## Propellant ratio to deliver propellant to LEO

The propellant ratio is defined here as the propellant mass burned by the launcher to achieve LEO, divided by the propellant load carried to LEO. Both the mass of propellant and the density of the propellant affect the size of the launcher, and

Launcher propulsion system characteristics 217 Table 5.3. Launchers sized to deliver 19 tons of propellant to LEO.

H2/O2 rocket LACE rocket RBCC Mach 10 FDL-7C/D FDL-7C/D airbreather

RBCC Mach 12 airbreather

Planform area

600 m2

370 m2 11.85 tons 57.9 tons 76.9 tons 379.2 tons 456.1 tons 20.0

301 m2

268 m2

Weng OEW OWE

wpp1 TOGW

27.95 tons 97.86 tons 116.9 tons 892.9 tons 1,010 tons 47.0

11.13 tons 46.73 tons 65.73 tons 235.2 tons 300.9 tons 12.4

8.92 tons 40.18 tons 59.18 tons

181.0 tons

240.1 tons 9.53

Propellant ratio

Design payload is 19tons (41,895 lb) of propellant with a bulk density of 999.4kg/m3 (62.4 lb/ft3).

this sensitivity was evaluated. The launchers were sized using the methodology of Vandenkerckhove-Czysz described in Chapter 4 and not repeated here. The vehicle assumptions were the same as Chapter 4 except that a permanent propellant tank replaced the accessible payload bay. For the design payload and payload density, the sizing results are given in Table 5.3.

The propulsion system selection determines the key parameter for an orbital tanker, the propellant burnt to lift the orbital maneuver propellant, divided by the propellant delivered. The LACE rocket is an adaptation of an existing, operational rocket engine, and requires good engineering design and testing, but it is not a technological challenge. The LACE rocket offers a greater than 50% reduction in the propellant required to deliver the design 19 tons of propellant to LEO, as shown in Table 5.3 and Figure 5.4. Because of the LACE rocket's greater thrust/drag ratio, the propellant ratio is slightly better than a rocket ejector ramjet utilizing atmospheric air up to Mach 6. A piloted vehicle is a disadvantage for an orbital tanker in that the provisions for the pilot increases the propellant required to deliver the orbital propellant to LEO. Transitioning to an airbreather vehicle and propulsion configuration offers the potential to reduce the propellant required to deliver the orbital maneuver propellant by 38% and 52% respectively. Proceeding beyond an airbreathing Mach number of 12 results in an increase in the propellant required to deliver the orbital maneuver propellant.

The important conclusion from this analysis is that a first step, based on an existing rocket motor (LACE rocket) offers a 57% reduction in the propellant required to deliver the orbital maneuver propellant. And that step does not require a technological breakthrough but only an adaptation of an existing operational propulsion system. The important observation is that even with the best propulsion system for the launcher, it requires 10 pounds of launcher propellant to deliver 1 pound of orbital maneuver propellant to LEO, so the orbital maneuver vehicle needs to be a very efficient user of orbital propellant.

In this exercise the design payload was 19 metric tons. If that payload mass is increased, there is a gradual decrease in the percentage of the propellant required to deliver the orbital maneuver propellant, as shown in the top graph of Figure 5.5.

P ropellani Required to Lift LEO Maneuver Propellant

30,0

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