by Paul Dye
However, if we want to raise the orbit all the way around, we can simply thrust again by the same amount when we reach our new height (referred to as the apogee). We will have raised our altitude at the starting point by a half mile as well, meaning that we will be in a new circular orbit a half-mile higher than when we started the pair of burns. We will have also increased our velocity by 2 feet per second total.
The math is really convenient if you’re trying to do it in your head—if you want to raise the orbit by 10 miles, you simply burn 20 feet per second (fps) initially, then another 20 fps at the new apogee, and voilà—you’re in a new circular orbit 10 miles higher, and it cost you a total velocity change (referred to as delta V) of 40 fps. Raising and lowering the orbit is how you execute a rendezvous. But for now, it’s important that we get into, and then know how to get out of, orbit.
Let’s take a look at the very end of the initial launch. There we are, thrusting all three main engines, accelerating at 100 feet per second every second. Knowing what we now know about orbital mechanics, we know that for every second we burn the main engines at this point, we are raising the orbital altitude by 50 miles when we get around the planet. We need about 100 miles of altitude to reliably stay out of the atmosphere—consider it the minimum safe altitude we want to end up in. The International Space Station is at an altitude of about 200 miles, the Hubble Space Telescope is about 350. The Orbiter lived in the altitude band between 100 miles and 350 miles—a difference of just 250 miles. In terms of orbital insertion speed, that is just five seconds of burn time.
A full ascent, from the launch pad to Main Engine Cutoff (MECO) was about eight and a half minutes, or about 510 seconds. The orbital altitude range of the vehicle meant that cutoff would be plus or minus five seconds, which is a very small percentage of that total burn time. Miss it by 1 percent and we were either not in orbit or we were going way too high, without enough thrusting capability to circularize the orbit—or to get home. So MECO was critical—you had to time it exactly right in order to get precisely into the orbit you wanted, and we considered precise to be within a couple of miles.
No problem, right? I mean, throw the switch at the right time to cut off the engines, and you’ve got it made. Well it’s not that simple, because you don’t just shut off an engine that’s putting out a half million pounds of thrust. It doesn’t go from full thrust to zero in an instant—it tapers off. Every engine tapers off a little differently, so you need to know the exact shutdown characteristics of each engine. You can measure these characteristics on a test stand on the earth, and then use that information to figure out when to command the engines off, in fractions of a second, so that you end up with exactly the amount of thrust you need to end up at the target altitude. When you look at the hundreds of variables involved, you quickly realize that it’s going to take a lot of smart people to figure this out. Fortunately, there are lots of engineers and physics guys who are smart enough to model it and come up with the right answer. In the earliest of rocket flights, way back to Mercury, they were happy just to know that they’d made it into orbit. In the Apollo and Shuttle eras, we needed to have precise control of where we were going to end up—and the accumulation of rocket flight experience made that possible.
Now those who are following closely have already figured out that spaceflight is much more complicated than this. Recall the basics of orbital mechanics, remembering that what goes up must come down. If we have thrusted ourselves from the ground up to an orbital altitude, say 200 miles, we are only at our apogee. Like throwing a ball straight up into the air, we’re going to be coming back down to our starting altitude. This will happen when we get all the way around Earth. The nitty gritty, of course, is that we need to do some shaping of the trajectory to make sure we aren’t going to come all the way back down to the ground. Remember we need to travel horizontally at a high enough speed so that we don’t fall back to the surface. If we really want to end up in a circular orbit (and not an ellipse), we need to do a burn about halfway around the planet. Since we go around the planet in ninety minutes, it means that forty-five minutes after launch we need to do that burn—and we can’t use the main engines to do it. For that, we switch to our Orbital Maneuvering System (OMS) engines.
