Monday, April 11, 2016

SLS Derived Artificial Gravity Habitats for Space Stations and Interplanetary Vehicles

Commercial space plane approaching a rotating AGH space station @ LEO;
a reusable Orion/ACES-41 OTV is docked at one of the  central ports.

by Marcel F. Williams

The inherently deleterious effects of a microgravity environment severely limit the human ability to remain healthy during several months or years in space.

Minor problems associated with long periods of time in a microgravity environment include: weight loss, a degraded sense of  taste and smell, the clumping of perspiration and tears, facial and speech distortions, and an increased frequency of  flatulence.

However, far more serious problems related to months or years in a microgravity environment include:

1. The loss of 1 to 1.5% of bone mass in a single month

2. The loss of up to 20% of muscle mass in just 12 days without regular exercise.

3.  Significant reduction in cardiovascular fitness

4. Fluid loss and bone demineralization,  increasing the blood's calcium concentration while increasing the risk of  developing kidney stones.

5. Increased frequency of common cold due to the fact that the infected spray from the cough or the sneeze from a person  floats in the air instead of falling to the floor, enhancing the spread of viral infections aboard ship, conditions already enhanced by the extremely confined environment. 

6. The hampered effect of medicines due to the changes in blood flow redistribution

7. Vision problems of varying degrees of severity can occur in men in their 40s or older. 

Returning to Earth after  a few months aboard the ISS, the blood pressure of some astronauts drops abnormally low when they move from a lying position to a sitting or standing position. Some astronauts even have problems standing up, walking, and turning and stabilizing their gaze.

But practically all of the problems associated with a microgravity environment could be eliminated if permanent space stations and crewed interplanetary vehicles  were configured to produced significant levels of simulated gravity.

Rotating a spacecraft in order to produce artificial gravity has long been proposed as a technological  solution to the health problems associated with a  microgravity environment. However, research has shown that rotations exceeding 2 rpm (rotations per minute) require several hours to several days for the human body to adjust. During those hours or days of  adjustment, a significant number of astronauts would experience nausea associated with the Coriolis effect.

Research suggest that rotations that are 2 rpm or less require no training or time to adjust to the simulated gravity environment. A slow rotation also makes it easier for spacecraft to dock at the central axis while allowing astronauts to enter and exit the rotating habitat without the need for several hours or days of physiological adjustment. So rotating a habitat at 2rpm or less, would appear to be the simplest way to avoid the nausea associated  with the Coriolis effect.

However, at 2 rpm, producing a simulated gravity similar to that experienced on Earth  would require habitat modules extending at least 224 meters from the central axis, a spacecraft 448 meters in diameter if  twin counterbalancing habitats were utilized.

But a 112 meter  rotational radius would only be required to produce an artificial gravity of 0.5 g at 2 rpm,  a simulated gravitational level higher than on the lunar surface (0.17g) or on Mars (0.38g). The rate of rotation could even be decreased to simulate levels of gravity on the surfaces of the Moon and Mars.

Rotating AGH with a standard 112 meter radius

2.0 rpm - 0.5g (50% Earth simulated gravity)

1.7 rpm - 0.38g (Mars simulated gravity)

1.17 rpm - 0.17 (Lunar simulated gravity) 

Notional SLS launch of a three module artificial gravity habitat (AGH).

But even  launch vehicles the size of the SLS wouldn't be able to deploy habitats with lengths longer than 40 or 50 meters (radii less than 20 to 25 meters from the central axis).  Attaching long cables or tethers has frequently been proposed as a convenient way of greatly extending the radius of a rotating habitats.   Even in its earliest incarnation, the SLS should be able to deploy large payloads up to 70 tonnes in mass. So, with a single launch,  it would  be relatively easy for the SLS to deploy three pressurized habitats that were attached to each other by cables that could be extended once the habitat begins to rotate in space.   

SLS derived pressurized habitats

Credit: NASA

SLS minimum class propellant tank derived:
Dry mass: 17.3 tonnes
Habitable volume: 353 m3
Habitat length:13.5 meters
Habitat diameter: 8.4 meters

Credit: NASA

SLS full class propellant tank derived:
Dry mass: 22.4 tonnes
Habitable volume: 519 m3
Habitat length: 16.5 meters
Habitat diameter: 8.4 meters 

OTV-400 prepares to be fueled with LOX/LH2 propellant at a WPD-OTV-400 propellant depot @LEO

Once the AGH (Artificial Gravity Habitat) is in low Earth orbit, a large reusable orbital transfer vehicle,  fueled with LOX/LH2 propellant at a LEO orbiting propellant depot,  could be used to transport  the AGH practically anywhere within cis-lunar space or even to the orbits of Venus or Mars.

OTV-400 transports an AGH to an EML1 halo orbit.

The notional AGH habitats described here would be derived from SLS propellant tank technology. The rotating habitat  would consist of two twin habitat modules connected by cables to a central habitat module.  Gaseous hydrogen and oxygen thrusters would be used to rotate or to maneuver the AGH in space.  The hydrogen and oxygen used for space  maneuvers could be directly supplied to the thrusters through the electrolysis of water normally used for the production of air (oxygen) for the crew.

