Darpa is still continuing different projects to start reducing launch costs in the United States. Today maybe one of the parts of such project has chance to begun-second Proposers' Day is finished !
AFRE is Advanced Full Range Engine project which could operate as combination of jet and rocket engine in little similar way like British SABRE by Reaction Engines Limited. SABRE engine combines advantages from rocket and jet propulsion and offers new possibilities in designing space launch vehicles. SABRE is able to use atmospheric oxygen as oxidizer for rocket fuel and fluently switch to rocket mode when oxidizer from tanks could be used. Vehicle with this propulsion could start as usual jet airplane, reach hypersonic speed to enter the space and switch propulsion to rocket mode. No launch sites, no assembling rocket one month before start, no tons of wasted oxidizer; only plane and payload on ordinary airfield. This vision is very attractive - ESA decided recently of starting cooperation with REL and investing to Company €10 million. AFRE seems to be less ambitious project. It will be combination of jet engine and ram jet. It means that under speed of 2.5 Ma propulsion will operate like jet engine and after passing speed of 3 Ma it could start working as scramjet to provide hypersonic flight capability (ramjet operates using pre compressed air which is generated by vehicle’s speed). It will make possible reaching hypersonic speed without additional rocket boosters. From the point of view of space industry this project seems of course less interesting than SABRE. Still engine will not be able to operate like rocket propulsion, but it would be base for reusable hyper speed planes; such planes could be easily used as base for two-stage launch vehicles delivering rockets to high altitudes. It is worth to remind that most of the rocket cost is generated by first, largest stage and by necessity of using expensive launch sites. Such concept was presented and patented by Boeing in 1989 as SSME project and covered lifting plane combined with attached smaller spaceplane launched from high speed and altitude. SSME could easily reduce price of the launch (it was designed as fully reusable vehicle) and could offer possibility of delivering payload to orbit from regular runways. Today, when SpaceX offers price of $61 million per flight of their Falcon-9 with utilization of experimental reusable first stage and still operating from expensive launch site we can assume that after resigning from launch site in favor of runway and switching to fully reusable vehicle price could significantly drop. This is why experimental hyper speed propulsion systems are still developed and still receive money from budgets of USA, China, India, UK, and Russia.
On picture above: amateur rocket covered with burned paint and stickers after reaching 2 Ma.