Can I start an amateur rocket using nitrous oxide as either oxidizer or monopropellant, using a hypergolic slug of something reasonable for use by an amateur rocketeer? What substance could it be?
This is a mix that apparently could work.
"When passed over a warm ruthenium catalyst bed, gaseous nitrous oxide and an ethylene-ethane gaseous blend combust instantly"
But I could not find sources for Ethylene, but it seems to be available for industrial uses. This could disqualify this combination depending on the meaning of 'available'.
The safety data for ruthenium seems to list is as quite poisonous, so that could disqualify this combination depending on the meaning of 'safe'.
Most if not all hypergolic fuels are toxic to varying degrees. Hydrazine, mono- methyl hydrazine, and unsymmetrical dimethyl hydrazine are all in this category. Virgin Galactic Space Ship One used nitrous oxide with aluminum grain but needed an igniter. A venting problem on Apollo-Soyuz return knocked out one astronaut and all three had to be hospitalized for a time. I would avoid hypergolic fuels. They are not for the amateur rocketeer.