April 24, 2024

Planning a soft landing on Mars

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Instantaneous solution quantities shown for a static Mach 1.4 solution on a mesh consisting of 33 billion elements using 33,880 GPUs, or 90% of Frontier. From left to right, the contours show the mass fractions of the hydroxyl radical and H₂O, the temperature in Kelvin and the local Mach number. Credit: Gabriel Nastac/NASA

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Instantaneous solution quantities shown for a static Mach 1.4 solution on a mesh consisting of 33 billion elements using 33,880 GPUs, or 90% of Frontier. From left to right, the contours show the mass fractions of the hydroxyl radical and H₂O, the temperature in Kelvin and the local Mach number. Credit: Gabriel Nastac/NASA

A US mission to land astronauts on the surface of Mars will be unlike any other extraterrestrial landing ever undertaken by NASA.

Although the space agency has successfully landed nine robotic missions on Mars since its first missions to the surface in 1976 with Project Viking, bringing humans safely to Mars will require new technologies for flying through the Martian atmosphere. But these technologies and systems cannot be comprehensively tested on Earth beforehand.

Since 2019, a team of NASA scientists and their partners have been using NASA’s FUN3D software on supercomputers located at the Department of Energy’s Oak Ridge Leadership Computing Facility, or OLCF, to conduct computational fluid dynamics, or CFD, simulations of Mars on a human scale. landing. The OLCF is a DOE Office of Science user facility located at DOE’s Oak Ridge National Laboratory.

The team’s ongoing research project is the first step in determining how to safely land a rover with humans on board on the surface of Mars.

“By their very nature, we don’t have validation data for this. We can do valuable but limited testing in ground-based facilities like a wind tunnel or in a ballistic field, but such approaches cannot fully capture the physics that will be found in Mars.” We can’t do flight tests in the real Martian environment – it’s all or nothing once we get there. That’s why supercomputing is so important,” said Eric Nielsen, senior research scientist at NASA Langley Research Center and principal investigator of the five-year effort at OLCF.

Unlike recent missions to Mars, parachutes are not part of the operation. Instead, the leading candidate for landing humans on Mars is retropropulsion — firing forward-facing rockets embedded in the craft’s heat shield to slow itself down.

“We’ve never flown anything like this before. The fundamental question from the beginning was, ‘Will we be able to control this vehicle safely?'” Nielsen said.

Credit: Oak Ridge National Laboratory

The reason NASA is investigating retropropulsion instead of conventional parachutes is a matter of physics. Previous Mars probes weighed about 1 ton; a vehicle carrying astronauts and all their life support systems will weigh 20 to 50 times as much, or about the size of a two-story house. Mars’ thin atmosphere – about 100 times less dense than Earth’s – will not support a parachute landing for such a large craft.

“With a conventional vehicle, we fly through a very clean and predictable environment. All of that disappears with this concept, where we will travel through an extremely dynamic environment consisting of high-energy rocket exhaust gases,” said NASA team member. and CFD specialist Gabriel Nastac.

With guidance from NASA mission planners, the team formulated a multiyear plan consisting of increasingly sophisticated simulations aimed at the key issue of controllability.

In 2019, the team conducted CFD simulations on the Summit supercomputer with resolutions of up to 10 billion elements to characterize the vehicle’s static aerodynamics at predicted acceleration settings and flight speeds ranging from Mach 2.5 to Mach 0.8, conditions in which the vehicle’s rocket engines will be required. for initial deceleration.

Throughout 2020, an intense code development effort focused on porting FUN3D’s general reaction gas capabilities to Summit’s graphics processing unit, or GPU, accelerators.

“Realizing efficient performance of an unstructured grid CFD solver in the face of complex physics-laden kernels is a huge challenge in a GPU-based computing environment. But ultimately, we were able to restructure critical segments of code to provide the performance we were looking for,” said NASA computer scientist Aaron Walden, who leads the team’s multi-architecture software development.

The work set the stage for a major 2021 campaign that allowed the team to address the complex interactions of liquid oxygen/methane rocket engines with the Martian atmosphere, which consists primarily of carbon dioxide and nitrogen. One petabyte (equivalent to 1,000 terabytes) of output data for each simulation conducted using 15,000–20,000 GPUs on Summit yielded important insights into critical differences in vehicle aerodynamics compared to those observed using the perfect gas assumption from the previous simulation.

