IAE (Inflatable Antenna Experiment) on Shuttle Flight STS-77
IAE was a pioneering NASA technology experiment in the field of inflatable antenna structures within NASA's IN-STEP (In-Space Technology Experiment Program) project to develop technologies that are critical for future national space programs requiring validation in the space environment. The IN-STEP IAE project was managed by NASA/JPL. Large deployable antennas in space open up desirable enabling technology applications in support of demanding future mission concepts that include mobile communications, Earth observation radiometry, active microwave sensing, and VLBI (Very Long Baseline Interferometry) techniques.
Inflatable structures have the advantages of low mass, low stored volume, and of low cost. They also have the potential to deploy much more reliably than the conventional mechanical systems used for deploying rigid structures. However, the deployment of high-performance antennas needs some supervision in form of antenna surface shape control to satisfy the requirements of surface accuracy.
The objective of the Spartan/IAE mission was to validate and characterize the mechanical function and performance of a 14 m diameter inflatable deployable antenna reflector structure in an operational orbit. Spartan (Shuttle Pointed Autonomous Research Tool for Astronomy) is a NASA freeflyer structure which can support freeflyer payloads - to be deployed and recovered from the Space Shuttle (Figure 1). The Spartan-207 carrier with payload had a total mass of 856 kg. The total payload mass of the STS-77 flight was 12,233 kg. 1) 2) 3) 4) 5) 6) 7)
Figure 1: Artist's view of the basic IAE architecture and the orbital experiment sequence (image credit: NASA)
Launch: IAE was carried aboard the Space Shuttle STS-77 (Endeavour, May 19-29, 1996), deployed on the freeflyer Spartan 207 platform, then inflated in orbit.
Other Shuttle payloads on this flight were: the pressurized research module SPACEHAB-4 and a suite of four technology demonstration experiments known as TEAMS (Technology Experiments for Advancing Missions in Space).
Orbit of the STS-77 Orbiter: Near circular orbit of altitude 278 km x 287 km, inclination = 39.1º, period = 90.1 minutes. The Spartan-207 orbit was slightly lower than that of the Orbiter.
Figure 2: Artist's rendition of the deployed IAE experiment in orbit (image credit: NASA, L'Garde Inc.)
The Spartan-207 freeflyer was deployed on May 20, 1996 (day 2 of mission) by the shuttle crew using the orbiter RMS (Remote Manipulator System) arm (also referred to as Canadarm). With all spacecraft functions operating correctly, the antenna was inflated at the proper time 1 1/2 orbits later, just after orbital sunrise. The IAE experienced unexpected dynamics during the initial ejection and inflation of the inflatable structure, but the correct final shape was attained. After full inflation of the antenna structure, the spacecraft began rotating unexpectedly. During the inflation process and throughout the science orbit, Endeavour's crew took extensive video, photographs, and motion pictures of their activities. 8) 9)
After reaching the desired orbit for science operations, the inflated antenna was jettisoned from the Spartan 207 spacecraft. On the following day, the rest of the Spartan 207/IAE payload was successfully retrieved and stowed for return to Earth. IAE was considered a prime example of a low-cost technology validation experiment.
The IAE experiment development was based on the L'Garde concept for a large, offset, parabolic reflector antenna and their associated technology data base at the time of initiation of the experiment in the early 1990's. NASA/JPL and L'Garde Inc. of Tustin, CA, worked together in the IN-STEP (In-Space Technology Experiments Program) project to conduct the IAE experiment.
The specific experiment objectives were to demonstrate a level of technology maturity for large, inflatable deployable, space antenna structures:
• Verify that large inflatable space structures can be built at low cost
• Show that large inflatable space structures have high mechanical packaging efficiency
• Demonstrate that this new class of space structure has high deployment reliability
• Verify that large membrane reflectors can be manufactured with surface precision of a few millimeters rms
• Measure the reflector surface precision on orbit.
The packaging techniques used for the IAE were based on a combination of recent experience at L'Garde with a large number of small inflatable structures and new techniques tailored for the unique geometry of the experiment hardware.
The main decision on packaging techniques was to separate the three strut structures from the cavity that contained the reflector structure. This approach was used to eliminate physical interaction between the struts and torus/reflector structure during deployment, so that separate control techniques could be used for each element of the support structure.
