A combination of procedure and equipment for loading liquid hydrazine into a spacecraft fuel tank that contains a diaphragm or bladder would be modified, according to a proposal. The purpose of the modifications is to enable fueling technicians to work safely, during all but a small part of the loading process, in less-restrictive protective attire.
Heretofore, for reasons too complicated to describe in this article, the task of loading hydrazine into fuel tanks that contain diaphragms has posed such a high risk of leakage and spillage of hydrazine that technicians engaged in this task have been required to wear protective garments called Self-Contained Atmospheric Protection Ensemble (SCAPE) suits, which are bulky and which restrict movement so much as to make the simplest tasks difficult. Recently a combination of equipment and a procedure that entails the wearing of less-restrictive, less-protective splash suits has been developed; however, this combination is applicable only to tanks that do not contain diaphragms. The essence of the proposal is to modify this recently developed combination to make it safe to load a tank that contains a diaphragm while wearing splash suits instead of SCAPE suits during most of the loading process.
The proposed modifications address two concerns that bear on safety — again, for reasons too complicated to describe in this article. These concerns are the following:
- A pressurant gas (ordinarily, helium) is used. This gas must be prevented from entering the spacecraft fuel tank on the fuel side.
- The equipment includes, among other things, a fuel-supply tank and a transfer tube called the "flexline." Once the spacecraft fuel tank has been loaded, the flexline must be purged with the pressurant gas to blow liquid hydrazine back into the fuel-supply tank.
According to the proposal, the equipment would be modified by three additions: (1) a liquid/gas detector would be incorporated into the flexline between the fuel-supply tank and a service valve on the spacecraft fuel tank, (2) an automated valve controlled by the liquid/gas detector would be incorporated into the flexline between the service valve and the liquid/gas detector (see figure), and (3) a new purge fitting would be installed at the service valve.
The liquid/gas detector would indicate whether the flexline holds liquid hydrazine or pressurant gas. The liquid/gas detector could be a capacitance, optical-density, flotation, or other sensor. If the fuel-supply tank were to become depleted during a transfer of liquid hydrazine from the fuel-supply tank to the spacecraft fuel tank, then the pressurant gas would begin to travel along the flexline toward the spacecraft fuel tank, posing a risk that the pressurant gas might undesirably enter the spacecraft fuel tank on the fuel side. To prevent the pressurant gas from entering the spacecraft fuel tank, the liquid/gas detector would generate a command to close the automated valve as soon as it sensed the arrival of helium gas. As a result, the portion of the flexline between the liquid/gas detector and the spacecraft fuel tank would be occupied by liquid hydrazine; that is, the pressurant-gas/liquid-hydrazine front would have been prevented from traveling farther along the flexline.
At this point, it would be necessary to close the service valve: This operation would have to be performed in a SCAPE suit because there would be liquid hydrazine under pressure behind the service valve; in other words, the technicians would be only one service valve away from liquid hydrazine under pressure. Next, the liquid/gas detector would be turned off, causing the automated valve to open. The pressurant gas would then be supplied through a ball valve and the purge fitting; the flow of helium would push the liquid hydrazine in the flexline back into the fuel-supply tank. The remaining cleanup of the hydrazine-contaminated pressurant gas in the flexline could then be performed in a manner similar to that of the unmodified combination of procedure and equipment.
This work was done by John Houseman of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.