Section 4.7 - Electric Propulsion
Electric propulsion typically has about ten times the fuel efficiency of chemical rockets. Thus they turn an exponential fuel requirement (fuel to push more fuel) into a nearly linear one for most Solar System missions. The timing of this step would be in parallel or soon after Orbital Assembly is started.
Near Term Electric Thruster Types
There are several kinds of electric thrusters that are good candidates for near term use. The selection here is based on state of development and usefulness:
- 49 Electrostatic Ion - This knocks electrons off of gas atoms making them charged, which is called "ionized". Once charged, they can be accelerated by metal screens with a large voltage difference. Ion thrusters are used on some communications satellites, and the Dawn spacecraft currently exploring the asteroids Vesta and Ceres.
- 51 Microwave Heated Plasma - This type uses microwave frequency heaters to heat the fuel. This is the same principle as a microwave oven, but much more intense. Above a certain temperature the heated atoms in the fuel will knock electrons off each other, turning it into a mixture of ions and electrons, which is called a Plasma. The plasma is contained and directed by magnetic fields. You need to do that because plasma is so hot it will melt anything it contacts, or cool itself down too much. In fact on Earth plasma is used as an efficient way to cut through metal. A version of this thruster is currently being developed on the ground, and will soon fly for testing on the Space Station. It's full name is Variable Specific Impulse Magnetoplasma Rocket, which is mercifully abbreviated to VASIMR. As a category they are called plasma thrusters.
- 72 Ionospheric Current - This operates like a motor, running a current in a wire in a magnetic field. The return path for the current is the ionosphere. This method is limited to places with suitable magnetic field and ionosphere density, but low Earth orbit fortunately is such a location. The attraction is it does not require direct fuel use, only a little leaked plasma to make electrical contact with the ionosphere. The equivalent exhaust velocity as if it were a fuel-using engine is 250 km/s. Since low orbit is the first place we want to use, developing this type of thruster is a high priority. Note it is not as fully developed as the other types.
Electric vs Chemical Thrusters
All rockets work by tossing mass in one direction, and by Newton’s Law (for every action, there is an equal and opposite reaction), the rest of the rocket gets pushed in the other direction. The faster you toss the mass, the more push (momentum) you get out of it. Conventional rockets burn fuel in a chamber then let it expand out a supersonic nozzle to get it going as fast as possible. The shape of the nozzle is governed by the physics of expanding gases, which is why they all look more or less the same. How fast it can get is limited by how hot the gas is and it's molecular weight. The best combination used today is burning Hydrogen and Oxygen in a ratio of 1:6 by weight. This produces mostly steam with a bit of Hydrogen left over to lower the average molecular weight. How fast the gas is going is technically called exhaust velocity, and is limited to about 4.5 km/s for this fuel type.
Electric thrusters are not limited by the energy produced by burning the fuel. They feed energy to the fuel from an external source, thus can get much higher exhaust velocity. This gives you more push from given amount of fuel. Since you have a finite amount of fuel to use this is more efficient in direct proportion to the increase in exhaust velocity. By analogy to automobiles, you are getting better "gas mileage".
The extremely high fuel efficiency is the key to why this type of thruster is important. If you are doing a lot of moving about in space the fuel savings outweigh (literally) the mass and cost of the power supply by a large margin. Conventional rockets only need a fairly lightweight fuel tank, but burn a lot more fuel. One drawback to electric thrusters when transporting humans is their relatively low thrust. This makes the trip times longer. There are various ways to work around that drawback. For example, passage through the Earth's radiation belts slowly would be unacceptable radiation exposure. So you can transport your main vehicle by electric thruster, taking weeks, and then deliver a crew in a small capsule taking hours once the main vehicle is outside the radiation belts.
Comparisons Between Types
All electric thruster types need an external power supply since the fuel is not self-heating as in chemical engines. The most common power supply used in space are photovoltaic panels. Those can get unwieldy at power levels of hundreds of kW or more, and their power output per area drops as the inverse square of distance from the Sun. So for some past and future missions a nuclear power source is preferred. Smaller size nuclear generators are based on isotope decay, and larger ones are full nuclear reactors. Any type of nuclear device brings both technical and political complications.
Electric thrusters cannot be used directly for launch or landing on large objects, because their thrust-to-mass ratio is significantly less than the local gravity acceleration. They can be used indirectly via space cable/elevator type systems. Chemical engines can reach vehicle thrust-to-mass ratios well above Earth gravity, which is one reason they have been the primary way to launch things to date.
