6.06.2021
Since Mariner 9 entered orbit around Mars on November 14, 1971, NASA has been continuously studying the Red Planet. The Viking landers reached the surface of Mars five years later and began sampling the soil. Since then, numerous landers and rovers with instruments from institutions across Earth have studied and traveled the red terrain in search of answers to many of our questions.
But all of the sample analysis has had to rely on the robotic laboratories and the data streams beamed back to Earth. Now, NASA and ESA (European Space Agency) are seeking to change that with the Mars Sample Return mission — for which a rocket launched from Earth and landed on Mars will be needed to bring Martian samples up to a waiting Earth Return Orbiter.
The complex mission — which will carry a fully-fueled, 3 meter long rocket — will require the recently-landed Perseverance rover to collect and bottle Martian surface samples for later collection by a Sample Fetch Rover that will be launched and landed with the Mars Ascent Vehicle (MAV).
On December 17, 2020, a full two months before Perseverance safely landed, NASA, in partnership with ESA, announced that it would move forward with this long awaited mission.
On the heels of the successful landing, NASA formally awarded the MAV contract to Northrop Grumman.
A 2019 graphic depiction the proposed mission architecture for Mars Sample Return. (Credit: ESA)
Other critical parts of the mission, the Earth Return Orbiter and Sample Fetch Rover, will be provided by ESA, along with the lander’s robotic arm. In addition to the MAV, NASA is providing the Sample Retrieval Lander and the Capture and Containment and Return System payload on the Earth Return Orbiter.
To discuss the initial concepts for the MAV, NASASpaceflight spoke with Northrop Grumman about the project and its current state of development and criteria.
“This won’t be our first rodeo, if you will, our first opportunity to provide propulsion systems that have flown within the Martian atmosphere,” noted Mike Lara, Space Programs Strategy and Business Development Lead, Northrop Grumman. “We supported the landing of the original Pathfinder/Sojourner rover. And then after that both the Spirit and Opportunity rovers were landing with the airbag-type arrangement that used our retrorockets and our gas generators to inflate the airbags as well as some small rockets to take out any sway in the parachute as it was coming down through the Mars atmosphere.”
David McGrath, Senior Northrop Grumman Fellow, added: “NASA started studying the mission in the late 1980s at a very low level, and then we got under contract in the mid 1990s. And we’ve been on contract off and on as the funding has ebbed and flowed. And what’s really special is to now see it go into full development and qualification and get ready to fly.”
McGrath noted the company’s long history with “long-term” missions that carried propellant for months or years before reaching their destinations — including the Magellan spacecraft to Venus which was in space for nearly 460 days before firing its rocket motor.
“This mission, because of the trajectory that we’re going to Mars on, will be about three years going outbound from Earth to Mars. It’s a different kind of trajectory. And then once we’re on the surface, we’re going to be there about a [Martian] year before the samples […] are collected, put into the rocket, and then launched,” said McGrath.
For this, Northrop Grumman will provide a 3 meter tall, two-stage, solid propellant rocket — with the first stage derived from the STAR 20 and and the second stage derived from the STAR 15 motors in Northrop Grumman’s propulsion systems catalog.
The propellant each stage will use has a long flight history for solar system exploration, having been used for the Surveyor lunar landings in the 1960s and on the Pioneer 10 and 11 and Voyager 1 and 2 spacecraft missions to the outer planets and reaches of the solar system.
“It’s a good propellant system. Well understood. Good for cold. Because it’s going to be cold,” added McGrath.
Additionally, long-term solid propellant aging is well defined and understood for a mission of this length thanks in large part to the Long Duration Exposure Facility (LDEF) experiment launched into low Earth orbit on April 6, 1984 by the Space Shuttle Challenger on the STS-41C mission.
LDEF was originally slated for retrieval 11 months later in March 1985; however, a series of mission reshuffles in 1984 and 1985 followed by the loss of the STS-51L crew in the Challenger disaster ultimately delayed its retrieval until January 1990 when Shuttle Columbia brought it back to Earth after nearly six years in orbit.
“The information from LDEF and some other aging studies gives us confidence that this longer than usual exposure to vacuum and cold won’t be a problem,” noted McGrath.
To the element of cold, McGrath noted that this is a tricky element to the mission — especially since the rocket will have less than 40 watts of power to keep both stage’s solid propellant within acceptable mission temperature ranges.
“Power is going to be limited because it’ll be solar powered. So we have basically less than 40 watts to keep our two stages warm enough such that they don’t crack and break under the cold. So that’s not a lot of power,” related McGrath.
And “cold” is relative. The allowed temperatures are “no colder than -40°C” with a preferred firing temperature of -20°C.
