Thursday, February 8, 2018

Efficient Utilization of the Space Launch System in the Age of Propellant Depots

by Marcel F. Williams 

SLS Block I and Block IB (Credit: NASA)

With the successful test launch of Space X's  Falcon Heavy, some have questioned why NASA continues to support the development of the Boeing/Orbital ATK Space Launch System (SLS). The Falcon Heavy is now the most powerful rocket in operation with the capability of deploying up to 63 tonnes of payload to Low Earth Orbit (LEO). But next year,  NASA will test launch an even more  powerful heavy lift vehicle.  In its earliest incarnation, the SLS will be capable of deploying at least 70 tonnes of payload to LEO. By the time its Exploratory Upper Stage (EUS) is developed in the early 2020s, the SLS will be capable of deploying more than 105 tonnes of payload to LEO. Future advances or alternatives to the SLS solid rocket boosters also promise to enable the SLS to  deploy more than 130 tonnes of payload to orbit.

Maximum payload deployment to LEO:

SLS Block 2: 130 tonnes

SLS Block 1B: 105 tonnes

SLS Block 1: 70 tonnes

Falcon Heavy: 63 tonnes

Delta-IV Heavy: 28.8 tonnes

Falcon 9: 22.8 tonnes

Atlas-V: 20.5 tonnes

But  the Space Launch System will have an additional advantage over other launch vehicles in its ability to also accommodate  payloads with substantially larger dimensions. While the Falcon Heavy and most other launch vehicles will continue to be  limited to housing payloads within a maximum diameter of 4.6 meters, the SLS will be capable of deploying payloads within its  fairing up to 9.1 meters in diameter.

NASA's Hubble space telescope has a 2.4 in diameter mirror. The SLS would be the only vehicle capable of  accommodating space  telescopes with a single mirror  8 meters in diameter mirror or  segmented mirrors up to 16.8 meters in diameter.  The SLS would also be only launch vehicle with a fairing size capable of deploying Bigelow's 65 to 100 tonne BA-2100 Olympus space station which requires a fairing diameter of at least 8 meters.

Beyond the payload fairing dimensions, the SLS would also be the only rocket capable of deploying  Lockheed-Martin's reusable Mars landing vehicle (MADV) to orbit.
Notional  MADV on to of SLS (Credit: Lockheed Martin)

Maximum (internal) payload fairing diameter:

Space Launch System (SLS): 9.1 meters

Falcon Heavy: 4.6 meters

Falcon 9: 4.6 meters

Atlas-V: 4.6 meters

Delta-IV: 4.6 meters

The cost of an SLS launch will largely depend on how frequently the heavy lift vehicle is launched. NASA launched as many as eight space shuttle (a heavy lift vehicle) missions in one year. But since the expendable RS-25 engines for the core vehicle won't be ready until the early 2020s, the SLS can't be routinely launched into space until that time. However, sixteen RS-25 engines derived from the old Space Shuttle program are available for four SLS launches until the new expendable engines are ready.

But the success of the SLS will depend on how efficiently and frequently it is utilized. Once the new RS-25 engines are in production, it will be essential for NASA to launch the SLS at least twice per year during the 2020's and a lot more frequently during the 2030s. 

Propellant producing water depots are still the key to opening up the rest of the solar system for eventual commercialization and colonization of the rest of the solar system. Since most propellant depots concepts utilize existing propellant tanks or existing propellant tank technology, the SLS would have a distinct advantage over other launch technologies because of the size and volume of its propellant tanks.

An EUS modified with the ULA's Integrated Vehicle Fuel ( IVF) technology (WPD-OTV-128) could be deployed by the SLS to LEO or EML1 with a LOX/LH2 storage capacity of 128 tonnes with a 450 Kwe solar power plant. At EML1, such a propellant producing water depot would be capable of producing approximately 45 tonnes of LOX/LH2 propellant per month plus an additional 12 tonnes of LOX per month. The notional WPD-OTV-128 would also be capable of redeploying itself in orbit around Mars or Venus to enable crew returns to cis-lunar space from those worlds. Propellant producing water depots (WPD-OTV-128) at LEO and EML1 could be supplied with water from private commercial launch vehicles such as the Falcon Heavy.

Notional solar powered propellant producing water depot (WPD-OTV-128) at EML1.

An SLS Block IB might be capable of deploying up to 35 tonnes of payload to EML1. An IVF modified EUS utilized as a reusable OTV (Orbital Transfer Vehicle) would be capable of transporting at least 70 tonnes of payload from LEO to EML1. Two such OTVs (positioned on opposite sides of the cargo) would be capable of transporting at least  140 tonnes of payload from LEO to EML1. So by utilizing propellant depots, anything the SLS can launch to LEO could also be transported practically anywhere within cis-lunar space.

