SLS Flexibility: Exploration roadmap focus taking center stage

by Chris Bergin

Teams at the main NASA centers are continuing to build up their involvement in the Space Launch System (SLS), ranging from mission operations in Houston, infrastructure evaluation at the Kennedy Space Center (KSC) and vehicle design at the Marshall Space Flight Center (MSFC). Exploration roadmap details remain under evaluation, although NASA claim the vehicle’s “flexibility” is providing an intriguing range of options.

SLS Mission Planning:

With the vehicle finally receiving its official unveiling by NASA’s administrator Charlie Bolden, the Agency’s own language is now publicly reflecting what has been the majority opinion from within the NASA engineering community that was tasked with creating the HLV options and finalizing the configuration.

“SLS Is a National Asset for Multiple Stakeholders and Partners. Vehicle Development and Acquisition Phased to Fit Budget Constraints and Schedule Targets. Technical Trade Studies and Business Planning Validated Independently,” noted just a few sound bytes NASA’s own documentation used to introduce the vehicle to the industry base recently, a far cry from the negative language of the Preliminary SLS response requested in the NASA Authorization Act of 2010.

While a finalized exploration roadmap for SLS is still under design – under the leadership of former Space Shuttle Program (SSP) manager John Shannon – SLS’ now-known capabilities has allowed for a basic overview of what the vehicle will be used for, as NASA returns – and expands on – it Beyond Earth Orbit (BEO) heritage.

The opening two missions are believed to be set in stone, with SLS-1 and SLS-2 tasked with missions to the Moon – the first being an unmanned flight for the Orion (MPCV), scheduled for a 2017 launch date, followed by a crewed mission, as early as 2018/2019 – as requested by mission planners, as much as this remains manifested as 2021 for the interim.

These flights will involve the Block I vehicle, which will loft Orion to its circumlunar trip around the moon, followed by a crewed lunar orbit mission. After those two flights, no definitive decisions have been made, but the options are starting to solidify.

Firstly, the Lunar flybys may become a precursor for a crewed return to the Moon’s surface. Advocated as “Lunar Surface First” internally, officially documentation also shows such missions are part of SLS’ capabilities.

“Earth’s Moon: Witness to the birth of the Earth and inner planets. Has critical resources to sustain humans. Significant opportunities for commercial and international collaboration.”

Such missions would provide a natural bridge to the favored NEO (Near Earth Orbit) mission in the mid-2025, working from the heritage of the Apollo missions, whilst testing out the new vehicles for long duration stays in BEO. However, without a known NEO target destination, the timeline and schedule of the mission remains flexible.

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The alternatives past SLS-1 and 2’s Lunar orbital missions would range from High Earth Orbit and beyond, potentially for a crewed mission to place an SLS-launched telescope at a Lagrange Point. However, a lot would depend on the costs, where some argue a “basic” Lunar Lander for Moon mission would likely be cheaper than an advanced telescope, along with providing greater public interest.

“High-Earth Orbit (HEO)/Geosynchronous-Earth Orbit (GEO)/Lagrange Point: Microgravity destinations beyond LEO. Opportunities for construction, fueling, and repair of complex in-space systems. Excellent locations for advanced space telescopes and Earth observatories,” added official documentation.

The reference to fueling provides a potential marriage between SLS and propellent depots, something which would partially appease some critics, given the remaining anti-SLS activists are now focusing on the use of the popular prop depot architecture as a full alternative to the HLV.

Ironically, some Prop Depot designers – along with several NASA HLV presentations over recent years – have advocated the use of both strengths in tandem. This was also a major element of what remains the most expansive HLV overview in recent times.

Per the 726 page HLV “final” assessment presentation (available on L2) – which mainly covers the use of a Sidemount HLV, as much as it covers the now confirmed In-line configuration – commercial vehicle “additions” to the HLV approach are heavily cited in the BEO mission options, namely higher energy missions.

In providing benefits to Lunar and Deep Space missions, the assessment notes the addition of propellant deports would significantly improve the overall exploration architecture. This is called the “Open Architecture – NASA Crew Elements and Commercial Propellant” example, showing “L1 Enables Anywhere, Anytime, Global Access.”

