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Plymouth Rock illustration
With Lockheed Martin’s “Plymouth Rock” mission architecture, two Orion spacecraft carry two people could fly six-month missions to near Earth asteroids. (credit: Lockheed Martin)

Charting a Flexible Path

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About a year ago a new term entered the space community’s lexicon: “Flexible Path”. The concept emerged from the deliberations by the Review of US Human Spaceflight Committee (better known as the Augustine Committee) as a more cost effective alternative to traditional approaches of going directly to the Moon and then on to Mars and other solar system destinations. Instead, a return to the surface of the Moon would be deferred in favor of nearer-term missions to Lagrange points and near Earth objects (NEOs). There was no single way to carry out this approach, hence the term “Flexible Path”.

Since the report’s release, the concept has been both praised and criticized. Proponents of Flexible Path note that by not going directly back to the Moon, the concept allows NASA to spread out exploration costs by deferring development of landers and surface systems, making the overall exploration approach more affordable and sustainable. Detractors, though, see Flexible Path as an aimless approach and thus more vulnerable to cancellation by a skeptical Congress and a bored public.

A key element of the HEFT approach to Flexible Path is the immediate development of an HLV using shuttle-derived designs and capable of placing 100 tons into LEO.

Despite that debate, though, something like Flexible Path seems almost certain to be in NASA’s future. The Obama Administration endorsed the concept by cancelling plans to return to the Moon by 2020, instead calling for a human mission to a NEO by 2025 and a Mars orbital mission in the mid-2030s. While other elements of the president’s NASA proposals have been sharply criticized in Congress, both the House and Senate appear ready to accept something like that: their versions of NASA authorization legislation, including a new “compromise” version introduced in House Science and Technology Committee leadership last week, make no effort to restore the Moon-by-2020 goal of the Vision for Space Exploration. The latest House bill, for example, talks about enabling missions “to the surface of the Moon and missions to deep space destinations such as near-Earth asteroids and Mars”, but sets no timetables for those missions or even in what order they should take place.

A HEFTy mission model

With NASA likely to move forward on Flexible Path, just how will the agency try to implement it? An agency task force, the Human Exploration Framework Team (HEFT), has been studying implementation strategies for several months, recently completing the first phase of their study. While the results of that analysis have not been formally released by NASA, a recent HEFT presentation to NASA leadership was published by NASA Watch earlier this month.

A key element of the HEFT approach to Flexible Path is the immediate development of a heavy-lift launch vehicle (HLV) using shuttle-derived designs and capable of placing 100 tons into low Earth orbit (LEO). That approach is aligned with Congress, which calls for development of similar vehicles in their authorization bills, but clashes with the administration’s original proposal to spend five years on HLV technology development before settling on a specific vehicle design. “There is no benefit to delaying work on the HLV, no technology needed for capability development,” the HEFT presentation states.

In particular, the HEFT presentation endorses an inline shuttle-derived approach not that different from an Ares 5. The HLV would use two five-segment solid rocket boosters attached to a core stage derived from the Space Shuttle’s external tank, with five Space Shuttle Main Engines attached to the base. HEFT estimates that development of such a vehicle would take 7.5 years from authorization to proceed to first flight and cost $11.6 billion (excluding reserves).

Another break from the administration’s plans is for the development of Orion. In April the White House announced that it would continue to support development of Orion, but initially only as an Earth return vehicle (ERV), or “lifeboat”, for the ISS. HEFT opposes this approach, calling it a “sub-optimum detour” for the development of a spacecraft ultimately intended for missions beyond LEO. “Pursuing an ISS ERV diverts near term resources that could be better aligned with advancing human (beyond LEO) exploration,” the presentation states.

“This is very much an approach to going to asteroids on the cheap,” Hopkins said of the Plymouth Rock concept.

The HEFT presentation includes a Design Reference Mission (DRM) concept for a human mission to a NEO. The concept would build up to a NEO mission with a series of robotic and human precursor missions into elliptical orbit and to the Earth-Moon L1 point. The NEO mission itself would require three HLV launches (plus one commercial crew launch) to deploy cryogenic chemical and solar electric propulsion systems, an Orion capsule, a habitation module, and a “Multi Mission Space Exploration Vehicle”. This mission would support approximately six-month journeys to and from an unspecified NEO, with a 30-day stay at the asteroid.

There are a couple of issues with this DRM, though. One is that it fails to meet the president’s goal, as stated in his April 15th speech at the Kennedy Space Center, of mounting a NEO mission by 2025. Instead, the HEFT mission model would push that mission out to 2031. In addition, the model is over its own budget. HEFT’s cost estimates project spending nearly $144 billion on the concept from 2011 through 2031, about $15 billion more than what the team estimates will be available based on budget proposals and projections.

Alternative approaches

The HEFT approach, though, isn’t the only way to do Flexible Path, and perhaps not the fastest or most cost-effective way of achieving the goal of a NEO mission by 2025. At the AIAA Space 2010 conference in Anaheim, California, last month, Josh Hopkins of Lockheed Martin presented a mission concept called “Plymouth Rock” that could enable relatively rapid, if barebones, human missions to nearby asteroids. “This is very much an approach to going to asteroids on the cheap,” Hopkins said.

