The Oberth Effect is a powered gravity slingshot. That is, a spacecraft does a gravity slingshot and then fires its rocket at the closest point to the planet. This is supposed to give an even greater boost than a rocket burn in other places. Lots of spacecraft have done slingshot maneuvers. I think every interplanetary space probe that went places other than the Moon, Mars, or Venus has done at least one and some have done multiple. But have any actually done the Oberth thing?
Okay I’m not one of the real space travel experts here but my guess would be No.
One example of a maneuver that “does the Oberth thing” is the Bi-elliptic transfer. I can’t think of a mission where the scenario would even be relevant, but even then, the minimal savings in the exact right circumstances would not be worth the downsides (longer time to complete).
Another maneuver is the two-burn maneuver. Here is a NASA study (pdf) about that maneuver. The Oberth effect can only be used to advantage in very high delta V budget missions - very fast escape trajectories or minimal transfer time missions. I am not aware of missions to date of that kind but there might be some in the future (PDF mentions examples).
There is a maneuver called supersynchronous orbit but I would barely call that doing the Oberth thing.
Spacecraft take advantage of the Oberth Effect all the time. Are you specifically only interested in those that completed a burn in conjunction with a gravity assist maneuver?
Can you give an example of a spacecraft that took advantage of the Oberth Effect?
Maybe it’s a matter of perspective. One perspective is you can’t not take advantage of the Oberth Effect, given you have a mission that thrusts at different orbital distances, or even thrusts at any distance within a gravity well.
I interpreted the question more narrowly. I took “taking advantage of the Oberth Effect” to mean either: deliberately thrusting more than you need at lower altitude (bi-elliptic maneuver) or deliberately lowering the periapsis when you actually want to go in the other direction (two burn maneuver).
I can’t think of a mission that did a bi-elliptic maneuver only for the small (maybe one percent) savings, or one that achieved large savings with the two-burn maneuver.
To be clear, your OP first mentions the Oberth Effect, but you seem to be talking about powered flybys (Oberth Manuever) around other planets. The Oberth Effect itself is just the name for the phenomenon in which rocket thrust is more efficient, the faster they are going. Because of orbital mechanics, a rocket will be going fastest at the point where its orbit is closest to the planet. So, firing the engines at this point is the most efficient use of fuel. Many missions take advantage of the Oberth Effect while still orbiting Earth. When their orbit is closest to Earth, they perform a short burn, which pushes out one side of their orbit a bit further. Then they go all the way around and fire again at the low point, making their furthest point a bit further than before. They’ll do this a few times until they are finally ready to leave the orbit of the Earth.
The picture here shows the Mars Orbiter Mission making several passes around the Earth. Each time, it performed a burn when closest to the Earth (at the top of the Earth in the diagram), pushing one end of its orbit further and further away (see the ellipses jutting outward toward the bottom of the Earth in the diagram). Each of these burns took advantage of the Oberth Effect. This took much less fuel than just making one single burn.
Chandrayaan 2 did something similar, as well as tons of others.
I’m not sure which (if any) spacecraft have performed powered flybys of other planets, though.
Thanks, that’s an example of using the Oberth Effect.
ISTM they used the Oberth Effect that way in order to be able to use smaller thrusters. AIUI if they could have produced the same delta-V in a single burn with a much larger thruster they would have achieved the same result. But even so (assuming this is correct) I agree this is still an example of taking advantage of the Oberth Effect.
And also, since the Oberth Effect works retrograde as well. It’s not just for speeding up. Spacecraft take advantage of the Oberth Effect when slowing down to orbit other planets. For example, The Mars Global Surveyor performed a 20 minute burn to slow itself down enough to enter orbit around Mars. The burn started 10 minutes prior to periapsis (that point where it is closest to the planet, where the Oberth Effect is strongest) and lasted for 10 minutes beyond it. Taking advantage of of the Oberth Effect in this way is the most efficient use of fuel. If you have a 30 minute burn, you’ll want to start 15 minutes prior to periapsis. You want the average distance from the periapsis to be smallest throughout the duration of the burn to maximize the effect.
