Many laboratory systems operate at or near sea level and at room temperature. The primary challenges of this project were related to operating in upper atmospheric conditions on a balloon gondola:
- Temperature fluctuations and pressure changes on seals during ascent and descent
- Size and weight restrictions of the assigned space on the gondola
- Manufacturing the instrument to withstand the forces experienced during flight and landing
- Very cold outside temperatures in the upper atmosphere can affect the pressure and temperature of the liquid cryogens
Indium Metal Seals Withstand Freezing Upper Atmosphere Temperatures
O-rings are used to seal case lids, fill ports and windows of laboratory dewars but very cold temperatures of the upper atmosphere freeze O-ring materials, creating vacuum leaks. For this project, we designed two types of seals: O-ring seals for the DLR team to be able to open and close lids, ports and windows with airtight seals in the lab and indium metal seals for an airtight seal in high-altitude flight. The indium metal seals require additional case pressure to seal, requiring a more robust bolting system with increased bolt size and frequency and thicker bolting surface materials to ensure sufficient pressure to seal.
Along with adding metal seals, windows were designed using brass instead of aluminum. Brass is more rigid for less flex to maintain vacuum pressure. The valve was designed with metal bellows and metal-on-metal sealing surfaces instead of rubber O-rings.
Weight and Size Budget Balanced with Extended Hold Times
DLR’s equipment had a defined position along with other experiments onboard the balloon gondola. The size and weight limits needed to be balanced with increased vessel capacity for extended hold times. Hold times included pre-launch time on the tarmac, ascent time to operational altitude, and an operational time of >24 hours total. To meet required hold times, LN2 capacity was 4.5 liters and LHe capacity was 5 liters.
In addition to performing at specifications for upper atmospheric conditions, the client needed to mount instruments to the LN2 and LHe vessel, requiring windows, ports, and connectors for each vessel. To accommodate this IRLabs added detector mounts between the LN2 vessel and LHe vessel so that components could be secured to either the nitrogen or helium vessels along with accompanying window ports. To keep the size and weight within the defined envelope, case mass was reduced by designing the exact number of bosses needed for external mounting of equipment and instruments. Reducing flat mounting surfaces on the cylinder keeps cases slimmed down.
Rigid Supports Secure Vessels for Impact on Descent and Landing
Balloon-mounted instruments are subject to impacts during flight and landing. Rigid internal supports were needed to secure the internal vessels in place. Internal vessels in laboratory cryostats are typically attached with minimal supports to increase cryogenic hold time. Rigid supports allow the instrument to survive the forces of landing, at a slight cost to hold time. Our experience lets us calculate the optimal balance between the two.
Evaluating Approaches to Maintain Cold Plate Temperature
Laboratory cryostats are used at room temperature and operate at 1 atmosphere of pressure at or near sea level. In upper altitude applications, the vessels are filled on the ground and then ascend to the upper atmosphere on the balloon gondola, experiencing significant temperature fluctuations and pressure drops. Temperatures and pressures for this application were low enough to freeze liquid nitrogen, which can interfere with contact to the cold plate. In addition, freezing of cryogens could create ice plugs in the fill tube, risking rupture during descent.
The DLR team worked with our experts to design a solution to fit their application and budget. For this project, different approaches were considered to maintain thermal conductivity between solidified nitrogen and the cold plate including:
- Filling the vessels with a media of fine-mesh copper wool to maintain cold plate contact
- Installing a cold finger in the vessel to increase surface area and cold plate contact
- Upgrading to motorized pressure regulators to improve control of the internal pressure
DLR and IRLabs teams collaborated through lab testing to determine the best solution to outfit the vessels. Absolute pressure relief valves, which maintain constant pressure on the vessel regardless of the outside pressure, were originally planned. After lab testing, the teams determined that upgrading the absolute pressure regulators to motorized regulators provided better control of internal pressure of the vessel. Absolute pressure regulators are passive devices that can hum or vibrate at certain pressure thresholds, potentially causing acoustic oscillations within the vessel. Active, motor-driven pressure regulators allowed the DLR team to reduce vibration risks by actively monitoring and commanding pressure relief valves to open or close. The cryostat was easily upgradable to allow modifications like this. We also added burst disks which are safety devices in case of pressure build-up in the cryostat.
Decades of Experience in Upper Atmospheric Research
Our collaborative approach to projects and our customizable solutions allow upgrades and modifications for the best fit for the application. This collaboration is well supported by our experience. Over 50 years ago, our founder Dr. Frank Low pioneered the use of bolometers for IR detection in astronomy. Our company’s experience with cryogenic systems started then, as the bolometers required cooling and continues today with other applications, including research.
The first successful flight of the OSAS-B onboard the HEMERA was in September 2022, with results to be published soon.