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February 1939 Popular Science
 [Table of Contents]
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This is the first of two articles submitted by Dr. von Braun; the first being
in the
July issue.
Dr. Wernher von Braun Answers Your Questions About Space
Why I am writing for Popular Science
If my daily mail is a suitable yardstick, space science is a popular science
indeed. I would not be able to get any other work done were I to try to answer systematically
all those questions that find their way to my desk.
Popular Science's invitation to write a monthly column on my favorite subject
was thus received as both challenge and relief.
Challenge, because it always intrigues me to reduce a complex problem to terms
that (I hope) anyone can understand. Relief, because I am frequently bothered by
a bad conscience for not having replied to some of the most enthusiastic, inquisitive,
curious, and penetrating letters.
Space science isn't like geography, or astronomy, or physics, or chemistry, or
medicine. It is a little bit of all of them and more. That is what makes it so fascinating.
But it is this kaleidoscopic aspect of space science that makes it almost impossible
to "organize" a monthly column such as this. Mr. Crossley and I have therefore agreed
not even to try to arrange the questions and answers in any systematic way. If the
result is a bit disjointed, it should at least be colorful.
Leave a capsule in space? Here are answers by Dr. von Braun
How do you steer a rocket?
Q How are large rockets steered during powered flight?
A All methods have one principle in common: The rocket exhaust
is deflected in a controlled fashion.
For a rocket to fly straight, the force of its thrust must be so aligned as to
point to the rocket's center of gravity. If the thrust force F is out of alignment
and passes the center of gravity at a distance L, a turning moment will result that
is equal to F x L. A large rocket is steered by shifting this turning moment to
the right and left (controlling yaw), or up and down (controlling pitch), depending
on which way we want it to turn.
The force of a rocket's thrust is always parallel to that of the flow of exhaust
gas, but acts in the .opposite direction. In a liquid-propellant rocket, the combustion
chamber with the exhaust nozzle is usually swiveled to and fro like the outboard
motor of a small boat. The swiveling forces are provided by hydraulic actuators
(oil-driven pistons) which are controlled by electrical signals from the rocket's
control computer.
Older types of liquid-fuel rockets were often controlled by jet vanes. Usually
there were four relatively small rudders of graphite, tungsten, or an ablative material
- one whose expendable outer surface is allowed to char or volatilize - that were
immersed in the main jet and rotated by electric actuators. Jet vanes do not deflect
the entire jet but only part of it. The effect of a jet vane can be compared with
that of a rudder located in the propeller down-wash of a larger inboard motorboat.
Steering solid-fuel rockets
Unlike liquid-fuel rockets, solid rockets do not have separate thrust chambers.
In a solid rocket the basic airframe serves simultaneously as propellant-storage
container and thrust chamber, and swiveling the thrust chamber would not be feasible.
For this reason designers of solid rockets have developed deflectable exhaust nozzles.
Often a single solid rocket discharges its exhaust gas through four parallel-mounted
swivel nozzles, permitting complete three-dimensional control in the up-and-down
(pitch), the right-and-left (yaw), and the rotational (roll) directions.
Q How does an astronaut enter or leave his pressurized crew compartment
in airless outer space?
A By means of an air lock. This is a sealed compartment with
access through two airtight doors, one from the pressurized cabin, one from the
outside.
IMAGE HERE
When leaving the cabin, the astronaut, clad in his pressurized space suit, enters
the air lock, closing the inner door behind him. He now depressurizes the lock either
by venting the air to the outside or by pumping it back into the cabin.
Once this is done, the pressure in the air lock is down to zero and equal to
that of outer space. He may now open the outer door and leave his spacecraft.
Returning, he enters the non-pressurized lock through the outer door, closes
it, and re-pressurizes the lock by opening a valve connecting it with the pressurized
cabin. After the pressures of cabin and lock have been equalized he may open the
inner door and enter the cabin.
For outside inspection: an air lock
Air locks will be used in advanced spacecraft (such as Apollo) to enable astronauts
to leave their pressurized cabin temporarily. This may be desirable for outside
inspection, docking maneuvers, rescue operations, and for crew transfer into another
vehicle such as the lunar-excursion "bug" designed for the letdown from lunar orbit
to lunar surface.
Q Why is liquid hydrogen such a good rocket fuel?
A There are two reasons. One is the high heat energy released
by the combustion of hydrogen. The other, equally important but less obvious, is
the low molecular weight of hydrogen and its combustion product, water vapor.
