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Comment by btilly | Hacker News Reader

The two basic facts to be aware of.

Momentum is mass times velocity. Energy is m v^2 / 2. Both with adjustments in the relativistic realm, which this barely gets to.

The maximum thrust per atom lost is going to be at insanely high velocity. The maximum thrust per unit energy is going to be at low velocity. Which makes ion thrusters great for slow changes of velocity when powered by unlimited solar power. But terrible for launch with a finite energy budget.

Useless trivia. A traditional rocket becomes more efficient if you mix some mercury into the fuel. The loss of energy is made up for by the extra mass. The collateral damage from the mercury is an environmental disaster though.

For a lot more, including the source of that trivia, I highly recommend reading https://library.sciencemadness.org/library/books/ignition.pd....

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I’m not sure that’s quite right (I work in this field and we also give all new employees a copy of Clarke) - unless you really squint about your metric for ‘efficiency’, adding Mercury would be a disaster - the specific impulse (how much velocity change you can get for a given mass of propellant) would fall off a cliff completely. You’d actually want to do the opposite- add extra free hydrogen - to improve efficiency.

I'm quite sure that if you start at the bottom of page 195 of the PDF, which is marked page 177 at the top, and read for a few pages, my claim will be verified.

It gets worse. The reasoning process actually started not with mercury, but with dimethyl mercury. My ex knew https://en.wikipedia.org/wiki/Karen_Wetterhahn while she was dying. And so that story brought particular horror for me, and made it particularly memorable.

Right, so your citation is specifically density-impulse which is rather different to specific impulse, the conventional metric when discussing 'efficiency' of an engine, but of course you can define efficiency to be many things depend on the context, like thrust to weight ratio or whatever. But density impulse is quite a specific and generally not useful metric for rocket engines out side of e.g. a missile that has to fit its propulsion unit into very constrained packaging. For a conventional rocket, e.g. to launch a satellite, it is absolutely the case that adding mercuty would make the performance much worse. If you read the page before the one you cited, Clarke says as much:

I calculated the performance of Cavea A with various proportions of mercury —up to six times the mass of the primary propellant. (It was easy to fit mercury into the NQD calculation method.) As expected, the specific impulse dropped outrageously as mercury was added to the system, but the density impulse (specific impulse X propellant density) rose spectacularly, to peak at 50 percent above that of the neat mono-propellant with a mercury/propellant ratio of about 4.8.

Specific impulse is overwhelmingly the metric that matters for rocket performance in all but a small number of applications, so I feel justified in saying that your original claim that:

A traditional rocket becomes more efficient if you mix some mercury into the fuel. The loss of energy is made up for by the extra mass.

is simply not correct!

Lol, mercury in rockets fuel reminds me of lead in petrol.

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