Since the focus on Martian exploration ramped up in the mid-1990s, most of the familiarity with the Entry, Descent, and Landing (EDL) sequence of landers and rovers to the surface of other planets has taken place against the backdrop of Mars.
But when NASA‘s upcoming Dragonfly mission arrives at Titan for its own EDL, it will experience a starkly different set of conditions that both add new complexity and ease some structural considerations for the system that will deliver the rotorcraft into Titan’s atmosphere.
Different from Mars
Three main considerations for an EDL sequence are the specific gravity, atmospheric thickness, and atmospheric composition of the planet where the landing will occur. While Titan is a moon of Saturn, for EDL, it can be thought of as a planet.
Compared to Earth, Mars is small, with 0.3 G surface gravity and an atmosphere less than one percent of our home planet’s.
Like Mars, Titan has weak gravity at only 0.1 G. However, their atmospheres are vastly different, with Titan’s measuring 1.19 times denser than Earth’s.
This combination of a dense atmosphere and low gravity change elements for Dragonfly’s EDL over those of the Mars missions.
“At Mars, you have to rapidly slow yourself down, rapidly have that descent because you don’t have much of an atmosphere,” said Dave Buecher, Lockheed Martin Dragonfly program manager, in an interview with NASASpaceflight. “In this case, we can take advantage of that dense atmosphere, have smaller parachutes, which also means lower loads.”
Those lower loads mean that portions of the aeroshell’s design don’t have to be as rigid as those used by Lockheed Martin on the aeroshells for the Mars missions.
The EDL Sequence to Titan
While this will be the largest lander sent to Titan and the first designed to fly autonomously through its atmosphere, it will not be the first probe to land on the moon. The honor went to the Huygens probe, a part of the Cassini mission which landed on Titan on Jan. 15, 2005.
Data from the success of that mission is feeding into the design for Dragonfly, but so are the past landings of Mars missions — to which Lockheed Martin has direct experience. Huygens was part of ESA’s element of the Cassini mission, and the aeroshell for Huygens was built by a consortium led by Aerospatiale.
At Mars, the EDL event is colloquially and somewhat lovingly called the “Seven Minutes of Terror,” referring to the length of time it takes a spacecraft to touch down on the surface after entering the atmosphere.
For Dragonfly, the first part of the EDL sequence, entry, will be somewhat similar to those at Mars, with similar heating and initial supersonic entry dynamics that are well understood at Mars and can be properly modeled and studied for Titan thanks in part to the Huygens probe’s landing.
After entry, the timelines diverge. EDL at Titan will last approximately 105 minutes, meaning the aeroshell will be under parachute descent and exposed to the cryogenic elements of Titan’s atmosphere for a prolonged period.
“Because we hang on to the parachute so much longer, we have to look at what that thermal environment does to all the different materials and elements and what we have to insulate or heat or change based on the thermal environments,” related Buecher.
The longer descent compared to Mars also affects parachute design, with Buecher noting that “because [EDL is] so much longer, it allows us to step our parachutes in size as well to shape them a little bit more for the speed that we’re coming in at.”
A smaller drogue chute will deploy first at supersonic velocities to begin slowing the aeroshell more and stabilize the craft ahead of main chute deployment.
Once under the main chute, the heatshield will be jettisoned (part of the lower “cup” of the aeroshell), similar to Mars missions, exposing Dragonfly and the interior of the backshell (the upper “cup” of the aeroshell) to the cryogenic conditions of Titan’s atmosphere.
“We want to hold on to the heatshield as long as we can so we can keep everything inside [the aeroshell] as warm as we can,” said Buecher. “Once you take the heat shield off, you start to ingest more of that environment, and you start to cool down. So there are reasons to keep it on longer if you can.”
After heat shield deployment, the backshell and parachute will continue to descend as the pose mechanism is deployed. This will ease Dragonfly below the backshell to expose the craft’s rotors ahead of activation.
The rotors will then begin to spin, helping stabilize the overall craft while also warming the rotor systems ahead of Dragonfly’s final release from the backshell.
Once the backshell and parachute system has descended to the appropriate altitude (which has not been finalized as of publication), Dragonfly will be released and will fly itself the final few meters down to the surface of Titan.
An important note regarding the current state of planning for Dragonfly’s EDL is that while the overall sequence is known, the precise timings are not.
This is in part due to mission-specific elements and mission planning overall continuing to work through various design reviews that are both informed by the needed EDL sequence and also inform the specific timings of the sequence.
Weather and Communication
Two other considerations for Dragonfly’s landing on Titan are the weather conditions the craft could be exposed to and its communication needs with Earth.
“There could be clouds or different weather,” said Buecher. But, “the season that we’ll be landing in is summer for Titan. And part of our mission design is to make sure that we’re hitting in a good time for us.”
In an interview with NASASpaceflight, Lockheed Martin discusses the upcoming Preliminary Design Review for Dragonfly: the mission to fly a nuclear-powered rotorcraft on Titan, a moon of Saturn.
By Chris Gebhardt (@ChrisG_NSF): https://t.co/q1MTnAz6rM
— Thomas Burghardt (@TGMetsFan98) January 13, 2023
While a summer landing increases the likelihood of stable weather conditions, that is no guarantee. And unlike human missions returning to Earth that can delay their landings based on observed weather conditions, there will be no such option to delay Dragonfly’s landing.
Furthermore, unlike Mars, where the constant surveillance of orbiting craft provides weather information ahead of landings on the red planet, there are no such spacecraft in orbit of Saturn or Titan.
Designing for various weather potentialities from the aeroshell perspective includes examining the atmospheric density side of the equation as well as the specific temperature loads and mass ingestions (i.e. descending through clouds) possible after heat shield separation.
Another difference that aeroshell and Dragonfly designers needed to account for is communication back to Earth during the EDL process.
At Mars, a direct line of sight to Earth for the entire EDL sequence — while preferred — is no longer a requirement due to the possible use of other multipurpose orbiting satellites that can serve as communications relays.
As no such satellites exist at Titan or Saturn, the aeroshell and Dragonfly itself must maintain a direct line of communication to Earth so that a constant stream of telemetry and information from the craft is maintained.
This data, excluding a blackout period during the plasma stage of entry, will allow mission controllers to know that Dragonfly executed the EDL sequence as planned and has successfully landed on Titan’s surface.
Conversely, should something result in the loss of the mission during EDL, it is this stream of returned data that will allow mission operators to hopefully diagnose what went wrong.
While this data is needed, it will not be received instantly upon transmission due to the distances between Earth and Saturn. At their closest approach, the two planets pass 1.2 billion km from each other, resulting in a one-way communication time of approximately 66.7 minutes.
This means the entire EDL process must be automated and controlled locally by Dragonfly’s computers.
Dragonfly is currently scheduled to launch in June 2027 on a yet-to-be-selected rocket for arrival on Titan in 2034.
(Lead image: Infographic showing Dragonfly’s EDL sequence. Credit: NASA)