by Paul Dye
Entry, of course, is where people really think about aerodynamics. The science and engineering that went into the design of the Shuttle’s entry trajectory was a tremendous challenge and was the result of the accumulated knowledge of thousands of aerodynamicists, thermodynamicists, and every other “cists” you could think of. Once your rocket engines had done their thing to drop you out of orbit, you had to prepare to kiss the atmosphere at Mach 25—25,000 feet per second (or about 18,000 miles per hour). At that speed, hitting gas molecules rips those molecules apart into a plasma—a super-heated gas. It is like applying a blow torch to the skin of the vehicle. Since the Shuttle was basically built like an airplane, there was no way the bare structure could survive such treatment, so the thermal protection system, a layer of silica tiles and reinforced carbon, was applied to protect the primary structure. These tiles were amazing—applied with a special adhesive, they could withstand the intense heat of entry without transmitting the heat to the aluminum to which they were attached. Unfortunately, these tiles and carbon were also relatively fragile, and they had to be treated with care—as we found out throughout the program, and tragically with the Columbia. The bottom line is that without the thermal protection, the Shuttle would have been vaporized in short order upon an attempted reentry. Most people who have an active interest in the Shuttle program have heard that before.
But there was more to entry aerodynamics than that. It was important that you hit the atmosphere at the proper angle. Too steep and it would capture you quickly, overloading the vehicle, breaking off the wings, and you would burn up and/or break up. Too shallow and you would literally skip off the atmosphere, enter again later, but this time too steeply… with the same results. The appropriate angle for the Shuttle entry was right about 40 degrees nose up, and you had to hold this through the hot part of the entry profile. Control was accomplished with maneuvering jets, and they did a good job of holding the necessary attitude. At the same time, the computers were recognizing increased loads on the movable control surfaces and had to adjust these surfaces to keep the vehicle in the appropriate attitude.
Now if all you did was hold the appropriate angle of attack and went straight ahead, your impact point with the earth—if you did nothing else—would be essentially determined. The beauty of a winged vehicle with lift capability is that you could point that lift in various directions to let you turn left or right. As you did this, you could also vary the direction of the lift vector to make your impact (or landing) shorter or longer. In fact, the Orbiter’s landing footprint from the time you did the deorbit burn was variable. It allowed us to land up to 750 miles on either side of the nominal aim point, 500 miles short, or about a thousand miles long. This footprint was sort of an odd-shaped ellipse that gave us plenty of capability in choosing landing sites.
The lift vector was pointed in various directions by rolling the vehicle left or right. If you rolled it into a steep left bank, you decreased the component of the lift vector that generated an upward force, and so you sank more quickly—increasing the rate of descent. You still went somewhat straight down the entry line, but you got a little bit of a left turn out of it. Eventually, you turned enough so that you were starting to point away from your intended landing site. When that happened, it was time for a “roll reversal”—you rolled from a left bank into a right bank, and that started a gentle turn back toward the runway—and eventually beyond it. Then you’d do another roll reversal to head back where you wanted to go again. By doing these rolls, you could vary the descent rate and manage energy and heating, all at the same time. It was a clever way to maintain a constant angle of attack yet still control where you were headed.
As the Orbiter dropped lower and lower into the atmosphere, it slowed down, and at the same time it got more bite from the thickening air. Wings respond not to airspeed as much as they do to what’s called dynamic pressure. Dynamic pressure is the product of air density multiplied by the speed of the aircraft squared. It is directly measured, just like it is when you had your hand out the car window—you can feel the resulting force. The more dynamic pressure you have, the more force you have acting on your wings and control surfaces. These forces provide you with the ability to control the aircraft. But you have to be careful with generalization like that, because odd things happen when you are flying at supersonic speeds. Shock waves form on various parts of the aircraft, and they can reduce the effectiveness of controls by effectively shadowing them from the airflow. This is sometimes referred to as the controls being “blanked out.” Sometimes, the controls will actually reverse their functions at certain speeds. This makes for real problems for pilots—it can be hard to figure out just how to move the controls depending on what speed you’re going—and the workload would be over the top. Fortunately, in the age of computers, we have the ability to put a machine in between the pilot and the controls. The pilot says, “I want to go left,” and tells the computer this by moving the stick left. The computer looks at the current speed and dynamic pressure (and a bunch of other factors), and realizes that, at this point in time, the best way to go left might be by moving the controls in a counter-intuitive direction—but because all the aerodynamics have been written into the computer’s program, the net result is a left turn. The pilot has no idea what is actually happening with the controls and doesn’t really care.
