|
 |
|
Review of Gravity Probe B
5
Concluding Observations
It is the unanimous opinion of the task group that Gravity Probe B is an extraordinarily well designed experiment. The science instrument design is very well conceived to minimize every known category of error. The spacecraft will roll around an axis passing through the gyros, so that all torques generated by the suspension system and spacecraft average out to high accuracy. The instrument package has extensive redundancy to guard against individual failures, and in order to protect against more general failures the redundant units are not all identical. The instrument and spacecraft are designed as far as possible with the flexibility of laboratory equipment, including remote adjustments for every important parameter and the equivalent of a portable oscilloscope able to examine every important waveform.
GP-B is a highly complex experiment, one that must work properly in orbit for many months. A majority of the task group believes that a credible analysis of expected errors has been performed and that the experiment has a high probability of achieving its accuracy goal of 0.5 milliarc sec/yr for both relativistic frame dragging and geodetic precession. Several members of the task group are worried that, despite heroic efforts, unspecified or unknown effects could seriously degrade the measurements made in orbit. This section concludes with some overall observations on the project.
SYSTEMS ENGINEERING
The systems engineering methodology used for the GP-B project appears to be excellent. Imposed and derived requirements for the hardware have been well defined and formally connected with relevant parts of the space vehicle. The requirements are currently maintained with a rigorous procedure, and the task group has not identified any significant outstanding problems. Numerous elements of the space vehicle use flight-proven hardware, or low-risk modifications of it. For most remaining subsystems, the critical design criteria necessary to meet mission requirements have been validated by technology projects or by prototyping. The task group's overall assessment of the spacecraft status is that it presents no significant technical or schedule risks.
The probe and Dewar units incorporate new technologies and require new fabrication methods for dealing with extremely low temperatures and extraordinary magnetic shielding over large volumes. The fabrication, integration, verification, and acceptance testing of the payload will be one of the more challenging space-hardware projects attempted in the U.S. space program. Quantitative assessment of risks associated with this part of the GP-B project is therefore very difficult. Detailed verification of the whole flight system, including hardware, software, and internal and external environments, must be carried out. Moreover, the entire system must be controlled and monitored throughout its final acceptance, transportation, preflight checks, and boost into orbit. The discipline with which the GP-B team addresses these issues will be crucial to the project's overall chances of success.
HELIUM THRUSTERS
Technology for the new helium thrusters has been adequately demonstrated. However, system interactions and precise thrust control still need to be verified in dynamic integrated tests. A suitable test program should include a full range of simulations at the 3 margins of "flying" the vehicle around the drag-free mass. Although the risks might appear to be low from a hardware standpoint, some of the margins available for deviations from expected behavior do not appear to be large, given the very small gap between the drag-free mass and its housing. The "safe mode" that uses magnetic torquers in place of the helium thrusters is a useful backup for attitude, roll, and pointing control, but not for drag-free flight around the proof mass. A careful risk assessment involving uncertainties in all crucial elements of this part of flight operations should receive close attention.
SAFETY MARGINS
Analysis of the safety margins for key system parameters shows that a few of them dominate the overall experimental errors. the available margins for most parameters are at least several times their 3 values. Under such conditions the dominant risks arise from the design-validated configurations associated with limitations. As noted above, the experiment duration (which is determined by performance of the liquid helium storage system) has by far the greatest influence on final accuracy. For durations much less than a year, other parameters dominate because of averaging limitations. If the system operates near its design-goal accuracy of 0.5 milliarc sec/yr should be achieved some 5 to 6 months into the experiment. Successful operation for 13 months under design conditions for the most critical parameters, even allowing multiple standard deviations for the others, could provide a 60 percent margin beyond the design requirements.
Analysis of the experimental errors under expected orbital conditions shows that parameters affecting the spurious gyro drifts have very low sensitivities. For this reason, ground-based testing can directly validate these measurement-error profiles. Conditions of gyro-support damping in the ground tests, which would influence the spurious drifts, appear to have low impact on the overall errors in orbit. Further analysis and updating of these sensitivities and margins should be carried out on a continuing basis as the validation programs proceed.
TECHNOLOGY TRANSFER
The task group is deeply impressed by the very careful thought, design, and testing invested in the cryogenic aspects of GP-B. The launch vehicle itself is an exceptionally interesting example of Dewar design. The container housing some 2000 liters of liquid helium for the 18- to 20-month mission contains many innovations. It is unusually efficient, despite the requirement that most of the helium be stored in the superfluid state. The low-conductivity shock absorbers used to stabilize the Dewar during launch are innovative and effective. The titanium alloy used in the narrow part of the container, near the top, is a new material that could improve many or even most liquid helium containers. the various glues, composite materials, and fasteners used in the design are unknown by much of the community of low-temperature experimentalists.
Designers of other cryogenic apparatus would profit from published reports of the materials used in the apparatus. A detailed discussion of thermal shielding used to optimize the cryogenic efficiency would be especially useful. The community of those who use nonmagnetic structures in other SQUID experiments could save a great deal of time by knowing which materials the GP-B team has found to be free of magnetic contamination. The task group strongly urges that the technology developed during NASA's support of GP-B be reported soon in the open literature for the benefit of the entire scientific community.
Last update 5/5/00 at 4:27 pm
Site managed by Anne Simmons, Space Studies Board
|
|
The National Academies
Current Projects
Publications
Directories
Search
Site Map
Feedback