Louis J. Salerno

Cryogenics and the Human Exploration of Mars

Current studies within NASA involve extending the human exploration of space from low earth orbit into the solar system, with the first human exploration of Mars proposed in 2014. The key cryogenic technology areas to be addressed in human Mars missions are long-term propellant storage, cryogenic refrigeration, cryogenic liquefaction, and zero gravity fluid management. Passive technologies such as advanced multilayer insulation (MLI) concepts, vapor-cooled shields (VCS), and catalytic converters will be combined with the development of active coolers (cryogenic refrigerators). The integration of passive and active technologies will form a hybrid system optimized to minimize the launch mass while preserving the cryogenic propellants. This paper will present a brief overview of the proposed Mars reference mission and the concomitant cryogenic fluid management technology, focusing on active cooling technology.

A piezoelectric microvalve for cryogenic applications

This paper reports on a normally open piezoelectrically actuated microvalve for high flow modulation at cryogenic temperatures. One application envisioned is to control the flow of a cryogen for distributed cooling with a high degree of temperature stability and a small thermal gradient. The valve consists of a micromachined die fabricated from a silicon-on-insulator wafer, a glass wafer, a commercially available piezoelectric stack actuator and Macor TM ceramic encapsulation that has overall dimensions of 1 × 1 × 1c m 3 .A perimeter augmentation scheme for the valve seat has been implemented to provide high flow modulation. In tests performed at room temperature the flow was modulated from 980 mL min −1 with the valve fully open (0 V), to 0 mL min −1 with a 60 V actuation voltage, at an inlet gauge pressure of 55 kPa. This range is orders of magnitude higher flow than the modulation capability of similarly sized piezoelectric microvalves. At the cryogenic temperature of 80 K, the valve successfully modulated gas flow from 350 mL min −1 down to 20 mL min −1 with an inlet pressure of 104 kPa higher than the atmosphere. The operation of this valve has been validated at elevated temperatures as well, up to 380 K. The valve has a response time of less than 1 ms and has operational bandwidth up to 820 kHz. (Some figures in this article are in colour only in the electronic version)

Thermal conductance of pressed metallic contacts augmented with indium foil or Apiezon grease at liquid helium temperatures

Abstract The thermal conductance of pressed contacts which have been augmented with indium foil or Apiezon-N™ grease has been measured over the temperature range 1.6–6.0 K, with applied forces from 22 to 670N. The sample pairs were fabricated from OFHC copper, 6061-T6 aluminium, free-machining brass and 304 stainless steel. Although the thermal conductance was found to increase with increasing applied contact force, the force dependence was less than in earlier work. The addition of indium foil or Apiezon grease between the contact surfaces resulted in an improvement over uncoated surfaces ranging from approximately a factor of 3 for stainless steel to an order of magnitude for copper contacts.

Cryocoolers for Human and Robotic Missions to Mars

Future missions to Mars will make use of a number of different cryocoolers. These cryocoolers will be used to liquefy and preserve propellants and for transporting liquid hydrogen feed stock. The earliest mission requiring a cryocooler is the 2003 robotic mission, which will demonstrate technology for In-Situ-Consumable-Production (ISCP) as part of the long-term goal of developing In-Situ-Resource-Utilization (ISRU). Later missions may require cryocoolers to preserve propellants on Trans-Mars Injection stages, Martian landers, and Mars return vehicles. In addition, ISCP may require liquid hydrogen feed stock as part of the chemical process to produce propellants from the Martian atmosphere. Cryocoolers will be required to minimize or eliminate the boiloff of this hydrogen in transit to Mars. Propellants produced on Mars will need liquefiers and cryocoolers for storage on the Martian surface until the propellants are used in the ascent vehicle.

