Space elevators, vacuum chutes: What next for big rocket tech?
How boffins hope to escape long shadow of the V2
So far we have, with the exception of the SR-71, considered purely rocket-powered vehicles. The advantage of the air-breathing jet engine is that it doesn't require an on-board oxidiser, instead relying on atmospheric oxygen to do the job of oxidising the fuel.
That air must be sucked into the engine poses particular problems as speeds increase. As noted, the SR-71's Pratt & Whitney J58's had intake shock cones to keep the flow of air inside the engine at subsonic speeds.
The engineering challenges associated with rotary air compressors and turbines, added to their upper speed limit of about Mach 2.5, make the ramjet an attractive alternative. The incoming air is simply slowed to subsonic speeds by the intake and fed directly into the combustion chamber.
The scramjet (supersonic-combustion ramjet) differs in that the airflow inside the engine is supersonic, offering the possibility of really hitting the gas pedal to above Mach 6, where ramjets' performance is seriously impaired by the dramatic fall in efficiency in slowing the intake air to subsonic speeds.
There is plenty of brainpower currently being dedicated to the scramjet. After a shaky start, NASA's hypersonic X-43 programme hit a high in 2004 when the hydrogen-powered vehicle reached 10,617km/h.
The X-43 and X-51 scramjet vehicles
In May 2010, Boeing's X-51 "Waverider", burning a JP-7 and ethylene igniter mix and then purely JP-7, thundered to approximately Mach 5 in the "longest supersonic combustion ramjet-powered hypersonic flight to date".
During a second flight in June 2011, the engine fired but didn't transition to pure JP-7 operation. In August 2012, the third launch test ended when the aircraft lost control and crashed into the Pacific.
The evident problems associated with scramjets confirm the assertion that their operation is comparable to "lighting a match in a hurricane and keeping it burning".
There are other impediments to viable scramjet vehicles, not least of which is that the engines require a kick-start. They won't fire unless they've got a good blast of incoming air, which is why the X-43 needed a modified Pegasus rocket first stage - itself carried skywards by a B-52 - to accelerate it to ignition speed.
The X-51 hitches a ride
The X-51 was likewise taken to test altitude under a B-52, where an adapted solid-propellant Lockheed Martin MGM-140 missile drove it to operational velocity.
For the foreseeable future, then, the dream of a scramjet-powered SSTO vehicle, or indeed the fabled London to New York in 30 minutes airliner, will remain just that.
Back on the drawing board, meanwhile, a cunning combination of jet/rocket propulsion is making its pitch for orbital glory. The Skylon team proposes to use "combined air-breathing and rocket cycles" to lift 15 tonnes from a conventional runway into LEO.
The Skylon's SABRE engines will use LH2 mixed with atmospheric air in a jet phase up to Mach 5.5, at which point they will go into LH2/LOX rocket mode, shoving the aircraft into space at Mach 25.
HOTOL: It looked good on paper
Failure of the lightweight composite fuel cells designed to hold the X-33's LH2/LOX led to its cancellation in 2001, although the planned aerospike engine showed promise.
An aerospike relies on ambient air pressure to form one side of a virtual exhaust nozzle, as the rocket exhaust exits across the surface of a wedge-shaped "spike", in the case of the X-33.
The advantage of the design is the elimination of the traditional bell nozzle "point design" restriction, in that they're "designed to provide optimum performance at one certain altitude or pressure".
NASA summarises that "since the combustion gasses only are constrained on one side by a fixed surface - the ramp - and constrained on the other side by atmospheric pressure, the aerospike's plume can widen with the decreasing atmospheric pressure as the vehicle climbs, thus maintaining more efficient thrust throughout the vehicle's flight".
As scientists continue to push the envelope of jet and rocket performance to the limit, there are other options for achieving orbit, some more plausible than others.