We have seen the use of fission reaction to produce thrust and propel rockets. Let us now see the use of fusion reactions to produce thrust to propel rockets.
In nuclear fusion, lighter nuclei (lighter than iron) fuse to form a heavier nucleus. In this process they release energy. This energy is used to heat the propellants forming gaseous products providing thrust to the rocket.
1H2 + 1H2 ® 2He4 + Energy
There is a difference in mass between the two original lightweight nuclei and single product nucleus. Here also, the energy is liberated according to mass defect concept explained in nuclear fission. This results in release of tremendous amount of energy. This energy is used to heat propellants, which form gaseous products. These gases are forced out through nozzle producing thrust.
• A fusion propulsion rocket will have much greater specific impulse. A chemical rocket engine has a specific impulse of about 450 seconds. A fusion rocket could have an estimated specific impulse of 130,000 seconds.
This large specific impulse would greatly increase the speed of rockets making the missions faster (it could cut down the number of days by about 50%) and make manned missions easier.
• Fusion-powered rockets would use hydrogen as a propellant. Hydrogen is present in the atmosphere of many planets. So as the spacecraft travels through space it could replenish its supply of hydrogen from any planet's atmosphere.
This propulsion technique is currently not being used, as it requires very high temperatures to initiate the fission process.
Electric propulsion is the next propulsion technique we are going to see..