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TABLE TWO
Delta v from NERVA Ships
(Km/second)
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Even with the lower figure, you can get a round-trip to Mars at a mass ratio of 4. With that kind of capability, commerce between the planets becomes economically feasible. It's still expensive, but given some kind of system to get materials into orbit at either end, it's more than just possible—it becomes likely.
There's another alternative to NERVA, though, and it too was studied extensively before it was abandoned. It was called ORION, and on first description it seems like the most unlikely method of space travel you ever could devise.
ORION was also known affectionately as Bang-Bang. It worked very simply: you take a big ship, and on the bottom you put a very thick metal plate. You hang the rest of the ship in such a way that there are a lot of shock absorbers between the base plate and the ship itself.
Then you set off an atomic bomb underneath the ship.
Believe me, the ship will move. When it starts to slow down, you fling another atom bomb down below- and detonate it. You keep doing that until you Ve got enough velocity to get where you want to go.
Silly as it sounds, ORION would have worked. There were some problems. Obviously that base plate and suspension system had to be carefully designed. You probably wanted a small shielded compartment for the crew and those things that couldn't take hard x-rays, and a larger compartment, unshielded, for the rest of the cargo. None of this is all that difficult.
Another problem with ORION was coupling the energy from the bomb to the ship. Atom bombs put out a lot of x-rays and neutrons and heat, and of course once out of the atmosphere there's no blast at all. But even that problem was solved: you have to put something between the bomb and the ship, something that will absorb energy from the bomb and whap! the bottom of the ship to keep it moving: something like Styrofoam, for example, which looks as if it would work despite its unlikeliness.
ORION works better from orbit, but it could lift from Earth—if it weren't for the Treaty of Moscow that prohibits surface detonations of nuclear weapons, and if you weren't worried about the possible fallout.
It has been calculated that ORION would put 5 million pounds in orbit, or land 2 million plus pounds on the Moon—and do it in one whack. That's enough for a fair-sized colony's consumables and machine tools.
Of course we won't use ORION to launch from the Earth's surface, but there's nothing wrong with using it from Earth orbit to plant Lunar and Martian colonies; it's the most efficient and cheapest form of space transportation known, believe it or not.
Moreover, ORION works us toward something even cheaper. The problem with ORION at the moment is that you're blowing off a kilo or so of weapons-grade U-235 with each bang, and that stuffs not cheap. Aviation Week and a few other publications have been hinting that fusion bombs with laser trigger are either already or about to be developed: with these, you don't need a U-235 primary, you just have a hot laser zap some tritium or deuterium. It's more bang for the buck, and it would power ORION nicely.
Dr. Greg Benford has also described a system that would be even cheaper: you have a big power source on the ship, say a small fission reactor. That feeds a big laser. Pellets of tritium or deuterium are ejected below the ship, and zapped from the laser, producing fusion to drive the ship. It's the ORION principle again, carried to its most efficient extreme.
When you've got ships like that, you don't even talk about Isp and exhaust velocities, and the mass ratios are actually fractional—that is, the ship that arrives weighs more than the fuel expended to get there.
All these ships were once seriously studied. Now, it's only in universities and among science fiction people that they're mentioned, and even there most don't take ORION-type ships very seriously. Yet any of these ships could have given us the planets—and until either the NERVA or the ORION principles are exploited, the black box boys have won.
Man can dominate near-Earth space using the shuttle and laser-launchers; but until we go beyond chemical rockets, interplanetary space will belong to unmanned probes.
Life Among the Asteroids
One of science fiction's biggest problems is consistency. Whenever we make an assumption, it's not enough simply to leave it at that; to be fair to the reader, the SF writer should also see what that assumption does to everything else.
This was brought home to me when Jim Baen called to ask for a column on "What happens if we get an economical space drive?" The result was not only the column, but the cover story for the issue. ("Tinker," included in HIGH JUSTICE, Pocket Books, 1977).
The problem is more complex than it sounds. In fact, until we have some idea of what kind of space drive, there's no real answer at all.
For example: let's suppose we have a magical space drive in which we merely turn on an electric motor and "convert rotary acceleration to linear acceleration." The Dean Drive, remember, was supposed to do just that.
Incidentally? the Dean Drive wasn't suppressed by big corporations, as I've heard some fans speculate. I am personally acquainted with two men who were given large sums by aerospace companies and instructed to buy the drive if they saw any positive results whatever in a demonstration.
After all, if the thing worked just a little bit, it would be worth billions. Think what Boeing could do with an anti-gravity machine! But, alas, no demonstration was ever given, although the prospective purchaser's had letters of credit just waiting to be signed.
However, couldn't we simply assume that it will work and write an article about the resulting space civilization?
No. The discovery of a "Dean Drive" would mean that every fundamental notion we have about physics is dead wrong. It would mean a revolution at least as far reaching as Einstein's modification of Newton. An anti-gravity device like that would have consequences reaching far beyond space drives, just as E = mc2 affected our lives in ways not very obviously associated with the velocity of light.
