# Future Directions for Electric Propulsion Research
## 3 Electric Propulsion Development Cycles  
In order to represent the relative maturity of the disparate electric propulsion systems in a common framework, we adopt the convention of the \"s-curve.\" This is a widely used metric [1] that can represent the qualitative stages of a technology development cycle. As illustrated in Figure 2, a technology begins in the preliminary formulation stage where theoretical arguments or scaling laws may indicate that the technology promises new capabilities, and at this point, there may even be unoptimized prototypes. In the subsequent part of the cycle, increased resources are devoted to the technology to try to achieve the performance indicated by theory. In this state, there are dramatic leaps forward in performance and implementation that are achieved through basic research. At the latter part of the curve, the technology transitions from the laboratory to flight. This leads to the inflection point in the curve where the technology approaches its theoretical limits in performance. To be sure, the plateau of the curve does not mean that the need for fundamental research ceases, but rather, the technical challenges no longer directly pertain to dramatic performance improvements. Instead, the remaining challenges stem from the operational use of the technology, e.g., trying to increase the lifetime, understanding the interaction of the thruster with its spacecraft, or improving testing reliability. Figure 1: The three major types of acceleration schemes for electric propulsion in terms of thrust per power and specific impulse. Throughout this article, we will present s-curves for various technologies at different power levels, namely Figures 3, 8 and 13. Therein, we show \"maturity\" as a function of time. To make these plots more quantitative, we pin the inflection point of the curve to the year at which three instances of a technology have flown on non-demonstration missions, according to [2]. We do this as a replacement for a more rigorous metric like technology readiness level, since assessing that quantity precisely is difficult for many immature technologies. In this way, the shape of the s-curve is largely qualitative, but the inflection is based on historical data. We categorize the s-curves respectively into low-power (\(<\)1 kW), moderate-power (1-20 kW), and high-power (\(>\)20 kW) concepts. We include gridded ion, magnetoplasmadynamic, Hall effect, pulsed inductive, electrospray, RF magnetic nozzle, and nuclear thermal (NT) thrusters. We additionally depict s-curves for common electrothermal devices (resistojets and arcjets) as examples of technologies that have been mature and flight-ready for many decades. We forgo any detailed discussion of them due to this maturity and the fact that the technical challenges they face are not representative of most other modern EP systems. Some technologies like Hall effect thrusters appear as multiple curves to reflect the parallel research efforts in developing these systems at different power levels, which often face distinct challenges worthy of a separate discussion. Finally, it is important to note that these curves are meant to reflect the typical, flight-ready state of a given class of thruster, not necessarily the state-of-the-art. We rely on these s-curves because future research directions in the field depend on the technology in question and its corresponding level of maturity. The research questions of interest for more mature technologies like Hall effect, electrothermal, and gridded ion thrusters are different than those for the less mature systems like pulsed inductive thrusters. In the remainder of this article, we discuss the current state of electric propulsion, as well as its future direction. In doing this, we refer to the s-curves introduced in this section to describe the current obstacles and opportunities faced by the major forms of electric propulsion reviewed in Section 2.