In Part One of Starhustling with SOFIA: A 747SP Star is Reborn, we talked about the NASA program and its unique Boeing 747SP, originally built to carry passengers on ultra-long haul routes but is now used to carry the world’s only flying infrared astronomy laboratory and flying infra-red telescope.
In Part two of Starhustling with SOFIA: A 747SP Star is Reborn, we explored in detail this unique Boeing 747SP, as the crew and staff make the preparations prior to its mission.
Back to the Future on a SOFIA Space Odyssey
With training, briefings, and more briefings behind us, we took our seats. At exactly on time at 20:08 local time, the doors were closed on OC4-B Flight 3.
We were offered seats for take-off in either the front nose section or the observation table. It’s a tough choice, but the IFE (In-flight Entertainment) system offered in mid-cabin makes the decision easy for us.
United’s cockpit audio channel 9 is novel, but SOFIA’s in-flight communications channel is on a whole other plain, with nearly everyone onboard on the same system – there is no audio PA employed in the cabin.
Co-pilot Ace Beall keys the tower with the words “NASA 747 Heavy ready to taxi to runway 25”. The only things pumping more than the four mighty Pratt & Whitney engines are our hearts with adrenaline. Even with no traffic on the field, taxiing out to the runway seems interminable. Finally, at precisely on schedule at 20:35, the four JT9D-7J’s, each rated at 50,000 pounds of thrust spool up, begin their 94% de-rated take-off.
SOFIA’s maximum take-off weight (MTOW) is 699,000 pounds versus 750,000 pounds for a typical SP. Our take-off weight with is only 630,000 pounds. With flaps set at ten degrees, V1 comes up at 143 mph, Vr at 150 mph, and V2 at 157mph. Our 747SP is light, so under the power of her four Pratt’s, it departed terra firma, thrusting into the cool desert air in less than 40 seconds.
The powered symphony of the engines, coupled with reduced cabin sound insulation, make for a palpably louder cabin than modern equipment. And thus, make it that much more enjoyable for this Airficcionado.
Just three minutes after takeoff and 5,000 feet off the ground, the seatbelt sign is extinguished. During the 30-minute climb to FL370, the science teams get busy running multiple checklists. The precious scheduled 6 hours and 55 minutes of observation time, once the telescope opens, is very busy and at times intense for most of the entire mission – there are no real breaks. So, this is the time not only for preparation, but also the time for a little nutrition. As the cabin is kept in the mid 60 degrees Fahrenheit at altitude, the crew members put on their sweaters and jackets.
Science is only able to initiate when the Upper Rigid Door and Lower Flexible Door (URD) open to expose SOFA’s telescope to the atmosphere. The upper door opens at a 25 degree aperture. That tracks the elevation of the telescope, at 15-60 degrees in elevation (U, V Axis), and cross elevation of 6 degrees in movement, (W Axis).
Throughout our briefings, Veronico and SOFIA crew proudly tell us that when the SOFIA’s door opens to expose the telescope to the atmosphere, there’s no detectable buffeting in the cabin. To prove their point, no one tells us when the door has opened until after it already has.
As SOFIA reached FL350 at 21:02, the telescope door opened in under one minute. Sure enough, there was no detectable turbulence created as a result of the door opening. The only indicator on the flight deck that the telescope is open is a green light on the flight engineer’s panel. On the main deck, things get a bit busier as the science observations begin, but other than that it’s imperceptible. Astonishingly, there is only a two percent increase in fuel burn with the door open. A laminar flow over the cavity allows this to be possible. Chalk it up to science!
The DLR telescope is so precisely balanced that it appears to be moving, when in fact, the airplane is moving around the telescope. Captain Manny Antimisiaris tells us that “up to moderate turbulence, the telescope can still make reliable observations.” The telescope has +/- 3 degrees cross X elevation Y axis and line of site Z axis to stay locked on its target. Its viewing range is 20 to 60 degrees. The aircraft has to make adjustments for cross X to keep it centered. If heavy turbulence is encountered, the telescope will go into standby and then its tracking target has to be re-acquired after the turbulence passes. Normally, the telescope operators adjust telescope at intervals of 30 minutes to keep affixed on target.
Once the telescope door opens, the Mission Director and Flight Crew operate in perfect synchronicity to facilitate the viewing of specific targets that must be done to the minute. The Mission Director crucially directs the flight crews to remain on course and exactly on time for each of the observation points. Pilots navigate by way points, while astronomers navigate by position of the stars and how local weather like winds affects the course. “The telescope has 6 degrees of movement of cross elevation, so we have to keep the star within the 6 degrees. We ask for 1 degree course changes about every four to five minutes,” Mission Director Grashuis said. This is vividly demonstrated as galaxies and stars are displayed across monitors throughout cabin. It’s an ethereal, surreal sight.
As I admitted before, my not so ulterior motive to fly a SOFIA Mission was to experience the rarest of rare birds – the 747SP, what it’s like to fly it and understand why it was chosen as the platform for a one-of-a-kind flying observatory. We visited the flight deck multiple times throughout the flight to get this side of the story.
