Michelle Munk is Entry, Descent and Landing Principal Investigator for the Space Technology Mission Directorate. Most recently, Munk was the subsystem lead and the Deputy Project Manager for the Mars Science Laboratory Entry, Descent, and Landing Instrumentation (MEDLI) flight payload.
NASA Tech Briefs: What is MEDLI?
Michelle Munk: MEDLI is an instrumentation system that was put on the heat shield of the Mars Science Laboratory. It measured the pressure and temperature of the front face of the vehicle as it flew through the Martian atmosphere to land Curiosity on the surface.
NTB: How were you able to determine the appropriate amount of thermal protection as the spacecraft landed?
Munk: The type of thermal protection system material is determined by the peak heating that we'll see as we fly thorough the atmosphere. The thickness of the material is determined by how long our heat pulses as we fly. Mars Science Laboratory had a PICA (Phenolic Impregnated Carbon Ablator) heat shield. That was first flown on the Stardust sample return capsule, but on MSL it was in a tiled form. That was the first time it ever flew that way, and it performed great. MEDLI had thermocouples at depth within the thermal protection system so we were able to see how the temperature rose in depth in the material during entry.
NTB: How did you prepare for the landing on Mars? How do you minimize risk, given the unpredictability of the environment?
Munk: We minimize risk in many ways, all along the design process. Each of the subsystems is given mass budgets and risk postures; those are balanced throughout the life cycle. One way we want to reduce the risk of future missions is to better understand where our margins are in the vehicle. If we don't instrument our entry systems, we only know that they "passed," as in a pass/fail test. With MEDLI, and hopefully other missions to follow, instrumenting the heat shield gives us information back about how well we did. So instead of a pass/fail, we know if we got an A, B, C, or D.
NTB: What kinds of data are the MEDLI sensors finding? How are you able to analyze the Mars environment that you're flying through?
Munk: MEDLI has seven pressure transducers, and those read the pressure through a tenth-inch hole in the heat shield, actually through the thermal protection system material. We did a lot of testing on the ground to make sure that putting that small hole in the thermal protection system would not do any harm to the vehicle. We had seven "plugs"; they're actually pieces of PICA that are cored out, and we put the thermocouples at four different depths for each one.
We had a charge sensor as well that sensed how the thermal protection material was getting burned, and to what depths, as we went through the atmosphere. We had 14 sensor locations, and the data we got back from MEDLI was actually less than a megabyte. By today's standards, that isn't really a lot of data, but it was orders of magnitude over what we've had since Viking [an earlymission that sent probes to Mars]. A couple of the other missions have had instrumentation, but it's been at the bond line between the thermal protection system and the aeroshell, which we never want to see get hot; it's not really all that useful in helping you improve your design.
From our thermal measurements, we could better refine our predictions of how hot the vehicle would get, and that's a function of many factors: the vehicle speed, the shape, the density, and its constituents in the atmosphere. Also, by measuring the pressure, we were able to separate out the aerodynamics of the vehicle from the atmospheric density. That gave us an independent measure of the atmosphere density as we were flying through that profile. And we were able to then compare that to the atmospheric models that were used, and see if those models are in the ballpark of what we actually saw. And they were. They bounded our trajectory quite nicely, so we know that those are good models for the future.
NTB: What were your biggest challenges when designing and testing the Mars landing technology?
Munk: MEDLI was a huge challenge because we never instrumented an ablating heat shield. Ablating means that the thermal protection system material is designed to burn away. We didn't know if that was going to corrupt the pressure measurements or block the holes. Again, the ground testing was key to convincing ourselves that we would get a good measurement and do no harm to the vehicle.
The other challenge was that we were mounted to the inside of the aeroshell, which faces toward deep space for the nine-month cruise out to Mars, and then heats up in a hurry when we enter the atmosphere. Those two extreme environments really challenged the amount of testing we had to do, as well as the actual configuration and technology that we use for the instruments. We had to pull the electronics out of the pressure transducers and build a custom electronics box, which we had to place cleverly in a warm area of the aeroshell because the transducers themselves got way too cold for the electronics to survive on the way out.
NTB: Can you take us through the day that Curiosity descended on Mars?
Munk: That night was really incredible: August 5, 2012. It was 10 pm [PST] entry. Most of our team was out at Jet Propulsion Laboratory [located in Pasadena, CA]. We had actually a couple MEDLI engineers who were receiving data as it came to down to Earth, and were able to do really up-to-the-minute assessment of how MEDLI was performing.
