The draft roadmap for Technology Area (TA) 14, Thermal Management Systems, consists of three technology subareas:1
• 14.1 Cryogenic Systems
• 14.2 Thermal Control Systems
• 14.3 Thermal Protection Systems
Thermal Management Systems are systems and technologies that that are capable of handling high thermal loads with excellent temperature control, with a goal of decreasing the mass of existing systems. TA14 is concerned with three broad areas of application of thermal management: Cryogenic Systems, which are systems operating below -150°C; Thermal Control Systems, operating near room temperature; and Thermal Protection Systems, which operate above about 500°C.
Before prioritizing the level 3 technologies included in TA14, the panel considered whether to rename, delete, or move technologies in the technology area breakdown structure (TABS). No changes were recommended for TA14, although corrections were made to the names of two technologies. The TABS for TA14 is shown in Table Q.1, and the complete, revised TABS for all 14 Tas is shown in Appendix B.
TOP TECHNICAL CHALLENGES
The panel identified seven top technical challenges for TA14, presented here in priority order.
1. Thermal Protection Systems: Develop a range of rigid ablative and inflatable/flexible/deployable Thermal Protection Systems (TPS) for both human and robotic advanced high-velocity return missions, either novel or reconstituted legacy systems.
1The draft space technology roadmaps are available at http://www.nasa.gov/offices/oct/strategic_integration/technology_roadmap.html
TABLE Q.1 Technology Area Breakdown Structure for TA14, Thermal Management Systems
|NASA Draft Roadmap (Revision 10)||Steering Committee-Recommended Changes|
|TA14 Thermal Management Systems||The steering committee made no changes to the structure of this roadmap, although NASA's draft roadmap had a different name for two technologies.|
14.1. Cryogenic Systems
14.1.1. Passive Thermal Control
14.1.2. Active Thermal Control
14.1.3. Integration and Modeling
|Rename: 14.1.3. Systems Integration|
14.2. Thermal Control Systems
14.2.1. Heat Acquisition
14.2.2. Heat Transfer
14.2.3. Heat Rejection & Energy Storage
14.3. Thermal Protection Systems
14.3.1. Entry/Ascent TPS
|Rename: 14.3.1 Ascent/Entry TPS|
14.3.2. Plume Shielding (Convective & Radiative)
14.3.3. Sensor Systems & Measurement Technologies
TPS is mission critical for all future human and robotic missions that require planetary entry or reentry. The current availability of high-TRL rigid ablative TPS is adequate for LEO re-entry, but is inadequate for high-energy re-entries to Earth or planetary missions. Ablative materials are enabling for all NASA, military, and commercial missions that require high-mach number re-entry, such as near-Earth asteroid visits and Mars missions, whether human or robotic (Venkatapathy, 2009a, 2009b). System studies have shown that large entry heat shields provide a potentially enabling means to increase landed mass on a planetary (Mars) surface (Jamshid et al., 2011; McGuire et al., 2011). In many cases, updating existing obsolete TPS materials and processes that were developed in the past may be faster and cheaper than the development of new materials or methods. Some are not now available either because of lost technology, new restrictions on material, or other factors. For example, carbon-phenolic recertification is needed before it can be used for future missions. Other new materials show considerable promise.
2. Zero Boil-Off Storage: Accelerate research on advanced active and passive systems to approach near-zero boil-off in long-term cryogenic storage.
Long-term missions that require cryogenic life-support supplies (e.g., LOX), cryogenic propellants (LH2), or very low temperatures for scientific instrument support will require near-zero boil-off rates. Multiple technologies are proposed in the TA14 roadmap, some of which provide incremental but desirable improvements in cryogenic technology. Emphasis should be on reliable, repairable, supportable active and passive systems that can be integrated into many missions. Many of the technologies are parallel in their impact. Some will emerge as top candidates.
3. Radiators: Develop improved space radiators with reduced mass.
Radiators are used for energy removal from spacecraft and planetary base systems, and are mission-critical for many proposed missions. To reduce radiator mass, area, and pumping power, research is needed on variable emissivity, very low absorptivity-to-emissivity ratio, self-cleaning, and high-temperature coatings, as well as research on lightweight radiators or compact storage systems for extending EVA capability.