This doesn't mean that "Dean Drive" systems are impossible, of course. It does mean that looking at their implications is a bigger job than I want to take on in a 5000 word chapter.
Jim's question was, "What happens if we have something that gives one gravity acceleration over interplanetary distances at reasonable costs per ton delivered?" Part of the question is easy to answer. Now that I've got my Texas Instruments SR-50 (by the way, they've come out with a really marvelous device called the SR-51, and I hate them) I can run off a couple of tables to show what we could do with such a system.
The figures in Figure 22 assume you accelerate halfway, turn end for end, and decelerate the other half, so that you arrive with essentially no velocity. The numbers aren't exact, because I haven't accounted for the velocities of the planets in their orbits—but after all, Pluto is moving about 5 kilometers a second, and Mercury about 50, and when you're playing with velocity changes like these, who cares about the measly 45 km/sec difference between the two? For shorter trips the effect is even less important, of course.
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Figure 22
TRAVEL TIMES AND DISTANCES AT
ONE GRAVITY ACCELERATION
* AU = Astronomical Unit (Average distance from Earth to Sun) = 150 million kilometer.
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Figure 23
HOW FAR CAN WE GO AT ONE GRAVITY?
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The numbers are a bit startling if you're not familiar with them. Twenty days to Pluto? They won't surprise old time SF readers, though. A full gravity is a pretty hefty acceleration. If you don't bother with turnovers but just blast away, the results are given in Figure 23, and they're even more indicative of what one gee can do.
Of course, long before you've reached light-speed at the end of a year, you'll have run into relativistic effects. Your ship gets heavier and your acceleration drops off. I don't care how good your drive is, maintaining a full gee for a year is going to take work. We're only concerned with the solar sys
tem, though, so we can ignore trips longer than a month and avoid relativity altogether.
Now we can write the article, right? Wrong. The problem is that last column in Figure 22. Just how do we expect to get delta v as big as all that?
Let's illustrate. As we've shown already, delta v can be calculated from mass ratio and exhaust velocity. (If you came in late, take my word for it; we'll get past the numbers pretty quickly.)
Now you could hardly call a drive economical if the mass ratio were much worse than, say, three, which means that if you start with 1,000 tons you'll arrive with 333. What, then, must our exhaust velocity be to make the simple trip from Earth to Mars?
It's horrible. About 2,204 kilometers/second, and what's horrible about that is it corresponds to a temperature of 50 million degrees Kelvin. The interiors of stars are that hot, but nothing else is.
Just how are we going to contain a temperature like that?
One answer might be that we'd better learn how; fusion power systems may require it. OK, and the fusion boys are working on the problem. However they solve it, we can be sure it won't be anything small that does the trick.
It's going to take enormous magnetic fields, superconductors, heavy structures, and a great deal more. After all, nothing material can hold a temperature like that without instantly vaporizing, and even containing the magnetic field that holds that kind of energy is no simple job.
Let's assume we can contain fusion reactions, though. We know immediately that energy is going to be no problem for our interplanetary civilization. With plentiful energy we'll find that a number of our other problems vanish.
There won't be many "rare" materials, for example; if they're rare and valuable enough, we'll simply make them out of atomic building blocks. Of course it may be cheaper to go find them somewhere, such as on Mars or among the asteroids, but we'll always be up against competition from the transmuters.
Life on Earth, at least among the people of the high-energy civilizations, will change drastically. Pollution will cease to be a problem (unless the fusion plants themselves are polluters, which isn't impossible). The Affluent Society will be with us, and possibly so will be regulations and rules, bureaucracy, and all the other niceties of a universal middle class.
All this comes as a result of assuming our space drive. More central to our immediate topic is the fact that the ships will be quite large—Queen Mary or supertanker size, not one-man prospector jobs. Someone is going to have to put up a lot of capital to build them, and it's not likely to be the Bobbsey Twins and their kindly uncle building ships in the backyard.
Only governments or very large international corporations will be in the space ship operating business, that's for sure. Thus there have to be profits in interplanetary travel. Not even governments will build more than one of these ships simply for scientific reasons. There's got to be commercial traffic.
Next, there's a technological problem: assuming we have fusion power and a method of getting electricity from it doesn't necessarily give us a space drive. Contrary to the notions of a lot of high school science teachers back in the 40's, rockets don't "need air to push against"; but the rocket exhaust certainly does need something on the rocket to push against.
What can that be? Perhaps some kind of magnetic field, but an open-end fusion system is at least two orders of magnitude harder to build than a "simple" system for generating electricity. It's one thing to take 50 million degrees and suck electricity out of it, and quite another to use that as a reaction drive.
Perhaps I'm not sufficiently imaginative, but for all these reasons I decided to shelve the one g system and design the article and story around something much simpler. In fact, if we had the electric power system, we could build these ships right now.
