Launch Failure of SpaceX Falcon 1 - Flight 3

So the crash was cut out BOOO!

Is launch control in Dr Fudds science class or something?

anti-illuminati and nwo !!!

That isn't important. A conspiracy always sounds better than sensible facts ; )

Sounds like they had a boisterous crowd there...

Dunno, really. For heat-driven exhaust I guess nothing beats H2, the lowest mass compound there is. Ideally, it'd be probably be monoatomic hydrogen, but that's rather non-doable.

Ion engines, yes noble gasses are mostly used. The reason why such high atomic number elements are used escapes me right now, though I did hear the explanation once and it did make sense at the time. Something to do with work to ionize and accelerate IIRC, i.e. they're less power hungry even if less efficient.

If you heat the H2 enough, it should become monatomic. A plasma, actually.

The heavier elements have bigger outer electron shells, so it should take less energy to strip one or more of them away to form ions. Is that the reason? Cs should be especially easy to ionize since it's an alkali metal with the lowest electronegativity of all elements except Fr.

I think you nailed it, the lower ionization energy of heavier elements. It supposedly more than offsets the lower velocity such an atom will achieve for the same accelerating voltage. Cs is very reactive, not convenient to handle as it's a solid at room temp and not worth the hassle. Similar to chemical engines employing fluorine as oxidant - superior performance but it's a b*tch to handle.

Cs melts at 28C, so it's effectively liquid. It's not reactive in a vacuum. Xe is inert, but as a gas it can't be stored as densely. But it must be easier overall.

Wow, didn't know about the melting point. I know it's not reactive in vacuum, I was implying it could pose problems to various materials used inside the ion engine assembly. With Xe you don't have to worry about pressurization, while a liquid would develop an ullage in the tank potentially posing all kinds of problems.