An unobtrusive model at DARPATech in Anaheim looks like the Hypersonic Cruise Vehicle (HCV) that is the far (very far) term goal of DARPA's Falcon program, but actually it's not.
Rather, it depicts HTV-3X, a new addition to a family of hypersonic test vehicles that has tended to lose and gain members since Falcon got started. HTV-3X, though, is by far the most ambitious member of the HTV clan, and although opinions clearly are varied as to the feasible timescale for the project, it could get a real hypersonic aircraft flying long before the B-52-sized HCV.
It's also the demonstrator that has been described as the lead-in to a new reconnaissance/strike aircraft nicknamed SR-72. For the moment, though, it's a proposal struggling for attention and money.
Key features - apparent from images snagged from DARPA video, and from other sources - include the fact that HTV-3X is an unmanned, fighter-sized aircraft. It's powered up to Mach 4 by high-speed turbojet engines, larger versions of the technology used in the RATTLRS high-supersonic cruise missile program. After that, ramjets take over, switching to scramjet mode above Mach 6.
The ramjets are of the inward-turning, circular type designed by ramjet/scramjet pioneer Fred Billig. Unlike the classic flying-doorstop shape of earlier scramjet designs, HTV-3X has an efficient waverider shape, and does not have to go 400 mph to get off the runway.
HTV-3X is powered by hydrocarbon fuel - basically similar to jet fuel - and takes off and lands on a runway. In part, the demonstrator proposal is a response to the checkered history and operational impracticality of rocket-boosted one-shot demonstrators. But it's also aimed at demonstrating to customers that practical hypersonics may not be as far off as they think.