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For payloads greater than 4.9 tons, the 19 tons of propellant payload delivered to LEO by the tanker launcher is insufficient for a LEO to GSO mission. This is shown for hypergolic propulsion because as advanced propulsion enters orbital operations, the propellant requirement will substantially reduce, even for the heavier payloads. The propellant load scales as the mass ratio minus one, so for nuclear electric the propellant load for the 7.5-ton payload OMV is only 1.07 tons and for the solar electric it is 4.71 tons. But as long as the principal orbital maneuver propellant of choice is hypergolic, the orbital propellant requirements will steadily increase. The ESA ATV meets a current need. With the Space Shuttle grounded, a more substantial thrust OMV is required to re-boost the International Space Station (ISS) and some mechanism to provide service capability to the Hubble Telescope is necessary. If Hubble were to be placed at the same orbital inclination as ISS, but at a higher altitude, Hubble could be serviced from ISS without an operational Shuttle.

The gross weight of the two-way OMV is more complex because the OMV must carry the return-to-LEO propellant to GSO. The sizing program balances the total propellant required versus the capacity of the propellant tanks that determines OEW. The sized OMVs for each of the propulsion systems transporting a 5000-lb (2.268-ton) satellite are given in Table 5.8. The gross weight for the two-way mission is:

= OEWomV • WR2 + WR-Wsatellite ( Wpropellant)to LEO = (WR - 1)(OEWomv)

( Wpropellant)toGSO = [WR(OEWomv) + W^te^R - 1) (5 .10)

As would be expected, the to-GSO and return OMV is significantly larger than the one-way vehicle, Table 5.8. Other than being larger, the same comments apply to the two-way OMV as the one-way OMV. In launches to GSO with the current multistage rockets, the propellant in the upper stage (usually third stage) contains the

Table 5.8. Sized OMVs for two-way mission from LEO to GSO to LEO.

Propulsion

Gross mass Propellant OWE OEW Two-way Thrust (tons) (tons (tons) (tons) mass ratio (kN)

Hypergolic

H2/O2

Solar electric Nuclear electric

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