Friday, December 15, 2006


Space tugs can enable habitats to switch orbits.
Orbiting habitats can indefinitely provide thousands with a friendly environment, but they can't switch orbits by themselves; thus, there is a definite need for space tugs.

    Due to centripetal spin, reflected sunlight, plentiful water and terrestrial flora and fauna; habitats can park indefinitely in many types of orbits around Earth and throughout the Solar System.  However, to transfer between orbits, habitats will need an external propulsion source; thus, TE's "space tug" concept will come into play.
    BACKGROUND:  "Space Tug" is a well used, modern day concept.
    BACKGROUND. space tug can transfer payloads between orbits. It was first envisioned in the post-World War II era as a support vehicle for a permanent, Earth-orbiting space station. Term "Space Tug" was initially coined by science fiction writer, Murray Leinster, as the title of a 1953 novel .   Since then, modern industry have leveraged this concept many times; a few examples are shown below.
    STS NASA first considered a reusable space tug as part of the Space Transportation System (STS). This was a basic propulsion module which could attach a crew module or other payload . This was never realized due to NASA's budget cuts in the wake of the Apollo program.  With propulsion module and a variety of cargo kits, NASA's Space Tug could have fulfilled many space applications.

    OMV Under NASA contract, TRW designed another space tug, the Orbital Maneuvering Vehicle (OMV), to retrieve satellites, such as Hubble, and bring them to a mother ship. It was also a victim of budget cuts.

    Parom In 2005, a Russian corporation, RKK, proposed a space tug, Parom, to ferry modules to the International Space Station (ISS).  Parom is "ferry" in Russian. 

    Sherpa  Spaceflight Inc. proposed Sherpa, an orbital tug for SpaceX's Falcon-9 launch vehicle. Plans called for SHERPA to ferry small payloads to secondary orbits away from the main payload's primary orbit.

    VASIMR Variable Specific Impulse Magneto-plasma Rocket (VASIMRuses plasma, the stuff of stars, to propel spacecraft.  It carefully controls high energy, plasma leak for small constant thrust throughout the trip;  i.e., constantly accelerate to midway; then, constantly decelerate for remainder.

    Manufactured by Dr. Chang-Diaz's Ad Astra Rocket Company, it might use 9 tons of a specialized propellant (Argon) to deliver 34 tons of cargo from Low Earth Orbit (LEO) to low lunar orbit (i.e. fuel is about 20% of mass). Exhaust particle speed is about 50,000 meters per second.
    VASIMR heats Argon gas to plasma state (millions of degrees) to greatly increase rocket performance. Thrust greatly exceeds chemical reactants which only reach thousands of degrees in a conventional rocket engine.
    TE's Space Tug
    Thought Experiment (TE) proposes a Space Tug which might look something like the one in the diagram.

    Such a vehicle could use traditional rocket  fuel (liquid/solid chemicals), but TE proposes an ion driven propulsion system augmented with simple particle acceleration devices (i.e., cyclotrons).
    TE assumes exhaust particles could easily attain velocity of one tenth light speed (.1 c), and typical fuel requirement would be less than 1% of total payload mass for each task.  TE further assumes expected technology upgrades could increase particle speed to as much as .5 c with even less fuel.

    Space Tug could be at times remotely controlled and other times autonomously controlled; thus, it could readily travel in various orbits around the Earth to position for desired tasks.
    It's possible that Space Tugs could move Physical Tether's "climber vehicle" from Up-Link tether to Down-Link tether. HOWEVER, TE's Space Tug could best be used in moving Space Habitats between terrestrial orbits or even to interplanetary orbits. 

    Space tug transports climber along Geosynchronous Equatorial Orbit (GEO) from Up-Link GEO Node to Down-Link GEO Node.

    ISEC proposes at least two unidirectional tethers, one for ascent and one for descent (perhaps others as spares).

    TE proposes an UP-LINK,  for ascending from Earth to Up-Link GEO Node, and a DN-LINK, for descent from Down-Link GEO Node back to Earth. To transfer between GEO Nodes, TE proposes Space Tug to push climber vehicle over the 3,300 km between them.

    ISEC suggests a minimal separation of 500 km between respective anchors on/near Earth's equator on the ocean.  If each tether extends straight "up" or outward from Earth's center, this distance (500 km) between the tethers would increase until approximately 3,300 km between respective GEO Nodes.

    UNFORTUNATELY, taut tethers (even when constructed of super strong Carbon Nano-Tubes (CNTs)) bear some risk of snapping.
    TE Proposes Virtual Tether
    Instead of traditional physical tether made of CNT strands for 1,000s  of kms, TE proposes a virtual tether using particle beams from Marine Base Stations located on the oceans of the Earth.  Instead of physical tether climbers which will take days to ascend from the ocean to the GEO; TE proposes particle beam riders which can make same trip in a few hours. Space tug could readily transport Beam Rider in a variety of ways; perhaps the most common way would be to engage Beam Rider to safely decelerate from high speeds attained by particle beam propulsion.

    Regardless of which method proves best for non-rocket transport of goods and people to the GEO, TE assumes their time in orbit would be best spent constructing Space Habitats, see TE's Habitat chapter. Habitat concept was first described by Dr. Gerard K. O'Neill in his 1976 book, High Frontier.
    Envisioned Habitats 
    ...should have following features:
    ●  Cylindrical shape can simulate Earth gravity by spinning around longitudinal axis.
    ●  Limitless energy from co-located mirrors to reflect sunlight as needed to the habitat.
    ●  Onboard agriculture from "terraforming" with plentiful food, oxygen and recreation.
    ●  Initial water exported from Earth's bountiful oceans, but eventually water will likely come from in-situ resources such as asteroids and comets.
    NOTE: Definitely not to scale.  Like modern day tugboats which service ocean liners many times their size, TE envisions space tugs pushing habitats many times larger.

    Habitats can park indefinitely in GEO and can provide many valuable services for Mother Earth. However, habitats will become even more useful when they can transition via an elliptical "transfer" orbit to an outer terrestrial orbit and eventually to interplanetary orbits.

    TE's Envisioned Space Tug 
    In associated diagram, Thought Experiment (TE) proposes a "space tug" to gently push habitat from GEO into an elliptical orbit with perigee (q) of GEO and apogee (Q) at an outer circular orbit.


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