The Shuttle flew entry pretty much like that—with a combination of maneuvering jets, the control surfaces on the wings, and the rudder and speedbrake on the tail, all being controlled by the computer, based on the pilot’s (or autopilot’s) wishes. In fact, most all entries were flown by the computer (autopilot) until the speed dropped to the subsonic regime, for reasons that will be discussed later on. The important things to take away from all this is that bringing a winged vehicle back from orbit is not a trivial task, and it took the combined knowledge of generations of aeronautical engineers to pull it off. It was a magnificent accomplishment that is often overlooked by those without a lot of background in supersonic flight.
In Mission Control, we had several positions devoted to navigating and guiding the Shuttle. Just like in the Apollo days, these folks lived in the front row, which became known as the Trench. The name is a holdover from the original control rooms, where this was the lowest row in the room, down on the floor. Even when we built the new Control Center with a flat floor, this front row was still referred to as the Trench—some traditions never die (and they shouldn’t).
The Trench was anchored by the Flight Dynamics Officer (FDO—pronounced “Fido”). Their job was basically navigation—figuring out where the vehicle was and where it was going. They were responsible for computing burn targets, they knew how to get us to landing sites, and they were the ones who kept track of everything having to do with orbital parameters, trajectory timelines, landing sites—you name it. If you wanted to know the best time to launch—ask FDO. If you wanted to know the current temperature at a landing strip overseas—ask FDO (the weather guys worked for him, as did the Landing Support Officers). If you wanted to know if you’re going to be in sunlight or darkness for a particular time on orbit, or if the vehicle is speeding up or slowing down—FDO is your person.
The FDO has always had one of the largest back rooms in the Control Center because they have lots of things to do and keep track of. When it came time to distribute mission crew patches (a traditional gift from the astronauts on a mission to the flight controllers and Mission Control Center personnel) to a team, most disciplines asked for a couple patches, maybe three, to cover themselves and their back room. FDO asked for about twenty or twenty-five—they sometimes seemed to have their entire office supporting every mission. That’s not a problem, actually: we wanted everyone in the operation’s organization to participate in missions. It made everyone aware that what they did contributed directly to our prime goal.
FDO lives in a world of vectors, and everything in this world is expressed in terms of vectors. We know where the vehicle is in space because
we can express its position as a vector—a description, a particular time, where the vehicle is (in three dimensions), and where it is going (also in three dimensions). If you know these elements (time, position, and velocity), and you know the equations of motion due to the laws of gravity, then you can use math to add the motion to the original position to tell where the vehicle is at any point later than the vector’s time. FDO can then compute the effects of future burns on the position and predict where the vehicle is going to be at some future time. FDOs are good at math—in fact, many of them aren’t engineers, but mathematicians and physicists.
FDOs who worked the ascent and entry shifts had additional responsibilities: determining abort parameters for ascent and watching over the entire entry trajectory during landing. The FDO position contributed a number of their alumni to the Flight Director office, as it was a position that encompassed a large portion of what went on in Space Shuttle operations.
Sitting right next to the FDOs were the Guidance Officers (GUIDO—pronounced “guide-o,” not like an Italian hit man). They were responsible for understanding and monitoring onboard guidance systems and determining how well they were doing. They also had all the responsibility for procedures used by the crew in navigating. It was often hard to separate responsibilities between the FDO and GUIDO positions, which is one reason why they sat next to one another. As a Flight Director, you could just lob a question in their general direction and the right person would pipe up with an answer.
GUIDO was used primarily for ascent and entry in the Shuttle program, and was replaced by the Rendezvous Officer (RNDZ) for orbit operations that included rendezvousing with another object or spacecraft. As an Orbit Flight Director, I rarely worked with an Ascent/Entry GUIDO, but I spent much of my career talking to RNDZ flight controllers. They watched over the onboard targeting and maneuvers, made sure all the i’s were dotted and t’s were crossed in the Rendezvous Checklist, and were experts in the esoterica of flying two spacecraft in close proximity. It’s nowhere near as simple as driving close to another car on the freeway—orbital mechanics gets very interesting. When you try to meet up with something else going 18,000 miles per hour around a planet, having help from a team of experts like these is essential to making it work.
The Third Leg of the Triangle
We liked to divide Shuttle mission control teams into three broad categories—Systems, Trajectory, and Operations. The one we haven’t yet talked about is Operations. Operations included all the folks who were dedicated to planning and executing timelines and serving the particular objective of the mission—the payload. Although everyone on the flight control team (including the trajectory and systems folks) served the mission, the operations team really had the reins when it came to making sure that the purpose of the flight was achieved. That group included the Flight Activities Officer (FAO), the Payloads Officer (referred to simply as Payloads), the Flight Director (Flight), and the Spacecraft Communicator (CAPCOM). There were others who came and went, and we’ll cover them as required—but this group had the overall job of beating the drum to make sure that everyone was pulling at the same tempo, and in the same direction.