Both Ion and Microwave Plasma thrusters have exhaust velocities in the range of 20 to 50 km/s, so are 4 to 10 times more fuel efficient than conventional rockets. Like electric devices on Earth, they are rated by how much power they use. The Dawn spacecraft has a 10 kW set of solar panels, and the VASIMR thruster in development is rated at 200 kW. Generally ion thrusters will maintain efficiency at lower power levels than plasma type thrusters because ion flow does not have to be restrained by containment fields, while plasma requires a field to keep it separated from the solid hardware. At small sizes the plasma volume vs total engine volume becomes small and efficiency drops.
For efficiency reasons, ion thrusters prefer high atomic weight fuels. The energy to ionize an atom is roughly constant across the Periodic Table, but does not contribute to thrust in this engine type. Thus using high weight fuels lowers the portion of total power used for ionization relative to acceleration. Typically Xenon is used as a fuel. Plasma thrusters can use most fuel types since their goal is to make the plasma extremely hot, on the order of a million degrees. By tuning the microwave generators, most atoms and molecules will absorb the energy. A key advantage of this is fuels like Oxygen or water are common in asteroids, so electric thrusters can be refueled locally, rather than having to bring all the fuel from Earth.
Electric Propulsion Applications
The following early missions can be performed starting with relatively small thrust levels, and working up to more ambitious missions.
This mission involves collecting air from the edge of the Earth's atmosphere for fuel and breathing. We start with a 50 kW solar array and a VASIMR type thruster which can generate 2 Newtons thrust at 40% efficiency and 20 km/s exhaust velocity. The solar arrays are assumed to use modern multi-layer cells with 30% efficiency, and have a power to mass ratio of 100 W/kg. The array will thus mass 500 kg, and we assume operates 30% of the time by intermittent use. The electric thruster can then produce an average thrust of 0.6 Newtons. At 200 km altitude, each square meter of collector generates 0.0129 Newtons of drag, so the total collector allowed area is 46 square meters to match the average thrust. This will collect 0.08 g/s, and the thruster consumes 0.03 g/s, leaving a net of 0.05 g/s. This amounts to 4.32 kg/day, or 3.15 times the solar array mass per year.
Later expansion would take the same thruster module to 200 kw power level and 5.7 N thrust at 50 km/s exhaust velocity and 60% operating time. The operating time is limited by the 40% of the orbit in the Earth's shadow. Thus average thrust is 3.42 N, and collection rate is 0.456 g/s. The thruster uses 0.114 g/s, leaving a net of 0.342 g/s. This is 29.5 kg/day or 10,785 kg/year, or 5.4 times the array mass per year. With a 15 year service life for the arrays, they can supply 75 times their mass in total. An electrodynamic thruster to make up for drag might improve on this even further.
For human transport, where speed is important going through the radiation belt, the collected air can be separated for Oxygen, and mixed with added Hydrogen from Earth in a chemical thruster. Alternately a lower exhaust velocity, higher thrust electric thruster could be used, sacrificing fuel efficiency for fast transit. There are several plasma and arc jet thrusters that could do that job.
Orbital Cleanup and Maintenance
Earth orbit has accumulated debris from spacecraft explosions and collisions, and there are a number of non-functional satellites which only need a single part repaired or new fuel to function again. This mission involves using a range of electric thruster vehicle sizes to collect the debris, repair or refuel satellites on location, or bring them to the orbital platform for maintenance. Debris mass ranges down to centimeter or less in size, so it would be inefficient to send a large vehicle to collect it. Alternately, satellites can range up to several tons in mass. Therefore we select electric vehicle sizes to match the size of what is being collected or moved. For debris collection, several pieces in similar orbits can be collected in one trip to minimize fuel use and mission time. The fuel for these cleanup missions comes from the atmosphere mining. Depending on what the target objects are, we perform one or more of the following tasks:
- Collect orbital debris and either deliver it to a low enough orbit that it will decay and burn up quickly, or feed the debris into a processing unit to extract useful materials from.
- Return non-working satellite hardware to the orbital platform to be salvaged for working parts.
- Repair non-working satellites at the orbital platform with salvaged or new parts.
- Repair, refuel, or attach a new propulsion unit to existing satellite at their current location.
- Transport new cargo to higher orbits.
The tasks above are approximately in order of size and difficulty. Before salvaging used satellites, you would need to get permission from their original owners. The legal regime for broken debris pieces is unclear. If they are considered a menace to navigation, they might be removed without permission, or the original owners charged for cleanup.