  In this paper, we present a real-time successive convexification algorithm
for a generalized free-final-time 6-degree-of-freedom powered descent guidance
problem. We build on our previous work by introducing the following
contributions: (i) a free-ignition-time modification that allows the algorithm
to determine the optimal engine ignition time, (ii) a tractable aerodynamics
formulation that models both lift and drag, and (iii) a continuous
state-triggered constraint formulation that emulates conditionally enforced
constraints. In particular, contribution (iii) effectively allows constraints
to be enabled or disabled by if-statements conditioned on the solution
variables of the parent continuous optimization problem. To the best of our
knowledge, this represents a novel formulation in the optimal control
literature, and enables a number of interesting applications, including
velocity-triggered angle of attack constraints and range-triggered line of
sight constraints. Our algorithm converts the resulting generalized powered
descent guidance problem from a non-convex free-final-time optimal control
problem into a sequence of tractable convex second-order cone programming
subproblems. With the aid of virtual control and trust region modifications,
these subproblems are solved in succession until convergence is attained.
Simulations using a third-party solver demonstrate the real-time capabilities
of the proposed algorithm, with a maximum execution time of less than 0.7
seconds over a multitude of problem feature combinations.
