Space Travel 101: Fundamentals of Space Travel
Overview
Propulsion systems powered by chemical reactions are quite inefficient in terms of their specific impulse. Scientists are developing new, more efficient systems which would be useful in long-term missions.

Analysis

Sections
1. Improving Efficiency
2. Ion Thruster
3. Variable Specific Impulse Magnetoplasma Rocket (VASIMR)



IMPROVING EFFICIENCY

For a basic action-reaction propulsion system, the initial mass (before using the system) M0, the final mass (after using the system) M, the final velocity v, and the velocity of the system's propellant vp can be related to each other using the equation:

M0 = M Exp(v/vp)
where Exp(x) is the exponential function ex and e is approximately equal to 2.718. For a spaceship, you can think of M as being the mass of the spaceship without fuel (i.e. the ship itself, the payload, etc.), and M0 - M is how much fuel you'll need to obtain a maximum velocity of v if the propulsion system being used shoots out the fuel at a velocity of vp. Although the mass of the ship without fuel plays a significant role in determining how much fuel is required, the ratio v/vp is far more important (compare doubling M and doubling v/vp). So the problem of reducing the amount of fuel required comes down to reducing v/vp, which requires making vp as large as possible. Although chemical propulsion systems provide a large amount of thrust, the propellant gases only reach a few kilometers per second, which translates to relatively low specific impulses. Scientists are currently researching new technologies that propel gases to much greater velocities, resulting in high specific impulses.
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ION THRUSTER
Specific Impulse: 1500-8000 sec
Thrust: 10-3-10 N
Atoms are made up of three types of basic particles: protons, neutrons, and electrons. The protons and neutrons are bunched together in the nucleus of the atom, while the electrons orbit around the nucleus. Two of these particles, protons and electrons, have an electric charge. Electric charges are what cause your hair to stand up when you rub it with a balloon. There are two types of changes, positive and negative, which act like the north and south poles of magnets. Objects with the same charge repel each other and objects with different charges attract each other. In atoms, protons are positive, and electrons are negative. In a neutral atom, the number of protons and electrons are equal, canceling each other out. However, since the electrons orbit the atom, they can be stripped away, throwing off the balance between protons and electrons. Since there are more protons than electons, the atom becomes positively charged. It's also possible for an atom to pick up extra electons, giving it a negative charge. When an atom becomes charged, it's known as an ion.

If you have a bunch of ions with the same charge, they can be controlled using an electric field. The ion thruster is based on this idea. A gas, such as xenon, which has a high atomic mass and is easily ionized, is injected into one end of a chamber where it is bombarded by electrons. The bombardment knocks off electrons from the xenon, ionizing it. The other end of the chamber is covered by a charged grid which attracts the ionized gas, accelerating it to speeds much higher than in chemical rockets. After the gas exits the engine, it's combined with a cloud of electrons, which neutralizes the gas to prevent a build up of charge.

Ion thrusters, like the one developed for NASA's Deep Space 1 probe, can accelerate gases up to ten times the speed of gases from chemical rockets. The resulting high specific impulses make these systems ideal for long term, distant missions. However, ion thrusters have some drawbacks. The thrust produced is very low (about equivalent to the force of a piece of paper on your hand), which means the acceleration is very low. Luckily, since there's no friction in space, this small acceleration, when continued for a long period of time, can result in great velocities.
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VARIABLE SPECIFIC IMPULSE MAGNETOPLASMA ROCKET (VASIMR)
Specific Impulse: 1000-30000 sec
Thrust: 40-1200 N
When you heat a solid to a high enough temperature, it eventually becomes a liquid then a gas. What happens when you continue to heat a gas? Eventually, the atoms in the gas lose their electrons and the gas becomes the fourth state of matter, plasma. Plasma is basically a gaseous mixture of ions and electrons. Plasma is actually the most common start of matter in the universe and is found in lightning and stars. Plasma has the special property that in can be influenced by a magnetic field. The VASIMR uses hydrogen in the plasma state as a propellant. Its unique design allows it to exchange specific impulse for thrust while maintaining a constant power requirement (in other similar systems, increasing thrust requires an increase in power). This feature sets itself apart from other propulsion systems because it can provide high specific impulse, high thrusts, or something in between.


The VASIMR is broken up into three parts: two chambers and a magnetic nozzle. In the first chamber, hydrogen gas is injected and heated to the plasma state using radio waves. The plasma is then sent to the second chamber where it becomes super heated by radio waves and magnets to a few million Kelvins (temperature in Kelvins = temperature in Celcius + 273). The super-heated plasma is then directed out of the engine by the magnetic nozzle. The magnetic nozzle has the ability to adjust how much exhaust is let out. Letting out large amounts of exhaust creates greater thrust. However, since the plasma spends less time in the second chamber, it is not heated as much, and so its velocity and specific impulse are lower. On the other hand, if less exhaust is let out, less thrust is produced, but the plasma is heated more, resulting in higher specific impulse.
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