NASA ARC Arc Jet Complex

NASA. Public domain (NASA / US government work).

The Arc Jet Complex at NASA ARC is the agency’s principal thermal protection system test installation: four named arc jet facilities across seven test bays in two laboratory buildings, drawing on shared direct current power supplies, a steam ejector vacuum system, deionized cooling water, high pressure gas and common controls, and used to simulate the heating rates of Earth or planetary hypersonic entry so that thermal protection systems and materials can be selected, validated and qualified [1][2].

Laboratory data

Parameter	Value
Operator	NASA ARC, Thermophysics Facilities Branch [1][4]
Location	Moffett Field, California, United States [1]
Commissioned	In thermal protection service since the 1960s; modernization study 2022 [1][2]
Type	Arc-heated hypersonic entry simulation for thermal protection systems [1]
Floor area	Not published; seven test bays in buildings N-234 and N-238
Capabilities	IHF, AHF, PTF, TFD, LEAF, mARC II [1][2]
Simulant or terrain	Not applicable; the working fluid is an arc-heated gas stream
Instrumentation	Water-cooled copper calibration plates with Gardon gauges [5]
Ground truth	Calibration plate run of the test article geometry ahead of the material sample
Fidelity limits	Monochromatic radiant source; no single leg spans the envelope [1][5]
Access	Not published as a schedule; modeled mean throughput 483 runs per year
Cited by	Not applicable

The scale of the supporting plant is the reason the complex exists as one installation rather than four. The facilities share two direct current power supplies, a steam ejector vacuum system, a deionized water cooling system, high pressure gas systems and controls, and the magnitude and capacity of that common equipment is the stated reason the complex is unique in aerospace testing. The steam vacuum system is a five-stage steam ejector pumping network about 300 ft long and 100 ft across at its widest, and it is what allows high-altitude flight conditions to be matched with test articles of relatively large size [2]. Cooling water is deionized because the arc heaters require coolant of very low electrical conductivity [2]. Building N-234 holds the Aerodynamic Heating Facility, on a 20 MW power supply and comprising three heaters, AHF Constricted, AHF Huels and TP3, and the Turbulent Flow Duct, also on a 20 MW supply. Building N-238 holds the Interaction Heating Facility on a 60 MW supply, which carries the highest traffic because of the flow rates and enthalpy it provides, and the Panel Test Facility on a 20 MW supply [2].

Capabilities

IHF

Dominic Hart. Public domain (NASA / US government work).

Parameter	Value
Working volume	Conical nozzles 152 mm to over 1 m exit diameter [1]
Test article limits	Flat panels to 610 x 610 mm on the semi-elliptical nozzle
Vacuum	Stagnation pressures 0.01 to over 1 atm, on the steam ejector system [1][2]
Temperature	7 to 47 MJ/kg bulk enthalpy; heat flux 5 to over 6000 kW/m2 [1]
Illumination	Radiant heating only in combination with LEAF [5]
Slope	Not applicable; article inclination set by the sting or panel holder
Gravity offload	Not applicable
Instrumentation	Gardon gauge calibration plate of matching geometry

The Interaction Heating Facility runs a 60 MW Ames-designed constrictor arc heater at 1 to 9 atm arc pressure, and it is the highest-traffic leg of the complex because of the combination of flow rate and enthalpy it delivers [1][2]. Its power supply can deliver 75 MW for 30 minutes or 150 MW for 15 seconds. Interchangeable conical nozzles set the stream size, and a semi-elliptical nozzle converts it into a panel facility.

AHF

Parameter	Value
Working volume	Conical nozzles 76 to 914 mm exit diameter
Test article limits	Samples to 203 mm diameter, or a 660 x 660 mm wedge
Vacuum	Chamber pressures 0.005 to 0.125 atm
Temperature	11 to 33 MJ/kg constricted; 3.5 to 9.5 MJ/kg on the Huels heater
Instrumentation	Five-arm fully programmable insertion system, up to five samples per run

Source: [1].

The Aerodynamic Heating Facility can run either a 20 MW constricted heater at 1 to 9 atm or a Hüls heater at 1 to 40 atm, which is the highest arc pressure in the complex and the reason the leg exists alongside IHF [1]. The five-arm insertion system is what makes it the throughput facility for materials screening: five articles can be exposed in a single run rather than one.

PTF

Parameter	Value
Working volume	Semi-elliptical nozzle on a 20 MW constricted heater at 1 to 9 atm [1]
Test article limits	Panels of about 355 x 355 mm, surface inclination -4 to +8 degrees
Vacuum	Surface pressures 66 to 4700 Pa
Temperature	7 to 35 MJ/kg; cold wall heat flux 6 to 340 kW/m2
Instrumentation	Runs of up to 30 minutes; Gardon gauge calibration plate [1][5]

The Panel Test Facility produces a flat-panel environment rather than a stagnation-point one. The same panel environment can be recreated in AHF or IHF with a semi-elliptical nozzle of the same family, so PTF is a routing choice rather than a unique capability, and its long run duration is what distinguishes it in practice [1][2].

TFD

Parameter	Value
Working volume	Supersonic rectangular duct, 2 by 9 in nozzle, on a 12 MW Huels heater [1][2]
Test article limits	Models 203 mm high by up to 508 mm long [1]
Vacuum	Surface pressures 0.02 to 0.15 atm
Temperature	3 to 9 MJ/kg; cold wall heat flux 20 to 700 kW/m2
Instrumentation	Shear stress 5 to 70 kg/m2 across the model surface

The Turbulent Flow Duct is the shear facility: instead of a free jet onto a stagnation point, it drives turbulent flow along a flat surface, which is the loading that erodes tile gaps, seams and coatings. It is the only leg for which surface shear stress is a published test parameter [1].

