NASA examines options and flight paths for SLS EM-2 mission
As NASA and its contracted partner agencies press forward toward the debut launch of the Space Launch System (SLS) rocket in 2018, the U.S. space agency is beginning to look toward preliminary planning and test objectives for the EM-2 mission of the SLS Program, which is expected to take place sometime in the opening half of the 2020 decade.
The road to EM-2
As originally conceived in the opening years of the SLS program, the EM-2 mission was to be the first crewed flight of the Orion Multi-Purpose Crew Vehicle that would take astronauts on a multi-day circumlunar mission.
However, the feasibility of this mission being one to carry crew was called into question as the EM-2 launch date slipped beyond the opening of the 2020 decade and NASA began investigating the potential advancement of the introduction of the Exploration Upper Stage (EUS) from the third to the second EM flight of SLS.
Originally, with timelines indicating that EM-2 would follow a couple years after EM-1, it was understood that EM-2 would make use of the same Interim Cryogenic Propulsion Stage (ICPS) as EM-1- with one key difference: The ICPS would be human rated for EM-2.
The feasibility of this was quickly challenged as the ICPS’ human rating would cost millions of dollars (USD) for a single-mission event before the switch to the EUS on EM-3 and all subsequent SLS flights.
Nonetheless, the human-rated ICPS for EM-2 would satisfy the requirement from the astronaut office that no crew fly on any variant of the SLS rocket when a major propulsion element was being used for the first time in flight.
However, NASA’s decision to advance the introduction of the EUS to EM-2 created a conflict with that rule – as EM-2 was viewed in its initial conception as the second flight of SLS, thus leaving no room between the EM-1 and EM-2 missions of SLS to test the EUS in flight.
The FY 2016 U.S. federal budget
However, following a protracted budget battle – which has become all too usual for the U.S. Congress – the final FY 2016 federal budget included language specifically addressing the Europa Clipper Mission and the vehicle upon which it would launch.
Originally, the notional mission to Europa was tentatively penciled in for launch aboard an Atlas V 551 variant from the Cape Canaveral Air Force Station.
However, the FY 2016 U.S. federal budget for NASA specifically directed funding “for the Jupiter Europa clipper mission and clarifies that this mission shall include an orbiter with a lander that will include competitively selected instruments and that funds shall be used to finalize the mission design concept with a target launch date of 2022.”
More surprisingly, the FY 2016 federal budget language also specifically mandated the vehicle upon which Europa Clipper will launch.
“The National Aeronautics and Space Administration shall use the Space Launch System as the launch vehicle for the Jupiter Europa mission, plan for a launch no later than 2022, and include in the fiscal year 2017 budget the 5-year funding profile necessary to achieve these goals.”
With SLS now legally mandated as the launch vehicle for Europa Clipper, the issue of testing the EUS in-flight prior to EM-2 could be rectified if SLS launches the Europa Clipper mission in 2022 ahead of EM-2.
This would place the EM-2 mission firmly in the No Earlier Than 2023 timeframe, approximately five years after the EM-1 mission, currently on track to launch by “No Later Than November 2018,” according to the Exploration Systems Development (ESD) presentation to the NASA Advisory Council Meeting on 2 March 2016.
EM-2 planning begins:
Despite uncertainty to the exact timeframe of EM-2, NASA has begun formal planning operations for the flight.
“Mission planning for EM-2 and beyond including on-ramp for low-cost opportunities for development tech objectives and capability enhancements” are underway, notes the ESD presentation.
Moreover, a dedicated mission planning team has been established, co-manifest payload options are being evaluated, and a mission planning resources watch item list is being created.
Critically, on-orbit Micro-Meteoroid and Orbital Debris (MMOD) risk exposure and related mission profile/trajectory planning efforts for the first crewed flight of SLS and Orion are in the initial phases of study, as are the precise objectives the EM-2 crew will achieve during their multi-day flight around the moon.
According to the ESD presentation, “With the selection of the EM-1 Mission being a DRO (Distant Retrograde Orbit) mission the goal of EM-2 is [to] complete residual FTOs (Flight Test Objectives) not accomplished on EM-1 [and] accomplish risk reduction activities for future more complex missions for EM-3+.”
(A DRO is a highly stable orbit that exists due to the interaction between the Earth and Moon’s gravity.)
Notably, the ESD presentation notes that EM-1 and EM-2 “should include capabilities relevant to potential near term deep-space missions.”
To this end, both EM-1 and EM-2 are classified as Design Reference Missions (DRMs), intended to be “design driving cases … to maintain current SLS/Orion FTOs while demonstrating as many Exploration Objectives as cost, schedule and risk allows.”
EM-2 flight options, in-flight abort requirements:
Presently, EM-2 is baselined as a High Lunar Orbit (HLO) mission.
Of particular note and importance for EM-2 is the fact that the vehicle will carry, at every point in its mission, the ability to directly return its crew to Earth within five days should such an emergency situation arise.
