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Saturday, April 23, 2016

Reading List: Coming Home

Launius, Roger D. and Dennis R. Jenkins. Coming Home. Washington: National Aeronautics and Space Administration, 2012. ISBN 978-0-16-091064-7. NASA SP-2011-593.
In the early decades of the twentieth century, when visionaries such as Konstantin Tsiolkovsky, Hermann Oberth, and Robert H. Goddard started to think seriously about how space travel might be accomplished, most of the focus was on how rockets might be designed and built which would enable their payloads to be accelerated to reach the extreme altitude and velocity required for long-distance ballistic or orbital flight. This is a daunting problem. The Earth has a deep gravity well: so deep that to place a satellite in a low orbit around it, you must not only lift the satellite from the Earth's surface to the desired orbital altitude (which isn't particularly difficult), but also impart sufficient velocity to it so that it does not fall back but, instead, orbits the planet. It's the speed that makes it so difficult.

Recall that the kinetic energy of a body is given by ½mv². If mass (m) is given in kilograms and velocity (v) in metres per second, energy is measured in joules. Note that the square of the velocity appears in the formula: if you triple the velocity, you need nine times the energy to accelerate the mass to that speed. A satellite must have a velocity of around 7.8 kilometres/second to remain in a low Earth orbit. This is about eight times the muzzle velocity of the 5.56×45mm NATO round fired by the M-16 and AR-15 rifles. Consequently, the satellite has sixty-four times the energy per unit mass of the rifle bullet, and the rocket which places it into orbit must expend all of that energy to launch it.

Every kilogram of a satellite in a low orbit has a kinetic energy of around 30 megajoules (thirty million joules). By comparison, the energy released by detonating a kilogram of TNT is 4.7 megajoules. The satellite, purely due to its motion, has more than six times the energy as an equal mass of TNT. The U.S. Space Shuttle orbiter had a mass, without payload, of around 70,000 kilograms. When preparing to leave orbit and return to Earth, its kinetic energy was about that of half a kiloton of TNT. During the process of atmospheric reentry and landing, in about half an hour, all of that energy must be dissipated in a non-destructive manner, until the orbiter comes to a stop on the runway with kinetic energy zero.

This is an extraordinarily difficult problem, which engineers had to confront as soon as they contemplated returning payloads from space to the Earth. The first payloads were, of course, warheads on intercontinental ballistic missiles. While these missiles did not go into orbit, they achieved speeds which were sufficiently fast as to present essentially the same problems as orbital reentry. When the first reconnaissance satellites were developed by the U.S. and the Soviet Union, the technology to capture images electronically and radio them to ground stations did not yet exist. The only option was to expose photographic film in orbit then physically return it to Earth for processing and interpretation. This was the requirement which drove the development of orbital reentry. The first manned orbital capsules employed technology proven by film return spy satellites. (In the case of the Soviets, the basic structure of the Zenit reconnaissance satellites and manned Vostok capsules was essentially the same.)

This book chronicles the history and engineering details of U.S. reentry and landing technology, for both unmanned and manned spacecraft. While many in the 1950s envisioned sleek spaceplanes as the vehicle of choice, when the time came to actually solve the problems of reentry, a seemingly counterintuitive solution came to the fore: the blunt body. We're all acquainted with the phenomenon of air friction: the faster an airplane flies, the hotter its skin gets. The SR-71, which flew at three times the speed of sound, had to be made of titanium since aluminium would have lost its strength at the temperatures which resulted from friction. But at the velocity of a returning satellite, around eight times faster than an SR-71, air behaves very differently. The satellite is moving so fast that air can't get out of the way and piles up in front of it. As the air is compressed, its temperature rises until it equals or exceeds that of the surface of the Sun. This heat is then radiated in all directions. That impinging upon the reentering body can, if not dealt with, destroy it.

A streamlined shape will cause the compression to be concentrated at the nose, leading to extreme heating. A blunt body, however, will cause a shock wave to form which stands off from its surface. Since the compressed air radiates heat in all directions, only that radiated in the direction of the body will be absorbed; the rest will be harmlessly radiated away into space, reducing total heating. There is still, however, plenty of heat to worry about.

Let's consider the Mercury capsules in which the first U.S. astronauts flew. They reentered blunt end first, with a heat shield facing the air flow. Compression in the shock layer ahead of the heat shield raised the air temperature to around 5800° K, almost precisely the surface temperature of the Sun. Over the reentry, the heat pulse would deposit a total of 100 megajoules per square metre of heat shield. The astronaut was just a few centimetres from the shield, and the temperature on the back side of the shield could not be allowed to exceed 65° C. How in the world do you accomplish that?

