The electric fuel cell propulsion system onboard the Ion Tiger has the low noise and signature of a battery-powered UAV, while taking advantage of hydrogen, a high-energy fuel
The common features of both of these new launcher systems include the cryogenic upper stage, which is powered by high-energy fuels
(liquid oxygen/liquid hydrogen).
The authors devote a chapter to each of these projects as well as another chapter to high-energy fuels
The group points out that low-energy materials like wood and charcoal actually release more pollution by volume than high-energy fuels
like propane and natural gas.