Engineering the Oberth Maneuver

by | Mar 3, 2022 | Missions | 23 comments

As we saw recently with the analogy of salt grains for stars, the scale of things cosmic stuns the imagination. But we don’t have to go to galactic scale. We can stay much closer to home and achieve the same effect. Because at our current technological levels, getting even as far as the outer planets taxes our capabilities. The least explored types of planet in our Solar System are the dwarf worlds, places like Ceres, Pluto and Charon, not to mention the enigmatic Triton. It takes years to reach them.

Beyond these objects we have a wide range of other dwarfs that merit study, at distances that push us ever farther. In a description of their NIAC Phase I study, just announced as a selection for 2022, Jason Benkoski and colleagues at Johns Hopkins University look into a combination heat shield and solar propulsion system that would perform a close Solar pass and use the Sun’s gravity to slingshot outwards at the highest possible velocity. It’s a maneuver familiar to Centauri Dreams readers, and one recently examined by the Interstellar Probe team at JHU’s Applied Physics Laboratory.

Benkoski is a materials scientist who has been working with the APL team, envisioning a tight solar pass around the Sun followed by the firing of a thruster to enhance the craft’s acceleration. This will require the probe to move within 1.6 million kilometers of the Sun’s surface, actually four times closer than the Parker Solar Probe plans to reach by 2025. In a 2021 article in Johns Hopkins Magazine, Benkoski explained the concept, which will preserve the heat shield by using channels filled with hydrogen gas that are built into the bulk of the shield itself. As the article puts it:

During the probe’s searing slingshot around the sun, the gas would heat up, expand, and course through the channels that all lead to a single exhaust nozzle. “The idea is to absorb all this heat with hydrogen,” Benkoski says, “and shoot it out the back of the probe.” In this way, the cooling setup also opportunistically doubles as an engine, thus supplying the thrust needed to complete the Oberth maneuver in the first place. “It’s like hitting two birds with one stone,” Benkoski says.

Image: Graphic depiction of combined heat shield and solar thermal propulsion system for an Oberth maneuver. Credit: Jason Benkoski.

The team believes that advances in materials science and engineering make their solar thermal engine concept a workable model for development. The 20 x 20 cm prototype they designed and fabricated is at benchtop scale, using liquid helium as coolant and propellant. The new study will extend this work, taking the concept into the realm of realistic materials and propellants. No small challenge, that, given that the contemplated Oberth maneuver would subject the probe to temperatures of 2500 degrees C, enough to melt even the Parker Solar Probe’s heat shield.

Benkoski points out that neither of our Voyagers was designed for observing the interstellar medium through which it now passes, while of course the Pioneers have long since ceased to function. New Horizons remains thankfully robust but will ultimately succumb to dwindling power levels and lose communications with Earth. The numbers are daunting: The Voyagers managed 3.6 AU per year, while even a full-stack SLS (which will never fly this mission) would push a 1 tonne spacecraft only to 8 AU per year.

The latter would require not just a working SLS but a Jupiter gravity assist, limiting the fly-out direction of our probes. Hence the need for a solar Oberth maneuver, in Benkoski’s thinking, which would be capable of surviving temperatures of 2800 K and use propellants now under study to widen the range of potential mission targets:

We…therefore propose a full trade study of alternate propellants in order to determine the maximum escape velocity for a given total system mass, including spacecraft, heat shield, propellant storage, and attitude control system. The main propellants of interest include H2, LiH, Li, CH4, NH3, and H2O. Methods: First we would determine material compatibility for each propellant with respect to its proposed storage system. We then calculate the efficiency (specific impulse) as a function of temperature for each propellant using Chemical Equilibrium Analysis (CEA).