A larger orbital transfer vehicles (OTV-400) and propellant depots (WPD-OTV-400) could also be derived from SLS propellant tank technology and  used for crewed missions to the orbits of Mars and Venus.
Notional OTV-400 orbital transfer vehicle for crewed interplanetary missions

 Reusable LOX/LH2 spacecraft deployed by the SLS or by private commercial launch vehicles could be utilized for cargo and crew missions between EML1 and the lunar surface and between EML1 and LEO. Lockheed-Martin's reusable MADV could be used to land astronauts on the Moon and Mars or  transport them between LEO and EML1.   Reusable vehicle concepts such as the XEUS spacecraft or a notional Altair-like reusable vehicle (ETLV-4) utilizing Boeing's 2.4 meter in diameter cryotanks could be used to transport astronauts to and from the lunar surface to EML1 or between LEO and EML1.

XEUS lunar lander (Credit: ULA)

NASA plans a crew launch of the SLS in the early 2020s with a plan to deploy the Orion Multipurpose Crew Vehicle (MPCV) to the vicinity of the Moon. But reusable spacecraft using propellant depots would make Orion's European manufactured Service Module (SM) obsolete. The Service Module could be replaced by a reusable LOX/LH2 ACES 68 which could also be utilized with  the ULA's future Vulcan spacecraft to deploy the Orion to LEO.

Notional ETLV-4 lunar lander

Notional CLV-7 lunar cargo lander

Another SLS advantage over other launch vehicles would be the inherent  ability to use SLS propellant tanks or SLS propellant tank technology to deploy large habitats into orbit and to the surfaces of extraterrestrial worlds.

SLS propellant tank derived 8.4 meter in diameter microgravity habitats would have enough internal space to easily accommodate 20 centimeters or more of water shielding to protect astronauts from the deleterious effects of heavy nuclei and major solar events while still providing enough room to accommodate hypergravity centrifuges up to 6 meters in diameter that could be used to mitigate some of the deleterious effects of microgravity on human physiology. At least two SLS propellant tank derived microgravity habitats with 662 m3 of internal volume (more than twice the internal volume of Bigelow's BA-330) could be deployed to LEO-- with a single SLS launch. 

Notional SLS propellant tank derived microgravity habitat (Credit: NASA)

8.4 meter in diameter  SLS propellant tank habitats designed for the lunar and Martian surfaces could be easily placed with the SLS  payload fairing for deployment to the surfaces of the Moon or Mars.
 Such SLS propellant tank derived habitats could provide spacious multi-level habitats for the surfaces of the Moon and Mars that can be easily protected from the dangers of excessive cosmic radiation, major solar events, micrometeorites, and extreme thermal fluctuations by dumping regolith into an automatically deployed regolith wall that could allow up to two meters of regolith shielding, reducing radiation exposure well below that of radiation workers on Earth. 
X-Ray of notional Lunar Regolith Habitat
Notional twin Lunar Regolith Habitats on top of a sintered regolith

Three 8.4 meter in diameter SLS  propellant tank derived habitats joined together by cables and a retractable boom. When rotating at 2rpm, the cylindrical rings of the telescoping booms would expand the AGH approximately  224 meters in diameter, producing a simulated gravity up to 0.5 g within the counter balancing habitat modules. For crewed interplanetary missions, the booms could be easily retracted so that the AGH can be re-docked with an Orbital Transfer Vehicle for trajectory burns during the beginning or end of any interplanetary mission. The 0.5g simulated gravity aboard an AGH would be higher than on the surface of the Moon (0.17g) or Mars (0.38g). Appropriate radiation shielding against the heavy ion component of cosmic rays could be internally provided by water. Permanent internal shielding against excessive levels of cosmic radiation exposure could be provided by iron extracted from lunar regolith. 
X-Ray of notional SLS propellant tank derived artificial gravity habitat
Notional rotating artificial gravity habitat at EML1

While the Falcon Heavy wouldn't even come close to the launch capabilities of the SLS, Space X is currently working on a Super Heavy Lift vehicle that could.  The two stage methane fueled  BFR would be 9 meters in diameter (0.6 meters wider than the SLS) and be capable of deploying up to 150 tonnes to orbit (20 tonnes more than the SLS Block 2. Clearly, Elon Musk understands the value of large super heavy lift space rockets.

Links and References 

Lockheed Martin's Reusable Extraterrestrial Landing Vehicle Concept for the Moon and Mars

(Part II) Practical Timelines and Funding for Establishing Permanent Outpost on the Moon and Mars using Propellant Producing Water Depots and SLS and Commercial Launch Capability

Reusable Heavy Cargo and Crew Landing Vehicles for the Moon and Mars

The ULA's Future ACES Upper Stage Technology

SLS Derived Artificial Gravity Habitats for Space Stations and Interplanetary Vehicles

Space X BFR (Rocket)

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