The aircraft -- the Falcon Hypersonic Technology Vehicle 2 -- is designed to fly at 13,000 mph.
"Here's what we know," said U.S. Air Force Maj. Chris Schulz, DARPA HTV-2 program manager. "We know how to boost the aircraft to near space. We know how to insert the aircraft into atmospheric hypersonic flight. We do not yet know how to achieve the desired control during the aerodynamic phase of flight.
"It's vexing. I'm confident there is a solution. We have to find it."
DARPA said the second test flight began Thursday with launch at 7:45 a.m. The Minotaur IV vehicle placed the aircraft into the desired trajectory. Separation of the vehicle was confirmed and the aircraft transitioned to Mach 20 aerodynamic flight.
More than 9 minutes of data were collected before the signal was lost.
"Prior to flight, the technical team completed the most sophisticated simulations and extensive wind tunnel tests possible," said DARPA Director Regina Dugan. "But these ground tests have not yielded the necessary knowledge. Filling the gaps in our understanding of hypersonic flight in this demanding regime requires that we be willing to fly.
"In the April 2010 test, we obtained four times the amount of data previously available at these speeds. Today more than 20 air, land, sea and space data collection systems were operational. We'll learn. We'll try again. That's what it takes."
Schulz said three technical challenges exist within the HTV-2 flight regime: aerodynamic; aerothermal; and guidance, navigation and control.
Each phase of flight introduces unique obstacles and as Thursday's test showed "high-Mach flight in the atmosphere is virtually uncharted territory."
In the coming weeks, the assembled independent Engineering Review Board will review and analyze the collected data.