4. Multifunctional Materials: Develop high-temperature multifunctional materials that combine structural strength good insulating ability, and possibly other functions.
Multifunctional systems can provide significant mass savings due to combining thermal and structural functions, allowing increased payload weight. Presently, these functions are separately incorporated in spacecraft
design. Multi-functional TPS and multi-layer insulation (MLI) systems that combine thermal, structural, micrometeoroid and orbital debris (MMOD), and crew radiation protection could provide significant weight savings and enable long-duration missions, and can also be used for planetary habitat thermal and multifunctional protection. This challenge is also ranked third by TA12.
5. Verification and Validation: Develop, verify, validate, and quantify uncertainty analysis requirements for new or improved comprehensive computer codes for thermal analysis.
Upgrades to predictive codes for ablation during re-entry heating are needed to include closely coupled multiphase ablation and radiative heating into the flow simulations, with careful attention given to verification, validation, and uncertainty quantification. All thermal analysis codes should include (1) verification that the codes have no internal errors, and accurately code the equations used for modeling and analysis; (2) predictions validating against all available experimental data, accounting for experimental error bands; and (3) quantifying the confidence in code predictions, accounting for uncertainties in the data used as model input, uncertainties in the mathematical models used in the analysis, and uncertainties caused by the numerical technique that is implemented (e.g., discretization errors in time and space). Without these attributes, the results generated by the codes are unreliable for design. This challenge is also addressed by TA10 and TA12.
6. Repair Capability: Develop in-space Thermal Protection System (TPS) repair capability.
Repair capability is especially important for long-duration missions, where no safe-haven repair facilities will be available. TPS repair developed for Space Shuttle Orbiter TPS (reinforced carbon-carbon/tiles) should be continued and expanded to provide a repair method for future spacecraft, both NASA and commercial.
7. Thermal Sensors. Enhance thermal sensor systems and measurement technologies.
Operational instrumentation is necessary to understand anomalies, material or performance degradation and performance enhancements, as well as for advanced science mission measurements. Ultra-lightweight sensor systems may provide data needed to identify on-orbit damage, measurement of liquid levels in a microgravity environment, in situ or self-repairing mechanisms, or adaptive control algorithms that can compensate for damage without repairing. Accurate instrumentation to monitor reentry TPS performance is necessary to validate emerging predictive codes for heat shield design. Each of these would improve flight safety and the probability of mission success.
QFD MATRIX AND NUMERICAL RESULTS FOR TA14
The averaged quality function deployment (QFD) matrix for the nine level 3 technologies in TA14 is given in Figure Q.1.
The weighted scores for all level 3 technologies evaluated with the QFD approach are listed in Figure Q.2. 14.3.1 Ascent/Entry TPS received a much higher score than all other level 3 technologies, creating an obvious break point in assigning the high rating. 14.1.2 Active Thermal Control is a needed technology to support zero boil off of cryogenic fluids. Though 14.1.2 Active Thermal Control did not achieve the high rating of 14.3.1 Ascent/Entry TPS, it is considered an enabling technology for a wide variety of long-duration missions, and was thus also assigned high priority. These two technologies are therefore discussed at length. The other seven technologies were rated as “Medium” or “Low.”
CHALLENGES VERSUS TECHNOLOGIES
In Figure Q.3, the technologies are listed in descending priority on the vertical columns, and the challenges are shown in the horizontal top row. The correlation between the two is indicated by high correlation (solid symbols), weak correlation (open symbols) or little or no correlation (no symbols). It is seen that the challenges correlate to
FIGURE Q.1 Quality function deployment (QFD) summary matrix for TA14 Thermal Management Systems. The justification for the high-priority designation of all high-priority technologies appears in the section “High-Priority Level 3 Technologies.” H = High Priority; H* = High Priority, QFD Score override; M = Medium Priority; L = Low Priority.
FIGURE Q.2 Quality function deployment rankings for TA14 Thermal Management Systems.
some degree with the priorities of the technologies, as seen by the roughly diagonal pattern of high and moderate blocks.
HIGH-PRIORITY LEVEL 3 TECHNOLOGIES
Panel 5 identified two high-priority technologies in TA14. The justification for ranking each of these technologies as a high priority is discussed below.
