Seems like getting something for free, doesn't it? And we all know that the universe is notoriously stingy with freebies. However, this maneuver does not violate any laws of physics.
Note that the fuel in the vehicle is also in orbit around the massive body. If the vehicle burns some fuel while dropping to a lower orbit, then much more propellant can be burned at the lower orbit. The propellant is then left behind in a much lower (less energetic) orbit. Oberth realized that releasing the chemical or nuclear energy of a fuel was not actually using all of the propellant's available energy. The propellant also has mechanical (kinetic and potential) energy that can be released — thus accelerating the vehicle — by use of his maneuver.
By using Oberth's maneuver around the sun, we can anticipate that a vehicle that can achieve a velocity of 5 percent the speed of light will actually blast out of the solar system at our target 15 percent the speed of light. However there are still a number of issues to be investigated.
What is the spacecraft's maximum acceleration? How close can the spacecraft actually get to the sun without suffering dire radiation and heating effects? How many stages will the vehicle need to have? And attaining 5 percent the speed of light before Oberth's maneuver is still a very challenging task. Future articles will discuss the propulsion systems that have potential to reach these velocities.
A possible mission profile using the options mentioned above would start in Low Earth Orbit (LEO) where the Icarus probe is built. Using conventional propulsion systems such as liquid (chemical) rockets, the craft begins its journey to Jupiter. The chemical rockets will be jettisoned immediately after this Trans-Jovian Injection (TJI).
Jupiter's gravity will drag the Icarus probe into a new trajectory perpendicular to the plane of the solar system. Additionally this gravity assist will shorten the Icarus orbit so that it starts falling back towards the sun. A short burst from its fusion engines will tighten the probes orbit, so that years later the probe will fly deep inside the sun's atmosphere, the solar corona.
A set of fuel tanks will be jettisoned at Jupiter after the short burst. As the probe nears this closest approach to the sun it will start firing at maximum thrust for weeks, or months. The probe will have to use some of its propulsive force to steer towards the sun as it accelerates. The probe will swing around the sun while continuing to accelerate. Multiple sets of drop tanks will be dropped along the way. The thrusting will not stop until the probe achieves the target velocity.
The probe will continue to thrust in short bursts on its long journey to the target star. These bursts will make up any drag caused by interstellar gases and dust, and to make minor course corrections as needed. As the probe approaches the target star it will conduct the Oberth maneuver in reverse, thrusting at maximum as it swings around the target star. If possible, the probe will slow sufficiently to be captured around the target star, and extend the time the probe has to study the star system.
It is not yet clear if the Icarus team will be able to find the right combination of propulsive technologies and mission options that will enable the full acceleration and deceleration at the target star. And many of the mission elements described above will be traded against numerous other options.
The Icarus team is committed to find the mission profile that will allow humanity to take those first few steps out to the stars.