INTRODUCTION Prediction of remaining propellant is critical to the phasing of orbital replacements in the telecommunications industry. Increases in demand for data flow capacity have led to the development of larger and more powerful satellites. When such systems are utilized to full capacity, the net annual revenues per vehicle may be in the billions of dollars. Thus it is highly desirable to have very accurate predictions of the end of useful life for each vehicle to best manage the procurement and launch of orbital spares and replacements. The liquid hydrazine propellant used for both orbit insertion and orbit station-keeping is stored in one or more large tanks with an internal passive capillary Propellant Management Device (PMD). The PMD controls liquid mass center and orients the liquid such that it can be extracted at the tank outlet. Typical PMDs are multiple thin vanes to orient the ullage bubble and a finer capillary structure near the tank outlet or sump. As the tank is emptied, a gas bubble grows in volume and is located away from the tank outlet by the PMD. The liquid collects in large fillet regions subtended by the vanes and tank walls. The liquid free surface forms a complex three-dimensional geometry not easily approximated by closed-form equations.

Transient liquid flow in a low-temperature, homogeneous wick heat pipe is investigated experimentally and analytically for pulsed heat load conditions. The continuous distribution of the liquid in the wick is modeled, where previous models had assumed a uniformly saturated structure. A heat pipe with beryllium walls was used to obtain transient measurements of the saturation distribution in the wick structure using x-ray radiography. Analytical and experimental results are presented for the axial liquid distribution in the wick as a function of time. Transient liquid distributions for the model and experiment compare favorably. These results show that significant reductions in saturation may occur in the evaporator region for higher heat loads. These reductions affect the wick-flow properties and must be included in the liquid-flow analysis. Nomenclature A -area, m 2 /(5) -moisture diffusivity, kg-m/s, Eq. (6) / = source term, s~l, Eq. (5) K = full permeability, m 2 K r -relative permeability L = length, m m = mass of liquid in pores, kg m -mass flow rate, kg/s P = pressure, Pa P c = capillary pressure, Pa Q = heat transfer rate, w S = saturation (ratio of liquid volume to void volume) 5, = irreducible saturation T = temperature, °C t -time, s x -position coordinate, m e = wick porosity A = latent heat of vaporization, J/kg IJL = dynamic viscosity of liquid, kg/m -s p = density of liquid, kg/m 3 a = surface tension, N/m = reduced saturation (<£ = S -5/1 -S,)

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The loop heat pipe (LHP) is known to have a lower limit on input power. Below this limit the system may not start properly creating the potential for critical payload components to overheat. The LHP becomes especially susceptible to these low power start-up failures following diode operation, intentional shutdown of the device, or very cold conditions. These limits are affected by the presence of adverse tilt, mass on the evaporator, and noncondensible gas in the working fluid. Based on analytical modeling correlated to startup test data, this paper will describe the key parameters driving this low power limit and provide an overview of the methodology for predicting a "safe start" design envelope for a given system and loop design. The amount of incipient superheat was found to be key to the enveloping procedure. Superheat levels have been observed to vary significantly based on evaporator design and even from unit to unit of identical designs. Statistical studies of superheat levels and active measures for limiting superheat should be addressed by both the hardware vendors and the system integrators. * Adverse conditions are often difficult to avoid during ground operations such as thermal balance testing and possible launch pad operations.

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