Success or failure can rest on a millisecond:

  • Published
  • By Philip Lorenz III
  • AEDC/PA
Arnold Air Force Base, Tenn. --  Space is a harsh and unforgiving environment, one that exposes satellites and space craft to powerful radiation, extreme temperatures and a host of other potentially destructive forces and phenomena.

This is why it is so important to rigorously test satellite, space vehicle and launch systems, materials and components before flight.

An essential aspect of ground testing is accuracy, and that is where Arnold Engineering Development Center's (AEDC) Precision Measurement Equipment Laboratory (PMEL) comes into the picture.

"Part of our job at the PMEL is to calibrate cryogenic sensors for testing in our space chambers, rocket motor test cells and other facilities using cryogenics," said Dave Compton, manager of Arnold's PMEL. "We have a cryostat for that purpose and a unique one at that."

So, what is a cryostat and how does it contribute to the mission?

According to Compton, a cryostat is a vessel, similar in construction to a vacuum flask or thermos used to maintain extremely cold temperatures.

At AEDC, cryostats are used to subject sensors that measure temperatures in the cryogenic range to precise temperature loads for calibration. These sensors measure the temperature of fluids and gases that are used in hypersonic test cells, vacuum space chambers and rocket test cells to simulate conditions of space or in the case of
rocket engine testing, fuel for launch.

Harry Clark, an Aerospace Testing Alliance (ATA) engineering specialist in the test IT Branch, recalled how PMEL used their cryostat to calibrate sensors for testing rocket engines with cryogenic fuels in the J-4 liquid rocket propellant test facility.

"In that case, the sensors were Resistance Temperature Devices (RTD) and they were used to measure the temperature of the propellants, liquid hydrogen and liquid oxygen (LOX)," he said. "Measurement accuracy of the temperature of these propellants was critical to the test program because the density of the propellants is very dependent upon the temperature and pressure of these fluids," he explained. "The density of the propellants factors directly into the calculation for the mass flow rates of the propellants and the mass flow rates of the propellants are direct contributors to the measurement of the performance of the rocket engine. Inaccuracies in the measured temperature would lead to inaccuracies in the reported engine performance, which could ultimately lead to payloads inserted into improper orbits."

Clark added, "With respect to the cryostat, the Precision Measurement Equipment Laboratory staff was always very exacting with their calibrations of the Resistance Temperature Devices for this test program and their data was used to qualify equipment used today on the Delta IV heavy lift launch vehicle. We always appreciated the work they did to assure the quality of the measurements we were making."

Cryogenic sensors at AEDC's Aerodynamic and Propulsion Test Unit measure the temperature of liquid oxygen in the facility. The liquid oxygen is added to the air downstream of a burner to replenish the oxygen content of the air that the burner has consumed as it burns butane to create the desired test conditions. The LOX is injected into this hot air stream so that the heated test medium it closely matches the physical constituents of the air that the hypersonic vehicle must mix with its fuel to generate thrust.
Leldon Kelley, IT department instrumentation engineer, said the sensors indicate the relative 'coldness' of the oxygen, i.e., whether it is in a gaseous or liquid state at a certain location in the system.

"Due to the criticality of maintaining a liquid flow throughout the system, it is important to have a sensor that is known to be good," he said. "The initial calibration is an important indicator that the Resistance Temperature Device [at APTU] meets manufacturer's specs and will perform as required before it is installed in the system.

"Removal of a bad sensor requires several days of downtime during which the system has to be depressurized, allowed to warm up and a nitrogen purge established on the system," he continued. "After installation, the system has to be re-cooled, or chilled down, prior to operation. PMEL's calibration of the sensor can prevent a maintenance nightmare."

Kelley said the lab's role is crucial to efficient and financially sound operations at AEDC.

"We rely on the Precision Measurement Equipment Laboratory to ensure we do not put in a bad or deteriorating sensor at any time," he said. "This could be done during the initial installation where PMEL has done an acceptance test on the Resistance Temperature Device, or a later when we replace a failed unit with a spare we've been keeping on the shelf and we want PMEL to verify the spare's integrity before we install it."