| Multi-Planet Voyages and Propulsion Systems Today's missions are planned with the limitations imposed by existing and near future launch vehicles and a lack of space based infrastructure for space craft assembly, refueling, etc. Earlier plans for Mars missions involved multiple Saturn V launches and the assembly of several modules with nuclear thermal propulsion. Zubrin's Mars Direct uses only two rocket launches and is as minimalistic as one can get. The best propulsion system seems to be nuclear thermal, solid core or gas core, and liquid hydrogen for reaction mass. The LH2 must be shipped up to LEO. The ships must have enough fuel for escape from Earth, braking at the target planet, escape from the target planet and braking upon return to Earth orbit. This demands lots of fuel and ships that are mostly fuel. Because they have to haul their own braking fuel they need even more fuel for escape. It's rough situation that can be partially remedied by using aerobraking and return fuel production from local materials at the target planet. FAST FORWARD If we go beyond this kind of thinking and envision a time when industry exists on other planets and space stations are present in orbit around the Moon, Mars and Venus, different strategies, propulsion systems and fuels become more appealing. Naturally, we will need heavy lift launch vehicles based on the Shuttle stack perhaps to industrialize space and means of conveyance. Ion drives accelerate slowly but use little reaction mass thereby making it possible to maximize payloads and they can operate continuously for long periods of time and reach high speeds that reduce total time of flight. Solar-mag sails are slow to accelerate and decelerate but reach high top speeds and use no reaction mass at all. Robotic freighters using ion or solar-mag sails will haul equipment to other worlds. Landers will use their own fuel supplies at first but use locally produced fuels like Al+LUNOX on the Moon and methane+LOX on Mars. Industry will grow by using local materials like oxygen, iron, cast basalt, glass-glass composites, magnesium, aluminum and titanium. A few thousand tons of equipment will grow to form industrial bases amassing millions of tons. Robots will be used extensively. Mass drivers will be built on the Moon and Mars to launch materials into space for construction and spaceship fueling inexpensively. Deimos and Phobos will be mined and perhaps even the planet Mercury and some asteroids. When enough infrastructure exists in space the time will come when thousands, even millions, of human beings travel to other worlds every year. Multi-planet voyages will be possible. We will not take off in single stage rockets that land on other planets like Flash Gordon. Large spaceliners made as low mass as possible will not be capable of landing on other planets. Small shuttles and space station terminals will exist above the Moon, Mars, Venus and even Mercury. A future space traveler will leave his home and fly by jet to an equatorial launch base and ascend aboard a VTOVL rocket to LEO that docks with a space station. From there he will board a small taxi rocket and within hours the taxi with dock with an inter-lunar cycling station, the preferred way of travel to the Moon. In a week's time the cycler will reach cis-lunar space and the traveler will board the taxi again and ride over to a space terminal at L2. There he will board a Moon Shuttle and descend to the surface. The taxis and Moon Shuttles will be powered by aluminum and LUNOX produced on the Moon and launched with mass drivers to stations in halo orbit around L2. This will be the propellant combination of choice because it is plentiful on the Moon and there is very little hydrogen in polar ice that must not be squandered. To make the trip worthwhile there will be hotels in sub-selene lava tubes and in rille valley settlements, railways on the Moon and scenic lookouts. Most travelers will not go beyond the Moon, but some will. How do we transport them to Mars or Venus? TWO PLANETS, ONE VOYAGE The best deal is a trip to Venus when aero-xities exist there that takes about five months followed by a three month stay on Venus, then a six month voyage to Mars. After eleven months on Mars it will be possible to return to Earth in seven months. These will be low energy trajectories. Faster trip times may be possible. Nuclear electric ion drive propulsion could speed things up. We will need advanced ion drives and nuclear powerplants that can get up to speed in a matter of weeks rather than months or years. Ships with ion drives cannot leave LEO and spiral out through the Van Allen belts due to radiation exposure. Ships could leave GEO. Travelers will ascend to GEO in Al+LUNOX powered taxis in just about five hours and avoid prolonged radiation exposure. If our Mars or Venus bound travelers are willing to travel to L2 by cycler and perhaps spend some time on the Moon also it will be possible for ion driven ships to use Al+LUNOX boosters to rocket out of L2 and obtain gravity assists from the Moon and Earth. This will speed them on their way. Empty boosters consisting of low mass aluminum tanks and engines will not add much parasitic mass to the ship. The ion drives will thrust continually for weeks afterwards and add speed to the ships. Reaction mass will be plentiful magnesium, a low boiling point metal. Upon reaching Mars or Venus the ships will decelerate into a tightening elliptical orbit that may take weeks to circularize for rendezvous with space stations. While