| LANTR Taxis for Interplanetary Cyclers Dave Dietzler External Tank Taxis Taxis for rendezvous with interlunar cycling stations could be propelled by a monopropellant mixture of lunar silicon and aluminum emulsified ultrasonically with LUNOX. Could the same taxis serve interplanetary cycling stations to Mars or Venus? Earth and Mars encounter speeds may be fairly high if the cyclers follow Aldrin's "up-down escalator" orbits for travel between Earth and Mars. These orbits allow six month "up" voyages to Mars. The stations then arc through the main asteroid belt for about a year and make another "down" encounter with Mars to allow another six month voyage to Earth. The cyclers might pass near Earth at a distance of several hundred thousand kilometers. We would want enough speed capability to return to Earth or a solar orbit from which we could be rescued if rendezvous with the cycler fails for some reason. The taxi hulls might be derived from Shuttle ETs, which could exist long after the Shuttle is dead as part of a Heavy Launcher. The ET makes one of the lightest, finest aerospace tanks ever designed and could be used to build spaceships and space stations someday, but what about propulsion and other matters pertaining to interplanetary cycler taxis? I calculate that a monopropellant powered taxi could reach a delta V of about 5 kps. This may not be enough. For one thing it may take several hours, even days to reach the cycler or rendezvous with an orbital station at the destination planet and we want to make that part of the voyage as short as possible. We might reduce the passenger load from 400 persons to 100 to extend life support capabilities and give the passengers more room to live in for a few days. That's simple enough. All it will do is add to the ticket price. Using LANTR Power The taxi as I have envisioned it amasses about 250 tons dry with 35 tons or 400 passengers included and only 30 tons of that is the ET itself. The polyethylene solar flare shield amasses about 90 tons. What if we alter it by equipping it with nuclear propulsion? Liquid oxygen augmented nuclear thermal rocket engines could supply high thrust for a rapid dash through the VABs with about 647 seconds ISP on a 3:1 oxygen/fuel ratio [1] Pressure feed is simple and reliable so I want to keep that feature. The 8.3 meter diameter spherical aft tank with 299 cubic meters of volume could hold 21.2 tons of LH2 with a density of 0.071 tons/cubic meter. The 3.25m by 35m cylindrical tanks with 290 cubic meters of volume each could hold 328 tons of LUNOX with a density of 1.13 tons/cubic meter for a total of 656 tons. We need a LH2 tank about ten times larger to carry 218 tons for a 3:1 ratio by weight. The spherical tank would only have to be stretched into a cylinder with spherical ends by 51 meters for a volume of 3071 cubic meters. That makes the taxi look much different. If we carry only 100 passengers instead of 400, we save about 25 tons and that could be devoted to the longer aft tank so the taxi with passengers still amasses 250 tons dry. I'm trying to keep things simple in this discussion. If we run the LH2 in the two lateral cylindrical tanks we get 20.6 tons in each or 41.2 tons and we can put 338 tons of LOX in the rear sphere. If we go that way we could fatten up the lateral cylinders till they hold 112.7 tons of LH2. They would each be 5.375 meters wide and have a volume of 794 cubic meters apiece. That might make a more aesthetic ship. If it looks right it flys right, or does it? Two LANTR Taxis Compared In the first case we would have a 250 ton ship with 874 tons of propellant and in the second case a 250 ton ship with 451 tons of propellant. With mass ratios of 4.5 and 2.8 respectively and an exhaust velocity of 6.34 kps (647 seconds times 0.0098) we get 9.54 kps and 6.5 kps. That's pretty fast. We'd need roughly 3.4 kps to escape from LEO and go on a Hohmann to Mars but the cycler will be going a bit faster. Intuitively it seems like either taxi could rendezvous and have enough speed left over for a maneuver that brought it into an orbit that it could be rescued from if it fails to meet the cycler. The taxis would even have enough fuel to taxi over to Mars orbit at the end of the voyage. However, we can't always trust intuition. Water plus NTR What if the faster taxi with its larger tanks ran on simple liquid water? Water has the great advantage of being space storable for extended periods of time. We could load 3651 tons of water and get a mass ratio of 15.6 but we would have an ISP of only 400 seconds for a delta V of 10.77 kps [2]. That's faster than either LANTR powered taxi. It would use 405 tons of hydrogen versus 112.7 tons for the slower taxi and 218 tons hydrogen for the faster taxi. If we trimmed the water "fueled" taxi down so that it used 218 tons of hydrogen like the faster taxi in 1962 tons of water it would have a MR of 8.85 and a delta V of 8.55 kps with 400 second ISP engines. The