In March, DARPA announced mission requirements for a new hypersonic aircraft dubbed “Blackswift”. This aircraft design proposal seems to have stemmed from a previousBlackswift concept hypersonic crusie missile known as Falcon. The Falcon, or Force Application and Launch from the Continental United States, project aims to fire a bunker-busting bomb into near-space, and then send it crashing into a target more than 3,000 miles away, at four times the speed of sound.

Blackswift will be reusable and make takeoffs and landings from an ordinary runway. The hyperplane will be required to reach Mach 6 – well in excess of the best speeds yet reached by runway aircraft – and maintain that stably for at least a minute.

In addition to these requirements (no mean feat that), DARPA would also require that Blackswift be able to do a barrel roll (program solicitation), in order to demonstrate that it is a “real airplane”, and this has been confirmed. In the new document, DARPA says that Blackswift “shall demonstrate testbed maneuverability at Mach 6+ including execution of an aileron [i.e. barrel] roll”.

Regarding propulsion, DARPA says that for subsonic and supersonic regimes, the new aircraft will use relatively ordinary turbojets. For “high supersonic and hypersonic” flight it will use engines burning fuel in a supersonic air stream – supersonic combustion ramjets, or scramjets.

DARPA has also been working on so-called “dual combustion ramjet” (DCR) which has two air flows running through the engine. In one, denser hydrocarbon fuel partially burns in a subsonic airflow; then the hot, light combustion products are further burned and release the rest of their energy in the other, supersonic stream. Since much of the air can move through the jet fast, drag doesn’t rise to the prohibitive levels which develop above Mach 3 or 4 in a regular ramjet – but you don’t have to carry hydrogen.

DCR has been tested successfully in the wind tunnel and small-scale flying prototypes, but has lately suffered a failure in flight tests of the “HyFly” hypermissile demonstrator. Nonetheless, it seems to be DARPA’s favored option for use in Blackswift, as the description document says:

In high supersonic and hypersonic flight regimes, propulsion is provided by a scramjet engine (also referred to as a dual mode ramjet engine)… The Blackswift testbed shall use a hydrocarbon-fueled… propulsion system…

In order to make Blackswift a reality, DARPA will need not only to ensure that the troublesome DCR hyperjets work: ordinary turbojet mode will also need to be available, as ram and scram jets need to be airborne and moving fast before they’ll kick in at all. Most present-day ramjets, used mainly in missiles, accomplish this by the use of rocket boosters, but DARPA want a fully reusable plane here.

Also see the following: Wired Blog Spaceplane