propulsion power is part of this picture,
the power needed to operate space vehicles after launching may prove to
be the larger and more important need. Progress has already been made in
this direction by use of special kinds of batteries and solar cells
which convert the sun's rays into electric current. But these will need
supplementing or replacing eventually as greater power becomes
necessary.

It would be rash to predict the outcome of this complicated field, but
certain very promising methods can be listed.

One is the fuel cell, which converts fuel directly into electric power
without the necessity for machinery or working parts. Much progress has
been made on the fuel cell in recent months. In England a 40-cell unit
has been used to drive a forklift truck and to do electric welding. It
develops up to 5 kilowatts.[31] In the United States a 30-cell portable
powerplant developing 200 watts has been delivered to the Army and
Marine Corps,[32] while a 1,000-unit cell has been developed in the
Midwest which provides 15 kilowatts and drives a tractor.[33]

Another method is plasma power, or power generated through the use of
hot ionized gas. Such gas acts as a conductor of electricity and when
employed as a "magnetohydrodynamics" generator it can be used for a
variety of purposes. It has the advantage of being simple, rugged, and
efficient. Some day it may also prove very economical. Already 10
municipal areas along the Mason-Dixon line are preparing to experiment
with electric power derived from this source.[34] It has been estimated
that "as much as 1 million watts could be generated by shooting a stream
of plasma at speeds three times that of sound through a magnetic field
only 3 feet long and with the magnetic poles 6 inches apart."[35]

[Illustration: FIGURE 7.--The possible power source for space
ships of the future, the ion jet, has significant counterpart uses for
the commercial world.]

Another possible source is photoelectric power. While a number of very
difficult problems block the practical generation of this kind of power,
the astronautics research division of one American company has now
succeeded in increasing the efficiency of photoelectric cells by a
factor of more than 300.[36] So the possibilities in this area are
looking up. As discussed in section II, photon power derived from the
ejection of electromagnetic rays may someday prove a source for
accelerating vehicles once they have escaped fr