Monday, September 7, 2015

Reusable Hoppers and Orbiters for Rapid Lunar Transportation and Exploration

by Marcel F. Williams

For NASA and its future SLS program,  developing  a reusable single staged extraterrestrial  landing vehicle (ETLV) could allow America to send astronauts  to the surface of the Moon, Mars, and even to the surfaces of the moons of Mars. Such an ETLV could be used to conveniently transport astronauts from EML1 (Earth-Moon Lagrange point 1) to the surface of the Moon and back to EML1 on a single fueling of LOX/LH2 propellant. NASA astronauts could reach EML1 and return to the Earth via an SLS launched Orion spacecraft.

Eventually, by deploying ETLV derived orbital propellant depots at points of departure and destination, such a reusable spacecraft could also be used as an orbital transfer vehicle, transporting astronauts between LEO and the Earth-Moon Lagrange points. This would allow Commercial Crew vehicles to shuttle NASA astronauts to LEO to dock with an ETLV destined for EML1 or to return astronauts from an ETLV returning from EML1. 

ETLV-2 (Extraterrestrial Landing Vehicle)

Inert weight with cargo and  crew (8 passengers): 10 tonnes

Maximum amount of propellant:  24 tonnes of  LOX/LH2

Maximum fueled weight: 34 tonnes

Specific Impulse of LOX/LH2 engines: ~ 450 seconds 
Top: ETLV-2 reusable lunar crew lander and lunar hopper; Bottom: CTLV-5B reusable LOX/LH2 cryotanker.

CTLV-5B (Cryotanker Landing Vehicle)

Inert weight: 8 tonnes

Maximum amount of propellant: 30 tonnes

Maximum fueled weight: 40 tonnes

Specific Impulse of LOX/LH2 engines: ~ 450 seconds

By utilizing  ADEPT  deceleration shields, such an ETLV could also be used to transport humans from low Mars orbit to the surface of Mars and back into  Mars orbit  on a single fueling.  With an ADEPT decelerator, the delta-v requirement to land on the lunar surface from orbit is only 0.51 km/s. The delta-v to travel from the surface of Mars back to Mars orbit is 4.4 km/s. Propellant depot located in Low Mars Orbit would allow the vehicle to refuel in order to travel to orbital habitats or interplanetary vehicles located in High Mars Orbits. Traveling between High Mars Orbit and the Earth-Moon Lagrange points has the lowest delta-v requirements between Mars orbit and cis-lunar space.

A reusable  ETLV located at propellant producing  lunar outpost that utilizes a reusable CTLV (Cryotanker Landing Vehicle) could also allow humans to continuously explore practically every region on the lunar surface without the need of any additional SLS launches from Earth-- dramatically reducing the cost of the human exploration of the Moon

Once a permanent outpost is established on the surface of the   Moon, the entire lunar surface, including its craters, could be continuously explored by robotic lunar rovers tele-operated from Earth. Such solar and nuclear powered mobile robots could also retrieve regolith samples  for return to the outpost for study and, eventually, transported back to Earth. Such mobile robots could also be used to  locate interesting sites for future human exploration.

Because annual levels of cosmic  radiation on the lunar surface can range from 11 Rem during solar maximum conditions to as high as 38 Rem during solar minimum conditions, astronauts living on the Moon for several months or several years will have to minimize their radiation exposure by mostly living inside regolith shielded habitats to reduce annual radiation exposure to less than 5 Rem (the maximum level of radiation exposure for radiation workers on Earth) during solar maximum and minimum conditions. This can easily be done by landing  habitats on the lunar surface that can automatically deploy regolith walls that can be easily filled with approximately 2 meters of lunar regolith.

If astronauts spend about 10% of their time outside of their shielded habitats  (2.4 hours per day or 16.8 hours per week), their additional exposure after a year would only range from 1.1 Rem to 3.8 Rem. A 25 year old female could live and work on the Moon for a decade and still not exceed her maximum lifetime limit of 100 Rem. Astronauts minimizing their radiation exposure by  exploring the lunar surface for just  four to eight hours per week could, therefore,  explore various regions on the Moon on a weekly basis-- if they could have easy access to such regions.

