ok; I just finished looking up some information on the Aurora at http://www.fas.org/irp/mystery/aurora.htm, and I learned a bit about why aircraft are better than satellites, how the engine/cooling systems work, and how the project was funded.
According to the Department of Defense, the SR-71 was retired due to financial reasons. Operation and Maintenance of the SR-71 was around $710,000,000 in 1990 and 1991, plus they claimed that the new reconnaissance satellites
could perform worldwide surveillance more efficiently and less expensively than manned reconnaissance aircraft.
Aeronautical engineers claimed that these satellites were not as good as aircraft, agreeing with what Mike had to say. Aircraft, for example, are inherently flexible and unpredictable. Though not as fast as satellites, they can fly lower and the interval between over the horizon arrival and time-over-target is just as short. Aircraft have a wide choice of routes, so tracking ships are unlikely to see it on the way in. Application of low observable technology could further reduce warning time. Thus, it appears plausible that aircraft may still have a role in global reconnaissance.
The first suggestion that these studies might be translated into operational hardware appeared in the Fiscal Year 1986 procurement program document, colloquially known as the P-1, dated 4 February 1985. A line item in this document, labeled "Aurora," was slated to receive $80 million in 1986, and over $2.2 billion in 1987. Since this line item appeared next to the line funding the TR-1 reconnaissance aircraft, it stirred up a hornet's nest of conjecture that a secret aircraft was being developed to replace the aging SR-71.
The Air Force quickly denied the existence of a secret program, and said the "Aurora" budget line was simply one site for B-2 bomber funds when that program was highly classified. One Air Force official commented, "I wish I could say it is (an SR-71 follow-on), because we'd love to have it. But it's just accounting, I'm afraid."
Keeping an aircraft sufficiently cool during extreme speeds is a primary challenge of hypersonic flight. According to studies done by General Dynamics and Boeing, an aircraft travelling at between Mach 5.5 and Mach 6 would have an average skin temperature of approximately 1100-1300 degrees Fahrenheit. One potential solution incorporated in the Air Force studies, also being explored by researchers at NASA's Langley Research Center and Wright-Patterson Air Force base, is the use of Methylcyclohexane (MCH) as both the fuel and the thermal management medium of the vehicle.
MCH has several advantages over other possible hydrocarbon or cryogenic fuels. Unlike standard hydrocarbon fuels, MCH has a very high capacity to absorb heat prior to combustion, up to 1800 Btu per pound of fuel, which is ten times the capacity of most hydrocarbon fuels. Cryogenic Methane and Hydrogen have high heat absorbtion capacities as well, but their use as an aviation fuel is limited by the logistical difficulties of handling, storage and fuel boil off.
The principle behind MCH thermal management is based on a catalytic reaction transforming MCH into Toluene and Hydrogen, which are then used to fuel the aircraft.
A fuel pump pressurizes the fuel to avoid boiling. The preheater heats the fuel to the proper reaction temperature while removing heat from a secondary coolant. After preheating, the fuel passes through the catalytic heat exchanger/ reactor.
The secondary coolant, Syltherm, circulates to the hot spots to maintain skin temperatures to within specified tolerances.
One aerospace journal says that an aircraft travelling at Mach 6 would be inside the combustion envelope of a subsonic-combustion ramjet. It suggests that the aircraft would thus need an accelerator to get it moving. One type of accelerator would be a ducted-rocket cycle into the engine. A fuel-rich, liquid rocket exhaust would be injected into a ramjet duct, pumping air through it even at rest. A second combustion then takes place, using atmospheric oxygen.
For more information, read the site at the top of this comment (yes, I know I copied and pasted from they're site. But they explained this stuff way better than I could ever hope to do).
