Most likely they're working with very hazardous fuels and oxidisers like hydrazine[0]. Edit: it seems that the orbiter was upgraded to use hydrazine and nitrog

Most likely they're working with very hazardous fuels and oxidisers like hydrazine[0]. Edit: it seems that the orbiter was upgraded to use hydrazine and nitrogen tetroxide[1] from the somewhat less toxic hydrogen peroxide/JP-8 combination originally specced for its AR2-3 engine. [0] https://en.wikipedia.org/wiki/Hydrazine https://en.wikipedia.org/wiki/Hydrazine [1] https://www.airspacemag.com/space/spaceplane-x-37-180957777/ https://www.airspacemag.com/space/spaceplane-x-37-180957777/