ABL RS1 stag 2 test
A nominal static-fire test of ABL Space System's RS1 upper stage on Jan. 7. A similar test Jan. 19 resulted in a "hard start" of the engine that destroyed the stage. Credit: ABL Space Systems

WASHINGTON — ABL Space Systems says a test incident that destroyed the upper stage of its RS1 rocket last week will delay that vehicle’s first flight by three months as it identifies and corrects the failure’s root cause.

ABL was in the middle of a test campaign for the RS1 upper stage built for its first launch when an anomaly destroyed the stage during a Jan. 19 static firing at the Mojave Air and Space Port in California. The accident created a dramatic plume of black smoke and prompted a response from local firefighters, but there were no injuries reported and no damage outside of the location where the stage was being tested.

Harry O’Hanley, chief executive of ABL, told SpaceNews that the stage’s E2 Vacuum engine suffered a “hard start” in the hot gas circuit of its turbopump. A hard start is when the flow of propellants and ignition fluid in an engine doesn’t allow for a gradual increase in energy, but instead an explosive rise. Hard starts can damage or destroy rocket engines.

In the case of this test, O’Hanley said, the hard start caused “a substantial fire on the aft end of the vehicle, resulting in a complete failure about 20 seconds later.” No personnel were near the test stand at the time of the incident, and there no significant damage to three adjacent test cells at the site.

Since the accident, engineers have been examining data to work through the fault tree to determine the root cause. “It appears that the failure was a result of multiple risk factors combining in an improbable way,” he said, although the company declined to comment on those risk factors while the investigation is still in progress. “This explains why we haven’t observed the failure after over 80 starts and 3,000 seconds of hot fire testing of the flight-design E2 engine over the past six months.”

Dan Piemont, president of ABL, said the company did see some small hard starts during earlier testing of the engine that caused minor damage. “Those were each traced to a simple cause, fairly typical engine R&D events,” he said. “The failure last week was larger and more complex.”

The company is work on what O’Hanley described as “manufacturing and operational improvements” to the E2 engine to reduce the chance that fuel could leak into the turbopump and cause a hard start. “We’re also implementing some simple changes to reduce the probability of an off-nominal startup.”

The incident took place in the seventh in a series of hot-fire tests of the stage in Mojave. The overall test campaign started in December with a series of fill-and-drain, cold flow and ignition tests, followed by the hot-fire tests. Piemont said that, at the time of the anomaly, at least five more tests were planned before the company completed the test campaign.

That upper stage was being tested ahead of the first RS1 launch from Kodiak Island, Alaska. “After some final engine design changes were identified last summer, we set an aggressive schedule to try to launch by the end of 2021,” Piemont said. “Our schedule slipped a bit in past few months, but our programs were converging towards a launch from Kodiak in February.”

After the incident, he said the company expects a three-month delay in that first launch. The upper stage being built for the second RS1 launch will now be used for the first launch. The lower stage is complete and currently in storage in Kodiak, while the interstage between the first and second stages is being shipped to Kodiak. The payload fairing has completed acceptance testing and will soon be shipped to the launch site.

“Our strategy in developing RS1 is to rapidly test, iterate and sometimes fail. An engine hard start was a known risk in this campaign,” O’Hanley said. “This strategy has been highly successful in helping us to uncover unknown unknowns and has been the underpinning of our rapid progress over the past three years.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...