by Paul Dye
The A in MMACS stood for Arm—they were responsible for the Remote Manipulator System (RMS—the robot arm) when it was not in use, or when it was needed in an emergency and the on-call arm specialists couldn’t get into the MCC in time. That was the theory anyway. In reality, the RMS position was usually staffed any time that the arm was on board, and the vehicle was on orbit. So, although MMACS Officers were trained for the arm, they rarely did anything with it.
The final letters in MMACS stood for Crew Systems—a broad category of equipment that accounted for pretty much anything that was installed in the cabin, or was loose in the cabin, that the crew worked with. From the seats used for ascent and entry to the orange suits (and helmets, and parachutes, and survival gear…) used for the same thing, the MMACSs were the experts. They also knew how to work the galley (and how to fix it). And they had responsibility for all the loose camera gear (still and video) carried into space. They were responsible for knowing where everything was stowed, and also for the lockers themselves. The escape pole—theirs. Tools—also theirs. The Crew Systems aspect of the MMACS job pretty much meant that if something was causing trouble and no one else had it clearly in their domain—the MMACS was the one the Flight Director turned to. (If you haven’t guessed already—I was the first certified MMACS Officer, and I served in that role for quite a few missions—and as the head of the MMACS section—for several years before being selected as Flight Director.) It was a great launching pad for the center seat because it literally put your fingers in everyone else’s systems when it came to IFMs, and the mechanical systems that supported all the ascent and entry work.
The systems we have talked about so far constituted the core Shuttle systems—but there were always a few specialists in the room to cover specific bits of the Shuttle when required. The Booster Officer (unimaginatively called BOOSTER) was present for every ascent. The Booster Officer was responsible for the three main engines (SSMEs), the External Tank (ET), and the Solid Rocket Boosters (SRBs). The SSME was an incredible engine—generating a half-million pounds of thrust. It was throttleable and reusable—no mean feat considering that its design was conceived and baselined only about a dozen years into the history of manned spaceflight. The three engines were mounted in the back of the Orbiter, but their fuel and oxidizer came from the big orange External Tank plumbed through pipes almost 18 inches in diameter.
These pipes actually penetrated the Orbiter’s belly, near the rear, and the holes had to be covered up with special doors after the ET was jettisoned. Of course, these doors belonged to the MMACS Officer, because they were mechanical and driven by motors. They were critical—if they didn’t close properly, you had a hole in the heat shield, and that was always going to be fatal.
We used to joke that a Booster Officer was like a moth—they only lived for a short period of time, and when the engines shut down eight and a half minutes into flight, they were gone as quickly as they could get the engines in a safe and inert configuration, which didn’t take long. Actually, they had everyone’s respect because of the high energies and the rapidity with which things could go wrong in their systems. While there were many, many pages of procedures to deal with engine problems, we all felt that, in most cases, if something was going to go badly wrong back there, it was going to happen so fast that you’d never have a chance to deal with it. But we trained for what could be done, and the Boosters were always there and working to make the ascent as safe as possible.
When the Boosters went home, one of the orbit-only specialists usually moved into their vacated console space. Who took that space varied over the years. The RMS operators took over the Booster console for some years, and the Extravehicular Activity (EVA) Officer took this space later in the program. The RMS operator did a huge amount of work before every mission, planning every move of the robot arm, which was itself a complex machine with its own computers, data, and command system. The arm, built by the Canadian Space Agency, was as long as the payload bay, and it mimicked the human arm by having a shoulder, elbow, and wrist that moved much like that of a person. The end effector or “hand” was a cylindrical contraption with moving snares that could grab a special grapple fixture that was installed on a payload before flight. The universal grapple fixtures can be seen all over the ISS today as the station was largely put together while in orbit by the robot arms on the Shuttle and the ISS itself. Many years from now, the technology might well have moved on to a different type of end effector and grapple fixture—but these will remain, mementos of the Shuttle that built the ISS.
The RMS was an extremely capable—and fairly complex—piece of machinery that had significant redundancy. But it also had some frustrating failure modes that could lead you to very slow operations under some conditions. But when it was all working (which was most of the time) a skilled operator could touch an egg set in the payload bay without breaking it. Although arm motion and trajectories were planned and thought out in great detail before every flight, the controllers and astronauts who operated the arm were always coming up with new trajectories in real time when the need arose.
The arm became an essential piece of equipment in the years after the Columbia accident because mission rules required an inspection of the Orbiter’s thermal protection system after every launch, before it proceeded to dock with the station. This was performed with cameras attached to an extension boom that was picked up by the RMS and positioned underneath the Orbiter. The Flight Day 2 Inspection became routine, and the arm proved its worth mission after mission until the end of the program.
