A truck from the first Hercules lander is unloading a cargo from the next one at right while a third lander arrives, with the surface base in the distance. Artistic license shows landings much too close together. (credit: Anna Nesterova)

Enabling a Mars settlement strategy with the Hercules reusable Mars lander

by John Strickland
Monday, October 17, 2016

NASA’s present strategy for Mars exploration promotes the idea of pioneering space in “NASA’s Journey to Mars – Pioneering Next Steps in Space Exploration.” This document describes NASA’s goals of expanding and sustaining human presence in space. However, NASA’s most recent plans resemble a series of Mars sortie missions with an unclear strategy for establishing a sustained presence that could lead to eventual settlement or colonization.

Two key driving constraints have led NASA’s design community to mission and architecture solutions that are more Apollo-like than settlement or colonization driven. One constraint is the mandate to utilize NASA’s Space Launch System (SLS) as the workhorse to get humans beyond low-Earth orbit. The projected cost of SLS, combined with anticipated flat budget profiles, means that the maximum number of annual flights of SLS will be limited to two per year (or three in a “surge” year). Another constraint, based on direction from the White House, is to send humans to orbit Mars and return safely by the mid-2030s, with a landing on Mars to follow. NASA subsequently set the goal for achieving the first human landing on Mars by the late 2030s.

To architect Mars mission scenarios with a limited launch cadence and still target a first human landing in the late 2030s, the design community is motivated to maximize the useful payload mass delivered to Mars to support each human mission. The rocket equation shows that staging is a great way to maximize payload for a given launch system capacity. The end result is that the transportation architecture is largely performance driven, relying on expendable, multistage systems, including the Mars lander and the two-stage Mars ascent vehicle. This results in a “boots-on-Mars” or “flags-and-footprints” as the best case mission scenario—a Mars super-sortie, defined as a mission measured in months-to-years, employing only those systems and provisions required to support a single crew. Subsequent missions would attempt to leverage assets or infrastructure from previous missions to eventually evolve to a permanent base, but in general each human mission to Mars surface will cost about the same. For the initial human landing, such a minimalist mission plan is unsafe. Over a campaign, this is unaffordable. Over multiple administrations, this is unsustainable.

Mars super-sortie missions, while potentially providing benefits associated with technology spinoffs, economic stimulation, and national pride, do little for long-term expansion of humanity into space. Science and exploration on Mars yield important knowledge, but much of this can be achieved with less expensive robotic missions. The present strategy promotes a developmental approach that minimizes technology investment, maximizes the degree of expendability in the system design, increases risk to the crew, and requires that we bring nearly everything we need from Earth for each mission. This may get humans to Mars surface by the late 2030s, but this program is not affordable or sustainable, nor does it establish the human race as a multi-planet species.

There’s a better path forward. An alternative strategy is proposed with the goal of affordably establishing a permanent and self-sustaining settlement on Mars in the next half-century, as a prelude to colonization, with NASA playing a major role. This strategy, referred to here as “Base-First,” briefly postpones early human landings on Mars until key technologies and systems are demonstrated and matured, and a significant amount of infrastructure is established on Mars to safely support humans. In addition, it could leverage emerging commercial capabilities along the way to improve the affordability of the campaign and potentially reduce the timeline for getting humans to Mars thought increased launch cadence and overall capacity.

This concept does not bypass science. The capabilities created will also enable a much lower cost for direct human presence (science outposts) on Mars, which will probably be a requirement to prove the viability of proposed settlement sites. However, to enable human operations, actual development of vehicles, modules, and equipment for use at Mars must begin soon.

Over the past two years, a team at NASA Langley Research Center has been developing a conceptual architecture with a Base-First strategy in mind. Employing both SLS and commercial launch capabilities in this architecture, the Langley team found that utilizing reusable space transportation systems, leveraging Mars resources to the maximum extent through in-situ resource utilization (ISRU), and developing significant robotic capabilities for autonomous operations both on Mars surface and in Mars orbit, are the keys to an affordable and sustainable campaign on the path towards permanent human settlement and eventual colonization.

Known as the “ISRU-to-the-Wall” study, the architecture utilizes a multi-phase campaign. Early phases emphasize technology development and demonstration, transportation system maturation, and autonomous surface systems operations, while follow-on phases focus on affordable growth of the base infrastructure and expansion of Mars industrial capabilities to enable the base to become self-sufficient.