The FAO’s primary product was the timeline—called the Flight Plan—which was the detailed listing of activities that each crewmember (and each control center) had to execute in order to make things happen. Sometimes, things went exactly according to that plan. These plans included voluminous information on which procedures had to be executed at what times, as well as communication plans, trajectory operations (such as maneuvers), and just about anything else that was important to execute at a particular time. It even included items that weren’t time dependent but had to be executed each day—things like cleaning air filters or reading the mail. We learned to give the crew this list and let them pick up these tasks whenever it was convenient for them.
The funny thing about the timeline was that it either worked perfectly or it was a complete disaster. If all the experiments and payload tasks worked, then you could execute the timeline from start to finish and make few changes. But if an experiment had problems or if an operation took longer than planned or if some other monkey wrench got thrown into the flight, well, all bets were off. It was the job of the FAO (and their busy backroom support folks) to replan the mission, while making sure that all the various dependencies (what needed to be done and in what order, and which things had to occur at particular times in the orbit) were met. The Shuttle timeline was a complicated beast, often a true house of cards that could only be put together one particular way. Well, actually, there were multiple ways to put it together so that it would work—but there were many, many more ways that it wouldn’t.
Early on in the program, the crew launched with several copies of the thick, overall detailed timeline. It was good for them to have this, but when the timeline had to be replanned, that got thrown aside. Eventually we learned that the crew really only needed an overview for the entire mission, so they launched with that—we sent up detailed pages every morning based on the results and replanning from previous days. Later in the program, with the advent of better data and communication techniques, we simply sent up files as PDFs that the crew could use on their laptops. These files could be printed out as they desired. Regardless of the method, the FAOs were always in charge of the timeline. They were like the editorial staff for the morning newspaper.
The FAOs owned no systems on board the Shuttle, but theirs was an important job. One of their backroom positions was Pointing—determining the Shuttle’s attitude at any time during the flight. Once a spacecraft is in orbit, it matters not (aerodynamically) which way the nose is pointed. But payloads needed to be pointed at targets, stars had to be put in the field of views of star trackers, and certain parts of the Orbiter needed to see sun or shadow depending on thermal requirements. It was the job of the Pointers to make sure that all the inputs that went into how the Shuttle was oriented were met—or at least could satisfy as many conflicting requirements as possible. Their attitude timeline went into the morning flight plan along with everything else, and the pilots keyed in maneuvers as required.
Now the Payloads Officer position was one that morphed and changed a bit from the beginning of the Shuttle program through to the end. Originally, it was simply a NASA flight controller, whose job was to represent the payload customer in the MCC’s front room. Of course, that sounds simple, and conceptually it is, but it was a huge job because the Shuttle rarely (if ever) simply carried one payload on a flight. Generally, there were one, two, or a few major payloads on every mission—but there were also scads of little things that went along for the ride. Sure, you were carrying the Hubble Space Telescope into orbit, and that was what everyone saw. But there were always little Mid-deck experiments that had to be dealt with—from a vial of fish eggs to sometimes baffling medical experiments—and each one had to have a knowledgeable presence in the MCC so that when there were questions, comments, or problems, we could get them dealt with and make the most of the time for the payload.
I always reminded my flight controllers when they were pooh-poohing a minor experiment that it probably represented some scientist’s life work. These scientists had probably begun looking into a problem in graduate school, proposed an experiment that would answer the questions they had, and then spent a dozen years developing the experiment and getting a grant to put it on the Shuttle. They would then spend ten more years analyzing the data they got and writing about it. That’s a significant portion of a professional life and was not to be dismissed lightly. So everyone got the full support of my flight control teams, and the Payloads Officer was their advocate in the front room.
Later on in the program, commercial satellite deployments went away, and the emphasis became less on commercial endeavors and more on visiting and assembling space stations. We still carried payloads, but they were really components that had to be put together as part of an International Space Station (ISS) construction project. In addition to these major
components, we had lots and lots of small cargo—supplies for the growing station, experiments that were going to be worked with by the Mir and ISS occupants, and all the other bits and pieces you needed to support a crew in space. In addition to carrying cargo, the new components that were being added to the space station had to have knowledgeable representation in the front room in regard to assembly and checkout.
So the Payloads position morphed and became the Assembly and Checkout Officer (ACO). The ACO was the single point of contact within the Shuttle Control Center for all questions relating to what we were carrying in the payload bay or in the cabin for transfer to the ISS. They stood with one foot in the ISS world and the other in the Shuttle world and bridged the gaps between the two. It was as monumental a job as it sounds. The ISS quickly became a monster in terms of size and complexity. The ACO and their back room worked closely with the ISS team and the Shuttle FAOs to make sure that all the assembly tasks were completed on time, and that cargo was transferred in both directions according to the plan. You didn’t want the Shuttle to come home with cargo that was intended to be left on the ISS—especially if it was some important small part that was desperately needed on orbit.
