The Magnetic Pioneer Venus Orbiter—Jack Fisher Revised September 18, 2018

The Pioneer Venus Orbiter incorporated a payload of 12 scientific instruments one of which was a fluxgate magnetometer provided by Chris Russell of UCLA, the principal investigator.  Previous flybys of Venus had revealed that the magnetic field of Venus was much weaker than Earth’s.  The resulting system requirements for the Orbiter magnetic fields are shown in Figure 4-2 in Reference 1.  The most challenging requirement is that the remnant field at the magnetometer (after a 50-gauss demagnetization of the spacecraft) be 0.5 gamma or less.  A Gauss is the usual measure used in magnetics—a gamma is 0.00001 Gauss.  The earth’s surface magnetic field varies from 0.3 to 0.6 Gauss.

These requirements presented some issues that Hughes had not dealt with previously.  At the beginning of the PV program no one at Hughes that I knew had experience in this area.  Very fortuitously at this time we received an application from a TRW engineer, Chris Thorpe, who had performed these tasks for the TRW Pioneer spacecraft and had worked with Chris Russell previously.  We hired him very quickly into the Perry Ackerman lab and assigned him to PV program.  Chris was a delightful Englishman with a wry sense of humor and supported me in systems engineering and Tony Lauletta in science integration throughout the program.

Chris quickly demonstrated his knowledge of spacecraft magnetics and instituted a magnetic control program that included:

  • Formulating and maintaining a magnetic model of the Orbiter that predicted the magnetic field at the magnetometer
  • Limiting the type and amount of magnetic materials used in fabrication.
  • Using a nonmagnetic electroless nickel plate
  • Controlling the location and orientation of magnetically troublesome units on the equipment shelf.
  • Separating the magnetometer from the spacecraft by a deployable boom
  • Provide for magnetic compensation of units that utilize permanent magnets in their operation to reduce their field contribution at the magnetometer

Based on Chris’ calculations the boom length was set at 15 ft 6 in. (4.72 meters).  As I recall Chris’s prediction was 14.5 feet and one foot was added to provide some margin.  Chris maintained the magnetic model throughout the Orbiter development.

The boom, consisting of three hinged segments, is folded together and stowed on the orbiter shelf until deployed shortly after launch.  The boom is secured by two redundant pyrotechnic pinpullers either of which when fired would release the boom for deployment. As the three segments extend, each hinged joint locks in the deployed position.  A spin rate of 6.5 rpm provides the centrifugal force that ensures deployment and positive latching.

System level testing of the magnetometer boom proved to be problematic.  The boom root hinge, when pyrotechnically released, was to deploy with the spacecraft spinning at 6.5 rpm.  However, aerodynamic drag prevented the boom from fully extending in sea level density air.  In order to validate the design it was necessary to encapsulate the spacecraft in a large plastic tent filled with 90% helium that provide a gas mixture with one fifth the density of air.  The deployment test in this environment was successful.

Two system level magnetic tests are required—remanent and stray field determination.  The remanent test is to determine the magnetic field of the quiescent spacecraft and requires a magnetic coil to cancel the earth’s magnetic field.  The NASA Ames facility Magnetic Standards Laboratory and Test Facility in Mountain View, CA was used for this test and of course this required shipping the spacecraft to that facility.  Tests were conducted with the spacecraft in a magnetized and demagnetized state.  The stray field test to determine the magnetic field of the operating spacecraft was conducted in the Hughes high bay in the early morning to provide a magnetically quiet environment.  The test results are presented in Figure 4.2 in from Reference 1.  Chris Thorpe oversaw these tests.

According to Chris Russell:  The most definitive measurements of the magnetic moment of Venus were obtained during the Pioneer Venus Orbiter mission in its first years of operation (1979-1981). Repeated low-altitude (~ 150 km) passes by that spacecraft over the antisolar region, coupled with dayside observations to the same altitude, proved the insignificance of a field of internal origin in near-Venus space. The observed fields for the most part could be explained as solar wind interaction-induced features. The new upper limit on the dipole moment obtained from the Pioneer Venus Orbiter wake measurements placed the Venus intrinsic magnetic field at ~ 10-5 times that of Earth.

At the conclusion of the Pioneer Venus program Chris and I were assigned to the newly started Galileo probe effort.  After I left Galileo I lost track of Chris.  Recently I learned that he passed away in 2000 at the age of 76.   If someone can provide any biographical details for Chris I can add them to this post.