Interior of an Artificial Gravity Habitat (AGH) configured for launch aboard the SLS
Once the AGH is rotating between 1.7 to  2 rpm, the rings connecting the two habitat modules will detach, allow the four sheaves on each side to extend their 100 meter long cables.  Light weight expandable and retractable booms composed of large aluminum cylinders only a few millimeters thick would conceal the connecting habitat cables from view. The twin light weight metallic booms would serve as levers, increasing or decreasing the AGH rotation provided by the hydrogen and oxygen thrusters.

Rotating AGH at EML1 as it begins to expand its interior cables and exterior booms.

Within the interior of each boom, a pressurized module, three meters in diameter,  would serve as an elevator  to transport astronauts from the peripheral habitat module to the central habitat module. The elevator system will consist of two electric drives and two sheaves with two deflector sheaves to provide a gap between the elevator module and the counterweight.

The top and at the bottom of the elevator modules will be equipped with active CBMs (Common Berthing Mechanisms) allow astronauts to enter and exit the elevator modules from the central habitat or the peripheral habitats. Large solar panel recharged lithium batteries will provide power for the elevator and boom cables. 

Once astronauts exit the elevator into the-- central habitat-- they would have access to the elevator module that could transport them to the counter balancing habitat or access to a spacecraft docked at the central axis.

An OTV-400 deployed AGH: Top: OTV-400 transports AGH; second from top: OTV-400 separates from AGH; Third: AGH begins to rotate at 2rpm; bottom: AGH expands its booms and its retractable solar panels.

Cosmic radiation exposure at the peripheral habitats would be mitigated by 30 centimeters of water surround the walls, the ceiling, and the floor. 30 centimeters should be enough shielding to reduce radiation exposure  to less than 25 Rem per year during solar minimum conditions. 30 centimeters of water could also protect the astronauts from the dangers of   major solar events. Circulating the water shield outside of the inhabited areas could also  serve as a heat radiator, transporting warm water from the habitat to a water loops  below the pressurized module where excessive heat generated inside of the habitat could be radiated into space.

During the last leg of an interplanetary journey, the water shielding can also be dumped into space just a few hours or a few days before the last trajectory burns into orbit around a planet. Since water shielding can add more than 100 tonnes of mass to an interplanetary vehicle, dumping it before the final trajectory burns to achieve orbit could substantially reduce the amount of propellant required for an interplanetary mission. Once in orbit,  the water shielding can be quickly restored from pre-deployed orbiting water/propellant depots.

Because of the Earth's magnetosphere and the Earth's mass, an AGH at LEO could reduce radiation exposure to less than 15 Rem a year for astronauts on board. But permanent habitats beyond the Earth's magnetosphere will require substantially more shielding. Forty centimeters of iron shielding derived from lunar regolith or imported asteroids combined with a few centimeters of temperature regulating water shielding  could reduce cosmic radiation levels within inhabited areas below  the maximum levels of radiation allowed for radiation workers on Earth.

AGH @EML1 with an Orion/ACES docked at one of its central ports while a crew carrying ETLV-2 moves away from the AGH, beginning its journey to a lunar outpost at one of the lunar poles.

Average Annual Station Keeping Delta-V Requirements

LEO --------------------------- less than 5 m/s

EML1 and EML2 ----------- less than 10 m/s

EML3, EML4, and EML5 - less than 1 m/s

In order for permanent space stations to  maintain their proper orbits, propellant for station keeping will still be required. Fortunately, within cis-lunar space, station keeping only requires a delta-v of less than 1 meter per second (Earth-Moon Lagrange points 3, 4, and 5) up to  10 meters per second (EML 1 and EML2). So even the heaviest iron shielded AGH (~2000 tonnes) would  require less than 5 tonnes of LOX/LH2 propellant annually for station keeping at EML1 and EML2.

An interplanetary crewed AGH is deployed to high Mars orbit for ETLV-2 exploration of the martian moons: Deimos and Phobos.

The extraterrestrial colonization of low gravity worlds with at least 0.1 of gravity, could restrict humans to the surfaces of the Moon, Mars, Mercury, and Callisto. But SLS propellant tank derived artificial gravity habitats could lead the way towards much larger artificial gravity habitats which could eventually allow humans to colonize and exploit extraterrestrial resources in practically every orbital region of the solar system. 

© Marcel F. Williams

New Papyrus Magazine

Links and References

If We're Serious About Going to Mars, We Need Artificial Gravity

 Gravity is a Massive Problem

 What if you were born in space?

What's the minimum spin hab?


SLS Fuel Tank Derived Artificial Gravity Habitats, Interplanetary Vehicles, & Fuel Depots

Deep Space Habitats

Habitat Concepts for Deep Space Exploration

Maintaining a Safe, Stable, and Human Accessible Parking Orbit 

Living and Reproducing on Low Gravity Worlds

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