Hubble Images a Grand Spiral

This NASA/ESA Hubble Space Telescope image features the glorious spiral galaxy NGC 5643, which is located roughly 40 million light-years away in the constellation Lupus, the Wolf. NGC 5643 is a grand design spiral, which refers to the galaxy’s symmetrical form with two large, winding spiral arms that are clearly visible. Bright-blue stars define the galaxy’s spiral arms, along with lacy reddish-brown dust clouds and pink star-forming regions.

As fascinating as the galaxy appears at visible wavelengths, some of NGC 5643’s most interesting features are invisible to the human eye. Ultraviolet and X-ray images and spectra of NGC 5643 show that the galaxy hosts an active galactic nucleus: an especially bright galactic core powered by a feasting supermassive black hole. When a supermassive black hole ensnares gas from its surroundings, the gas collects in a disk that heats up to hundreds of thousands of degrees. The superheated gas shines brightly across the electromagnetic spectrum, but especially at X-ray wavelengths.

NGC 5643’s active galactic nucleus isn’t the brightest source of X-rays in the galaxy, though. Researchers using ESA’s XMM-Newton discovered an even brighter X-ray-emitting object, called NGC 5643 X-1, on the galaxy’s outskirts. What could be a more powerful source of X-rays than a supermassive black hole? Surprisingly, the answer appears to be a much smaller black hole! While the exact identity of NGC 5643 X-1 is unknown, evidence points to a black hole that is about 30 times more massive than the Sun. Locked in an orbital dance with a companion star, the black hole ensnares gas from its stellar companion, creating a superheated disk that outshines the NGC 5643’s galactic core.

NGC 5643 was also the subject of a previous Hubble image. The new image incorporates additional wavelengths of light, including the red color that is characteristic of gas heated by massive young stars.

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Media Contact:

Claire Andreoli (claire.andreoli@nasa.gov)
NASA’s Goddard Space Flight Center, Greenbelt, MD

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6DOF Check Cases

This article is from the 2024 Technical Update.

In 2015, the NESC released benchmark Earth-based check-cases for well specified, rigid-body, six-degree-of-freedom (6DOF) aero/spacecraft models to promote consistent and accurate flight simulations across multiple Agency tools and facilities. Recently, the NESC expanded upon that effort to add Lunar-based check-cases to support new lunar exploration initiatives. This study produced a smaller, focused set of cases that exercise new and unique features of missions in the lunar environment in comparison with 8 high-fidelity NASA simulation tools and provides a measure of validation for simulations supporting Human Landing Systems.

Results
The primary output of the check-cases is a time history of each output variable, which can then be plotted with any data plotting software. For simulation comparison, the results from multiple simulations are
plotted together. A static website was developed as a tool for the simulation groups to perform quick data comparison using interactive plots,access scenario specifications, and catalogue the results.

Benefits for the FM Community
Utilizing benchmarking check-cases improves the simulations being assessed, reduces errors, builds confidence in solutions, and serves to build credibility of simulation results per NASA Standard 7009A Standard for Models and Simulations. Simulation comparisons can benefit from utilizing common standards for defining parameters and sharing models and elevates the validation for critical simulations used to support insight or requirement compliance through analysis.

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Meagan Chappell

Helium Conservation by Diffusion Limited Purging of Liquid Hydrogen Tanks

This article is from the 2024 Technical Update.

The NASA Engineering and Safety Center (NESC) has developed an analytical model that predicts diffusion between two gases during piston purging of liquid hydrogen (LH2) tanks. This model helps explain dramatic helium savings seen in a recent Kennedy Space Center (KSC) purge, shows that undesired turbulent mixing occurred in Space Shuttle External Tank purges, and is applicable to future helium purges of the Space Launch System Core Stage LH2 tanks.

Background
In 2023, work was completed on a new 1.3-million-gallon (174,000 standard cubic feet (scf)) liquid hydrogen tank at KSC in support of the Space Launch System[1], see Figure 1. Per contract, the vendor delivered this tank filled with gaseous nitrogen, leaving KSC ground operations the task of replacing the nitrogen with helium: a necessary step prior to introducing liquid hydrogen, which would freeze the nitrogen. Prior helium/nitrogen purges on the Apollo/Space Shuttle era 850,000-gallon (114000 scf) LH2 tanks were performed by pumping
out the nitrogen, introducing helium, drawing samples, and then repeating if necessary. However, the new tank did not have a vacuum port, so instead, it was decided to introduce the helium from the top of the tank and push the nitrogen out of the bottom. Two million scf of helium was obtained and made ready for fear the two gases would mix, resulting in a long and expensive purge. Surprisingly, this top-down, or piston purge, resulted in a rapid replacement of the nitrogen with helium, using only 406,000 scf of helium, about 1.6 million scf less than planned (at $1/scf this is a $1.6 million dollar savings). To better understand this remarkable result, the NESC was asked to address the questions; why did this work so well and can it be improved further?