The OMS engines are mounted in pods on the upper rear corners of the Orbiter. There are two of them, and each produces about 8,000 pounds of thrust. You can burn them together, or separately, depending on how much thrust you need and how finely you want to manage the final velocity. These engines burn propellant stored in their pods, the same kind of fuel and oxidizer used for the attitude control jets—in fact, the tanks for the OMS and Reaction Control System (RCS) jets can be shared (or interconnected) between the two systems, if need be. Once you have shut down the main engines, and jettisoned the big External Tank (about eight and a half minutes into the flight), the OMS and RCS are all that you have. Because these engines are so much smaller than the main engines in terms of thrust, you have to burn them longer to get the same amount of velocity change—in fact, the acceleration available from the OMS engines is barely noticeable inside the vehicle.
But getting back to circularizing the orbit. In the simplest terms, when you get halfway around Earth from your launch site (over the Indian Ocean when launching from Kennedy Space Center), you point yourself in a direction to thrust ahead, and then burn the OMS engines to add the velocity that you need. It can be in the neighborhood of 100 feet per second or more. In the earliest Shuttle flights, we were happy to see that it worked to keep us in orbit—later on, we had learned enough about trajectory shaping and burn times that we were pretty unhappy if we missed our orbital parameters by more than a couple miles. We used ground and space tracking to confirm the orbit we needed to be up in, and we added what we learned into planning for every flight until we became very good at putting the Orbiter exactly where we wanted it to be.
Once you made it to orbit, changing that orbit was simply a matter of adding or subtracting velocity by adding speed with a burn or taking it away (you did that by thrusting backward). When you wanted to come home, you needed to thrust backward. That’s called retrograde. This is where we got the term retrofire, which was done in the early years of space travel with retrorockets. The backward thrust lasts until you slow down the vehicle enough to lower its perigee to about 80 miles, which is where you are assured of being captured by the atmosphere. If you decrease velocity by too much, you lower the perigee by too much, and that means you enter the atmosphere too steeply. Too steep of an entry means that you decelerate too quickly and have to dissipate your orbital energy in a shorter amount of time, which means that you get much higher temperatures on the skin and you burn up. Too little velocity change means that you enter the atmosphere at too shallow of an angle, and you could effectively skip off its surface, much as a rock skips off a pond. The problem with a skip is that you scrub off speed, which makes you slower, so you drop back into the atmosphere more steeply the second time, and eventually you end up with that steep entry again and burn up—sort of how those stones always drop into the water and sink.
By now, however, you can see that to bring the Orbiter home from a 200-mile orbit, you need to drop the perigee (the low point of the orbit) by about 120 miles (200 minus 80), and to do that, you need to slow it down by about 240 feet per second. If you filled the Orbiter’s tanks before launch, then you had approximately 600 feet per second total orbital maneuvering capability (the total delta V) that could be used throughout the mission—to raise and to lower the orbit. The key to mission planning and execution was to use that delta V wisely.
Spacecraft Evolution
The earliest manned spacecraft were, by today’s standards, pretty crude. Both the Russian Vostok and the American Mercury were essentially pressure vessels that could keep a human alive as the craft was lobbed into a low orbit, circled around Earth a few times, and withstood the heat of reentry to come down safely to the earth. The craft had limited maneuvering capabili
ty. Mostly they had the capability to orient themselves, or control attitude, so as to point windows and cameras in particular directions. They had the capability to execute a reentry (we now call it a deorbit) burn using retrorockets dedicated to the purpose. They were simple but effective vessels that carried out their missions.
Spacecraft technology matured and evolved relatively quickly. The Gemini series of US spacecraft included not only the ability to control attitude, but to change the altitude of the spacecraft using maneuvering engines. With this capability came the ability to rendezvous with other spacecraft or targets—but in order to do this, they needed to be able to navigate—so sensors and computers had to be added to allow the astronauts a way to figure out where they were, where they needed to go, and how to effect the necessary changes to close the gap. This same maneuvering capability provided them a wider and better control of where and how they came back to Earth, which made piloting an essential part of spaceflight.
