Swinging around the sun would make a spacecraft the fastest ever

NASA is very interested in developing a method of propulsion that would allow spacecraft to go faster. We’ve reported several times on different ideas to support that goal, and the most successful ones have put the sun’s gravity to good use, usually by swinging around it with a slingshot, as is often done with Jupiter at the moment.

But there are still significant hurdles to this, not least the energy that radiates through the sun and vaporizes anything that comes close enough to utilize gravity. That’s the problem that a project supported by NASA’s Institute for Advanced Concepts (NIAC) and led by Jason Benkoski, now of Lawrence Livermore National Laboratory, is trying to solve.

The project was awarded a NIAC Phase I grant in 2022, aimed at combining two separate systems: a heat shield and a thermal propellant system. According to the project’s final report, combining these two technologies could enable a spacecraft to perform a so-called Oberth maneuver around the sun.

In this orbital mechanics trick, a spacecraft uses the sun’s gravity to hurl itself at high speed in the direction it is pointing. It is similar to the sundiver technology discussed in other articles.

What makes this project unique? One thing is the heat shield: Dr. Benkoski and his team have developed a material that can withstand temperatures up to 2,700 K. While that’s still nowhere near the temperature of the Sun’s surface, which can reach 5,800 K, it’s enough to get pretty close arrive and thus unlock a room. the ability of spacecraft to use an Oberth maneuver at all.

Samples have already been made of the material with these thermal properties. However, further research is needed to understand whether they are suitable for spaceflight. And a heat shield alone is not enough to perform the maneuver; a spacecraft must also have a propulsion system that can withstand these temperatures.

A solar thermal energy system could potentially do this. These systems use the sun’s energy to pressurize their own propellant and then expel those propellants to provide thrust, which is a necessary part of an Oberth maneuver. There are several types of fuels that could work for such a system, and much of the research in the Phase I project looked at the different costs/benefits of each.

Hydrogen is one of the most common fuels considered for a solar thermal system. Although lightweight, it requires a bulky cryogenic system to store the hydrogen as it is heated to the point where it is used as propellant. Ultimately, the tradeoffs made it the least effective of the propellants considered during the project.

Lithium hydride was the surprise winner for the fuel that allows the fastest escape velocity. Calculations show that this could result in a rate of more than 12 AU/yr. However, there are limitations regarding the storage and handling of the fuel.

Dr. Benkoski chose a more mundane fuel as the overall winner of the modeling he did: methane. Although it generally results in a lower terminal velocity than lithium hydride, at over 10 AU/yr the terminal velocity is still respectable. It also eliminates many storage problems with other propellants, such as the cryogens needed to store hydrogen.

There are some drawbacks, however: the calculated maximum speed is only about 1.7 times faster than what would already be possible using Jupiter’s gravity assist, which wouldn’t require all the fancy thermal shielding.

However, there are other disadvantages, such as the direction in which the spacecraft can travel is limited by where Jupiter is located in relation to other objects of interest. In contrast, if you orbit the sun, it is possible to reach almost anywhere in the solar system and beyond with proper controlled combustion.

As Dr. Benkoski notes in the final report, he made many assumptions in his modeling calculations, including that the system could only use already developed technologies rather than speculative technologies that could dramatically affect the results.

For now, it does not appear that NASA has selected this project to advance to Phase II, and it is unclear what future work is planned for further development. If nothing else, it’s a step toward understanding what it would take to truly send spacecraft beyond the sun and into deep space at a speed far faster than anything else has ever gone. Given NASA’s continued attention to this topic, one of its missions will undoubtedly succeed one day.