1. Theoretically, five modes of ram accelerator operation, which span the velocity range of -0.7-12 km/sec, have been developed and studied by the authors. These concepts differ from the prior art noted above i n that the projectile flies through a premixed fuel-oxidizer mixture and utilizes energy release modes and configurations not heretofore considered. These include two subsonic combustion mdes (one of which involves thermally choked combustion), a normal overdriven detonation mode and two oblique detonation modes. The fundamental difference between these concepts lies i n the method of heat release. In the subsonic combustion ram accelerator modes the combustion of propellant gas i s stabilized with respect to the projectile by flame holders. I n the overdriven detonation mode all the heat release occurs i n a very thin layer inmediately following the normal shock i n the diffuser. The heat release i n the oblique detonation modes occurs in the immediate vicinity of either the reflected bow shock or an oblique shock generated further back On the projectile. The basic principles of all five acceleration modes are sumnarized i n Section 11 of this paper. Section Ill contains the theoretical and experimental results for the subsonic modes of operation. Theoretical results for the three detonation driven modes are presented in Section IV. Section V presents a discussion of features i n cowon between the subsonic combustion and detonation driven ram accelerator modes.
2. The velocity range of operation depends upon the specific ram accelerator mode, i.e., thermodynamic cycle, employed for generating thrust. The subsonic combustion driven modes are capable of operating effectively i n the velocity range of -0.7-3 km/sec. Projectiles have heen propelled up t o velocities of -1.9 km/sec using the thermally choked mode of the ram accelerator principle in recent experiments conducted by the authors. Around velocities of -2 km/sec the detonation driven modes of the ram accelerator become promising. though to date, none of the detonation driven modes have been experimentally investigated. In principle, the overdriven detonation mode has an operating range of -2-5 km/sec and the oblique detonation modes operate i n the range of -2-12 kmisec. This section of the paper presents the various ram accelerator drive modes i n order of increasing operational velocity ranges. Subsonic Combustion Driven Modes
3. (a) Type I oblique (b) Type 11 oblique i s not initiated by the compression process. Instead, a small squat frustum of a cone, with a relatively blunt cone angle, i s inserted into the projectile profile to initiate an oblique detonation wave. This frustum or bump could be located anywhere along the cylindrical section of the projectile. To reduce the drag produced by the bump, it may be desirable to make the the bump non-axisymmetric and have it divided into, say, the four separate regions occuping only 20-25% of the circumferance of the projectile. The detonation waves would then spread out i n both the azimuthal and radial directions from the four initiating regions. In this oblique detonation mode, as well as i n the type I mode, the flow i s supersonic throughout. 80th oblique detonation modes can, i n principle, attain velocities approaching 12 kmlsec. Performance Parameters of the Ram Accelerator
4. For the ram accelerator, specific impulse does not have i t s usual meaning, since no fuel or oxidizer i s carried onboard the projectile. Rather, the performance of the device. can be characterized by two main parameters: thrust pressure ratio, 4. and ballistic efficiency, n. The thrust pressure ratio i s the net average drive pressure on the projectile (the thrust divided by the marlmum projectile cross-sectional area) divided by the maximum cycle pressure. This ratio i s an inportant performance parameter because it provides a measure of the device's launch capability versus the maximum pressure the project i l e and launch tube must survive. The ram accelerator operates with high performance only within a narrow range of flight Mach numbers for a given projectile (typically. these MI ranges are 2.5-3.0 for subsonic combustion modes, 5-6 for the overdriven detonation mode, 10-12 for the type I oblique detonation mode. and 6-7 for the type 11 oblique detonation mode). In principle, the performance of the ram accelerator can be maximized by operating the projectile through propellant gases that have a continuous composition gradient chosen to coincide with the optimum progression of the projectile along the length of the tube. This results in the speed of sound of the gas mixture increasing along the length of the tube and the projectile Mach number remaining at the peak performance point. For computational simp 1 i c ity, the determination of the ram accelerator performance i s based on a segmented tube having different propellant mixtures tailored for the velocity increments expected within the segments.