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Mars mission illustration
An illustration of a Dragon capsule with an inflatable hab module, attached by a tether to its upper stage, leaving Earth for Mars under Zubrin’s proposal. (credit: The Mars Society)

A transorbital railroad to Mars


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One of the biggest challenges for the effective exploration and exploitation of space has been the high cost of space access. Over the last several decades governments and companies alike have made numerous attempts to lower the cost of reaching orbit, from the Space Shuttle program to various proposed commercial launchers, with little success.

Zubrin’s concept is, at its core, a space access subsidy program.

At the same time, efforts to move ahead with human exploration of Mars—widely regarded as the long-term goal for human spaceflight—have been stymied in part by the perceived need to develop a heavy-lift launch vehicle similar in capacity to the Saturn V to carry out such missions. The high cost and long development time of such a vehicle, along with the need to develop various other components such as landers and habitat modules, have deferred those plans for decades. Now, though, one leading space advocate believes he has a solution to both problems that could enable low-cost space access and near-term human missions to Mars.

Building the transorbital railroad

In a presentation Sunday at the International Space Development Conference (ISDC) in Huntsville, Alabama, Mars Society president Robert Zubrin unveiled a concept he calls the “Transorbital Railroad” to provide low-cost and regular access to space. The name, he explained in an interview at ISDC the day before his presentation, is intended to draw a parallel with the development of the transcontinental railroad in the United States in the 1860s. “How do we create something to open the way to space in as profound a way as the transcontinental railroad?”

Zubrin’s concept is, at its core, a space access subsidy program. Rather than spend billions on new launch vehicles, he envisions NASA instead spending a modest amount of money—he suggested $1.2 billion a year, about six percent of its current $18.5-billion annual budget—buying the most “cost-effective” launch vehicles available. That cost effectiveness would be some function of its price and payload capacity; Zubrin has a particular preference for SpaceX’s proposed Falcon Heavy, which could launch up to 53 metric tons into low Earth orbit (LEO) for as little as $80 million a launch.

NASA would then, in turn, resell that launch capacity to itself, other government agencies, and the private sector, at the artificially low price of $50 per kilogram, or about $2.65 million per fully-loaded Falcon Heavy. Those launches, he said, would take place on a regular schedule, regardless if the capacity on each vehicle is fully subscribed. “You don’t hold the train in the station until it fills up,” he explained. Any excess capacity would be filled with consumables like water, oxygen, and propellant, which could be stored on orbit for use by any interested parties.

“All kinds of space ventures become possible,” Zubrin said. “They may succeed or they may fail, but launch cost is not going to be the problem.”

In Zubrin’s example of the Falcon Heavy, his proposed $1.2-billion annual budget would allow the launch of 15 vehicles, with a total capacity of 795 metric tons. The cost to customers for that capacity would be just under $40 million, or somewhat less than the going price of a single Falcon 9 that can place a little over 10 metric tons into LEO.

Those low “user fees” for accessing his transorbital railroad pay only a tiny fraction of the real launch costs, he acknowledged. However, he believes that those launch costs will enable a whole new range of commercial space applications that would otherwise not be possible at current launch prices. “All kinds of space ventures become possible,” he said. “They may succeed or they may fail, but launch cost is not going to be the problem.”

The cost to the federal government for the program could be offset by an increase to the Gross Domestic Product (GDP) of $7.2 billion a year, assuming a ratio of $1 in increased tax revenue per $6 increase in GDP. “It’s very reasonable to argue that the tax revenues would pay for the railroad program many times over,” he said. The required GDP increase “is a very modest estimate given the kinds of things you would be unleashing here.”

Riding the railroad to Mars

One potential use of the “transorbital railroad”, of course, would be a human mission to Mars. While not strictly dependent on that concept, Zubrin has developed a new variation of the Mars Direct concept he first proposed in the early 1990s that could allow for a relatively near-term, albeit economy-class, human expedition to Mars.

Zubrin’s “Mars Semi-Direct” concept would make use of three Falcon Heavy launches every two years. In the first phase, the three Falcon Heavy rockets would launch to Mars an unmanned hab module and ascent vehicle, which would land on the Martian surface, and an Earth return vehicle (ERV) that would go into Martian orbit; all three would be based on SpaceX’s Dragon spacecraft. Two years later, three more Falcon Heavy rockets would carry another ERV and ascent vehicle as well as a crewed Dragon that would land on the surface. At the end of their stay on the surface, the astronauts would fly one of the ascent vehicles to orbit, where they would rendezvous with one of the ERVs for the trip home.

