This op-ed originally appeared in the Oct. 22, 2018 issue of SpaceNews magazine.

The release of Space Policy Directive 1 in December 2017 redirected the U.S. civil space program to pursue a sustainable program of exploration with a near-term emphasis of returning humans to the moon. Since then, the related details indicate an aggressive schedule to initiate lunar activities, and a desire for much of the architecture between here and the moon to be reusable. Reusability has been partially implemented before, but recent SpaceX and Blue Origin booster landings have reignited hopes that reusability can change the economics of space activity simply by switching from expendable to reusable launch vehicles.

Aggressive schedules and reusability are well understood and directly relate to space exploration. However, sustainability has subtle but important implications in the space exploration domain. Sustainability is the ability to maintain competency levels and the delicate balance for all supporting elements of a program over a long-term program in an affordable manner. For space exploration, this balance is hard to achieve as these programs are expensive and funding-constrained. Especially in large-scale programs such as those involved in human space exploration, budget constraints can drive short-term decisions that result in atrophy of knowledge and expertise, needed equipment, and processes that are part of the program life cycle, leading to limited or no retained capacity. Such decisions are sometimes unavoidable, but will ultimately undermine programs long-term.

Despite the challenges associated with sustainability, great strides have been made in human spaceflight since the first mission of NASA’s Mercury program in 1961. This November, the U.S. space program will soon achieve a milestone of 18 years of continuous human presence in space on the International Space Station (ISS). That accomplishment has been built from the lessons learned through NASA’s Gemini, Apollo, space shuttle and ISS programs, but there is still need for improvement. As the U.S. space program now looks to the challenges of long-duration exploration beyond low Earth orbit, sustainability concerns need to drive the early decision making.

As with any space exploration endeavor, returning astronauts to the moon and onto Mars all starts with the rocket. Human space exploration to the moon and beyond requires a super heavy-lift rocket. The Saturn 5 rocket, designed by Wernher von Braun and the brightest minds working at NASA and industry, remains to this day the world’s most powerful rocket ever launched. Today, NASA, Russia, China, and even new entrants SpaceX and Blue Origin, have reached the same conclusion as von Braun regarding the need for a super heavy-lift rocket. NASA is now nearing completion of such a vehicle, the Space Launch System (SLS). The SLS is even more capable than the Apollo-era Saturn 5 and the space shuttle, which was used to build the International Space Station. The SLS is the only rocket that can provide the needed lift and support the need dates for the deep space exploration missions, but is it sustainable?

Like the Apollo Saturn 5 launch vehicle, the SLS is expendable, and some may argue that a reusable launch vehicle would offer a more sustainable solution. While reusability appears to offer advantages, reusability also includes performance penalties when compared to an expendable rocket. A reusable rocket requires additional systems including thermal protection, flight stabilization, and the considerable fuel reserves needed to maneuver and land. Mission payload weight must then be sacrificed to offset the additional vehicle weight, and these penalties only increase as a rocket travels farther from Earth. To match SLS lift performance, a reusable rocket would need to be even larger and in the end become more complex and costly. The other option is to break the in-space exploration payloads into smaller pieces, but doing so adds further complexity, weight, cost to the payloads and increases risk to the program. Statistically, more launches and higher complexity increases the overall mission failure risk.

Then there is the issue of flight rate. Before starting SLS development, NASA anticipated the SLS flight rate would be as low as one flight per year. Under these conditions, sustainability necessitates streamlined production for affordability, a steady level of activity to maintain critical skills over the life cycle of the rocket for reliability, a stable industrial base, and the ability to incorporate product improvements as needed. NASA selected an expendable rocket architecture as the best design to meet their mission objectives and overall sustainability needs.

Like the original expectations for space shuttle, there are proposals for large reusable launch vehicles with a life expectancy of up to 100 flights. However, a reusable vehicle used at a low flight rate is counterproductive to sustainability. The combination of reusability and low flight rates creates production gaps between builds, discontinuities in workforce and loss of critical skills. The production gaps also pose additional challenges to attracting and maintaining a healthy supply chain. During the space shuttle program, suppliers were unable to provide parts later in the program due to loss of tooling, technology or processes that became obsolete due to the long gaps since their original production. This occurred because these processes and tooling were not needed each time the vehicle was reflown. These programmatic and industrial base factors are often overlooked, but essential for sustainability. A low but constant production rate of building one rocket a year, such as the SLS, serves to maintain key critical skills, keeps the industrial base supply chain active, and offers opportunities to fix problems, on-ramp new technologies or block improvements.

There are also additional costs and risks associated with reusability which should be taken into consideration. Successful reusability is built upon a detailed understanding of system degradation over the useful life. This requires additional analysis and design efforts, use of more expensive materials to endure the rigors of use, and an on-going test program to demonstrate and validate the design and manufacturing processes. If a significant change is made to an element or system of the rocket, then the test-experience clock for that is reset to zero. There are the additional recovery and refurbishment infrastructure and personnel costs to factor in as well. The bottom line is launch vehicle reusability comes at a cost and is not always the best solution for every scenario. Alternatively, it is important to look at opportunities for in-space hardware reuse, such as satellites, propulsion systems and habitats that operate in more benign environments than launch vehicles, and therefore do not have the extensive reuse overhead.

For human space exploration to be affordable, sustainable and therefore to succeed, every possible advantage to reduce cost must be studied and understood. New ideas for efficiencies must be solicited, explored and demonstrated to reach realistic and reliable solutions. Deep space exploration is a highly complex endeavor involving complex trade-offs in cost and risk.

Von Braun and the team that led the successful Apollo missions had it right. Although a lot has changed since the 1960s, the laws of physics and the architecture to conduct successful crewed missions to the moon remains unchanged. NASA is on the right track to support President Trump’s goal of returning astronauts to the moon in the early 2020s and preparing for even more ambitious missions to Mars. The Space Launch System is the right rocket for this mission.

Doug Cooke is a former NASA associate administrator for exploration systems and principal of Cooke Concepts and Solutions.