Plans and programs are being formulated with the goal to ensure the nation is pursuing the best trajectory for the future of human spaceflight — one that is safe, innovative, affordable and sustainable. The pursuit of that goal requires a vision of what human spaceflight should be. Whatever that vision be, the pursuit of that goal with evolved transportation systems — evolved 20th century technologies — would be extremely difficult. Instead, it would take a combination of evolutionary and revolutionary space transportation systems to achieve that goal. The latter systems would use revolutionary propulsion systems; they would be reusable; and they would manifest aircraft-like operations for Earth to low Earth orbit (LEO) transportation. These are the key innovations necessary for safe, affordable and sustainable human spaceflight.

A vision of human spaceflight could be formed with evolved transportation systems. Humans could conduct missions beyond LEO assisted with a new expendable launcher, an Evolved Expendable Launch Vehicle (EELV)-derived launcher, or a shuttle-derived launcher to reach LEO. Humans could land on the Moon again with “Apollo on steroids.” Space entrepreneurs could provide transportation capabilities to LEO and develop space tourism. The space industry could develop two-stage-to-orbit (TSTO) launch vehicles with rocket engines on both stages to provide some degree of operationally responsive spacelift. Such transportation systems would neither be affordable nor sustainable.

The vision of human spaceflight should be to have humans ultimately conduct activities in space routinely. Space must, therefore, be accessible. Accessible space is also assured space access. Accessible space necessitates revolutionary space transportation capabilities and appropriate infrastructures on Earth and in space to make human spaceflight safe, affordable and sustainable. The vision for space access should be to make most space launches almost as ordinary as an airplane taking off from a runway.

The history of transportation teaches us that the country that excels in transportation does well economically, in national security and in exploration. Progress in transportation has been made only by revolutionary changes in modes of propulsion. The development of the Western United States in the 19th century was made possible when steam engine-driven train systems replaced horse-driven Conestoga wagons; and transportation with aircraft in the 20th century changed the aviation paradigm when jet-powered aircraft replaced propeller-driven aircraft. Likewise, revolutionary transportation systems are essential for further development and exploration of space.

The “Sputnik” moment simply was a catalyst to help formulate a set of fundamental factors. These factors were prestige and confidence in the U.S. scientific, technological, industrial and military systems; the instinctive need to explore and discover; scientific observation and experimentation to enhance knowledge of the Earth, solar system and universe; and national defense. These factors are eternal. In the 21st century, revolutionary space transportation systems are needed to continue the pursuit of most of these factors with human spaceflight.

The trajectory for the future of human spaceflight would be best only if it is based on transportation systems manifesting the following figures of merit: affordability and reliability for exploration, security and commerce; safety for humans and critical cargo; operational responsiveness for emergency human spaceflights and security; and sustainability for human exploration.

A paradigm change in space transportation can be made, only if existing chemical rocket engines are replaced by revolutionary modes of propulsion. Nuclear-thermal propulsion and air-breathing propulsion are such modes for human missions beyond LEO and for hypersonic flight in Earth’s atmosphere on the way to LEO, respectively.

Nuclear-thermal propulsion has the potential to significantly shorten in-space travel times for human missions to Mars, asteroids in Earth’s neighborhood, or to deep space destinations. It could be used for transportation between the surface of a planet, a moon or an asteroid and a parking orbit around it. A nuclear-thermal propulsion system could be converted to generate power on the surface. Human missions to the Moon also could be conducted with nuclear-thermal propulsion in preparation for human missions to other distant destinations.

A significantly improved access for humans to LEO is a foundation for building the best trajectory for the future of human spaceflight. Launch vehicles assisted by hypersonic air-breathing propulsion have the potential to meet the aforementioned figures of merit for access to low Earth orbit, thereby transforming the space launch industry. A way to build that foundation would be the development of a reusable TSTO launch vehicle with air-breathing engines on the first stage booster and with chemical rocket engines or rocket-based combined cycle engines on the second stage. Such vehicles would conduct aircraft-like operations.

The proposed TSTO vehicles would replace the space shuttle, transfer crew to the Crew Exploration Vehicle, supplant a majority of existing operational expendable launch vehicles and would be used to deliver medium-weight components of exploration spacecraft to LEO where they would be assembled. The proposed vehicles also would be used to establish and maintain infrastructures in LEO.

In the near term, the trajectory of the future human spaceflight would include the retirement of the space shuttle after currently available external tanks are used. An evolved crew or cargo transportation vehicle is developed as soon as possible. Its use is discontinued after a revolutionary system is developed. The international space station is used until it is able to provide significant scientific dividend at an affordable cost. The development of revolutionary transportation systems and infrastructures is initiated. Depending on how and where such a transportation system is assembled in LEO for human missions beyond LEO, an expendable heavy-lifter with chemical rocket engines is developed.

In the long term, human spaceflights beyond LEO are internationalized and conducted by governments. Two independent revolutionary space transportation systems are developed for human missions to LEO. Only revolutionary in-space transportation systems are used. Infrastructures on Earth, in LEO and other locations in Earth’s neighborhood are setup. The space industry commercializes LEO and assists in maintaining infrastructures for human spaceflights beyond LEO.

The trajectory of the future of human spaceflight ought to assure leadership in innovation and technology for spaceflight. Revolutionary propulsion modes, reusable vehicles and aircraft-like operations for Earth to LEO transportation would change space transportation paradigm, open up space beyond imagination, and provide leadership in space transportation. National will and excellence in leadership are essential to execute the developed human spaceflight trajectory.


Unmeel Mehta, Ph.D., is an associate fellow at American Institute of Aeronautics and Astronautics.