I was actually here in Virginia at the Virginia Air — Space Center [in Hampton, VA], and I was emceeing the entry for an auditorium of about 300 people as we waited. It was really exciting. They call it the "Seven Minutes of Terror" because the entry takes about 7 minutes, and it's harrowing the whole way.
For the past several years, there's been a requirement for the vehicle to send back information as it goes through each step of the entry. Just kind of a ping that says: "I'm ok. I've passed this point." As those were being received on the Earth, it was really exciting. I thought the "7 minutes" were going to last a long time, but it went really fast. Before I knew it, they said "MEDLI Thermal," which meant we were starting to feel heat. That was really exciting for me because 1) we were actually entering [the atmosphere] and 2) they used MEDLI to help tell them that.
Then, before I knew it, the parachute was out and the heat shield was ejected. My hardware was then trash on the Martian surface. We were all just really astonished and happy that it had gone so well, and that we got that first image back from Curiosity, just a few seconds after we sensed a landing.
NTB: Did it all go as planned? Were there any surprises?
Munk: Not from the MEDLI perspective really. I would say it all went flawlessly. There were a couple of things that went better than planned. I think we had more fuel leftover in the descent stage than we had expected. This data came back in the days following, as did our full set of MEDLI data after Curiosity safely landed on the surface. Our team worked for a few months afterward to really understand the trajectory that we followed, and then fully analyzed the data, and published lots of papers with our results. I think everything went extremely well.
NTB: What did you learn from the landing that will you help you design future spacecraft?
Munk: Our MEDLI thermal and our pressure measurement told us two different classes of [information]. On the aerodynamics side, our computational fluid dynamics tools predicted our aerodynamics very, very well. Especially in the high-speed regime. In the lower-speed regime, around parachute deploy, we'd like to understand the aerodynamics better because the back part of the vehicle becomes more important then, and that's not a place where we have good prediction capabilities. Our sensors were not calibrated in that flight regime for MEDLI, so we're hoping to improve that in the future by calibrating them a little bit differently.
In the thermal area, this was a very complex heating environment. That was expected because MSL was the biggest vehicle we've ever sent and so that gives you some flow-field characteristics that you don't usually see on smaller vehicles like MER [Mars Exploration Rover] or the Phoenix [Mars lander], and it was a guided trajectory. This was the first time we had actively guided through the atmosphere. That made things a lot calmer, in some respects, because we had some control and we were able to fly out any errors that we had in our trajectory. We expected the flow to go turbulent, and it did. It happened at a little different time than we predicted, and the heating was not quite as high as we predicted.
On the other hand, in the stagnation region of the vehicle, we underpredicted what the heating was. Luckily we've had some other efforts, through the Space Technology Mission Directorate, to do some ground testing and fully understand better why we underpredicted and overpredicted. Those studies are continuing, but our results are influencing the insight design and margin policy. We also found that the PICA thermal protection system did not recede or burn away as much as we expected for Mars Science Laboratory. The next mission, Mars 2020, will be a build-to-print of MSL so we do not expect to change the heat shield thickness based on our data. We know now, though, how much margin we have built in.
NTB: What are you working on currently?
Munk: I'm currently the Entry, Descent, and Landing Principal Investigator for the Space Technology Mission Directorate, which means that I look across all nine programs within Space Technology, and the EDL investments that we're making, and try to put together a strategy and influence the upcoming solicitations and content that we invest in. We want a cohesive program from low TRL [Technology Readiness Level] up through middle and high TRL, all the way to flight demonstration and mission infusion. It's a big job, but we're doing really well. We're making a lot of investments in Entry, Descent, and Landing technologies and we're moving forward quite nicely.
NTB: What is your favorite part of the job?
Munk: MEDLI was just fantastic because it was a small project — small enough that you can get your hands around it. You weren't on a huge program. It was a small team at Langley Research Center and Ames Research Center, working together with JPL and Lockheed. And you actually got to see something fly, and there's nothing to compare to that. That's the ultimate in the career, I think. But I also enjoy the technology development because we're always doing new things. Just seeing connections between the capabilities that we have, both with our vendors and on the NASA side of the house, and what we can do and how we can make missions better, is really fascinating and fun.
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