Of course, if you were going to stay in orbit long enough to do anything useful, you needed more than just a sealed can of air to do so. You needed life support systems to keep the air fresh and circulating, and you had to control humidity and temperature. And to do these things, you needed power—which meant not only an electrical system to distribute power but also the ability to generate that power, be it from storage batteries, fuel cells, or solar arrays. Of course, if you are going to keep a human alive for a few days in space, you need to feed them and provide them with water, and have to provide ways for them to eliminate waste. You also need the humans to be able to communicate with each other, and with the earth. From those requirements follow all sorts of mechanical and electronic systems that can keep the humans occupied, doing useful work, and recording data so that the missions have value in the long term.
After the Gemini program came Apollo, and with it came an increased complexity by an order of magnitude. You now had two crewed spacecraft—one to take the humans to the moon, and another to get them down to the surface. You had space suits that allowed the astronauts to exit their craft and work outside (these were much more sophisticated than the ones that allowed the astronauts to do this from Gemini, where they were attached to the spacecraft for essential life support). You had multiple rocket engines that had to work to get the men down to the moon and back up. You had different navigation schemes for getting to the moon, landing there, and leaving. Just the technology of the science experiments themselves represented entirely new levels of complexity to operations.
The Space Shuttle was conceived in the time frame of Apollo and was intended to be part of a multipronged system to allow routine access to space. Originally, the Shuttle was to be built concurrently with a space station and a space tug—the station was to be a place to take humans, and the tug was going to be used to move satellites and other spacecraft from Shuttle orbits to other orbits where they could do their work or launch to other planets. In order to serve a set of yet-to-be-defined missions and objectives, the Shuttle became a very complex vehicle, far more sophisticated than anything that had come before, and far more capable as a result. In order to serve the widest possible array of missions, the Shuttle had to have the widest possible array of capabilities—and that meant a wide variety of systems.
Aerodynamics
Early spacecraft designers realized that they had two problems from a physics and aerodynamics standpoint. The first was how you accelerated an object from zero to orbital velocity. They solved this with the rocket—a way of channeling the high energy of what amounts to a controlled explosion into a propulsive force that will last long enough to put you in orbit. The second problem was, however, somewhat harder—that is, decelerating the object and returning it safely to the earth. The laws of physics say that if you are going to put all that energy into an object, you need to figure a way to get it back out of that object—without melting or otherwise destroying it in the process. Meteors are a good example of something that fails to make it through the atmosphere safely—they enter the atmosphere at a high speed and usually at an angle that creates so much friction that they burn up completely before making it to the ground. This is fine if you’re trying to prevent falling rocks from hitting people on the head, but it’s a bad thing if you want to bring a spacecraft home in one piece.
The first generation of crewed spacecraft took lessons from the earlier designs of nuclear-tipped Intercontinental Ballistic Missiles (ICBMs). Nuclear devices are complex devices that don’t react well to extremely high temperatures (until, of course, they generate those temperatures themselves). Launching a hydrogen bomb on a missile and lobbing it halfway around the world meant that you had to get it back into the atmosphere without it burning up—so that it could do its thing on the intended target. The earliest rocket work by the Pilotless Aircraft Research Division (PARD) at NACA (National Advisory Committee for Aeronautics, NASA’s predecessor) Langley in the late 1940s and into the 1950s was dedicated partly to solving the problems of reentry heating. This led directly to better ICBM nose cones, but also to the shape used to design the first Mercury spacecraft: a blunt body with an ablative coating that burned off with high heat, carrying the heat energy away so that the soft, pink human body inside made it all the way to the ground. Well, not to the ground but to the water, actually; or more accurately yet, to the point where a parachute could be deployed to lower the craft into the ocean.
Similarly, the Russians used blunt bodies or spheres to reenter. They ended their flights with parachutes as well, although the very first manned missions required the cosmonaut to eject from the capsule and use a personal parachute for the final landing.
