A Thought Experiment: SPACE TUG: Solar Orbits

Space tugs can enable habitats to launch from terrestrial orbits to interplanetary orbits and thence among different orbits around the Sun.

OVERVIEW:

Due to centripetal spin, reflected sunlight, plentiful water and terrestrial flora and fauna; habitats can park indefinitely in many types of orbits throughout the Solar System. However, to transfer between Solar orbits, habitats will need an external propulsion source; thus, TE's "space tug" concept will come into play.

CURRENT EXAMPLE: VASIMR
VASIMR uses 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 PROPOSES A SPACE TUG ...
	.... which might look like the diagram. Such a vehicle could use traditional rocket fuel (liquid/solid chemicals), but TE proposes an ion driven propulsion; perhaps as shown: Interface specially designed to fit habitat hull. Tokamak, an plasma storage container Cyclotrons to speed particles up to one half light speed (.5 c). Ions exit the exhaust. Such a Space Tug could best be used in moving Space Habitats between interplanetary orbits. TE assumes space tug has access to habitat’s plentiful water (H2O) supply as a source of protons. NOTE: Go to The NASA Psyche mission: Journey to a Metal World for an excellent blog on "Electric Thrusters" by Dan Goebel, Spacecraft Chief Engineer on Psyche.
LAUNCH INTO DEEP SPACE
Once habitat is constructed in one of Earth's many circular orbits, it can park there for as long as necessary due to: simulated gravity (via longitudinal spin), energy (via exterior mirrors to reflect sunlight), agriculture (via terraforming and water) fellowship (of 1,000's of co-resident colleagues), When ready to leave Earth's orbit and enter another orbit around the Sun, the Space Tug will enable the habitat to make the transition. The diagram shows a transition to a highly ecliptic, cycler orbit, which is a great way to gain ready access to another planet's orbit around the Sun.
SPACE TUG PROPELS HABITAT
	When Space Tug connects to the Space Habitat, TE assumes ready access to the ample water supply and numerous protons. TE further assumes exhaust particles could easily attain velocity of one half light speed (.5 c), and that typical fuel requirement will likely be far less than 1o% of total stored water for each task.
FROM CYCLER ORBIT TO MARS
Space tug burns and turns from cycler orbit to Mars orbit. Once on Mars orbit, space tug can power off. HOWEVER, some more "burns" will be needed to slow down and enter orbit about Mars itself.