Measure of Success
An SwRI-developed fuel gauge for spacecraft will save fuel and money
By Danny M. Deffenbaugh and David B. Walter
The most expensive phase of any space mission, no matter how far it intends to go, is the high cost to attain orbit. The National Aeronautics and Space Administration (NASA) has deemed the development of improved launch systems critical for its next generation of space exploration vehicles, with a goal of reducing the cost to orbit by a factor of 10. Because the largest portion of any launch vehicle is the initial mass of rocket fuel, any reduction in mass delivered to orbit will reduce the quantity of rocket fuel needed, and consequently the overall mission cost.
Different space missions have different on-orbit requirements. Reusable launch vehicles for manned missions, such as the space shuttle or future shuttle replacements, must carry liquid propellants into orbit to provide the propulsion required for on-orbit maneuvering and for return to Earth. Precise knowledge of the quantity of propellants available for the "de-orbit burn" for reentry into Earth's atmosphere is crucial and is required for planning on-orbit operations. To accommodate inaccuracies in the gauging of these on-orbit propellants in the weightless environment of space, additional propellant reserves must be launched to ensure a safe return. These reserves result in added launch fuels and thus increased launch mass and cost.
SwRI Gauging Program
Southwest Research Institute (SwRI) has supported NASA in launch vehicle and on-orbit propellant dynamics since the 1950s. The Institute's first involvement in on-orbit fuel gauging was to address a gauging problem on one of the early Apollo lunar landing missions. The gauging technology employed had falsely indicated insufficient fuel to complete the critical landing maneuver because of fuel sloshing. With help from Institute engineers, the problem was solved before the next lunar mission.
During the past decade, SwRI has collaborated with NASA to develop a concept known as the compression mass gauge (CMG). This collaboration began with an SwRI internal research and development project and a NASA Space Act Agreement. This effort culminated in a program designated as the Micro-Gravity Advanced Upper-stage Gauging Experiment (µGAUGE). The purpose of this program was to advance the CMG technology to flight-like hardware with realistic mission constraints.
On-orbit Gauging Concepts
The gauging of liquids in containers on Earth is a simple, everyday occurrence. Typically, the method used is a float that "rides" on the flat liquid surface. The float is attached to a lever arm that senses its position. As the liquid quantity in the container changes, the float position moves up or down and the sensor indicates a change in fill level. In space, without the influence of gravity, the liquid surface is not flat and this simple method is not viable.
There are a number of in-space alternatives. One approach is to activate the orbit maneuvering propulsion system to "settle" the propellants and create a flat surface caused by acceleration of the vehicle. However, this method uses up valuable fuel that has been lifted into orbit. It also may alter the mission by repositioning the spacecraft for the sole purpose of gauging fuel. This method is extremely limited and has its own launch mass penalties.
There are three non-settling gauging methods: bookkeeping, pressure-volume-temperature (PVT) and CMG. The bookkeeping method uses a flow meter in the tank exit line to measure flow rate. The measured flow and duration of a flow event are used to calculate the total mass of flow used. When subtracted from the initial fill level, this gives the balance of liquid remaining. For the PVT method, a non-condensable gas is injected to increase the tank pressure. The mass of gas injected, and the resulting pressure increase in the tank, are then used in a pressure-volume-temperature relationship to calculate the ullage volume. This volume is subtracted from the total tank volume and, with the liquid density, is used to determine the remaining liquid mass. The CMG method uses a small tank volume change and the resulting pressure change with a thermodynamic relationship to calculate the ullage volume. The liquid mass is then determined the same way as with the PVT method. All three non-settling methods do require some added hardware and thus a small amount of added launch mass. However, the improved gauging accuracy, and the resulting reduction in propellant reserve, more than offset this additional hardware.
There are significant differences in the accuracies of these methods, as illustrated in the following example: A manned mission to the Space Station will require on-orbit propellants for de-orbiting, on-orbit operations, boil-off and 10-percent reserve. The de-orbit burn requires approximately 10 percent of the original fluid inventory. At this fill level, the accuracy of the CMG is within 2 percent, while the bookkeeping method accuracy is 8 percent and the PVT method 11 percent. Using the best of these methods, CMG, the reserve propellant can be reduced significantly. The launch cost implications of this can be calculated for both current launch vehicles, with an estimated payload launch cost of $10,000 per pound, and the next-generation goal of $1,000 per pound. Typical propellant loading for the orbital maneuvering system is about 24,000 pounds of propellant. A six-percent reduction results in a savings of $14 million per mission for current launch systems, or $1.4 million per mission for the next generation launch vehicle goal.
