A Stirling cryocooler is under development for small satellites where weight and packaging volume must be minimized while achieving high thermodynamic efficiency. The challenge is exacerbated by the requirement to operate efficiently down to at least 80K. Small size and weight are achieved by operating at a frequency well in excess of the current state of the art, up to and beyond 200 Hz. The efficiency requirement motivates the implementation of a moving Stirling displacer instead of a pulse tube. The latter has the benefit of simplicity, but at the unacceptable expense of thermodynamic efficiency since the expansion power in a pulse tube is dissipated, not recovered as in a Stirling. While using a Stirling displacer avoids several critical loss mechanisms inherent in a pulse tube, it does introduce the “shuttle loss,” which arises from the motion of the displacer within a fixed cylinder/bore. While both the displacer and the bore have nearly linear axial temperature distributions, the relative motion results in instantaneous radial temperature gradients across the clearance gap, which causes heat transfer between the structures. The direction of this heat transfer changes during a cycle, but ultimately “shuttles” heat from the warm end to the cold tip. This paper reports on a computational fluid dynamics (CFD) study aimed at quantifying this loss in the size and frequency range of interest as a function of relevant geometries and piston materials, and assessing its sensitivity to various design parameters. Parametric calculations show that among the various design parameters the overall conductance of the displacer has the most significant effect on the shuttle loss.
The optimum small satellite (SmallSat) cryocooler system must be extremely compact and lightweight, achieved in this paper by operating a linear cryocooler at a frequency of approximately 300 Hz. Operation at this frequency, which is well in excess of the 100-150 Hz reported in recent papers on related efforts, requires an evolution beyond the traditional Oxford-class, flexure-based methods of setting the mechanical resonance. A novel approach that optimizes the electromagnetic design and the mechanical design together to simultaneously achieve the required dynamic and thermodynamic performances is described. Since highly miniaturized pulse tube coolers are fundamentally ill-suited for the sub-80K temperature range of interest because the boundary layer losses inside the pulse tube become dominant at the associated very small pulse tube size, a moving displacer Stirling cryocooler architecture is used. Compact compressor mechanisms developed on a previous program are reused for this design, and they have been adapted to yield an extremely compact Stirling warm end motor mechanism. Supporting thermodynamic and electromagnetic analysis results are reported.
<p> As the scientific requirements of microsatellites migrate closer to those of larger, more-expensive traditional satellites, the technical requirements on the key enabling components and subsystems are becoming more demanding. If the utility of microsatellites is ever to expand to include high performance mid-wave infrared (MWIR) and short-wave infrared (SWIR) sensors, significant advancement in the state of art of small cryocooler systems is required. The Microsat Cryocooler System (MCS) is a radiation hard, space-qualified integrated cryocooler assembly (ICA) for CubeSat and microsat applications. The ICA includes a high reliability tactical cryocooler, a miniature set of Low Cost Cryocooler Electronics (mLCCE), the thermal management components, and the isolation structure. As is the case with the larger LCCE from which it was derived, the mLCCE supports any of a wide range of linear cryocoolers in its design output power range (nominally 25W). With minor adaptation, rotary coolers are also supported. </p><p> This paper presents the initial results from the brassboard phase of the MCS Program. A high fidelity set of cryocooler electronics with a well-defined upgrade path to a space-compatible design has been built and tested with the target cryocooler. Those data are presented. In addition to reducing risk for the spaceflight design to follow, these electronics are being released as an intermediate product for high-end tactical applications where the plug-and-play operability among different coolers and the enhanced level of control and programmability (relative to typical tactical cooler electronics) are desired. </p><p> The overall CubeSat-compatible mechanical subsystem design is also presented, including descriptions of the thermal
management and vibration isolation approaches.</p>
Closed-cycle cryogenic refrigerators, or cryocoolers, are an enabling technology for a wide range of aerospace applications, mostly related to infrared (IR) sensors. While the industry focus has tended to be on the mechanical cryocooler thermo mechanical unit (TMU) alone, implementation on a platform necessarily consists of the combination of the TMU and a mating set of command and control electronics. For some applications the cryocooler electronics (CCE) are technologically simple and low cost relative to the TMU, but this is not always the case. The relative cost and complexity of the CCE for a space-borne application can easily exceed that of the TMU, primarily due to the technical constraints and cost impacts introduced by the typical space radiation hardness and reliability requirements. High end tactical IR sensor applications also challenge the state of the art in cryocooler electronics, such as those for which temperature setpoint and frequency must be adjustable, or those where an informative telemetry set must be supported, etc. Generally speaking for both space and tactical applications, it is often the CCE that limits the rated lifetime and reliability of the cryocooler system. A family of high end digital cryocooler electronics has been developed to address these needs. These electronics are readily scalable from 10W to 500W output capacity; experimental performance data for nominally 25W and 100W variants are presented. The combination of a FPGA-based controller and dual H-bridge motor drive architectures yields high efficiency (>92% typical) and precision temperature control (± 30 mK typical) for a wide range of Stirling-class mechanical cryocooler types and vendors. This paper focuses on recent testing with the AIM INFRAROT-MODULE GmbH (AIM) SX030 and AIM SF100 cryocoolers.
