Cu-Enhanced 3-D Printed Fuels for Green SmallSat Propulsion

The Propulsion Research Laboratory at Utah State University (USU) has recently developed a promising High-Performance Green Hybrid Propulsion (HPGHP) technology that derives from the novel electrical breakdown property of certain 3-D printed like acrylonitrile butadiene styrene (ABS). This electrical breakdown property has been engineered into a proprietary, power-efficient system that can be cold-started and restarted with a high degree of reliability. One of the issues associated with ABS as a propellant is its low burn rate. It is well documented in technical literature that hybrid rocket systems generally have fuel regression rates that are typically 25-30% lower than solid fuel motors in the same thrust and impulse class. Lowered fuel regression rates tend to produce unacceptably low equivalence ratios that lead to poor mass-impulse performance, erosive fuel burning, nozzle erosion, reduced motor duty cycles, and the potential for combustion instability. To achieve equivalence ratios that produce acceptable combustion characteristics, hybrid fuel ports are often fabricated with large length-to-diameter ratios. The resulting poor volumetric efficiency is incompatible with Small Satellite (SmallSat) applications. This paper presents preliminary results from a collaborative development program between the University of Miami (UM) and USU. In this reported work, modern extrusion and 3-D printing techniques are used to fabricate sample ABS fuel grains with varying levels of copper-metallization. Hybrid-ABS fuel grains were printed from Cu-infused feed stock with 2%, 4%, and 6% Cu-mass concentrations. As baseline control, 100% pure ABS fuel grains (0% Cu) were also printed. Heat conduction via the additive copper (Cu) provides an efficient heat transfer mechanism that augments surface convection from the flame zone. Forced convection, the primary mechanism for pyrolysis for hybrid fuels, is generally inefficient due to wall-blowing associated with the radially emanating mass flow from fuel pyrolysis. Wall-blowing pushes the flame zone away from the fuel surface and significantly reduces the rate of enthalpy exchange. Homogeneously mixing a high conductivity metal such as Cu into the ABS fuel provides an efficient heat transfer mechanism, and allows radiant heat from the flame zone to be transferred deep into the fuel material. This process significantly increases the pyrolytic efficiency of the fuels. The Cu-infused fuels were tested at USU using a legacy 12-N hybrid thruster system. Fabrication and manufacturing methods are described, and results of hot fire tests are presented. The top-level conclusion is that Cu-infusion of the printed fuels measurably increases the fuel regression rate, allowing for a higher thrust level with no increase in the required volume. The Cu-infusion has negligible impact on the propellant characteristic velocity and the overall system specific impulse. The increased burn rate and overall increase in solid-fuel density resulting from Cu-infusion allows a measurable increase in the propellant impulse-density. This increase in volumetric efficiency is potentially significant for small spacecraft applications where available space has a premium value. Follow-on methods that infuse lower-molecular weight and higher thermal conductivity materials like graphene and carbon-nanotubes are proposed

Potential effects of optical solar sail degredation on trajectory design

The optical properties of the thin metalized polymer films that are projected for solar sails are assumed to be affected by the erosive effects of the space environment. Their degradation behavior in the real space environment, however, is to a considerable degree indefinite, because initial ground test results are controversial and relevant inspace tests have not been made so far. The standard optical solar sail models that are currently used for trajectory design do not take optical degradation into account, hence its potential effects on trajectory design have not been investigated so far. Nevertheless, optical degradation is important for high-fidelity solar sail mission design, because it decreases both the magnitude of the solar radiation pressure force acting on the sail and also the sail control authority. Therefore, we propose a simple parametric optical solar sail degradation model that describes the variation of the sail film's optical coefficients with time, depending on the sail film's environmental history, i.e., the radiation dose. The primary intention of our model is not to describe the exact behavior of specific film-coating combinations in the real space environment, but to provide a more general parametric framework for describing the general optical degradation behavior of solar sails. Using our model, the effects of different optical degradation behaviors on trajectory design are investigated for various exemplary missions

UltraSail - Ultra-Lightweight Solar Sail Concept

UltraSail is a next-generation high-risk, high-payoff sail system for the launch, deployment, stabilization and control of very large (sq km class) solar sails enabling high payload mass fractions for high (Delta)V. Ultrasail is an innovative, non-traditional approach to propulsion technology achieved by combining propulsion and control systems developed for formation-flying micro-satellites with an innovative solar sail architecture to achieve controllable sail areas approaching 1 sq km, sail subsystem area densities approaching 1 g/sq m, and thrust levels many times those of ion thrusters used for comparable deep space missions. Ultrasail can achieve outer planetary rendezvous, a deep space capability now reserved for high-mass nuclear and chemical systems. One of the primary innovations is the near-elimination of sail supporting structures by attaching each blade tip to a formation-flying micro-satellite which deploys the sail, and then articulates the sail to provide attitude control, including spin stabilization and precession of the spin axis. These tip micro-satellites are controlled by 3-axis micro-thruster propulsion and an on-board metrology system. It is shown that an optimum spin rate exists which maximizes payload mass