My illo for this will probably be a diagram of the early NERVA engine, the labels are close enough that it sort of works.
“…they use a minute amount of uranium or radium as catalyst to release the energy in the fuel. Uranium has low activity; it will set off only metals like the alkalis, and ships using uranium motors burn salt. And radium, being more active, will set off the metals from iron to copper; so ships using a radium initiator usually burn one of the commoner iron or copper ores."
"I know all that," I grunted. "And the heavier the metal, the greater the power from its disintegration…"
"…Well, Gunderson wanted to use still heavier elements. That required a source of rays more penetrating than those from radium, and he knew of only one available source—Element 91, protactinium…”Redemption Cairn
The general principle of the atomic blast is simple; a reactor chamber in which radioactive rays break down the atoms of fuel to release energy. The radiation released by disintegrating atoms bombards the surrounding fuel and in turn helps to break it down. Uranium-based atomic blasts break down sodium compounds into energy; radium blasts generally use iron or copper compounds; the most recent experimental protactinium blasts use lead as fuel.
Engines are at their most efficient and reliable when the fuel passes through relatively slowly and there is time for all of the fuel to disintegrate and convert to energy, but this produces a relatively slow output of energy, insufficient for takeoffs and landings. Higher power outputs can be achieved by pumping the fuel through the engine much more rapidly, but this is inherently wasteful; at full emergency power (the term “emergency” is used advisedly, since over-prolonged use leads to damaged engines) less than 20% of the fuel is disintegrated, the rest is simply lost. One side effect is a flare of waste energy – a bright exhaust in space, flames in atmosphere – as unused fuel is released as superheated gas. This limitation applies to all atomic blasts; the latest engines don’t produce much more energy than their predecessors; they produce roughly the same amount but when all of the bugs are fixed ought to be able to work efficiently at higher power outputs. 0.05g radium blasts and 0.1g protactinium blasts should eventually be possible.
|Earth – Mars|
|The Red Peri||0.050g?||5.0g?||9.0|
If speed was the only consideration all current ships would have radium blasts and burn iron or copper salts, but things aren’t that simple. The advantages of uranium engines include cheapness, reliability – they’ve been developed over more than a century, and most of the bugs are now engineered out – and very cheap fuel; burning sodium salts means that you can literally fill your tanks with concentrated sea water or salt from the flats of Venus or the silted canals of Mars. Sodium is abundant and readily available on Earth and most of the rest of the solar system; iron is even more abundant, but much less likely to be found in a form that doesn’t require a good deal of preliminary processing. Although protactinium drives will be even more powerful, the radioactive component is likely to be in short supply for the foreseeable future, and lead is comparatively rare and expensive; it seems unlikely that these drives will be economic for flights anywhere in the inner solar system, with the possible exception of Mercury.
When planning a flight it’s necessary to take numerous factors into an account; these include the positions, distances and orbital speeds of the planets, whether the ship is going “uphill” away from the Sun or “downhill” towards it, the gravity and escape velocities of the planets themselves, availability of fuel at the destination, etc. Routes between Venus, Earth, and Mars are the easiest to plan; distances, gravitation and escape velocities are comparatively small, and orbital speeds aren’t wildly different. Mercury’s high orbital speed and position deep inside the Sun’s gravity well make it a much more difficult destination, while the huge distances to the outer planets, and their high escape velocities, add their own problems. The table compares these factors for the planets and for Ceres, the largest asteroid.
|Pluto||29.7 to 49.3||7.8 or less||1.2g||12.9||4.7|
For example, a flight from Venus to Earth requires takeoff acceleration to reach an escape velocity of 10.3 KPS (or, rather, to reach an orbit where the escape velocity is lower, then accelerate more gently), a 5.2 KPS change in orbital velocity, and a 7 KPS change in velocity to overcome the Sun’s gravity and reach a “higher” orbit, and another 11.2 KPS change to land. Some of these changes add up, others cancel each other out, and all ignore the additional velocity changes needed to make the trip usefully fast. Calculating courses and velocity changes for an entire journey is a hugely complicated process, made possible only by specialised mechano-electrical calculating engines, regularly updated astrogational tables, and accurate charts. Fortunately most of the work is now routine; every ship comes with a set of simplified calculation tables, as in the example below, which state roughly how much fuel is needed for a given journey, accurately enough to give their owners an approximate cost for the voyage. All other things being equal fuel use is proportional to travel time and distance; there are complications such as the extra power needed to overcome the intense gravity of the Sun at Mercury’s orbit, the gravity of Jupiter and other giant planets, eccentricity and inclination of orbits, and so forth, but the simplified table shows the most common routes and compares travel time and fuel consumption when planets are at their closest, with Earth-Mars taken as the “standard” journey: the “fudge factor” is a rough correction, the extra fuel needed to compensate for gravity and other factors in these calculations. Sometimes these fudge factors may be counter-intuitive; for example, Ceres might seem an easier destination than (for example) Mars because its gravity is low, but this is cancelled out by the fact that the asteroid’s low gravity makes it harder to match orbits with it.