This paper describes a series of experiments during which a particular cryocooler control electronics (CCE) was shown
to successfully drive several very different cryocoolers and simulated cryocooler loads, including a space pulse tube
cryocooler, long life tactical Stirling coolers, and even a simulated reverse turbo Brayton (RTB) cryocooler compressor.
This CCE is an early brassboard version of a low cost, radiation hard cryocooler electronics module being developed
primarily for cost-constrained, but nevertheless mission critical military and civilian spaceborne applications. This
design is also applicable for tactical applications which seek to support multiple cryocooler types and/or vendors with a
given CCE. The CCE provides high efficiency DC-to-AC conversion, automated cool down, and precision temperature
control. The results demonstrate convincingly that this CCE design is broadly supportive of a wide range of
thermodynamic-mechanical cryocooler units (TMUs) for a subsequently broad range of payloads and missions.
Cryogenic coolers are often used in modern spacecraft in conjunction with sensitive electronics and sensors of military,
commercial and scientific instrumentation. The typical space requirements are: power efficiency, low vibration export,
proven reliability, ability to survive launch vibration/shock and long-term exposure to space radiation.
A long-standing paradigm of exclusively using "space heritage" equipment has become the standard practice for
delivering high reliability components. Unfortunately, this conservative "space heritage" practice can result in using
outdated, oversized, overweight and overpriced cryogenic coolers and is becoming increasingly unacceptable for space
agencies now operating within tough monetary and time constraints.
The recent trend in developing mini and micro satellites for relatively inexpensive missions has prompted attempts to
adapt leading-edge tactical cryogenic coolers for suitability in the space environment. The primary emphasis has been
on reducing cost, weight and size. The authors are disclosing theoretical and practical aspects of a collaborative effort to
develop a space qualified cryogenic refrigerator system based on the tactical cooler model Ricor K527 and the Iris
Technology radiation hardened Low Cost Cryocooler Electronics (LCCE). The K27/LCCE solution is ideal for
applications where cost, size, weight, power consumption, vibration export, reliability and time to spacecraft integration
are of concern.
The operation of the thermo-mechanical unit of a cryogenic cooler may originate a resonant excitation of the
spacecraft frame, optical bench or components of the optical train. This may result in degraded functionality of the
inherently vibration sensitive space-borne infrared imager directly associated with the cooler or neighboring
instrumentation typically requiring a quiet micro-g environment.
The best practice for controlling cooler induced vibration relies on the principle of active momentum cancellation.
In particular, the pressure wave generator typically contains two oppositely actuated piston compressors, while the single
piston expander is counterbalanced by an auxiliary active counter-balancer. Active vibration cancellation is supervised
by a dedicated DSP feed-forward controller, where the error signals are delivered by the vibration sensors
(accelerometers or load cells). This can result in oversized, overweight and overpriced cryogenic coolers with degraded
electromechanical performance and impaired reliability.
The authors are advocating a reliable, compact, cost and power saving approach capitalizing on the combined
application of a passive tuned dynamic absorber and a low frequency vibration isolator. This concept appears to be
especially suitable for low budget missions involving mini and micro satellites, where price, size, weight and power
consumption are of concern.
