Date: July 9, 2010
Title: A Champion at Keeping Cool
Podcaster: Bob Hirshon
Organization: American Association for the Advancement of Science (AAAS) – www.aaas.org
Description: July is hot enough here in Washington, DC, but nothing compared to the heat the MESSENGER spacecraft must endure as it prepares to orbit Mercury. This episode of the 365 Days of Astronomy podcast explores the techniques used to keep the spacecraft operational at temperatures hot enough to melt lead, along with new work on an even hotter mission: the Solar Probe, which will have to withstand temperatures four times hotter.
Bio: Bob Hirshon is Senior Project Director at the American Association for the Advancement of Science (AAAS) and host of the daily radio show and podcast Science Update. Now in its 23rd year, Science Update is heard on over 300 commercial stations nationwide. Hirshon also heads up Kinetic City, including the Peabody Award winning children’s radio drama, McGraw-Hill book series and Codie Award winning website and education program. He oversees the Science NetLinks project for K-12 science teachers, part of the Verizon Foundation Thinkfinity partnership. Hirshon is a Computerworld/Smithsonian Hero for a New Millenium laureate.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the Education and Outreach team for the MESSENGER mission to planet Mercury. Follow the mission as the spacecraft helps to unlock the secrets of the inner solar system at www.messenger-education.org.
Transcript:
365 Days of Astronomy Podcast
July 9, 2010
“A Champion At Keeping Cool”
by Bob Hirshon, AAAS
Welcome to 365 Days of Astronomy podcast. I’m Bob Hirshon, host of the AAAS Radio show and podcast, Science Update.
It is steamy here in the Washington area. We’ve got triple-digit heat. But not at Jack Ercol’s office at the Applied Physics Lab in Laurel, Maryland. There, it is freezing. Despite his attempts to correct the problem, cold air pours out of his ceiling vents so fiercely—and noisily—that we have to conduct our interview in a nearby conference room.
Which is a strange irony, since Dr. Ercol’s specialty is controlling thermal conditions on spacecraft operating in some of the solar system’s harshest environments—from the inferno of the planet Mercury, to the icebox that is Pluto.
MESSENGER is the spacecraft now en route to Mercury, slated to go into orbit in March of 2011. MESSENGER passes less than 50 million kilometers from the sun, just a third the distance of the sun from the earth. Near Mercury, the sun appears eleven times brighter than it does from earth, and MESSENGER is exposed to temperatures hot enough to melt lead.
Dr. Ercol says he began work on MESSENGER’s thermal design in the mid-90s, when the idea of a mission was first proposed. He explains that it was planned as a Discovery-Class mission, which specifies both cost and development timeline
Ercol:
And of course some of the challenges were mass of the spacecraft, the propulsion, because in order to meet a Discovery type mission you’re stuck with a kind of launch vehicle that was available, you can’t really just overload it with a thermal design, so the thermal design had to fit within, or dovetail with that mass requirement.
Hirshon:
He set about designing a passive solar shade to protect the delicate instruments that would ride in the “bus” or spacecraft cavity. Even this close to the sun, a simple shade is all it takes to keep everything behind it icy cold. Ercol mounted the shade on fixed arms.
Ercol:
It’s bigger than the craft itself, so you get an extended area, so you don’t have to keep it directly pointed at the sun all the time, you can tilt it plus or minus I think it’s 15 degrees. So you basically can move it left and right, up and down, so that’s why the shade’s bigger, so that it protects the spacecraft over some variable angle displacement.
Hirshon:
But while everything behind the shade stays cool, you have to keep the shade itself from melting. Ercol selected a ceramic fabric from 3M that can easily withstand the high temperatures, and treated it with an aluminum coating that made it completely opaque.
The next trick involved several spacecraft parts that couldn’t hide behind the shade—they needed to see the sun to do their job. These included the Digital Solar Attitude Detectors, or DSADs. They pinpoint the location of the sun, so MESSENGER can make sure it keeps the sunshade pointed at it. Ercol found detectors that could do the job, but they were meant to operate on earth, not in the much brighter environment near Mercury. So he modified them with filters.
Ercol:
So when the sun sensor is looking at the sun at our closest approach, at our perihelion, or .3 au, the sun is about eleven times brighter there than it would be at earth, so what this / filter does is it takes that eleven suns and makes it look like one sun at the sensor head.
Hirshon:
Of particular concern were the solar panels. They need to see the sun to generate electricity, but too much sun will fry them. The solution designed for MESSENGER consisted of arrays that could rotate toward and away from the sun, to achieve an optimum angle.