Technology 14.3.1, Ascent/Entry TPS
Effective heat shields and thermal insulation during ascent and atmospheric entry are mission-critical for all robotic and human missions that require entry into a planetary atmosphere. This is an area that has suffered the loss of previous technology (from the Apollo era) due to the ageing and retirement of personnel. Newer safety and environmental regulations have also required changes to earlier TPS fabrication and formulation processes.
FIGURE Q.3 Level of support that the technologies provide to the top technical challenges for TA14 Thermal Management Systems.
Further, higher velocity re-entry for more advanced missions requires development of more TPS with higher limits on temperature, and radiative and total heat flux.
Current TRL is approximately 3 for all except LEO missions. New approaches such as using multiple layers with thermal protection gradients, inclusion of various additives (e.g., various nanotube or nanoparticle materials) to promote anisotropic conduction are below TRL 3, but show promise for improved performance.
NASA maintains the test facilities necessary for qualifying new systems, but the test facilities must be modified to incorporate high radiative heat fluxes to simulate conditions expected for high-velocity re-entry.
Other potential users are USAF and possibly commercial space developers.
There is little or no need for the Space Station in developing this technical area, although one could envision using the station as a base for preparing a high-Mach test re-entry mission using an accelerated return trajectory.
Ascent/Entry TPS is game-changing because it is necessary for every planetary atmospheric ascent and/or entry mission, including every mission for return to Earth. Because the necessary level of effort for developing appropriate TPS is high, a joint NASA-industry development and testing program with careful coordination would maximize efficiency for NASA.
Particularly critical level 4 technology items are Rigid Ablative TPS, Obsolescence-Driven TPS Materials and Process Development, Multi-Functional TPS, and Flexible TPS (crosscutting with TA09-EDL and TA12). Supportive are In-Space TPS Repair and Self-Diagnosing/Self Repairing TPS.
This assessment of 14.3.1 Ascent/Entry TPS as a high priority agrees with the TA09 report (Appendix L), which also lists Rigid TPS as a high priority.
Technology 14.1.2, Active Thermal Control of Cryogenic Systems
Low to zero boil-off of cryogenic fluids will be mission-critical for long-duration missions, and cannot be achieved with present technology. Active thermal control will enable long-term storage of consumables such as LOX for human missions, for cryogenic propellants for both human and robotic missions, for supporting lunar or planetary surface stations, and for supporting scientific instruments that require cryogenic conditions. Active control (recondensation) of cryogenic systems will be necessary to counter remaining heat leaks after effective passive thermal control technologies are applied. A goal of this technology is to develop an overall cryogenic system design that integrates active and passive technologies into an optimal system, as well as instrumentation and sensors to monitor fluid mass. Minimization of active system capacity through effective use of passive control should help increase overall system reliability.
Many level 4 technology items are proposed in the roadmap for active thermal control, but most provide incremental improvement over existing technology. Taken as a whole, they may provide significant reduction in boil-off rates. Most are at TRL 3.
NASA will be the de facto lead in guiding improvements in this technology because of the need for the technology for long-term missions although there are many potential users in non-NASA aerospace who can benefit. However, their needs are less critical to mission fulfillment; generally they can accept some loss rate, unlike NASA.
The Space Station can provide a platform for testing in actual conditions, although less costly Earth-based testing in cryogenic vacuum test chambers can be used for most initial testing.
The panel overrode the QFD score for this technology to designate it as a high-priority technology because the QFD scores did not capture the value of this technology in terms of its ability to enable a wide variety of long-duration missions. This technology generally received high scores because it is mission critical, but lower scores in some areas because the projected gains are incremental for many of the level 4 technology items.
Many of the proposed technologies are inter-related, and careful monitoring and systems integration possibilities should be developed to allow continuing support of those that appear most promising.
MEDIUM- AND LOW-PRIORITY TECHNOLOGIES
Five of the nine level 3 technologies in TA14 ranked medium priority (14.2.3 Heat Rejection and Energy Storage, 14.1.3 Systems Integration, 14.1.1 Passive Thermal Control, 14.3.3 Sensor Systems and Measurement
Technologies, and 14.3.2 Plume Shielding). The two remaining level 3 technologies scored low priority (14.2.2 Heat Transfer and 14.2.1 Heat Acquisition).