Since ground vehicles transporting crews across the lunar surface are not likely to exceed 20 km per hour in average speed, the maximum area that could be explored by pressure suited astronauts is not likely to exceed a distance of more than 80 kilometers away from a shielded lunar  outpost.

However, rocket powered sub-orbital Lunar Hoppers hurtling along parabolic arcs have long been advocated as a way for humans to explore more distant regions on the Moon-- far beyond a permanent lunar outpost. But such missions would require the reusable vehicle to have-- enough propellant-- to:

1. take off from the outpost on a suborbital trajectory,

2. land at the site intended to be explored,

3. take off again on a suborbital trajectory,


4. land back at the lunar outpost.

The delta-v and travel times for possible crewed suborbital hops on the lunar surface.
David Hop, on his popular space blog, has done some interesting calculations, suggesting  that lunar hoppers could transport humans anywhere on the lunar surface in less than an hour with a maximum delta-v of only 1.68 km/s. So  transportation between lunar outpost and lunar cities could be conveniently fast and easy-- as long as every lunar outpost or  lunar city can refuel the Hopper for its next suborbital destination.  

An ETLV fueled with a maximum of 24 tonnes of LOX/LH2 propellant (originally designed for round trips between EML1 and the lunar surface) could transport astronauts within a 1300 kilometer radius from a lunar outpost and back.  Beyond 1300 kilometers (45 degrees), however, such an ETLV  would not have enough propellant for its return trip to the lunar outpost.

Since the distance from the poles to the lunar equator would be 2700 kilometers away and to  the opposite pole, more than 5400 kilometers away,  a single polar outpost would pretty much  confine  human exploration via Hoppers mostly to it's polar region.

One way to overcome such geographical limitations would be to launch crewed ETLVs to EML1.  There it would add additional rocket fuel  from a  propellant depot (WPD-OTV-5A) located at EML1 for a round trip mission from the Lagrange point to the lunar site chosen to be explored. After the completion of the exploratory mission, the ETLV would return to EML1 to add propellant for its return trip to its original lunar outpost.

Lunar Exploration via lunar outpost and EML1 propellant depot

1. Crewed ETLV-2 launched from lunar outpost to EML1 (less than 12 hours at high delta-v or two days at a lower delta-v))

2. ETLV-2 rendezvous with WPD-OTV-5A adding enough propellant for a round trip from the lunar surface and back to EML1

3. ETLV-2 travels to lunar orbit and lands at lunar site (~2 days of travel) for a few hours or a few days of exploration

4. ETLV-2 launches itself back to EML1 (~2 days of travel) and rendezvous with WPD-OTV-5A to refuel for trip back to lunar outpost

5. ETLV-2 departs from EML1 to return to lunar outpost (less than 12 hours or up to two days)

This scenario requires only one vehicle (ETLV-2). In theory, it would  allow sorties to be conducted practically anyplace on the lunar surface on a weekly basis.  This method, however,  would require at least five to eight days of travel time-- excluding the time spent exploring the region on the lunar surface. So each lunar sortie would expose astronauts to five to eight continuous  days cosmic radiation outside of a regolith shielded outpost-- for perhaps just a few hours or a few of days of exploration at a particular lunar site.

Alternatively, pre-deploying a mobile propellant depot (MCT) at the site intended to be explored could minimize astronauts radiation exposure during a lunar sortie.

 ETLV-2 lands near a pre-deployed mobile cryotanker (MCT) after it's suborbital flight to a predetermined lunar exploratory site. The MCT will provide the ETLV-2 with additional LOX/LH2 for its return flight to a polar outpost.

 Lunar exploration utilizing mobile cryotankers

1. A solar and fuel cell powered mobile cryotanker (MCT) with up to 12 tonnes of propellant is sent to a lunar exploratory site (distance traveled: 300 km/day) in less than a month

2.  Crewed ETLV-2 launched from lunar outpost to lunar exploratory site (less than an hour)  with a few tonnes of extra fuel for the return trip to the outpost. 

