Part II: Free Flight Trajectory Design–Jack Fisher

According to the terms of the Surveyor contract Hughes was responsible for Surveyor mission design and mission flight path operations. This was not JPL’s preference, but NASA headquarters dictated this due to JPL’s heavy involvement at the time with the Ranger and Mariner missions. Hughes had no experience with lunar trajectories prior to the start of the Surveyor contract. JPL’s RFP provided reference trajectory data for use in the design of the spacecraft so there was no need to become involved in this area during the proposal effort. The Hughes effort was rightly directly solely towards the spacecraft design. Mal Meredith had requested permission during the proposal to gain some experience in this area, but his request was denied. At the beginning of the Surveyor contract there was evidently no background or experience in this realm. When I first became involved with Surveyor in early 1962, about one year after the contract start, the engineers that I looked to for guidance were Mal Meredith and Paul Wong. At that time responsibility for Surveyor trajectory design resided outside the Surveyor project within Eli Botkin’s section in Ed Marriott’s Engineering Mechanics and Preliminary Design Department.

In May 1962 JPL requested that the Hughes Surveyor program office provide several Hughes engineers to take up residence at JPL to learn more about JPL mission design and operations for the Ranger lunar mission and subsequently apply this knowledge to Surveyor. The areas of concern were trajectory analysis, midcourse guidance and orbit determination. The engineers selected were myself, John Ribarich and Mike Horstein.

The purpose of my assignment at JPL was to design the lunar trajectories for Ranger 6 that was scheduled for launch in January 1963. The Ranger mission was intended to obtain pictures of the lunar surface prior to impact on the lunar surface. I spent about six months working at JPL primarily with Bill Kirhofer and Vic Clarke learning the ins and outs of lunar trajectories and mission operations. At that time Vic Clarke was conceded to be the leading expert for lunar and planetary trajectories in the country.

As a part of this assignment I took part in mission operations for Ranger 5 that was launched on October 18, 1962. A malfunction after 15 minutes of operation caused the irreversible transfer of power from the solar panels to the battery. After about 9 hours of operation the battery was depleted and operation of the spacecraft ceased. This failure resulted in the replacement of JPL’s Ranger project manager and a postponement of the Ranger 6 mission that was not launched until January 1964. Due to this delay I terminated my assignment at JPL in early 1963 and returned to Culver City.

Upon my return to Hughes I wrote a detailed description of JPL lunar trajectory operations. Shortly thereafter Mal Meredith convinced me to join Surveyor project team and assume responsibility for the design of Surveyor lunar trajectories under Bill Grayer in the Guidance and Trajectory Department in Jim Cloud’s Systems Engineering and Analysis Laboratory. My experience at JPL formed the basis for the Hughes Surveyor lunar trajectory efforts.

Hughes tasks included selecting the Surveyor lunar transfer trajectories, specifying trajectory parameters to General Dynamics for the Centaur guidance equations, developing the logic and software for midcourse guidance as well as terminal descent and guidance. The major thrust was to provide the necessary data for the overall design of the Surveyor spacecraft. Further, for mission operations to be conducted in JPL’s Space Flight Operations Facility (SFOF) Hughes would have the responsibility for staffing and the managing the Flight Path Analysis and Command (FPAC) group and the Spacecraft Analysis and Command (SPAC) group.

Within Bill Grayer’s Guidance and Trajectory Department I assumed responsibility for the free flight trajectory design, John Ribarich for midcourse guidance analysis, and Len Davids for terminal descent and guidance. Mal Meredith assumed the role of “systems engineer” to assure that our efforts were properly coordinated. Mal later was assigned the role of FPAC director and played a major role in planning and conducting Surveyor flight operations.

The successful design of the Surveyor lunar trajectories required the establishment of the relevant requirements and constraints. For Surveyor these were:

• The launch vehicle is the Atlas Centaur utilizing either the direct ascent or the parking orbit operational mode.

• For the parking orbit mode the minimum coast time is 116 seconds and the maximum coast time is 25 minutes.

• Launch will take place from Pad 36 of the Air Force Eastern Test Range (now referred to as the Kennedy Space Center).

• Launch azimuths from Pad 36 are restricted to the range from 80 to 115 degrees east of true north for direct ascent and 78 to 115 degrees for parking orbit.

• The Centaur payload capability is limited by the requirement for a 3-sigma propellant reserve.

