Helium-3 fusion

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Travelling in space requires the generation of thrust. This thrust has varying effects on the spacecraft, depedant on the mass being ejected and the velocity of the ejacta. This is basic reaction mechanics (momentum), and is defined by the following equation (Newton's Third Law):

Newton's Third Law Applied to Rocketry
mshipvship = (mship-mejecta)vship + mejectavejecta

Where v is a vector quantity. Thus, the greater the value of vejecta, the greater the increase in vship.

Since a ship is limited by its reaction mass, the best way to get increases in velocity are by somehow propelling the ejecta at faster rates. The resultant atoms and subatomic particles from nuclear fusion can be ejected at more than 10,000 Km/s, much faster than any chemical rocket ever could.

The Basic Reactions

Hydrogen is the most abundant element in the universe, and can even be found in interstellar space. Normal Hydrogen has 1 proton and no neutrons. Deuterium is an isotope of Hydrogen that has a neutron next to its proton.

Normal Helium has 2 protons and 2 neutrons in its nucleus, giving it an atomic weight of 4, thus the name Helium4.

Now, if the Helium atom loses a neutron somehow, you get Helium3. This happens once in a while in very energetic nuclear reactors. About one in every ten thousand helium atoms ejected from the sun comes out missing a neutron, thus naturally occuring Helium has 1.3 parts in a million of Helium3.

Helium3, lacking a neutron to be optimally stable will attempt to grab one if it can. At high temperatures, the nucleii of HeM3 and Deuterium will get close enough for that transfer to take place.

Nuclear Fusion

D + He3 --> p + He4 + 18.4 MeV

This is a nuclear fusion reaction. The Deuterium and Helium3 atoms come together to give off a proton and Helium4. The products weigh less than the initial components; the missing mass is converted to energy. 1 kg of Helium3 burned with 0.67 kg of deuterium gives us about 19 megawatt-years of energy output.

A reactor built to use the D-He3 reaction would be inherently safe. Helium4 is not dangerous, and the proton is highly reactive and will not remain in a free state for very long. Unlike other reactions, there are no neutron emmissions. The worst-case failure scenario would not result in any civilian fatalities or significant exposures to radiation.

Outward Bound

With this kind of power at our fingertips, the reaction mass vs. ship's velocity ratio drops to numbers that make interplanetary flight quite reasonable (and speedy!). But how would it work exactly?

The ship would have two tanks of fuel. One of Deuterium and another of Helium3. The two gasses would be introduced into a reaction chamber in the rear of the craft, where the gasses would be heated to fusion temperatures with the use of a pulse laser (and perhaps a fission based catalyst).

As the reaction progressed and the gasses expanded, they would be ejected through an opening in the reaction chamber. This opening would direct the ejecta, allowing for an optimal use of the generated thrust.

Additional thrust could be obtained by introducing a non-reactive reaction mass, such as an inert gas or captured interstellar hydrogen. This injected mass would also become plasma and expand, forcing its way out of the reaction chamber and giving additional thrust.

Way Ahead

With controlled nuclear fusion, the gateway to the planets -- perhaps even the stars -- will open and mankind will finally be able to attain his dream of exploring new worlds and expanding his realm. New societies and civilizations are possible due to the interactions of some of the smallest atoms in the periodic table.

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