- 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.
April 2016 