When the thruster is operated below 200 kW, the self-induced magnetic field becomes only marginally sufficient to provide the desired body force, and external fields are frequently added to enhance performance in this range. High efficiency (above 30%) is typically reached only at high power levels (above 100 kW) consequently, the steady-state version of the MPDT is regarded as a high-power propulsion option. The MPDT has demonstrated its capability of providing specific impulses in the range of 1500–8000 sec with thrust efficiencies exceeding 40%. Choueiri, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 IV.C.2 Present and Projected Capabilities They are actually not needed for missions of this class, while being enabling technologies for true interstellar missions. Finally, it is possible that other propulsion concepts based on substantial advancements in physics will be available in the future, likely too late for interstellar precursor missions. Other concepts requiring greater technological developments are those based on laser or microwave beamed energy systems, while nuclear thermal rockets, based on fusion, and antimatter devices require even greater effort. On the contrary solar sails, nuclear-electric and -thermal (fission) propulsion are all viable alternatives requiring no actual breakthrough in propulsion technology. Although solar electric propulsion has already been used with success in the Deep Space 1 mission, it is not adequate for a precursor interstellar mission, where low thrust must be applied for a long time, with the probe at very large distance from the Sun, where solar panels lose their efficiency. Several alternatives to chemical propulsion will be available in a short time. This final velocity is still low and the perihelion of the trajectory must be quite close to the Sun, with all the problems related to heating, radiation and long mission time. The clever use of gravity assist is unquestionably a success, but it doesn’t come without drawbacks, particularly when used to reach the outer solar system via Venus: the large increase of the mission duration and the need of travelling in the radiation-rich regions of the inner solar system affect the reliability of the probe and raises the costs linked with a very long mission, as testified by the problems encountered by Galileo.Ī Sun flyby, perhaps preceded by a flyby of Venus, accompanied by a perihelion burn, and followed by a flyby of Jupiter can send a probe out of the Solar system with a hyperbolic excess velocity up to about 10 A.U./year even with chemical propulsion. This performance was made possible by clever use of gravity assist: no missions beyond Mars orbit was performed without it and the lack of availability of a powerful enough rocket compelled to exploit the gravity assist of Venus (twice) and of the Earth even for the Jupiter mission Galileo. Note that both the table and the figure are indicative and give only orders of magnitudes some points are controversial, like the line labelled ‘gas core nuclear’.įour spacecraft (the Voyager and Pioneer probes) are now travelling into interstellar space, with speeds between 2.2 and 3.5 A.U./year (10.5 and 16.6 km/s). The relationship between specific impulse and thrust is shown in Figure 1. To assess some orders of magnitudes, the minimum requirements for various missions in terms of Δ V and specific impulse are reported in Table 1 together with the specific impulse of some of the propulsion concepts presently used or under study. Performances of various propulsion concepts. The mentioned figures must be considered just as a rough order of magnitude, as both a lower (not much) or higher value can be obtained depending on how the actual mission is designed.įigure 1. However, a far higher Δ V is required to avoid very long mission times: to obtain a hyperbolic excess velocity of 20 A.U./year (95 km/s) with a single burst at the surface of the Earth, a Δ V of about 97 km/s is required, a performance completely beyond chemical propulsion. Apparently, missions outside the solar system are not so demanding from the viewpoint of propulsion: to exit the solar system from the surface of the Earth a Δ V of just 16.5 km/s is sufficient. Although the near interstellar space can be reached using chemical propulsion, aided by gravitational assist, no mission in interstellar space can be performed in a reasonable time without improvements in propulsion. Chemical propulsion, characterized by low specific impulse 1 ( Table 1) but enabling to build engines with very large thrusts ( Figure 1), falls short for deep space missions.
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