Stephen M. Volz

Cryogenic on-orbit performance of the NASA Cosmic Background Explorer

NASAs Cosmic Background Explorer (COBE) was launched into a polar orbit from the Vandenberg Air Force Base, California on November 18, 1989. The COBE conthin three scientific instruments. Two of these are infrared instruments housed within a 660 liter toroidal superfluid helium cryogen tank. The tank is designed to maintain the base of the instruments below 1.6 K for the duration of the planned one year mission. Boil-off helium is vented from the cryogen tank through a porous plug liquid vapor phase separator, and then overboard from the spacecraft. We discuss here the initial thermal set-up and operation of the dewar in general, and the helium vent system in particular. During the initial cooldown of the dewar from 1.72 K to 1.41 K, short term (1 mm ≤ t ≤ 3 mm) temperature and pressure oscillations were observed in the porous plug and in the vent line. These oscillations have continued throughout the mission life. A detailed flow model was developed to describe this phenomenon and is described below. We further detail the slow establishment of a steady state, mission mode operation of the dewar. The various factors leading to a two week time to mission mode equilibrium for the dewar and cryogenic instruments are discussed. Finally we summarize the performance of the dewar and instruments through the first six months, and we project the expectations for the remainder of the mission through the final depletion of the liquid helium.

The X-Ray Spectrometer (XRS): a multi-stage cryogenic instrument for the Astro-E X-ray astrophysics mission

Abstract The XRS cryogenic system has undergone numerous system configuration changes since its inception in the early 1980s. The Astro-E XRS is a high precision X-ray spectrometer with better than 20 eV resolution between 0.3 and 10 keV. It is a single photon counting solid-state calorimeter with pixel elements cooled to ~ 0.065 K by a unique three-stage cooling system. The low temperature is produced by an adiabatic demagnetization refrigerator (ADR) operating between 0.065 K and 1.3 K. The 1.3 K temperature is maintained by a 33 litre helium tank. Graphite/epoxy straps suspend the helium tank from an outer cryogen tank containing 120 litres of solid neon at ~17 K. The neon tank is in turn suspended from the dewar mainshell by low-conductivity straps. The system has a minimum design lifetime of 2 years, with a 2.5-year goal. The instrument design is severely constrained by mass limitations (≤ 400 kg) and high launch loads. The Astro-E will fly in 2000 on an ISAS M-V solid rocket. The M-V is a new launch vehicle, with the first launch scheduled for 1996.

Anomalous on-orbit behaviour of the NASA cosmic background explorer (COBE) Dewar

Abstract The cryogenic operation of NASAs Cosmic Background Explorer (COBE) ended on September 21, 1990, with the depletion of the liquid helium cryogen, after COBE had completed more than 10 months of successful Dewar and instrument operation. In this report we provide a brief summary of the nominal cryogenic performance of the Dewar (for more detail consult Reference 1). We present in detail several aspects of the helium and spacecraft dynamical behaviour. We discuss the occurrence of temperature and pressure oscillations in the Dewar porous plug. We review the impact of internal instrument malfunctions and of external radiation sources on the performance of the Dewar and of the instruments. From measurements of the COBE spacecraft spin rate we are able to monitor the spin coupling of the liquid helium to the walls of the Dewar. We analyse the spin measurements and present a model for the coupling. Finally, we review a number of helium Dewar ‘lessons learned’ from the COBE mission, and remark on the applicability of these lessons to future missions involving cryogenic payloads.

A calorimetric mass gauge system for the Cosmic Background Explorer (COBE)

We have designed a system for the COBE flight dewar to measure its liquid helium fill. We apply a small known amount of heat to the helium tank and monitor the temperature rise in the liquid and the tank. Working with a detailed thermal model of the tank and liquid we can determine the amount of liquid present. COBE uses a 117 mW, 7 mA heater to warm the helium. It is planned to use the mass gauging system only after the projected midpoint of the mission, after one full sky survey. The system is optimized for use with 50–75 liters of helium (≈8–12% of the cryogen tank volume). Ground testing of the system in a one gravity environment is difficult, but from tests conducted so far we estimate an on-orbit temperature rise of ≈2.5mK/min. A similar system is planned for the Superfluid Helium On-Orbit Transfer (SHOOT), a Shuttle-based experiment. The SHOOT’s specific requirements call for a high power pulse heater, applying 40 W for approximately 20 seconds.

Shoot Performance Testing

The Superfluid Helium On-Orbit Transfer (SHOOT) Flight Demonstration is a shuttle attached payload designed to demonstrate the technology necessary to resupply liquid helium dewars in space. Many SHOOT components will also have use in other aerospace cryogenic systems. The first of two SHOOT dewar systems has been fabricated. The ground performance testing of this dewar is described. The performance tests include measurements of heat leak, impedances of the two vent lines, heat pulse mass gauging accuracy, and superfluid transfer parameters such as flow rate and efficiency. A laboratory dewar was substituted for the second flight dewar for the transfer tests. These tests enable a precise analytical model of the transfer process to be verified. SHOOT performance is thus quantified, except for components such as the liquid acquisition devices and a phase separator which cannot be verified in one gravity.