Reference 1.  Pioneer Venus Final Report, Contract No. NAS 2-8300, December 1978, Bernard J. Bienstock.

 

Pioneer Venus Mass Properties—Jack Fisher

The two Pioneer Venus spacecraft were designed to be launched by the Atlas-Centaur for the 1978 Venus opportunity.  Earlier studies had considered the Thor-Delta launch vehicle, but the Atlas-Centaur was judged by NASA to provide superior science performance and potential cost savings due to the greater payload capability.  The starting point for spacecraft design is the allowable mass for the two spacecraft that is determined by the performance of the designated launch vehicle.  Our customer, NASA’s Ames Research Center, adopted a specification weight for us to work to allowing for a cushion or contingency below the Atlas-Centaur launch capability.  The ARC specification values, as a function of time, are shown in Figures 4-1 and 4-2 for the Orbiter and Multiprobe spacecrafts.

The 1978 Venus launch opportunity can be divided into two phases.  The earlier launch opportunity, late May-early June, has a greater flight time to Venus and is a Type II interplanetary trajectory traversing an arc of more than 180O about the sun.  However, this launch requires greater launch vehicle performance and provide less payload capability.  The later launches in August, use a Type I interplanetary trajectory (less than 180O solar arc) and provide more than a 50% greater payload capability.  As neither Hughes nor NASA Ames could support two simultaneous launch campaigns the Orbiter and Multiprobe require using both the early and late launch opportunities for the 1978 Venus opportunity.  The Orbiter weight was significantly less than the Multiprobe and could launched during the earlier opportunity.  An advantage is the 60% lower ∆V required for orbit insertion at Venus.  The August launch opportunity is then available for the 60% heavier Multiprobe.

The final mass properties measurements for the two spacecraft are shown in Table 4-1 from the Reference 1.  Note that the first row in the Table which is labeled “Spacecraft height” should read “Spacecraft weight.”  Both spacecraft are stable spinners based upon the HS-333 design.

Joe Lotta was responsible for the Pioneer Venus mass properties analyses.  This involved collecting inputs from each design area on a monthly basis and calculating the overall mass properties for each spacecraft.  As shown in Figures 4-1 and 4-2 from Reference 1, over the course of the nearly four-year program weight growth was a constant concern.  Considerable effort was devoted to trying to control weight growth and finding weight savings.  At every opportunity trade-offs were considered and lists of weight savings with the cost detailed for each saving would be considered.  Those weight savings characterized by lower dollars per pound would be implemented.  Reference 1 documents 90 pounds of savings implemented for the Orbiter and 105 pounds for the Multiprobe.  NASA ARC was able to provide increases in their specification weights to accommodate our weight growth.  Some must have been due to Atlas Centaur performance improvements and the rest due to reduced contingencies and weight reserves.  In retrospect it all came together and we witnessed two very successful missions.

Reference 1.  Pioneer Venus Final Project Report.  HS507-7970. December 1978, Bernard J. Bienstock.

Pioneer Venus Cost Growth Analysis

The attached letter, dated 27 August, 1979, from Dr. Wheelon to C. A. Syvertson, director of NASA’s Ames Research Center, analyzes the cost growth in Hughes’ Pioneer Venus program.  The contract was cost plus award fee and what was at stake here was the determination by the Ames Performance Evaluation Board of Hughes’ award fee.  This analysis was undertaken at the invitation of Ames to provide the causes for the program cost growth.

The following comments have been added by Steve Dorfman:

Of course I wrote the letter and NASA took mercy.  They appointed Tom Young from NASA HQ to adjudicate and he came up with 2% fee in a Solomon-like decision which gave us a $2M profit instead of a significant loss due to our aggressive cost share proposal which, in hindsight, was way too risky.  We had proposed this aggressive cost share, which had us going to negative fee (that is losing money) to be consistent with NASA HQ desire to have a management experiment in reducing the cost of planetary programs. However Charlie Hall ignored the “management experiment” and ran the program just as he had all Pioneer programs.  In hindsight Charlie was right and NASA HQ was wrong and naive and so were we. Tom Young knew all this and that is why he gave us a modest fee.
In fact the program was an outstanding success in keeping costs low when compared with other NASA Planetary Programs.  For $105M  Hughes achieved a technically ambitious and difficult program.  A bargain.