Slinging around the Sun would make the spacecraft the fastest ever

NASA is very interested in developing a propulsion method that will allow spacecraft to go faster. We’ve reported several times on various ideas in support of this goal, and most of the more successful ones have made good use of the Sun’s gravity, usually by slingshotting around it, as is currently commonly done with Jupiter.

However, there are still significant obstacles, not the least of which is the energy emitted from the sun simply vaporizing anything that gets close enough to use gravity assistance. That’s the problem 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, focusing on the combination of two separate systems – a heat shield and a thermal propulsion system. According to the project’s final report, combining the two technologies could allow the spacecraft to perform what is known as an Oberth maneuver around the Sun.

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

So what makes this project unique? One thing is a heat shield – Dr. Benkoski and his team have developed a material that can withstand up to 2700 K. While that’s still nowhere near the temperature of the Sun’s surface, which can reach up to 5800 K, it’s enough to get pretty close to unlocking the spacecraft’s ability to use the Oberth maneuver on the first place.

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

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

Hydrogen is one of the most common fuels considered for a solar thermal propulsion system. Although light, it requires a bulky cryogenic system to store the hydrogen as it is heated to the point where it is used as thrust. Its compromises ultimately made it the least efficient of the propellants considered in the project.

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

Dr. As the overall winner of the modeling he did, Benkoski opted for a more common fuel – methane. Although it generally results in a lower terminal velocity than lithium hydride, its terminal velocity is still respectable at over 10 AU/year. It also removes many of the difficulties of storing other propellants, such as the cryogenics needed to store hydrogen.

There are some drawbacks though – the calculated top speed is only about 1.7 times faster than what could already be done with Jupiter’s gravitational assistance, which wouldn’t require all the fancy heat shielding.

However, it also has other drawbacks, such as the direction in which the spacecraft can travel because it is limited by where Jupiter is in relation to other objects of interest. On the other hand, by orbiting the Sun it is possible to reach almost anywhere in the solar system and beyond with the right controlled burn.

As noted by Dr. In the final report, Benkoski made many assumptions when performing his modeling calculations, including that the system would only be able to use already developed technologies rather than speculative ones that could dramatically affect the results.

For now, it does not appear that NASA has selected this project to move 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 actually send a spacecraft around the Sun and into deep space at speeds far greater than any before. With NASA’s constant attention to this topic, no doubt one of the missions will succeed one day.