Final cryogenic performance report for the NASA Cosmic Background Explorer (COBE)

The cryogenic operation of NASA’s Cosmic Background Explorer (COBE) ended on September 21, 1990, with the depletion of the liquid helium cryogen. The COBE had successfully completed more than 10 months of dewar and instrument operation. We report on the cryogenic performance of the COBE dewar and of the two cryogenic instruments throughout the mission lifetime. We discuss the steady state dewar performance, and the dewar and instrument response to a variety of transient thermal phenomena, including external radiation (from the earth and the sun) and instrument power variation. We present the effectiveness of using approximate mass gauging techniques in determining the liquid helium content. Finally we discuss the dewar behavior during the depletion of the helium, and the expected thermal performance of the dewar cryogen tank and the cryogenic instruments as they approach final thermal equilibrium.

Mechanical properties of solid neon and structural modeling of the XRS solid neon dewar

Compression tests have been performed on solid neon to determine Youngs modulus and compressive strengths in the temperature range from 8 to 23 K. This data was used for structural analysis of the X-Ray Spectrometers (XRS) solid neon dewar. The dewar is nearly full of a 2.5% dense aluminum foam, so measurements were made for samples where solid neon was formed within the aluminum matrix as well as pure solid neon samples. The latter measurements were made in order to determine, at least qualitatively, how the mechanical properties of solid neon are affected by the aluminum foam and to allow direct comparisons with Youngs moduli obtained from literature data as a check on the experimental technique. As a further check and to verify the structural model, thermal cycle tests were conducted on a small-scale toroidal dewar filled with solid neon. Mechanical properties measurements and data are described, along with model predictions and actual performance of both the engineering unit XRS dewar and the sub-scale dewar.

Verification testing of the superfluid helium on-orbit transfer (SHOOT) experiment

Abstract The Superfluid Helium On-Orbit Transfer (SHOOT) project is a secondary shuttle crossbay payload which flew on the STS-57/Endeavour mission. It was designed to develop and demonstrate the technologies required to resupply liquid helium containers in space, and to develop new technologies that may be used in other future space cryogenic systems. The SHOOT payload consists of two superfluid helium Dewars with helium management cryostats connected by a transfer line, and six avionics boxes for valve and heater control, temperature, pressure and fluid position monitoring and data processing and telemetry. The cryostats contain numerous specialized helium management components; including high and low flow phase separators, liquid/vapour discriminators, flowmeters, liquid level detectors, cryogenic mechanical valves and cryogenic relief valves and burst discs, and two varieties of fluid acquisition systems. To prepare the SHOOT payload for launch a series of functional, structural, thermal and reliability tests were conducted at every level of hardware assembly, from materials tests to system level thermal, structural and functional performance tests. We present here the verification tests and analyses developed and completed at each level of assembly. We discuss the trade-offs considered for, and the success (or failure) of, models and analyses to predict performance results. Finally, we present some lessons learned of potential interest to future cryogenic missions, whether on the Space Shuttle or on expendable launch vehicles.

Performance predictions for spaceborne, long-lifetime helium dewars containing large-aperture telescopes

The evolution of design approaches for high-performance superfluid helium dewars containing large-aperture telescopes are discussed. Particular attention is given to thermal-math modeling for the IRAS and the Cosmic Background Explorer (COBE) dewars. Correlation of the recent COBE flight data with the dewar thermal-math model is presented, and apparent predictive deficiencies of the model are discussed.

Crate: a Superfluid Helium Dewar for a Small Explorer (SMEX) Satellite

In recent years there has been a growing interest within NASA to design and fly spacecraft and instruments that are smaller, cheaper, and faster to design, develop and implement. The impetus is a desire to offer more opportunities for the scientific community to conduct astrophysics and space physics research, and to minimize the time between project proposal and fruition. The development time for orbiting observatories has increased steadily over the years to the point where the total mission lifetime may extend to decades (for observatories such as the Hubble Space Telescope or the Advanced X-Ray Astronomy Facility). Even “smaller” observatories can take many years to complete. For example, the Cosmic Background Explorer, or COBE, took sixteen years between the original proposal in 1973 and the successful launch in 1989. The Small Explorer (SMEX) Program at NASA is pursuing one approach to significantly shorten the orbiting observatory development cycle, and to reduce spacecraft costs in the process. One SMEX satellite was launched in July 1992, the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX). Two others are currently in advanced stages of fabrication and testing.