On-orbit Assembly is Expensive and Dangerous

We often hear this objection when someone wants to promote a certain mission that involves a big rocket. The story goes that assembling a spacecraft in Earth orbit involves extraordinary cost and poses severe hazards to the crew and the mission. However, there are no historical data to substantiate the claim.

On-orbit assembly is routine. Apollo did it on every flight. The Russians did it with Mir. The United States has done it dozens of times on Space Shuttle flights, and is in the process of assembling the most complex spacecraft ever conceived in the International Space Station Program.

Assembly in orbit does add some additional cost to the program. A space launch -- any space launch -- has certain overhead costs associated with it. We have to pay those costs regardless of the size of the launch. We reduce the effect of this financial phenomenon by building commonality into our payloads and mission profiles; from flight to flight, the amount of recertification and replanning that needs to be done is minimized.

Moon ship assembled in Earth orbit Artwork by Vik Olliver

We also have to consider the additional weight of docking and berthing interfaces. In the Artemis Project reference mission, we eliminate additional weight for these mechanisms by dividing the launch packages at the places where the spacecraft necessarily has to be coupled and uncoupled.

We also reduce the weight of these mechanims by using an orbiting assembly fixture. The assembly fixture, with its cradle for aligning parts of the spacecraft and large robot arm for gently berthing them, eliminates the need for mechanisms to absorb the force of a high-speed docking maneuver and off-axis alignment when two spacecraft come together.

Near the end of the mission, when the Ascent Stage arrives back at the Lunar Transfer Vehicle, we eliminate the need for docking mechanisms entirely. The Ascent Stage approaches the LTV slowly and engages a small latch to keep the two spacecraft together. Then the crew, in their space suits, maneuver the Ascent Stage onto the flat part of the LTV and secure it to latches for the return home.

Ascent stage arrives at Lunar Transfer Vehicle Artwork by Vik Olliver

So, no, on-orbit assembly does not need to be either expensive or dangerous. It has been done dozens of times before, and will continue to be the mainstay of space operations for all time to come.

Beware of one-shot thinking

There is another major problem with the belief that we need huge rockets that can put an interplanetary spacecraft into Earth orbit in a single launch. Like the Apollo program, reliance on huge launchers leads to one-shot missions. Single-launch missions do not provide a growth path, and at least today, large rockets mean enormous costs for each launch.

In contrast, if we continue to develop and perfect on-orbit assembly techniques, we can build anything in space. Orbiting space complexes can be as large and complex as we want; we just need to assemble them one step at a time. This gives us time to evaluate what we have at each step in the process and adjust our plans as necessary to make our spacecraft even better.

On-orbit assembly also allows us to build reusable spacecraft. In the Artemis Project, all the spacecraft are designed to be refueled in space by swapping out empty fuel tanks -- or sometimes entire propulsion assemblies -- for fresh ones. This modular design allows the transportation system to be developed and improved each step of the way, and updated as new technologies become available.