Space Propulsion Laboratory
The basic principles
of rocket propulsion stem from Newton's 3rd law of motion. The so-called
law of action and reaction. A rocket propels itself by expelling
material at high velocity, in the opposite direction to its motion.
The material is usually a gas and the heat of a chemical reaction
generally imparts the velocity. The heat builds pressure in a combustion
chamber and is converted to exhaust velocity by the action of a
properly designed nozzle. The principle is quite general and applies
equally well to the motion of a simple garden sprinkler as it does
to the powerful engines of the space shuttle.
Family Tree for Rocket Propulsion
for imparting energy to the exhaust is not always a chemical reaction.
In fact, numerous approaches exist, giving rise to a family tree
of rocket propulsion, which features many branches
and sub-branches. This ordering helps in building a basic understanding
of the various rocket concepts. It also helps in drawing comparisons
(and distinctions) among them. Rockets can be thermal and non-thermal,
depending on if the fluid is heated and accelerated, or accelerated
only without heating. Further still, thermal rockets can be active
or passive, depending on whether the heat is generated internally
by the working fluid or given to it by some external source. For
example, the chemical rocket is in the active branch, whereas the
nuclear thermal rocket belongs to the passive branch. The chemical
reaction heats the working fluid in the former. In the latter, the
nuclear reactor core provides the heating. Non-thermal rockets,
are more like particle accelerators. Their exhaust is a beam of
fast moving particles where random motion (and hence temperature)
may be negligibly low. Most of the low thrust electric rockets,
such as the ion engine and the Hall effect thruster lie in this
branch of the family.
of a rocket is the simple product of its exhaust velocity (relative
to the ship) and its propellant mass flow. Thus one can achieve
the same thrust by either ejecting more material at low velocity
or less at high velocity. Clearly, since the material must be carried
on board, the latter approach is preferred. Thus, the fundamental
goal of rocket propulsion is to achieve the highest possible exhaust
velocity. Generally, in thermal rockets (chemical, nuclear or otherwise)
this leads to high exhaust temperatures. Thus, these motors must
be quite sturdy, yet lightweight enough for flight. This implies
that a careful choice of materials must be made. In non-thermal
rockets, other forms of materials issues are still present, having
to do with the bombardment of walls and components with very fast
(albeit cold) moving particles.
a rocket launch makes one aware of the extreme conditions under
which they operate. Indeed, their design is a tribute to human ingenuity.
These devices operate at the limits of known materials, in an exquisite
yet unforgiving balance. For example, a space shuttle main engine
has an exhaust velocity of about 4500 m/sec, at an exhaust temperature
of about 1400° C. Its high-strength alloy engine nozzle must
be actively cooled by liquid hydrogen, in order to prevent melting.
The power level of such an engine is about 10 GigaWatts, enough
to provide electrical power to a large city. Yet, all of this power
comes in a package no bigger than a small automobile. Our society
has colloquially recognized the remarkable achievement of our early
rocket pioneers by the popular term "rocket science."
of the Chemical Rocket
In spite of
these impressive numbers, the exhaust velocity of modern chemical
rockets is at least one order of magnitude too small for the needs
of fast interplanetary travel. The amount of propellant that must
be carried along today constitutes most of the ship's mass and hence
leaves little room for useful payload. Also, its flow rate is so
high, that the rocket ship spends it all in a few short bursts,
traveling mostly in a free-fall condition. This restrictive form
of flight does not lend itself well for human transportation. Currently,
a one-way mission to Mars takes about 10 months using these devices.
velocity can be achieved by the use of a plasma, where the atoms
of the gas have been stripped of some of their electrons, making
it a soup of charged particles. The temperature of a plasma starts
at about 11,000° C. But present day laboratory plasmas can be
a thousand times hotter. Particles in such plasmas move at velocities
of 300,000 m/sec. These temperatures are comparable to those in
the interior of our Sun. No known material could survive direct
contact with such a plasma. Fortunately, a plasma responds well
to the presence of electric and magnetic fields. A magnetic channel
can be constructed to both heat and guide a plasma, without ever
touching material walls. Magnetized plasmas are envisioned to, some
day generate abundant energy on Earth, by controlled thermonuclear
fusion. Their complex physics is the subject of intense study.