Umeel B. Mehta OpEd
The fundamental goal of U.S. space exploration policy is to advance U.S. scientific, security and economic interests through a robust space exploration program.
The pursuit of this goal with evolved transportation systems would be extremely difficult, for example, for human missions to Mars, for responsive, flexible and resilient Earth-to-orbit transportation, and for space tourism.
Instead, it will take a combination of evolutionary and revolutionary space transportation systems to achieve this goal.
A viable space exploration program will require 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 security and emergency human spaceflights; and,
– sustainability for human exploration.
Meeting these figures of merit requires revolutionary space transportation capabilities.
Historically, progress in transportation has been made only by revolutionary changes in modes of propulsion. Progress in space transportation can be made only if existing chemical rocket engines are supplemented with, or replaced by, revolutionary modes of propulsion.
Revolutionary transportation systems are as essential to the development of space, just as the development of the Western United States was made possible when horse-driven Conestoga wagons were replaced by steam engine-driven train systems .
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. Likewise, nuclear propulsion has the potential to revolutionize in-space transportation for human exploration beyond low-Earth orbit .
To conduct human exploration beyond low-Earth orbit in a sustainable manner, the first principal requirement is to develop systems that will make it possible to get from Earth’s surface to low-Earth orbit in an affordable, reliable and safe manner. Low-Earth orbit is in many ways halfway to the Moon, Mars or asteroids in the Earth’s neighborhood because so much energy is required to transport people and payloads off the planet’s surface and to that orbit.
The Columbia Accident Investigation Board (CAIB) also observed, “The United States needs improved access for humans to low-Earth orbit as a foundation for whatever directions the nation’s space program takes in the future.”
The best way to solve that problem would be the development of a two-stage-to-orbit (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. That would be a revolutionary space transportation system. The booster should be reusable; the second stage could also be reusable. Reusable TSTO vehicles and hybrid two-stage vehicles could conduct aircraft-like operations.
The proposed TSTO vehicles would be substantially safer and significantly more reliable, cost-effective and operationally responsive than vertical rocket launchers. The proposed vehicles could replace the space shuttle, transfer crew to the crew exploration vehicle (CEV), supplant a majority of existing operational expendable launch vehicles and could be used to deliver components of exploration spacecraft to low-Earth orbit where they could be assembled. The proposed vehicles also could be used to establish and maintain infrastructures in low-Earth orbit .
The success of the NASA X-43A (Hyper-X) flights accomplished major steps toward producing TSTO boosters. Based on those promising results, it should not be that difficult if properly funded to develop a first-generation TSTO space plane with air-breathing propulsion on the first stage that could propel the vehicle to a speed of Mach 7 prior to second-stage separation . Such a vehicle could possibly even be ready to achieve an initial operational status by 2017.
A second-generation TSTO space plane could then be developed with the ability to reach a speed of Mach 10 prior to second- stage separation or carry rocket-based combined cycle engines on the second stage.
It should be possible to have such a space plane with an initial operational capability by 2025.
Human missions to Mars or asteroids in Earth’s neighborhood will require revolutionary in-space propulsion systems to achieve short in-space travel times. Human missions to the Moon also could be conducted with revolutionary in-space propulsion systems in preparation for human exploration to Mars and other destinations.
The status of transportation capabilities for exploration in 1989, the year the ill-fated Space Exploration Initiative was introduced, and the state of space transportation in 2005 are essentially the same. The second incarnation of the Space Exploration Initiative in the form of the Space Exploration Policy can be made affordable and sustainable by supplementing evolutionary transportation systems with revolutionary transportation systems.
In the Apollo program, the lunar-orbital rendezvous ensured that the program would be a dead end.
If the Earth-rendezvous technique had been chosen instead, the work on deploying a space station could have begun much earlier. A lesson to be learned is that it is necessary to ask about what follow-on programs or growth potential are planned before discarding options. To advance U.S. scientific, security and economic interest, revolutionary transportation systems are needed sooner, not later.
Revolutionary space transportation systems can be developed in a timely manner, but only if the 2004 U.S. Space Transportation Policy is amended to require that, by 2020, the United States must develop a revolutionary Earth-to-orbit space transportation system for deployment of spacecraft and other payloads in low-Earth orbit, inclusive of human spaceflight, and to develop a revolutionary in-space transportation system for human space exploration beyond low-Earth orbit.
These objectives can indeed be achieved, only if aeronautics and astronautics resources are appropriately directed and adequately supported.
Unmeel B. Mehta is an associate fellow of the American Institute of Aeronautics and Astronautics and holds a doctorate in Mechanical and Aerospace Engineering.