While this approach does include the complication of Mars orbit rendezvous for the trip home when compared to the original Mars Direct concept (where the ascent vehicle launched directly to Earth) this Semi-Direct approach does have some advantages beyond its use of the low-cost Falcon Heavy. Among the biggest advantage is that it does away with the development of a 100-kilowatt nuclear reactor needed in Mars Direct. Zubrin estimates that the power needs could be met by Soviet-era 10-kilowatt Topaz nuclear reactors or even solar power, “which is just barely possible with ten kilowatts,” he said.

The Zubrin proposal includes the use of an inflatable hab module to provide extra living space for the two-person crew.

Zubrin has done basic calculations of the logistics required to show that it is possible to send two people to Mars with enough supplies for up to three years. Those calculations, though, do require recycling of oxygen and water. The mission also makes use of in situ resource utilization (ISRU) to produce propellant for the return mission. Unlike the original Mars Direct concept, which carried liquid hydrogen to Mars to produce methane and oxygen, this proposal brings the methane to Mars and produces liquid oxygen by extracting it from carbon dioxide in the Martian atmosphere.

Zubrin first publically discussed this proposal in an op-ed in the May 14th issue of the Wall Street Journal. One of the criticisms in online discussions of that proposal is that it appears to require the two-person crew to spend the six-month transits between Earth and Mars in the cramped volume of a Dragon capsule. That struck many as uncomfortable at very least, if not unsafe.

One aspect missing from the Journal piece, though, is the use of an inflatable hab module. Zubrin said the module, eight meters long and six meters in diameter, would inflate from the nose of the Dragon capsule. It would provide 56.5 square meters of floor space and 180 cubic meters of overall volume, pressurized at a third of an atmosphere. That makes its volume comparable to Bigelow Aerospace’s proposed Sundancer volume, although Zubrin said the module would not have the solar panels, attitude control systems, or other components needed for a free-flying module. “This is why it’s not a question of two people being crammed in the ten cubic meters of the Dragon,” he said.

That hab module, featuring a Kevlar skin, weighs only 200 kilograms, but Zubrin said it has ten times the strength needed to maintain its integrity. The hab and Dragon would fly to Mars attached to its upper stage by a tether, spinning to provide Mars gravity. Once at Mars the crew would sever the tether and either deflate and store the hab module prior to reentry or simply detach the module. Once on the surface, the crew would re-inflate the hab or deploy a new one to provide the same living space during their stay on Mars.

Because this architecture would make use of components that either already exist or are under development (such as the crewed version of the Dragon spacecraft and the Falcon Heavy launcher), and would not require the expense and time involved with developing a new space-rated nuclear reactor, such a mission could be flown relatively soon. In his Journal op-ed Zubrin suggested that “we could have our first team of human explorers on the Red Planet by 2016.” However, that may be overly optimistic given that the crewed Dragon and Falcon Heavy are still in their early phases of development and other technologies, including oxygen and water recycling, would need to be demonstrated in space.

Enabling every vision

Zubrin’s two ideas—the “transorbital railroad” and the Falcon Heavy Mars mission architecture—are technically independent from one another. The transorbital railroad could serve multiple customers; Zubrin notes that the Mars missions, which require three Falcon Heavy launches every two years, would account for only ten percent of the system’s capacity. And even without the railroad, that Mars mission architecture could be flown relatively affordably simply by buying Falcon Heavy vehicles at SpaceX’s listed price.

“I think this idea is something the entire space community can rally behind, because it enables every vision,” Zubrin said.

Combined, though, Zubrin believes offer some interesting possibilities for Mars exploration. “If NASA didn’t want to do it, certain private organizations could say, ‘We want to do a Mars mission, so we’ll buy three launches of this railroad,’” Zubrin said. For perhaps two to three times to cost of mounting an entry in the America’s Cup yachting race, he estimated, some organization could decide to mount their own human Mars mission.

Zubrin admitted that there are a number of details that have to be worked out for both concepts. The transorbital railroad, for example, requires some work on the logistics of integrating multiple payloads and also accommodating the items that would fill the excess capacity on each launch. The specifics of contracting the launches, including whether to include a second launch provider instead of being reliant on a single vehicle, also require some work, he said.

Zubrin believes, though, that several factors mean that the time has come for both ideas. SpaceX’s announcement of the Falcon Heavy opens the door to mission applications that could take advantage of relatively low cost heavy lift. Concepts like the transorbital railroad has been proposed in the past, he noted, but with the impending retirement of the shuttle, potential institutional opposition to them may be fading. And the upcoming presidential election cycle could provide an opportunity for a candidate to make a statement about human space exploration that takes advantage of this proposal.

“I think this idea is technically possible, enormously beneficial, and politically possible,” he said of the transorbital railroad. “I think this idea is something the entire space community can rally behind, because it enables every vision.”


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