Upon realizing that the purge was diffusion limited and could be modelled, variations were studied, leading to three important conclusions. The flow rate should be increased until the onset of turbulent mixing; once started, the purge should not be stopped because this allows additional diffusion to occur; and trying to improve the purge by varying temperature or pressure has little benefit. Purging of the huge LH2 spheres is rare, but purging of flight tanks is common. In 2008, purge data from three Space Shuttle External Tanks was measured using a mass spectrometer and the NESC was asked to apply the diffusion model to this data. Doing this showed
evidence that turbulent mixing occurred indicating that the flow rates needed to be decreased. Having such a model has provided insight into the use of piston-type helium purges at KSC, with the goal of saving helium and manpower. This work is now directly applicable to purging the LH2 tank on the Space Launch System Core Stage.

The Binary Gas Sensor
During past purges, gas samples were taken to a lab to indicate the status of the purge but doing that for a piston purge would introduce time delays, allowing unwanted diffusion to take place. Fortuitously, an independent NESC assessment[4] was evaluating a binary gas sensor, with an excellent combination of cost, size, power, and weight to implement in the field, providing rapid real-time monitoring of the purge gas ratio. Using this sensor made the piston purging of the new LH2 tank successful.

References

1. Fesmire, J.; Swanger, A.; Jacobson, J; and Notardonato, W.: “Energy efficient
   large-scale storage of liquid hydrogen,” In IOP Conference Series: Materials
   Science and Engineering, vol. 1240, no. 1, p. 012088. IOP Publishing, 2022.
2. Youngquist, R.; Arkin C.; Nurge, M.; Captain, J.; Johnson, R.; and Singh, U.:
   Helium Conservation by Diffusion Limited Purging of Liquid Hydrogen Tanks,
   NASA/TM-20240007062, June 2024.
3. Singh, U.: Evaluation and Testing of Anaerobic Hydrogen Sensors for the
   Exploration Ground Systems Program, NASA/TM-20240012664, Sept. 2024.

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Meagan Chappell

Considerations for Using Autonomous Flight Termination Softwarein Crewed Launch Vehicles

This article is from the 2024 Technical Update

Autonomous flight termination systems (AFTS) are being progressively employed onboard launch vehicles to replace ground personnel and infrastructure needed to terminate flight or destruct the vehicle should an anomaly occur. This automation uses on-board real-time data and encoded logic to determine if the flight should be self-terminated. For uncrewed launch vehicles, FTS systems are required to protect the public and governed by the United States Space Force (USSF). For crewed missions, NASA must augment range AFTS requirements for crew safety and certify each flight according to human rating standards, thus adding unique requirements for reuse of software originally intended for uncrewed missions. This bulletin summarizes new information relating to AFTS to raise awareness of key distinctions, summarize considerations and outline best practices for incorporating AFTS into human-rated systems.

Key Distinctions – Crewed v. Uncrewed
There are inherent behavioral differences between uncrewed and crewed AFTS related to design philosophy and fault tolerance. Uncrewed AFTS generally favor fault tolerance against failure-to-destruct over failing silent
in the presence of faults. This tenet permeates the design, even downto the software unit level. Uncrewed AFTS become zero-fault-to-destruct tolerant to many unrecoverable AFTS errors, whereas general single fault
tolerance against vehicle destruct is required for crewed missions. Additionally, unique needs to delay destruction for crew escape, provide abort options and special rules, and assess human-in-the-loop insight, command, and/or override throughout a launch sequence must be considered and introduces additional requirements and integration complexities.

AFTS Software Architecture Components and Best-Practice Use Guidelines
A detailed study of the sole AFTS currently approved by USSF and utilized/planned for several launch vehicles was conducted to understand its characteristics, and any unique risk and mitigation techniques for effective human-rating reuse. While alternate software systems may be designed in the future, this summary focuses on an architecture employing the Core Autonomous Safety Software (CASS). Considerations herein are intended for extrapolation to future systems. Components of the AFTS software architecture are shown, consisting of the CASS, “Wrapper”, and Mission Data Load (MDL) along with key characteristics and use guidelines. A more comprehensive description of each and recommendations for developmental use is found in Ref. 1.

Best Practices Certifying AFTS Software
Below are non-exhaustive guidelines to help achieve a human-rating
certification for an AFTS.

References

1. NASA/TP-20240009981: Best Practices and Considerations for Using
   Autonomous Flight Termination Software In Crewed Launch Vehicles
   https://ntrs.nasa.gov/citations/20240009981
2. “Launch Safety,” 14 C.F.R., § 417 (2024).
3. NPR 8705.2C, Human-Rating Requirements for Space Systems, Jul 2017,
   nodis3.gsfc.nasa.gov/
4. NASA Software Engineering Requirements, NPR 7150.2D, Mar 2022,
   nodis3.gsfc.nasa.gov/
5. RCC 319-19 Flight Termination Systems Commonality Standard, White
   Sands, NM, June 2019.
6. “Considerations for Software Fault Prevention and Tolerance”, NESC
   Technical Bulletin No. 23-06 https://ntrs.nasa.gov/citations/20230013383
7. “Safety Considerations when Repurposing Commercially Available Flight
   Termination Systems from Uncrewed to Crewed Launch Vehicles”, NESC
   Technical Bulletin No. 23-02 https://ntrs.nasa.gov/citations/20230001890

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Meagan Chappell