Feedthrough Challenges on the A-3 Test Stand
With potentially thousands of control and sensor data channels entering and exiting in engine test-stand vacuum chamber applications, it is essential for hermetically sealed conductor feedthroughs to solve cross-talk and leakage challenges as well as provide flexibility to meet future custom data needs. NASA worked with Douglas Electrical Components to provide such solutions for its feedthrough challenges on the A-3 test stand at the Stenner Space Center.
One of the bright spots in the US’s post-recession economy is its technology innovation and leadership, including space vehicles, launches, and exploration. From civilian communication satellites to deep-space exploration to military applications, the private and public space industry infrastructure forms a vital part of this country’s ongoing economic engine. NASA continues to play a large role in this industry, operationally as well as in the development and testing of launch vehicles, propulsion systems, and payloads – for both commercial and government projects. Current NASA plans include the Multi-Purpose Crew Vehicle, the Space Launch System (SLS), and continued participation in the International Space Station, just to name a few.
One of the key areas of US engineering expertise is in the design and development of propulsion systems. Maintaining technology leadership while meeting budget constraints is an ongoing challenge and has led to some innovative re-engineering of legacy propulsion systems, including the J-2 engine first used in the Apollo moon missions. The newest J-2 design was engineered to be more efficient and simpler to build than its Apollo predecessors. The engine called for the removal of beryllium, a redesign of all the electronics, and the use of the latest joining techniques and materials (Figure 1). The latest variant of this engine is the probable choice for the upper stage of the SLS, which requires true mission-duration testing at atmosphere (vacuum) of these large propulsion systems.
While upfit and expansion of several existing test facilities was considered, it was discovered that the ability to accommodate a 300,000-pound class engine with the steam generation capacity to create vacuum for a full 10-minute mission simulation presented many retrofit hurdles. After several feasibility studies, the development of a new greenfield test platform – the A-3 test stand located at Stennis Space Center – was determined to be the most cost-effective solution.
The A-3 Test Stand
The A-3 test stand (see Figure 2) was designed to simulate a 100,000-foot altitude at 0.16 psi pressure. The A-3 stand enables true vacuum testing of up to 300,000-pound class engines for a typical 550-second mission firing duration. With a vacuum base on a chemical steam generation process, which provides 650 seconds of steam, the A-3 test stand is the only test cell in North America that combines the ability to accommodate 300,000-pound class engines with vacuum capacity for a full mission profile from ignition at altitude. Allowing time for ramp-up and ramp-down, a 10-minute engine test can be conducted at altitude for simulation of full-mission duration.
“For the test stand customer, data is our product,” says Phillip W. Hebert, A-3 lead electrical design engineer. “Therefore our biggest challenge during the design phase was ensuring clean data signals.” That’s a tall order, particularly considering the number of channels and the number of threats to data integrity. Because it required elimination of cross-talk and leakage across hundreds of sensors, controls, signal types, cables, and connectors entering and exiting the test environment, as well as facing extreme vibration, long cable runs, vacuum penetrations, and signal mix, the project presented a formidable engineering challenge.
The final feedthrough solution was based on using a number of port plates with pretested penetrations on each plate (Figure 3). Designed and constructed by Douglas Electrical Components, each of the 12 port plates had pre-wired feedthroughs that permitted pretesting of all connections with verified sealing of multiple penetrations. “Early in the design phase we made the decision to isolate each data channel to a single penetration. While this does increase the number of feedthrough penetrations, it greatly reduces the risk of cross-talk,” says Hebert. Total penetrations number close to 1,500 to accommodate current and anticipated requirements. Spare cables and penetrations are in place for additional data and control channels as required and to accommodate custom requirements of future projects.
Spare plugged penetrations allow for unanticipated variations in cabling requirements. “Troubleshooting a vacuum leak in a system with multiple feedthroughs and thousands of wire connections can be frustrating, time-consuming, and expensive,” says Ed Douglas, president, Douglas Electrical Components. “That’s where certain solutions such as port plates can really cut the risk of leakage and provide great flexibility to handle future requirements.”
On the A-3 test stand, the low voltage signals (30mV and lower) require continuous feedthrough to prevent signal attenuation, while the 28VDC control signals are routed using connectors. To prevent cross-talk, each channel is routed through an individual penetration rather than via multi-pin connectors. With atmospheric cable runs of several hundred feet, signal attenuation was a factor influencing the selection and testing of the controlled impedance conductors (see Figure 4).
Engine test programs can run for a year or longer, as in the case of the J-2X, to cycle through a complete evaluation of all engine components. The powerpack test alone demands the total range of test-stand capabilities. “By varying the pressures, temperatures, and flow rates, the powerpack test series will evaluate the full range of operating conditions of the engine components,” says Tom Byrd, J-2X engine lead in the SLS Liquid Engines Office at NASA’s Marshall Space Flight Center in Huntsville, Ala. “This will enable us to verify the components’ design and validate our analytical models against performance data, as well as ensure structural stability and verify the combustion stability of the gas generator.”
While the engineering challenges of designing, building, testing, and operating a facility as complex and critical as the A-3 test stand is not an everyday occurrence, the advice from Jody Woods, the Chief Engineer for A-3, can be applied to any level of project complexity. “Quantify the requirements, and then design the solution based on consensus standards. Compliance with consensus standards across all the challenges of pressure vessels, vacuum, steam, electrical, plumbing, and feedthroughs followed by conformance testing will lead to a robust solution that will provide reliable service for years to come.”
“It’s exciting to play a role in the development of the new A-3 test stand. It is this type of project that positions the American economy for continued growth and spawns new technological developments that NASA continues to contribute to industry,” says Douglas.
With the expansion and commercialization of the space industry, one of the greatest challenges faced by feedthroughs on the A-3 test stand is the ability to cater to custom requirements. Provisioning for the future takes care and planning, but building in flexibility is always less costly than retrofitting at a later date. The port plate solutions as designed for the A-3 project will provide years of service for all kinds of conductors and signal requirements.
This article was contributed by Douglas Electrical Components.