question now becomes, how much delta V do we need? If the faster LANTR taxi represents overkill, why bother? Why deal with cryogens that are hard to store and transfer to space vehicles at fuel depots? Why not use water? The slower LANTR taxi used 451 tons propellant, the faster one used 874 tons and the taxi using water for reaction mass used 1962 to 3651 tons. It would require about twice as much Moon mining and mass driver activity for the faster LANTR taxi than the slower taxi and about two to four times as much as for the faster taxi with the water "fueled" taxi. Although we have plenty of oxygen in regolith and moonrock it all has to be mined and extracted as well as launched into space. We cannot make a decision until the astrodynamics involved are analyzed. I can be sure of one thing, the faster LANTR taxi with a delta V of 9.54 kps will get us to and from the cycler faster! Since passenger comfort is paramount, this could be the deciding factor. Astrodynamics: On a Wing and a Prayer My own crude analysis of the astrodynamics involved using some free ware computer programs from Delta-Utec and another authored by "eagle" indicate that a solar orbit 150 million km. by 350 million km. would take 2 and 1/7 years like Aldrin's escalator orbits. Taxis would need to speed up by 4.476 kps over LEO velocity of 7.8 kps to reach a hyperbolic excess speed of 5.45 kps and match velocity with a cycler bound for Mars. Nearing Mars the taxis must slow down by 6.3 to 7.3 kps to match the planet's heliocentric speed, but wait a minute, things aren't so simple! Since the taxis will be approaching the planet at 6.3 to 7.3 kps and Mars has an escape V of 5 kps we find that they will be drawn into the martian gravitational field and slung past the planet at 10 kps to 11 kps or faster. ( for exiting a gravitational field hyperbolic excess speed squared=burnout V squared - escape V squared and even worse, for entering a gravitational field hyperbolic excess speed squared={approach speed+escape V squared }-escape V squared! ) A gravity assisted flyby is not what we want for the taxis! Since my programs indicate that Mars encounter speed will be 6.3 to 7.3 kps, we will take the average - 6.8 kps. If it retro-rockets by 5 kps down to 1.8 kps it will then fall towards Mars from a few hundred thousand km. out, practically infinity, and pick up 5 kps again. It will be moving at 6.8 kps again when it skims over Mars and have: VhSQRD=(1.8+5)SQRD-5SQRD= 4.6 kps Unfortunately it will be traveling at 4.6 kps at some distance from Mars, a few hundred thousand kilometers where martian Vesc is much lower and it will escape into space. This isn't good. There are only four solutions: 1) a trans-Mars trajectory that involves much slower approach speeds 2) aerobraking. The taxi must lose 3.3 kps by aerobraking to enter low Mars orbit at 3.5 kps. Or it can lose less speed to go into an elliptical orbit from which it can be rescued. 3) a faster taxi rocket. 4) the use of mag-sails and solar sails by the cycling station to reduce encounter speeds. The fifth possibility is that my computer program is in error. What do you expect from freeware? A combination of the above strategies may be necessary. Interplanetary flight becomes complicated! It is overwhelmingly apparent that the sleek sharpnosed ET LUNOX augmented nuclear thermal rockets will be good for 24 hour flights to the Moon but not for Mars. LANTR Taxi #1 is Best All Around If the taxi aerobrakes into a highly elliptical orbit a nuclear thermal rocket powered tug from Deimos or Phobos base can rescue the taxi and circularize its orbit for docking at a mooonlet base. We will need taxis that can perform a delta V of 9.5 kps at least and use aerobraking if we are to use escalator orbits. It seems that the LANTR taxi can do the job without as much mining as the water "fueled" taxi. This will require space cryogen production, storage and transfer technology, but that's probably unavoidable in the distant future or large scale space transportation. Heavy cooling devices on the cycler can keep the taxis' propellants cold during the voyage. Other Orbits There are also cycler orbits based on minimum energy ellipses but they allow trips between Earth and Mars only every three to five years. Other orbits involving a swing by of Venus and gravity assist are possible and the use of mag-sails and solar sails could allow the cyclers to change orbits for more favorable harmonization with the positions of Earth and Mars for more frequent voyages. Aldrin's escalator orbits already use gravity assists to "rotate" their orbits and allow encounters with Mars twice during and Earth once every 2 and 1/7 years. Earth-Moon cycler orbits are much simpler and straightforward. The Politics of Nuclear Energy and Hydrogen Two other issues come up. One, will we be permitted to use nuclear rockets in LEO? Or will the fear of uranium reactors from space crashing into the hearts of big cities vanish