Propulsion

The whole system for the propulsion is composed of two parts: the launch vehicles (it usually is the chemical rocket) and the propulsion system of the spacecraft. Launch vehicles provide the energy for the spacecraft to launch leaving the earth. Once the spacecraft escapes from the earth, it uses engine on it for the propulsion and the gravity assist to travel out the solar system and heading toward the Epsilon Eridani. The basic idea of rocket engines work is pushing propellant away from the spacecraft. The action of the propellant leaving the engine causes a reaction that pushes the spacecraft in the opposite direction. It is based on Newton’s 3rd law. All current spacecraft use chemical rockets for launch.

Launch Vehicle

We choose Delta IV as our launch vehicle to encapsulate the satellite in it. The first stage of a Delta IV consists of one, or in the Heavy variety three, Common Booster Cores (CBC) powered by a RocketdyneRS-68 engine. Unlike most[citation needed] first-stage rocket engines, which use solid fuel or kerosene, the RS-68 engines burn liquid hydrogen and liquid oxygen.

Typically, the RS-68 runs at 102% rated thrust for the first few minutes of flight, and then throttles down to 58% rated thrust before main engine cutoff. On the Heavy variant, the core CBC's engine throttles down to 58% rated thrust around 50 seconds after liftoff, while the strap-on CBCs remain at 102%. This allows the core CBC to conserve propellant and burn longer. After the strap-on CBCs separate, the core CBC's engine throttles back up to 102% before throttling back down to 58% prior to main engine cutoff.

Source: http://upload.wikimedia.org/wikipedia/commons/d/d5/Delta_rocket_evolution.png

The RS-68 engine is mounted to the lower thrust structure of the vehicle by a four-legged (quadrapod) thrust frame, and enclosed in a protective composite conical thermal shield. Above the thrust structure is an aluminum isogrid (a grid pattern machined out of the inside of the tank to reduce weight) liquid hydrogen tank, followed by a composite cylinder called the centerbody, an aluminum isogrid liquid oxygen tank, and a forward skirt. Along the back of the CBC is a cable tunnel to hold electrical and signal lines, and a tube to carry the liquid oxygen to the RS-68 from the tank. The CBC is of a constant, 5-meter, diameter.

The L-3 Communications Redundant Inertial Flight Control Assembly (RIFCA) guidance system used on the Delta IV is common to that carried on the Delta II, although the software is different because of the differences between the Delta II and Delta IV. The RIFCA features six ring laser gyroscopes and accelerometers each, to provide a higher degree of reliability.

The upper stage of the Delta IV is nearly identical to that of the Delta III, however the tanks are friction stir welded and either stretched (in 4-meter variants), or have a larger diameter (5-meter variants). The second stage is powered by a Pratt & Whitney RL-10B2 engine, which features an extendable carbon-carbon nozzle to improve specific impulse. Depending on variant, two different interstages are used to mate the first and second stages. A tapering interstage which narrows down from 5-meters to 4-meters in diameter is used on 4-meter variants, where a cylindrical interstage is used on 5-meter variants. Both interstages are built from composites.

Ion Engine

We uses nuclear reactor as the power source of the spacecraft and the ion engine as its propulsion system. To date, ion engine is one of the best engine for deep space travel. It uses the same theorem as the rocket propulsion, but having higher efficiency. The principle of the ion engine is that the gas xenon flows into the ion engine, where it is given an electrical charge. As soon as the xenon atoms become xenon ions, they can be pushed around by an electrical voltage. A pair of grids in the ion engine, electrified to almost 1300 volts, accelerates the ions to very high speed and shoots them out of the engine. As the ions race away from the engine, they push back on the spacecraft, propelling it in the opposite direction. By carfully modifiyng the direction of the ion thrust, the spcaecraft could enter the destination orbit of the Epsilon Eridani correctly.

Source: http://www.nasa.gov/centers/glenn/images/content/102690main_ion_thruster_diag_516x366.jpg

The xenon ions travel at about 77,000 mph. This is about 10 times faster than the exhaust from conventional rocket engines, so the xenon gives about 10 times as much of a push to the spacecraft as chemical propellants do. It takes only one tenth as much propellant for an ion engine to work as it does for a chemical propulsion system. However, that means that the thrust is very low. Current ion thrusters can provide only 0.5 newtons (or 0.1 pounds) of thrust, which is equivalent to the force you would feel by holding 10 U.S. quarters in your hand.

Since we are dealing with the deep space traveling, the most important feature of the ion engine is its long life time. Unlike chemical engines, which can be operated for minutes, or in extreme cases, for an hour or so, ion engines can be operated for years. The effect of the gentle thrust slowly builds up, eventually attaining speeds far beyond the reach of conventional propellants.