“HLV capabilities for higher energy missions beyond LEO were examined. This required the use of upper stages. Existing stages from Delta IV and Atlas V and new stage designs using RL-10B2 (existing), RL-60 and J-2X LOX/LH2 rocket engines (as now chosen) were used to estimate payloads to Geosynchronous Transfer Orbit, Geosynchronous Orbit, Trans-Lunar Injection and to the Earth-Moon L1 point,” the presentation noted.

Launched by a single HLV, the depot – based at Lagrange Point L1 – could be refuelled using commercial vehicles, reducing the mass required to launch from Earth’s surface on a Lunar or deep space mission via “dry” – and potentially reusable – landers.

“A propellant depot at the Earth-Moon L1 point would significantly improve lunar and deep space exploration mission operations by providing an infrastructure capability for deep space transportation and by opening up participation to international partners and commercial vehicles. This propellant depot outside of Earth gravity well would act as a refuelling station for spacecraft on the way to the lunar surface or Mars,” added the presentation.

Such a plan would result in SLS with an EDS (Earth Departure Stage) delivering Orion and a dry lunar lander to L1. The dry lunar lander would be loaded with 25 mt of propellants at the depot to complete the lunar phase of it’s mission. Dry launch of the LSAM element to L1 would dramatically reduces the spacecraft weight constraints, permitting more flexible and robust operational capabilities to be designed into the lander.

This approach also plays into the growth requirements of SLS, with Block 1 capable of 70mt, and Block IA capable of 105mt.

An alternative to requiring the full 130mt SLS – which may not be ready to launch until the 2030s – would allow for a redirection of vehicle funding towards mission content/architecture. For the interim, the 130mt SLS is officially classed as required.

Mars itself remains the ultimate destination, as much as one of its moons may be the opening mission – with Phobos highlighted in the Flexible Path approach.

“Mars and its Moons Phobos and Deimos: A premier destination for discovery: Is there life beyond Earth? How did Mars evolve? True possibility for extended, even permanent, stays. Significant opportunities for international collaboration. Technological driver for space systems,” is how NASA presented the case for the Red Planet to the SLS industry base.

However, the main hurdle is the timeline, with such missions still not expected until the 2030s, by which time several government and NASA administrations will have come and gone, raising concerns on the impossibility of predicting budget levels two decades into the future.

While the roadmap is an internal effort, NASA will take outside input on the mission content, opening with a three-day Human Space Exploration Community Workshop in San Diego starting on Monday, November 14.

As noted by NASA on Tuesday, the agency will introduce the International Space Exploration Coordination Group’s Global Exploration Roadmap during the event.

“The goal is to review the work done developing international exploration scenarios while seeking community input on the long-term scenarios represented in the roadmap,” noted NASA.

“NASA is seeking industry and academia feedback to shape strategy, assist with investment priorities and refine international exploration scenarios for human exploration and operations through the 2020’s.”


Most of the work at the Kennedy Space Center (KSC) is related to Ground Operations, which will be tasked with caring for the vehicle through integration, rollout and launch.

Planning for the modifications to the Crawler Transporters (CTs) is ongoing, as are evaluations into the Ares Mobile Launcher (ML) which will launch SLS from Pad 39B, following rollout from the Vehicle Assembly Building (VAB).

Early notes point to the changes required for the ML, which was designed to launch the “stick” configuration of the Ares I from a specifically tailored launch mount. Such changes include moving the centerline away from the Fixed Service Structure by eight feet, to the old Saturn V centerline.

Such a change is related to the potential SLS will become an all-liquid vehicle, as will be decided when the booster competition awards the long-term contract to either ATK and an evolved Solid Rocket Booster, or a liquid – likely RP-1 – alternative.

KSC engineers are also studying the umbilicals between the ML and the for the SLS, some of which may be the Ares I designed TUUAs (Tilt Up Umbilical Arms), potentially in tandem the Saturn V-style swing arms, and/or even the flexible arms as used by EELVs.

Currently, engineers are planning to reuse the Shuttle TSM (Tail Service Masts) for the core LOX and LH2 stage, while a new flame deflector will be designed, replacing the current shuttle flame deflectors at Pad 39B.


