液体水素
Although liquid hydrogen gives a high Isp, its low density is a disadvantage: hydrogen occupies about 7x more volume per kilogram than dense fuels such as kerosene. The fuel tankage, plumbing, and pump must be correspondingly larger. This increases the vehicle's dry mass, reducing performance. Liquid hydrogen is also relatively expensive to produce and store, and causes difficulties with design, manufacture, and operation of the vehicle. However, liquid hydrogen is extremely well suited to upper stage use where Isp is at a premium and thrust to weight ratios are less relevant.