Flying through methane

In 2021, the Ingenuity helicopter flew over the surface of Mars, becoming the first aircraft to ever fly on another planet.

“The Ingenuity drone is only 20 inches tall,” said Chris Goldenstein, associate professor of mechanical engineering at Purdue. “Dragonfly is different; it’s about the size of a Honda Civic. And it’s not just flying around to scout locations for a separate rover. It’s a totally self-contained airborne scientific laboratory, which will fly itself to promising locations to conduct experiments at different locations on Titan's surface.”

How can such a large aircraft travel solely by rotor blades? The key is Titan’s atmosphere.

“Titan’s atmosphere is mostly nitrogen (like Earth’s), with about 1.5% to 2.5% methane by volume,” said Vishnu Radhakrishna, a Ph.D. student in Goldenstein’s lab and lead author of a recently-published paper on detailed spectroscopy relevant to entry on Titan. “But it’s about 4 times denser than Earth’s — and hundreds of times denser than Mars’ — which makes rotor-based flying easier.”

But it's not flying that concerns the Purdue researchers — it's that initial atmospheric entry.

“The vehicle will enter Titan’s atmosphere at extremely high velocity, forming a shock layer with unique and highly non-equilibrium chemistry,” Goldenstein said. “That will induce a significant amount of heat flux to the vehicle. It’s our job to try to understand that chemistry and the interactions of the shock layer plasma, so that NASA can more accurately predict the radiative heat transfer and design an appropriate heat shield for Dragonfly.”

In order to recreate the extreme conditions and temperatures encountered during atmospheric entry into Titan, Goldenstein’s team uses a shock tube: a long metal tube with high-pressure helium at one end and a mixture of gases resembling Titan’s atmosphere at the other end. A metal diaphragm divides the two sections; when it ruptures, a shock wave is sent down the tube, instantaneously heating the mixture to temperatures like those during Titan atmospheric entry. Goldenstein’s team then uses an ultrafast laser to measure the number of molecules in specific quantum states or energy levels, which helps them understand the chemical physics taking place in the gas mixture. This information can then be used to evaluate the models used to predict the amount of radiative heating experienced during entry into Titan’s atmosphere.

Their research has been published in The Journal of Chemical Physics.

(Left to right) Vishnu Radhakrishna and Chris Goldenstein use a shock tube to recreate the extreme temperatures, pressures, and velocities involved in atmospheric entry on Titan. (Purdue University photo/Jared Pike)

Shocking science

Goldenstein’s team developed a diagnostic called ultrafast laser absorption spectroscopy to study the chemical processes occurring behind shock waves. “This diagnostic has a unique ability to quantify the number of molecules in a huge number of energy levels and on a very short timescale, less than one nanosecond,” Goldenstein said. “This results from using femtosecond laser pulses, which inherently possess many wavelengths of light.”

Timing is critical in their experiments. After bursting the diaphragm, the shock wave travels toward the test section at nearly Mach 6 — that’s about 1.6 miles a second. The team has a very short window to fire the ultrafast lasers and collect data. The whole test is over in less than a millisecond.

“We’re very proud of our shock tube facility here,” Goldenstein said. “We can operate at burst pressures of over 200 atmospheres to produce very strong shock waves. There aren’t many shock tubes in academia that can operate at such high pressures.”

As a result of their experiments, they witnessed cyanide (a compound with one carbon atom and one nitrogen atom) being created in high-energy quantum states, and surviving there longer than conventional models predict.  “This is something that hadn’t been conclusively observed previously,” Radhakrishna said. “These results could help NASA develop improved radiative heat transfer models for Dragonfly.”

Dragonfly is scheduled to launch no earlier than 2028 and arrive on Titan in the mid-2030s. That may sound like a long way off, but Goldenstein’s team is excited at the opportunity to help NASA solve these challenges now.

“This mission’s main goal is to explore how life came about in the solar system,” Radhakrishna said. “It’s exciting to be able to contribute to that and, hopefully, one day to see the results and know that we played a role in making that happen.”

NASA’s Dragonfly hopes to become the first aircraft to conduct scientific research on another planet. But first it has to survive atmospheric entry on Titan. (Photo courtesy NASA / Johns Hopkins APL)

Dragonfly will be designed, built and operated for NASA by the Johns Hopkins Applied Physics Laboratory (APL). This research is part of NASA’s Space Technology Mission Directorate (STMD) Early Career Faculty Program, and in collaboration with NASA’s Ames Research Center via the Entry Systems Modeling Project, which supports the Dragonfly team as they develop systems to land on Titan.

Writer: Jared Pike, jaredpike@purdue.edu, 765-496-0374

Source: Chris Goldenstein, csgoldenstein@purdue.edu

Characterization of non-Boltzmann CN X²Σ⁺ behind shock waves in CH₄–N₂ via broadband ultraviolet femtosecond absorption spectroscopy
Vishnu Radhakrishna, Ryan J. Tancin, and Christopher S. Goldenstein
https://doi.org/10.1063/5.0150382
ABSTRACT: This article describes the temporal evolution of rotationally and vibrationally non-Boltzmann CN X²Σ⁺ formed behind reflected shock waves in N₂–CH₄ mixtures at conditions relevant to atmospheric entry into Titan. A novel ultrafast (i.e., femtosecond) laser absorption spectroscopy diagnostic was developed to provide broadband (≈400 cm⁻¹) spectrally resolved (0.02 nm resolution) measurements of CN absorbance spectra belonging to its B²Σ⁺ ← X²Σ⁺ electronic system and its first four Δv = 0 vibrational bands (v′′ = 0, 1, 2, 3). Measurements were acquired behind reflected shock waves in a mixture with 5.65% CH₄ and 94.35% N₂ at initial chemically and vibrationally frozen temperatures and pressures of 4400–5900 K and 0.55–0.75 bar, respectively. A six-temperature line-by-line absorption spectroscopy model for CN was developed to determine the rotational temperature of CN in v′′ = 0, 1, 2, and 3, as well as two vibrational temperatures via least-squares fitting. The measured CN spectra revealed rotationally and vibrationally non-Boltzmann population distributions that strengthened with increasing shock speed and persisted for over 100 μs. The measured vibrational temperatures of CN initially increase in time with the increasing CN mole fraction and eventually exceed the expected post-shock rotational temperature of N₂. The results suggest that strong chemical pumping is ultimately responsible for these trends and that, at the conditions studied, CN is primarily formed in high vibrational states within the A²Π or B²Σ⁺ state at characteristic rates, which are comparable to or exceed those of key vibrational equilibration processes.