What are the problems with using the sun for Oberth maneuvers?

Presumably, it has something to do with the sun being rather hot. All the same, are there any potential cases with current technology where swinging close by the sun would be worth the shielding for the velocity imparted? How close could an uncrewed spacecraft get to the sun with shielding?

If ultralight shielding technology comes along, would such maneuvers have broader applications?

The question 'Could the sun be used as a gravity assist outside the solar system (with current tech)?' got me thinking about this.

The main problem is thermal control: you need a heatshield on your spacecraft to survive through the close pass. It is a solved problem though, meaning that NASA is planning to send a mission in 2018 to explore the close environment of the sun as low as 8 solar radii:

http://solarprobe.jhuapl.edu/mission/index.php

https://en.wikipedia.org/wiki/Solar_Probe_Plus

There was a project called the Realistic Interstellar Explorer, which would have used a probe to explore the interstellar medium. The mission would have started with a gravity assist from Jupiter to get the perihelion down to 3-4 solar radii, and then a perihelion boost using solids or solar thermal propulsion (which turns the heating constraints into an advantage):

presentation here

The goal was to cross 1000 AU in 50 years.

• I think the 3-4 solar radii is extremely telling. You could potentially get twice the Oberth effect by reducing this further, which would be massive for such a mission. What this tells us is that the thermal issues can't be engineered around easily. – AlanSE Nov 30 '14 at 2:35
• And to make use of the Sun's gravitational focus as a lens for a radio telescope, it is "only" 550+ AU away. – LocalFluff Nov 30 '14 at 10:13
• Note for most targets within the solar system, the solar shielding would be a dead weight that needs to be lifted beyond the orbit, and is entirely useless at the target. Using that weight for more fuel is deemed more practical in most cases. especially that the maneuver would set you on a very elongated, steep elliptical orbit, while to rendez-vous with most objects in the solar system a more circular orbit is far better. – SF. Dec 2 '14 at 11:15
• ...also, solar flares are entirely a gamble, and that close to the Sun, the risk is pretty high. – SF. Dec 2 '14 at 11:18
• Getting close to the Sun is difficult too. Either go by Jupiter (Ehricke trajectory) or like Solar Probe Plus spend 6½ years to make 7 flybys of Venus. Starting at the Sun 6½ years after launch needs a very distant target in order for the Oberth effect to pay off, given that New Horizons reached Pluto after 9½ years. (Although SPP enters solar orbit and the Jupiter way can be done faster). – LocalFluff Jul 19 '15 at 15:47

Edit: This answer refers to an earlier version of the question when it was about a gravity assist maneuver instead of an Oberth maneuver.

No, the answer is very simple. The problem is physical, not technical. In order to use an object for a gravity assist, you need to have escape velocity with regard to that object to begin with. You can fly close to the sun (no reference intended), but it won't give you any net momentum.

If it was otherwise, we wouldn't need thrusters to reach GTO from LEO, we could just swing by the earth several times, but that would violate conservation of momentum.

Maybe you could swing by other stars on an interstellar voyage, but I gather that this is not the question you want to ask.

• I'm sorry - i think i should have asked about an Oberth maneuver. I'm still unfamiliar enough with this to have trouble with distinctions. Would that make the question valid? – kim holder wants Monica back Nov 29 '14 at 17:05
• Yup, that makes it a much better question. Maybe not practically relevant, because of the long travel times from the outer solar system back to the sun, but theoretically possible. – Rikki-Tikki-Tavi Nov 29 '14 at 17:12
• I feel your answer here is still useful in that the explanation of why gravity assists wouldn't work is easier for me to get than the one PearsonArtPhoto gave to the question i linked to - brief and i can imagine it, reference frames are more challenging to picture. – kim holder wants Monica back Nov 29 '14 at 17:27
• You might make it a little more explicit in your answer that a gravity assist is all about the transfer of momentum (just a suggestion). – Jerard Puckett Nov 29 '14 at 18:35

What are the problems with using the sun for Oberth maneuvers?

A number of recent papers have explicitly advocated using the Oberth effect near the Sun to achieve very, very high velocities. The only problem is heat. Conceptually, all we need is unobtanium to overcome the heating problem. The current lack of this fantasy material is of course a big problem.

Some of those papers advocate using conventional thrusters with a spacecraft passing very close to the Sun. An economical way to achieve this is to use a gravitational assist from Jupiter to cancel almost all tangential velocity. It's a lot cheaper from a delta-V perspective to go to Jupiter than it is to dive straight toward the Sun. The basic idea is to send a vehicle towards Jupiter, let Jupiter send the vehicle diving almost straight toward the Sun, and fire massive set of thrusters near perihelion. The resulting $V_{\infty}$ is huge compared to anything yet achieved by humankind.

Other papers advocate using solar sails in combination with sun-diving to achieve very high escape velocities. A gravity assist from Jupiter is not needed here. In fact, it's counterproductive; solar sails don't work very well beyond Mars' orbit. On the other hand, solar sails work quite nicely (and that's putting it mildly) inside of Mercury's orbit. All we need are solar sails made of unobtanium as opposed to a very thin layer metal on plastic so the solar sail can dive very close to the Sun, and voila! the solar sail can easily escape the solar system at velocities that make those obtained by the Pioneer and Voyager satellites look miniscule.

I'll try to dig up some references to these papers later.

• So unless you are going beyond Jupiter, there is no point, i gather. Not even by doing some of your braking to head sunwards with solar sails, or ion drives? – kim holder wants Monica back Nov 29 '14 at 21:42