transfer ellipse. The shortest time corresponds to a speed approaching escape speed, 10.946 km/s, in Table 6.2.

If and when a nuclear electric rocket or a nuclear thermal rocket becomes available (see Chapter 7), the reduction of the propellant required for the translunar trajectory will be significant. As with the orbital maneuver vehicles (OMVs) described in Chapter 5, the major hurdle for the nuclear electric propulsion system is thrust and the magnitude of the rejected heat, that determines the space radiator mass. The propellant mass in terms of the operational weight empty (OWE) will reduce from about 2.0 times the OWE to about 0.17 times the OWE, a reduction of some 91.5% in propellant mass. The difficulty with all elliptical transfer orbits is the time it takes to return to Earth if the trajectory is not precisely corrected at the intersection with the lunar sphere of influence. For the Hohmann transfer ellipse, 119.5-hour trip time, the elliptical orbital period is approximately 10 days, 5 hours. For the 70-hour lunar trip time the injection speed is 10.88 km/s and the transfer ellipse orbital period is approximately 16 days, 15 hours. For the 58.5-hour lunar trip time, the transfer ellipse orbital period is approximately 40 days, 22 hours. And finally, for the 54.0-hour lunar trip time, the transfer ellipse orbital period is approximately 135 days, 21 hours: the faster you go, the larger the orbit eccentricity and length if the trajectory to the Moon is not precise All of these elliptical trip times are greater than the resources carried by the Apollo spacecraft, so either a redundant or very reliable rocket system, or a sufficient resource reserve is necessary. There is a propellant requirement for the transfer to the lunar sphere of influence trajectory with the proper selection of the arrival angle (A) that can be almost negligible, or at least sufficiently manageable not to affect too much sizing the total propellant mass. Only a numerical analysis for a specific trajectory will yield that quantity correctly; such analysis does not affect the selection of the propulsion system and therefore need not be done for the purposes of this book. The last table (Table 6.3) deals with the propellant requirements to land on the Moon's surface and to take off from it.

Table 6.3 lists the minimum mass ratios to the lunar surface from the lunar parking orbit and back, from the lunar surface to the lunar parking orbit. As for the Apollo lunar ascent module, a hypergolic propellant is a reasonable choice until nuclear rockets or other non-chemical launching systems are operational. The hyper-golic rocket requires no igniter and is the most reliable starting engine available,

Table 6.3. Arriving or departing the Moon, hypergolic propellant rocket.

Altitude Altitude (km) (nautical miles)

Mass ratio orbit

Mass ratio escape

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