The folding technique used for the reflector structure, which consists of the lenticular structure and the torus, was driven by the folding pattern of the torus structure. In order to interface the ejection plate with the most durable inflatable structure, the torus, the three segments of the torus, located between the joints with the struts, were folded into short/wide-pedestal type packages and placed parallel to each other on the ejection plate. This packaging technique resulted in a near rectangular configuration that fit into the rectangular canister structure (Figure 3).
The complete IAE experiment was packaged into a canister of size: 2.04 m x 1.08 m x 0.5 m. The launch mass of IAE antenna was 60 kg, providing a drastic decrease in weight over traditional mechanical systems. The deployed antenna structure had roughly the size of a tennis court.
Figure 3: Schematic view of the packaging configuration (image credit: NASA, L'Garde)
Payload on STS-77: The payload consisted of two main pieces of hardware: the Spartan free flyer and IAE experiment and the SFSS (Spartan Flight Support Structure), which held the Spartan spacecraft in the Space Shuttle Cargo Bay.
The SFSS had three assemblies: the main, across-the-bay support structure known as the MPESS (Mission Peculiar Equipment Support Structure), the REM (Release Engage Mechanism), and the interface hardware between the MPESS and REM known as the MPE (Mission Peculiar Equipment). The SFSS was attached to the payload bay through keel and sill trunnion fittings. The REM interface allowed the Spartan-207/IAE to be attached to and detached from the SFSS.
The reflector assembly formed a 14 m off-axis parabolic aperture with a f/d of 0.5. The surface accuracy goal was I.0 mm rms as compared to a best fit parabola. The reflector film, 1/4 mil aluminized mylar, was stressed to about 82.5 bar (1200 psi) by the inflation pressure of 20,600 bar (3 x 104 psi). This stress level was sufficiently high to assure a good reflective surface for the accuracy measurement system.
The canopy was also constructed with 62 gores of 1/4 mil mylar but was left transparent. The torus/strut structures were 61 cm and 46 cm in diameter, respectively, and were made with 12 mil thick neoprene coated Kevlar; they locate the reflector assembly at the effective center of curvature of the reflector parabola as required for operation of the Surface Accuracy Measurement Subsystem. The torus also provides the rim support for the reflector assembly without which the reflector assembly, when inflated, will take a spherical shape.
Figure 4: Some details of the IAE elements (image credit: NASA, L'Garde)
Deployment control: The basic deployment scheme for IAE was based on ejecting the stowed reflector structure, as a package, away from its launch container prior to initiation of its own deployment. In this way, the inertially-loaded reflector structure would essentially “pull” the struts out of their launch containers in a near uniform manner. When the struts were stretched to about 80% of their deployed length, a gas flow path would be developed and at that time inflation gas was to be introduced to all three struts at their intersection with the canister structure.
The plan was that when the struts were nearly completely deployed, inflation gas would be introduced to the already partially deployed reflector structure in order to complete its deployment.
On orbit, it turned out that the magnitude of residual gas and the strain energy in the stowed structure was significantly more than anticipated. As a consequence, the planned deployment sequence did not materialize. Instead, the reflector structure deployed prematurely so that its planned ejection away from the canister did not take place. By the time the struts migrated from their pods as a consequence of residual gas and material strain energy, the reflector structure was over half deployed. However, due to the robust nature of this type of space structure, the torus and two of the struts completed deployment at about the same time and complete deployment of the third strut followed within a minute or two. The actual deployment sequence is illustrated in Ref. 1).
Figure 5: Illustration of the planned antenna deployment sequence (image credit: NASA, L'Garde)
The results of the IAE deployment strongly suggest that to achieve precise control of large inflatable structural elements, directional deployment control devices are needed.
When the assembly of the reflector was complete, the determination of its precision was done by placing the reflector structure on a ring-shaped fixture the same diameter as the structure. It was then stretched and attached to the fixture along its outer edge. Then the same pressure differential, planned for orbit, was applied to the edge supported membrane. The surface precision was measured with photogrammetric techniques. The surface precision achieved was on the order of 2 mm RMS, for the portion of the reflector about a meter away from its edge. - Naturally, future applications of this technology can expect much higher reflector precision than demonstrated by the IAE.
