The analysis of midcourse maneuver requirements had been previously well defined by JPL for their Ranger and Mariner missions and was primarily dedicated to correcting injection errors resulting from launch vehicle performance dispersions. The Surveyor mission introduced another level of complexity to the problem of midcourse guidance with the requirement for a lunar soft landing at a pre-selected location. For the selected lunar trajectories the spacecraft will approach the moon at a velocity as great as 2700 meters/second. To achieve a soft landing this velocity would be reduced to zero in two stages. The spacecraft design utilized a solid propellant rocket motor to remove the bulk of this speed before the start of the second stage terminal descent utilizing the liquid propellant vernier engines. The start of the terminal descent is signaled by an Altitude Marking Radar signal at a slant range of 60 miles above the lunar surface at which point the propulsion system operation is enabled after a ground commanded time delay. The remainder of the terminal descent sequence is fully automatic with no means to alter by ground command. Thus the only means to control terminal descent parameters after the midcourse maneuver is the ground commandable time delay that can range only from 0 to 20 seconds. This emphasizes the importance of the midcourse maneuver in ensuring a successful terminal descent.
Unbraked lunar approach speeds can vary by as much as 80 meters per second over a given launch period. Since the solid propellant loading cannot be modified during a launch period the midcourse maneuver can be utilized to make adjustments in the main retro burnout speed so that a fixed solid propellant loading would prove to be appropriate.
JPL’s Surveyor specification required the capability to perform midcourse maneuvers and a controlled soft landing on the lunar surface at a designated location. Hughes had the tasks of performing the analysis to determine the maneuver requirements, developing the software to be used during real-time mission operations, and providing the personnel to conduct the operations in JPL’s SFOF.
Requirements for the propulsion subsystem include attitude stabilization during the main retro firing and terminal descent and led to a system design with three throttleable vernier engines that would also be used for the midcourse maneuver. Thus the requirements for the midcourse maneuver must take into account the predicted vernier propellant requirements for the remainder of the mission.
The first step in planning a midcourse maneuver is to select an execution time. Surveyor specifications state the maneuver could be performed as early as 4 hours and as late as 40 hours after injection. The nominal correction time is 15 hours after injection for direct ascent and 20 hours for parking orbit missions. The nominal times are based upon the desire for Goldstone DSIF visibility for the maneuver and most desirable would be in the middle of the view period so as to allow immediate assessment of the executed maneuver. Two other variables are the orbit determination accuracy on which the maneuver will be based and the maneuver ∆V magnitude. The first argues for allowing sufficient time to for accurate orbit determination and the second for accomplishing the maneuver earlier as the required ∆V for a given correction increases with decreasing distance to the moon.
With the selection of maneuver time the position of the spacecraft is fixed and there remains the capability of adjusting the three velocity components. Two velocity components can be used to adjust the landing location and these define what is called the critical plane. The other velocity component, normal to the critical plane, can adjust the time of flight or other related parameters such as the unbraked lunar impact velocity. These two corrections are almost independent and to the first order can be determined separately.
The first step in the midcourse analysis is to determine the maneuver required in the critical plane to place the landing site at the desired location. This is accomplished by determining the sensitivity of the landing site location to velocity increments in the critical plane and using this data to formulate a covariance matrix can be used to determine the required maneuver. With the critical plane component known the velocity component normal to the plane can be determined by considering the applicable constraints. These include time of flight considering Goldstone visibility limits, the main retro burnout velocity constraints and the vernier propellant requirements for the remainder of the mission. Further the consumption of vernier propellant for the midcourse maneuver lowers the spacecraft weight and results in a greater main retro ∆V and a lesser main retro burnout velocity.