Compression Mass Gauge
The application of the CMG concept is to compress the vapor in a tank using a small bellows and sense the resulting pressure change. The physics behind this approach is known as the perfect gas law. This law simply states that a closed container filled with a quantity of gas at a fixed temperature will exhibit a pressure inversely proportional to the vapor volume. The accuracy of this method is a function of the accuracy in sensing the pressure change and the magnitude of the volume change. The more accurate the sensor, the smaller the volume change required for a given accuracy. Current sensor technology requires a volume change of about two hundredths of a percent of tank volume. The smaller the volume change required, the smaller the size of the gauge; the lower the launch mass of the gauge, the less power needed to operate the gauge.
One of the major challenges of a flight CMG unit is the severe operating condition imposed by the temperature of the preferred propellants. The most effective propellants for launch vehicles and on-orbit operations are liquefied hydrogen and oxygen, which are stored at cryogenic conditions. For testing purposes, SwRI engineers designed the gauge to operate at both ambient temperatures and at various cryogenic conditions. The challenge was to accommodate various levels of contraction as temperatures decrease. The temperature of liquid oxygen is -297 degrees F; liquid nitrogen is slightly colder at -321 degrees F and liquid hydrogen considerably colder still, at -423 degrees F. To minimize the power requirement, the internal gauge pressure and the propellant tank pressure are automatically equalized before each gauging event. Additional complexities for realistic space applications include provisions for mass transfer between the liquid and vapor phase, non-adiabatic and non-reversible compression process, slightly compressible liquid, slightly elastic tank, sloshing induced by bellows operation, acoustic wave effects and non-homogeneous stratification.
The flight configuration selected for design of the SwRI gauge was a 67-cubic- foot hydrogen tank operating over a pressure range of 14.7 to 50 pounds per square inch. The volume of the CMG was to be less than 1 cubic foot, with a peak power consumption of less than 100 watts. The accuracy for this flight configuration was to be 2 percent at a 90-percent fill level. This design requirement resulted in a 2.2-cubic-inch swept volume of the bellows. In its completed form, the gauge is 0.25 cubic feet, weighs 17.5 pounds and consumes 55 watts of power.
To meet the required temperature range, the design had to match the thermal expansion properties of all the materials that would be used to construct the gauge. SwRI engineers used various grades of stainless steel. Off-the-shelf parts, such as bellows, bearings, springs and seals were also of different grades of stainless steel. SwRI-designed parts as well as the gauge housing were fabricated by Institute machinists.
The SwRI-designed gauge was tested in a 2.5-cubic-foot liquid nitrogen tank operated at ambient pressure. The reference measurement system is a load cell, which measures the mass of the liquid in the tank. The accuracy of this reference system is ±0.15 percent. The expected measurement accuracy is estimated to be ±2 percent. Based on the tests conducted, researchers documented the accuracy at ±1.4 percent of fill level, which meets the project objective.
The results of this program are encouraging and demonstrate the feasibility of this promising concept, but additional work is still needed. The critical sensor in the CMG is an off-the-shelf commercial pressure sensor that was used at the bottom 2 percent of its operating range. A sensor custom-designed for the actual operating range will greatly improve performance. Furthermore, the accuracy stated is for all operating conditions. The team discovered that when the sensor is located in the vapor, the accuracy is twice as good as when the sensor is located in the liquid. This phenomenon should also be explored to further improve performance. The next stage in maturing this technology is to integrate the custom-built pressure sensor and to test the CMG in a larger tank filled with liquid hydrogen. The expectation is to improve the accuracy to within less than one percent.
With the redirection of NASA toward manned exploration, the requirement for on-orbit operations, a lunar base and eventual Mars missions will generate scenarios for on-orbit propellant depots that need accurate mass gauging. These programs will have an even greater requirement for improved cryogenic fluid management, long-term storage, propellant refueling and precise knowledge of fluid inventory. Improved mass gauging will be extremely valuable to these future efforts, and SwRI is ready to play a role in furthering this technology.
Comments about this article? Contact Deffenbaugh at (210) 522-2384, or firstname.lastname@example.org.
The authors gratefully acknowledge the contributions of SwRI staff members Staff Engineer Steven T. Green, Technical Advisor Dr. Franklin T. Dodge, Research Engineer Russell Burkey, Engineering Technologist Marion Burzynski, Engineer Shane P. Siebenaler, SwRI retiree A.C. Rogers and former SwRI staff member Steven P. Petullo. The authors also appreciate the support and collaboration of the NASA Glenn Research Center.
Published in the Summer 2004 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.