Ercol:
The solar arrays were a very specialized thermal design because they have to survive through a full range of sun angles and solar distances.
Hirshon:
Still, when MESSENGER is at its closest to the sun, the difference between high performance and failure is very tight, and the angle of the arrays and the position of the entire spacecraft have to be carefully monitored.
Now, if all MESSENGER had to do was cruise near the sun, that would be all that was needed. But once it goes into orbit, Ercol says the requirements change.
Ercol:
So you get these large heat spikes which last, you know, tens of minutes, and then they disappear.
Hirshon:
Why?
Ercol:
Because the orbit takes you away from the planet, you move away from the point where it gets hot.
Hirshon:
So the planet’s surface itself is variable…
Ercol:
Right, over certain portions of the planet, you get extremely hot, and over other portions of the planet, due to the angle and the distance, it’s very cold, very benign.
Hirshon:
Withstanding these spikes was built into the design parameters for each instrument, once Ercol provided the teams with the information they’d need.
Ercol:
I calculated all the thermal environments for both the cruise phase which we’re in right now and the orbiting phase. Then there are instrument teams that actually had thermal engineers who took that information and used that to design the instruments.
Hirshon:
So far, MESSENGER has completed three successful fly-bys of Mercury in preparation of orbital insertion in March.
At the other end of the spectrum, Ercol also worked on the thermal design for New Horizon, a mission to the outer reaches of the solar system. While you might imagine that temperatures below minus 200 Celsius would present a major problem, Ercol says when it comes to spacecraft, no heat is no problem.
Ercol:
Thermally, it’s in a very benign environment, there’s no real sun environment to speak of, because we’re not close to any planets, there’s no planetary environments to speak of, so it’s very quiescent.
Hirshon:
The craft has a Radioisotope Thermoelectric Generator, or RTG, that provides all the heat and electricity it needs. In fact, Ercol says the only difficulty with New Horizon was protecting it right after launch, when he had to deal with the solar environment, plus heat dissipation from the craft itself.
Ercol:
Once we started to get away from the earth, and point the high gain back to earth, which sort of creates a mini-sun shield—when the antenna was pointed at the earth, it shades the bus from direct solar, so that helped to keep things pretty cool. And then after we got past, I guess a mars-type orbit, or mars type solar distance, temperatures have been pretty flat for three or four years now.
Hirshon:
Well, while New Horizon may have given Ercol a breather, it won’t last. Because now he’s preparing for his biggest challenge yet: the Solar Probe Plus mission. The goal is to send a spacecraft to within about 6 million kilometers of the sun—remember that Mercury itself gets no closer than 46 million kilometers.
Ercol:
So we’re trying to leverage some of our knowledge from the MESSENGER mission, because we did a high temperature solar array development for MESSENGER and we’re trying to use once again what we’ve learned from the MESSENGER mission to transfer over to the Solar Probe mission.
Hirshon:
But the conditions that Solar Probe Plus will face are many times more formidable—especially the cooling requirements for the solar arrays, which will require an active, rather than a passive, solution.
Ercol:
MESSENGER’s worst case is about 300 C, 350 C. Solar Probe’s worst case is more like 1400 C. So that’s the difference. So you’re, you’ve moved through a band of technology; the MESSENGER scenario will not work here.
Hirshon:
Therefore, Ercol is looking at liquid cooling system.
Ercol:
Which is basically a water loop system that takes heat away from the solar arrays and pulls it to a radiator system that is remote, and then dissipates that heat to space.
Hirshon:
That’s right—pretty much the same way your car keeps cool. Remarkably, he chose water as the coolant over a variety of other liquids they considered.
Ercol:
We did a complete study on what potential we had for working fluids and some of em work good hot, but don’t work good cold, some of ‘em work cold but not good hot, so you’re trading problems. And the water system is probably the most stable for the time frame that we’re looking at and there’s no real contamination issues with the water. It has very good heat transfer characteristics, very good heat capacity, so forth.
Hirshon:
Solar Probe Plus is still in its early design phase. It’s not scheduled for launch until 2015. But the challenge is so formidable that his work is already well underway.
Ercol:
We’ve built piece parts and so far everything is working out, so hopefully it continues. It’s a challenge, but it’s a good challenge.
For the 365 Days of Astronomy, I’m Bob Hirshon.
End of podcast:
365 Days of Astronomy
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