• Arrival at the moon shall be with Goldstone visibility a minimum of 2 hours prior to and 3 hours following unbraked impact with a time of flight near 66 hours. Goldstone is the DSIF station located near Barstow, California.

• For the initial Surveyor missions the landing sites were selected to be within the Apollo landing zone of plus or minus 45 degrees lunar longitude and plus or minus 5 degrees lunar latitude. Note: the first six Surveyor missions with four successful landings satisfied Apollo requirements so Surveyor VII successfully landed in the lunar highlands at 40 degrees south lunar latitude to provide science data.

• For direct ascent trajectories lunar landing shall be between 20 hours before the morning terminator and 72 hours before the evening terminator.

• For parking orbit trajectories lunar landing shall be between 20 hours after the morning terminator and 150 hours before the evening terminator.

• The unbraked lunar impact speed shall be between 2615 and 2692 meters per second for direct ascent missions and 2600 and 2690 meters per second for parking orbit missions.

• The unbraked flight path angle at impact measured from the vertical was constrained to 25 degrees for the early Surveyor missions and 45 degrees for the later missions.

The establishment of lunar trajectories can proceed with several levels of precision. Many important characteristics can be determined with simple earth-centered conics including establishment of the dates on which it was possible to launch towards the moon and the times on those days during which it was possible to launch. Given that the moon orbits the earth with a period of 27.3 days launch opportunities should repeat on a near-monthly basis. In JPL terminology the times during a day during which a launch is possible are called the launch window while the days during a month on which a launch is possible are called the launch period. Once the launch opportunities are determined the trajectory characteristics can be established and made available for the detailed design of the spacecraft and the planning of mission operations.

Of special interest are the conditions at the moon for initiation of the terminal descent. To determine the lunar trajectory characteristics it is necessary to add a further degree of complexity to the analysis. The so-called patched conic approach requires transferring the trajectory from an earth-centered phase to a moon-centered phase at the point where the earth’s and moon’s gravitational attractions are equal. At this point an earth-centered ellipse becomes a moon-centered hyperbola and the lunar approach characteristics can be determined.

Most of the studies conducted were based upon the patched-conic approach. In 1965 Ron Gillett and I published our studies of Surveyor lunar trajectories. Direct ascent launch opportunities from January 1966 to December 1968 and parking orbit launch opportunities from January 1967 to January 1969 were presented in two separate reports.

More precise simulations of lunar trajectories were required to establish the accuracy of the simpler solutions and also to provide data to General Dynamics for the determination of the Centaur guidance equations and firing tables for each mission. JPL provided Hughes with a all-encompassing trajectory simulation that satisfied these accuracy requirements. However, the running time on the computers of that time was prohibitive. Fortunately, Paul Wong with Gladys’ Perkins programming assistance was able to create a simpler simulation of sufficient accuracy based upon Encke’s method. This simulation took into account several perturbing factors including the Earth’s oblateness and the Sun’s gravitational attraction. The accuracy of this program was established by comparing the results with those of JPL’s program. Initially there were serious discrepancies between the two programs, but these were eliminated with the use of double precision integration.

It was necessary to specify to General Dynamics the launch parameters that would result in a successful mission based upon the desired mission parameters at the moon.   These were the landing site location determined by lunar latitude and longitude and the arrival time and arrival speed. For launch on a given day with parking orbit ascent the four factors that can be varied to achieve the desired landing conditions are launch time, launch azimuth, parking orbit coast time and the injection energy defined by JPL in terms of C3, a measure of the energy remaining after escape from the Earth’s gravitational field. For direct ascent trajectories the four launch parameters were launch time, launch azimuth, flight path angle at injection and injection energy, C3. Trajectories were calculated varying the launch parameters to determine the impact on the lunar arrival conditions. A covariance matrix was created that related the launch conditions to the arrival conditions so that launch parameters resulting in the desired landing conditions could be determined. These trajectory parameters were then specified to JPL and GDC. A post-injection standard trajectories report was generated for each mission—these reports were written by Carl Winkelman for the first two Surveyors and Stan Dunn for the last five.

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About Jack Fisher

Jack was a systems engineer at Hughes from 1961 to 1992. He contributed to various programs including Surveyor, Pioneer Venus, Galileo, Intelsat VI and innumerable proposals. He was the manager of of the Spacecraft Systems Engineering Lab until his retirement. Upon retirement Jack taught systems engineering at a number of national and international venues.