•   A 200 volt spacecraft power system, and

•   A long-life/high-power thruster.

Al Herzl (Lockheed Martin) discussed how past technological advances are often made to directly support an identified mission need. He presented numerous examples, such as the advances in hydrogen propellant operations that were achieved during the development of the space shuttle external tank. He suggested that it is important to identify the technologies actually needed by future NASA missions, seek out adjacent commercial markets, and have those projects invest in the technology. At the same time, he indicated that projects should only be approved when required technologies are mature. He then reviewed his proposed list of near term technology needs:

•   High-pressure, low-mass systems,

•   Autonomous and integrated health management,

•   Long-life cryogenics storage and refueling capabilities

•   EP, and

•   Development tools.

Jim Berry (Northrop Grumman) emphasized the need to prioritize technology investments based on benefit and cost impacts to reference missions for both exploration and science. He said NASA can only afford to invest in a limited number of technologies, and the selection of which technologies to fund should be based on approved missions. Once NASA has committed to conduct a particular mission, Berry suggested that NASA should stick with those decisions and carry them through. He also proposed choosing suites of technologies that work well together, and he urged NASA to establish technology backup options when a preferred technology is particularly risky. Berry also saw value in improving the operational lifetime of cryogenic engines and their supporting systems, including long-term in-space storage and propellant transfer. For EP systems he suggested that advances in power processing units and radiators will be keys for high-power systems. Berry also observed that small flight experiments can demonstrate the potential to operate at larger scales while validating the small systems.

The group discussion spent much of its time focused on cryogenic storage. Some speakers suggested that long-term in-space cryogenic storage should be addressed as a systems problem. One participant stated that a flight demonstrations should validate models before architectures with cryogenic storage move forward. Several speakers asserted that cryogenic fluid transfer will be required for almost any future human exploration beyond LEO. One participant questioned the value of high-power SEP in an era with low flight rates.

Session 4: In-Space Advanced Concepts

Terry Kammash (University of Michigan) started the session on advanced concepts with a discussion of fusion-based propulsion. He presented four paths that might be used to realize a fusion-based system, all of which involve technologies that are much less mature than the propulsion technologies discussed in the earlier sessions.

Rob Hoyt (Tethers Unlimited) presented an overview of three potential uses for space tethers:

•   Electrodynamic tethers: a current is applied along the tether which interacts with Earth’s magnetic field, imparting a force without using propellant.

•   Momentum exchange tethers: enable the kinetic energy of one spacecraft to be transferred to another.

•   Formation flying: tethers join multiple spacecraft that need to fly in a tight formation.

Kammash presented multiple operational scenarios using tethers. For example, a tug could boost payloads using momentum exchange tethers, and then the tug could restore its kinetic energy using an electrodynamic tether. Kammash also observed that tether flight tests had experienced a 70 percent success rate; all the failures were caused by engineering problems, not unexpected physics problems. He said that an operational demonstration of electrodynamic tethers is possible in the near term; demonstration of momentum exchange would be a long-term project.

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