Ongoing H3 arc heater developmental testing hits major milestone, sets record

Blackanthem Military News, ARNOLD AIR FORCE BASE, Tenn., April 18, 2006 13:01

Arnold Engineering Development Center reached a significant milestone recently while conducting a series of independent tests on the H3 arc heater and a multiple model positioning system to expand the facility operating envelope and validate a major arc heater system upgrade.

At the end of a series of recent runs of the H3 arc heater under a technology development program, Aerospace Testing Alliance's (ATA) reentry materials testing group and supporting team members succeeded in getting the large arc heater facility up to 167 atmospheres, setting a new world record and keeping with their validation goals.

"This is further proof of what this system will be able to accomplish for our customers," said Mark Smith, the project engineer in ATA's reentry materials testing group. "Validation like this will ultimately lead to our ability to use H3 to conduct increasingly accurate aerothermal testing in support of mission critical work, including operational testing for the Naval Surface Warfare Center's Reentry Systems Applications Program and the Air Force's Reentry Vehicle Applications Program."

The H3 arc heater, which has a larger flow field than the H1 heater - is used to subject material samples of missile nose tips, reentry vehicles and structures to the severe environment encountered during hypersonic flight or reentry from high altitudes and space. The objective of this testing is to cause ablation, which is a burning away of the protective covering of a missile nose tip or the leading edge of a spacecraft, hypersonic aircraft or reentry vehicle.

Ablation is a normal, expected re-entry phenomenon, and the use of an ablative heat shield is often the only way to protect a vehicle during reentry. Because energy is absorbed as the heat shield surface is vaporized, energy, which would have been absorbed by the craft is absorbed in the ablation process. The objective of operational arc heater testing is to find materials that will burn evenly and slowly enough for a missile to successfully intercept a target or to allow a space vehicle or other structures to survive hypersonic flight or atmospheric re-entry.

"The ongoing work on the H3 heater could be described as high-pressure arc heater developmental testing," explained Dr. Joseph Sheeley, ATA facilities technology engineer. "The idea is to expand the operating envelope of the H3 arc heater from its current capacity to simulate higher surface pressures and heating rates. It's a continuing process because as materials get better and technologies get better, missiles and spacecraft are going to come in on a steeper reentry trajectory - coming in faster and hotter - so, you're going to need increased (testing and flight simulation) capabilities."

AEDC's engineers reproduce ablative conditions in a two-stage process. The arc heaters use a high voltage, electric arc discharge to heat air to temperatures up to 13,000 degrees Rankine (thermodynamic temperature scale). High-pressure test flows are achieved by confining the electrical arc discharge in a water-cooled channel capable of withstanding high chamber pressures.

Prior to the current round of developmental tests, it was possible to get the arc heater chamber pressures up to 120 atmospheres reliably. The success of the current phase of H3 improvements will validate further military contract (MILCON) investments necessary to meet higher demands placed on the High Temperature Laboratory's power supply, provide the needed air pressure overhead and flow rate, and the infrastructure required for cooling H-3's components.

For development, a novel configuration is used where the H3 facility's core is comprised of a 10-module heater, which is 80 inches long with a smaller nozzle. This configuration allows the heater to be run at higher pressures than is currently possible with the standard configuration by keeping conditions within existing utility limits.

Each upstream module is made up of 18 copper segments, while downstream modules are made up of 24 thinner segments. Modules can be added or removed to vary the heater's operating characteristics, and a large part of the development is the determination of a configuration in which the heater will operate reliably.

The large arc heater is operating in a modified configuration, which is dictated by the current limits of power and cooling capabilities, but should allow testing to be conducted up to 150 atmospheres reliably, according to Smith, who is also in charge of the customer requirements for model positioning checkouts.

"We're now in the mode of trying to ultimately expand that demonstrated pressure range up to the 200-250 atmosphere range, which simulates conditions more realistic to flight," he explained. "These are all demonstration runs to validate that H3 can handle the expanded test envelope."

The team conducting the testing on H3 has faced an array of challenges. Operating a higher pressure arc heater results in more accurate and useful test data, but the fielding of such a capability is difficult.

"One big issue we face is arcing to the walls," Sheeley said. "The arc heater becomes more unstable as you push up the pressure because the resistance of the air column inside the heater goes up, along with heater voltage and power. As those go up, the internal environment becomes more unstable - the arc starts wandering inside the heater. It can warm the wall which in turn reduces the electrical resistance between the gas and the wall, initiating a destructive cycle that can eventually result in the arc entering the wall causing severe damage to the heater. We try to keep the arc in the center of the heater."

Maintaining higher pressures and temperatures inside the arc heater also presents the dual challenge of ensuring segment walls are thick and strong enough to withstand these extreme forces while simultaneously being thin enough to allow rapid cooling to prevent overheating and destructive arcing.

Earlier arc heater developmental testing initially employed a simple model positioning system to evaluate the test environment. H3 uses a new multiple model positioning system.

"I think we had four or five test runs with the single model positioning system," Smith said. "Basically, H3 had been secured for a year and a half while the investment group's been installing and checking out this new model positioning system. They're about 90 percent done with the check outs."

The next challenge involves designing the nozzle which directs the high temperature and highly pressurized air onto the test article attached to the model positioning system. The nozzle has to withstand the extremes in temperature and pressure long enough to collect data, sufficient in quantity and quality, to meet the objectives for a test or series of runs.

In January, one test run was successfully conducted on three Navy test articles using the current arc heater configuration with the new model positioning system.

Jeff Stewart, a project engineer for the arc technology program, put the large arc heater's considerable capabilities into perspective - in terms anyone might appreciate.

"I called the Tullahoma Utilities Board recently and found that at any point in time their average power consumption is 35 megawatts as compared to the 70 megawatts required by this arc heater," he said. "Tullahoma's average water usage is about 1,700 gallons a minute; we're running about 3,100 gallons a minute - just through this arc heater alone.

By Philip Lorenz III
Arnold Engineering Development Center Public Affairs