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identify the configurations. Examining the images of the launchers and spacecraft we find an excellent cross-section of the past 50 years. There are three configurations that have variable-geometry features employing retractable straight wings for improved landing and takeoff, i.e., numbers 2, 10 and 11. All of the spacecraft are delta planforms, except for Harry Stine's horizontal takeoff and landing concept, number 3. Configurations 5, 7 and 9 are two-stage-to-orbit (TSTO) concepts that are very similar. The German "Saenger" configuration (7) by MBB employs a hypersonic glider that carries onboard the propellant necessary to achieve orbit, maneuver and return. Lozinski (5) and Dassault (9) both have a different philosophy from MBB with respect to the propellant to reach orbit. In their studies it was more economical to carry the ascent propellant in an expendable rocket and to carry maneuver and return propellant on board the spacecraft. In fact, the question of propellant has many answers, depending on flight rate, and has yet to be determined today. If the flight rate postulated as needed in 1965 were real (74 flights per year) the answer would probably favor the MBB approach. All three of these designs had the idea to use the first stage (which staged the second stage at Mach number from 6 to 7) for a Mach 4.5 to 5 hypersonic cruise aircraft. If sub-cooled liquid methane were substituted for the hydrogen, with the same total energy content, the methane would occupy only 36% of the hydrogen tank volume. The 64% of the hydrogen tank would now make a perfectly well insulated cabin for either carrying cargo or human passengers. The useful range of such an aircraft would easily be in the 6,500 nautical mile (12,040 km) category.

Of the vertical launch rockets in Table 1.1, one is expendable, the Vostock launcher from the former USSR. The Vostock launcher is designated SL-3. The growth version of this launcher is the SL-4, the Soyuz launcher. It is in fact from the former USSR, as the companies that supply the hardware and launch facilities for the Soyuz are now in separate nations. However, it is show because Soyuz has achieved the launch rate required to support the 1965 space station (it is noteworthy that in 1991 there were 92 launches from the three Soyuz pads at the Baikonour launch facility). The other two, the MDC Delta Clipper and the GD Millenium Express are intended to be sustained use vehicles, although not at the rate required to support the 1965 space station. Reusable vertical launch vehicles are important because they can lift heavy payloads to orbit when required by the mission, such as orbital assembly of space stations, or of the deep space and Mars vehicles represented by configuration 17.

We have now established that the launchers and propulsion to get to Earth's orbit is neither beyond current capability (nor was it beyond 1965 capability!) nor limiting in establishing a space transportation system or infrastructure. So now it is to the future to achieve the dreams of the past generation.

Still in the context of reusable versus "throwaway" launchers, it is a fact that the expediency of launching another expendable rocket historically has always won over the will to develop a commercial, sustained-use, multiple-launch spacecraft. As a consequence, the current "progressive" path is still an expendable rocket, albeit with some parts reusable. In October 1999 at the International Astronautics Federation (IAF) Congress in Amsterdam, an IAF paper reported that US-Russian cooperation resulted in a hydrogen/oxygen rocket engine (the RD-0120, in the Russian classification) for the Energia launcher that had been fired on a test stand for 80 simulated launches and returns, with a throttle up during ascent to 135% rated thrust (the US Shuttle engine, the SME, throttles up to about 109% rated thrust). A manager from one of the US rocket launcher companies exclaimed, ''This is terrible, we would have lost 79 launcher sales!" [Davis, 1999]. That explains why sustained operational use spacecraft never developed. The rocket launcher organizations never proceeded along a path analogous to that taken by the Douglas Aircraft Company with the DC-3, DC-4, DC-6, DC-7 and DC-8 commercial transport family, to cite one example. From 1934 to 1974 this series of commercial transports went from reciprocating engines with propellers, with 150 mph speed and 1000 miles range, to gas-turbine-powered jet aircraft, flying for 7,000 miles at 550 mph. In the 50 years from the first artificial satellite (Sputnik) the launcher is still the liquid-rocket-powered ballistic missile of the late 1950s. The aerospace establishment has forgotten the heritage of its pioneers and dreamers. It has forgotten to dream, preferring to rely on a comfortable status quo (and certainly perceived safer by shareholders). These historical motivations and current perceptions will have to be reassessed if man is to travel in space for longer distances than those typical of the near-Earth environment. A synthetic description of distances and time in our Solar System and our galaxy will illustrate this point.

1.4 OPERATIONAL SPACE DISTANCES, SPEED, AND TIMES

Envisioning the time and space of our Solar System, our Milky Way galaxy, and intergalactic space is a challenge for anyone. In terms of our current best space propulsion systems, it takes over one year to travel to our planetary neighbor, Mars. It can take up to 12 minutes for a microwave signal to reach Mars from Earth. Consider a rover on Mars that is approaching an obstacle or canyon. When the picture of that is received on Earth it is already 12 minutes behind actuality. By the time a stop signal reaches the rover, between 24 and 30 minutes

Orbit of Moon, 477,680 miles in diameter

Sun, 856,116 miles in diameter Moon, 2,158 miles in diameter

Figure 1.2. Diameter of the Sun compared with the Moon's orbital diameter.

Earth, 7,927 miles in diameter

Sun, 856,116 miles in diameter Moon, 2,158 miles in diameter

Figure 1.2. Diameter of the Sun compared with the Moon's orbital diameter.

have elapsed, depending on the speed of the project team. It is another 12 minutes, or a 36 to 42 minute elapsed time, before the project team knows whether the rover was saved, stalled, damaged or destroyed. With the control center on Earth, the time interval is too long to assure the rover remains operational, so an independent intelligent robot is a necessity. Traveling to our remotest planetary neighbor, Pluto, requires a daunting 19 years. In terms of light speed, it is a mere 5 hours 13 minutes, at Pluto's average distance from Earth. And this is just the outer edge of our planets, not our Solar System. To the edge of our Solar System, the boundary between our Solar System and the oncoming galactic space medium, the Heliopause, the light time is 13.46 hours. Envisioning the size of our Solar System is also a challenge. For example, our Sun is 109 times the diameter of the Earth and 1.79 times the diameter of the Moon's orbit around Earth, as depicted in Figure 1.2, and the Sun represents the single most massive object in our Solar System. From the Sun, we can proceed outward to the outer edge of our Solar System and our nearest star, Proxima Centauri. Proxima Centauri is a very dim star; its slightly more distant neighbor, Alpha Centauri is instead very bright, but they are near the Southern Cross and only visible from the Southern Hemisphere. A cross-section of our local galactic space is presented in Figure 1.3. Remember that an astronomical unit (AU) is the distance to an object divided by the Earth's distance from the Sun, so Jupiter is 5.20 AU from the Sun means that Jupiter is 5.2 times further from the Sun than Earth is. Figure 1.3 spans the space from the Sun to our nearest star, Proxima Centauri. The space is divided into three zones. The first zone contains the terrestrial planets; those are planets that are rocky, Earth-like in composition. These are Mercury, Venus, Earth, Mars and a band of rocky debris called the Asteroid Belt. The second zone contains the Jovian planets; those are planets that are essentially

Earth Mars

Venus

Mercury \

Jupiter

• Terrestrial Planets

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