The software to determine midcourse maneuver requirements during mission operations was developed by Mal Meredith, John Ribarich and Len Davids and is described in the document, Midcourse and Terminal Guidance Operations Programs (Hughes SSD 4051R) published in April 1964. This program will determine a velocity increment at a specified time that will result in a landing at the desired landing site and ensure that:
1) Main retro burnout velocity constrained by the successful operation of the radar altimeter and Doppler velocity sensor (RADVS) and the desired descent profile.
2) The remaining fuel must be sufficient for the main retro and terminal descent operations including a reserve for possible dispersions.
3) The unbraked arrival incidence angle must be less than a defined maximum value that will allow the altitude marking radar to operate satisfactorily.
4) The arrival time must satisfy Goldstone DSIF visibility requirements.
The midcourse and terminal guidance program also performs a number of other calculations including required attitude maneuvers to implement the midcourse and terminal maneuvers, determine potential errors in the terminal conditions, vernier propellant requirements for the remainder of the mission, potential alternate landing sites, and investigate alternate mission scenarios including possible hard landings or flybys.
Midcourse Guidance–Flight Experience
For six of the seven Surveyor missions a midcourse maneuver was successfully accomplished. The maneuver for the Surveyor II mission resulted in a vernier engine failure that caused the spacecraft to tumble . However, the analysis for this planned maneuver is included in this discussion.
The performance of the Atlas-Centaur launch vehicle was outstanding. Not only were all Surveyors successfully launched, but the injection accuracy was remarkable. If only the injection errors were to be corrected the average midcourse velocity increment would have been 3.1 meters per second for the seven missions. For the last 3 missions the increment was only 1.1 meters per second.
Actual maneuver planning was more comprehensive taking into landing site accuracy and concern for time of flight, main retro burnout speed and the vernier propellant required for the terminal descent phase.
Landing accuracy upon lunar trajectory injection was expected to be a 99% circle 50 kilometers in radius. For later missions this was reduced to a 30-kilometer radius. As could be expected there would be quite a variation in lunar terrain in an area of this size some of which would not be suitable for a soft landing. It was therefore important in maneuver planning to attempt to reduce this uncertainty. Also as Lunar Orbiter photos became available they were used to select areas of suitable terrain. For example, with the Surveyor I mission the landing site was biased about one degree to the north to avoid several craters. The spacecraft actually landed about 15 kilometers from this aim point. The Surveyor I non-critical velocity component was selected to be 20 meters per second to reduce the main retro burnout velocity by about 100 feet per second. This maneuver component also provided close to the maximum vernier propellant margin and a time of flight that resulted in landing in the middle of the Goldstone view period.
For the seven missions the average non-critical plane maneuver was 9.2 meters/second and the critical plane component was 3.3 m/s, but this includes a maneuver of 11 m/s for Surveyor VII. For this mission the landing site was moved from the equatorial region to 41 degrees south latitude to allow scientific exploration of the lunar highlands region. Maneuver times for six of the seven missions were between 16 and 21 hours after launch in the first Goldstone pass. For Surveyor IV the maneuver was conducted during the second Goldstone pass at 38.5 hours in order to take advantage of the small maneuver requirement and improve landing accuracy.
The Surveyor V mission provided the most challenging midcourse guidance situation. The planned and executed maneuver at 17 hours after launch was 0.5 m/s in the critical plane and 14 m/s in the non-critical direction for a total maneuver of 14.01 m/s. It was determined after this maneuver that helium regulator valve was stuck open and the helium pressurant required for operation of the vernier propulsion system was leaking. In an attempt to reseat the valve three more maneuvers were conducted over the next three hours—none were successful in stopping the leak. A fifth maneuver was performed at 24 hours after launch to reduce main retro burnout velocity and maximize usable propellant in the terminal phase by increasing gas volume in the propellant tanks. A sixth maneuver of 5.3 m/s was performed at 39 hours after launch totadjust spacecraft weight and burnout velocity and further increase gas volume to optimal values, and correct a 267-km residual miss to move the impact point from an area of mountainous terrain to the desired landing site. The terminal descent and landing were successful.