Ion drive systems solve the "something to push against" problem by shooting charged particles out the back end. The ship is charged, the particles are charged, and they repel each other. You can get very high exhaust velocities, in the order of 200 km/sec, with ion systems. They're among the most efficient drives known.
The trouble with present ion drives is that electricity costs weight. As an example, a currently useful system needs about 2100 kilowatts of power to produce one pound of thrust. Since the power plant weighs in the order of four tons, the total thrust is not one g, but about 1/10,000 of a gravity.
It works, but it's a little slow getting there. Not as slow as you might think: it would take about 140 days to go a full AU, and your ship would reach the respectable speed of 12 km/sec. Still, it's hardly interplanetary rapid transit.
Suppose, though, we had a fusion system to generate the electricity. It would undoubtedly weigh a lot: let's say 1000 metric tons, or about two million pounds, by the time we've put together the fusion system and its support units. We'd still come out ahead, because we'd have lots of power to play with. Assuming exhaust velocities of 200 km/sec, which we can get from present-day ion systems, we'd still have quite a ship.
She wouldn't be cheap, but it's not unreasonable to think of her as on a par with modern supertankers. She wouldn't be enormously fast: I've worked out the thrust for a ship massing about 100,000 tons with that drive, and she'd get only a hundredth of a g acceleration. Still, a trip from Earth to Ceres would take no more than 70 days, and that includes coasting a good part of the way to save mass.
A world-wide civilization was built around sailing ships and steamers making voyages of weeks to months. There's no reason to believe it couldn't happen in space.
IBS Agamemnon (Interplanetary Boost Ship) masses 100,000 tons as she leaves Earth orbit. She carries up to 2000 passengers with their life support requirements. Not many of these will be going first-class, though; many will be colonists, or even convicts, headed out steerage under primitive conditions.
Her destination is Pallas, which at the moment is 4 AU from Earth, and she carries 20,000 tons of cargo, mostly finished goods, tools, and other high-value items they don't make out in the Belt yet. Her cargo and passengers were sent up to Earth orbit by laser-launchers; Agamemnon will never set down on anything larger than an asteroid.
She boosts out at 10 cm/sec2, 1/100 gravity, for about 15 days, at which time she's reached about 140 km/second. Now she'll coast for 40 days, then decelerate for another 15. When she arrives at Pallas she'll mass 28,000 tons. The rest has been burned off as fuel and reaction mass. It's a respectable payload, even so.
The reaction mass must be metallic, and it ought to have a reasonably low boiling point. Cadmium, for example, would do nicely. Present-day ion systems want cesium, but that's a rare metal—liquid, like mercury—and unlikely to be found among the asteroids, or cheap enough to use as fuel from Earth.
In a pinch I suppose she could use iron for reaction mass. There's certainly plenty of that in the Belt. But iron boils at high temperatures, and running iron vapor through them would probably make an unholy mess out of the ionizing screens. The screens would have to be made of something that won't melt at iron vapor temperatures. Better, then, to use cadmium if you can get it.
The fuel would be hydrogen, or, more likely, deuterium, which they'll call "dee." Dee is "heavy hydrogen," in that it has an extra neutron, and seems to work better for fusion. We can assume that it's available in tens-of-ton quantities in the asteroids. After all, there should be water ice out there, and we've got plenty of power to melt it and take out hydrogen, then separate out the dee.
If it turns out there's no dee in the asteroids it's not a disaster. Shipping dee will become one of the businesses for interplanetary supertankers.
Thus we have the basis for an economy. Whatever people go to the Belt for, they'll need goods from Earth to keep them alive at first. Later they'll make a lot of their own, and undoubtedly there will be specialization. One rock will produce water, another steel, and yet another will attract technicians and set up industry. One may even specialize in food production.
Travel times are long but not impossible. They change, depending on when you'r
e going where. It costs money to boost cargo all the way, so bulk stuff like metals and ice may be put in the "pipeline": given enough delta v to put the cargo into a transfer orbit. Anywhere from a year to several years later the cargo will arrive at its destination. If there are steady supplies, the deliveries are quite regular after the first long wait.
Speculators may buy up "futures" in various goods, thus helping capitalize the delivery system.
People wouldn't travel from rock to rock much. Thus each inhabited asteroid will tend to develop its own peculiar culture and mores. On the other hand, they will communicate easily enough. They can receive educational television from more advanced colonies. They can exchange both technical and artistic programs, and generally appreciate each other's problems and achievements.
What kind of people will go out there? Remember that life on Earth is likely to be soft: those going out will be unhappy about something. Bureaucracy, perhaps. Fleeing their spouses. Sent by a judge who wants them off the Earth. Adventurers looking to make a fortune. Idealists who want to establish a "truly free society." Fanatics for some cult or another who want to raise their children "properly."
A Step Farther Out Page 21
