Wednesday, July 29, 2009


Unlike a rocket that quickly passes mostly vertically through the atmosphere or a turbojet or ramjet that flies at much lower speeds, a hypersonic air breathing vehicle optimally flies a "depressed trajectory", staying within the atmosphere at hypersonic speeds. Because scramjets have only mediocre thrust-to-weight ratios, acceleration would be limited. Therefore time in the atmosphere at hypersonic speed would be considerable, possibly 15-30 minutes. Similar to a re-entering space vehicle, heat insulation would be a formidable task. The time in the atmosphere would be greater than that for a typical space capsule, but less than that of the space shuttle.

New materials offer good insulation at high temperature, but they often sacrifice themselves in the process. Therefore studies often plan on "active cooling", where coolant circulating throughout the vehicle skin prevents it from disintegrating. Often the coolant is the fuel itself, much in the same way that modern rockets use their own fuel and oxidizer as coolant for their engines. All cooling systems add weight and complexity to a launch system and reduce its efficiency. The increased cooling requirements of scramjet engines result in lower efficiency.
The efficiency of a launch vehicle depends greatly on its weight. Calculating the efficiency of an engine system is mathematically complex, and involves trade offs between the efficiency of the engine (takeoff fuel weight) and the complexity of the engine (takeoff dry weight)

Scramjets have few to no moving parts. Most of their body consists of continuous surfaces. With simple fuel pumps, reduced total components, and the re-entry system being the craft itself, scramjet development tends to be more of a materials and modeling problem than anything else.

A scramjet cannot produce efficient thrust unless boosted to high speed, around Mach 5, depending on design, although, as mentioned earlier, it could act as a ramjet at low speeds. A horizontal take-off aircraft would need conventional turbofan or rocket engines to take off, sufficiently large to move a heavy craft. Also needed would be fuel for those engines, plus all engine associated mounting structure and control systems. Turbofan engines are heavy and cannot easily exceed about Mach 2-3, so another propulsion method would be needed to reach scramjet operating speed. That could be ramjets or rockets. Those would also need their own separate fuel supply, structure, and systems. Many proposals instead call for a first stage of droppable solid rocket boosters, which greatly simplifies the design.

Unlike jet or rocket propulsion systems facilities which can be tested on the ground, testing scramjet designs use extremely expensive hypersonic test chambers or expensive launch vehicles, both of which lead to high instrumentation costs. Launched test vehicles very typically end with destruction of the test item and instrumentation.

There is no published way to make a scramjet powered vehicle (or any other hypersonic vehicle) stealthy- since the vehicle would be very hot due to its high speed within the atmosphere it should be easy to detect with infrared sensors. However, any aggressive act against a scramjet vehicle would be difficult because of its high speed.

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