EM propulsion

Electromagnetic propulsion

  1. Introduction

Basic principle of electromagnetic (EM) propulsion is to generate ionized mater and by a suitable architecture of electromagnetic field produce collimated jet that produces force on the space craft in the opposite direction. Outside source of energy is used to ionize mater and accelerate it to a certain velocity. As opposed to this mechanism is chemical propulsion where the basic mechanism is transforming thermal energy that is produced by chemical reaction of two reactants into the translation energy of the products. The essential difference between those two types of propulsions is that whilst the EM essentially depends on the outside source of energy the chemical is limited by the energy released in reaction. Since the thrust force is proportional to the velocity of the exiting stream of matter there is an upper limit to it in chemical propulsion whilst in the EM depends only on the abundance of energy source. This has great consequence on deep space travel because the ratio of the payload to fuel is of essential importance. In order to see this more clearly one derives relationship

where v is the sum of all changes of the space craft velocities that is required from getting from A to B and V is velocity of  the jet. By fuel is meant both oxidizer and fuel per se. Therefore, how much payload is transported for a given mass of fuel depends only on the ration of the two velocities, and not the mass flow rate at the exhaust.  The mass flow rate enters in two important parameters: time to attain terminal velocity

and the maximal acceleration when the fuel is exhausted

mt is mass flow rate.

  1. Power requirement for propulsion

Minimum power that is required for propulsion is estimated from the kinetic energy of the mass of mater that flows out of rocket engine. Thus if the mass flow rate is 1 kilogram per second then the minimum power is V2/2.  For most powerful chemical propulsion engine that is based on the mixture of Hydrogen and Oxygen the exit velocity of jet is V=4400 m/s  and the minimum power is 9.68 MW. Burning of the two reactants produces around 14.4 MW which is sufficient for propulsion, the excess cannot be used because it is dissipated into environment and heating up of fuel. On the other hand in the EM propulsion exit velocity could be anything and for V=100 000 m/s the minimum power is 5 GW. However, here it is expected that additional power is needed for ionizing mater.

  1. Ionization

EM propellant fuel comes in the form of neutral atoms or molecules that are ionized to be accelerated by the electromagnetic field. The main criterion for choosing a fuel type is its ionization energy; should be as small as possible. It is equally important that fuel is abundant but this criterion is not taken into account here. Ionization is very energy-intensive if a large amount of ions is required, such as for space travel. For a type of fuel that has 1eV the ionization energy of one electron (the unit used for atoms and molecules), the required power to produce 1 kg of substance per second is 96.5 MW / M (M is the molar / atomic mass). The ionization energy of the atom is in the figure

whilst for some two-atom molecules they are listed here. The energy required to ionize 1 kg of matter could be reduced by ionizing particles with higher mass. Although less energy is required for ionization, it is necessary to invest more energy to accelerate them to a certain speed, since it is a priority to maximize the payload at the expense of fuel. The power required to drive, the time to reach the ultimate speed and the ultimate acceleration (if humans are part of the payload) are all other factors to consider when choosing the right fuel.

There are many mechanisms for the production of ions, but most of them are geared toward minimal quantities compared to what is required for the efficient mobility of spacecraft. What is needed is the production of kilograms of ions, but in addition to power, it is also necessary to control high current electric currents. The removal of electrons from the fuel involves a current of 96 500 A / M per kilogram of ionized matter.

  1. Power source

The only candidate as a source of energy that could supply energy to EM, other than solar energy for micro thrust systems, which satellites use to orbit correction, is nuclear energy. The candidates are a fission chain of uranium, plutonium or thorium with an energy density of about 80,000 G Joule / kg. This is quite sufficient for even very demanding propulsion systems. However, there is a problem of converting the kinetic energy of the fission fragments into electricity, and so far no very effective mechanism has been found for this, which would also take into account the need for the least demanding space and mass. Thorium is a very promising source of power, but the technology to harness its energy content is not yet sufficiently developed for widespread use.

The only candidate as the source of energy that could supply the power for EM propulsion, except solar power for micro thrust propulsion systems used by satellites for orbit corrections, is nuclear power. The candidates are Uranium, Plutonium or Thorium fission chain with the energy density of about 80 000 G Joule/kg. This is well sufficient for even very demanding propulsion systems. There is, however, a problem of conversion of kinetic energy of fission fragments  into electricity and so far not a very efficient mechanism was found for this that would also take into account the need to be the least demanding on space and mass. Thorium is a very promising source of power but the technology for harnessing its energy content is not yet sufficiently developed for use on large scale.