for various reasons? Cancers might all be curable decades from today and genetic screening of fetuses could prevent the birth of radioactive mutants. That means abortion and that is a whole other issue. Greater use of nuclear fission to combat global warming might lead to a world less terrified of uranium. In addition, a reactor from space would bury itself in a crater so deep that the soil would shield out radioactivity. Anybody close enough to be exposed to radiation would be killed outright by the impact! Two, can we afford to start using lunar polar ice and solar wind hydrogen implanted in the lunar regolith for taxis serving interplanetary cycling stations to Mars or Venus? If we can overcome the nuclear issue, could Lunans make an agreement with Martians to sell hydrogen in amounts equal to or greater than those used for travel to Mars? Would this agreement be honored in the long run? Or would the few billions of tons of lunar hydrogen be plundered and never returned? Who or what political body would enforce this trade agreement? If the Martians were not acting in good faith and didn't want to sell water or more exactly, hydrogen, to the Lunans would the Lunans embargo hydrogen sales to Mars bound vehicles? Could trade wars erupt into violence? Would nations whose citizens were stranded by the embargo get involved? What does Mars have to sell to the Moon besides hydrogen, carbon and nitrogen anyway? Would they cut their own throats by failing to meet hydrogen trade agreements? Or would ice miners on Callisto flood the market with so much hydrogen, carbon and nitrogen hauled down to the inner solar system by mag-sail/solar sail combination "barges" that nobody had to worry about the supply of these vital light elements so scarce on the Moon at all? An interesting mercantile and political future emerges and it all depends on our choice of propulsion just as we chose gasoline engines over electric cars and mass transit today. Some people might just side step the whole issue by traveling to Mars in ships using nuclear electric propulsion and abundant lunar magnesium, a low boiling point metal, for reaction mass! Some people will wonder why we bother with hydrogen and interplanetary hydrogen trade at all. "They" might think we can travel in taxis unlike the ones I envision that have enormous fuel tanks filled with low ISP monopropellant. It seems as if the cost factors involved in shipping massive amounts of monopropellant from the Moon to LEO for taxis fast enough to rendezvous with interplanetary cyclers would be prohibitive, but when we bring in AI robots to do most of the work anything might be affordable. "They" will say that the only reason we burn hydrogen, be it in the form of water or LANTR, in nuclear powered spacecraft is because some rich fat cats wanted to make their ships look a certain way, sell uranium and sell hydrogen. They just might be right; however, I really doubt that non-nuclear taxis would be fast enough for interplanetary cycling stations. Until then, I have a hunch that high speed LANTR taxis and cycling stations will win the race and I can only hope the hydrogen trade goes smoothly. At least the Martians have no cultural or religious prejudices that can be foreseen. I'm not a prophet, so I might just be all wet. My purpose for writing is not to create the future but to inspire others to do so. I won't even be alive when interplanetary tourism to Mars is reality. I confess, cycling stations and LANTR just seems like cool stuff to me! Travel to the Moon and Mars is also exhilarating just to think about and the idea of interplanetary trade to make it all possible rather than massive government programs reveals my Jeffersonian belief that the government that governs least governs best! Less government is more freedom, if you can make any money in this nation! Another argument evolves over travel to the Moon. If LANTR can get you to the Moon in 24 hours, why take a cycling station? The Moon miners and hydrogen-less monopropellant (Si, Al, in LUNOX) makers can still sell their products to the Moon Shuttle operators who haul passengers and freight down from halo orbiting L1 or L2 space docks. This is all going to be a matter of who gets first dibs on the hydrogen supplies. If Mars travelers cannot do without it, they will pay top dollar to get it. The interlunar cycling station cruise lines and monopropellant taxi operators might use political muscle to block the activities of the competing companies that offer 24 hour interlunar flights, but that would be like the owners of the Titanic trying to stop the development of overseas air travel by jet! Time and money will become the deciding factors. Business travelers might demand 24 hour flights by LANTR powered rockets while vacationers choose a more leisurely interlunar cycling station. People don't like having their options blocked. Somebody is going to get that hydrogen and sell it Interplanetary Hydrogen Trade It has