LEAF

Parameter	Value
Working volume	Four beams routed into the IHF test box
Test article limits	152 x 152 mm wedge, or 432 x 432 mm panel in the spread configuration
Illumination	Four 50 kW continuous-wave ytterbium fiber lasers, 1070 +/- 4 nm
Temperature	Radiative flux to 375 W/cm2 on a wedge; 551 W/cm2 combined with IHF
Instrumentation	Six Gardon gauges each on radiative and convective calibration plates

Source: [5].

The Laser Enhanced Arc jet Facility adds what an arc jet cannot produce. Uniform radiative flux is delivered to within plus or minus 6 percent across the article [5]. In the wedge configuration the system has provided up to 375 W/cm2 on a 152 by 152 mm article, and combining that with the 160 W/cm2 of convective heating IHF produces on a wedge behind a 9 in nozzle gives a combined 551 W/cm2; the panel configuration spreads the same four beams over a 432 by 432 mm square and is designed for 100 W/cm2 [5]. That separates radiative from convective heating in a way the plasma stream alone does not, and it is a requirement for EM-2 certification of the Orion thermal protection system.

mARC II

Parameter	Value
Working volume	Segmented constricted heater on a Hypertherm MAX200 torch cathode
Test article limits	Small material and instrument samples; scale not published
Vacuum	Chamber held below about 3 kPa through a test
Temperature	Bulk enthalpy by energy balance on the cooling loop; 40 A and 100 A at 0.25 g/s air run
Simulant or terrain	Working gas air, nitrogen or carbon dioxide
Instrumentation	Differential temperature sensor and flowmeter on the 750 L water loop

Source: [3].

mARC II is the small-scale counterpart, built because operational complexity accompanies the high-power facilities and some programs are better suited to a smaller scale, whether for budget reasons or to test many materials or instruments quickly. Strongback, constrictor disks, anode and nozzle are oxygen-free high conductivity copper [3]. The primary distilled water loop cools both arc heater and chamber walls at about 150 L/min, and a dedicated 45 L high-pressure loop cools sensors at up to 5.7 L/min during insertion into the jet; gas supply runs through Sage Prime SIP-030-DC24 thermal mass flowmeters.

Instrumentation

Heat flux in a panel or wedge test is calibrated by running a water-cooled copper calibration plate of the same geometry as the test article, instrumented with Gardon gauges at fixed locations, ahead of the material samples; in LEAF the radiative and convective calibration plates carry six Gardon gauges each at matching positions, and the convective plate also takes surface pressure [5]. The complex publishes a measurements handbook as controlled branch-level documentation [4].

mARC II is instrumented for enthalpy as well as heat flux. A differential temperature sensor and flowmeter on the primary 750 L distilled water loop provides an estimate of the bulk enthalpy delivered to the plasma by energy balance, which is a facility-level measurement the larger legs obtain differently [3].

What it does not reproduce

Coverage in one place. No single leg spans the envelope. The four facilities differ in heater, pressure range and enthalpy range, and a panel environment that PTF produces with a semi-elliptical nozzle can also be recreated in AHF or IHF with the same nozzle family, so a campaign is routed to whichever bay reaches its conditions rather than to one large facility [1][2].

The radiation spectrum, in the laser facilities. Shock-layer radiation during entry spans the ultraviolet into the infrared, and there is no light source that can illuminate a wedge or panel at high heat flux across that range. LEAF’s fiber lasers are monochromatic at 1070 nm, which was accepted as the price of reaching the power levels required; a broader-spectrum ground facility awaits practical multi-kilowatt visible and ultraviolet lasers [5].

Chamber pressure at the small scale. mARC II’s as-built vacuum configuration holds the chamber below approximately 3 kPa through a test, which the operators note is higher than optimal, producing a slightly over-expanded jet; modifications to increase system conductance were under consideration [3].

Run duration and throughput. A PTF run can last up to 30 minutes, and exposures across the complex have run from a few minutes to over an hour [1]. Throughput is the binding constraint at the complex level: a Monte Carlo model of the legacy complex, on notional inputs of 250 working days, three test runs per day for complex testing or five for simple, and per-facility availability of 20 to 40 percent, put the mean at 483 test runs a year [2].

Campaigns run there

Thermal protection system development, 1960s to present. The complex has supported Apollo, Space Shuttle, Viking, Pioneer-Venus, Galileo, Mars Pathfinder, Stardust, X-33, X-34, SHARP-B1 and B2, X-37 and the two Mars Exploration Rovers, in the roles of material selection, validation and qualification [1].

Orion thermal protection, LEAF, 2018 onward. LEAF testing of Orion thermal protection, including the tiled Avcoat heatshield and its compression pads, is a requirement for EM-2 certification of flight readiness, and it is run as a combined radiant and convective exposure in the IHF test box rather than in either mode alone [5].

mARC II characterization, published 2020 [3]. Initial characterization of the upgraded second-generation miniature arc jet reported measured arc current and voltage, column and chamber pressures, mass flow rates and initial column diagnostics, at settings including 40 A and 100 A at 0.25 g/s of air, with the facility undergoing integrated systems testing at the time [3].