A piezoelectric microvalve with integrated sensors for cryogenic applications

This paper describes a normally open, self-encapsulated, gas valve that has embedded sensors for pressure and temperature monitoring. The valve has been validated at operating temperatures over 80-380 K, and at pressures up to 130 kPa. A perimeter augmentation scheme for the valve seat has been implemented to provide higher flow modulation. Two kinds of suspensions are described for the valve seat. In tests performed at room temperature, the flow was modulated from 980 mL/min. with the valve fully open (0 V), to 0 mL/min. with 60 V actuation, at an inlet pressure of 55 kPa. Cryogenic flow rate tests show similar modulation with flow from 166 mL/min. with the valve fully open, to 5.3 mL/min. with 120 V actuation voltage, at an inlet pressure of 70 kPa. The platinum RTD temperature sensor is independently tested from 40-450 K with sensitivity of 0.23 %/K in its operational range of 150- 450 K. The pressure sensor has sensitivity of 250 ppm/kPa at room temperature.

ANALYSIS OF CONTINUOUS HEAT EXCHANGERS FOR CRYOGENIC BOIL-OFF REDUCTION

Cryogenic boil-off reduction systems (CBRS) employing continuous heat exchangers in pressurized helium distributed cooling networks for active thermal control of large surfaces such as propellant tank walls and light-weight radiation shields have been studied for some time. Usually, very simple and intuitive relations are used to derive such quantities as the pressure drop across the network and the required flow rate for a given heat load. Here, detailed thermal-fluid and heat transfer relations for such systems are formulated and then studied term by term in order to determine the conditions under which various approximations to them may reasonably be made. It is found that in most applications of interest, use of the simplified relations is justifiable.

A Low Power, Microvalve-Regulated Drug Delivery System using a SI Micro-Spring Pressurized Balloon Reservoir

This paper reports on a drug delivery system that provides modulated delivery of liquid-phase chemicals. The device uses silicon torsion springs on a 2times3 cm2 chip to pressurize a soft polymeric reservoir and regulate flow with a piezoelectricaly actuated silicon microvalve that is 1.5times1.5times1 cm3. Using the finished device, regulated diffusion of a fluorescent dye into agar gel was demonstrated. Fluid flow out of the 500 muL reservoir could be regulated from 10-500 muL/min with up to 80 kPa of delivery pressure. Typical regulation consumes 0.136 muW of power. Analysis of the valve, reservoir springs, and a model based on pressure-enhanced diffusion are presented and are validated by experimental data.

Performance Testing of a Lightweight, High Efficiency Cooler

A lightweight, high efficiency cryocooler has been tested. The design operating point of the cooler is 10 W at 95 K with a rejection temperature of 300 K, and the goal is a 10 yr operating lifetime. The cooler has a mass of less than 4.0 kg. The cooler efficiency at the design operating point is 12.6 W/W. These values represent approximately a one-third decrease in mass and a one-third increase in efficiency over previous designs.

Thermal conductance of pressed bimetallic contacts at liquid nitrogentemperatures

Abstract The thermal conductance of one aluminum and one stainless steel pressed metal contacts has been measured near 77 K, with applied forces from 8.9 N to 267 N. Both 5052 or 5083 aluminum were used as the upper contact. The lower contact was 304L stainless steel. The thermal conductance is linear in temperature over the range of measurement and ranged from roughly 9 to 21 mW/K. There is no difference in conductance between the two aluminum alloys. Extrapolating the data to zero applied force does not result in zero thermal conductance. A possible cause of this anomalous effect is discussed.

CHARACTERIZATION OF AN ACTIVELY COOLED METAL FOIL THERMAL RADIATION SHIELD

Zero boil‐off (ZBO) or reduced boil‐off (RBO) systems that involve active cooling of large cryogenic propellant tanks will most likely be required for future space exploration missions. For liquid oxygen or methane, such systems could be implemented using existing high technology readiness level (TRL) cryocoolers. However, for liquid hydrogen temperatures (∼20 K) no such coolers exist. In order to partially circumvent this technology gap, the concept of broad area cooling (BAC) has been developed, whereby a low mass thermal radiation shield could be maintained at temperatures around 100 K by steady circulation of cold pressurized gas through a network of narrow tubes. By this method it is possible to dramatically reduce the radiative heat leak to the 20 K tank. A series of experiments, designed to investigate the heat transfer capabilities of BAC systems, have been conducted at NASA Ames Research Center (ARC). Results of the final experiment in this series, investigating heat transfer from a metal foil fi...