Advanced Space Propulsion Laboratory

PROPULSION
Newton's Rocket

by Dr. Franklin Chang-Diaz

Principle

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.

A Family Tree for Rocket Propulsion

The mechanism 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.

Rocket Fundamentals

The thrust 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.

Rocket Engineering

Witnessing 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."

Limitations 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.

Plasma Rockets

High exhaust 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.