A larger thruster would have weighed more. Which starts the mass spiral: More weight… more fuel… more weight…
And getting it to orbit in the first place would have taken more fuel. . . or possibly have been too heavy to even lift into orbit.
And it may have been more powerful, but not necessarily more efficient. Remember, it’s all about efficiency. You will get more DeltaV out of a rocket if you take advantage of the Oberth Effect. With a given thruster and a given amount of fuel, you will produce more DeltaV in the gravity well (going faster) than when you are further away (going slower).
With any given rocket motor, the faster it is traveling, the less fuel it will need to achieve a particular change in velocity.
It was best explained to me with a video that showed the test firing of a rocket while on the ground. The rocket engine was clamped down, of course, so it didn’t go anywhere. It was stationary, and none of the energy from the exhaust become kinetic energy for the rocket. All of the energy basically stayed with the exhaust as it flew out the back. So, that’s the extreme example of a rocket that isn’t moving at all. All of the propellant energy goes out the back, and none of it is transferred to the rocket. Very inefficient! The extreme example on the other end is a rocket going very fast. The faster it goes, the higher percent of that exhaust energy goes toward propelling the rocket instead of just propelling the exhaust.
Since faster rockets are more efficient, it’s most efficient to burn when going fastest. That fastest point occurs at the closest point in an orbit.
OK, it looks like what I want to know is if any spacecraft has used the Oberth maneuver during a gravity assist.
I can’t think of any powered gravity-assist maneuvers. Ulysses didn’t even need one! I guess the large planets provide enough free DeltaV, that there’s no sense in spending one’s own valuable resources on it. I think they will mainly come into play with manned missions where time can become much more important than efficiency. When there are humans aboard with finite life support resources, trajectories are going to sacrifice efficiency somewhat in order to shorten the trips.
OK, thanks Bear. That pretty much confirms my research, but I thought I may have missed one. After all, there’s been at least two dozen spacecraft that have done gravity assists, some multiple times.
MESSENGER used one. It performed a 316 m/s burn on the first Earth flyby. There were several other gravity assist maneuvers but I think that was the only one with a significant delta V (not including the insertion at Mercury).
Are you sure? It did perform a deep space maneuver (DSM) 4 months after passing Earth. It was a 316m/s burn, so I think you might be thinking of this one.
Huh–indeed you’re right. I misread the timeline. A bit surprising that there seems to be no example of a powered flyby.
I thought that some probes did a small burn for change in trajectory at closest approach to line up for the next target. It’s also most effective to do that then.
Anyone know if this actually happened?
I thought the opposite.
Trajectory targeting and cleanup are done to hit certain precise orbital parameters, not to gain the maximum possible orbital potential energy.
Doing these maneuvers near the closest point would give you the most energy, which you don’t need, but the lowest accuracy, which you do need.
I can’t find a systematic list of Cassini TCM’s and OTM’s but here is an article about targeting a particular event in the mission:
http://articles.adsabs.harvard.edu/full/2006ESASP.606E..79S
Note how the OTMs are planned before and after the closest approach, and at the APOapse, but not the periapse. The opposite of the Oberth Effect.
(ETA: In contrast, of course, the Saturn Orbit Insertion was done precisely around the closest approach to the planet, taking advantage of the Oberth Effect, because that’s exactly where you do need the maximum change in orbit for your propellant.)
Japan’s Nozomi spacecraft did a powered swingby of Earth. But this case may illustrate why it’s not done more often. As Frankenstein Monster noted above, powered flybys are less precise to begin with. If something goes wrong during the burn, the probe ends up in a very wrong orbit. In this case, one of the fuel valves on Nozomi jammed, resulting in less delta-V than intended. There was not enough fuel to put it back into the intended trajectory, so they had to change plans and add 2 more flybys, arriving at Mars 4 years late. (And during the delay, many other systems on the probe broke down.)