Chinh T. Nguyen

A NEW DENSIFIED PROPELLANT MANAGEMENT SYSTEM FOR AEROSPACE VEHICLES

Sierra Lobo is developing the Densified Propellant Management SystemTM (DPMS) for the National Aeronautics and Space Administration (NASA) under the Space Launch Initiative 2nd Generation Reusable Launch Vehicle Program. The DPMS utilizes a number of innovative densifier and mass gauging system technologies including the patented Cryo-Tracker TM temperature and liquid level sensing probe that will safely, efficiently, reliably, and cost effectively produce, maintain, and mass gauge densified cryogenic liquids for use on board aerospace vehicles. Densified cryogenic liquid propellants have the benefit of increasing vehicle payload performance or decreasing vehicle mass, decreasing system-operating pressures, increasing engine system life, and increasing engine and vehicle safety margins. The patent-pending DPMS technology presented here can be scaled for a variety of system applications such as reusable and expendable launch vehicles, upper-stages, space-based platforms, orbit transfer vehicles, and interplanetary vehicles.

High-Power Thermoacoustic Stirling Heat Engine Results

We are developing a thermoacoustic Stirling heat engine (TASHE) to drive a pulse tube refrigerator (PTR) and electrical linear alternator for instrument cooling and power generation on a Venus lander. The TASHE will produce acoustic (PV) power and deliver it both to the linear alternator to generate electrical power and to the PTR to generate refrigeration power. Thus, this duplex system will consist of a TASHE, a PTR, and a linear alternator, simultaneously producing cooling and electrical power from an input heat source. The system is expected to be highly efficient, and will have no moving parts at high temperature, which will be very reliable. This paper will discuss our Venus Lander Duplex System (VLDS) conceptual design, results of a trade study that included a single-stage refrigerator, the coldbay thermal performance option with a multiple-stage refrigerator, and initial test results of the Sierra Lobo 12 kW input-power TASHE. The TASHE was designed using DeltaEC software to use 12 kW of input power at the hot heat exchanger to produce 4 kW of acoustic power, with a resulting efficiency of 30%. The TASHE hot heat exchanger operates at 950 K and the cold heat exchangers operate at an ambient temperature of 300 K. The system mean pressure is 3.45 MPa and the pressure amplitude at the TASHE outlet is 12% of the mean pressure. The operating frequency is 30 Hz and is controlled by a gas resonator. For the Venus application, the gas resonator will be replaced by a free-piston resonator, which will reduce acoustic loss significantly, as well as being smaller than the gas resonator, making it more feasible for space applications.

Thermoacoustic Duplex Technology for Cooling and Powering a Venus Lander

Sierra Lobo, Inc. is developing a technology that can provide both cooling and electric power generation using heat. When coupled with a radioisotope heat source, the technology is ideally suited to the needs of a long-lived Venus lander. The heat source powers a Thermoacoustic Stirling Heat Engine (TASHE), which is directly coupled to a Pulse Tube Refrigerator (PTR) in a duplex configuration. A unique feature of the Venus Duplex System is the use of the supercritical carbon dioxide Venus atmosphere as the working fluid. A linear alternator, also directly coupled, generates electricity. The initial SBIR Phase I detailed thermoacoustic modeling results indicate that a TASHE working at 23 percent efficiency and a PTR operating at 23.7 percent efficiency can effectively produce 20 W of electrical power and 154 W of cooling at a temperature of 350°C using the heat from 15 General Purpose Heat Sources (GPHS). The Venus Duplex System thermoacoustic model was used to design a Similitude Duplex System that has the same geometry but uses supercritical nitrogen as the working fluid. This results in a Similitude Duplex System that operates at reduced pressure and temperature enabling the manufacture and testing of Similitude Duplex System hardware for validating the thermoacoustic models without the need for exotic high temperature materials. Physical models of the resulting Similitude Duplex System hardware configuration have been developed.

4 kW Power Thermoacoustic Stirling Heat Engine Test Results

A Thermoacoustic Stirling Heat Engine (TASHE) that produces 4 kW of acoustic power is being developed to drive a pulse tube refrigerator (PTR) for instrument cooling and a linear alternator for power generation on a Venus lander. This duplex system is expected to be highly efficient and has no moving parts at high temperature, improving reliability. Test data on a modified 4 kW TASHE using a dummy acoustic load indicates the engine has a 36.4 percent actual efficiency (acoustic to electrical input power) at 945 K operating temperature and 287 K heat rejection temperature, resulting in a 52.2 percent Carnot efficiency.

