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a The product of specific impulse and the specific gravity of the propellant is termed density specific impulse and was used by the late V. Glushko of the GDL OKB to show the performance advantages of hypergolic propellants. All the Isp are in vacuo.

a The product of specific impulse and the specific gravity of the propellant is termed density specific impulse and was used by the late V. Glushko of the GDL OKB to show the performance advantages of hypergolic propellants. All the Isp are in vacuo.

hypergolic propellants in bold. Hypergolic propellants are those that spontaneously ignite on contact with each other, monopropellants are in italics.

The chamber pressure assumed in Table 4.5 is 1000 psia (about 68 atmospheres), yielding the specific impulse values given in a nozzle with optimum area ratio. The Isp is the thrust developed per unit mass flow and per second (lb/(lb/s)) or kg/(kg/s)). The Isp is a function of the combustion temperature, chamber pressure, and the thermodynamics of the products of combustion. Since the thrust per unit mass flow is constant, the rocket engine thrust is a function of the total mass flow. Given the combustion temperature, the mass flow depends on chamber pressure and engine throat area. To obtain more thrust either the pressure can be increased for the same size engine, or the size of the engine can be increased. The rocket motor is necessary for space propulsion because it is independent of any atmosphere. Although a turbopump rocket engine is shown, for some, if not most, space applications the propellant tanks are pressurized to feed propellant into the engine and there are no turbopumps. This is to clarify that the question of airbreather engines versus rocket applies only to flight in the Earth's atmosphere and concerns the large weight of oxidizer required by rockets, which increases the gross weight of the vehicle and increases the thrust of the rocket engines accordingly. Thinking along these lines, it appears intuitive that one way to increase the thrust of the rocket, for the same propellant flow, is to make it an ''air augmented'' rocket.

2. Air augmented rocket. Figure 4.10 employs the rocket motor as a primary ejector [Harper and Zimmerman, 1942; Nicholas et al., 1966] so some of the external airstream can be mixed with the rocket exhaust to increase mass flow and thrust and increasing the specific impulse. These systems are generally operated up to Mach 6 or less because of pressure and temperature limits of the air induction system. At Mach 6 the inlet diffuser static pressures can typically equal 10 to 20 atmospheres and 3,000°R (1,666 K). The rocket motor operates on its normal oxidizer-to-fuel ratio. The reduction of the mass averaged exhaust velocity increases propulsion efficiency. This simple concept is not designed to burn the oxygen in the entrained air. The weight ratio is reduced for an SSTO from 8.1 to 7.5. The sketch in Figure 4.10 is notional, but the use of an inward-turning inlet with a variable capture area offers high mass capture tailored to the Mach number and provides high-pressure recovery. The retractable feature eliminates inlet drag at higher Mach numbers. True, the external air inlet system adds empty weight, but with a mass ratio reduction of 0.60, the air induction system weights less than the rocket, if the inlet system is less than 8% of the dry weight.

3. Ram rocket. Figure 4.10 is an air augmented rocket cycle where the rocket is operated at a fuel-rich oxidizer-to-fuel ratio, so the oxygen in the entrained air can now burn the excess fuel at the normal airbreathing air/fuel ratios for the fuel used. Scherrer gives an excellent evaluation of the air augmented rocket and the ram rocket based on ONERA research [Scherrer, 1988]. The external airstream is mixed with the rocket exhaust to increase mass flow and with the combustion of the excess fuel thrust and specific impulse increase at lower Mach numbers (M < 6).

The weight ratio is reduced for an SSTO from 8.1 to 6.5. The sketch in Figure 4.10 is notional, but the use of an inward-turning inlet with a variable capture feature offer high mass capture tailored to the Mach number and provides high-pressure recovery. The retractable feature eliminates inlet drag at higher Mach number. The external air inlet system adds empty weight. But with a mass ratio reduction of 1.6, the air induction system weights less than the rocket if the inlet system is less than 24% of the dry weight. This is the better operational mode than the air augmented rocket.

Neither of these latter two rocket configurations has found any significant applications yet, because of the opinion that the air induction system is too heavy for the benefit provided. That is very close to true for the air augmented rocket but it is not true for the ram rocket. A significant reduction in mass ratio can be realized for about a 5% increase in empty weight. Aircraft such as the Saab-Scania Viggen, in fact, employ this method to increase the thrust of the gas turbine engine. The exhaust nozzle is an ejector nozzle, where the primary gas turbine exhaust induces ambient air into a secondary nozzle-mixer flow.

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