Additionally, according to the “Evolution of Orion Mission Design for Exploration Missions 1 and 2” report, Orion and its crew, through the Trans-Lunar Injection (TLI) burn, will be placed into a free return trajectory to a nominal Earth Entry Interface condition.
This specific free return trajectory will allow the crew to safely return to Earth in the event the Service Module’s (SM’s) engine fails to ignite post-TLI.
In this case, in a free return trajectory, a minimal series of thruster burns can accomplish precise positioning of Orion to allow for a passive swing and gravity assist maneuver around the Moon to slingshot the vehicle and crew back toward a landing point on Earth without any major burns.
Baselined HLO mission:
Under this profile, Orion and the EM-2 crew would launch aboard SLS and complete one full orbit of Earth before the TLI burn on the second orbit would propel them toward the Moon.
After jettisoning the EUS, the SM would perform the Outbound Trajectory Adjust (OTA) burn to lower the lunar flyby altitude to 100 km (62.13 miles).
Orion and its crew would then coast through the void between the Earth and the Moon before swinging around the backside of the Moon.
This baseline would see the crew and vehicle perform a Lunar Orbit Insertion (LOI) burn at an altitude of 100km above the lunar surface to enter a 100 x 10,000 km (62.13 x 6,213.712 miles) HLO trajectory.
After three days in lunar orbit, the Trans-Earth Injection (TEI) burn would propel Orion and its EM-2 crew into a return trajectory back to Earth to wrap up an approximate 9-13 day mission.
However, NASA is also evaluating two other options for the flight path and trajectory the EM-2 flight could take.
EM-2 Option 1 – DRO/NRO
Under this option, the EUS stage would be used to deliver Orion and a four-person crew into a DRO DRM flight.
In this manner, EM-2 would repeat the EM-1 DRO trajectory, only this time it would carry a crew through those maneuvers and flight path.
Additionally, NASA could also choose to send EM-2 to other destination orbits such as the L2 Halo, a Near Rectilinear halo Orbit (NRO), or a high-energy cislunar orbit that long-duration habitat missions might experience.
Under this mission option, the “Evolution of Orion Mission Design for Exploration Missions 1 and 2” report states that the increased performance gained from the use of an EUS on EM-2 could allowed for a co-manifested payload in the form of additional consumables to allow Orion and its four person crew to perform a mission greater than the 21 day limit currently afforded to the spacecraft.
This kind of DRO mission would result in a total mission duration of 25-26 days, so additional consumables would be needed assuming NASA opted to go with a maximum crew size of four.
EM-2 Option 2: Hybrid
However, since EM-2 will be the first Orion mission to carry people, the “Orion Mission Design” report notes that “the Orion Program and NASA may choose to fly a mission that is less risky in some aspects.”
The exact meaning of “less risky” could range from a desire to fly a path that conserves propellant for contingency abort cases to opting to fly a trajectory that avoids HLO and allows the crew to return to Earth faster than an HLO flight path would permit.
Under this consideration, a hybrid mission for EM-2 would preserve some elements of an HLO DRM flight and reject others.
This kind of mission would see the crew launch into three different Earth elliptical orbits.
The first orbit would be close-range, with a first-orbit-complete-perigee engine firing from the EUS to raise the craft’s orbit to 391 x 71,333 km (242.9 x 44,324.3 miles).
After this burn, Orion and its SM would separate from the EUS.
This second orbit of Earth would carry a total orbital period of 24 hours and would allow the EM-2 crew to fully vet and check their spacecraft and its systems before committing to the swing around the moon.
During this 24 hour orbit, if a system does not check out, Orion and EM-2 can remain in this orbit until the situation is resolved.
Conversely, if a serious issue develops while in this 24 hour orbit, Orion can return the EM-2 crew to Earth within 12 hours.
However, if all systems check out, Orion’s SM engine would, at a perigee of 391 km, fire for the TLI burn and propel EM-2 toward the moon.
This TLI would put Orion and EM-2 on a “near-free return trajectory,” notes the “Orion Mission Design” report, and would result in a swing through the lunar L2 point.
Concretely, it would result in Orion and EM-2 performing a lunar flyby at the L2 point at a distance of 61,548 km (38,244.15 miles) from the lunar surface.
At this point, Orion’s SM engine would perform the Return Trajectory Adjust (RTA) burn with a deltaV of 77 m/s (253 f/s) to aim the craft for the location Earth will be at for Entry Interface and landing.
This Hybrid mission would last approximately 15-16 days and would carry an added risk of radiation and MMOD as Orion would make multiple passes through the Van Allen Radiation Belts and Earth proximity space.
However, the “Orion Mission Design” report notes that the “radiation dosage to the crew for two revolutions in the intermediate orbit is roughly equivalent to a six-month stay on the ISS.”
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