The authors reveal the results of theoretical study and experimentation on the attainable performance using a fullscale
technology demonstrator relying on a Ricor model K527 tactical split Stirling cryogenic cooler. The theoretical
predictions are in fair agreement with the experimental data. From experimentation, the residual vibration export is quite
suitable for demanding wide range of aerospace applications. The authors give practical recommendations on heatsinking
and further maximizing performance.
Closed-cycle mechanical cryogenic refrigerators, or cryocoolers, are an enabling technology for next generation
infrared (IR) sensors. Passive cryoradiators and stored cryogen systems have been used successfully in the past, but the
increased cooling requirements for emerging systems cannot practically be met with these passive techniques. Modern
systems are employing much larger focal plane arrays that dissipate more energy and have higher parasitic thermal loads
than in the past. Additional "on chip" FPA data processing capability, such as time delay and integration (TDI) and
analog-to-digital conversion (ADC), is further driving up the heat loads. While loads are going up, temperatures are
going down. The desire to operate at long wave infrared (LWIR) wavelengths (>9 microns) for a broader range of
remote sensing missions is driving the need for 35-40 K refrigeration, significantly colder than past systems that operated
at shorter wavelengths. Unfortunately, the use of a mechanical rather than passive cryocooler introduces an additional
jitter source that must be properly mitigated. Techniques include the use of inherently low vibration cryocoolers, closedloop
active vibration cancellation servo systems, damping struts, soft mounts, or a combination of these techniques.
Implementation of these techniques within a proper system engineering context is presented.
Development of the Dual-Use Cryocooler (DUC) system has progressed substantially over the past two years, including
the design, build and testing of a brassboard thermo-mechanical unit (TMU). Early design efforts were undertaken with
simplicity as a goal, and as a result the brassboard TMU contained significantly less parts than typical space-level
cryocoolers. Build time for the brassboard unit was extremely short, with the compressor being built in a matter of days
as opposed to the more traditional timescale of weeks. The brassboard TMU was subjected to characterization testing in
both horizontal and vertical orientations (to address sensitivity of the pulse-tube cold head to gravitational effects), and
results from that set of tests have been correlated to the thermodynamic model. Several lessons were learned as the
testing and correlation activities progressed, and improvements necessary to meet the intended performance objectives
were identified for implementation in the deliverable system.
Significant progress was made in terms of electronics development as well. Existing tactical assets were heavily
modified for use with the DUC, including the addition of separate drive circuits for each compressor motor. The
operating software was modified to enable features not found in typical tactical systems such as first-order active
vibration cancellation. Ultimately, the brassboard electronics were used to drive passive loads as well as an actual
(representative) tactical Stirling cryocooler.
Raytheon has manufactured closed-cycle cryocoolers for both tactical military and space applications for over thirty years. Tactical and space cryocooler technologies have historically been treated as distinct both at Raytheon and throughout the industry. Differing technical requirements, operating lifetimes, and order quantities have driven these types of coolers to dramatically different design approaches and cost levels. For example, a typical space cryocooler system today costs approximately $2M as compared to roughly $10,000 for a tactical cryocooler. However, stimuli from both the tactical and space cooler user communities are driving the markets together. Tactical cryocooler requirements are starting to push towards operating lifetime requirements more characteristic of the space coolers (e.g., 20,000+ hours). Space cryocooler users, in particular Missile Defense Agency, are pushing for substantial cost reduction. In response, Raytheon is developing a low cost space cryocooler with an intended dual-use capability to also serve the tactical marketplace. This cooler leverages proven flexure-suspension technology to achieve long life, and a low cost concentric pulse tube cold head design has been developed that can be packaged into the existing Standard Advanced Dewar Assembly, Type One (SADA-I). The cooler meets or exceeds the SADA-I operational requirements (capacity, efficiency, etc.) as well. For the space-version of the cooler, the electronics cost has been reduced by an estimated 80% versus current designs, largely by approaching the vibration cancellation requirement from a dramatically different perspective. Fabrication of the brassboard expander is nearly complete, and the prototype design is well underway. The design approach, development progress, and proposed applications are presented.