Cryogenic Systems Enable SOFIA Space Project

NASA 747 for the SOFIA Project

NASA 747 for the SOFIA Project

Following is an interview with Nans Kunz, Senior Engineer at NASA Ames Research Center, nans.kunz@nasa.gov. Kunz is the former SOFIA Chief Engineer. The Stratospheric Observatory for Infrared Astronomy (SOFIA) project is a telescope carried in a 747 aircraft up above the atmosphere. Quotes are from www.sofia.usra.edu. Cold Facts’ questions are in italics.

1. One of the modifications to the aircraft was “installation of a complex liquid-nitrogen cooling system used to pre-cool the telescope cavity to match thermal conditions when the cavity door is opened at altitude.”

Please describe how cryogenics is used in SOFIA. What challenges were encountered and how were they surmounted?

It’s expensive to operate a 747, as you can imagine, and the 747 in the beginning of the flight when it’s fully loaded burns about 25,000 pounds of fuel per hour. We don’t want to fly around at altitude waiting for the telescope to cool down before we begin observing. In infrared astronomy, it is desirable to have all the optics and everything in the cavity at the temperature expected in the cavity at the altitude you’re flying and when you fly in the stratosphere, the ambient temperature is about -60˚F or so. The telescope is operated from within an open port cavity, so with the recovery temperature (the temperature of the air in the cavity when you’re cruising) it’s going be around -40˚. It’s preferable because of the thermal mass of the optics and the telescope to have the telescope already be at -40˚ before you take off so that the science observing can begin as soon as the desired observing altitude is reached. The temperature uniformity is required for both performance of the optics and to minimize “seeing.”

“Seeing” is the distortion of the image seen by the telescope caused by a varying index of refraction (the speed of light through the air). It’s like when you look over the hood of a hot car and you see little heat waves rising…and of course a large telescope is much more sensitive to “seeing” than the naked eye. If you have a warm telescope up at altitude, the heat waves rise and cause distortion of the image of the objects you’re looking at—”seeing”. To avoid this, the optics, mostly the big piece of glass that is the primary mirror, need to be the same temperature as the surrounding cavity air. Additionally, warm surfaces are a noise source for detectors that are looking in the IR (infrared). For these reasons it is desired to have the telescope at the ambient temperature of the cavity and for operational efficiency the cavity should be pre-cooled before you leave the ground.

The choices for pre-cooling the SOFIA cavity were either a standard HVAC system (such as a building air-conditioning system that includes a compressor, refrigerant, heat exchangers and ducts that would circulate cold air in the cavity) or a system that uses cryogenics as the cooling medium. In other words, the cavity cooling system was a choice between a mechanical refrigeration system or a consumable cryogenic system. To save money for the development phase, and perhaps until we characterize how much heat removal is required for a cool down cycle, it was decided to implement the cryogenic system. There still may be a future upgrade to build a mechanical A/C system, but at this time the approach is to use liquid nitrogen as the consumable medium to cool off the telescope. It’s plentiful, it’s relatively cheap, and it’s got a lot of heat removal capability.

Using liquid nitrogen as the primary cooling source presents challenges, including the plumbing and the control of the flow of nitrogen, and avoiding direct LN2 contact with telescope or telescope components. To control the flow and to avoid the requirement for a variable flow cryogenic metering valve, we developed an approach that consisted of a series of three spray bar assemblies, one with one spray bar, one with two spray bars, and one with four spray bars. With a simple on/off solenoid valve to each spray bar assembly we are able to achieve seven different discrete flow rates with different combinations of the three on/off solenoid valves. To mitigate concern about liquid nitrogen getting into the cavity and having direct contact with any of the telescope surfaces or systems, potentially causing damage, these spray bars are in a duct away from the cavity where the liquid nitrogen is vaporized and the cold nitrogen gas is blown into the cavity and recirculated through the cavity via the duct system.

Another challenge is to determine the most efficient way to get the LN2 from the ground storage dewar up into the onboard cooling system. If you’ve ever been on the ground near a 747, you have probably noticed that the fuselage is about 25 feet up in the air. And the LN2 dewar, the source of the LN2 needed for the pre-cool, which is estimated to be about 1000 gallons of liquid nitrogen per pre-cool/mission, is on the ground, of course. The challenge is to get the pressure necessary to lift the liquid up to the onboard cooling system in the most efficient way.

