CAPE CANAVERAL, Fla. - Engineers probing the trouble experienced on the Boeing Delta 4-Heavy rocket's test flight have cleared 40 potential causes of the main engines cutting off prematurely, leaving 9 scenarios on the table, including the leading theory that bubbles formed in the liquid oxygen plumbing.

"The investigation team is making solid progress in identifying the cause of the early engine shutdowns. The investment the Delta 4 team made in additional telemetry has helped support the engineers as they narrow in on the cause of anomaly. I remain confident we will then be able to find solutions that allow us to avoid this problem on future flights," said Col. John Insprucker, Evolved Expendable Launch Vehicle program director at the Air Force's Space and Missile Systems Center.

The shortened firing time of the main engines meant the rocket failed to reach the proper altitude. Two student-made nanosats hitching a ride on the rocket's demonstration test flight were lost and the mission's primary payload -- an instrumented satellite mockup -- fell short of its intended orbit. The launch served as a full-up test of the powerful new booster before national security satellites are placed aboard the rocket in the future.

The investigators are using a Fault Tree analysis, which is a standard tool for such inquiries to pinpoint the root cause of an incident. Parts of the rocket's propulsion system, avionics, launch environments and other categories have been under suspicion as part of the Fault Tree, the Air Force said.

"Forty of the 49 branches of the Fault Tree have been reviewed, ruled out as potential causes of the anomaly and determined not to be a factor. These Fault Tree closures continue to clear the flight control systems (flight software and engine cutoff sensors), the RS-68 main engine and the basic structure," the Space and Missile Systems Center said in a statement Friday.

It is believed the flow of super-cold liquid oxygen in the rocket's three core boosters was disturbed, creating "bubbles" in the oxidizer feed lines that tricked internal sensors into thinking the motors were out of fuel and causing them to command an early engine shutdown.

The rocket's internal plumbing could have caused this so-called cavitation where the liquid oxygen changed to gaseous oxygen near the engine cutoff sensors. During the launch, the sensors registered the liquid oxygen supply was depleted, triggering the engine cutoff sequence. But moments later the sensors returned to "wet" conditions indicating the flow of liquid oxygen. All three boosters experienced this same scenario.

"This disruption of the smooth flow of liquid oxygen from the propellant tank to the engine is caused by a unique combination of vehicle acceleration, liquid oxygen level in the propellant tank, tank pressures and flow rates in the feed line when the RS-68 engine is at full power. The engine cutoff sensors reacted to the cavitation, indicating a depletion of liquid oxygen propellant. In reality, there was adequate propellant in the tank to support the expected duration of the RS-68 engine burns," Friday's Air Force statement said.

"Based upon progress to date, the combined Air Force and Boeing investigation team aims to complete the Fault Tree closeout and identify the cause by mid-February."

Computer simulations are being conducted to better understand the liquid oxygen flow between the bottom of the propellant tank and the engine cutoff sensors that are positioned about five feet downstream. This testing and analysis could continue through March.

Ways to fix the Delta 4-Heavy vehicle to prevent the cavitation bubbles on future launches won't be decided upon until the investigation identifies the official root cause, the Air Force said.

"We're making great progress in working through our Fault Tree," said Dan Collins, vice president of Boeing Expendable Launch Systems. "Our Air Force and Boeing investigation team remains confident that we will identify the cause of the early (main engine cutoff) in a timely manner and determine an appropriate solution prior to our next scheduled Delta 4 launch later this year."

The Air Force had planned to fly the first operational Delta 4-Heavy rocket in the August-September time frame to deploy the final Defense Support Program missile warning satellite. What, if any, delay this launch could encounter remains unknown.