The Thermal Management Systems technologies that were ranked as medium and low priority are useful in supporting future NASA spacecraft and missions. These technologies apply to all or nearly all NASA and non-NASA space missions in all or most mission classes, but in a supporting role. These technologies can provide incremental improvements in overall thermal management system performance, but they do not appear to be mission critical or game-changing. 14.1.3 Systems Integration, 14.3.3 Sensor Systems and Measurement Technologies, and 14.1.1 Passive Thermal Control received medium ratings because the technology items are incremental improvements without breakthrough ideas. Passive Thermal Control is a necessary adjunct to 14.1.2 Active Thermal Control, which is listed as a high priority, and passive control improvements can reduce the capacity needed in the active systems, but by themselves cannot achieve the zero-boil-off goal.
If breakthrough ideas come forth in some of these medium- and low-ranked technologies, then they can be pursued more vigorously.
OTHER GENERAL COMMENTS ON THE ROADMAP
Software validation and the use of ground test facilities are two overarching, crosscutting issues pertinent to TA14 that are addressed in detail in Chapter 4.
Budgetary and staffing restraints make it impossible for NASA to carry out all of the tasks proposed in the roadmaps. It will be necessary to coordinate and cooperate with other organizations that can fund and carry out major parts of the research for their own purposes. NASA can then piggy-back on this work. However, it will be necessary to proactively interact with these organizations. Determining which organizations can best help NASA carry out its missions is a daunting task, and will require significant management effort.
The choice of tasks for direct NASA research support will depend to some extent on which tasks can be expected to be performed by others, making NASA research moot. However, there will always be areas where NASA needs will not correlate with external research agencies, and NASA would maximize its return on investment by focusing its funding support in these areas. Such areas as re-entry thermal protection systems for high-velocity re-entry, radiation shielding, reduced mass structures, low-temperature cryogenic radiators, etc. will probably require either internal research teams or funding for contracted work.
The Office of the Chief Technologist should continuously monitor progress on the technology items outlined in the roadmap for it to remain relevant. Some items will prove to be unfeasible; others will progress faster than expected, so priorities for support and funding will shift in out years. NASA does conduct such reviews, and is encouraged to continue and expand this oversight.
Many of the tasks could (and perhaps should) be combined. The draft roadmap breaks Technology 14.1.1 Passive Thermal Control into eight items: large-scale MLI, advanced MLI systems, multifunctional MLI/MMOD, ground-to-flight insulation, low conductivity supports, low conductivity tanks, in situ insulation, and low-temperature radiators. All of these items deal with minimizing heat leaks, and the research should be attacked as an overall systems problem rather than on a technology item-by-technology item basis.
TA14 is very interdependent with other research roadmaps, and coordination across these lines will require careful management to assure cooperation and avoid duplication. In particular, many of the TA14 technologies are dependent on or synergistic with TA01 (Launch Propulsion Systems), TA02 (In-Space Propulsion Systems), TA09 (Entry, Descent, and Landing Systems), TA10 (Nanomaterials), and TA12 (Materials, Structural and Mechanical Systems, and Manufacturing), and have significant interactions with the others.
PUBLIC WORKSHOP SUMMARY
The workshop for the TA14 Thermal Management Systems technology area was conducted by the Materials Panel on March 11, 2011, at the Keck Center of the National Academies, Washington, D.C. The discussion was led by panel chair Mool Gupta. Gupta started the day by giving a general overview of the roadmaps and the NRC’s task to evaluate them. He also provided some direction for what topics the invited speakers should cover in their presentations. After
the introduction, the day started with an overview of the NASA roadmap by the NASA authors, followed by several sessions addressing the key areas of each roadmap. For each of these sessions, experts from industry, academia, and/or government provided a 35 minute presentation/discussion of their comments on the NASA roadmap. At the end of the day, there was approximately 1 hour for open discussion by the workshop attendees, followed by a concluding discussion by the panel chair summarizing the key points observed during the day’s discussion.