small manned and unmanned vehicles of the early days might use aerobraking to eliminate the need for retro reaction mass, this is somewhat dangerous and would require a large heavy heat shield for the large ships of the future. It creates some less than aesthetic designs for spaceliners also. Rather than force the travelers to wait for weeks while decelerating with ion drive, the liners will rendezvous with chemically propelled taxis and the travelers will fly over to a space station in low orbit. The liners will remain on automatic control while they complete their long braking maneuvers. Our travelers will then descend to the surface of Mars or the aero-xities of Venus in various kinds of landing craft. On Mars they will use methane+LOX powered VTOVL landers that have replaceable heat shields and parachutes for soft landing on the deserts of Mars. Aerobraking from low orbit is much safer than aerobraking from high interplanetary speed. At Venus they will fly down in jet atomic+atomic rocket with liquid CO2 reaction mass combination powered winged spaceplanes that glide to the surfaces of balloon borne flat tops. On Mercury, Al+LOX powered VTOVL rockets will be used. When returning, they will ascend in various rocket powered craft to low orbiting space stations where the ion liners have successfully docked after several weeks of low thrust braking. The liners orbiting Mars will be reloaded with magnesium that is mined from Deimos or Phobos if possible, or the magnesium will be mined on Mars and launched up with mass drivers on Pavonis Mons. If Mars is ever terraformed it won't be possible to use a mass driver anymore, so let's hope that the moonlets contain plenty of magnesium, aluminum and oxygen like many silicate bodies do. The boosters will be loaded with Al+LOX or a monopropellant consisting of methane and LOX. Escape from Mars will not demand lots of delta V because of the planet's low gravity. The ships will run their ion drives and return to Earth where they will enter into a tightening elliptical orbit again. They will aim for orbits that return them to L2. Taxis will rendezvous with the ships and travelers will either return to L2 space stations and then take cyclers back to Earth or the taxis will be able to enter LEO where the travelers can board VTOVLs and descend to their home planet's surface. If the demand for delta V for the taxis exceeds the capability of Al+LUNOX they might use LANTR. Although LANTR requires hydrogen it uses much less hydrogen than a nuclear rocket running straight LH2. At Venus the gravity well is too deep to ship fuel up from her hellish surface or from her atmosphere. Or is it? We can use NTRs on Venus. Single stage to orbit LCO2 filled atomic rockoons might ship carbon, oxygen, nitrogen and sulfur up to LVO economically. These could be used to build plastic and graphite space stations and fuel LANTRs. Diverting asteroids into Veneran orbit to use for reaction mass and construction material might be possible, but there could be other ways. The best strategy may be to mine Mercury and launch payloads of aluminum, LOX and magnesium up to orbit with mass drivers and then use the powerful light and solar wind of the nearby Sun to propel robotic freighters/tankers with combination solar and magnetic sails to Veneran orbit. Thus, ships will be able to refuel at Venus and taxis will have fuel also. Escape from Venus will require more delta V than from Mars or even the Earth-Moon L2 region, but Venus is revolving rapidly around the Sun and its heliocentric speed will be added to ships bound for Mars or Earth and reduce flight times. A Hohmann from Venus to Mars actually takes less time, about 220 days, than a Hohmann to Mars from Earth which lasts 260 days! Interplanetary ships will be able to decelerate into tightening ellipses with their ion drives while travelers take taxis to low orbital space stations and then fly down to the aero-xities. When they are ready to return to Earth or go on to Mars after a few months in the air of Venus the ships will have completed their autopilot guided braking maneuvers and be all fueled up and ready to go again. FUTURE INFRASTRUCTURE In space there must be orbital stations, Lagrange point stations, cycling stations, robotic freighters and tankers, fuel depots and taxis. On other worlds there must be mines, refineries and mass drivers. There must also be landers or shuttles, launch and landing bases, habitations, farms, factories and modes of surface and aerial transportation. Centuries of development will be required. Major technological breakthroughs like low mass high thrust fusion drives or even some kind of four dimensional hyper-space drive could occur, but this is how the picture looks now. Large spaceliners that use solar-mag sails and some kind of accessory drive like a mass driver are possible. These too would undergo long slow decelerations and require taxis to get the passengers over to the planet. Hydrogen may become more available. Mars has a lot of water ice and tanks of LH2 could be launched into space and robotic solar-mag sail tankers could haul it down to Venus. The first tanker may take a long time to get to Venus but once a series of freighters is established a sort of pipeline will emerge. Since launch windows from Jupiter to Venus and Mercury come more often than from Mars to Venus and Mercury and the ice moons of Jupiter which have such low gravity and no atmospheres at all, mining bases and mass drivers there could set an even better pipeline in motion. There