NASA’s Artemis I mission is nearing completion, and so far Orion’s daring flight far beyond the Moon has gone about as well as the space agency could hope. However, to earn a passing grade, the mission must still pass its final test.
That final review will take place on Sunday, when the spacecraft begins to enter Earth’s atmosphere at 12:20 p.m. ET (17:20 UTC). Over the next 20 minutes, before Orion plunges into the Pacific Ocean off Mexico’s Baja Peninsula, it will need to slow from a speed of Mach 32 to, essentially, zero before plummeting into the water. .
This is an achievement. Orion has a mass of 9 metric tons, about the same as two or three large elephants. Its base, covered with a heat shield designed to carbonize slowly during passage through the Earth’s atmosphere, must withstand temperatures close to 3,000 degrees Celsius.
There are two main elements for this reentry that NASA aims to test: the performance of this heat shield and its parachute system. For mission planners, the heat shield is the biggest concern.
“Re-entry is our top priority for a reason,” said Mike Sarafin, who leads the Artemis I mission management team. “There are no arc-jet or aerothermal facilities here on Earth. capable of replicating hypersonic re-entry with an Orion-sized heat shield. And it’s a completely new heat shield design. It’s safety critical equipment. It’s designed to protect spacecraft and astronauts on board. So the heat shield has to work. We can reduce some of that risk on the ground, but not in terms of getting back to Mach 32.”
A new design
NASA tested a go-anywhere version of the Orion spacecraft in December 2014, launching it to an altitude of nearly 6,000 km. From this orbit, Orion re-entered the Earth’s atmosphere at a speed of 9 km/s. For Artemis I, Orion will return at a speed of 11 km/s. That may not seem like such a large increase, but for reentry velocity, the increase in convective and radiative elements is exponential as velocity increases, said Jim Geffre, vehicle integration manager at Orion.
“So the velocity effect is huge, and that’s why the increase in heat load from entering low Earth orbit at lunar velocity is so much higher,” he told Ars.
The Orion vehicle flown during the EFT-1 mission featured the same basic ablative material, an epoxy known as AVCOAT that was also used by the Apollo capsules when they returned from the Moon half a century ago. Like the Apollo capsule, this AVCOAT material was injected into honeycomb cells at the base of the spacecraft.
For the Artemis I flight and future missions, however, NASA switched to a “molded” block design of AVCOAT for the Orion base. This was done, in part, to make the production of these heat shields faster and more efficient. Unlike the honeycomb design, these molded block heat shields can be built in parallel with the base of the spacecraft, rather than having to be affixed afterwards.
There are 186 different molded blocks on the bottom of Orion, a real puzzle to cover the bottom of the 5 meter wide spaceship. Sunday’s reentry will test the design of NASA’s method for filling the joints and gaps between these molded blocks.
Parachutes and grabs
Another key element of Orion’s reentry involves deploying its parachutes approximately 1,600 meters above the Earth’s surface. These falls are intended to slow Orion to a speed of 30 km/h as it falls into the ocean.
However, unlike Orion’s heat shield, NASA officials believe they have correctly characterized the risk to the parachutes through an extensive testing campaign. Geffre said NASA has conducted 47 drop tests of Orion’s parachute system to date.
NASA announced Thursday that it plans to land Orion further south in the Pacific than originally planned. This is due to poor weather conditions further north off the coast of California. As a result, Orion will crash near Isla Guadalupe, about 240 km west of the Baja Peninsula in Mexico.
As part of its descent, Orion will follow a jump entry technique instead of a direct descent followed by the Apollo missions. This will allow Orion to land closer to shore and subject the astronauts to lower gravitational forces – about 4 G – than occurred during Apollo’s re-entry.
NASA will provide live coverage of Orion’s return on Sunday starting at 11 a.m. ET (4 p.m. UTC), with a splashdown scheduled for 12:40 p.m. ET.
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