propulsion systems, in terms of takeoff size and weight required for a specified payload.

The focus of the discussion so far has been on a space transportation system. As with the railroad analogy, that implies efficient two-way travel to and from LEO. The vehicle configurations discussed have all had high hypersonic lift-to-drag ratios. The reason for that is the corollary to the argument that if waiting times and launch delays are economically penalizing to commercial launch vehicles, the waiting times and return delays are also economically penalizing. However, the way the continents and national boundaries are distributed on the surface of Earth means that a returning vehicle may have to wait until its landing site comes within the lateral range (cross range) capability. Figure 2.24 shows the waiting time in terms of orbits, as functions of the spacecraft lateral range capability and orbital inclination. This chart was salvaged from the original 1964 work done for the MOL support vehicle. For Cape Kennedy orbital inclinaion, the waiting times for an Apollo type ballistic capsule (with very limited lateral range capability) can be 14 orbits or about 21 hours. For nominal lifting bodies the wait times vary from 11 orbits or about 16.5 hours to 8 orbits and about 12 hours delay. The class of vehicles discussed in Chapter 3 would have no wait times. They could return at any time, any location in the orbit they were in, and land in CONUS (Continental U.S.A). The longest return would be if the spacecraft were directly overhead the landing site: the spacecraft would have to circumnavigate the Earth in space, that is one orbital period of about 1.5 hours. The spacecraft hypersonic aerodynamic performance and its resultant glide performance is shown in Table 2.1 in terms of lateral range (LR) and down range (DR) together with the maximum waiting time.

The implication of commercial operational requirements is to be able to return to the landing site from any orbital location on the current orbit. That requires a high hypersonic lift-to-drag ratio glider. The Space Shuttle had a hypersonic L/D sufficient to land at its intended site after 1 missed orbit, or a 1,500 nautical mile lateral range. The hypersonic lift-to-drag ratio performance of spacraft discussed in Chapter 3 have hypersonic L/Ds of from 2.7 to 3.2, meaning they can land in CONUS from any position on a low Earth orbit (400 nautical miles or less).

So, this class of spacecraft can have a scheduled launch and return capability that minimizes waiting time and, more importantly for commercial passengers and crew, can return in an emergency without waiting time.

The correlation of lateral range and L/D and the resulting down range is given in equation (3.1).

LR = lateral range (nautical miles)

DR = 4,866.6 + 4.704 17LR = down range (nautical miles)

For continental Russia, the longitudinal span is twice that of the U.S.A, so the L/D requirement for any time return is less at approximately an L/D of 1.7. Lozino-Lozinski was a strong advocate of no waiting emergency return, and his BOR vehicles were capable of meeting the Russian L/D requirement. He had a forceful way of making his emergency return requirement much as Mr McDonnel had for the MOL support vehicle in 1964.

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