Captain Troy Asher passionately espouses the SP as a very capable platform because, “We fly between 2,000 and 4,000 feet higher, based on the same weight. We try to get as high as we can, as fast as we can.” But make no mistake this is challenging flying for reasons beyond the more labor intensive technology of an older aircraft like the SP. Asher continues, “To conduct science, we have a narrower band of air speed +/- Mach 0.02 for the telescope to be able to fix. We fly many straight legs, alternating with turning 1 degree at a time to stay on a curved path. One to 2 degrees of bank and roll, we keep it shallow not to affect science. We plan to fly at Mach 0.85. Our waypoints are predictions that we file but are not actual. We get some priority (from ATC) based on a scientific mission. Controllers help and cooperate because they feel like they’re part of the team.”
Indeed the flight deck, in particularly the throttles, gets noticeably busier as the observatory encounters moderate mountain wave effects over the Rockies a few hours into the mission. Asher tells us “We don’t have auto-throttle for cruise.” SOFIA has a very narrow 13-knot flight envelope during cruise compared to that of a normal commercial aircraft at 40 to 50 knots. “The SP does a fantastic job but it’s not the most efficient operation due to the weight of the airplane and that we are trying to fly at such a high altitude.”
With such an unusual, bespoke configuration, it would be understandable that SOFIA would have very different flying characteristics than a standard 747 Classic. Asher defends the SP: “We are a little under powered, but in flight there’s no difference between flying this and a classic. The only real difference is in landing with single-piece flaps on the trailing edges rather than the smaller triple-slotted flaps of standard 747s- we land a bit faster with a firmer landing.”
As we clear the turbulence of the Rockies and burn off fuel, SOFIA reaches Leg 7 over Southwestern Idaho. The mighty SP willingly, if not enthusiastically, climbs to FL 410. On the telescope status display, we can see the streaking of the stars as the telescope moves while it acquires its next target. On the flight deck, one pilot dons his oxygen mask as it’s mandatory to wear them above FL 410. SOFIA will eventually reach its final flight level of FL430.
I didn’t expect to enter the cockpit of a 1977 vintage Boeing 747SP and bear witness to a modern Honeywell Primus Epic glass avionics suite, similar to that found on the Embraer E175/190 jets.
“When we purchased the airplane, it was analog. It’s been plagued by cost over-runs. The plan was always to upgrade it, but we were under pressure to get to be able to do science and get first light on the telescope. Once we proved it worked, we were able to upgrade,” explained Asher.
“But it’s more modern than it looks, and there is still a flight engineer and flight navigator in the front office. The FE’s console is as analog as a 1970s Pioneer high-fi system. “We switched to glass displays which have improved our situational awareness and navigation. We have new Flight Management Systems (FMS) and a custom system to work with NextGen ATC systems as well. But it’s certainly not an all-digital, automated aircraft. The auto-pilot and air data computers (ADC’s) are still analog. Because of this, we have to calculate maximum speeds, V1’s, V2’s, etc. There are a lot of legacy systems still onboard.”
The SOFIA program will hopefully live on indefinitely as long as there is funding, but what of the 747SP platform itself that is designed for a service life of twenty years? As meticulously maintained as it is, N747NA is nearly forty years old, utilizing first generation wide-body technology dating back to the 1960s. The crews acknowledge the more maintenance intensive issues with the aircraft: its relatively under-powered engines, dwindling parts supplies, its programmed fuel inefficiency, and sometimes reliability. SOFIA was befallen by a crack in the number 1 engine on its Summer Southern Hemisphere sortie in June 2016 and grounded for a few days, resulting in the cancellation of a few missions.
Newer aircraft such as the 747-400 and even the 747-8 would not only resolve many of these problems, but allow an aircraft to climb even faster and fly longer. Given the prohibitive purchase and conversion costs, coupled with the fact that SOFIA is only in its third operational year of a planned 20-year life, the space agency is not even thinking in these terms.
The flight continued on rather uneventfully until we reached Leg 7, when a computer issue having nothing to do with the 747SP platform caused a loss of data for forty minutes. This is a rare, but not an unheard of occurrence. Once the computers rebooted, it was “science as usual” and all remaining targets were met.
At 05:55 local time on Saturday February 27, the de-throttling of the engines indicated not only our initial descent from FL430, but that our scientific mission would be drawing to a close. At FL350, as dawn began to subtly paint the horizon, the fuselage door closed, putting the telescope to bed. The closing sequence took under 1 minute and was again imperceptible.
With flaps fully deployed to 30 degrees and 135 knots on approach, and rays of sunlight just peaking out over the California desert horizon, we touched down on time at 06:22 just as planned after 9 hours, 46 minutes aloft.
The SOFIA program certainly made it clear to me, as it has to many in the scientific community, that the sky is isn’t the only thing that’s no longer the limit – nor is the stratosphere. A certain 747SP, an eye in the sky, has made certain of that.
The author wishes to thank Nicholas Veronico, SOFIA Public Affairs Officer and Cody Diamond for their invaluable assistance in reporting this story.