|Days at 0.015g||18.2||12.4||16.5||31.0||47.8||68.0|
|“fudge factor” 1.0 x||2.0||1.25||1.0||1.0||1.2||1.1|
|1Plus optional reserve for asteroid avoidance|
|Moons of Jupiter to:||Another|
|Days at 0.015g||1 to 2||48.4||86.7||116.2||136.7|
|“fudge factor” 1.2 x||0.5||1.1||1.22||1.22||1.1|
|2Assumes a landing on the world itself, not a moon|
|Days at 0.015g||71.9||105.7||127.8|
|“fudge factor” 1.1 x||1.2||1.2||1.1|
Needless to say, all of the above is hugely over-simplified. While these rough calculations are adequate for estimating rough costs and travel times, a precise analysis may involve twenty or more pages of tables, and dozens of complex equations.
Example: The freighter Newton’s Apple normally uses 125 tons of supersaturated salt solution as fuel for the Earth-Mars run. If it’s chartered to fly an Earth-Jupiter run instead the minimum fuel requirement goes up to 3.5 x125 tons, about 438 tons. Adding in a 10% reserve for asteroid avoidance takes this to 482 tons. This tonnage must be subtracted from the available cargo capacity.
Example: After unloading at Ganymede the ship needs to fly to Io to load cargo there. The fuel needed for this is minimal, less than 10% of that needed for a trip to Mars, about 12 tons. There’s more than that left in the tanks since most of the reserve hasn’t been used.
It’s often asked why salts are preferred to metallic solids – for example, why salt is used instead of pure sodium in uranium engines. In this case the explanation is simple; metallic sodium is less dense (and would take up more room aboard ship) and more dangerous to handle than a salt solution, and very expensive to manufacture. When the atomic blast is operating at its highest efficiency all of the salt solution is eventually converted to energy; the sodium, the chlorine, and the hydrogen and oxygen in the water. When the extra complications of working with molten sodium are taken into account it’s obvious that it’s more efficient and considerably cheaper to run on salt. In an ideal world solid salt would be preferred, since it’s denser than salt solutions, but there’s no easy way to liquefy it for pumping etc. or spray it into engines. In consequence most spaceship engines use brine (saturated salt solution), with density around 1.25 tons per cubic metre.
With radium engines the answer isn’t so simple. Iron and copper are both much denser than their salts, so theoretically it would be preferable to feed these engines the pure metals. Indeed, some experimental engines have been built which run on copper or iron wire, iron filings, etc. The problem is that all of the prior engineering art relates to liquid-fed engines, and solid-fuelled engines have a distressing tendency to overheat, jam, explode or otherwise malfunction. Insurers refuse to cover their use in civilian spacecraft. For now most radium engines run on saturated solutions of copper or iron salts, usually the sulphates, with with density of both averaging 1.3 tons/m3. If these problems can be solved iron-fed engines would store up to 7.8 tons of fuel per cubic metre, copper-fed engines up to 8.9 tons/m3.
The development of protactinium engines is focusing on designs which will use pure lead as fuel, either molten metal or fed in as wire or lead shot, with density 11.3 tons/m3. So far progress is slow.
Does this seem coherent? What I want to avoid doing is giving actual power outputs for engines in the text, since I'm pretty sure that even at total conversion the "high" acceleration I'm using can't be sustained for long, you'd need vast quantities of fuel. Bearing this in mind, does this seem OK?
If any of the astronomy seems a bit off, blame Weinbaum - he gave Uranus a solid surface, not sure about Neptune but I might as well treat it that way, and made Pluto more massive than Earth
Later Edited to remove a couple of bits left over from an earlier draft which had the new engine designs produced more energy from a given mass of fuel. Not sure how that got past the techno-bullshit detector, now changed.