Wednesday, July 4, 2012

Fascinating article

In April 2001, NASA's Jet Propulsion Laboratory (JPL) awarded $1-million Mars Sample Return (MSR) study contracts to industry teams led by Ball Aerospace, Boeing, TRW, and Lockheed Martin (LM). In Phase 1 of the study, the four teams independently assessed a range of MSR mission concepts. JPL stipulated that all should include a rover capable of gathering samples across several square kilometers of Mars's surface over several months. The MSR mission would begin no earlier than 2011. In Phase 2, each team fleshed out one or two of its concepts to permit cost estimates to be made and to identify areas requiring technology development.

The LM industry team looked at three concepts: Libration Point Rendezvous (LPR), Low Mars Orbit (LMO), and Deep Space Rendezvous (DSR). In LPR, the Mars Ascent Vehicle (MAV) bearing the Mars samples would rendezvous with an Earth-Return Vehicle (ERV) waiting in a loose orbit about the Sun-Mars L1 point, about one million kilometers nearer to the Sun than Mars. LMO was the traditional Mars Orbit Rendezvous (MOR) MSR architecture, which saw the MAV meeting up with the ERV in close Mars orbit. DSR would see the MAV rendezvous with the ERV as the latter zipped past Mars on a flyby trajectory that would carry it back to Earth. DSR outwardly resembled the FLEM and piloted flyby/MSSR concepts of the 1960s; according to study participant Benton Clark, however, his team was unaware of the 1960s plans when it developed DSR.

In LPR and LMO, the MAV could remain on Mars for months waiting on the JPL rover to gather samples; DSR, by contrast, enabled only a short surface stay before the ERV flew past Mars, so did not enable effective use of the rover's capabilities. Thus, in Phase 2, LM narrowed its focus to the LPR and LMO concepts.

Even as the contractor teams performed their studies, however, budget pressures forced changes in NASA's Mars plans. Aided by a Science Steering Group, JPL trimmed its MSR requirements to permit a cheaper "groundbreaking" "science floor" MSR mission in 2013. Sample collection would be by robot arm mounted on the lander, not by rover. This would reduce to a few weeks the time the lander needed to stay on Mars. The shorter stay-time led the LM team to revive the DSR MSR concept.

The company scheduled launch of its DSR MSR mission for September 14, 2013. The DSR MSR spacecraft would consist of an 810-kilogram Earth Return Vehicle (ERV) and a 2078-kilogram lander with Mars Ascent Vehicle (MAV). A General Dynamics Atlas 5-521 rocket would place the spacecraft on a low-energy Type IV trajectory needing nearly 32 months to reach Mars. The ERV, based on the LM Mars Odyssey spacecraft design (top image above), would provide course correction propulsion and electricity during the long voyage.

The MSR spacecraft would approach Mars on a collision course. The lander, encased in a 4.5-meter-diameter conical aeroshell, would separate from the ERV on February 4, 2016, four days out from Mars. The ERV would then briefly fire its rocket engine to place itself on a Mars flyby trajectory.

On February 8, 2016, the MSR lander would enter the martian atmosphere directly (that is, without first entering Mars orbit). Atmosphere entry, descent, and landing would benefit from experience and software developed during the 2009 Mars Science Laboratory mission, the LM team stated (bottom image above). Touchdown would take place within 10 kilometers of a pre-selected target near the beginning of martian northern hemisphere summer.

The DSR MSR lander would include three landing legs, six descent engines, twin 10-sided solar arrays, and a "Sample Handling Workbench" with twin robot arms. The arms would fill small cylindrical sample containers with a total of 0.5 kilograms of rocks and dirt.