There are different ways of accomplishing that. One is to add the heat to the dewar system and allow the pressure of the gaseous nitrogen created to build up and push the liquid up. As the liquid is delivered, more heat is required at the lower tank to vaporize enough LN2 to keep the pressure up. Of course, adding this heat also consumes some of the LN2 which is where the efficiency issues arise.

Included as part of the cavity cooling system is a set of liquid nitrogen dewars in the back of the aircraft to store the LN2 required to keep the cavity cool after the SOFIA aircraft is disconnected from the ground LN2 source. These dewars have been sized to be able to provide about half an hour of cooling, during taxi, take off and the climb to altitude.

Completely separate from the cavity cooling system are the cryogenic requirements of the nine different science instruments. Most of the time there will only be one science instrument on board in the cabin at a time. However, the science instruments are located in the occupied, pressurized cabin, not the open port cavity, therefore creating additional safety concerns and corresponding mitigations. As a further cryogenic challenge, some of these science instruments require the detectors to be cooled down to 2° Kelvin. This is done using liquid helium, in addition to LN2, in nested dewars. To get below the nominal 4K temperature of liquid helium to 2K absolute requires an additional special procedure involving a salt crystal and magnetic field. This process involves the thermodynamic laws of entropy, and alternately applying a magnetic field to align the polarized salt molecules, decreasing the entropy and affecting a temperature change and removing the magnetic field. This allows an increase in entropy, resulting in the opposite temperature change from the nominal 4K inner dewar environment. The detector is mounted to the salt crystal and the observations are made during the 2K cold portion of this cycle. This is pretty amazing when you think about it.

In the case of liquid helium, we know that proper pressure controls are necessary to avoid having a potential bomb on board the aircraft. How did you get around this problem?

Since it takes so little energy to vaporize the liquid helium, we needed to make sure that the helium could escape from the enclosed volume in case of an insulation failure so that we don’t end up with a bomb. Analyses were done to size the relief valves, but it’s very difficult to exactly nail down these thermal characteristics at these extreme conditions, so we conducted a series of tests. These tests involved releasing the vacuum in the inner dewar jacket, causing the helium to vaporize nearly instantly while we monitored the pressure in the inner dewar to verify that the relief valve system was big enough to prevent excessive pressure buildup. The tests were kind of exciting, but that’s a pretty standard way of dealing with those types of hazards. You just have to make sure you have a relief valve that’s big enough to allow the flow of gaseous helium to keep from having the pressure build up to unsafe levels.

2. “SOFIA is the successor to the much smaller Kuiper Airborne Observatory (KAO), which was operated by NASA from 1974 to 1996.”

What capabilities does SOFIA have that will exceed the observation capabilities of Kuiper? Does it have to do with the size of the telescope, or just new innovations in technology?

The primary reason for replacing Kuiper with SOFIA was telescope size. The telescope is three times the diameter, which allows you to do some science that you couldn’t do with a smaller telescope. Even from just an efficiency point of view, you collect light at nine times the rate, so even similar science as Kuiper’s can get done nearly ten times faster. Additionally, the new instruments themselves have capabilities that the Kuiper instruments didn’t have.

3. “Because of the telescope motion limitations and the fact that SOFIA is a flying observatory, planning observations is far more difficult than at ground-based observatories. Flight restrictions constrain the amount of time possible to spend observing any given object, and moving to a new object requires turning the aircraft and embarking on a new flight path.”

Do you think these limitations will have a negative effect on what SOFIA could potentially observe? Does operating within these limitations still provide better data than would be collected at a ground-based observatory?

The reason that it’s more difficult to do observing planning for SOFIA as opposed to a ground-based telescope is because it’s more capable. You simply have more variables. A ground-based telescope doesn’t move and it’s fixed to Earth, so you can only view things that are within range of the telescope on the ground. With an observatory like SOFIA, you can be anywhere you need to be, anytime you want to be there, which gives you more variables to plan a night of observing.

