Solid core reactor SCR

Figure 8.4. Power and Isp of chemical, fusion and fission system (adapted from [Kammash, 1995]).

So, at fixed power and depending on the type of mission, a trade-off exists between the combination of high Isp (low mass of propellants) and low F (low acceleration) and its reverse, that is lower Isp but faster acceleration due to larger F. Figure 8.4 shows such a trade-off immediately, because thrust power is reported on the vertical axis.

Whatever the trade-off, power scales with 13p, because F ~ (dm/dt)Isp, and dm/dt ~ Isp. In fact, whether the propulsion system accelerates only the mass of products of energy conversion, or adds to them inert propellant, or even scoops mass from space, tremendous power is needed to support large Isp. In Chapter 7 it was seen that increasing Isp by means of electric propulsion does not pose insurmountable problems. Isp in the 105 s range are assumed feasible in NASA studies [El-Choueiri, 2002; Mikellides, 2004]. Electromagnetic acceleration is inherently suited to produce large exhaust speeds, based as it is on applying a direct Lorentz body force to each charged particle: that is, exhaust speed is no longer tied to the rocket engine thermodynamic cycle. Powering such an electromagnetic system, producing large exhaust speed and Isp, is instead the real challenge, since power scales as 13p = V3. To illustrate this point, a propulsion system capable of 20 tons (44,100 lb) thrust with Isp = 105s needs about 200 GW (200 billion watts) to function, assuming 100% efficiency. For reference, the total electric power installed in the US is of order 1,000 GW [Trumbull, 2000].

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