On-Orbit Cryogenic Refueling: Potential Mission Benefits, Associated Orbital Mechanics, and Fuel Transfer Thermodynamic Modeling Efforts

The placement of cryogenic fuel/propellant depot stations in Earth orbit has the potential to transform the nature and operations for many types of spaceflight missions. Today, spaceflight missions are almost universally required to carry the entire amount of fuel required for the mission, for the entire duration of the mission, from the point of launch. This is the rough equivalent of making a drive from Ohio to California, requiring the traveler to bring along the total sum of gasoline required for the entire trip, without being able to `fill-up’ anywhere along the route. Obviously, this framework of travel greatly encumbers the breadth, scope, and efficiency of potential journeys. Cryogenic fuel/propellant depots have not been implemented because many technical, operational, and engineering challenges still exist. These must be overcome prior to the placement of usable on-orbit propellant depots. This thesis investigates three specific engineering challenges related to on-orbit propellant depots, and presents the current state, technological challenges, and ultimate benefits of on-orbit cryogenic refueling.This thesis begins with a literature review of past and present research endeavors being undertaken to realize on-orbit refueling depots, focusing on the technologies necessary for and the orbital mechanics associated with cryogenic fuel depot operations. Then, an orbital dynamics study is conducted, and a method for computing refueling orbits to optimize total mission architecture mass savings over a no-refueling, single rocket case is presented. A MATLAB script has been written that allows for calculation and assessment of optimal refueling orbits around the Earth and Moon for deep space missions, utilizing specific impulses of engines and mass ratios of stages as inputs. Python is also used in conjunction with this MATLAB script to compute launch windows and to create dedicated plots to find optimal mission windows for minimizing mission energy requirements (“porkchop” plots). Next, the results of a parametric study analyzing the effects of staging, mass ratio, and specific impulse on optimal refueling orbit placement and mass savings are shown and discussed. Specifically, this parametric study confirms that orbital refueling can offer significant launch vehicle mass savings, potentially providing equivalent missions for 1.4-7.3 times less total mass than the traditional single rocket architecture for two-stage rockets and enabling utilization of single-stage to orbit (SSTO) launch vehicles for more demanding missions. Additionally, upcoming missions, such as NASA’s Artemis 1 mission and a SpaceX Starship Mars mission are assessed with refueling in mind, and potential mass savings are tabulated for applicable optimal refueling architectures. Finally, the idea of sustainable, on-orbit cryogenic refueling infrastructures is discussed as a whole, with long-term effects on the human exploration of the solar system theorized and presented.The second topic of research in this thesis concerns itself with developing technologies and methods needed to achieve on-orbit refueling. Specifically, the storage and transfer of cryogenic fuels between spacecraft tanks has been identified as a key issue, and so is addressed in this thesis. In the environment of microgravity, propellants are unsettled in their tanks and cannot be transferred with the receiving tank’s vent valve open. For normal (storable) fuels few problems would arise. However, for cryogenic fuels, rapid conversion to a vaporized state can occur, causing the pressure to build within a tank. This process can ultimately force the vent valve on the receiving tank to open, relieving pressure but allowing for liquid fuel to escape. One method of solution here is to pre-chill the receiver tank to some “target temperature” that is sufficiently cold to then allow a non-vented fill (NVF) to take place. Predicting this “target temperature” is a key goal of this work, and a derivation based on the 1st Law of Thermodynamics is undertaken. This derivation is similar to what was done in Kim et al. (2016) but is extended to include heat leak from the space environment as well account for initial fill levels in the receiver tank (a no-vent top off fill). Accounting for these specific factors subsequently allows for application of this prediction parameter to the entire no-vent fill and no-vent top off experimental database. The “target temperature” is computed for 158 historical tests over a wide range of fluids, injection methods, and tank geometries. Additionally, a parametric study is conducted to determine the influential factors that affect NVF, and an efficiency parameter is derived to determine the efficiency of a given no-vent fill process. Results indicate that the predicted “target temperature” (also known as the prediction parameter) can always predict the failure of a non-vented transfer if the thermal energy needing to be taken from the tank metal is too large for the transferred fluid to absorb. It is concluded here that the resulting pressure in the receiver tank depends on the specific filling method used and as such is path dependent. Therefore, transient modeling of the no-vent fill process is needed to sufficiently predict how a tank/injector/cryogen pair will behave.The third technical chapter of this thesis presents work done on a trajectory subroutine for such a model, allowing a user to select tank geometry, fuel injection method, and specify dimensions of each. The program subsequently outputs parameters necessary for a transient 1st order no-vent fill model, including the height of the liquid-vapor interface in the tank, the average angle that injected fuel impacts the tank wall, and the average distance the fuel travels through vaporized fuel. Additionally, the subroutine is then applied to the available NVF historical tests for validation. In each research area of this thesis, limitations of the research and recommended future work to improve and supplement the findings in this thesis are addressed.