SOFIA is flexible and able to be at the right place at the right time. There are airspace flight restrictions that will influence the observing plan to some extent, mostly near the airport at the beginning and end of each mission. Further, a given flight plan/observing plan will need to take into account that most SOFIA missions will take off and land from the same location, so it is undesirable to just randomly put together a series of objects to view and then end up having to fly back to home base for three hours without being able to observe anything interesting. So for maximum operating efficiency, it is necessary to put together a series of observations where you’re able to observe a series of desirable objects the entire flight and still end up over the destination (usually home base). When that last observation is over, SOFIA will descend rapidly and land without the need to fly further to get to a destination without observing.

To clarify, the flight heading for SOFIA while observing is determined by the astronomical object being observed. The telescope looks out the left side of the aircraft perpendicular to the direction of flight, with an elevation range to observe of 15° to 70° above the horizon.

These related flight issues are not so much of a difficulty for planning observations, it’s just that they result in more variables that need to be considered—variables that are not available from ground-based observatories. A prime example of this flexibility as a big advantage is the ability to observe what is called an occultation. An occultation occurs when one of the planets or objects in our solar system happens to line up with a bright star that’s outside of our solar system and the shadow of that planet from that star crosses the earth. The opportunity to view that ephemeral event only comes when those two objects line up with a location on Earth and then it is necessary to be at the right place at the right time on Earth to observe it. Unless the shadow of the solar system object happens to cross an observatory on the ground in the right spot, which is extremely unlikely, you need a mobile observatory like a SOFIA that can be moved to be at the right place at the right time. Occultation observing from Kuiper is how it was determined that Pluto has an atmosphere and it was discovered that Uranus has rings.

One of the advantages that SOFIA has over Kuiper, because of its larger diameter, is that these occultation observations are possible with smaller stars, thanks to the increased sensitivity. Therefore, there will be many more observable events like these that make it possible to study the planets and other objects in our solar system.

4. “SOFIA is expected to have a lifetime of 20 years.”

Since new innovations are constantly changing technology, can we expect SOFIA to be as advanced a scientific tool in the future as it is now, or do you think the technology will be outdated toward the end of the aircraft’s second decade in operation? Can new technology be easily incorporated into the observatory?

SOFIA’s actually designed as a tool to keep the new technology new. The science instruments are not static. When they’re not flying, they’re being upgraded by the principal investigators. So SOFIA’s like a mountaintop observatory and the highest technology part is the instrument that you bring to it. Even 20 years from now, you can bring out a new instrument with the latest technology and just bolt it on.

5. “The first generation of science instruments for SOFIA consists of nine imagers and spectrographs spanning the entire wavelength range.”

Are these instruments more sophisticated than those currently used in ground-based observatories and in space telescopes?

Yes, they’re definitely more sophisticated from a detector perspective than those of space telescopes, because you can’t put instruments this complicated in space. SOFIA instruments can be designed to be hands-on and be tuned while flying. Conversely, to put things into space, they have to be really robust because after they are launched they can only be operated remotely. The sophistication for the space-based instruments is reserved for supporting the remote operations. Further, to minimize mission risks for space-based missions, the detector technology has to be fairly mature. Typically, with space-based instruments, you don’t get to touch them after they are launched, so the opportunity to tweak or tune them is limited by what can be done remotely.

On the other hand, SOFIA instruments can be state-of-the-art, because they can be touched, tuned, tweaked, etc., in real time while observing. Furthermore, the worst-case risk from a complete failure of an instrument to perform on SOFIA during a flight is limited to a wasted take-off and landing—an aborted single flight.

Ground-based instruments can also be state-of-the-art for the same reasons as the SOFIA instruments. However, they are limited to observing wavelengths that reach the ground and do not have access to wavelengths that are available from the stratosphere that will be available to SOFIA. Actually, some of the SOFIA instruments can and have also been used on the ground-based observatories.

In addition to the continuous evolution and improvement of these first generation instruments, there are going to be future generations of science instruments to be developed for SOFIA that will continue to keep SOFIA at the state of the art. With these instruments, SOFIA is going to open our eyes to parts of the universe that we, mankind, haven’t really had a chance to study, and facilitate many discoveries, further increasing our understanding of the universe in which we live.