3. MCT adds enough additional  fuel to the ETLV-2 for it to return to the lunar outpost

4. With added fuel, the ETLV-2 launches itself back to lunar outpost in less than an hour of travel time

5. The mobile MCT returns to the lunar outpost after a few weeks of travel time.

This scenario dramatically reduces  astronaut's travel time  to less than two hours of continuous radiation exposure. Preparing for such lunar sorties, however, would require a mobile cryotanker to be deployed to the exploratory site a few weeks before the crewed mission. And then a few weeks would have to be allowed for the cryotanker's return to the lunar outpost.

However, there is another way that lunar sorties from a lunar outpost could be conducted on a daily basis while also minimizing cosmic radiation exposure. This scenario would require an ETLV to be launched in an orbital plane above the intended site to be explored  along with a reusable  CTLV (Cryotanker Landing Vehicle).

Crewed ETLV-2 rendezvous with an unmanned CTLV-5B cryotanker in the same orbital plane as the intended  lunar exploratory site and the lunar outpost. After the lunar exploratory mission is completed, both the ETLV-2 and the CTLV-5B will return to the lunar outpost to be used again for future lunar exploratory missions.

 Lunar exploration utilizing an ETLV-2 and CTLV-5B in lunar orbit

1. CTLV-5B launched from lunar outpost into an orbital plane directly above the intended landing site

2. Crewed ETLV-2 launched from lunar outpost  into the same orbital plane

3. ETLV-2 rendezvous with CTLV-5B adding enough propellant for a round trip from the lunar surface and back into orbit

4. ETLV-2 lands at lunar exploratory site for a few hours or a few days of exploration

5. ETLV-2 launches itself back into orbit along the same orbital plane

6. ETLV-2 rendezvous with CTLV-5B adding enough propellant to return to the lunar outpost

7. CTLV-5B uses the its remaining amount of propellant to land back at the lunar outpost to be eventually refueled to assist in the next sortie mission on the lunar surface

ETLV-2 at an exploratory site on the lunar surface. Distances exceeding  1300 kilometers away from a propellant producing lunar outpost will require the ETLV-2 to use its remaining fuel to launch itself back into the same orbital plane as an orbiting CTLV-5B cryotanker, in order to add the needed fuel necessary for it to return to the lunar outpost.

The CTLV is simply the CLV (Cargo Landing Vehicle) without the cargo. So no new extraterrestrial vehicle would have to be developed in order to utilize the CLV as a reusable propellant vehicle (CTLV).

While this scenario requires two reusable launch vehicles (ETLV-2 and the CTLV-5B), it has the advantage of being able to  deploy astronauts quickly to an exploration site in just a few hours. Most of the few hours of travel time for astronauts would be spent in lunar orbit while rendezvousing with the CTLV-5B propellant  depot to add more propellant.

In the early 2030s, I imagine that most of the water produced at a lunar outpost  would probably be exported to one of the Earth-Moon Lagrange points to provide water for future interplanetary missions to Mars, Venus, ESL4, ESL5, and the NEO asteroids: water for drinking, washing, the production of air, radiation shielding, and LH2/LOX propellant.

But some of the water derived from the lunar poles could also be used for the production of lunar propellant intended for the domestic human exploration of the lunar surface.  This could allow reusable Extraterrestrial Landing Vehicles to  cheaply and conveniently transport astronauts to practically every region on the lunar surface for a few hours or even a few days of exploration. So the production and export of lunar water could not only greatly enhance NASA's ability to send humans to Mars but it could also usher in a new renaissance of human exploration-- on the lunar surface.

Links and References

Trajectory Optimization for Adaptive Deployable Entry and Placement Technology (ADEPT)

Lunar Hopper

Travel on airless worlds 

Lunar pogo hopper

Drones on the Moon
Is it possible to explore the Moon with low-altitude flying spacecraft?

Lunar Lander Designs for Crewed Surface Sortie Missions in a Cost Constrained Environment

The SLS and the Case for a Reusable Lunar Lander

An SLS Launched Cargo and Crew Lunar Transportation System Utilizing an ETLV Architecture

Utilizing the SLS to Build a Cis-Lunar Highway

Cosmic Radiation and the New Frontier

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