The right rocket for the moon and Mars
Almost fifty years ago, Apollo and the Saturn V Moon rocket demonstrated both that deep space could be an accessible frontier for human endeavor and that being there permanently would be very challenging. Establishing a continuous human presence on the Moon and Mars will be among humanity’s greatest achievements, but there is only one modern rocket specifically designed for such complex missions. Although many companies are developing launchers that may be adapted for human use later, NASA’s Space Launch System (SLS) was developed from the start to push human presence beyond low Earth orbit. This is the first launch system with that purpose since the development of the Saturn Moon rocket began in early 1960.
Since the test flight of SpaceX’s Falcon Heavy launch vehicle a few short months ago, many have questioned why we need SLS when commercial vehicles boast “bargain” prices. Their arguments center on the price-per-pound to orbit of commercial vehicles compared to SLS. However a price-per-pound comparison is practically meaningless in the context of real deep space mission requirements. We need to launch crew along with the systems and supplies needed to support human life for longer than a couple of days in order to begin building our next “home away from home” in deep space. Depending upon location we will also need to launch a lot of infrastructure. For example, if lunar resources are to be used to support terrestrial fusion power, lunar settlement, and Mars exploration, large scale production and refining equipment and habitat and power facilities will be required. SLS is designed to evolve to meet these needs. For purposes of comparison, let’s assess just the current capabilities of SLS and SpaceX’s Falcon Heavy in the context of each of deep space mission requirements.
We need to launch crew, life support systems, and supplies
The evolution of an American presence in deep space requires greater capability even than the remarkable performance of the Apollo Saturn V. The Orion spacecraft to fly atop of the SLS is specifically designed to safely take four crew members to lunar orbit and beyond and return to Earth. It has a mass of approximately 27 mT (metric tons). Orion’s mass aligns closely to the 28.8 mT Apollo three-crew-member spacecraft but is far more efficient and capable as measured by functionality, safety, and automation. The first version of SLS - SLS Block 1 – will be the only vehicle capable of taking the Orion spacecraft to the Moon. The next version of the SLS - Block 1B - can take both Orion and an additional 10 mT+ payload to the Moon. This additional capability enables establishment of an initial lunar surface delivery capability as well as the logistics needed with every Orion launch. In comparison, even if it were human-rated, which it is not as yet, the Falcon Heavy could take only a partial Orion - roughly 18 mT- which doesn’t meet the minimum mission requirement.
Those not familiar with spacecraft might argue that Orion weighs too much and that the mission should be done with lighter spacecraft. However, vehicles designed to carry crew to the International Space Station (Dragon-2 and CST-100 Starliner) – each weighing between 18 and 20 mT - do not incorporate the additional fuel needed for lunar orbit entry and return to Earth, the continuous communications capabilities designed for deep space navigation and guidance, life support systems, volume for consumables to sustain the spacecraft and crew required for lengthy deep space missions, a heat shield capable of lunar return speeds or greater, and hygiene systems. When combined with the unprecedented level of avionics and software redundancy, these systems also enable Orion to safely support multi-day contingency operations to return a lunar crew to Earth in case of emergency.
All of these subsystems bring added mass; however, they are necessary to transport humans to deep space and to conduct sustained operations there. They are not required for low Earth orbit spacecraft.
We need to launch a lot of infrastructure to deep space
There are two components to this requirement: Mass performance to deep space, and volume capability. The most recent comparison is that SLS Block 1 can launch ~95 mT while Falcon Heavy can launch ~64 mT to Low Earth Orbit. However, using LEO performance is not the correct measure for deep space mission requirements. However, using LEO performance is not the correct measure for deep space mission requirements. Some launch systems designs enable high performance to LEO, but are not efficient for missions beyond LEO. Also, SLS Block 1 is not the right vehicle to use in comparison, as it is a test vehicle. Future exploration mission launches will utilize the SLS Block 1B with the Exploration Upper Stage (EUS). As such, a more accurate performance comparison is that SLS Block 1B can launch ~40 mT to the Moon, with future additional growth potential that can include crewed landings. The Falcon Heavy can launch only ~20 mT to the Moon (assuming full performance without recovery of its first stage booster). As such, the true performance comparison is that it would take 2 Falcon Heavy launches to lift the same payload mass as an SLS Block 1B.
However, mass is just the beginning of the story. The SLS Block 1B fairing can accommodate 537m3, a little more than 3 times that the Falcon Heavy fairing can accommodate. Large volume is important because some systems aren’t well suited to being shipped in parts and assembled in space. It can be done—much as a canoe can be cut up, packed inside an SUV, and driven to a lake—but there is added hardware needed to join the pieces and seal the joints. Greater complexity means added cost and risk of delay or mission failure, and extra hardware adds mass and takes up otherwise usable space. For deep space systems, these are serious constraints. SLS Block 1B can launch a fully outfitted ~40mT habitat with ~270 m3 usable volume to the Moon. To provide the same capability with the Falcon Heavy, therefore, would take 4-6 launches and re-assembly at the final location, once both mass and volume constraints are taken into account.
Cost comparison
Once one understands the key mission requirements, a price-per-pound comparison simply doesn’t make sense; rather, a use-based cost comparison is clearly the better metric. Assuming a fully expendable Falcon Heavy launch costs $150-200M, the total comparative price of carrying infrastructure to deep space would be $600M to $1.2BM for 4-6 Falcon Heavy launches compared to $500-$1,000M for a SLS Block 1B launch. The SLS approach is similarly priced but less risky, as there is a significant increase in logistics and risk when a mission requires 100 percent success of 4-6 launches versus a single launch. There are also risks in operations with assembly of numerous components. Interfaces may not match up in zero g and different temperatures the way they did in Earth-based testing. This was a considerable risk for the International Space Station, adding significant cost and complexity to the program. In deep space, difficulties in integration become amplified due to the distances and risks involved in mitigation.
With regard to carrying human beings into deep space for extended missions, as it stands now SLS is the only vehicle capable of meeting inherent mission requirements. SLS and Orion were designed from day-1 for human rated missions to the Moon and Mars. The SLS growth potential even beyond that of the SLS 1B, the risk mitigation of human-rated design constraints from the get-go, and a national commitment to the maintenance of indefinite production lines will assure the future dominance of the United States in deep space.
Final Thoughts
Human activity in deep space always will be difficult and highly risky relative to now familiar, but still challenging activities in LEO. Performance and mission requirements drive the need for innovative, cutting edge technology and engineering solutions to unprecedented challenges in performance and safety. Performance and mission requirements also distinguish the design and uses of one vehicle from another. Opening the Moon and inner solar system for exploration and commercial development always has, and will continue to, require the leadership of the United States as well as collaboration between space agencies, government and private institutions, and industry, but it is important to understand the use requirements of all space systems in order to make wise architectural choices. Regardless of any price-per-pound comparison that may be alleged, SLS is not only competitive with Falcon Heavy and other commercial rockets, but superior to them for human exploration mission requirements in deep space.
Harrison “Jack” Schmitt is a retired geologist and NASA astronaut who was on the Apollo 17 mission to the moon.