Key attributes of the architecture include cislunar aggregation of Mars-bound payloads, reusable interplanetary transportation between cislunar space and Mars orbit, reusable landing systems that transport cargo and crew between low Mars orbit (LMO) and the Mars surface, and a Mars surface resource utilization system that produces, among other things, propellant for the reusable lander. The system would use raw materials such as Mars ice and carbon dioxide from the atmosphere.

Early investments are required in technologies that enable ISRU, reusable transportation, and automation, recognizing that these attributes enable the campaign to transition from Earth dependency to Earth independence and a sustained human presence on Mars. The extensive use of ISRU greatly reduces the logistics supply chain from Earth in order to support population growth at Mars. Reusable transportation eliminates the need to deliver new transportation elements for each mission. Reliable and autonomous systems, in conjunction with robotics, enable ISRU and reusable architecture systems that operate and maintain themselves while the crew is not present. Some level of direct crew support for the robotic operations can also be provided by crewmembers at a habitat in low Mars orbit.

Initial landings would be uncrewed, focusing on delivery of critical payloads for the surface infrastructure. This includes landing, autonomously deploying, and initiating key systems, including power, thermal, habitation, mobility, in-situ resource acquisition and processing, and propellant production and storage infrastructure. The reusable lander (discussed in detail below), operating initially as an “expendable” lander, uses these early flights to build system maturity and reliability on its way to being human-rated. The expended lander hardware elements, designed to be modular and multi-functional, are re-purposed for use as part of the base infrastructure. Once a functioning base that is generating propellant is established, the campaign will transition into a build-up phase where reusable systems become operational so that the base can grow affordably.

The lander, a multi-functional, single-stage reusable vehicle (SSRV) known as Hercules, is designed to operate between LMO and the Mars surface base, utilizing oxygen and methane propellants manufactured at the Mars base from Martian resources. Its primary function is cargo and crew transport between LMO and the Mars surface base. In the cargo mode, Hercules is designed to deliver 20 tons payload from LMO to the surface, and then go back to LMO using propellant produced at the base. It would ascend without payload but carry up to five tons of additional propellant that is used to resupply an orbital taxi or Earth return stage. In the crew delivery mode, a five-ton module supporting four crewmembers is included in the nose section and can be used for both ascent and entry. The crew module is designed to be separable and is used in the event of a catastrophic vehicle failure during Mars ascent or entry to safely recover the crew by abort to surface or to orbit. Hercules is configured for vertical takeoff, mid lift-to-drag (mid-L/D) nose entry, and vertical landing, supported by landing legs that extend from the vehicle’s base. The Hercules SSRV vehicle, 6.5 meters diameter and 20 meters long, is 20 tons dry, 120 tons fully loaded for Mars ascent, and about 50 tons at entry.

A Hercules lander during entry into the Martian atmosphere. (credit: Anna Nesterova)

One of the primary motives for configuring Hercules as a mid-L/D, nose-entry vehicle was to allow it to package within SLS’s 8.4-meter-diameter shroud for Earth launch. Other key design considerations include the proper location of the center of gravity for both Mars entry and Mars ascent and the proximity of the payload to the surface (ease of payload offloading.) Balancing these constraints and design attributes lead to the selected design, although it is worth stating that assuming the use of alternative launch vehicles or payload mounting options that allow increased packaging diameter enables consideration of aerodynamic configuration options for the SSRV, such as a large-diameter conical capsule or other base entry shapes, that allow the lander’s payload to be closer to the surface. The main design requirement for the vehicle is that it be reusable.

The vehicle is divided into four sections:

1. Nose Section: includes the abort/terminal landing system tanks and engines and a separable/recoverable crew module for crew missions only.
2. Payload Section: four-meter-high payload bay, which would first hold a truck with a large scissors jack for lowering later payloads to the surface.
3. Ascent Tank Section: propellant tanks for ascent that feed propellants to the main base-mounted engines through the interconnected descent tank system.
4. Aft Section: includes the descent tank systems, main engines, power generation (via a set of internal combustion engines burning oxygen and methane), and the body flap and actuation systems.

Each of these sections is modular (separable) and serves multiple functions in the architecture. For example, some early demonstration flights of the SSRV will be expendable, thus the ascent tank section for this one-way demonstration mission will be used for propellant storage as part of the propellant production infrastructure. The nose section for the crewed variant is essentially a separable spacecraft that serves multiple functions. Nominally it is the cockpit for both crew descent and ascent. But, during the demonstration and infrastructure buildup phases, the nose section can be used as a surface mobility system to relocate or position payloads, as a rescue hopper vehicle that recovers the crew from a downrange abort landing site, or to perform exploration sortie missions and return to the primary base.