Roadmap Overview by NASA
During the overview of the NASA TA14 roadmap, the NASA team noted that they had a large trade space to cover, ranging from milliwatt cryogenic systems to zero boil-off (ZBO) to crew/vehicle thermal management and more. For these reason, they split the roadmap into three primary categories based on temperature: Cryogenic Systems for temperatures less than -150°C, Thermal Control Systems for temperatures between -150°C and a few hundred degrees C above zero, and Thermal Protection Systems for temperatures above several hundred degrees C. The NASA team also noted how they tied their roadmap to the NASA Strategic Goals and Agency Mission Planning Manifest, as well as utilizing the design reference missions from the NASA Human Exploration Framework Team (HEFT) efforts.
In terms of the top technical challenges, the NASA team categorized these into different categories based on timing:
— Mid-density ablator materials and systems for exo-LEO missions (>11 km/s entries)
— Innovative thermal components and loop architecture
— 20 K cryocoolers and propellant tank integration
— Low conductivity structures/supports
— Two-phase heat transfer loops
— Obsolescence driven TPS materials and processes
— Supplemental Heat Rejection Devices (SHReDs)
— Hot structures
— Low-temperature/power cryocoolers for science applications
— Inflatable/flexible/deployable heat shields
The NASA team also indicated that this roadmap is crosscutting with several others, and that they had discussions with the teams for TA6, TA9, TA11, and TA12. The NASA team also described how many of these technologies can provide a benefit to NASA:
• Reduced mass
— 20 K cryocoolers with 20 W capacity
— Single-loop thermal control systems (elimination of interface heat exchanger)
— Supplemental Heat Rejection Devices (both vehicle and EVA)
— Large-scale multi-layer insulation (MLI) and low conductivity supports and tanks for cryogenic systems
• Increased reliability
— Single-loop thermal control systems
— Reliable heaters/controllers reduce multiple strings
• Improved performance
— 150 W/cm2 flexible TPS
— Liquid metal heat pipes
— Two-phase flow systems for tight temperature control
Additionally, the NASA team highlighted how these technologies could benefit the energy, construction, environmental, and automotive sectors.
During the discussion following the NASA team’s presentation, there was some discussion of the types of collaboration going on between different groups regarding the technical properties of mid-density ablators, flexible heat shields, and multifunctional systems. On the latter in particular, some follow-on comments from the NASA team were that for multifunctionality incorporating radiation protection, it is important to make sure material properties are not degraded by the multifunctionality. One workshop participant asked the NASA team for their views on the near- and long-term impacts of nanotechnology in thermal management. The response was that thermal straps and phase change materials are areas where mass savings may be realized with carbon nanotubes; in general, though the NASA team indicated that many of their thermal technologies could benefit from nanotechnology. Additionally, there was some discussion on radiators, in particular related to MMOD impacts, redundancy, and reliability (e.g., it was commented that the reliability for the ISS was 0.9999 over 10 years of life).
Regarding technologies near a “tipping point,” the NASA team indicated that many of their technologies are in the TRL 4-5 range and have already experienced small on-ground demos, and that the next step is to flight test some of these. When one of the workshop participants asked about the kinds of flights required, the NASA team responded that identifying push missions has been difficult, and that some technologies might benefit from using the ISS or suborbital vehicles, while others (e.g., cryocoolers) can be advanced with additional ground testing for integration and other aspects.
Finally, there was substantial discussion on facilities. One of the workshop participants commented that while he understood that the NASA team was asked not to address facility issues in their roadmap, advancing some technologies to higher TRLs (e.g., mid-density ablators) requires facilities that do not exist. In responding to his question on what NASA was doing about this, the NASA team responded that there have been two teams looking at arcjet facilities initially, but that this has now morphed into an oversight/implementation group. In general, though, one NASA team member indicated that if demand/throughput is not there, than sustaining the business case for these facilities is difficult, and each NASA center is struggling with this. Another team member concurred, and supplemented that you need to have assured capability, facilities to test in relevant environments, and throughput. Based on the agency risk posture, this team member noted that having multiple facilities spreads out the risk, while also allowing different physics to be investigated at different locations.
