The Balloon-Borne Cryogenic Telescope Testbed Mission: Bulk Cryogen Transfer at 40 km Altitude

TL;DR

This paper reports on a high-altitude balloon experiment demonstrating bulk cryogen transfer at 40 km altitude, advancing technology for future cryogenic space telescopes.

Contribution

It presents the first successful transfer of cryogens at stratospheric altitude, testing ultra-light dewar performance for cryogenic space observatories.

Findings

Liquid nitrogen transfer cooled a dewar to 65 K at 40 km altitude.

Liquid helium transfer was successfully performed at high altitude.

Calorimetric measurements quantified heat leaks during transfer.

Abstract

The Balloon-Borne Cryogenic Telescope Testbed (BOBCAT) is a stratospheric balloon payload to develop technology for a future cryogenic suborbital observatory. A series of flights are intended to establish ultra-light dewar performance and open-aperture observing techniques for large (3--5 meter diameter) cryogenic telescopes at infrared wavelengths. An initial flight in 2019 demonstrated bulk transfer of liquid nitrogen and liquid helium at stratospheric altitudes. An 827 kg payload carried 14 liters of liquid nitrogen (LN2) and 268 liters of liquid helium (LHe) in pressurized storage dewars to an altitude of 39.7 km. Once at float altitude, liquid nitrogen transfer cooled a separate, unpressurized bucket dewar to a temperature of 65 K, followed by the transfer of 32 liters of liquid helium from the storage dewar into the bucket dewar. Calorimetric tests measured the total heat leak to the LHe bath within bucket dewar. A subsequent flight will replace the receiving bucket dewar with an ultra-light dewar of similar size to compare the performance of the ultra-light design to conventional superinsulated dewars.