isn't much solar pressure out at Jupiter, but the planet has a strong magnetic field and electrodynamic tethers could be used to spiral away from the planet without expending fuel. Automated hydrogen tankers could be coming in to the inner solar system, including the Moon, and heading back out to Jupiter empty on long slow voyages, but robots are patient. Taxis will be of no use to heavy tankers as these are only good for carrying loads of people and they don't use much fuel because of their small size. You can't just stop and drop off a massive load of hydrogen either. The tankers must undergo long slow spirals until they can dock with orbital depots. Many tankers would be in transit and any given time, but arrivals will be frequent. This kind of advancement would not require any stranger than fiction technology and it would revolutionize space travel by allowing the use of gas core nuclear thermal rockets using LH2 for reaction mass and LANTRs. Such rockets have high thrusts and can reach high speeds in just hours rather than weeks as would be the case with the best ion drives. Travelers could simply rocket to LEO, transfer at a space station to the nuclear powered LH2 fueled ships and rocket off to Mars or Venus, retro-rocket quickly into low Mars or Venus orbit, and transfer at a space station to a lander or shuttle then descend to the planet. This would simplify things. It's as close to Flash Gordon as you can get. All we need are people who want to mine hydrogen and supply it to us. We can imagine pioneers living off the ice and under the ice of Europa selling hydrogen, other elements found in the ice and subsurface waters, salt, magnesium, sodium, sulfur and carbon. Perhaps they could even mine helium 3 from the stormy atmosphere of Jupiter. In their remote places they could become fabulously rich and the space travel economy of the solar system would center around them. Perhaps the ice and subsurface sea of Europa which Peter Kokh has named the Rhadamanthic* and imagined Atlantean cities there is the golden key to the future of interplanetary travel. Who would make the massive investment in Europan colonies and fleets of robotic tankers? How rich and powerful would they become in the space operas of the future? CYCLING STATIONS Interplanetary cycling stations to Mars and Venus are also possible. No reaction mass is required and their orbits can be adjusted with solar-mag sails. As the cyclers pass a planet, taxis rendesvouz with them and people board or disembark. Taxis use far less fuel than a huge liner designed for conveying people in comfort for several months at a time. Cyclers could be very large. Their only drawback is that the planets are in line for flight by cycler every few years or so. Interlunar cyclers can make one trip to the Moon and one trip to the Earth-Moon L3 region every month, but interplanetary cyclers face a different challenge. If they can use their sails to alter their orbits in such a way as to make more frequent rendesvouz with other planets this would make them more useful. A combination of liners with their own propulsion systems and cycling stations could make interplanetary travel more flexible. While a liner is braking a cycler might be coming by, so our business traveler might get things done quickly and jump on a taxi to the cycler and head on home. That would be great timing. Schedulers with computers will work these things out for travel agents in the future. BEAMRIDERS Another possibility that requires no fusion or four dimensional magic is beam rider propulsion. Massive solar satellites could be built and Planet-Sun Lagrange regions and they could power tight microwave beams at the sails of ships and push them to escape or brake them into orbit when approaching the destination. The beamrider stations would fire opposing beams to stay on station lest they push themselves away into space. This would require some mammoth engineering accomplishments, but we are looking at the solar system of the 22nd or even 23rd century and beyond. Beamrider propulsion using the immense energies of the Sun that harvest solar winds and direct beams of relativistic plasma to propel mag-sails whose magnetic fields and bubbles of beam catching plasma reach thousands of kilometers in diameter could even drive ships to the stars at incredible velocities someday. Upon reaching the destination star system the starships will use their mag-sails to brake in the tenuous plasma of interstellar space. Colonists could build a beam rider station at the destination and return to Earth. Round-trip interstellar travel could be realized. No string theory magic is required for this, only incredible amounts of money and extraordinary engineering. Will AI make the people of the future so rich that they can embark on such giant projects? I don't have a crystal ball that reliable, but I think it's possible. |
 |
 |
| ABOVE) Ships based on Shuttle LOX tank and complete ET with gas core nuclear rocket engines using LH2 reaction mass for rapid interplanetary travel. They could also serve as taxis if high delta Vs are needed for docking maneuvers with interplanetary liners and LH2 supply is not great enough to fuel interplanetary traffic which continues to rely on ion drives and solar-mag sails. |
| Excellent web page: Living "Off the Ice" on Europa |
| more thoughts: Interplanetary Fuel Costs |
| *after Rhadamanthes, mythical son of Europa sired by Jupiter |
| For the source of this inspiration, see: Visits to Venus en Route to Mars by Peter Kokh |