The Hercules SSRV de-orbits from LMO and enters Mars atmosphere at a 55-degree nose-up angle of attack. During entry, body flaps at the base of the vehicle, wrapped around the windward side about 180 degrees, offer trim and downrange control in addition to protecting the main engines from heating during the entry phase. Entering Mars atmosphere from LMO enables a rigid and durable hot structure heat shield, designed for multiple atmospheric entries, using an advanced carbon-carbon (ACC-6) aeroshell for thermal protection over fibrous insulation. As it slows to around Mach 2.5, the vehicle pitches up to 180 degrees and the main engines at the base of the vehicle are ignited to slow the vehicle’s rate of descent and horizontal velocity, aimed at a spot 50 meters above the landing site. The propulsion system has single engine-out capability and sufficient reserve propellant and reserve thrust to enable reliable precision landing at the designated landing area for a Mars surface base.

The terminal landing phase is performed using eight abort terminal landing system (ATLS) engines installed at a 30-degree cant angle in the nose section. The key advantage of the forward ATLS engines used during terminal landing is the elimination or significant mitigation of plume interaction with the surface (such as minimizing the surface ejecta hazard from gravel thrown by the plume) relative to lander configurations with base-mounted terminal landing engines. The eight ATLS engines are sized to provide 6 g’s of separation during an abort either during Mars descent or Mars ascent. For terminal landing, however, only four ATLS engines are required, and each must be throttled down to 50 percent in order to land. Using the ATLS engines (as opposed to the base-mounted main descent/ascent engines) simplifies the engine system developments by alleviating the need for deep throttling (down to 10–20 percent) of either the main engines or the ATLS engines. As a backup to the ATLS engines, terminal landing can be accomplished using a single main engine throttled to 60 percent. However, the pressure-fed ATLS engines are designed for reliable start and operation and thus offer fault tolerance (engine-out) and high reliability during the critical landing phase.

Once the vehicle is fully loaded with Mars manufactured propellants, the Hercules SSRV is ready to ascend to LMO to pick up additional payload for the base. Nominal ascent to LMO requires the use of only four of the five main engines, thus enabling engine-out capability. Once in orbit, Hercules picks up a new payload and delivers propellant to the on-orbit reusable systems, such as a high Mars orbit-to-LMO taxi or the Earth return vehicle. It then initiates a de-orbit maneuver, re-enters, and lands at the base again using the Mars-derived descent propellant it carried up into orbit itself.

The ISRU-to-the-Wall study illustrated how working towards reusability from the start, and focusing the early campaign phases on establishing a thriving robotic infrastructure on Mars that leverages in-situ resources, will pave the way to affordable human exploration, settlement, and eventual colonization. The Hercules SSRV vehicle concept offers a high degree of capability, operational flexibility, and eliminates the need to deliver a new descent and ascent stage with each cargo and crew delivery to Mars, reducing the mass delivered from Earth. As part of an evolvable transportation architecture, this investment is key to enabling continuous human presence on Mars.

SpaceX recently unveiled a large, fully-reusable Mars transportation system design to support Mars colonization efforts (see “Elon Musk’s road to Mars”, The Space Review, October 3, 2016). There are key similarities between the SpaceX plan and the Hercules study. First, the vision to move humanity toward large-scale colonization as a strategic objective stands out as a common theme. Second, both state that the keys to success reside in utilizing Mars resources to manufacture oxygen and methane propellant and employing highly reusable systems that can be resupplied. Third, both studies preferred a vehicle configured for vertical takeoff, mid-L/D nose entry and vertical landing, albeit the Langley study focused on a significantly smaller scale lander responsible only for taxi between LMO and the surface. The value of smaller lander vehicles is supported by the usual need for a variety of vehicle sizes and types, which generally enhances the operations of any base or expedition.

A partnership with NASA on developing this capability is possible in the future, but is not currently planned. Considering this, efforts by NASA to study other reusable Mars transportation approaches are appropriate.

John Strickland is an independent space and energy analyst. He is a member of the National Space Society Board of Directors and is an Advocate with the Space Frontier Foundation. However, his views are his own; he does not speak for any group.

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