Deep space exploration

editorial | Reusability and Sustainability in Deep Space Exploration

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

The release of Space Policy Directive 1 in December 2017 refocused the US civilian space program to pursue a sustainable exploration program with a near-term focus on returning humans to the moon. Since then, related details point to an aggressive timeline for initiating lunar activities and a desire that much of the architecture between here and the moon be reusable. Reusability has been partially implemented before, but recent SpaceX and Blue Origin booster landings have rekindled hopes that reusability could change the economics of space activity simply by switching from expendable to reusable launchers.

The intertank that will be flown on Exploration Mission-1 as part of NASA’s Space Launch System heavy-lift rocket has completed functional avionics testing at the Michoud Assembly Center in New Orleans. Avionics, shown here inside the intertank structure, guide the vehicle and direct its power during flight. Credit: NASA

Aggressive schedules and reusability are well understood and directly related to space exploration. However, sustainability has subtle but important implications in the field of space exploration. Sustainability is the ability to maintain skill 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 difficult to achieve because these programs are expensive and limited in terms of funding. Especially in large-scale programs such as those involved in human space exploration, budget constraints can lead to short-term decisions that result in atrophy of knowledge and expertise, necessary equipment, and processes that are part of the program life cycle, leading to no retained capacity. Such decisions are sometimes unavoidable, but will ultimately undermine long-term programs.

Despite the challenges associated with sustainability, great strides have been made in human spaceflight since NASA’s Mercury program first mission in 1961. In November, the US space program will soon reach a milestone of 18 years of human presence. continues in space on the International Space Station (ISS). This achievement was built on lessons learned through NASA’s Gemini, Apollo, Space Shuttle and ISS programs, but improvements are still needed. As the U.S. space program now grapples with the challenges of long-duration exploration beyond low Earth orbit, sustainability concerns must guide early decision-making.

The Saturn 5 rocket, designed by Wernher von Braun and the brightest minds working at NASA and in industry, remains to this day the most powerful rocket the world has ever launched.  Credit: NASA
The Saturn 5 rocket, designed by Wernher von Braun and the brightest minds working at NASA and in industry, remains to this day the most powerful rocket the world has ever launched. Credit: NASA

As with any space exploration venture, the return of astronauts to the Moon and Mars begins with the rocket. Human space exploration to the moon and beyond requires a super heavy rocket. The Saturn 5 rocket, designed by Wernher von Braun and the brightest minds working at NASA and in industry, remains to this day the most powerful rocket the world has ever launched. Now NASA, Russia, China, and even new entrants SpaceX and Blue Origin have come to the same conclusion as von Braun regarding the need for a superheavy rocket. NASA is 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 capable of providing the necessary lift and supporting the dates of need for deep space exploration missions, but is it durable?

Like the Apollo Saturn 5 launch vehicle, the SLS is expendable, and some might argue that a reusable launch vehicle would provide a more durable solution. Although reuse seems to offer advantages, reuse also includes performance penalties compared to an expendable rocket. A reusable rocket requires additional systems, including thermal protection, flight stabilization, and the massive fuel reserves needed to maneuver and land. The weight of the mission payload must then be sacrificed to compensate for the additional weight of the vehicle, and these penalties only increase the farther a rocket gets from Earth. To match the lift performance of the SLS, a reusable rocket would have to be even larger and, in the end, become more complex and more expensive. The other option is to split the space exploration payloads into smaller pieces, but this adds even more complexity, weight, cost to the payloads and increases risk to the program. Statistically, more launches and greater complexity increase the overall risk of mission failure.

Then there is the issue of theft rate. Before beginning development of the SLS, NASA predicted that the flight rate of the SLS would be as low as one flight per year. Under these conditions, sustainability requires streamlined production for an affordable price, a stable level of activity to maintain essential skills throughout the life cycle of the rocket for reliability, a stable industrial base and the ability to integrate product improvements as needed. NASA selected an expendable rocket architecture as the best design to meet its mission objectives and overall sustainability needs.

Workers prepare to install an RS-25 engine on the A-1 test bed at Stennis Space Center, Mississippi, in preparation for fire testing in support of the SLS program.  Credit: NASA
Workers prepare to install an RS-25 engine on the A-1 test bed at Stennis Space Center, Mississippi, in preparation for fire testing in support of the SLS program. Credit: NASA

Like the initial expectations for the 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 theft rate is counterproductive to sustainability. The combination of reusability and low theft rates creates production gaps between releases, discontinuities in the workforce, and loss of critical skills. Production gaps also pose additional challenges to attracting and maintaining a healthy supply chain. During the Space Shuttle program, suppliers were unable to supply parts later in the program due to loss of tooling, technology, or processes that became obsolete due to long gaps from production of origin. This happened because these processes and tools were not needed each time the vehicle was flown. These basic programmatic and industrial factors are often overlooked, but essential for sustainability. A low but steady production rate of building one rocket per year, like the SLS, serves to maintain key critical skills, keeps the industrial base supply chain active, and provides opportunities for problem solving, accelerating new technologies or block improvements.

Wernher von Braun standing in front of F-1 engines mounted on a Saturn 5 test vehicle on display at the US Space & Rocket Center in Huntsville, Alabama.  Credit: NASA
Wernher von Braun standing in front of F-1 engines mounted on a Saturn 5 test vehicle on display at the US Space & Rocket Center in Huntsville, Alabama. Credit: NASA

There are also additional costs and risks associated with reuse that need to be considered. Successful reuse relies on a detailed understanding of system degradation over its useful life. This requires additional analysis and design effort, the use of more expensive materials to withstand the rigors of use, and an ongoing test program to demonstrate and validate design and manufacturing processes. If a significant change is made to any element or system of the rocket, the corresponding test experiment clock is reset. Additional recovery and refurbishment infrastructure and personnel costs must also be considered. Ultimately, reusing launchers comes at a cost and isn’t always the best solution for every scenario. Alternatively, it is important to examine opportunities for the reuse of hardware in space, such as satellites, propulsion systems and habitats that operate in more benign environments than launchers, and therefore have no costs. extended reuse generals.

For human space exploration to be affordable, sustainable and therefore successful, all possible benefits to reduce costs must be studied and understood. New ideas for efficiency must be solicited, explored and demonstrated to arrive at realistic and reliable solutions. Deep space exploration is a very complex undertaking involving complex cost and risk trade-offs.

Von Braun and the team that led the successful Apollo missions were right. Although much has changed since the 1960s, the laws of physics and the architecture for conducting successful crewed missions to the Moon remain unchanged. NASA is on 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. Space Launch System is the right rocket for this mission.


Doug Cook is a former NASA Associate Administrator for Exploration Systems and director of Cooke Concepts and Solutions.