In keeping with JPL's instructions, the LM-led team devoted much attention in its "science floor" study to preventing possible martian microbes from reaching Earth. Each sample container would be placed into an "ashing chamber," where heat would sterilize its exterior, then would be pushed into a "bagging chamber," where it would be sealed inside a cylindrical sleeve-like plastic bag. After bagging, the container would ride an elevator to the cylindrical Sample Canister Assembly (SCA) in the MAV's conical nosecone. Sample loading would occur within a "Bio Enclosure" "cocoon" pressurized above Mars's atmospheric pressure to ward off Mars dust and microbes that might infiltrate the SCA.

On March 1, 2016, 20 days after landing on Mars, the 266-kilogram two-stage MAV would blast skyward from a launch tube at the center of the lander's triangular frame. A modestly enlarged Star 17A motor with 146.2 kilograms of solid propellant would form the MAV's first stage, and would form its second stage. Cold-gas thrusters would provide attitude control. Mars atmospheric friction heating during ascent would help to sterilize the MAV's exterior. Prior to first-stage separation, the MAV second stage - a Star 13B motor with 54.2 kilograms of solid propellant - would be spun up to 150 rotations per minute to provide gyroscopic stabilization.

When the MAV second stage exhausted its propellant, it would be about four million kilometers from the ERV. Solar cells on the MAV nosecone would power a radio transponder to enable tracking. On March 6, 2016, the ERV would fire its rocket motor to place itself on course to intercept the passive MAV second stage and SCA.

About 80 days after MAV launch (June 1, 2016), when the MAV second stage and ERV were about one million kilometers apart, the ERV would perform another rendezvous maneuver. The ERV would then intercept the MAV second stage about 20 days later. The SCA would eject from the MAV nosecone and the ERV would scoop it up using a "funnel" that would channel it into a "vault" in the Earth Entry Vehicle (EEV). The team explained that its EEV design was based on the Stardust and Genesis sample return capsule designs.

The ERV would then begin a series of Earth targeting maneuvers. Earth return would occur on November 13, 2016. The ERV would spin up and release the EEV, then fire its rocket motor to ensure that it miss Earth. This would help prevent any martian microbes it carried from contaminating Earth. If the ERV failed, timers on board would automatically fire a solid-propellant rocket motor to divert it from Earth impact. If telemetry from the EEV indicated that the SCA had leaked, the samples would be heat-sterilized to prevent biological contamination of Earth.

The EEV would enter Earth's atmosphere directly, cast off its backshell and heatshield, and deploy a parachute. If the EEV broke up during Earth atmosphere entry, the vault containing the SCA would serve as a backup entry vehicle capable of surviving Earth impact. Assuming a successful EEV reentry, however, a helicopter trailing a hook would snag the parachute in midair in the manner planned for September 2004 Genesis and January 2006 Stardust sample recoveries. The industry team stated that the probability of a successful midair recovery was 99.9%, and that this could be further enhanced by additional testing and helicopter pilot training. After recovery, the EEV would be placed inside a containment carrier and transported to a Bio Safety Level 4 handling facility.

The LM team briefly considered a "split" DSR scenario in which the lander and MAV would leave Earth in September 2013, and the ERV would leave Earth in November 2016, after the MAV had successfully placed the SCA bearing Mars samples into orbit around the Sun. The split scenario called for a "long-life SCA" and a three-stage MAV. The additional MAV stage would be used to refine the SCA's orbit, making it easier for the ERV to locate.

The team estimated that, at $791 million, its baseline DSR MSR mission would cost $480 million less than its LPR mission. The largest single savings ($72 million) would be achieved by deleting all lander science instruments. Switching from a Delta 4050H rocket to an Atlas 5-521 would save $64 million, and dispensing with the rover would save $51 million. The industry team did not include in its cost estimate the $135-million "Pre-Project Technology Development Program" it believed to be necessary to help ensure a successful DSR MSR mission.

In September 2004, after a successful Earth reentry, the Genesis sample return entry vehicle failed to deploy its parachute and crashed in Utah, breaching its sample container (image below). U.S. space policy changes, meanwhile, caused NASA to push its target date for the launch of the first U.S. MSR mission to the late 2010s or the 2020s.


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