FORTE: The All-Composite Satellite
Dateline: 09/15/97Guide Introduction
This week's feature represents a departure from my previous features. The article tells the story of how composites came to be used on the FORTE satellite and what went into the design of that structure.
I had originally called this article FORTE: The World's First All-Composite Satellite. However, I received some feedback that other all-composite satellites may have been launched before FORTE. FORTE is definitely the first all-composite satellite to be widely publicized as such, and thus is an important milestone.
However, I have not been able to confirm or deny that FORTE was the first all-composite satellite to be launched. If you have specific knowledge of another all-composite satellite, or know for a fact that there is not another, please send me an e-mail at composite.guide@about.com. Please provide as much detail as possible (such as a program name) so I can verify the information.
This feature includes more graphics than usual, so I have divided it into two parts for faster downloading. If you have a fast connection, you may want to load the one part version instead. If you have already read the article and want to look at the indexed links, you may go directly to the Additional Information section.
Launch Day
On August 29, 1997, at 7:00 AM local time, Orbital Sciences Corporation's L-1011 carrier took off from a runway at Vandenburg Air Force Base. As is usual for Vandenburg, the fog was so heavy that observers standing near the runway couldn't even see the plane take off. Launch veterans stayed in the control room, knowing they would get better views from the chase plane once it was airborne.
The L-1011 was carrying a Pegasus XL rocket, or launch vehicle as it is more commonly known in the industry. The Pegasus, in turn, was carrying the FORTE satellite, built by Los Alamos National Laboratory (LANL) and Sandia National Laboratories (SNL).
FORTE had shipped to Vandenburg only a few weeks before launch. During that time, the launch team had taken to calling the Pegasus-FORTE stack the "Plastic Fantastic": Both FORTE and Pegasus were made almost entirely from graphite composite materials, or reinforced plastics.
This was not just the world's first all-composite launch--it was also one of the first deployments of an all-composite satellite. Composites have been used in a number of space structures, but FORTE represents one of the first missions in which the primary bus structure is made from composites.
The Composite Decision
Composites offer a number of advantages over metals for space structures. The primary advantage is that, pound-for-pound, composites are stiffer and stronger than metallic structures. That means the satellite structure can be made lighter, allowing for more experiments.
Despite these advantages, few satellite manufacturers want to risk their mission on an all-composite structure. Satellites are expensive to build and launch, and primary composite structures do not have a long space heritage.
LANL had primary responsibility for the FORTE satellite. SNL provided some instruments, testing facilities, and the ground station, but the program was run out of NIS Division, with Stephen Knox as Program Manager.
In early 1993, the FORTE design called for an aluminum structure. Weights were high, but not quite pushing the margins. But as any aerospace veteran can tell you, satellites have a way of growing. Thus, when a new structural design team joined the program, they pushed for an all-composite structure.
This new design team was led by Tim Thompson, who was acting Group Leader for the Design Engineering Group ESA-8 (now ESA-DE). Thompson had worked on many composite structures program, the most recent of which was the GEM detector for the Superconducting Super Collider (SSC). Although the SSC had been cancelled before anything but subscale prototype hardware had been built, Thompson knew that the GEM structural designs would work on FORTE.
When the composite design was presented at the FORTE Critical Design Review (CDR), many people on the program wanted to stay with the original aluminum structure. Weight wasn't seen as a critical issue at that time, and the composite was seen as much more risky.
Although Thompson and others pushed for the composite structure, the decision was ultimately up to the program office in NIS Division. Steve Wallin of NIS-4 was on the CDR review board and ultimately became Project Engineer for the FORTE program. He was one of the proponents of a composite structure.
The composite structure was risky but, according to Wallin, taking that risk "is the type of thing a National Laboratory should be doing." Despite some other disadvantages, such as the lack of inherent RF shielding (aluminum makes a good natural shield, graphite doesn't), the composite structure was ultimately chosen as the design baseline.
In the end, the composite structure weighed about 95 pounds, whereas the aluminum structure would have weighed about 142 pounds (these weights are the bus structure only--solar arrays and instruments are not included). The 47 pounds were available as extra margin and for additional instruments.
Enter Composite Optics
After the CDR, Thompson's team was ready to start building the structure. Because LANL is a research institution, it contracts out much of its manufacturing work. The design went out for competitive bid and Composite Optics Incorporated (COI) of San Diego was chosen to build the FORTE structure.
COI has been building composite space structures for over
22 years. Their specialty is highly stable structures: for example,
they built the composite platform for the fine guidance system on the Hubble
Space Telescope.
COI takes a unique approach to building composite structures. Most composites are custom molded for each application. A hard tool in the shape of the part is built, uncured composites are layed up on the tool, and the whole thing is cured. Both tooling manufacture and part layup are expensive.
In contrast, COI starts with flat, pre-cured composite
panels. They cut the panels to size and include mortis and tenon
features (tabs and slots) for joints. The pieces are then bonded
together to form the structure. The resulting part is basically a
number of H-beams with flat panels (either monolithic composites or
sandwich panels) for mounting instruments.
COI recognized that this approach could be adapted to inexpensive space structures if they found lower cost materials, improved the adhesive dispensing system, used CAD patterns for cutting the flatstock, and made other production-friendly changes. This revised process was ultimately patented as SNAPSAT: Short Notice Accelerated Production Satellite.
The SNAPSAT process was successful. Lead times and costs were cut to nearly one-third that of traditional composite structures. Compared to aluminum structures, lead times were slightly shorter and costs only slightly higher.
Gary Tremblay, COI Program Manager for FORTE, said that the hardest part about selling composite satellite structures, both before and after FORTE, is "interacting with customers who are not used to working with composite bus structures."
With FORTE, of course, the sale had already been made within LANL. Tremblay also noticed that the LANL design "looked exactly like a SNAPSAT," requiring only minor changes to accomodate the mortis and tenon joints. COI sealed the winning bid by donating surplus flatstock, so LANL did not have to pay for materials. The FORTE program had become a true industrial collaboration, with shared responsibilities and resources among the partners.
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