Session 1: Cryogenic Systems
Ray Radebaugh (NIST, retired) provided a presentation on his experiences and views on cryogenic systems. He started with an overview of the benefits/applications, which in particular for NASA he highlighted as densification (e.g., liquefaction and separation), quantum effects (e.g., fluids and superconductivity), and low thermal noise (e.g., for sensors). Regarding insulation materials, Radebaugh highlighted the need to investigate ways of reducing the mass of multi-layer insulation (MLI), as well as noting that while foams and aerogels may be lower cost, they might not be as effective as MLI. For radiators, he showed a graph indicating how there is a lower temperature limit to radiating in space, and that development is needed to get to lower temperatures. Radebaugh then talked about active thermal control, and how it is important to look for ways to reduce the specific power, mass, and vibration for 20 K cryocoolers. He also provided examples of how Hubble uses Turbo-Brayton cryocoolers and Plank uses Joule-Thomson cryocoolers, and that investments could allow cryocoolers for scientific instruments to work with higher input temperatures. He also noted that Turbo-Brayton designs need to move away from using Neon to other fluids, and that pulse-tube cryocoolers require improvements in performance and efficiency. Radebaugh later observed how the NASA roadmap did not appear to address using O2 and CH4 (useful for ISRU) for high-power liquefiers, and that there some trade space exploration is required between low-temperature radiators and active cooling. Another area he felt the roadmap did not address was the need for thermal expansion matching over wide temperature ranges (e.g., matching materials). Other gaps in the roadmap identified by Radebaugh included cryogenics for zero boil-off and liquefaction applications, as well as a technology path for cold compressors. The top technical challenges that Radebaugh highlighted included reducing the mass of cryocoolers, increasing cryocooler
efficiency, lightweight insulations in a wide range of atmospheres, flexible radiators, and heat transport over long distances. Radebaugh concluded his presentation noting that NASA is good in performing overall system studies, and has expertise in specific areas as well; he also suggested utilizing expertise at other groups (e.g., USAF, NIST, private industry).
After his presentation, Radebaugh was asked about his thoughts on the importance of thermal interfaces. Radebaugh responded that many of the applications of interest (e.g., cryocoolers) do not generate much heat, but that for some applications heat spreaders or similar items may be required to transfer heat into the system. Radebaugh was also asked about cryocooler vibration, and whether this is an aspect of compressor design. In this case, Radebaugh indicated that vibration can be a significant issue for space observatories. He noted that the Hubble Turbo-Brayton cryocooler is a rotary system with very low vibrations, but that pulse-tube and Stirling cryocoolers may have reciprocating parts that may generate vibrations, and that typically space applications will use multiple pistons to damp these vibrations.
Session 2: Thermal Control Systems and Modeling/Simulations
The session on Thermal Control Systems started with a teleconference discussion with David Gilmore (The Aerospace Corporation). A workshop participant asked Gilmore about his observations on the roadmap. Gilmore indicated that the roadmap was largely consistent what he had seen in NASA centers and DOD, and that many of the technologies outlined seemed to be applicable to DOD as well. On the other hand, Gilmore noted that there appeared to be gaps, including (1) the need for ultra-reliable thermal management, which is required for deep space missions and will drive thermal design; structural-thermal-optical analysis codes, because faster, integrated codes could reduce analysis cycle times from months down to much lower durations and (2) science applications, because many scientific missions have unique thermal requirements (e.g., thermal stability requirements in order to maintain the sensitivity on future decadal survey space observatories, and techniques for thermal balance testing of large passive cryogenic observatories). Gilmore responded to a question about which decadal survey missions might be enabled by these technologies, and indicated that a Venus lander might require some of the insulation and phase change technologies, as do probes to Jupiter; he also noted that applications such as terrestrial planet finding and imaging require significant cryogenic technologies. Gilmore then suggested that a focus on how widespread the utility is might help in prioritizing technologies. For example, he noted that radiators see wide usage. Other high-payoff technologies he discussed included two-phase pump loops (e.g., enabling for high-power space systems), and advanced pumps (both low- and high-power applications). Another participant asked about reverse cooling for habitats, to which Gilmore responded that there does not appear to be much research on this, and that in general the design philosophy is to keep things as simple as possible for reliability. When asked to comment on the state of the art and future directions for MLI and insulations, Gilmore noted that this area is important for science missions. He also commented that today these materials are custom made for each application, and that simplifying the process of building and installing insulations could lead to cost savings on missions. Finally, there was some discussion of moving absorbance down to 0.01. Gilmore indicated that while this is desirable, it is uncertain how far down this can go. He noted that coatings with low absorbance can be developed, but then methods to keep the materials/coatings clean are also required (e.g., lotus coatings to minimize dust issues). There was some discussion that these coatings can make radiators smaller, and have the potential to reduce mass and cost for missions.
