Feeling the “Burn” of Solar Thermal Propulsion

Integrated Solar Upper Stage Concept Credit: NASA

Integrated Solar Upper Stage Concept
Credit: NASA

For a brief period in the 1990’s, Solar Thermal Propulsion appeared to be on the cusp of becoming the next big thing in space technology. While that obviously has not happened yet, a recent NASA Future in Space Operations (FISO) presentation from Marshall Space Flight Center suggests it may still be an approach whose time will come.

Solar thermal propulsion is in many ways the simplest form of in space propulsion imaginable, with the possible exception of solar sails. The sun’s energy is harnessed and concentrated by a form of lens or mirror, and then focused on a chamber containing a working fluid, which can be nearly anything from hydrogen to water. The result, at least in terms of ISP, or specific impulse can be impressive. Whereas the Space Shuttle Main Engine (SSME) now being repurposed for the Space Launch System operates at a very repectable ISP of 452 seconds in vacuum, a solar thermal engine using gaseous hydrogen propellant (no oxidizer is required) can deliver performance twice that number.

There are drawbacks of course, with the principle one being that the level of thrust generated is only a tiny fraction of that produced by the SSME. In this respect STP is similar to another form of in-space propulsion which also gained popularity in the 1990’s, but then went on become mainstream technology now in widespread use.  From commercial satellites such as Boeing’s all electric models, to NASA’s Dawn spacecraft now in orbit around the dwarf planet Ceres, solar electric, or ion propulsion, is changing the way we think about moving automated spacecraft.

Might STP one day do the same?

Whereas solar electric engines such as the 30 cm ion thrusters powering NASA’s Dawn spacecraft deliver outstanding ISP as 3100 seconds, the thrust level, at 91 Millinewtons, or .02 lbf, is barely perceptible, leading to highly efficient but lengthy trip times.

Solar thermal offers a different sort of tradeoff, one that may not be very applicable today, but could come to occupy a valuable niche in the future. In the 1990’s as the Space Exploration Initiative was closing down, engineers at the Marshall Space Flight Center tested a near flight weight STP engine which generated .5 lbs of thrust in a high altitude vacuum chamber. The test, labelled STT-1, examined what is called a Direct Gain engine, which only works when exposed to sunlight. The alternative, labelled a Storage engine, captures and retains heat to energize the working fluid even when a spacecraft is in Earth’s shadow, but offers considerable lower average ISP. Notably, as spacecraft move into higher orbits, solar exposure increases, yielding increased efficiency over time.

Results from the STT work suggested there was quite a bit more to learn about how to maximize the heat transfer across the entire thrust chamber, but did demonstrate a core concept of using the natural boiloff of liquid hydrogen to feed the engine, eliminating the need for pumps or additional insulation. That was as far as it got however, as the opportunity to move beyond STT-1 to an actual in-space test was lost with the end of SEI.

Deploying an Inflatabe Antenna During STS-77 Credit: NASA

Deploying an Inflatabe Antenna During STS-77 Credit: NASA

At least one component of solar thermal propulsion has been demonstrated in orbit.  On STS-77, NASA successfully tested a lightweight, inflatable thin-film antenna as part of the L’Garde experiment. What is an inflatable antenna to one engineer can be mirror/concentrator to another. (and perhaps a solar sail to a third).

One of the reasons STP never really took off may have been that engineers at the time were looking at the wrong application, focusing on alternative upper stages for payloads which had been launched into low Earth orbit by conventional boosters such as the Delta II. In those cases, analysis indicated that the efficiency gains in ISP were offset by the increased mass and larger payload fairings required to house the hydrogen propellant, limiting any incentive to further develop the technology. Given the performance increases subsequently offered by newer boosters, and now augmented by solar electric propulsion on spacecraft themselves, there is even less reason to consider STP for satellite launch today.

Tomorrow may be a different story however. Where STP really could shine is in applications which don’t require the power, expense and complication of chemical combustion engines, but still need a higher thrust level than can be achieved with solar electric engines. The key, as with so much else, lies in developing In-Situ Resource Utilization (ISRU) to harvest propellants from local materials  and eliminate the need to haul literally everything uphill out of Earth’s gravity well.

 

Posted in: Advanced propulsion

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