A Physics Perspective on Future Propulsion Systems

In recent years there has been a proliferation of proposals in the literature promising new advanced propulsion sch emes and extremely low -cost access to space and unrefueled high speed global reach for both military and civilian uses. We examine some of the claims using new propulsion technologies (fission, fusion, antimatter, Casimir force, magnetohydrodynamics, hydr inos, etc). Physics and engineering constraints are severe on these claims. The conclusions are that in the immediate future, propulsion advances probably will be by more conventional approaches (e.g. combined cycles, higher energy density and endothermic fuels, higher temperature materials, improved modeling, flow control, advanced automated controls and changed protocols , and extreme miniaturization of payloads). However, the results can become very nonlinear and revolutionary when several changes are imp lemented synergistically. Progress may be slowed by several factors: the need to amortize the investments for the latest generations of designs; the probable increase in the cost of fuels , the continued emphases on durability and safety , the emphasis on r eductions of emissions , the infrastructure changes which would be required by most of the new technologies and not least, the constraints imposed by the applicable range -and velocity -scaling laws. Nevertheless, the synergy between the integration of advanc es in high power electrical systems especially applied superconductivity, flow control, high temperature (possibly nano) materials, controlled combustion , and accurate modeling may permit advances such as higher system efficiencies, self -diagnosis and repa ir of systems , and a distributed power grid in space using lasers. Also very important for the economics of space access will be the development of reusable highly instrumented robotic space vehicles and semiautonomous launch systems. Introduction: Adva nced propulsion concepts are quite abundant in the literature and even more so through the world -wide -web. There are many schemes involving new cycles, new fuels and sometimes new forces. It has become popular to claim that a new scheme will reduce the cos t of access to space by one or even two orders of magnitude. The impetus to explore new concepts has been good ; however are such dramatic breakthroughs possible? Well we will have to see: as Scotty says in Star Trek "Captain, I cannot change the laws of Ph ysics ." Hypersonic airbreathing propulsion is very difficult. The public image is that the parameter space is defined in terms of Mach number and hence the difficulty may increase with Mach number. However the energy required scales approximately as the ai r density times the square of the Mach number. Even worse, to first order, the thermal load scales as the gas density times the cube of the Mach number! This means that very high speed travel can only be at high altitudes. These constraints led to the acce ss to space corridor mapping illustrated in Figure 1 , adapted after H. Metcalfe (1960) (1) . This figure shows the altitude versus velocity ( Mach number ) operational corridor and the achievable flight combinations. The upper limiting curves are determined b y aerodynamics to provide lift or thrust and the lower curves are determined by the maximum permitted forces and hence by the materials structural and thermal properties. The typical values for the ramjet -scramjet portions of the power curve are ~ 100 psia internal duct pressure at M ach number greater than 6, and by a 750 psia limit at maximum velocity for a selected altitude. In contrast, on the upper limiting curve in Fig ure 1, at maximum speed for a selected altitude, the dynamic pressure is ~180 psia. N ote that Metcalfe already in 1960 had indicated the additional flexibility of a skip vehicle, where the departure below the skin temperature limit is determined by the skip duration and the thermal insulation. The next physics constraints that we consider are the energy density and average mass of the fuel and the combustor temperatures (actually the square root of the temperature/mass). These determine the exhaust velocity and consequently the velocity of the vehicle. The propulsion physics is captured by Tsiolkovsky's simple rocket equation (2) :