If you reentered using a blunt body with a symmetrical design, and maybe you even introduced a little roll about the line of symmetry, you had a predictable reentry trajectory over which you had little control. You knew where you were coming down, but you couldn’t change it. It was soon discovered that if you had an asymmetrical design, you actually could generate some lift—and you could point the lift vector in different directions to effectively change your impact point. You now could control the vehicle’s final touchdown spot (at least by a little bit). Gemini used such an asymmetry by offsetting the center of gravity, and so did the Apollo spacecraft. But even though you now had some control over your landing point, it was limited—and the parachute system still required a water landing to keep the final impact loads survivable for the crew. The Russians land on solid ground because they fire a final braking rocket at the moment of touchdown to cushion those last loads—but it is still a pretty good impact.
The dream of almost everyone in the modern spaceflight era (most of us had been inspired by science fiction writers in the mid-twentieth century) was to have a fully reusable spacecraft that could land like an airplane, on a runway, and be flown again. It was recognized that the use of parachutes and water landings required a tremendous amount of recovery resources and was impractical in the long run if you wanted routine access to space. In order to effect a routine landing capability without parachutes, you needed not only the capability to move your target point around, you also needed to generate enough lift to break the descent rate—to flare for a landing, in other words. But wings are not great at dissipating the heat of reentry, so having them in space didn’t mean you were going to have them after the fiery part of reentry. What was needed was a stubby wing that could survive the heating and be around to execute the flare for landing.
NASA had worked with wingless, lifting body reentry vehicles in the 1960s. Although they worked, they had very high landing speeds and unforgiving control characteristics. So work began on developing a truly winged vehicle that was shaped to survive reentry and provide a reasonable amount of lift so as to be controllable and provide a reasonably soft touchdown so that the vehicle could be reused. The Shuttle Orbiter was the result of that research.
The Shuttle has been described as a heavy glider. It is, in fact, a glider for entry and landing, generating lift with its
wings as it reaches the thicker part of the atmosphere. Gliding capability is often referred to as a ratio of lift to drag. Whereas a typical light airplane might have a lift-to-drag ratio of 10 to 1, and a sailplane might be in the neighborhood of 25 to 1 (or far better—in the case of competition sailplanes, 50 to 1 is normal), the Shuttle has a relatively poor glide performance that topped out at about 4 to 1. In truth, it usually flew in the 2 to 1 regime. That means, in aeronautical terms, the Shuttle was a rock. The joke about a low lift-to-drag airplane is that if the engine quits and you’re looking for where to land, just throw a brick out and follow it down. The Shuttle was a lot like that brick, but when you compared it to a ballistic reentry, it was a heck of a good glider.
Now there are a few challenges with launching a winged glider into space and bringing it back. First, there are potential aerodynamic load problems on ascent. In order to make the Shuttle light enough to fly significant payloads into space, it had to have a fairly light structure. The airframe was therefore not a fighter plane that could be thrown around the sky with abandon—you had to be careful not to exceed the design limits—and that included aerodynamic loading. Almost everyone has stuck their arm out a car window and felt the lift and drag generated by cupping your hand like a little airfoil. The faster the car goes, the more work you have to do to keep your hand from blowing backward. The same thing is, of course, true with airplanes—and the Shuttle was an airplane. The problem was that if you were going to travel at speeds approaching or exceeding Mach 1 (the speed of sound), you could easily rip the wings off with the air loads.
That’s exactly what happened in the Challenger accident—the leaking solid rocket motor cut a hole in the bottom of the External Tank, which exploded. The explosion threw the Challenger off the fireball at a high angle of attack (the angle between the wing and the oncoming air) and that put so much pressure on the wings that they failed—followed, of course, by the rest of the structure. I have often been asked if the crew could punch off the External Tank in a situation like that, and the answer is always the same—take a look at the Challenger crash and you’ll see the results. The aerodynamics of winged rockets on ascent is very unforgiving. In the Shuttle, we had very specific limits to angle of attack and sideslip (the angle of wind hitting the nose from the left or right) and very little margin of error. This is why upper air winds were an important consideration on launch day—a wind shear (a sudden change in wind speed or direction over a short altitude span) could easily create an overload situation in the vehicle.
