Combining Heat Shield and Propulsion for a Fast Solar Slingshot

NASA Advanced Innovations Concepts has funded research to combine a heat shield and solar thermal propulsion system to perform a solar Oberth maneuver in order to achieve the highest possible escape velocity from the solar system. This will enable missions ten to hundreds of times further than Pluto out to the Kuiper Belt Objects or interstellar space.

A solar slingshot maneuver is to fly as close as possible to the sun in order to get more speed. A gravity assist around a Sun changes a spacecraft’s velocity by entering and leaving the gravitational sphere. The spacecraft’s speed increases as it approaches the sun departs in the opposite direction.

The team has designed and built working solar thermal propulsion prototypes out of materials that can survive 2800 K at a 20 x 20 cm scale. These benchtop-scale demonstrations have thus far validated our thermal and propulsion models. Despite growing confidence that a full-scale heat shield/heat exchanger can survive an Oberth maneuver (solar slingshot), many questions remain regarding the feasibility of long-term cryogenic storage.

The goal is for the craft to move within 1.6 million kilometers of the Sun’s surface. This is 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 (or other gas) that are built into the bulk of the shield itself. Absorb heat with coolant gas and shoot it out of the probe as an engine burn as they are leaving the sun. The cooling setup also opportunistically doubles as an engine.

The closer you can get to the Sun then the more speed you can get with a slingshot. The sun has a radius of 696,000 kilometers. They want to pass within 2.3 solar radii.

There have been studies where lightweight spacecraft with solar sails make the slingshot move. A solar sail slingshot maneuver in the vicinity of the sun, just ~2-5 solar radii distant from the sun, solar sails can propel light-weight cubesat class spacecraft to near-relativistic speeds, over 0.1% of the speed of light (over 300 km/s or over 60AU/year characteristic velocities). The new study is for a heavier spacecraft making a tight slingshot but without a solar sail. I am guessing 20-40 AU/year might be achievable with the 2.3 solar radii pass. I also think this configuration can be a heavier craft which can carry larger payloads. Some of the payloads could be lightweight solar sail craft. They could launch multiple ultralight solar sail craft after making the closest approach. This would let a partially protected solar sail mission to launch with velocity and achieve most or all of the speed of the solar sail only slingshots.

Below is the graphic for a solar sail slingshot plan:

Section 7.2.2 of a comprehensive 329 page interstellar probe report goes of Solar Oberth (slinghshots) for interstellar missions. They had a 4 solar radii pass reaching about 12-14 AU/year.

They 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 they would determine material compatibility for each propellant with respect to its proposed storage system.

They then calculate the efficiency (specific impulse) as a function of temperature for each propellant using Chemical Equilibrium Analysis (CEA). Using information from published studies of propellant storage systems,

They would determine how the mass and volume of the tank scale with the quantity of propellant for realistic tank designs.

Next, they devise an equation for the area of the heat shield as a function of propellant quantity.

Finally, the above inputs would be used to perform a full system mass trade to determine the maximum propellant fraction that can be achieved within the payload limitations of an SLS transit to Jupiter Significance. Approximately 130 known dwarf planets inhabit the trans-Neptunian region. Dwarf planets are the most common type of planet in the solar system, far outnumbering giant planets and terrestrial planets. They are also the least explored. Ceres, Pluto, Charon, and Triton (the latter two often considered as both satellites and dwarf planets) remain the only explored dwarf planets.

Also little explored is the local interstellar medium. Although the Voyager 1 and 2 probes have exited the heliosheath, neither were designed for heliophysics observations. The Pioneer 10 and 11 probes, while in interstellar space, have long ceased functioning; New Horizons is expected to meet a similar fate.

The feasibility of deep space exploration hinges on a high solar system escape velocity (over 10 astronomical units per year), as the potentially long transit times pose extreme challenges for both hardware reliability and staffing.

The development of efficient propulsion systems with high thrust can thus generate significant cost savings and risk reduction. Consider a mission to the near interstellar medium (~550 AU). Voyager 1–the current record-holder at 3.6 AU/yr–will not journey this far for another 100 years. An SLS rocket with a Jupiter gravity assist offers improvement, but it tops out at 8 AU/yr for a ~1 tonne spacecraft payload. Crucially, the fly-out direction depends on Jupiter’s orbital position, which greatly limits the possible scientific objectives. A solar Oberth maneuver offers much more freedom for the fly-out direction.

SOURCES- NASA NIAC, Interstellar Probe Study Team
Written by Brian Wang,

14 thoughts on “Combining Heat Shield and Propulsion for a Fast Solar Slingshot”

  1. This is almost exactly the idea I "invented" about seven years ago, when I got interested in missions to to the gravitational focal radius of the sun. (Which starts at ~ 600 AU). I never carried through on the idea or wrote about it, however. I don't have the academic credentials or connections to have any chance of winning a NIAC grant.

    Brian is right: 20 – 40 AU/year should be achievable on a 2.3 solar radii Oberth flyby. I was aiming for a more aggressive 2.0 solar radii, in order to launch a heavy spacecraft without a punishing fuel-to-payload mass ratio. I figured one launch for the payload and the materials for the solar shield / rocket engine, and a second launch for the fuel. But I wasn't going to have the solar shield manufactured on earth and stuffed inside the faring of an SLS launch. No way! I would have it 3D printed in space, from titanium. It would have an area of ~1000 square meters. Oh, there would also be a third launch, for a booster to launch the assembled payload / shield-rocket / loaded fuel tanks on a Jupiter flyby to kill most of its solar orbital velocity. The Jupiter flyby and long fall toward the sun add a few years to the mission, but there's no other way to get a heavy payload that close to the sun.

    The fuel I would use could easily keep for several years in space with only passive solar shielding. The team proposing this new study missed a bet. They should look at water + methane.

  2. What we need now are orbiters for the Outer Planets. I don't think we need solar flyby to get it done. Best way would be small nuclear reactors and rocket engines.

  3. What's the 'Oberth effect' (with kinetic energy of propellant transferred to space ship for acceleration) getting towards speed of light?
    (mentioning dislike about Oberth's (public) political role)

  4. It could be accelerated farther with a propulsion system that turns it around few times back to the Sun.

  5. It feels like cheating, but it's totally within physics and conservation laws.

    You just use your fuel at perihelion, losing mass at the bottom of the gravity well and gaining a significant extra speed when you leave, stealing a little bit from the fuel's potential energy, which will be none the worse from it when you leave it behind. And the faster you go at perihelion, the better it gets.

  6. To undestand the benefit of this proposal, you have to know about the "oberth effect". In a gravitational field, you gain more momentum by burning your propellant while at higher speed respectively to the same maneuver at lower speed. Its weird, but you believe it after doing the math. The higher isp by heating the propellant is just on top, and not nesessary for this type of maneuver.

  7. Would be neat to use thermoelectrics to harness energy to power a mag-sail grid to accelerate plasma in front of the craft for extra free thrust. Ain't no propellant like free propellant.

  8. So it sounds like it’s not a slingshot, since that would not actually produce any net velocity itself, but a propulsion system that uses the energy of the sun to heat propellant. Sounds like a solution in search of a problem. Melting metal is easy to do with chemical rockets.

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