The final system specialist (and special system) that was essential to orbit operations was EVA—the guys with the space suits. Space walks have been a part of the US manned spaceflight program since Ed White floated out of the Gemini capsule for America’s first twenty-minute walk in space back in the 1960s. Learning to work outside the spacecraft was essential to getting to the moon, and it is amazing to think that this technology, which was developed in such a short period of time, would allow humans to walk on the moon and then ride in a buggy on the moon. After Apollo, because we were no longer going to be on a lunar surface, space suits became specialized for use floating in a zero-G environment. The Shuttle usually carried two suits until it was time to build the ISS—then the great “wall of EVAs” appeared. They were used on a seemingly endless series of space walks that were required to build the station, assembling it from components brought up in the payload bay. That wall seemed daunting to those of us who lived through the early years of learning how astronauts could work in space—but darned if it wasn’t conquered in style with all the ISS assembly EVAs completed successfully.
Once the ISS assembly got underway it was not uncommon to have four space walkers on a given Shuttle mission, so up to four suits were put aboard. We carried two in their usual spots in the airlock, and two more were stored in the Mid-deck. Sometimes, crewmembers were close enough in size that they could share a suit. If so, they only had to swap out the soft components (the lower torso, legs, arms, and certain internal pads) between work sessions.
The EVA space suit or Extravehicular Mobility Unit (EMU) was itself a complete little spaceship, comprising a full life support system, instrumentation, communications—in later years it even had an emergency propulsion system that could be used if an astronaut found themself suddenly free-floating, untethered in space. Such an event was unlikely since astronauts were trained from day one in tether protocol (always being attached with two tethers). But the fact that the ISS could not maneuver to pick up a separated astronaut forced the development of the SAFER—a little emergency jet pack that was attached to all EMUs during the ISS time frame.
The suits really were marvels of engineering, with pumps, oxygen systems, cooling, and communications all rolled into one package the size of a human being. The torso was rigid, and you entered by breaking the suit at the waist. The lower portion—below the beltline—was soft. You got into the lower portion before snaking your arms and head thr
ough the upper torso portion. It took care (and a long checklist) to get into a suit and be ready to go outside—nothing like the movies where a person hops into a suit, zips it up, and finds themselves floating in a matter of minutes. It just doesn’t work that way because there is really no margin for error.
The EVA Officers in MCC are the same people who build the procedures for each mission, and they train the crew to perform the specific tasks needed for each flight. They are truly an integrated, all-around operations engineer—involved with the design of the equipment, developing the timeline and procedures, training the crew, and then watching over them when they are actually in space. EVAs are dangerous because you have a person outside with nothing but a nonredundant suit to protect them—but the dangers are mitigated because the EVA Officers have been involved with the specific mission plan, the hardware, and the crew from the beginning. They truly are experts in that specific day’s work—and it shows.
The many and various systems officers who knew the Shuttle intimately were, of course, only part of the team. Equally important were the guidance and navigation flight controllers, the trajectory officers who knew the mathematics and physics of spaceflight like the backs of their hands.
Chapter 2
Spaceflight 101
Orbital Mechanics
The phrase orbital mechanics—like nuclear particle physics or the theory of relativity—is something that makes many people’s eyes glaze over. The average person thinks that subjects like these are far too complex to understand. But if you strip away all the math and simply try to understand what is going on, they are actually not that hard to grasp. As for orbital mechanics, it is the physics of spaceflight—it describes how objects move relative to one another in space. Once you understand the mechanics of how planets and spacecraft interact with gravity, it’s easy to make a spacecraft do what you want it to do.
If you take a baseball and throw it straight away from yourself, it will eventually hit the dirt—it runs out of speed, and then gravity pulls it to the earth. If you’re a major league pitcher, your throw will probably go farther than mine because it launches off your hand faster. But gravity eventually does its thing, and the ball will come down to the earth. Now, let’s build a machine to throw the ball faster—it will go farther, of course… but the ball will always curve down and hit the dirt. That curve the ball follows, by the way, is known as a ballistic path. Because of gravity, any object thrown or fired into the air will eventually be pulled back to the earth. With every increase in speed, the ball goes farther. If we change the ball to a projectile, and start firing it from a gun, then it will go farther still.
Naval guns can fire projectiles so far that they go over the horizon before falling back to the earth (hopefully on their targets). That phrase, over the horizon, is key to understanding orbital mechanics. The spherical shape of Earth means that as you travel horizontally, the surface is always curving down. If you can throw the object fast enough that its drop, its ballistic path, is equivalent to Earth’s curvature, then it will never hit the planet—and voilà—it has gone into orbit!
