Human spaceflight has become increasingly complicated over the past decade. This month marks a transition from Expedition 71 to Expedition 72 on the International Space Station (ISS), as well as a privately funded Polaris Dawn operating in high orbit. ISS incorporates modules from Russia, the United States, the European Space Agency (ESA) and Japan. Expected launches include Soyuz MS-26 and Dragon Crew-9, and the next returns to earth from ISS include Starliner (sans crew), Dragon Crew-8 and Soyuz MS-25.
At this writing, ISS hosts seven astronauts and cosmonauts participating in Expedition 71, plus two astronauts arriving from Starliner on its crewed test flight. In the Meantime, China operates its third and current Tiangong space station comprising three modules launched between April 2021 and October 2022. Tiangong currently hosts three taikonauts who arrived on Shenzhou 18, a Chinese-designed spacecraft modeled on Soyuz. May 2023 holds the current record for number of persons in orbit with eleven aboard ISS and six at Taingong.
Private Industry Launches into Space
The title photograph (above) shows Boeing’s Starliner (top) and SpaceX’s Crew Dragon Endeavour (right) both docked to the Harmony node of the ISS. In relation to the viewer, the Destiny laboratory (left), and Columbus (front) and Kibo (rear) modules respectively connect to the Harmony node. The truss assembly obscures the Unity node that attaches to Destiny and supports the solar panel arrays and thermal radiators to provide power and enable heat dissipation.
With the retirement of its Space Shuttle in 2011 after thirty years in service, the United States has had to rely on Russian Soyuz TMA (subsequently replaced by the MS variant) for ferrying astronauts to the ISS – an uncomfortable circumstance given rising international tensions. Consequently in 2014, NASA embarked on commercial competition to provide reusable spacecraft for bringing personnel to orbit under fixed-price contracts. Ultimately, contracts for ISS missions were awarded to SpaceX's Crew Dragon for launch atop Falcon 9 (also produced by SpaceX), and Boeing for its Starliner atop Atlas V (managed by the consortium known as United Launch Alliance or ULA).
Crew Dragon and Starliner both sport modestly larger volumes than the Apollo command module flown from 1969-1975. Both are designed to carry seven adults, in contrast to Apollo and Soyuz with only three aboard, although NASA limits each flight to four persons. (It should be noted that the Soviets restricted early Soyuz capsules to only two cosmonauts, as a result of an undocking fatality that occurred back in 1971.) Relative sizes of the modules can be compared between the Starliner, Dragon, Soyuz and Space Shuttle below.
Other vehicles in development are Orion for a manned lunar return under the Artemis program, and Sierra’s miniature spaceplane Dream Chaser, which is also designed to dock with ISS after launch from a Vulcan Centaur (by ULA). Dragon is designed to splash into the sea, while Starliner is designed to land on ground (as does Soyuz), and Dream Chaser is engineered to glide onto a runway, as did the various Space Shuttle orbiters. In Russia, development commences on Oryel for earth and lunar orbits, with maiden crewed flights planned in 2028.
Manned spacecraft must be accelerated into low earth orbit using chemical rocket-powered launchers. Achieving thrust-to-weight ratios that exceed unity for a sustained period previously presented an engineering challenge. The solution eluded mankind until about a century ago when Robert Goddard tested the first liquid propellant engine in March 1926. The boosting of payloads of several tons, necessary to provide a survivable environment for human passengers, awaited military imperatives which weren’t realized until after the second World War.
Desires to reduce the enormous expenses of engine development and launch operation have encouraged several private sector entrepreneurs to develop reusable rockets. These have been funded both privately and by government contract, and include private entities as diverse as Boeing Defense, Lockheed Martin, SpaceX, Blue Origin, Relativity, Firefly and Sierra. International competitors include Russia (Soyuz and Proton), China (Long March series), the ESA, Japan, and India. A representative handful of launchers from across several decades is illustrated below, with rocket heights shown in meters and payload capability in metric tons. The Vulcan Centaur with Blue Origin’s BE-4 engines is expected to replace current Atlas V and Delta IV launchers to compete with SpaceX’s Falcon 9.
Successes and Setbacks in Space Programs
Elon Musk founded SpaceX in 2002 and introduced launcher Falcon 1 six years later, before producing larger rockets along with a reentry capsule equipped with an expendable trunk for propulsion and power. Dragon’s first variant delivered cargo on twenty-three missions to the ISS between 2010 and 2020. Musk took the name of the capsule design from the “Puff” song in response to criticism regarding the impossibility of his projects. SpaceX introduced an improved successor in 2019, and began launching astronauts in May 2020 with crewed test flight Demo-2 on Endeavour in May 2020.
