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Hydrazine and its various derivatives have been used as hypergolic fuels.

Would symmetrical dimethylhydrazine also theoretically work as a hypergolic fuel? If so, why hasn't it been used in practice? If not, why wouldn't it work?

symmetrical dimethylhydrazine

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    $\begingroup$ Yes, it can be used but its freezing point is only -8.9C so, it isn't often used. $\endgroup$
    – Auberron
    Apr 2, 2020 at 4:47
  • $\begingroup$ @Auberron: Would you like to add some sources and write that in a proper answer? Then you can earn reputation. $\endgroup$
    – DrSheldon
    Apr 2, 2020 at 11:34
  • $\begingroup$ @Auberron That doesn't sound like the driving reason as ordinary anhydrous hydrazine, used as a monopropellant and a hypergolic bipropellant in countless satellites, has a freezing point of +2degC. $\endgroup$
    – Puffin
    Apr 2, 2020 at 11:43
  • $\begingroup$ @DrSheldon I haven't found exact resource but some resource suggest SDMH is less experimented and data are less.They are both carcinogenic and toxic but report on effects of SDMH on human health aren't done yet. SDMH are less known compared to that of UDMH.This forum suggests SDMH are difficult to synthesize. $\endgroup$
    – Auberron
    Apr 2, 2020 at 14:32

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This website suggests SDMH is incompatible with oxidizing agents, water. So it can be used for hypergolic purpose.This quote is from Sittig's Handbook of Toxic and Hazardous Chemicals and Carcinogens:

Properties of SDMH

Forms explosive mixture with air. A
strong reducing agent and strong base. Violent reaction
with strong oxidizers, strong acids, metallic oxides. Attacks
some plastics, rubber, and coatings. May accumulate static
electrical charges, and may cause ignition of its vapors.

And UDMH is also miscible with water and reacts with oxidizing agent. UDMH is often used in hypergolic rocket fuels as a bipropellant in combination with the oxidizer nitrogen tetroxide and less frequently with IRFNA (inhibited red fuming nitric acid) or liquid oxygen. The real difference between them is their melting point. The freezing point of SDMH is -8.9°C and UDMH is -57.7°C .UDMH has the lowest freezing point and has enough thermal stability to be used in large regeneratively cooled engines. Lower melting point ensures storing at low temperature for longer time & greater safety of this toxic chemical.So, this may be the prime reason.

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enter image description here

To draw out a quote:

Symmetrical dimethyl hydrazine turned out to be a dog (it's [sic] freezing point was only--8.9 [degrees])

Not only have I found a rare typo in the remarkable Ignition! (and in the second edition, to boot!), but there you go: SDMH was a good propellant, but it froze far too readily. To answer your question: yes, it was probably a good hypergolic, but since hypergolic developing was sponsored by a military trying to ensure launch-readiness under any circumstances, the elevated freezing point was a no-go.

The reason for this is simple to reason: symmetry lends itself more readily to crystal formation. It's harder to organize the unsymmetrical form, so it stays molten at lower temperatures.

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