|SLS propellant tank derived DSH @ EML1 (credit NASA)|
NASA has viewed a DSH as a necessary component for safely transporting humans from cis-lunar space to Mars orbit in the 2030's and also as a gateway to the lunar surface and beyond. The Earth-Moon Lagrange points EML1 and EML2 have most often been proposed as the place where a Deep Space Habitat should be deployed.
EML2 (L2) has the advantage of requiring the lowest delta-v from LEO in order to deploy the DSH into a halo orbit around the Lagrange point. But crewed journeys from LEO to L2 also has the disadvantage of taking as long as 8 days to reach the habitat if the low delta v of 3.43 km/s is to be taken advantage of. Such a long journey would expose astronauts to two to four times as much cosmic radiation as journeying to EML1. A higher delta-v of 3.95 km/s could transport a crew to EML2 in just four days. But this would be higher than the 3.77 km/s delta-v requirement to transport crews from LEO to EML1. EML1 also has the advantage of a fast 2 day journey from LEO at 4.41 km/s. Such fast journeys would reduce radiation exposure while also reducing the chance of traveling during a major solar event in half.
|The Earth-Moon Lagrange points (Credit the Artemis Project)|
Another, long term, disadvantage of a DSH at EML2 is that radio transmissions between the habitat and Earth could interfere with future radio telescopes deployed on the back side of the Moon in order to avoid radio interference from the Earth's surface, Earth orbit, and space craft traveling to and from the Moon.
Delta- V budgets between LEO and EML1 or EML2
LEO to EML1 (~ 2 days) - 4.41 km/s dv
LEO to EML1 (~ 4 days) - 3.77 km/s dv
EML1 to Lunar Surface (~3 days) - 2.52 km/s dv
Lunar Surface to EML1 (~3 days) - 2.52 km/s dv
LEO to EML2 (~ 8 days) - 3.43 km/s dv
LEO to EML2 (~ 4 days) - 3.95 km/s dv
EML2 to Lunar Surface (~3 days) - 2.52 km/s dv
Lunar Surface to EML2 (~3 days) - 2.52 km/s dv
Aesthetically, a Deep Space Hab positioned at EML1 would probably have the most spectacular views within cis-lunar space. An astronaut at EML2 would view an Earth that is slightly smaller than viewed from the front side of the Moon while the view of Earth at EML1 would be slightly larger than is seen from the lunar surface. Both EML1 and EML2 would view a Moon that is titanic in size relative to its view from the Earth. But EML2 would only be able to view the back side of the Moon while EML1 would only be able to view the front side of the Moon.
Because of the reduced time and radiation exposure to get there, the fact that an EML1 habitat wouldn't interfere with radio telescopes on the back side of the Moon, plus the aesthetic view, I think NASA should deploy the Deep Space Hab at EML1 rather than at EML2.
The primary purposes for an EML1 (L1) Deep Space Habitat (DSH) should be to:
1. Serve as a gateway to the lunar surface. Astronauts traveling from the Earth or from the lunar surface could dock their spacecraft at the EML1 habitat, taking advantage of the larger accommodations at the DSH while transferring from one vehicle to another.
2. Serve as a storm shelter during the occurrence of major solar events. This will probably require at least 30 cm of water shielding for the areas within the habitat that the astronauts will be occupying. Major solar events can last for several minutes or up to several hours.
3. Serve as a maintenance and repair station for reusable lunar shuttles (ETLV) and orbital transfer vehicles. Flex Craft docked at the DSH could be utilized for extravehicular repairs to nearby water/propellant depots and associated solar arrays at EML1.
4. Test the effectiveness of various levels of water shielding required to mitigate cosmic radiation and potentially brain damaging heavy nuclei. In theory, 30 cm of water would be enough shielding to to stop the penetration of the heavy nuclei component of cosmic rays, reduce the annual exposure of cosmic radiation in general to less than 25 Rem per year, while also significantly mitigating the effects of major solar events. While an even thicker shielding of water could reduce cosmic radiation exposure, a minimal amount of shielding will be required to minimize the mass for crewed interplanetary vehicles.
5. Test the integrity and reliability of the pressurized habitat structure which could also be used for habitats on the surface of the Moon and Mars and for rotating interplanetary artificial gravity habitats.
Its probably the intent of Congress for NASA to design the habitat module that will transport humans safely to Mars. But because of the inherently deleterious physical and psychological effects of a microgravity environment on human beings, its unlikely that any microgravity habitat will ever be able to accomplish this goal.