Engineers have investigated a wide variety of ways to beat the heat. The simplest are completely passive systems: they have no moving parts. An example of a passive system is a “heat sink”. You simply have a mass of some substance with high heat capacity (which means it can absorb a large amount of energy with a small rise in temperature), usually a metal, which absorbs the heat during the pulse, then slowly releases it. The heat sink must be made of a material which doesn't melt or corrode during the heat pulse. The original design of the Mercury spacecraft specified a beryllium heat sink design, and this was flown on the two suborbital flights, but was replaced for the orbital missions. The Space Shuttle used a passive heat shield of a different kind: ceramic tiles which could withstand the heat on their surface and provided insulation which prevented the heat from reaching the aluminium structure beneath. The tiles proved very difficult to manufacture, were fragile, and required a great deal of maintenance, but they were, in principle, reusable.

The most commonly used technology for reentry is ablation. A heat shield is fabricated of a material which, when subjected to reentry heat, chars and releases gases. The gases carry away the heat, while the charred material which remains provides insulation. A variety of materials have been used for ablative heat shields, from advanced silicone and carbon composites to oak wood, on some early Soviet and Chinese reentry experiments. Ablative heat shields were used on Mercury orbital capsules, in projects Gemini and Apollo, all Soviet and Chinese manned spacecraft, and will be used by the SpaceX and Boeing crew transport capsules now under development.

If the heat shield works and you make it through the heat pulse, you're still falling like a rock. The solution of choice for landing spacecraft has been parachutes, and even though they seem simple conceptually, in practice there are many details which must be dealt with, such as stabilising the falling craft so it won't tumble and tangle the parachute suspension lines when the parachute is deployed, and opening the canopy in multiple stages to prevent a jarring shock which might damage the parachute or craft.

The early astronauts were pilots, and never much liked the idea of having to be fished out of the ocean by the Navy at the conclusion of their flights. A variety of schemes were explored to allow piloted flight to a runway landing, including inflatable wings and paragliders, but difficulties developing the technologies and schedule pressure during the space race caused the Gemini and Apollo projects to abandon them in favour of parachutes and a splashdown. Not until the Space Shuttle were precision runway landings achieved, and now NASA has abandoned that capability. SpaceX hopes to eventually return their Crew Dragon capsule to a landing pad with a propulsive landing, but that is not discussed here.

In the 1990s, NASA pursued a variety of spaceplane concepts: the X-33, X-34, and X-38. These projects pioneered new concepts in thermal protection for reentry which would be less expensive and maintenance-intensive than the Space Shuttle's tiles. In keeping with NASA's practice of the era, each project was cancelled after consuming a large sum of money and extensive engineering development. The X-37 was developed by NASA, and when abandoned, was taken over by the Air Force, which operates it on secret missions. Each of these projects is discussed here.

This book is the definitive history of U.S. spacecraft reentry systems. There is a wealth of technical detail, and some readers may find there's more here than they wanted to know. No specialised knowledge is required to understand the descriptions: just patience. In keeping with NASA tradition, quaint units like inches, pounds, miles per hour, and British Thermal Units are used in most of the text, but then in the final chapters, the authors switch back and forth between metric and U.S. customary units seemingly at random. There are some delightful anecdotes, such as when the designers of NASA's new Orion capsule had to visit the Smithsonian's National Air and Space Museum to examine an Apollo heat shield to figure out how it was made, attached to the spacecraft, and the properties of the proprietary ablative material it employed.

As a NASA publication, this book is in the public domain. The paperback linked to above is a republication of the original NASA edition. The book may be downloaded for free from the book's Web page in three electronic formats: PDF, MOBI (Kindle), and EPUB. Get the PDF! While the PDF is a faithful representation of the print edition, the MOBI edition is hideously ugly and mis-formatted. Footnotes are interleaved in the text at random locations in red type (except when they aren't in red type), block quotes are not set off from the main text, dozens of hyphenated words and adjacent words are run together, and the index is completely useless: citing page numbers in the print edition which do not appear in the electronic edition; for some reason large sections of the index are in red type. I haven't looked at the EPUB edition, but given the lack of attention to detail evident in the MOBI, my expectations for it are not high.

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