## 1 Introduction  
The use of advanced space propulsion systems, which are able to give a steerable thrust vector, whose magnitude can be selected within a prescribed range during the flight, allows robotic spacecraft to cover complex, non-Keplerian, space trajectories and to complete scientific missions that would be difficult using more common chemical thrusters, both in planetocentric and heliocentric contexts [1, 2, 3, 4, 5]. Among the continuous-thrust propulsion systems proposed or actually used in recent decades, such as the well-known photonic solar snails [6, 7, 8] or the more exotic electric solar wind sails [9, 10, 11, 12], solar electric thrusters actually occupy a prominent position thanks to the maturity reached by this specific important technology [13, 14, 15]. This maturity has allowed its operational use as a primary propulsion system, even in challenging interplanetary scenarios, since NASA's mission Deep Space 1 launched almost thirty years ago [16, 17, 18, 19]. In addition, the continuous and throttleable propulsive acceleration vector provided by a solar electric thruster has also been proposed for the de-orbiting of small satellites [20, 21], or for achieving an optimal reconfiguration and maintenance of complex spacecraft formation structures [22, 23, 24]. Technological advances in the miniaturization of both spacecraft components [25] and scientific payloads [26, 27] have recently enabled the installation of a continuous-thrust propulsion system on board a CubeSat [28], such as a medium-sized photonic solar sail [29, 30, 31] or a miniaturized solar electric thruster [32]. In this context, the Busek's BIT-3 RF ion thruster (Busek Co. Inc., Natick, MA, USA) represents an interesting, efficient, and compact iodine gridded engine that allows us to achieve a maximum thrust magnitude of about 1.1 mN, with a specific impulse of 2150 s and a rather small occupied volume of 1.6 U [33]. A cutaway of the BIT-3 engine unit, which also houses a propellant tank with 1.5 kg of solid iodine, is shown in Figure 1. In this figure, one can observe the presence of a two-axis gimbal system that allows the (possibly) small spacecraft's reaction wheels to be desaturated. The technology readiness level achieved by this type of miniaturized RF ion thruster [34; 35; 36] allowed its operational use in two scientific space missions launched by NASA a few years ago, that is, the Lunar Polar Hydrogen Mapper (LunaH-Map) [37] and the Lunar IceCube [38; 39], which were designed to perform remote sensing of the lunar surface using two (small) 6 U CubeSats whose launch mass was 14 kg. In the recent literature, the performance of a CubeSat equipped with a propulsion system based on the BIT-3 thruster technology has been studied considering a simplified mathematical model in which the thrust magnitude has a fixed value during the flight [40] or varies continuously within a prescribed range [41] as a function of the available input power given by a solar array-based subsystem [42]. However, the operating levels and the performance characteristics of a BIT-3 engine unit, in terms of specific impulse and the electric power required for the system to function, are finite in number, according to the typical thrust table of the electric thruster which can be retrieved from the recent works of [PERSON] et al. [43; 44]. In particular, as detailed in Section 2, according to Ref. [44], the thrust table of the BIT-3 RF ion engine has five distinct operating levels to which a sixth is added that, although technically a thrust command, can be considered a sort of fictitious thrust level, as it produces almost zero thrust while consuming a small amount of propellant. This additional fictitious operating level is inserted into the actual thrust table to model a BIT-3 operating condition in which the engine unit is essentially in a sort of idle mode that, however, allows it to reach normal thrust conditions without going through the entire ignition sequence [44].