Over in Houston, the famed Mission Operations Directorate (MOD) team have outlined a draft Memorandum Of Understanding (MOU) with the SLS management, ranging from their involvement from a Flight Operations standpoint, through to Ascent Operations and participation with the Systems Requirements Review (SRR).

“Flight Operations Technical Team: MOD proposed a joint Flight Ops Tech Team. Working group-level forum for discussing ascent operations technical topics, flight ops products. MOL personnel agreed in principle. Specific structure of the team requires further definition,” noted MOD’s latest SLS presentation (L2).

“Reports to SLS Operations. Discipline Lead Engineer (DLE) – DLE reports to SLS Chief Engineer. Forward Work: MOD will draft Team charter jointly with SLS Operations group.”

While it was expected, MOD was backed by the Marshall Space Flight Center (MSFC) – who are the lead center for SLS – on their “prime” role for mission ops when the SLS clears the tower. This will match how Shuttle operations worked, with KSC’s Firing Room at the Launch Control Center (LC) handing over to the Flight Control Room (FCR) in Houston almost immediately after the vehicle launched.

In fact, it appears MOD’s foundation of “Plan, Train, Fly (P/T/F)” is assured for Orion and SLS missions, as much as some of the workload may include Marshall, again matching Shuttle missions.

“SLS Ascent Operations: MSFC reinforced that MOD will be the prime SLS ascent flight control team for MPCV (Orion) and Cargo missions. Includes all elements of the integrated vehicle (SLS, CPS, MPCV). Includes commands, ops products, training. MOD will be responsible for iCPS and CPS flight control. There may be flexibility for MSFC involvement on non-crewed, non-NASA payloads,” added the presentation.

“(Future Work). MSFC/HOSC is responsible for providing engineering support. Includes real-time support to MOD. If a blended team is required, all flight controllers will perform their duties from JSC/MCC. Discussed option for MSFC MPSR (Multi-Purpose Support Room) for some cargo missions.”

Further work will be required on fine-tuning MOD’s role, with a second meeting due before the end of November to review the agreements.

“Agreements: Memorandum of Understanding (MOU). Reviewed a draft MOL-MOD MOU. Descoped the document from an MOU with SLS Program. Now focused on Organization-to-Organization. Laid out proposed roles and responsibilities for SLS Operations between MOD and MOL,” the summary continued.

“High-level tasks identified. Captures all SLS ascent flight functions that affect MOD and MOL. Lead roles identified where appropriate. Introduces SLS Flight Operations Technical Team. Further details are spelled out in a White Paper. This content is also being copied into the SLS Program Plan.

“Summary Results: Reinforced MOD roles in SLS ascent flight control. Identified and Established interfaces with SLS Operations. Began Charter for Flight Operations Technical Team. Completed first drafts of MOU and White Paper defining roles and responsibilities between MOD and MOL.

“Forward work: Complete MOU, White Paper, and Charter. Have signed by MOD, MOL, and SLS. Conduct second F2F with SLS Ops in Oct.-Nov.. May occur around SRR checkpoint, discuss Vehicle Management and Aborts. Participate in SLS document reviews and SRR.”

SLS at Marshall:

The Alabama center was already deep into their SLS work well before General Bolden officially announced the configuration, not least because the Design Reference Vehicle (DRV) was designed by MSFC’s winning Requirements Analysis Team (RAC-1).

The current effort is focused on the combined SRR/SDR (Systems Design Review), which will continue through until early next year, with numerous trades being evaluated for the purpose of fine-tuning the configuration of the vehicle.

Due to the nature of trades, current documented configurations are open for changes, with one example noted for the 105mt Block IA SLS.

According to performance analysis notes, this configuration may have added flexibility, such as the potential to launch somewhat smaller payloads than the 105mt capability.

Options exist – or are under current evaluation – to launch the Block IA minus the core engine (four, as opposed to five, RS-25s), host a reduced propellant load, or utilize both methods.

This flexibility is important, as it is likely the Block IA will be the main SLS workhorse configuration, ahead of the fully evolved 130mt SLS coming on line.

(Images: Via L2 content, driven by L2′s new SLS specific L2 section, which includes, presentations, videos, graphics and internal updates on the SLS and HLV, available on no other site. Other images via NASA.)

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