The first operational flight, Crew-1, arrived on Resilience in November that year. The maiden flights Crew-3 and Crew-4 of the respective third and fourth manned Dragons Endurance and Freedom occurred November 2021 and April 2022. All four Crew Dragon spacecraft have been reused, as have the Cargo versions. Endeavour has flown Crew-2, Axiom-1, Crew-6 and Crew-8, while Resilience has flown Inspiration 4 and Polaris Dawn. Endurance has flown Crew-5 and Crew-7, and later this year plans Fram2 for the first manned polar orbit. Freedom has flown Axiom-2 and Axiom-3, and is scheduled to bring Crew-9 to the ISS this month. (The Axiom flights to its module on the ISS are privately funded ventures.) At this moment, SpaceX is currently constructing a fifth unnamed crew Dragon.
Meanwhile, Boeing has fared less well. In the aftermath of its 1997 merger with McDonnell Douglas, Boeing transformed from a premier airliner designer and manufacturer and spacecraft integrator to a beleaguered corporation selling airplanes. Its descent into ignominy gained notoriety with a pair of international 737-MAX crashes in Indonesia in October 2018, and one in Ethiopia in March 2019 (which was caused by reliance on attitude controlling software on a faulty pitot-tube pressure sensor). Although space business constitutes a much smaller portion of their business, public visibility from such technology highlights these setbacks.
Initially planned for operation in 2017, Starliner underwent a pair of unmanned orbital flight tests in 2019 and 2022. Its first manned test flight launched two astronauts in June 2024, but faulty thrusters and ongoing helium leaks in the unrecoverable service module prompted weeks of ground tests to rectify these problems. Eventually to Boeing’s chagrin, NASA decided to return the two astronauts with Crew-9 next March of 2025, while deorbiting an unoccupied Starliner Calypso earlier this month. With only a partial success, Boeing can expect to be tasked with a mere supplemental test flight, before being permitted to embark on regular missions to the ISS, which is currently scheduled for re-entry destruction in 2030.
To provide a better understanding of the difficulties encountered, a brief explanation of rocketry follows. Unlike air-breathing jet engines, rockets carry both oxidizer and fuel. These ignite in a combustion chamber to generate hot exhaust gases, which compress through a throat and expands supersonically, thereby producing thrust. Military missiles combine these chemicals into a homogeneous rubbery mixture for rapid launch and continuous storage, with a commensurate penalty in performance. Commercial and civilian launchers contain their propellants separately – most commonly oxygen and a common hydrocarbon for better thrust per propellant flow rate. To augment performance, turbo pumps powered by a pre-burner deliver the oxidizer and fuel to the chamber.
By contrast, maneuvering thrusters for a satellite payload emphasize reliability and frequent restarts. Their designs typically involve hypergolic propellants that ignite spontaneously on contact, with a pressurizing tank that pushes these room-temperature liquids into the chamber. Such propellants have toxic and corrosive properties that complicate long-term equipment longevity and personnel safety. The tank uses helium due to chemical inertness and low molecular weight. However, the near-zero viscosity and small size of helium atoms present design challenges to prevent leakage. Both valve degradation from corrosion and helium leakage from heated joints have plagued the Starliner service module.
The Future of Orbiting Spacecraft
Although Boeing’s expenditures have exceeded its commercial fixed-price contract award with NASA, eventual completion at Boeing's expense seems likely. And it should be noted that NASA has interest in maintaining the competitive environment it created nearly a decade ago. Additional entries, such as Dream Chaser may further exacerbate these complicated circumstances. Moreover, SpaceX may experience modest market erosion from Firefly for high-priority short-notice satellite launches, and from Relativity for rapid additive-manufacture production (via three-dimensional metal printer) of integrated engine and structure assemblies.
The advent of reusability has encouraged an important shift from kerosene (C12H26–C18H38) as rocket fuel, to the cleaner methane (CH4) that deposits less soot residue in the combustion chamber and nozzle upon booster recovery. (Note that cryogenic liquid oxygen is used to burn both fuels.) With help from private enterprise, further scientific developments may yet remain on the horizon. Although spaceflight is likely to remain a preserve for hyper-qualified pilots and technical specialists for decades to come, the most exciting news is that private industry at the international level is paving our current path to space. This expansion of technology and innovation by the world's brightest specialists (hopefully unburdened by overly bureaucratic constraints; governmental or otherwise) seems destined to make a broader participation in space flight a reality in the near future – perhaps during some of our lifetimes.
Photo Credit- NASA Space Flight. com