The use of Proton Exchange Membrane Fuel Cell Technology to Recover Gaseous Helium from Rocket Testing Systems

Many rocket testing systems used by both the National Aeronautics and Space Administration (NASA) and commercial space agencies utilize liquid hydrogen (LH2) as a propellant. Subsequent to testing, significant quantities of LH2 and gaseous H2 (GH2) from boil-off may remain which must be removed through gas purging. This is accomplished with gaseous helium (GHe) because it is chemically inert and the boiling point of liquid helium (LHe) is lower than that of LH2. This ensures that the He will remain in the gaseous phase throughout the purging process. The resulting purge stream consists of a mixture of GHe and GH2 which must be sufficiently diluted and vented to the atmosphere or recaptured and separated.

Advanced Insulation Techniques for Cryogenic Tanks

[Abstract] The ability to store large amounts of cryogenic fluids for long durations has a profound affect on the success of many future space programs using propellant, reactant, and life-support cryogens. These missions will require on-orbit systems capable of long-term storage of cryogens for applications such as: space transportation, orbit-transfer vehicles, space-power systems, spaceports, lunar-habitation systems, and in-situ propellant systems. The high cost of delivering payload mass to orbit will require storage systems capable of limiting cryogenic losses due to boil-off to less than two percent per year for mission durations of up to ten years; or in some cases, completely eliminating boil-off losses. Highperformance insulation of 150 layers or more of multilayer insulation (MLI) will likely be needed to meet the requirements of future long-term missions. Limited data exists on the performance and physical characteristics of these thick MLI systems. Sierra Lobo, under a Missile Defense Agency, SBIR Phase I research program, developed such a tool called the MLI Design Code. This MLI Design Code uses empirical estimates of the heat flux due to basic butt seams and un-insulated penetrations, based on test results reported by Sumner. However, this analysis needed to be expanded to include higher fidelity models of more efficient seam and penetration insulation concepts. In thick MLI systems, the seams and penetrations are expected to be the major sources of heat flux into the cryogenic-storage system. Sierra Lobo, in a NASA SBIR Phase I research program, developed models that analyzed the heat transferred through selected seam and penetration concepts, which can be major contributors to cryogenic losses in MLI systems. The importance of minimizing heat leak through seams and penetrations is best illustrated with an example. In a 50-cubic-meter, cylindrical, (L/D=2) liquid-hydrogen tank insulated with 150 layers of MLI, the heat flux through the basic insulation would be approximately 7.3 watts for a warm-boundary temperature of 300 K. Using the data reported in Reference 2, the heat flux through one 3.81 cm outside diameter fiberglass penetration (66 cm long) would be 0.403 watts and 4.44 watts through 26.3 meters (considered to be minimum for this tank configuration) of a basic butt seam. Assuming it would require a minimum of eight fiberglass support penetrations to support this tank, the seam and penetration heat fluxes would represent at least 51 percent of the total heat flux into the tank. As the MLI thickness requirement increased, the contribution from seams and penetrations would also increase. Minimizing these contributions becomes very important in storing cryogens for long periods of time. This paper presents the results of this effort, including stand-alone analysis and the incorporation into the main MLI design code.

Modeling RL10 Thrust Increase with Densified LH2 and LOX Propellants

Densified propellant technology has been demonstrated to improve performance capabilities of launch vehicles and employment of densified propellants is under investigation within several launch vehicle programs. Analysis of rocket engine performance using densified propellants is a key component of the research necessary for optimization of engine design and is required in order to advance the maturity of densified propellant technology. A model of the RL10A-3-3A rocket engine utilizing the Rocket Engine Transient Simulator (ROCETS) mathematical modeling program has been modified to simulate densified propellant inlet conditions to determine the effects on overall system operating conditions. The nominal operating conditions of the RL10A-3-3A will be compared to operating conditions obtained at various levels of densified propellant inlet conditions. This system analysis will determine the effects of the colder inlet conditions on the overall system performance and will identify rocket engine system component interactions as influenced by the presence of densified propellants. The research results presented last year at the 38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference (Ref. 1) quantified some of the key benefits for densified propellants in engine systems including pressure drop reduction and decreased turbine/pump speeds, which results in increased engine life and reliability. This second paper on this research will present the effects of densified propellant on increased engine thrust. Flowing densified hydrogen and oxygen through the RL10 turbo-machinery operating at nominal rotational speeds can result in a nominal increase in thrust level of 12 percent. This provides an additional degree-of-freedom for aerospace vehicle designers to determine the optimal usage of densified propellants; increased thrust or decreased turbomachinery speeds and pressure drops. In addition, the RL10 model is being used to examine a variety of densified oxygen and hydrogen propellant inlet conditions for a given set of required engine operating conditions such as thrust level, mixture ratio, and chamber pressure.