“The atomic blast got weak. I started losing altitude right away, and suddenly there I was with a thump right in the middle of Thyle! Smashed my nose on the window, too!” He rubbed the injured member ruefully.
“Did you maybe try vashing der combustion chamber mit acid sulphuric?” inquired Putz. “Sometimes der lead giffs a secondary radiation—”
A Martian Odyssey
The general principle of the atomic blast is simple; a tube in which radioactive rays break down the atoms of fuel to release energy. The radiation released by disintegrating atoms bombards the surrounding fuel and helps to break it down. Uranium-based atomic blasts break down sodium compounds (most typically sodium chloride, ordinary salt) to split atoms into combinations of the following elements:
Sodium (11) - Neon (10) - Fluorine (9) - Oxygen (8) - Nitrogen (7) - Carbon (6) - Boron (5) - Beryllium (4) - Lithium (3) - Helium (2) -Hydrogen (1)
For example, one sodium atom might split into Neon + Hydrogen, another into Oxygen + Lithium, and so forth. These atoms in turn break down into lighter elements – one Neon atom might break down to Oxygen plus Helium, another to Fluorine plus Hydrogen. Usually these atoms are unstable isotopes.
Like all radioactive processes it’s essentially random, and some sodium atoms may get through the engine without breaking down at all, others may break down unusually fast. In practice the process never reaches completion, with hydrogen ions as the only exhaust; at its most efficient the exhaust products are likely to be a lithium / helium / hydrogen mix with traces of the heavier elements, plus chlorine ions from the salt. All of these products are released as superheated gas plasma, the exhaust that drives the ship.
Engines are at their most efficient and reliable when the fuel passes through relatively slowly and there is time for several stages of this reaction to occur, but this is achieved via a relatively slow output of energy, insufficient for takeoffs and landings. The most practical way to get high power outputs is to pump the fuel through much more rapidly, but this is inherently wasteful; in the example above the progression might stop several stages earlier, with the main product carbon or boron. At full emergency power (the term “emergency” is used advisedly, since it often leads to damage engines) fuel efficiency is typically less than 10% of that at economical cruising output.
Radium power plants work by a similar process, beginning with iron-based compounds; while power output for a given weight of fuel is better, the relationship between fuel consumption at cruising acceleration and emergency power is much the same. Range is improved, efficiency and reliability are not. When and if protactinium power plants using lead compounds as fuel enter service it’s likely that the relationship will remain much the same.
There are several ways to increase fuel efficiency at higher outputs, but all of them are expensive, decrease reliability and add significantly to the proportion of the size and weight of the ship taken up by the engines and fuel. While progress is being made, no commercial or military spacecraft whose details are known currently has a sustained cruising acceleration better than 0.03g – it’s believed that the Red Peri may achieve 0.05g or better, but features of its design suggest that it does so at the expense of safety and reliability. The table shows accelerations for the Ares (the first ship to Mars) and some modern vessels, and typical travel times from Earth to Mars when the worlds are at their closest.
|Earth – Mars|
|Red Peri||0.05g ?||5g ?||9|
Remember, I want something that might seem plausible by 1930s SF standards, not hard SF!