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In particular, I'm thinking about lithium, but sodium could be considered as well.

Lithium has low molecular weight, very high propellant density, and low melting point. At sufficiently high temperatures (above 500 degrees Fahrenheit) it ignites spontaneously in air with explosive force.

A lithium scramjet/ramjet would require an onboard power supply to heat the fuel to liquid and then pump it into the combustion chamber. The idea is that the combustion chamber would need to be hot enough so that the lithium droplets would quickly evaporate and ignite making the mixture hypergolic.

I'm surprised that I have found no current R&D into alkali metal fuels for rocket propulsion(let alone hypersonic propulsion). It seems to me that to make longer range hypersonic flight possible, exotic fuels will be absolutely necessary.

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    $\begingroup$ Energy per unit weight pf lithium is actually half of jet fuel. You might have better luck with aluminum. Technically it's not too difficult since solid ramjet is already done, you add metal into the base fuel. $\endgroup$
    – user3528438
    Commented Mar 31, 2019 at 21:31
  • $\begingroup$ Hydrogen has such a high energy density than anything else that hydro carbon is almost next-to-ideal, which is why kerosene and methane are both considered very good rocket fuels. $\endgroup$
    – user3528438
    Commented Mar 31, 2019 at 21:40
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    $\begingroup$ @user3528438 But it has a very low physical density. Which is why it is impractical for usage as a fuel because of the weight and volume of a sufficiently large fuel tank. Liquid H2 is cryogenic which has its own set of problems as seen by the Space Shuttle. $\endgroup$
    – Mr X
    Commented Mar 31, 2019 at 22:58
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    $\begingroup$ @user3528438: For example, the Space Shuttle solid rocket boosters, which used aluminum metal with an ammonium perchlorate oxidizer. $\endgroup$
    – jamesqf
    Commented Apr 1, 2019 at 4:37

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How long does the lithium take to evaporate and then to react with oxygen? Air in a scramjet is moving extremely fast (supersonic by definition!), so there is very little time left for mixing and combustion if you desire to create thrust from the the combustion heat energy.

Your lithium engine will most likely trail a bright contrail of burning lithium but develop rather little thrust. The high flame speed of a mixture of hydrogen and oxygen makes hydrogen the preferred scramjet fuel.

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    $\begingroup$ In that case, it would be better to make it a ramjet where the combustion chamber temperature is > 2000 degrees Fahrenheit. Hydrogen has low density and this makes it not very practical for use in a ramjet/scramjet. So for using something like lithium this would require subsonic, compressed, heater air and possibly spark plugs to rapidly vaporize the lithium fuel droplets and ignite them. $\endgroup$
    – Mr X
    Commented Mar 31, 2019 at 20:10
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    $\begingroup$ @Mr X your thinking is good, just consider Hydrogen as a metal! Now check the electrons available for chemical combustion vs weight. Liquifying it helps the density issue. But they do use aluminum in solid fuel mixtures rather effectively. $\endgroup$
    – Robert DiGiovanni
    Commented Mar 31, 2019 at 20:56
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    $\begingroup$ @RobertDiGiovanni I certainly do. But liquid hydrogen is cryogenic which means that the tank needs insulation which adds to size and weight. Lithium is much more reactive than aluminum. $\endgroup$
    – Mr X
    Commented Mar 31, 2019 at 22:56
  • $\begingroup$ There are some good charts on Google about Mj energy per kg of various elements and also energy density. What PK said about mixing and burn time in a very short time frame factors in, no problem vaporizing liquid H2. Cryrogenics can also be used to help cool the combustion chamber before injection. But a lithium solid booster is interesting. $\endgroup$
    – Robert DiGiovanni
    Commented Apr 1, 2019 at 0:50
  • $\begingroup$ Of course, if you burn the fuel using a detonation wave (which has a speed a couple orders of magnitude greater than that of a wimpy flame front)... $\endgroup$
    – Vikki
    Commented May 28, 2019 at 3:28

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