shape inner diameter. External surfaces are ground when needed to meet dimensional or other requirements.

Several large companies in the satellite industry, such as TRW/Lockheed Martin, Kaiser Marquardt/Hughes and Kaiser Marquardt/Loral, as well as NASA, have invested nearly $25 million in the development of this technology. The payoff for the satellite industry may be quite large; Ultramet estimates that as much as a 3-percent improvement in the life of a satellite could be gained. That can be equated to a 50- to 100-kilogram increase in the weight of communications mass on a satellite.

Ultramet has such iridium/rhenium thrusters currently in production, and they have been successfully operated on latest generation satellites. This immediate application is only one of the potential commercial and government applications of this SBIR-initiated technology. The possibility of enhancing the performance of Space Shuttle maneuvering thrusters by use of this technology is currently under study.

The iridium/rhenium technology also has benefits beyond that of thrust chambers, including the potential for leading edges of hypersonic structures. And the inside-out fabrication method is not limited to thrusters. For example, in current work, the method is being applied to the development of high-aspect-ratio, axially symmetric structures. In some cases, the technology is being extended to the development of refractory metal lined, ceramic or composite overwrapped structures that maintain the benefits of long life at high temperature while providing reduced weight through the use of lightweight composites.

For more information, contact Dr. Steve Schneider at Glenn Research Center. 216/977-7484, steven.j.schneider@grc.nasa.gov Please mention you read about it in Innovation.