been said repeatedly that the Moon is lacking in light elements-hydrogen, carbon and nitrogen as well as metals like copper, lead and zinc. Mars has plenty of H, C and N and copper is believed to exist there based on Viking results and the study of SNC meteorites which are thought to have come from Mars. Of all the elements Mars has to sell to the Moon and Earth orbital space, hydrogen may be the biggest money maker. Hydrogen will be in great demand if we use it for LUNOX Augmented Nuclear Thermal Rocket propulsion that can drive ships to the Moon in 24 hours. The Moon has no shortage of oxygen in its regolith and rocks. There are many ways oxygen can be extracted from lunar materials. Magma electrolysis may be the simplest way. Some scientists have suggested mining near Earth asteroids for hydrogen and carbon. Unfortunately, those asteroids are not near Earth much of the time and launch windows to and from them are many years apart. Mars offers launch windows every 2.13 years or 25 months. The moonlets, Diemos and Phobos might contain water. If not, the planet Mars has water at one of its polar caps and below the surface as indicated by the Mars Odyssey probe. Ice or regolith could be mined, the hydrogen extracted by electrolysis powered by fission, fusion or aerothermal powerplants. We could even combine it with nitrogen to make ammonia which is liquid at only minus 33 C or combine it with carbon to make methane which is liquid at minus 162 C. LH2 is liquid at below minus 253 C., so this would ease up on boil-off cooling system requirements and get some carbon and nitrogen to the Moon also. Mars also offers livable gravity, unlike asteroids, and a thin atmosphere that provides some galactic cosmic ray shielding. On a planet, half the GCRs are blocked by the planet itself. In space you are surrounded by GCRs. Farming on Mars in inflated plastic greenhouses is possible and martian caves could make excellent natural shelter once they were sealed and pressurized. The challenge of keeping people alive and healthy on Mars is much less than that of sustaining asteroid mining crews. It will also be possible to build a mass driver atop Pavonis Mons to launch payloads up to Mars orbit where they will be loaded on mag-sail/solar sail combination propelled "barges" for transportation down to Earth-Moon space without using any fuel at all. This will take many years of progress to achieve, but Mars needs money if it is every going to buy anything from Earth. Radicals might insist upon a completely independent Mars economy. If so, then they won't have any cash to spend back on Earth and they will become exiles of Mars rather than free entrepreneurs who do a multi-year stint on Mars and come back home wealthy. I don't think permanent emigration to Mars will be as popular as going there to work a number of years for high wages. Earthly manufactured items like cell phones and computer chips might not be available on Mars for decades if ever. Skilled and professional services of Earth people may in short supply. The radical martians could live simply and cheaply in a more rugged place than Antarctica. Or they might be fooling themselves if they think they can do without doctors, lawyers and engineers who are not willing to exchange acreage on Mars for their services. The martians are going to need cold hard cash and they have nothing to offer Earth that Earth does not already have. They do have plenty to offer the Moon and Space Oases dwellers who trade with Earth for energy and tourism in real dollar and euros. Chances are there are some vital elements in short supply on Mars or hidden away that the martians cannot find that they will only be able to get by importing them from Earth! Labor and professional services to be sure, but does Mars really have enough indium or tantalum? By some quirk of martian geology is it not possible that certain ores found on Earth never formed on Mars? Fate works in mysterious ways. And what about music and art? If the martians want to buy CDs and DVDs from Earth, or just buy the downloaded data by radio or laser links, they will need real money. If they pirate their entertainment, Virgin Galactic which is associated with Virgin Records will send its goons to Mars to smash up their computers...well maybe they will be milder than gangsters. Earth could always jam interplanetary Internet links and really punish the martian pirates! I'm sure Richard Branson the 8th will see it that way. What a coincidence that a British record dealer is getting into the space travel business. Fate works in mysterious and entertaining ways. The Interplanetary Worlds Trade Organization is going to be one hell of a free-for-all someday. |
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| References 1) Advanced Propulsion Concepts http://www.islandone.org/APC/Nuclear/04.html 2) NIMF by Robert Zubrin Moon Miner's Manifesto 30 Nov. 1989 |
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