by Wesley Johnson, NASA Glenn Research Center, wesley.l.johnson@nasa.gov

The currently stated exploration plan for NASA includes towing an asteroid into lunar orbit and having astronauts visit it there to gain experience with new technologies and methodologies for human exploration of the solar system [1]. While this mission does not need the super high thermal performing insulation of a Martian mission (where the large quantities—hundreds of cubic meters—of cryogenic propellants must be stored with minimal loss for up to nine months), it does need more than the spray-on foam insulation that has been applied to the Saturn V space shuttle’s external tank and multiple expendable launch vehicles [2, 3]. As such, NASA has developed a plan to develop multilayer insulation (MLI) at a level it can be engineered for large spacecraft and upper stage mission durations between several hours and several days.

While MLI for large in-space cryogenic applications has a few differences from most applications (namely that the mass of a vacuum jacket is unacceptable and thus the MLI must be protected from the aerodynamic and heating environments experienced during launch in other ways and must also have more provisions for rapid venting), many of the thermal concerns are very similar to terrestrial applications.

This development is currently bookkept under the Evolvable Cryogenics (eCryo) Technology Demonstration Mission. Known as the Improved Fundamental Understanding of Super-Insulation (IFUSI), this task will analyze both numerically and experimentally the design details of insulation systems. Figure 1 gives an overview of various design details being investigated. While these details have all been investigated to some degree, the data is in most cases sparse and scattered throughout the literature.

Multiple sources over the years have investigated the bulk or ideal thermal performance of insulation systems, and this is not the main goal of IFUSI. In 2012, W. L. Johnson, A. O. Kelly and K. M.
Jumper investigated the integration of small penetrations such as fill lines, engine feed lines and vent lines [4]. Similarly the effects of integrating large penetrations such as structural skirts will need to be understood as those are the desired structural solutions for existing and planned upper stages. These structural “skirts” go around the circumference of flight tanks up to 8.4 m in diameter and transfer the structural loads from the rocket through the stage tank. The skirts can then also act as radiative fins on orbit and must be insulated to minimize the heat load into the cryogenic tank.

Every insulation system is structurally held together by a combination of tape, pins, Velcro and other attachment methods. The thermal penalties of these, while often individually small, can add up for large MLI blankets. Taping every layer to itself in a temperature-matching type fashion has been shown to be an effective solution [5]; however, it would be very labor-intensive when installing a system on an 8-m-diameter stage. Thus it is expected the blankets will need to be prepackaged in panel-type sections for installation. Additionally, these attachment mechanisms must survive any structural and vibration loads they might encounter during launch.

Various specific seams have been studied [6], but beyond a single butt seam [7, 8, 9], no effort has been made to understand complicated seams. Most types of seams that are used today on insulation systems are difficult to analyze due to the anisotropic thermal properties of multilayer insulation. Practical experience has shown that staggering seams (whether butt joints or overlapped joints) can reduce the heat load, but it has not been experimentally pursued. A closed form analytical solution is probably not achievable; however, with a combination of experiments and system analytical models, it is hoped that simplified form solutions for staggered seams can be obtained.

Due to the expense of thermal vacuum testing, MLI data to date has always been a “one-off” type test. While some vendors have established general ranges of performance of their specific insulation scheme based on experience, these ranges have not been experimentally established. Repeatability of identical systems installed multiple times is necessary for statistical performance predictions of the insulation system.

The ultimate goal of the IFUSI task is to provide a method for predicting the heat load of the insulation for testing on the Structural Heat Intercept-Insulation-Vibration Evaluation Rig (SHIIVER). SHIIVER is envisioned as a 4-m-diameter test article with representative fluid and structural attachments that might be seen on a cryogenic propulsion stage. It is currently being designed by NASA, with thermal vacuum and vibro-acoustic testing planned for 2018. SHIIVER’s objective is to demonstrate various heat intercept and insulation concepts that improve the performance of short-duration mission cryogenic propulsion stages. Detailed information from the IFUSI effort will open the design space for the insulation blankets for the SHIIVER test effort.

References

1. National Aeronautic and Space Administration, “Asteroid Redirect Mission,” [online]. Available: http://www.nasa.gov/mission_pages/asteroids/initiative/index.html. [Accessed 21 August 2015.]
2. J. E. Fesmire, B. E. Coffman, B. J. Meneghelli and K. W. Heckle, “Spray-on Foam Insulations for Launch Vehicle Cryogenic Tanks,” Cryogenics, Vol. 52, pp. 251-261, 2012.
3. J. S. De Kruif and B. F. Kutter, “Centaur Upperstage Applicability for Several-Day Mission Durations with Minor Insulation Modifications,” in AIAA 2007-5845, 2007.
4. W. L. Johnson, A. O. Kelly and K. M. Jumper, “Two Dimensional Heat Transfer around Penetrations in Multilayer Insulation,” National Aeronautics and Space Administration, Kennedy Space Center, FL, 2012.
5. W. L. Johnson and J. E. Fesmire, “Cryogenic Testing of Different Seam Concepts for Multilayer Insulation Systems,” in Advances in Cryogenic Engineering, Vol. 55, Melville, NY, Plenum Publishers, 2010, p. TBD.
6. I. E. Sumner, “Degradation of a Multilayer Insulation Due to a Seam and a Penetration,” Lewis Research Center, Cleveland, OH, 1976.
7. R. B. Hinckley, “Liquid Propellant Losses During SpaceFlight, Final Report,” Arthur D. Little, Inc, Cambridge, MA, 1964.
8. Q. S. Shu, “Systematic study to reduce the effects of cracks in multilayer insulation. Part 1: theoretical model,” Cryogenics, Vol. 27, No. 5, pp. 249-256, 1987.
9. Q. S. Shu, R. W. Fast and H. L. Hart, “Systematic study to reduce the effects of cracks in multilayer insulation Part 2: experimental results,” Cryogenics, Vol. 27, No. 6, pp. 298-311, 1987.