Future Mobility

Beyond Low Earth Orbit: Transportation Technologies for Getting Around the Solar System

Some companies are working on different concepts and strategies to reduce the cost of sending material into low Earth orbit, or LEO, but for destinations beyond, there remains an essential problem of propellant mass that these technologies do not address: that is, even if the rocket is reusable, if all the propellant needed for the entire journey is taken along from Earth, there is a tremendous disadvantage, because most (for a round trip to Mars, more than 99 percent) of what is lifted into orbit is propellant. Is this a fundamental limitation?

Fortunately, no. One way to solve it is to produce fuels elsewhere than on Earth. A lot of people are thinking about this now, and some have even formed businesses based on selling water from the Moon to make hydrogen and oxygen. This makes a lot of sense, but there are alternatives to making fuel in space that hold even greater long-term promise, and those are advanced propulsion technologies that could greatly reduce mass and cost.

Is electric propulsion a feasible solution to get spacecraft into orbit?

In this approach, like the beamed microwave propulsion system discussed in the previous article, most of the energy of a propulsion system is provided in the form of electricity, by something other than the chemical energy in the propellant. That energy can come from a compact nuclear power source, solar energy or beamed from Earth.

This energy is used to heat (or in some cases, ionize and accelerate) a much smaller amount of propellant than in chemical propulsion, achieving higher exhaust speeds. The drawback is that the thrust, which produces acceleration, is much lower, so it takes longer to achieve the desired change in velocity. (In a chemical propulsion system, the velocity change is accomplished with short “burns” of only a few minutes each. With electric propulsion, “burns” can take days, weeks or even months.)

This is why this approach is not useful to get spacecraft into orbit from the ground: the thrust is just too low, but once in space, one can in principle take as long as desired, though to be a viable transportation strategy, it should not make the journey itself much longer that it already needs to be. (For journeys to the Moon, this is about three days; for journeys to Mars, it can be six months or more.)

Electric propulsion is already in use in some small, robotic spacecraft, but research is underway to increase thrust so that the technology is practical for larger spacecraft.

Are solar sail spacecraft cost-effective?

This technology is akin to sailing using winds on earth, but literally uses the solar “wind” (streams of high-energy particles) or its light energy to push an enormous sail in space. Like electric propulsion, it is a very low thrust technology, but does not require any fuel at all, so can in principle be very low mass, and thus cost-effective.

Space experiments with solar sails have begun, but the technology is not yet mature. A variant on the solar sail uses laser power beamed from Earth, and is in principle much higher thrust than solar-based technology, but also more costly because producing the energy can be expensive.

Conventional propellants vs very high-energy density fuels

It turns out that there are higher energy density chemical fuels than conventional propellants: individual atoms of hydrogen, boron or carbon, or metastable nitrogen compounds, trapped in a cryogenic matrix of solid hydrogen or noble gases to prevent bonding. At high densities (greater than 10 percent by mass), such fuels could be used to make powerful engines at a fraction of the mass of the best conventional propellants (for example, liquid hydrogen and oxygen). Currently, such concepts are still in the research stage because needed densities are not high enough.

Likewise, the use of fission fragments from a nuclear reactor directly as propellant guided with a magnetic nozzle (rather than first converting it into thermal energy to heat a propellant as described above under “electric propulsion”) could lead to vastly more efficient engines. Numerous studies on fission fragment concepts have been conducted since the 1980s, but the technology requires materials that can endure extremely high temperatures, radiation, and exposure to hydrogen.

A number of other fascinating technologies are discussed in the 2015 NASA technology roadmaps, including, among other things, fusion and antimatter propulsion, but these technologies are also very far from a practical device stage. Nonetheless, any of these technologies might become a reality with continued research, with vast implications for cost reduction and higher-speed space travel.

Could space elevators be a solution?

While not yet technically feasible on Earth, a space elevator could be a “one-stop shop” solution for placing objects anywhere from low Earth to interplanetary orbits, with minimal energy requirements. By simply climbing a cable that extends from the ground to more than 50,000 miles into space (held in place by the balance of gravity and centrifugal force) and detaching at different altitudes, a wide range of speeds, and hence orbits, is achievable: the higher one climbs, the higher one’s speed, including velocities that can reach the outer solar system.

In the lower gravity of the Moon and Mars, companies may even have the necessary technology to build such a system today, so this concept may become a reality there first.

The impact of new innovations in the field of space transportation

The impact of these technologies could greatly lower the cost of sending spacecraft beyond LEO, ushering in business opportunities that are not even conceivable today.

The importance of any of them to the commercial development of space cannot be understated, and could revolutionize not only commerce both in space and on Earth, but also fundamentally change people’s relationship with space, if off-Earth travel becomes an everyday reality.

What is your reaction to these developments? Share your thoughts in the comment section.

Please note that this article expresses the opinions of the author and does not reflect the views of Move Forward.


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