The idea of using a network of outpost space stations (also known as deep space habitats) scattered around the solar system to assist in making manned spaceflight between the planets affordable has been around for at least 25 years. Who came up with the idea originally I don’t know, but it is an idea that has its roots in the network of coaling stations that were built all around the world for refueling steamships back in the 1800s. The steam engines that powered the early steamships were not very efficient and they burned a lot of coal. So much coal that if a ship had to carry the coal for an entire voyage there wasn’t much room left for cargo to pay for the voyage. To solve this problem coaling stations were built at strategic locations all around the world so that the steamships would only have to carry enough coal for the journey to the next station. This reduced the steamship’s propellant fraction and increased its payload fraction, which reduced the cost of shipping people and cargo all around the world to an amount that just about anyone could afford.
This idea also applies to automobiles. The average automobile only carries enough fuel to give it a range of about 300 miles. Not a lot of range if you are planning a cross-country trip. If it was necessary to design a car so that it could go across the country and back on a single tank of fuel, the car would be much larger and much more expensive to build, buy, and operate. So much more expensive that very few people could afford them. To solve this problem, a network of gas stations was built around the country. It is the existence of this network of gas stations that makes automobiles so effective and affordable. They decrease the car’s propellant fraction and increase its payload fraction.
This is what outpost space stations are about. They are the refueling stations on the trade routes to the planets and asteroids. They allow the spaceships traveling between the planets and asteroids around the solar system to only carry the propellant that is needed to travel to the next outpost space station along the route. This reduces the amount of propellant the spaceship needs to carry and allows it to carry a much larger payload. This reduces the cost of space travel to an amount that just about anyone can afford.
An example of this are the spaceships that were designed by Wernher von Braun for going to Mars in 1948. These ships were designed for four major propulsive events: first, to accelerate from Earth orbit velocity to Mars transfer orbit velocity; second, to decelerate from Mars transfer orbit velocity to Mars orbit velocity; third, to accelerate from Mars orbit velocity to Earth transfer orbit velocity; and fourth, to decelerate from Earth transfer orbit velocity to Earth orbit velocity. The total amount of change in velocity required for these four maneuvers is 11.48 kilometers per second. The amount of propellant required to do this using the low-performance rocket motors of the day required that the spaceships be over 98 percent propellant when departing from Earth orbit. This meant the empty weight of the spaceship, plus the weight of the crew and all their supplies and equipment, had to be less than 2 percent of the departure mass. In all of human history, no one has ever come close to building any type of vehicle with a propellant fraction that high and with an empty weight fraction that low. Even if it were possible to build such a vehicle with existing technology, it would not be affordable on a commercial basis due to the extremely low payload fraction.
Now assume a non-rotating Skyhook in Earth orbit that has an upper endpoint velocity of just short of Earth escape velocity, an outpost space station at the Earth-Moon L2 Lagrange Point with a local source of propellant (either the Moon or a near-Earth asteroid that has been moved to L2), and a non-rotating Skyhook in Mars orbit that has an upper endpoint velocity of just short of Mars escape velocity as well as a local source of propellant (either Mars or one of the Martian moons), and look at what happens to the change in velocity requirement and the propellant fraction of an Earth-Mars spaceship.
Passengers and cargo bound for Mars, fly to the lower end of the Skyhook that is in Earth orbit using the fully reusable combination launch system described in previous posts. From there they transfer to the upper end of the Skyhook where they transfer to the spaceship that will take them to the outpost space station at the Earth-Moon L2 Lagrange Point where they board the Earth-Mars spaceship.
On the day of departure, the Earth-Mars spaceship leaves the halo orbit at L2 and heads toward Earth where it will perform a gravitational slingshot maneuver as it accelerates to Mars transfer orbit velocity. The change in velocity for these two maneuvers is approximately 1,500 meters per second. When the Earth-Mars spaceship gets to Mars it slows down to just under Mars escape velocity and enters a high Mars orbit. The change in velocity required for this maneuver is approximately 900 meters per second. The total change in velocity required for this voyage is 2.4 kilometers per second. That is 1/5th the amount of change in velocity required by the spaceships used in Wernher von Braun’s original Mars fleet. This difference is due to both the Skyhooks and the outpost space stations with local sources of propellant at both Earth and Mars.
The transfer of passengers and cargo from the Earth-Mars spaceship to Mars will be performed by smaller spacecraft operating from the upper end of the Martian Skyhook using locally supplied propellant. These spacecraft will also be used to carry locally supplied propellant to the Earth-Mars spaceship for its return trip to Earth. If it is assumed that the Earth-Mars spaceship uses existing LOX/LH2 chemical rocket motors with oversized expansion nozzles (a specific impulse of 480 seconds), the propellant fraction for the journey to Mars will be 40 percent. This will leave plenty of room in the mass budget for the Earth-Mars spaceship to make it completely reusable (no drop-off propellant tanks or stages), allow for a spinning section with artificial gravity for the crew and passengers, a hydroponics garden for fresh vegetables, plus plenty of radiation shielding. If it is assumed that the empty weight fraction for this Earth-Mars spaceship is 40 percent of the departure mass, that will leave 20 percent for the payload.
If a nuclear thermal rocket motor that uses water for reaction mass is used for the propulsion system for the Earth-Mars spaceship, the propellant fraction for the trip to Mars drops to 25 percent and the payload fraction increases to 35 percent. When used with a combination launch system that includes an escape velocity capable Skyhook, either of these Earth-Mars spaceships would make a trip to Mars mass market affordable on a commercial basis and would operate at a profit for the owners.
Either of these spaceships would also be capable of making trips to the asteroid belt, and to dwarf planets like Ceres, both affordable and possible. Put an outpost space station in orbit around Ceres with a reusable lander that has the ability to lift water from the surface of Ceres into orbit, and these spaceships would have the ability to take a manned expedition to the moons of Jupiter and to explore the Greek and Trojan asteroids.
As hard as this might be to believe, we really are on the cusp of another Great Age of Exploration. Only this time, instead of exploring a single planet, we will have people exploring the entire solar system. All that is required is a combination launch system with a Skyhook and a couple of outpost space stations. It truly is the beginning of our reaching out to the stars.
Index of Articles
- Opening the High Frontier
- Skyhook, a Journey to Orbit and Beyond
- In the Beginning . . .
- Why do Rockets Cost so Much?
- Combination Launch Systems
- It’s All About Speed!
- Visions of the Future
- The Call of an Unlimited Future
- Combination Launch Systems, part 2
- Outward Bound: Beyond Low Earth Orbit
- and someday . . . Starships!
- Mars: how to get there
- Outpost Space Stations
- Dreams of Space
- The Moon or Mars?
- Skyhooks and Space Elevators