The exhaust velocity of a rocket engine is the best yardstick of its fuel economy.
Each gas molecule spurting from a rocket motor's exhaust nozzle can be looked upon
as a tiny bullet fired from a gun. The higher the muzzle velocity, the more recoil
will be exerted on the gun barrel. As the thrust of a rocket motor is made up of
the total of all the little recoils produced by millions of molecule bullets, and
the exhaust gas is produced by burning fuel, it follows that the higher the exhaust
velocity with a given amount of fuel, the greater will be the rocket motor's thrust.
For high velocity: hydrogen
The exhaust nozzle of a rocket motor can be looked upon as a device that orients
the all-directional movement of the gas molecules in the combustion chamber into
one predominant direction. The exhaust velocity is, therefore, directly related
to the velocity at which the gases whirl around in the chamber immediately after
combustion but prior to entering the exhaust nozzle.
Now a fundamental law of physics states that at a given temperature the average
kinetic energy of the whirling molecules of any two gases must be the same. The
kinetic energy, or energy of motion of a body, depends on two factors: its weight
(or mass) and its speed. This means that in order for a light and a heavy body to
have the same kinetic energy, the light one must be faster. It follows that, for
a given combustion temperature in a rocket engine, the propellant combination that
produces lighter exhaust products will also produce a higher exhaust velocity.
Of course, a high combustion energy is needed to obtain a high combustion temperature.
The combustion of hydrogen is one of the most powerful reactions known in chemistry.
But, as we have seen, a light combustion product with a low average molecular weight
is equally important. Hydrogen, with a molecular weight of only 2, is the lightest
gas in existence. Even its combustion product, water vapor, resulting from reaction
between two hydrogen atoms and one oxygen atom, has a molecular weight of only 18,
which is quite low compared with that of combustion products of other fuels. Moreover,
rocket engines using liquid hydrogen as fuel and liquid oxygen as oxidizer operate
at maximum efficiency when running at a fuel-rich mixture. This means there are
more hydrogen atoms around than there are oxygen atoms available with which they
could react. The exhaust jet of such a rocket engine is therefore composed of a
mixture of water vapor and unburned hydrogen, with a molecular weight some-where
between 18 and 2. It is because of these advantages that rocket engineers put great
faith in liquid hydrogen.
More answers by Dr. von Braun: Dead moon? Erupting sun?
Q Is the moon a dead world?
A It is certain that the conditions on the lunar surface are
prohibitive for any kind of higher animal or plant life. It is not impossible, however,
that soil bacteria might exist on the moon, and we have no way of knowing whether
there are subterranean deposits of ice that might be capable of supporting certain
low forms of growth.
Some lunar craters are the center point of ray-like features. These light-colored
bands, sometimes several hundred miles long, seem unaffected by terrain over which
they pass. Astronomers conclude they are made by dust thrown up by volcanic explosions
or by volcanic gases frozen to the lunar soil.
Q What are solar flares?
A Although the sun seems never to change in appearance, it is
actually subject to erratic behavior. Its surface may be perfectly clean today;
a month later it may be covered with dark spots. Sun spots are an indication of
activity that bears some resemblance to volcanic eruptions on earth. The difference
between the two phenomena is that the gas expelled by the sun - predominantly hydrogen
- is so hot that the hydrogen atom (consisting of a proton and an orbital electron)
is deprived of its electron. As a result, the solar gas burst, or flare, consists
of protons or electrons.
Under "quiet" conditions, there is a more or less steady flow of these particles,
called "solar wind." This flow travels all the way from the sun to the earth and
beyond. During average solar eruptions the density of this flow increases a hundred-fold
or more, and the velocity at which the particles reach the earth is also markedly
higher. Once a year or so the eruption of a gigantic flare is observed, with particle
densities and speeds far exceeding those in normal flares.
For manned space flight, only these giant flares are considered hazardous. A
program to predict such flares has been initiated, and it is planned to time short
trips (such as round trips to the moon) so that they won't coincide with the superflares.
On long interplanetary space voyages it may be necessary to take along "storm cellars,"
into which the crews could withdraw during the hours of peak intensity of the flare.
Dr. von Braun will consider answering questions from readers of Popular Science
in the magazine, but he cannot undertake to answer each one by mail. Letters to
him should be addressed in care of Popular Science, 355 Lexington Ave., New York
17, N. Y.
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