External heat input in a TVAC test can be applied with a solar simulator, an infrared source or a conductive heater. These methods do not reproduce the same boundary condition. A solar simulator addresses irradiance, spectrum and direction; infrared systems can reproduce a calibrated absorbed heat flux without solar spectral fidelity; conductive heaters impose power or interface temperature. The verification objective and thermal model determine which approximation is acceptable.
Why external heat flux must be simulated
At Earth's distance, total solar irradiance is about 1361 W/m², while absorbed power at a spacecraft surface depends on incidence angle, spectral absorptance, attitude, albedo and planetary infrared. A cold shroud provides the radiative sink; separate sources or controlled interfaces provide the heat inputs required for the selected orbital case.
The guiding question is what the test must prove. Verifying a thermal design and correlating a model requires the correct absorbed power on each surface. Verifying wavelength-dependent behaviour – coatings, optics, solar cells – requires the correct spectrum as well.
Solar simulators: spectral fidelity at a price
A solar simulator uses a lamp source and optical system to approximate a specified solar reference spectrum, irradiance, beam geometry and uniformity. It is appropriate when wavelength-dependent response, illumination direction, shadowing or optical performance is part of the requirement. The required class and calibration method must be stated in the test specification.
What that fidelity costs
- High capital and operating cost: lamps, power supplies, optics and their maintenance dominate the budget.
- Limited beam size and uniformity: illuminating large articles uniformly requires large, complex optics.
- Chamber integration effort: a suitable window or in-chamber optics, stray-light management and alignment.
- One-sided illumination: the beam lights one aspect of the article, as the sun does – other faces still need separate provisions.
IR lamp arrays and infrared heaters
Infrared lamp arrays and radiant heater panels apply a controllable radiative load without reproducing the solar spectrum. The thermal model converts the required absorbed orbital load into source power or incident flux for each zone. This can be a valid verification method when spectral response is not part of the requirement and surface properties and view factors are known with adequate uncertainty.
- Far lower cost and complexity than solar simulation, and scalable to large or irregular geometries.
- Zoned arrays allow different fluxes on different faces, following the orbit-driven thermal cases.
- Lamp cages partially block the view to the cold shroud – this blockage must be captured in the thermal model.
- Flux conversion depends on assumed surface properties, so uncertainties in α and ε feed directly into the test.
Heater plates and skin heaters
Conductive methods skip radiation entirely: thermal plates control the interface temperature under mounting feet, while film or skin heaters bonded to surfaces – or to enclosing dummy panels – inject defined local power. They are precise, inexpensive and easy to control, but they impose temperatures or powers rather than a radiative environment, so view-factor-driven gradients are not reproduced.
- Well suited to unit-level tests where the environment is specified as an interface temperature.
- Skin heaters replicate absorbed flux locally when attachment and instrumentation are acceptable on the article.
- Often combined with a cold shroud so radiative losses stay realistic while inputs are conductive.
- Typical limitation: heater mass and wiring add their own thermal paths, which the model must include.
When spectral fidelity actually matters
Spectral fidelity is required when the verification result depends materially on wavelength. If only the net absorbed heat load and temperature response are required, a calibrated infrared or conductive method may be sufficient. That decision belongs in the test rationale and uncertainty analysis.
Cases that justify a solar simulator
- Optical coatings and thermal-control surfaces whose α/ε ratio drives the design.
- Solar arrays, where part of the absorbed energy converts to electrical power and the spectrum affects the balance.
- Optical instruments where stray light, glint or aperture heating under sun illumination must be verified.
- Materials or mechanisms whose degradation under solar-spectrum exposure is part of the question.
Control, calibration and combined setups
Whatever the source, flux must be measured, not assumed. Radiometers and calorimeters at reference planes establish the delivered flux; lamp arrays are then run closed-loop on flux sensors or on fixture temperatures, with lamp ageing compensated by calibration at defined intervals. Solar simulators additionally need beam uniformity and spectrum verification.
Combined setups are common when different boundary conditions must be controlled independently, for example a radiative source on one face, controlled interface temperatures and simulated internal dissipation. Each source, sensor and control loop must be represented in the test configuration and data package.
Takeaway
The heat-input method should follow the verification objective. Spectral simulation is necessary when wavelength-dependent absorption or optical response is being tested; otherwise, calibrated heat-flux equivalence can provide cleaner boundary conditions with less complexity.
Frequently Asked Questions
A solar simulator is a light source – typically xenon or metal-halide lamps with collimation optics – that reproduces the intensity and approximate spectrum of sunlight in space, around 1361 W/m² at Earth distance. Inside a TVAC chamber it lets each surface absorb heat according to its real solar absorptance, which matters when coatings, optics or solar arrays are under test.
Infrared sources can replace a solar simulator when the requirement is a calibrated absorbed heat load and wavelength-dependent effects are not being verified. They are not equivalent for solar cells, optical coatings, stray-light cases or other tests in which spectrum and direction influence the result.
With flux measurement at reference planes: radiometers or calorimeters map the delivered flux before the test, and reference sensors monitor it during the test. Lamp arrays are typically run closed-loop on these sensors or on fixture temperatures, with periodic recalibration to compensate lamp ageing. Solar simulators are additionally checked for beam uniformity and spectral content.
Skin heaters are thin film heaters bonded directly to a surface – of the test item or of a surrounding dummy panel – to inject a defined local power. They are used when absorbed flux can be translated into electrical power per area, typically at unit level or for internal dissipation, and they are precise and inexpensive but do not reproduce a radiative environment.
Technical references
Primary standards, agency material and current Deepvac product pages used to verify the technical statements in this article.