To maintain these temperatures, the rocket will be contained within a thermal covering to assist with temperature control. And the rocket’s liquid-fueled attitude control pod for the first stage will actually assist with radiative heating into the solid propellant.
“The rocket will be held in what they call an igloo. So the power that they need to keep the liquid warm will actually give us some radiative heating for both the first and second stage because it’ll be in the middle. So that’ll help as well, but for the solids themselves we’ll have less than 40 watts to deal with to keep us warm specifically.”
The thermal covering is also designed to open at the top so that the collected Martian samples can be loaded into the sample return capsule the Mars Ascent Vehicle will deliver into Martian orbit for retrieval by the Earth Return Orbiter.
Timing there will be tricky as the covering cannot be open too long as temperatures will drop to unacceptable levels. Multiple loading operations could be necessary based on the in-situ weather conditions at the Martian launch site.
“So that’s part of the challenge, as well as: how do you do all of this and not break the system physically before you have to use it.”
Part that will be addressed in the way the propellant is designed and poured into the adapted STAR 20 and STAR 15 stages of the Mars Ascent Vehicle. Exactly what the modified STAR 20 and 15 motors will ultimately look like is still somewhat “to be determined.”
The specifications for the “off-the-shelf” STAR 15 motor from Northrop Grumman, a modified version of which will serve as the second stage for the MAV. (Credit: Northrop Grumman Propulsion Products Catalog)
As McGrath noted, mission architecture is not yet finalized, but once it is, Northrop Grumman can then take the requirements given to them by NASA and finalize how the motors have to be designed and the nozzles sized for the needed ISP (specific impulse) the rocket’s stages need to attain.
Overall, NASA has asked for a first stage burn time of approximately 70 seconds and a stage two burn time of roughly 25 seconds — a sharp departure from the STAR 20’s 27 second “off-the-shelf” burn time. The STAR 15’s needed burn time is 8 seconds less than its “off-the-shelf” variant.
Additionally, like an Earth orbit ascent with an all-solid vehicle, after first stage burnout and separation, the second stage will coast up to apogee before igniting its engine for the orbit insertion burn.
However, the rocket first has to get off the ground — something never before attempted on another planet (though natural impacts have “launched” rocks and material from Mars to Earth over the course of the solar system’s 4.6 billion year evolution).
“As you can imagine, if you’re on a slope, what do you do about how you erect the rocket and do certain things along those lines? How do you [balance] the load against a lander that’s in contact with the ground and has to absorb all that launch load?” asked McGrath.
“The solution was actually quite brilliant by the NASA folks.”
Throw the rocket up into the air and then light the first stage.
“Basically, they’re going to use a scissor-type spring, and they’re going to push the rocket up into the air. And because the Martian gs are one-third that of Earth, you’ve got a little bit longer time before it’ll re-contact the ground. So there’s plenty of time for it to ignite,” related McGrath.
“And then the [Thrust Vector Control system] will allow us to get on to the right trajectory. It takes out all the issues with the lander that we worried about, quite frankly pretty seriously as a team internally and with NASA for the last 25 years. It’s quite brilliant because it designed out a bunch of problems that were going to be hard to solve.”
“It’s really all about timing. And, you know, we scoffed at the idea initially, too. Then when you run the math, it’s like, ‘oh, there’s plenty of time for the motor to ignite normally. There’s nothing really special about what we’re doing for ignition.’ So it’s very well within our experience base,” added McGrath.
Weather will also be a key consideration for liftoff from Jezero Crater, just as it is on Earth.
“It takes a lot to get a launch vehicle off the ground here on Earth, and many, many times we see holds with seconds to go, or a minute or a few minutes to go in the countdown,” stated Mr. Lara. “And all of that occurs with a variety of inputs — the weather folks, the range safety officers, the principle investigators. All of that will have to be pretty much automated for this liftoff.”
McGrath added, “I can imagine the various orbiting spacecraft at Mars are going to have a play in ‘what’s the weather like?’ Maybe what has to happen is the lander itself will have to have some anemometers for wind. Maybe the Ingenuity helicopter will still be operational at that point. Hard to say, we’re talking several years out now.”
“Right now we’re dealing with it robustly with the design to have enough control authority to ensure the rocket will get to where it’s supposed to go. Because they have a reasonable idea of various conditions on Mars, and I’m certain the window that they’re waiting for by staying on the surface for about a year will give them some good launch opportunities away from the dust storm time frames and with conjunctions of Earth and Mars.”
The portion of the mission with Mars Ascent Vehicle and Fetch rover is currently slated to launch no earlier than July 2026, followed in October 2026 with the launch of the Earth Return Orbiter on an Ariane 6 rocket.
(Lead image: Artist’s impression of a potential architecture for the Mars Ascent Vehicle. Credit: NASA/JPL-Caltech.)
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