It takes a tremendous amount of speed to reach that point where your object is not going to hit the earth—in the neighborhood of 25,000 feet per second (that’s over 17,000 miles per hour) if you’re talking about flying in the region known as low-Earth orbit (LEO). Let’s call that anywhere from about 100 miles above the surface to about 400 miles—give or take. That’s where the Space Shuttle did all its work. If we take our imaginary baseball or projectile and keep upping the speed, eventually it will go so fast that it never falls back to Earth. The speed at which that occurs is known as escape velocity. We never worried about escape velocity with the Shuttle—it didn’t have the capability to go that fast. Unlike the Apollo spacecraft that took men to the moon, the entire Shuttle spacecraft was always coming back to Earth. It would have taken approximately its own weight in fuel to propel the Orbiter to the moon. (I asked my navigators to figure that out one night…)
So the first element of orbital mechanics isn’t that hard—make an object move fast enough relative to the planet, and it will never hit the planet. It will just keep going around and around and around—until, of course, something slows it down to where it begins dropping toward the surface. What might slow it down? Well, one thing might be running into the thinnest wisp of the atmosphere, the widely spaced molecules of gas that reach for hundreds of miles into space. The atmosphere doesn’t just end abruptly, it gradually gets thinner and thinner as you move away from the planet’s surface. It never really goes away completely, it just eventually fades in density until it no longer has any effect. Just about 100 miles above Earth’s surface, there are enough air molecules that if a spacecraft runs into them, an infinitesimal amount of energy is lost with every collision. You can’t measure the energy from any one collision, but if you add up enough of them you can eventually discover that you have lost some speed. And that speed loss adds up.
The slower you go, the more you fall back to Earth. So the effect of running into the atmosphere is that it drags you back down. If you don’t add some velocity with a rocket motor burn every once in a while, you won’t stay in orbit. It doesn’t take a lot—just a couple of feet per second every day—but if you don’t account for it, your mission isn’t going to last very long. We use this effect to our advantage, of course—it’s how we bring a spacecraft home. If you point your spacecraft so that when you ignite an engine it slows the craft down significantly, then you will drop lower into the atmosphere where the gas is thicker, which then slows you down even more—eventually to the point where you have been captured by the atmosphere, and you fall to the earth. This is what happens when a meteor becomes a meteorite: it generates enough heat from the friction of the atmosphere to quickly burn up, which creates the streak we see in the night sky. Of course, burning up on reentry is the last thing we want a spacecraft to do, so we have to enter in a controlled fashion. We’ll talk about that later.
For now, let’s remember that if we go fast enough horizontally, we end up orbiting Earth rather than falling back to the surface. The faster you go, the higher you go. The slower you go, the lower you go (until you fall out of orbit and are captured by the planet’s atmosphere). These are the basics of orbital motion (or mechanics). With that mental picture, you can understand almost anything else we’ll talk about when it comes to the Space Shuttle’s trajectory.
To get the Shuttle into orbit, you have to do two things: get it out of the atmosphere and accelerate it to orbital velocity. In the simplest terms, the first part of that is done with the Solid Rocket Boosters (SRBs). These two monsters had enough energy to loft the entire Shuttle stack up to an altitude where the air was so thin as to be negligible. They did this in just a little over two minutes. Each SRB put out about 3 million pounds of thrust, for a combined total of 6 million pounds. By comparison, the three Shuttle main engines contributed another half-million pounds of thrust each, for a total of 1.5 million pounds—a much smaller portion of the overall thrust available at liftoff. Not insignificant, of course, but still—the SRBs dominated during what we referred to as the first stage.
When the SRBs dropped off, the vehicle was up where the air was not a real factor, but it was only going a couple thousand feet per second horizontally. It was now the job of the main engines to accelerate the ever-lightening stack (the Orbiter and External Tank) to that magic number of 25,000 feet per second to get it to stay in orbit. That “ever lightening” part is important—as you burn fuel, you get lighter, but with a constant thrust (about 1.5 million pounds, remember?) you are going to accelerate more quickly. That’s a consequence of Newton’s basic law of motion—if the force remains the same and the mass decreases, the acceleration goes up! Now the Orbiter was designed for a maximum acceleration of three times the force of gravity (3 Gs). A “G” is about 32 feet per second per second, so 3 Gs is just about 100 feet per second per second—you’re really gaining velocity quick! When the acc
eleration reached that point, the only thing you could do to keep from over stressing the vehicle was to throttle the engines back, and that is what we did. In the last two minutes or so, the throttles would come back to make sure we didn’t break anything.
In the simplest terms, when you reached the desired velocity to make orbit, you shut down the engines and coasted into orbit. It sounds simple—but it isn’t. Let’s stretch our knowledge of orbital mechanics a bit. Let’s assume that you are in a circular orbit—the same altitude above Earth at all points in the orbit. If you decide that you want to go higher, then you have to increase your speed. This is done with a thrust event, known as “doing a burn,” because we thrust by firing—burning—an engine. If you squint and allow yourself to approximate, doing a burn to increase your speed by 1 foot per second will increase your altitude by about a half mile. That’s not an instantaneous gain—what you are actually doing is driving yourself uphill until you reach that new altitude, which you will reach when you are halfway around Earth. But you won’t remain there. Think of the ballistic path that a ball takes when you throw it—it first increases in height, then gravity pulls it back down, and so it comes back down again. The same thing happens when you increase the Orbiter’s velocity—it will go uphill to the new altitude, but it eventually comes back to where it started… right to the altitude where you increased the speed. It so happens that you’ll reach your new altitude halfway around the world, and then you’ll be back where you started when you complete the orbit. It will continue in this elliptical path for as long as you let it.
