200 3 00 4.00 500 6 00 7.00 800 9 00 Weight ratio to orbital speed

Figure 3.23. Takeoff and landing speeds of minimum-sized launchers. TO = takeoff; LND = landing; SWB = switchblade wings.

mass ratio values. With this approach the landing and takeoff speeds are essentially equal, adding a degree of operational simplicity. Landing and takeoff speeds correspond to those of current military aircraft and civil transports, at least for the lower mass ratios (five or less). However, the landing speeds do increase with takeoff mass ratio, since the operational empty weight of the vehicle increases with mass ratio. An approach to make the landing speed approximately constant and a lower value is to deploy the switchblade wing for landing (diamonds). Then the landing speed becomes very modest, even lower in most civil transports and military aircraft.

Takeoff speeds for blended bodies in the 200- to 230-knot ranges were postulated in the 1960s by using a very large gimbaled rocket motor to rotate upwards causing the body to rotate, lifting off the nose wheel and eventually the entire vehicle with a thrust-supported takeoff. This concept was not implemented in an actual system. If the takeoff speed is too high for the propulsion system chosen (because of weight ratio) then the only way to decrease the takeoff speed is to increase the planform area for the system volume, that is, to reduce the Kiichemann tau. This unfortunately introduces a cascade of incremental mass increases that result in an exponential rise of the takeoff gross weight. This is illustrated in Figure 3.24.

Figure 3.24 begins with a solution map of vertical takeoff launchers, as represented by the shaded area in the lower part of the figure. All of this data is for converged solutions, where the SSTO mission requirements are met and the mass and volume of each solution are converged. These solution areas represent a spectrum going from all rocket systems (far right) to advanced airbreathing systems (far left). These solution areas are for vertical takeoff, horizontal landing (VTOHL) with thrust to weight ratio at takeoff (TWTO) of 1.35 and tau equal to 0.2. For comparison, the gross weight trends are shown for five different takeoff wing

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