Revolving around the sun would make a spaceship the fastest ever

NASA is very interested in developing a method of propulsion that allows spacecraft to go faster. We’ve repeatedly reported on different ideas to support this goal, and most of the most effective have made good use of the sun’s gravity, usually by going around it, as is common with Jupiter today.

But there are still significant obstacles to overcome, not the least of which is the sun’s radiant energy which simply vaporizes anything that gets close enough to use a gravity assist. 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 received a NIAC Phase I grant in 2022, focused on combining two distinct systems: a heat shield and a thermal thruster system. According to the project’s final report, the combination of these two technologies could allow a spacecraft to perform what is known as an Oberth maneuver around the sun.

In this orbital mechanics trick, a spaceship makes good use of the sun’s gravity to launch itself at high speed in the direction it’s aiming. This is similar to the sundiver technology discussed in other articles.

So what makes this project unique? One thing is the heat shield—Dr. Benkoski and his team developed a material capable of withstanding up to 2,700 K. Although this temperature is still far from the temperature of the surface of the sun, which can reach 5,800 K, you only need to get close enough and thus unlocking the spaceship’s ability to use an Oberth maneuver in the first place.

Samples of the material with these thermal properties have already been produced. However, more research is needed to understand whether they are made for spaceflight. And a heat shield alone is not enough to perform the maneuver: a spacecraft must also have a propulsion system capable of withstanding these temperatures.

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

Hydrogen is one of the most commonly considered fuels for a solar thermal propulsion system. Although it is lightweight, it requires a bulky cryogenic system to store the hydrogen because it is heated to the point of being used as thrust. Ultimately, its compromises made it the least effective of the thrusters considered during the project.

Lithium hydride was the surprise winner for the fuel that allows the fastest leak rate. Calculations show that this could result in a speed greater than 12 AU/year. However, there are constraints related to fuel storage and handling.

Dr. Benkoski chose a more mundane fuel as the big winner in the modeling he carried out: methane. Although this generally results in a final velocity slower than that of lithium hydride, its final velocity remains respectable at over 10 AU/year. It also eliminates many of the storage problems of other propellants, such as the cryogenics needed to store hydrogen.

There are some drawbacks, however: the calculated maximum speed is only about 1.7 times faster than what could already be done with a Jupiter gravity assist, which wouldn’t require all the sophisticated thermal shielding.

There are other drawbacks to this, however, such as the direction in which the spacecraft can move, as it is limited by Jupiter’s position relative to other objects of interest. In orbit around the sun, on the other hand, it is possible to reach almost anywhere in the solar system and beyond with well-controlled combustion.

As Dr. Benkoski notes in the final report, he made many assumptions during his modeling calculations, including that the system could only use already developed technologies rather than speculative technologies that could have a considerable impact on the results .

As of now, it does not appear that NASA has selected this project to move on 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 would be needed to truly send a spacecraft beyond the sun and into deep space at a speed much faster than anything that has been done before. Given NASA’s constant attention to this topic, there is no doubt that one day one of the missions will achieve this.