Seven months after the first lunar module failed to reach the moon, Astrobotic announced Tuesday that the probe suffered a valve failure that caused a fuel tank to rupture in orbit. Repairs will be made to prevent a similar failure during the company's next landing attempt with a much larger probe.
Astrobotic's first Peregrine lander, which the company called Peregrine Mission One, launched on Jan. 8 aboard United Launch Alliance's first Vulcan rocket. But shortly after separating from the rocket in space, the lander ran into trouble as it went through an activation sequence to prepare its propulsion system.
An investigative committee concluded that “the most likely cause of the malfunction was a failure of a single helium pressure control valve called PCV (Pressure Control Valve 2) in the propulsion system,” said John Horack, a space industry veteran and professor of aerospace engineering and mechanical engineering at Ohio State University.
The helium was intended to pressurize Peregrine's propulsion system and transport fuel and oxidizer from the lander's onboard tanks into the spacecraft's small rocket engines so that they burn and generate thrust.
“PCV2 experienced a loss of sealing capability that was most likely due to a mechanical failure in the valve caused by vibration-induced relaxation between some threaded components inside the valve, a defect deep inside the valve itself,” said Horack, who led Astrobotic's investigation into the Peregrine lander failure.
It didn't take long for the valve malfunction to have catastrophic consequences for Astrobotic's Peregrine lunar lander, which was attempting to be the first U.S. spacecraft to make a soft landing on the Moon since 1972.
“As the PCV2 was being actuated, opened and closed, helium began to flow uncontrollably into the oxidizer tank, causing significant and rapid overpressurization of the tank,” said John Thornton, CEO of Astrobotic. “Unfortunately, the tank then ruptured and subsequently leaked oxidizer for the remainder of the mission.”
Astrobotic ground controllers, working from a control center at the company's Pittsburgh headquarters, responded quickly to stabilize the situation on the spacecraft. The lander's engines used hydrazine fuel mixed with nitrogen tetroxide to generate thrust, but due to the diminished nitrogen tetroxide supply, Peregrine was unable to maneuver into orbit around the moon and attempt a landing.
But the company kept the lander running, and ground teams were able to make small adjustments to ensure Peregrine's solar panels were pointed at the sun to generate power as the probe orbited a loop roughly the distance of the moon. Ten and a half days after launch, Earth's gravity pulled it back into the atmosphere, and it burned up over the distant Pacific Ocean.
Astrobotic designed and built the Peregrine lander under contract to NASA, which awarded the company a $108 million contract to deliver a series of government-sponsored scientific payloads to the lunar surface. Peregrine Mission One was the first mission launched under NASA's Commercial Lunar Payload Services (CLPS) program, which purchases transportation assets for scientific payloads to the Moon from commercial suppliers.
Flying to the moon on a small budget
As it turns out, Astrobotic officials were aware of the risk that a pressure regulator valve on the Peregrine spacecraft could fail. The lander had two such valves, one controlling the flow of helium into the fuel tank and another into the oxidizer tank. During pre-mission ground tests, the pressure regulator valve on the fuel side began to leak, so engineers replaced it with a new one. The similar valve on the oxidizer side that failed in space showed no problems during ground tests, said Sharad Bhaskaran, Astrobotic's mission manager for Peregrine Mission One.
Although the pressure control valve on the oxidizer side was of the same design, Astrobotic decided not to replace it because doing so would have required dismantling large parts of the Peregrine lander, further delaying the launch of the mission, which was already several years behind schedule.
Tests of a replacement pressure control valve conducted following the Peregrine mission confirmed that it could be leaking after engineers subjected it to vibrations similar to those that would occur during a rocket launch.
“There's a threaded part inside the valve,” Horack said. “So imagine a screw and a washer or any threaded part. If you shake it enough, you can get some changes in the mechanical configuration that prevent the valve from seizing up. And it's really no different than when your kitchen sink starts to leak. Water gets through the seal and comes out the other side. In this case, it's helium and it's under high pressure, so it's much harder to contain.”
Astrobotic did not name the third-party vendor that supplied the pressure control valve, but officials said the company is working with its supplier to redesign the component. “It's slightly different than the actual valve that flew on Peregrine, from the same vendor, but we worked closely with them to redesign the internal workings,” said Steve Clarke, Astrobotic's vice president of landers and spacecraft.
Astrobotic
Astrobotic's next lander, called Griffin, is larger and more complex than Peregrine. It will use the newly developed pressure-regulating valves, and Astrobotic will install pressure regulators and so-called interlock valves in Griffin's helium system. These new components would regulate the flow of helium into the fuel tanks in case a similar pressure-regulating valve fails on Astrobotic's next mission, officials said Tuesday.
“We have now increased the reliability of the system to mitigate this single point failure,” Clarke said.