Next, Robert Moser (University of Texas at Austin) provided a presentation on “Modeling and Simulation: Verification, Validation, and Uncertainty Quantification.” Moser started out by taking several quotes from the NASA roadmap text, noting that many statements deal with modeling and simulation. He indicated that modeling and simulation is important, as it is used to develop science-based predictions to support decision making. He also suggested it is also valuable when experiments in specific regimes cannot be performed, but then this leads to the need to understand the uncertainty in modeling. Moser mentioned that the roadmap appears to pay minimal attention to quantifying or improving the reliability of computational predictions, and that this may be a gap. Next, Moser talked about uncertainty quantification, and the need for verification and validation. He said that code verification is critically important, but generally not given enough attention; some methods he suggested include good software practices, doing end-to-end testing of models, and potentially using manufactured solutions. On the validation side,
Moser indicated that there are several needs, including: math models for uncertainty, algorithms and software tools for computing with uncertainty, and characterization of experimental uncertainties. He presented an example of a heat flux gage where he was able to get an idea of the uncertainty in the measurements of that system. Another example he provided was on the NASA Orion vehicle, where uncertainty quantified in results lets NASA make more rational decisions regarding margins in the system design. Moser concluded with several recommendations: (1) ensure that rigorous code validation is applied to computational simulations; (2) develop modeling software with modern a posteriori analysis and adaptivity; (3) develop/adopt formulations and software tools for uncertainty quantification; and (4) tightly integrate physical modeling, uncertainty analysis and experimental programs to ensure reliable uncertainty assessments. During the discussion following the presentation, Moser was asked by a workshop participant how one deals with the absence of some physics. Moser responded that this is a challenge, and that in general you calibrate with the data available to the extent that you can do so. When asked about his comments on the NASA roadmap, Moser indicated that what he thought was missing was defining what is needed to simulate, and how it should be done. He commented that obtaining data to quantify uncertainty and reliability calculations can be difficult. Finally, responding to another question on the role of modeling and simulation as part of the design process, Moser suggested that in some situations modeling and simulation might be used to provide confidence in the system to be fielded.
Session 3: Thermal Protection Systems
The session on Thermal Protection Systems started with a presentation by one of the panel members, Don Curry (Boeing). Curry started with a table showing historical thermal protection system (TPS) mass fractions for several human-rated vehicles. In general, this was on the order of around 10 percent. Curry provided some discussion on different ablative materials, including the Apollo AVCO and Ames’ PICA materials. For carbon phenolic TPS, Curry noted that in many cases this is the only feasible TPS material for specific missions, yet the difficulty obtaining aerospace-grade Rayon is a significant issue for future missions. In terms of TPS testing, Curry mentioned how reusable TPS materials have had mission lifespans quantified via arcjets and other testing. He also provided some data on AVCO used for Apollo; in this case thousands of hours of testing were performed, along with multiple facilities. Curry noted that this was all necessary in order for the material to be available in time. Curry discussed the importance of testing, noting that the final design values for many properties (e.g., thermal conductivity of char) come from arcjet testing; likewise understanding material properties such as compression, shear, etc., is required. Related to TPS design, Curry highlighted many important considerations, including: aerothermal environment, strength/stiffness, thermal gradients, venting characteristics, outgassing, space environment, damage tolerance, repairability, and refurbishment. Finally, Curry concluded by emphasizing that test facilities are important to TPS development, and that eliminating facilities will lead to significantly increased risk. After his presentation, Curry was asked why Orion did not use PICA, as it has a low density but high heat of ablation. Curry’s response was that PICA is a tile system, and can potentially crack due to tension in the structure (there are also typically gaps between tiles to account for this). Curry also noted that in the past, PICA has had problems with shockloads during separation pyros.