Credit: Oak Ridge National Laboratory

For the 2022 campaign, the team took a big step by incorporating NASA’s cutting-edge flight mechanics software known as Program to Optimize Simulated Trajectories II, or POST2, into the workflow. Going beyond simulations that assume a static flight condition, the team now sought to “fly” the vehicle in the virtual supercomputing environment. This test would represent a first attempt to quantify and address critical unstable dynamics that would be encountered during an actual descent to the Martian surface.

The team recruited key experts from Georgia Tech’s Aerospace Systems Design Laboratory; this group was led by Brad Robertson. These experts had already spent several years developing a docking algorithm to replace the low-order aerodynamic models in POST2 with real-time physics-based FUN3D simulations to ultimately perform high-fidelity trajectory simulations that take advantage of sophisticated control algorithms. of flight.

“Coupling FUN3D and POST2 was quite a challenge. We had to juggle five or six frames of reference and the data transformations between them. But the reward was being able to adopt all the hard work done by other NASA engineers on detailed guidance, navigation , control, and propulsion models and bring them all into a single, unified multiphysics simulation,” said team member Zach Ernst, a Georgia Tech doctoral student at the time, who worked with NASA intern Hayden Dean on the effort.

The incorporation of POST2 brought an additional challenge. Because POST2 is subject to more restrictive export control regulations than FUN3D, team member Kevin Jacobson was tasked with developing a remote docking paradigm in which POST2 would run at a NASA facility while communicating in real time. with FUN3D running at a leading scale in the OLCF. .

Establishing and maintaining this connection considering firewalls, network outages, and task schedulers presented numerous challenges. This work required approximately a year of planning and coordination with cybersecurity personnel and systems administrators at both facilities.

The additional effort paid off when the team achieved its long-term goal of flying a substantial portion of the descent phase in the virtual environment.

The arrival of OLCF’s Frontier supercomputer could not have come at a better time for the project. With exascale computing power (a quintillion or more calculations per second) now a reality, the team could afford to reintroduce the desired physical modeling and other lessons learned over the life of the project.

Credit: Oak Ridge National Laboratory

In 2023, the team focused on the ultimate simulation they had been hoping for years before: a truly autonomous, closed-loop test flight leveraging the world’s most powerful supercomputing system.

While the eight main engines are used to control pitch (up and down rotation) and yaw (lateral rotation) while the guidance system points to the designated landing zone, POST2 also issues commands to instruct FUN3D. to periodically fire four reaction control system, or RCS, modules arranged circumferentially around the rear of the lander to perform in-flight rotation corrections.

“These capabilities will be critical for evaluating the controllability of future vehicles,” said Alex Hickey of Georgia Tech, who led the development of the RCS modeling.

The team’s long-term goal became a reality in late 2023, when OLCF personnel helped coordinate a careful sequence of high-priority work over a large-scale two-week period at Frontier.

“For the first time, we were able to return to the original question of safely controlling this type of vehicle in autonomous flight,” said Nielsen. “In a typical aerospace CFD simulation, one can calculate a second or two of physical time. Here, Frontier allowed us to successfully fly 35 seconds of controlled flight, descending from 8 kilometers (about 5 miles) altitude to about 1 kilometer (0.6 miles) as the vehicle approached the landing phase.

“The resolution, physical modeling and temporal duration are beyond anything we could attempt in a conventional high-performance computing system,” added Nielsen. “The sheer speed of GPUs deployed at leading scale is truly empowering, and we are deeply grateful for the many opportunities and world-class insights that OLCF has provided.”

More information:
Jan-Renee Carlson et al, High-fidelity simulations of human-scale lander descent trajectories, AIAA AVIATION Forum 2023 (2023). DOI: 10.2514/6.2023-3693

Ashley M. Korzun et al, Application of a separate eddy simulation approach with finite-rate chemistry to Mars-relevant retropropulsion operational environments, AIAA SCITECH 2022 Forum (2022). DOI: 10.2514/6.2022-2298

Gabriel Nastac et al, Computational Investigation of the Effect of Chemistry in Mars Supersonic Retropropulsion Environments, AIAA SCITECH 2022 Forum (2022). DOI: 10.2514/6.2022-2299

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