Under microgravity conditions, astronauts can lose between 1 to 1.5% of their bone mass in a single month and without regular exercise, astronauts can lose up to 20% of their muscle mass in just 5 to 11 days. A microgravity environment can reduce cardiovascular fitness-- possibly increasing the chances of heart attaches. And vision problems of varying degrees of severity can occur-- especially in older men. The infected spray from the cough or the sneeze an ill person on board floats in the air instead of falling to the floor, enhancing the spread of infection aboard ship-- especially in a confined environment. Unfortunately, blood flow redistribution in a microgravity environment can effect medicines ingested or injected into the human body to treat illnesses.
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.
Added to the serious problems above, there are other annoying problems in a microgravity environment that could enhance discomfort and psychological stress aboard ship such as:
1. Weight loss: the less strenuous conditions diminish appetite, resulting in weigh loss which could become excessive if astronauts don't exercise and eat regularly.
2. A degraded sense of smell and taste: your favorite foods could taste a little different under microgravity
3. Clumping of sweat and tears and perspiration: there's no gravity to force trickles of water to run off the human body
4. Facial and speech distortions: the face becomes puffy and the voice tone and pitch becomes more nasal. This could cause some to misinterpret another individuals expression, possibly causing tension between two individuals aboard a multiyear mission.
5. Increased flatulence: since digestive gasses no longer rise towards the mouth, their is an increase in gas being expelled through the posterior orifice
The problems listed above could be viewed as only a minor inconvenience on short missions into space. But during long interplanetary journeys lasting months or years, such problems could be annoying enough to enhance stress and increase tension aboard ship.
It might be possible to eliminate all of these deleterious microgravity related problems aboard an interplanetary vehicle by simply rotating pressurized habitats in counter balancing pairs to produce a significant level of simulated gravity. The additional benefit of having two pressurized modules is that it also provides a back up module in case there are serious life threatening malfunctions at the other habitat module.
Pressurized habitats capable of being used in space and on the surface of the Moon or Mars could also be used as counter balancing habitats for rotating spacecraft and space stations that produce some levels of artificial gravity. And development cost could be greatly reduced if the basic habitat pressurized tank can be used for microgravity habitats, low gravity surface habitats, and for artificial gravity habitats.
NASA could significantly reduce development cost by utilizing SLS propellant tanks for both a DSH but also for lunar and martian habitats. The lunar and martian regolith habs that I've previously proposed would use an SLS propellant tank as a pressurized habitat. Once the habitat module is properly placed on the lunar surface, a kevlar regolith wall sandwiched between eight three meter wide aluminum panels would automatically deploy, allowing a lunar backhoe to deposit regolith shielding within the two meter cavity between the outer wall and the inner cylindrical wall.
Since crewed interplanetary voyages to Mars probably won't take place until the 2030's, serious funding by NASA for the development of artificial gravity habitats for interplanetary journeys probably won't have to start until the early 2020's. However, this doesn't mean that a DSH habitat couldn't be designed to function as a microgravity habitat, as a low gravity habitat, and as a simulated gravity habitat in order to reduce cost for both the cis-lunar program in the 2020's and for the Mars program in the 2030's.
If such habitats are to be used for long interplanetary journeys in the future, they must be as comfortably spacious as possible while also minimizing mass. NASA is evaluating several types of potential Deep Space Habitats derived from current technology:
Habitat Modules Derived from Current Technologies
SLS full class propellant tank derived:
Dry mass: 22.4 tonnes
Habitable volume - 519 m3
SLS minimum class propellant tank derived:
Dry mass: 17.3 tonnes
Habitable volume: 353 m3
Dry mass 20 to 23 tonnes
Habitable volume: 330 m3
ISS node & MPLM:
Dry mass: 35.5 tonnes
Habitable volume: 108 m3
ISS hab & MPLM:
Dry mass: 32 tonnes
Habitable volume: 90 m3
The ISS derived habitats only provide between 2.8 meters to 3 meters cubed of habitable volume per tonne. The Bigelow BA-330 would provide significantly more volume, between 14 m3 and 17 meters cubed of habitable volume but within severely confined areas. The SLS propellant tank derived habitats, however, would provide between 20 m3 and 23 m3 of habitable space per tonne (35% to 64% more habitable volume per tonne). Since SLS propellant tanks will already be in production for SLS launches, manufacturing more tanks for Deep Space Habitats and for surface habitats for the Moon and eventually for Mars should greatly reduce development cost for a Deep Space Hab.
Four of the RS-25 engines will be dedicated to an SLS launch in 2018 to test the MPCV (Multipurpose Crew Vehicle). Another four engines will be used by NASA for the first crewed MPCV mission beyond the Earth's magnetosphere. That only leaves enough engines available for two additional SLS launches until new engines are in production.