Next, Bill Willcockson (Lockheed Martin) gave a presentation on TPS materials. Starting out his discussion talking about past experience in robotic missions, Willcockson noted that Viking had hundreds of tests (potentially up to a 1,000), and that tests might be a good metric relative to human missions. For Jupiter/Venus entry (e.g., 10,000 W/cm2 class), he noted that carbon phenolic can no longer be made, and that these types of materials cannot be tested without arcjet facilities. Relative to affordability, he commented that PICA is three times the cost (process-wise) versus SLA-561V, and that Lockheed Martin has been developing new materials such as MonA to address this. He noted that while SLA-561V was developed in the 1970s, it is important to keep older technologies like this up to date to avoid obsolescence issues. Regarding flexible materials, Willcockson suggested that these are at relatively low TRLs and maturing slowly. During his wrap-up, Willcockson highlighted the importance of investment in TPS; in particular: (1) the need for a resurrection of carbon phenolic, which may require rebuilding facilities as well; (2) the need for a mechanism to take advantage of experienced folks at large companies (e.g., similar to the SBIR program); (3) continued funding to maintain existing TPS materials; and (4) NASA program
support needed to offset arcjet costs. Willcockson also noted that the NASA roadmap does not mention in-flight instrumentation use, but that this was done on Pathfinder and is being done on MSL because funding was available. After Willcockson’s presentation, a workshop participant provided an additional comment that shock loads can force design changes (e.g., the use of RCC instead of tiles for attach points on the space shuttle), and that testing and modeling need to go together.
Chris Mangun (CU Aerospace) next provided a presentation with a materials perspective on the NASA roadmap. For rigid ablative TPS, he noted that PICA is the current state of the art, but posed the question on whether it will work for the next generation. Mangun noted that for reentry with high heating rates, the thermosetting resin must char, and outgassing of resin is advantageous, as it thickens the boundary layer and reduces heat flux. He listed several desired TPS properties, including low thermal conductivity, high heat of ablation, mechanically tough— not brittle (i.e., resin must adhere well to reinforcement)— and monolithic construction (i.e., avoiding tiles). He provided some discussion on aromatic thermosetting copolyesters, and noted several benefits and potential future applications of this material. Another topic that Mangun commented on was the use of AlB2 as a planar reinforcement for metal matrix composites (MMCs). Regarding self-healing materials for applications such as micrometeoroid and orbital debris protection, structural recovery, and self-sealing cryotanks, Mangun noted that dual-microcapsule systems in composites are one option; he also mentioned that new microvascular approaches can continuously deliver healing agents. (Note that a microvascular network in a structural composite can also introduce dynamic, reconfigurable functionality, such as damage sensing, thermal management, and radiation protection.) Mangun concluded his presentation noting that it may be possible to accelerate some technologies (e.g., multifunctional TPS, structurally integrated TPS, and self-repairing composites).
Public Comment and Discussion Session
The following are views expressed during the public comment and discussion session by either presenters, members of the panel, or others in attendance. (Note that due to an early end time for the last day of the workshop, there was limited time available for the public discussion period.)
• Roadmap funding assumptions. A participant asked the NASA team what the funding assumptions were, as the roadmap lists timeframes to specific TRL numbers for some of the technologies. The NASA team responded that there was no guidance on this, but in general they asked their staff to develop the details of each technology development assuming a “reasonable” funding profile.
• Importance of dual-use technologies. One workshop attendee posed the question on how much importance NASA puts on dual-use of the technologies, i.e., the applicability for a technology to benefit others outside NASA. The NASA team responded that while they are always looking for potential spinoffs, that will not drive the development of a specific technology.
Jamshid, A., Samareh, J.A., and Komar, D.A. 2011. Parametric mass modeling for Mars entry, descent and landing system analysis study. AIAA Paper 2011-1038. 49th AIAA Aerospace Sciences Meeting, Orlando, January 4-7, 2011. American Institute of Aeronautics and Astronautics, Reston, Va.
McGuire, M.K., Arnold, J.O., Covington, M.A., and Dupzyk, I.C. 2011. Flexible ablative thermal protection sizing on inflatable aerodynamic decelerator for human Mars entry descent and landing. AIAA Paper 2011-344. 49th AIAA Aerospace Sciences Meeting, Orlando, January 4-7, 2011. American Institute of Aeronautics and Astronautics, Reston, Va.
Venkatapathy, E. 2009a. Thermal Protection System Technologies for Enabling Future Sample Return Missions. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C.
Venkatapathy, E. 2009b. Thermal Protection System Technologies for Enabling Future Outer Planet Missions. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C.