Usages of electric propulsion versus chemical engines

Question

Both electric propulsion systems and chemical engines have been used in the history of space travel. What are the specific areas where one would have an advantage over the other?

Basic Principles of Each Type of Engine:

Chemical engines:

• Combustion engines have been used more widely throughout the history of space travel.
• Chemical engines have a much higher thrust-to-weight ratio than electric propulsion systems.

Electric Propulsion engines:

• Electric thrusters typically use much less propellant than chemical rockets.

• Electric propulsion provide little thrust, but over a long time.

• An electrically powered spacecraft propulsion system uses electrical energy to change accelerate propellant to high speeds.

This research shows that electric propulsion would be ideal for long duration space flight, but be almost completely ineffective for reaching escape velocity of a planetary body. And on the flip side, that chemical engines would be suitable for reaching escape velocity, but inefficient for long duration space travel due to the amount of fuel consumed by the thrusters. Am I correct in my assumptions?

Are their any notable missions that used one distinct kind or the other?

Answer 1

Your question really contains most of the answer.

As you note, electrical propulsion is unusable for liftoff from large bodies; the engines have far less than 1:1 thrust-to-weight ratios (in Earth gravity) by themselves, let alone with the added weight of propellant, payload, and power systems.

Once you've gotten into orbit on chemical rockets, though, the thrust becomes somewhat less important. Time becomes the next major constraint. Radiation exposure is a significant constraint on any crewed mission beyond low Earth orbit; so are life support consumables. Therefore, chemical rockets will remain the appropriate choice for those missions.

Flyby missions such as New Horizons don't need to carry significant propulsion of their own -- just small thrusters for minor course corrections -- so the mass of a power system for an electric thruster isn't justified. If you need to enter orbit around a distant planet, though, electric propulsion is very attractive because of its specific impulse advantage.

Regarding notable missions:

There are a bunch of satellites using ion engines or Hall thrusters for orbit correction. Relatively few interplanetary missions so far have used them: Deep Space 1, Dawn, and Hayabusa 2 are I think the most significant. Some information on those thrusters is collected here.

Answer 2

Once you're in space (let's say at least as far as low-Earth orbit), if you need something like 2 to 3 km/s mission $\Delta V$, then chemical propulsion is probably the cheapest approach. If you need a fair bit more than that, and you can expend it in the inner solar system with ample sunlight, then solar electric propulsion will be the cheapest. (This may even be true for missions with time constraints, e.g. with humans -- you may just need to have very high power, i.e. lots of solar array area, to support sufficient thrust.) If you need lots of $\Delta V$ and you need it in the outer solar system, then you will likely need nuclear electric propulsion, but we have no space nuclear power sources big enough for that at this time.

Answer 3

Further to the previous answers, it is helpful in early system studies to look at the mass trade-off for various chemical and electric systems.

• The hardware mass for an electric system grows with specific impulse because the power source scales with the propulsive beam power.
• This is offset more readily by propellant savings for missions needing a large delta V
• This means that the most appropriate choice (in terms of minimum total system mass) of electric specific impulse grows with delta V

e.g.

• ion thrusters are a better match for large delta V,
• hall effect thrusters and arc jets match lower delta V missions and fit in between ion thrusters and chemical thrusters.

The practical choice is often swayed by other system considerations such as the thrust, total impulse, propulsion needs for other functions such as attitude control, storage volume, presence of solar arrays for other purposes