Electric (Ion) Propulsion
Electric propulsion works by using electrical energy to accelerate a propellant to much higher velocities than is possible using chemical reactions. The most common propellant used in ion engines is xenon. Early ion engines used mercury and cesium, but they proved hard to work with. At room temperature, mercury is liquid and cesium is solid; they both must be heated to turn them into gases. Also, as the mercury or cesium exhaust cooled, many of their atoms would condense on the exterior of the spacecraft, contaminating solar cells and instruments. Eventually researchers turned to xenon as a cleaner, simpler fuel for ion engines. (DeFelice, 1999a)
While current ion propulsion systems use solar panels to create the required electricity, there are plans for nuclear-powered ion systems. These would be of great use in exploring the outer solar system, where solar power is no longer efficient.
The development of ion thrusters began in the 1960s. A U.S. Air Force satellite was equipped with an ion thruster in 1979. The first commercial satellite equipped with a xenon ion engine was launched in 1997. It uses the thruster to maintain the satellite in its proper orbit and orientation.
As stated above, ion propulsion involves ionizing a gas to
propel a craft. Instead of a spacecraft being propelled with standard chemicals,
the gas xenon is given an electrical charge, or ionized. It is then electrically
accelerated to a speed of about 30 km/second. When xenon ions are emitted at
such high speed as exhaust from a spacecraft, they push the spacecraft in the
NASA's Deep Space 1 probe is testing a new type of ion thruster, shown in the schematic above. This thruster is quite different from the thrusters found on satellites, in that is it being used as the spacecraft's primary propulsion system, rather than just a station-keeping device. DS-1 is also the first spacecraft to use continuous thrust. Rather than requiring a very high acceleration for a short period of time (as is normal in chemical propulsion), DS-1's ion drive was operated in a continuous mode for weeks and months at a time. Although the trust is very low, by accelerating steadily for long periods of time the spacecraft was able to reach very high velocities. The following description of DS-1's ion thrusters is from the official DS-1 Web site:
Its ion propulsion system (IPS) uses a hollow cathode to produce electrons, used to ionize xenon. The Xe+ is electrostatically accelerated through a potential of up to 1280 V and emitted from the 30-cm thruster through a molybdenum grid. A separate electron beam is emitted to produce a neutral plasma beam. The power-processing unit (PPU) of the IPS can accept as much as 2.5 kW, corresponding to a peak thruster operating power of 2.3 kW and a thrust of 92 mN. Throttling is achieved by balancing thruster and Xe feed system parameters at lower power levels, and at the lowest thruster power, 500 W, the thrust is 20 mN. The specific impulse decreases from 3100 s at high power to 1900 s at the minimum throttle level. (DeFelice, 1999b)
Although very efficient, an IPS does require a significant power source. Solar-powered IPSs are fine for small spacecraft such as satellites and probes, but if humans want to use this technology to explore the Solar System, power systems with greater output capacities will be required.
One possibility for increasing the power available to an IPS would be to use a remote power source, essentially leaving your power generator at home, where it can be repaired or upgraded, if needed. The remote power source, whether it is the Sun or a ground- or space-based transmitter, transfers power to the spacecraft via a beam of electromagnetic radiation, in either near-visible or microwave wavelengths, using a laser or maser, respectively.
Once the energy reaches the spacecraft, it must be converted into electricity. This is accomplished by using some sort of conversion technology. A few possibilities include:
● Photovoltaic cells - convert sunlight or laser energy
● Rectenna - used to convert microwave (maser) energy
● Heat Engine - uses incoming energy to power electrical generator
All beamed-energy systems can used for orbit-to-orbit in-space applications; however, only the laser or microwave systems have sufficient power density to allow their use as Earth-to-orbit (ETO) launch systems. Interestingly, the beam power requirements for the beamed laser/microwave in-space systems are quite modest (typically 0.1 to 10 MW). By contrast, ETO launch systems require very large powers (on the order of 0.1 MW of beam power per kilogram of vehicle mass). (Frisbee, 2000c)
Hybrid Nuclear-Thermal / Nuclear-Electric Propulsion
The last ISP that we will discuss is actually a hybrid
method, combining a nuclear-thermal rocket (NERVA-derivative) with an ISP.
This might be an excellent compromise for human exploration of the Solar
This hybrid system would use the NERVA-type engine for
maneuvers in a high-gravity field, where its high thrust-to-weight (T/W) ratio
would minimize gravity losses and trip time.
Once outside of a plant's gravity well, the system uses the nuclear
reactor to produce electricity for an ISP that is well suited for interplanetary
transfers, due to its low T/W ratio and high Isp.
The mission benefits of this approach are highly mission dependent, because there is a trade-off between the high T/W (e.g., vehicle T/W>0.1) and relatively low Isp (e.g., 800-1000 lbf-s/lbm) of the NTR mode, and the low T/W (e.g., vehicle T/W<10-3) and relatively high Isp (e.g., 2000-5000 lbf-s/lbm) of the NEP mode. (Frisbee, 2000e)