Overall, the experiment was considered successful - as flown on the recoverable Spartan spacecraft. All the experiment objectives were met with the exception of the inflation of the lenticular reflector structure. The basic antenna support structure achieved full and complete deployment and maintained its design configuration for the duration of one orbit. The results of the IAE were used specifically to establish the technology data base and were the basis of a technology road map for the continued development of this type of space structure.
In retrospect, the IAE experiment laid the groundwork for future inflatable structures in space, such as telescopes and satellite antennas, since a number of totally new technologies were demonstrated and evaluated.
Figure 6: Photo from Shuttle of the Spartan-207/IAE freeflyer with the deployed antenna system (image credit: NASA)
Spartan is a Shuttle-deployed retrievable freeflyer platform (an autonomous subsatellite, three-axis stabilized) providing short-term LEO observation opportunities for instrumentation in various disciplines and fields of applications, such as astronomy, remote sensing, and technology demonstrations. The Spartan program is managed by NASA/GSFC. The multi-purpose Spartan carrier provides an attitude control system, a data handling and power system, and a thermal control system.
A typical Spartan configuration consists of two main pieces of hardware, a Spartan Flight Support Structure (SFSS) and a Spartan freeflyer with the experiment. Part of the SFSS is the Release Engage Mechanism (REM) which allows the freeflyer to be removed from and returned to its berthing position in the Orbiter cargo bay. The freeflyer was deployed and retrieved by the Remote Manipulator System (RMS) operated by an astronaut. 10)
Spartan-207 represented the eighth flight of NASA's Spartan project flown on Shuttle representing one of the primary payloads on mission STS-77 and the most ambitious Spartan mission to date.
Figure 7: Photo of the Spartan-207 subsatellite capture with RMS (image credit: NASA)
Figure 8: The IAE payload on top of the Spartan carrier during integration tests at NASA (image credit: NASA)
Table 1: Some characteristics of the SPARTAN platform series 200
2) R. E. Freeland, G. D. Bilyeu, G. R. Veal, M. M. Mikulas, “Inflatable Deployable Space Structures Technology Summary,” 49th IAF Congress, Melbourne, Australia, Sept. 28 to Oct. 2, 1998, URL: http://www.lgarde.com/papers/iaf-98-I501.pdf
3) R. E. Freeland, G. D. Bilyeu, G. R. Veal, M. D. Steiner, D. E. Carson, “Large Inflatable Deployable Antenna Flight Experiment Results,” 48th IAF (International Astronautical Congress), Turin, Italy, October 6-10, 1997, IAF-97-13.01, also in Acta Astronautica, Vol. 41, No 4, Aug. 1997, pp. 267-277, URL: http://www.lgarde.com/papers/iaf-97-1301.pdf
4) R. E. Freeland, G. R. Veal, “Significance of the Inflatable Antenna Experiment Technology,” AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, AIAA/ASME/AHS Adaptive Structures Forum, April 20-23, 1998, Long Beach, CA, USA, paper: AIAA-98-2104, URL: http://www.lgarde.com/papers/aiaa-98-2104.pdf
5) R. E. Freeland, G. Bilyeu, “IN-STEP Inflatable Antenna Experiment,” Proceedings of 43rd IAF Congress, Washington D. C., USA, Aug. 28-Sept. 5, 1992, paper: IAF-92-0301, URL: http://www.lgarde.com/papers/instep.pdf
6) R. E. Freeland, G. Bilyeu, G. R. Veal, “Development of flight hardware for a large Inflatable Antenna Experiment,” Proceedings of the 46th IAF (International Astronautical Congress) Congress, Oslo, Norway, Oct. 2-6, 1995, URL: http://www.lgarde.com/papers/iaf-95-1501.pdf
7) G. Veal, R. Freeland, “IN-STEP Inflatable Antenna Description,” AIAA Space Programs and Technologies Conference, Huntsville, AL, Sept 26-28, 1995, paper: AIAA-1995-3739
9) R. E. Freeland, G. D. Bilyeu, G. R. Veal, M. D. Steiner, D. E. Carson, “Large inflatable deployable antenna flight experiment results,” Acta Astronautica, Vol. 41, Issues 4-10, August-November 1997, pp. 267-277
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.