One SLS launch would be enough to deploy the DSH to EML1. But a the final engines available for one more SLS launch would not be able to transport enough water to appropriately radiation shield the DSH. This could mean that DSH deployment might have to be delayed until 2022 or 2023.
In previous articles, I have suggested that NASA needs to commit itself to developing a reusable single staged Extraterrestrial Landing Vehicle (ETLV) for crewed and robotic missions to the surface of the Moon, the moons of Mars, and to the Martian surface (with an ADEPT or HIAD deceleration shield). An essential component of a reusable ETLV would be an ETLV derived water/propellant depot (WPD) that would be capable of using solar electricity to produce LOX and LH2 from water. Serious funding from Congress for the development of the ETLV and the associated landing vehicles and orbiting depots derived from it should start in 2017, in my opinion, at a funding level of at least $1.5 billion per year.
|An ETLV derived and SLS deployed water/propellant depot at EML1 near solar power station. The solar powered facility would be capable of storing up to 100 tonnes of water while also producing and storing up to 60 tonnes of LOX/LH2 propellant.|
Once a WPD has been deployed to EML1 then private commercial providers could deliver water to EML1. Space X will be testing its new Falcon Heavy in 2016. Such a vehicle might be able to supply 10 to 15 tonnes of water to EML1 per launch. The ULA also has plans to develop a heavy lift version of their future Vulcan rocket (the Vulcan Heavy) which should also be capable of delivering 10 to 15 tonnes of water to EML1 per launch. So starting in 2020, commercial launches could be used to deliver 10 to 15 tonnes of water per month to EML1 (120 to 180 tonnes per year). Monthly commercial water deliveries will continue to EML1 until lunar water manufacturing and exporting facilities on the lunar surface are complete in the middle or late 2020's.
|Artist rendition of Space X Falcon Heavy (Credit: Wikipedia)|
|Artist rendition of ULA Vulcan Heavy|
Once the water has been delivered to EML1 and fairly close (within a few hundred meters) of the water/propellant depot, the WPD will rendezvous with the water tankers, extracting and depositing the water with WPD's water tank. The WPD will dock at an solar power station where it will use that power to convert some of the water into LOX and LH2 while storing the rest.
After the DSH is deployed to EML1, the WPD will also rendezvous with the DSH, transferring water to the habitat for radiation shielding, drinking, and air production. Once ETLV spacecraft are ready for robotic and crewed missions to the lunar surface, they will be fueled by the WPD at EML1.
The last remaining engines from the Shuttle era can then be used to launch the MPCV to EML1, testing the ability of the SLS to deliver astronauts safely to the Earth-Moon Lagrange points while also checking out the integrity and functionality of the Deep Space Habitat.
Since long periods of time under microgravity conditions is inherently deleterious to human health, time aboard the DSH at EML1 should be constrained. For astronauts over the age of 40, the most vulnerable astronauts to microgravity visual damage, I'd limit missions confined to microgravity environments to only 16 days. For astronauts under the age of 40, I'd limit the stay at EML1 to less than 31 days. Such short stays at EML1 would ensure that the astronauts would not receive enough radiation in the DSH to prevent them from participating in future interplanetary missions where they will be exposed to months and even years of cosmic radiation bombardment.
While thicker water shielding for the DSH could further reduce radiation exposure, it would also add substantial amounts of mass to an interplanetary vessel. One of the goals of the DSH should be to replicate conditions for astronauts aboard an interplanetary vessel. So the DSH should only provided with enough water shielding similar to that of an interplanetary vehicle. And an interplanetary habitat only has to be shielded to a level that would enable astronauts to complete a three year round trip to and from Mars and Mars orbit without exposing them to more than 50% of the recommended lifetime radiation exposure recommended by NASA-- which would be about 100 Rem for the least vulnerable passengers (women 25 years of age).
Links and References
Spending Bill To Accelerate NASA Habitation Module Work
Deep Space Habitats
Commonality between Reduced Gravity and Microgravity Habitats for Long Duration Missions
Building an L1 depot in phases
Habitat Concepts for Deep Space Exploration
NASA Mega-Rocket Could Lead to Skylab 2 Deep Space Station
Solar Storm and Space Weather
Cosmic Radiation and the New Frontier
NASA Contracts Production of New RS-25 Engines for the Space Launch System
SLS Fuel Tank Derived Artificial Gravity Habitats, Interplanetary Vehicles, & Fuel Depots
ULA Future Full Spectrum Lift Capability
The SLS and the Case for a Reusable Lunar Lander