The Plymouth Rock concept has little of the infrastructure of the HEFT approach: no solar electric propulsion modules, no hab module, and no Multi Mission Space Exploration Vehicle. Instead, an HLV would launch one Orion and a departure stage, while a second vehicle would launch a crewed Orion that would dock with the other. “It would probably be very hard to justify spending the money to develop a customized deep space spacecraft” just for an asteroid mission and still be inexpensive, Hopkins said. “It would make more sense to try to figure out how to do this mission with the same kind of spacecraft that we can use in Earth and lunar space.”

Akin calculated operating costs of no more than $3 billion a year by the early 2020s for his modulat architecture, with two human missions a year to the Moon and one Flexible Path mission to a NEO or elsewhere in cislunar space.

The two Orion spacecraft, loaded with supplies, would be just capable enough to support two people on an approximately six-month roundtrip mission, including five days in the vicinity of the asteroid. The baseline mission uses two identical Orion spacecraft, with one jettisoned after the asteroid encounter, although Hopkins said they’ve looked at developing a deep space variant of Orion that would replace one of the two Orions for such missions. The deep space model would exchange the systems needed for reentry with additional pressurized volume, as well as better support for EVAs.

Hopkins said his group has looked at a number of potential NEOs, identifying “three or four” that could be reached by a dual-Orion approach. The earliest, to the asteroid 2008 HU4, could fly in 2016. Hopkins said his favorite, though, was a 2029 mission to asteroid 2000 SG344: it has one of the lower delta-V requirements, allowing a round-trip mission in under 150 days. The asteroid is also among the largest considered, with a diameter of 30 to 45 meters, and happens to be among those known asteroids that pose the greatest impact hazard to the Earth, a factor that could generate additional interest for a human mission.

The Plymouth Rock approach, though, does require development of an HLV, which would likely rule out the 2016 and other immediate opportunities (Hopkins estimates the earliest feasible mission opportunity is a late 2019 mission to asteroid 2008 EA9.) Others, though, are looking for ways to do Flexible Path architectures without the development of an HLV.

In a separate presentation at AIAA Space 2010, David Akin of the University of Maryland proposed a more incremental approach that uses a larger number of smaller building block modules that could be launched on existing EELVs, such as the Delta 4 Heavy. Such a modular approach, he argues, permits a wide range of missions without the need to take years and spend billions on an HLV.

Akin said he modeled this approach on the construction of a new base at the South Pole. The NASA approach to building such a base, he said, would have concluded that what was needed was a large airplane that could deliver the base in one piece. Instead, what the National Science Foundation did was rely on an existing aircraft, the LC-130, that flew 925 flights delivering components and equipment needed to build the base. “Rather than design something custom for this,” he said, “they just did it.”

Akin’s approach uses three basic modules: a crew module, an orbital maneuvering stage, and a lander that is based on the maneuvering stage. Using multiple modules and launches, he argued it’s possible to do a variety of missions, including a lunar landing. “This is a kludgey system,” he admitted, but his analysis showed that its reliability was higher than Constellation for lunar landing, as the modular approach allowed for the introduction of spares. It was also relatively affordable: he calculated operating costs of no more than $3 billion a year by the early 2020s, with two human missions a year to the Moon and one Flexible Path mission to a NEO or elsewhere in cislunar space. Most of that money goes to procuring launches: up to 13 Delta 4 Heavy and four Atlas 5 vehicles a year.

Flexible Path, Sherwood said, “puts Mars decades away, many decades away.”

However, for the time being the HEFT concepts appear likely to be the basis for a baseline Flexible Path architecture, and one that aligns well with the NASA authorization bills in Congress that call for immediate development of an HLV. In any event, the path forward appears to shun the central goal of the previous administration’s Vision for Space Exploration: a human return to the Moon by 2020.

In some respects, though, Flexible Path is an extension of the Vision concept: both had a long-term goal of sending humans to Mars, but not in a direct-to-Mars approach sought by some space exploration advocates. Sending humans to Mars requires “a big jump up in capability,” said Brent Sherwood of JPL, who was part of an internal NASA study team last fall examining Flexible Path concepts, in an AIAA Space 2010 presentation. The Vision and Constellation sought to break that big step into a few smaller steps. Flexible Path breaks that goal into more, and smaller, steps.

Flexible Path is like a mortgage, he argued, that breaks a huge purchase into a large number of smaller payments, even if it means taking longer and spending more than if you bought the house with cash. “The beauty of the mortgage is that it lets you buy it a piece at a time, and that’s the gist of Flexible Path,” he said.

Such an approach, though, requires patience, since by fitting human exploration into a tight budget, it pushes missions out well into the future. Flexible Path, he said, “puts Mars decades away, many decades away.” That creates, more than ever, a need to develop a rationale for human space exploration that can be sustained and supported by multiple administrations and Congresses—a challenge that could prove far more formidable than any technical obstacle to human spaceflight beyond Earth orbit.


ISPCS 2015