I read information about Prof. Robert Benny Gerber and his coauthors’ discovery on Space Daily and asked him a few questions for Transterrestrial.

Dinkin: May I ask you a few questions about your new Xenon compounds? Can you summarize your discovery of the ability to combine noble gases with hydrogen for the lay rocket scientist?

Gerber: Thank you for your kind interest. I am going on a lecture trip to France tomorrow [4/1], so I shall have to reply in haste. I should say at the outset, with apologies, that I know nothing of rocket science, and have not considered at all any applications in that direction. I would be very happy if anybody has got creative ideas in that respect.

Our results led to the discovery of a new family of xenon compounds. These are molecules formed of a noble gas atom (xenon or krypton) with hydrocarbons. A typical case is HXeCCH, a molecule made of xenon and acetylene. Another specially interesting material that we predicted most recently (but was not made yet experimentally, is polymeric xenon acetylide, a polymer built of the unit -(XeCC)-. The only applications we are pursuing actively are medical applications (anesthetic agents), which is completely irrelevant here. There is some relevance to your points to the fact that these are high energy density materials.

Thus, our results do not combine just H and a noble gas atom. HXeCCH for example, is expected to decompose mainly into Xe + acetlyne. Depending on the conditions, it may also decompose in a secondary channel, giving H₂ and HCCCCCH. Both channels release a very large amount of energy, but since Xe has a large atomic weight, I would not expect any relevance to rocket fuels.

Dinkin: Can you combine hydrogen with helium?

Gerber: There is no compound to my knowledge consisting only of He and H. We predicted stability of the compound HHeF in pressurized solid helium, about 3 years ago. However, this compound was not made yet, and its preparation even in very small quantities is expected to be a major challenge.

Dinkin: Can you combine hydrogen with Neon?

Gerber: There is no known chemically-bound, neutral molecule of Ne, with hydrogen or with any other element. There are quite a few weakly-bound complexes, but I think this is a very different matter (I assume you are only interested in chemically bound, neutral species).

Dinkin: How do the disassociation energies compared with the combination energy of water? (Forgive me if I am using the wrong word, it has been 13 years since I took freshman chemistry at Caltech.)

Gerber: The energy released in the decomposition of HXeCCH into HCCH + Xe is 4.5eV is somewhat lower than that obtained in the recombination of OH + H into water (I think the state of the art number is 5.1 eV).

Dinkin: What is the highest boiling point you have found so far compared to liquid oxygen and liquid hydrogen? The boiling temperature of HXeCCH is not known, but should be much higher than that of liquid O₂ (and certainly H₂).

Dinkin: How much quantity could you produce if I gave you $1 million, $10 million or $100 million? How long before industrial quantities could be made available with $10 million, $100 million or $1 billion in development money?

Gerber: I do not know, I have never done applied research or dealt with such questions.

Dinkin: Would any of the molecules be suitable to use as a monopropellant, but safer and easier than peroxide? Would you describe your discovery as revolutionary to propulsion economics? Are there catalysts that make it easy for these molecules to burn as a monoprop? Do you think H-Xe would be any easier to work with in an ion engine than straight Xenon? Any other exciting possibilities for rocket reactions? He-CH₃?

Gerber: Here I can only offer a disappointing opinion. As I understand it, a desired property for a rocket propellant is high Specific Impulse. Because of the high atomic weight of Xe, HXeCCH should be poor in this respect. (It may have some promise as energetic material for other applications, but that is another matter).

I also asked Jeff Greason, CEO of XCOR (the rocket company, not another Xenon compound), about it and he said something to the effect of ‘I like propellant that does not combust at room temperature.’

Xenon leads me into thinking about Ion Drives.

The Xe is useless from a combustion standpoint, but if there's a dramatic increase in energy density where the 'waste product' is itself useful once you're outside atmosphere, that might be interesting.

Xenon? Who cares? Britney Spears is pregnant!

That's the important thing!

From my limited knowledge of propulsion, the nice thing about an Ion Drive is that you are separating the reaction mass from the energy that propells it. The energy can even be harvested in transit (e.g. with solar panels), while mass cannot (lets ignore the Bussard Ramjet and automated remote mining for now).

The more energy you have, the faster you can shoot that Xenon to the back, giving you more thrust (F = m x a). This also means it is easy to throttle thrust. Unlike, say, a solid rocket engine.

You use Xenon because it is inert and dense. That also makes for a smaller vehicle.