A relatively simple and effective design is to mount a small rocket engine inside a ramjet-like tube so that ram air is further compressed and then accelerated by the rocket. This method is more efficient than a ramjet alone, can double the specific impulse of a conventional rocket, and reduce the weight of the launch vehicle by half. The main working mass becomes the ram air, rather than the onboard propel-lants. Sometimes called a ramrocket, the hybrid device is more fuel efficient than either a ramjet or a rocket alone. As early as 1965, the Soviet Union was working on such a device, called Gnom, which resulted in a multistage ICBM that weighed only 30 metric tons. This allowed the vehicle to be launched from a mobile platform, with resulting strategic advantage. The air-augmented second stage used a solid-propellant rocket and exhibited a specific impulse of 550 s, double what could be achieved with rocket alone. It boosted Gnom from its ram air ignition speed to 1 km/s using 300 kg of solid propellant. The final design involved a 29-metric-ton launch vehicle that would lob a 535-kg nuclear warhead 11,000 km. The air-augmented stage, with a ram airflow of 1200 kg/s, would accelerate the missile from Mach 1.75 to Mach 5.5 in just over a minute on an optimum aerodynamic trajectory, after which two more solid rocket stages would take over.7 The great advantage in air-augmented rockets is that they can reduce total lift-off mass by over half. The Martin company, as early as 1962, calculated that air-augmented rockets could reduce lift-off weight by 56% with a given payload.8
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