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."
"Well?" I asked. "Where do I fit in this mess?"
"I don't quite know, Jack. Let me finish what I know, which is all Stefan would tell me. Gunderson succeeded, they think; he's supposed to have worked out the formula by which protactinium could be made to set off lead, which would give much more power than any present type of initiator. But if he did succeed, his formula and notes were destroyed when the Hera crashed!"
If I'm reading this right the engines use radiation to somehow trigger the breakdown of elements in what amounts to slow total conversion - I'm visualising some sort of cascade effect where (for example) Uranium rays strip off a proton and breaks sodium down to Neon, releasing some energy, then Neon to Flourine, with another big energy release, and so forth. So with uranium the ideal fuel would be pure sodium but in practice they use salt, which is a hell of a lot cheaper, though the Chlorine is unwanted weight.
I don't really need too much of an explanation, and I know that this doesn't work with current atomic theory, but I'm wondering if Weinbaum got his ideas from some theory of the time. Any suggestions? Preferably one that doesn't involve having flourine hanging around for too long, considering how nasty it is...
The one reference to a similar idea I can think of is "Blowups Happen," but I think that was building up the heavier elements, not breaking things down.