Stiffening The Spines Of Large Space Structures

Much larger orbital structures, like the international space station, are now onthe drawing boards. As these larger space habitats are constructed, a newproblem will become increasingly important: stability.

If two sections of a space structure begin moving to the beats of differentdrummers, the structure could easily be severely damaged.

One solution to the problem is to design in added rigidity, but that adds weight,which can significantly increase cost.

An alternate, and potentially more cost-effective, approach is to build in adynamic control system that actively controls errant oscillations. E. HarrisonTeague, a Stanford doctoral student in aeronautics and astronautics, hasdeveloped such a system, which employs signals from the Global PositioningSystem, the Department of Defense's satellite navigation system.

Using inexpensive GPS receivers attached to different portions of a spacestructure, the method can detect wayward motions with centimeter-levelprecision and then automatically fire thrusters to compensate for them. Thesystem also can be used to change the orientation of a flexible structure withsuch accuracy that it moves almost as if it were rigid.

Teague developed the GPS system as part of his doctoral thesis, which hecompleted in June. An article describing the work will appear in the summerissue of the Navigation Journal, which is still in press. His thesis advisers wereaeronautics and astronautics professors Jonathan How and BradfordParkinson.

Previous methods that provided centimeter-level measurements of position andattitude using GPS relied on the object in question being a rigid body. Teagueadapted these techniques to provide the same level of precision with a flexiblestructure.

Next he had to identify the shapes and frequencies of the various modes ofoscillation that could develop in such a structure. Although researchers hadsome general ideas of what such modes should be, they were not known withenough precision for effective control.

Finally, the student came up with procedures that could control such motionswhile automatically accommodating processes such as docking and undockingof capsules and the addition and consumption of consumables, processes thatcan cause major changes in the dynamic properties of space structures.

To try his system, Teague built an ungainly-looking test bed that allows him tosimulate the movement of a light structure in weightlessness. The test bedconsists of three 100-pound blocks of aluminum connected by two 15-footlong rods. Each block has two arms that are about five feet long extendingperpendicular to the rods. On the end of each arm is a small GPS receiver anda cluster of four compressed-air thrusters.

The entire assembly is hung by extremely strong thread. The top of eachaluminum block is milled out in a cone shape so that the thread can be attachedat its center of mass and the block can rock without contacting the thread.Threads from each of the three blocks extend upward where they are attachedto a 30-foot length of heavy steel pipe. Straps from each end of the pipe aretied onto a thrust bearing that allows the entire assembly to rotate. The bearing,in turn, is supported by a heavy, overhead crane.

Because the rods connecting the three blocks are extremely flexible, the testbedcan simulate a wide variety of motions. Each of the blocks can be set rockingvertically and horizontally. The rods transmit some of this motion to the otherblocks. So waves of motion can travel from one end of the assembly to theother. When the blocks are set rocking in different directions, the waves cancombine and cancel in unexpected ways.

Teague's test area is indoors, so he had to use pseudo-satellites, antennas thatproduce imitation GPS satellite signals. The receivers use these signals to keeptrack of their precise position. All the positions are sent to a desktop computerthat contains a model of the assembly. The computer identifies the oscillationmodes when they are still very small and calculates the timing and duration ofthe air blasts necessary to dampen them out.

The most dramatic demonstration of the system's capabilities comes whenTeague vigorously sets the assembly rocking and rolling. When he activates thecontrol system, the thrusters begin hissing, the motions get smaller and smallerand the assembly returns to rest within 5 seconds.

A less dramatic, but more realistic test cares when Teague turns the controlsystem on and then manually moves one of the arms. Thrusters begin hissingimmediately and the arm rapidly returns to its proper position when he lets go.

Teague also can use the system to rotate the flimsy assembly as if it were rigid.When he enters the proper command, the thrusters begin to hiss and theassembly begins to turn like a rigid body, with very little shuddering or deviationfrom its base configuration.

The research was funded by the National Aeronautics and SpaceAdministration.