Benkoski intends to discover how the mass and storage volume of the tank scale with the quantity of propellant to produce a series of realistic tank designs, devising an equation for the heat shield area and maximum propellant fraction that can be achieved given the limitations of existing heavy boosters. We’ll see how this study fares in producing a full-scale heat shield/heat exchanger design with robust long-term cryogenic storage. A tight Oberth maneuver is not going to be easy. See Assessing the Oberth Maneuver for Interstellar Probe for some of the myriad reasons why.

tzf_img_post

Lowering the Laser Barrier

by | Mar 1, 2022 | Sail Concepts | 5 comments

The continuing release of papers related to or referring to the Breakthrough Starshot sail concept is good news for the entire field. Interstellar studies as an academic discipline has never had this long or sustained a period of activity, and the growing number of speakers at space-related conferences attests to the current vitality of starflight among professionals and the general public alike.

Not all interstellar propulsion concepts involve laser-beaming, of course, and we’ll soon look at what some would consider an ever more exotic concept. But today I’m focusing on a paper from Ho-Ting Tung and Artur Davoyan, both in the Mechanical and Aerospace Engineering Department at UCLA. You could say that these two researchers are filling in some much needed space between the full-bore interstellar effort of Breakthrough Starshot, the Solar System-oriented laser work of Andrew Higgins’ team at McGill, and much smaller, near-term experiments we could run not so far from now.

Of the many potential show-stoppers faced by a mission to another star at our stage of development is the need to develop the colossal laser array envisioned by Starshot. The Higgins array is at a smaller scale, as befits a concept with nearby targets like Mars. What Tung and Davoyan envision are tiny payloads (here they parallel Breakthrough), some no more than a gram in mass, but the authors push the sail with a 100 kW array about a meter in size. Compare this with Breakthrough’s need for a gigantic square-kilometer array of 10 kW lasers with a combined output of up to 100 GW.

Image: In this illustration, a low-power laser (red cone) on Earth could be used to shift the orbit (red lines) of a small probe (grey circle), or propel it at rapid speeds to Neptune and beyond. Credit: Ho-Ting Tung et al.

The UCLA work takes us to a consideration of operations with spacecraft in Earth orbit as well as payloads sent on interplanetary trajectories. Thus we are in the realm of the kind of missions that today would demand chemical or electric propulsion, and we are looking at a system that might be used, for example, for orbital adjustment of Earth satellites after launch, or in the case of chip-class payloads, interplanetary missions with surprising velocities, up to 5 times that of New Horizons. As noted, the needed laser aperture is, by the standards of the missions we’ve discussed earlier, small:

…a sail with w = 1 m would require a laser with an aperture D ? 26 m (compare with the 30 m diameter primary mirror of the Thirty Meter Telescope under construction). However, we stress that most practical scenarios are limited to low and medium Earth orbits that require a much shorter operation range (z ? 1000km), and therefore a significantly smaller laser array.

Indeed, an array a meter in size could be efficient, maneuvering small satellites in Earth orbit, or being used to bring small chip-craft up to Solar System escape velocity. Thus we have the potential to create laser propulsion experiments and missions with array powers of ? 100 kW and array sizes that do not require kilometers of desert for their construction. Payloads can range from 1 to 100 grams depending on the mission, though the focus here is wafer-scale, on the order of 10 centimeters.

As to sail materials, the authors calculate that for maximum reflectivity coupled with rapid cooling, silicon nitride and boron nitride are the materials of choice:

Broadband spectral emissivity of silicon nitride…results in a better heat rejection (i.e., lower temperature) as compared [to] narrow band BN thermal emitters. However, boron nitride being lighter than silicon nitride allows design of very light-weight light-sails, which eventually translates onto higher velocity gain, ?v.

The paper offers possible ways to create these structures, including using metamaterials formed into nanostructured architectures with nanometer-scale ‘sandwich’ panels between material layers, or using ‘micro pillars’ within the photonic structure.

The broader picture is that we’re mapping out how to experiment with lasers and materials that may begin moving up the ladder of mission complexity. There are innumerable issues to be overcome, but the early theoretical work is crucial to making what may become an interplanetary infrastructure a reality. These examinations should also feed into the ambitious work on projects that aim at interstellar missions.

The paper is Ho-Ting Tung et al, Low-Power Laser Sailing for Fast-Transit Space Flight, Nano Letter,” Nano Letters 22, 3 (31 January 2022), 1108–1114 (abstract).

tzf_img_post