Fun with Atoms

[ffutures]

OK, let's say up front that I know that this isn't how atomic power really works. It's my best guess as to how Weinbaum thought it worked. Does this seem vaguely plausible from the perspective of 1930s SF?



“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.

	Cruising Acceleration	Emergency Power	Earth – Mars (days)
Ares	0.01g	2.5g	20.2
Modern Freighter	0.015g	3g	16.5
Modern Liner	0.03g	3g	11.7
Military Vessels	0.03g	3.5g	11.7
Red Peri	0.05g ?	5g ?	9

Remember, I want something that might seem plausible by 1930s SF standards, not hard SF!

Comments

[dsample.livejournal.com]

Did Weinbaum really have his atomic blasts breaking up elements lighter than iron into even lighter elements? Even in the 30s I think it was pretty well known that you have to add energy to do that (4 hydrogen atoms weigh more than one helium atom.) Doing something like that puts most of your energy into making your exhaust heavier, rather than moving it faster.

[ffutures.livejournal.com]

That's the impression I got. A passage from Redemption Cairn.

She nodded. "Well, you know how a rocket motor works, of course. How 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."

"Exactly." She paused a moment. "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. And it happens that the richest deposits of protactinium so far discovered are those in the rocks of Europa; so to Europa he came for his experiments."

[ffutures.livejournal.com]

Actually, that's a much better introduction than the passage I originally used. I shall amend accordingly

[ffutures.livejournal.com]

On re-reading this it's obvious I've made it more complicated than it should be. Weinbaum is describing conversion of matter to energy, not to lighter particles.

[ffutures.livejournal.com]

Revised version:

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 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, but this produces 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 the engine much more rapidly, but this is inherently wasteful; at full emergency power (the term “emergency” is used advisedly, since it often leads to damaged engines) less than 10% of the fuel is disintegrated, the rest is simply lost. One side effect is a huge flare of energy – a bright exhaust in space, flames in atmosphere – as the remaining superheated fuel becomes superheated gas plasma. This limitation applies to all types of atomic blast; the latest engines don’t produce much more energy than their predecessors, they produce roughly the same amount but expend considerably less fuel to do so, and represent an improvement in range rather than acceleration.

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. While progress is being made, no current commercial or military spacecraft 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 above 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.

[robertprior.livejournal.com]

That looks better.