beyond reach, advances in materials technology and construction techniques have shown them to be possible. For example, calculations show that the Titan II first stage, launched on its own, would have a 25-to-1 ratio of fuel to vehicle hardware. It has a sufficiently efficient engine to achieve orbit, but without carrying much payload. Dense versus hydrogen fuels Hydrogen fuel might seem the obvious fuel for SSTO vehicles. When burned with oxygen, hydrogen gives the highest specific impulse of any commonly used fuel: around 450 seconds, compared with up to 350 seconds for kerosene. Hydrogen has the following advantages: Hydrogen has nearly 30% higher specific impulse (about 450 seconds vs. 350 seconds) than most dense fuels. Hydrogen is an excellent coolant. The gross mass of hydrogen stages is lower than dense-fuelled stages for the same payload. Hydrogen is environmentally friendly. However, hydrogen also has these disadvantages: Very low density (about of the density of kerosene) – requiring a very large tank Deeply cryogenic – must be stored at very low temperatures and thus needs heavy insulation Escapes very easily from the smallest gap Wide combustible range – easily ignited and burns with a dangerously invisible flame Tends to condense oxygen which can cause flammability problems Has a large coefficient of expansion for even small heat leaks. These issues can be dealt with, but at extra cost. While kerosene tanks can be 1% of the weight of their contents, hydrogen tanks often must weigh 10% of their contents. This is because of both the low density and the additional insulation required to minimize boiloff (a problem which does not occur with kerosene and many other fuels). The low density of hydrogen further affects the design of the rest of the vehicle: pumps and pipework need to be much larger in order to pump the fuel to the engine. The end result is the thrust/weight ratio of hydrogen-fueled engines is 30–50% lower than comparable engines using denser fuels. This inefficiency indirectly affects gravity losses as well; the vehicle has to hold itself up on rocket power until it reaches orbit. The lower excess thrust of the hydrogen engines due to the lower thrust/weight ratio means that the vehicle must ascend more steeply, and so less thrust acts horizontally. Less horizontal thrust results in taking longer to reach orbit, and gravity losses are increased by at least . While not appearing large, the mass