Langley ResearchCenter to discuss the future mannedspacecraft program. (See
(ref. 4) for a history of the Apollo program.) Dr. Goett was eager to get
feasibility studies under way at the NASA research centers to _efine guidelines for
the manned lunar mission.
Of major interest to the present authors were the areas in which the Dynamics
Analysis Branch at Ames should concentrate its research efforts. The principal
outcome of many meetings of the Space Task Group was the identification _ two
potential areas of research for the Branch. The first was midcourse navigation and
guidance for the circumlunar mission, and the secor._ was the autopilot design for
large, flexible body liquid-fuel boosters.
The Dynamics Analysis Branch had only eight analytically oriented research
persons--too few to carry out two very complex research programs. The most logical
choice, considering personnel experience, seemed to be the booster autopilot design
problem. But after many discussions, enthusiasm grew for the midcourse navigation
and guidance problem, and there was finally a unanimous decision for the Branch to
work in that area. Certainly, it was an ambitious undertaking, one that presented a
greater challenge to Branch personnel than had been faced before.
The primary emphasis in the Branch was rapidly brought to bear on the need tc
develop the concepts and technology for a completely self-contained system. This
meant the software for the mission would have to be resident in a reliable, on-board
digital computer with considerable memory and relatively high computational speed.
The midcourse algorithms would have to be as efficient as possible. The system,
with pilot/navigator inputs, would have to navigate and guide the spacecraft from
injection into a circumlunar trajectory, around the Moon and back to Earth, satisfy-
ing very restrictive entry corridor requirements on return to Earth.
Having selected the problem area and directed our attention to a conceptual
design for a solution to the problem, it was clear that we were facing a rather
massive effort with a staff that was utterly inexperienced in many of the required
tasks (such as lunar trajectory analysis). Perhaps if we had fully comprehended the
necessary effort we would have reconsidered our choice of problem area for the
Branch research effort. Basically, we were starting with nothing in the way of
analytical software tools. Ames had an IBM 704, which would be our simulation
computer. Fortunately, Fortran was made available to us early in our research
efforts, but software for the machine was quite limited. For example, later on,
when we were preparing the software to do the Kalman filter matrix operations, we
had to write all of our own matrix-handling subroutines. We soon found, however,
that the double indexing needed for matrix operations ran so slowly that our matrix-
handling subroutines had to be rewritten to use single indexing. Otherwise, a
circumlunar trajectory with a Kalman filter would have taken hours of computer time.
The first step was to build a trajectory analysis program capable of simulating
a trajectory to the Moon and return. Fortunately, Dr. Clarence Gates and others at
the Jet Propulsion Laboratory (JPL) gave us invaluable assistance and counseling
based on their work in this area. Probably the most timely aid was in the form of
an ephemeris tape containing the positions of the Sun and the Moon versus time. By