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In the article under Example calculation it says then the specific kinetic energy per unit mass after the burn is. Should that be either just specific kinetic energy without the words per unit mass, or kinetic energy per unit mass without the word specific? I mean, isn't the per unit mass part already stated implicitly with the word specific? Doesn't the current wording mean, strictly speaking, kinetic energy per unit mass squared? 130.234.5.138 ( talk) 21:03, 18 December 2007 (UTC)
I cannot chime in on the Oberth Effect, as it is over my head. I'd like to propose A rewording of the introduction, that leaves out the efficiency-at-velocity point, because that is already discussed in the article on Thrust and in Rocket, and the wording here is ambiguous. The OE seems to be about firing in a gravity well, so better leave it at that. The thrust of a rocket remains the same, velocity notwithstanding, and this is not made clear enough in the text. —Preceding unsigned comment added by 134.2.125.243 ( talk) 12:38, 1 April 2009 (UTC)
I am bookmarking this to review later, but let me flag that the first paragraph makes no sense on first reading. Out of the many possible interpretations of the effect, "usable energy" strikes me as the least accessible, and thus not a good lead-in to the topic. I suggest starting from the core idea that propellant imparts a certain amount of momentum, and thus delta-V as it is ejected. Then consider the effect of that delta-V on energy at low V vs high V scenario.
Something along the lines of:
A rocket works by transferring momentum to its propellant. At a fixed exhaust velocity, this will be a fixed amount of momentum per unit of propellant. For a given mass of rocket (including remaining propellant), this implies a fixed change in velocity. Because kinetic energy = mv^2, this change in velocity imparts a greater increase in kinetic energy at a high velocity than a low velocity. For example, if you have a 1kg rocket, at 1 m/s, adding 1 m/s increases KE from 1J to 4J, for a gain of 3J, but at 10 m/s, you start with a kinetic energy of 100J, and end up with 121J, for a net gain of 21J. This greater kinetic energy can then carry the rocket higher in the gravity well than if it were burned at a lower speed.
Bob Kerns ( talk) 05:35, 11 November 2010 (UTC)
Aerospace engineer checking in here. The explanation in this article is incorrect and misleading (I'm not trying to cause any offense here). The Oberth effect has nothing to do with how fast a rocket is going when it burns its fuel so as to achieve the effect - it only occurs when there is a change in potential energy. In deep space - far from any gravitational source (i.e potential energy) it wouldn't matter if the rocket is at a dead stop or flying through space at high speed when the impulse is given - no Oberth effect can occur. Indeed velocity is entirely relative anyway, such that "at high speed" means nothing: an observer traveling at the same speed (same frame of reference) would see the 'fast' rocket as being stopped. A trade of potential energy of exhaust gases at a lower point in a gravitational energy well must occur to gain a boost from the Oberth effect, and this is done by dropping down to a lower orbit point where the impulse is given. The end result is a bigger delta-V than had the impulse otherwise been given at a higher orbit point. You are basically gaining all the energy of the exhaust gases by 'leaving them' at a lower potential energy point than a higher potential energy point. Yes, by doing such a maneouver the impulse is given at a point when the rocket is indeed going faster - but that is incidental. It is the unloading of exhaust gases at a low point in the energy well that gives rise to Oberth. — Preceding unsigned comment added by Magmaiceman ( talk • contribs) 05:24, 1 January 2015 (UTC)
There is a disagreement between the explanation of the paradox, as would be given in the last paragraph of the description. It is (at the moment) a disagreement between User:Wolfkeeper and User:Wingedsubmariner. The current version of the page is by Wolfkeeper, the most recent by Wingedsubmariner is here.
The discussion to this point has been on user pages, here it is copy-and-pasted:
from User_talk:Wingedsubmariner
You're sort of half right. The kinetic energy of the propellant is transferred to the vehicle in the way you indicate, but the potential energy of the propellant is where the energy originates from if you trace it through.- ( User) Wolfkeeper ( Talk) 03:08, 25 February 2009 (UTC)
from User_talk:Wolfkeeper
I hope I didn't give offense: I only replaced the explanation because I was certain I was right. The paradox does need a resolution.
The potential energy difference does not explain the effect. Consider a modified version of the example I give on Oberth effect, where the difference in speed comes from the speed being invested into potential energy, that is, the spacecraft was moving away from some mass. We now have two different cases. One, the rocket at a certain distance from a mass moving away from it below escape velocity, two, the rocket after its speed has reached zero, converting all of the speed from case one into potential energy, 1/10 of which was invested into the propellant. It started with a speed of 9. We don't need to know the size of the mass or the exact distances. A speed of 9 gives initial kinetic/potential energy of 81, 8.1 of which is held in the propellant. The rocket fires in the same way as in my previous example, and kinetic energy is assigned in the same way. Potential energy does not change because nothing has changed its height. We still have an 89.1 difference in kinetic energy of the rocket between the two cases, and the difference in potential energy of the propellant is 8.1 between the two cases, insufficient to explain the effect.
-- Wingedsubmariner ( talk) 03:55, 25 February 2009 (UTC)
Above by Wingedsubmariner ( talk) 16:06, 1 March 2009 (UTC)
The thing is, I don't think the term Oberth effect is used to simply mean the fact that a rocket body gains more kinetic energy at high speed from its burn than at low speed (although that is true); I think it's used specifically with respect to a gravity field. In that case the vehicle can even give the propellant negative total energy, and the rest of the energy goes into the body of the rocket; when not close to a gravitating body it can't do that. It's a slightly more nuanced understanding.
However, I think that we simply need more good referenced POVs we can add here. If anybody has any we can add that to the article that would be very desirable.- ( User) Wolfkeeper ( Talk) 17:10, 1 March 2009 (UTC)
Here, does this seem correct to you?
It covers the Oberth effect operating with a gravitational field, and explains where the energy for the rocket comes from; the paradox disappears once you include changes in the propellants energy. Wingedsubmariner ( talk) 18:13, 1 March 2009 (UTC)
I suggest this paragraph to replace the current last two paragraphs in the description:
It should cover every single situation any of us have come up with to this point. Wingedsubmariner ( talk) 16:49, 4 March 2009 (UTC)
3PO (this was at the top of this list, but...) I'm not sure how much the 3PO can help here. This seems to be a nuanced issue in astronautical engineering, so it may make sense to email a professor in the field. The reference desk may also be able to help (or at least find a good professor). Also, since the 3PO was requested I see a third person has entered the conversation. Although the tone of discussion here is very congenial, if you do find that an agreement is not forthcoming, consider an RFC. (removing 3PO request) NJGW ( talk) 03:24, 6 March 2009 (UTC)
I'm finally coming back to my edit that got immediately reverted. I offer a more detailed explanation of the correct mathematics as I see it.
The residual speed at infinity is always , where e is the specific total energy and is the specific gravitational potential energy. For an instantaneous change in speed, the return in the form of per investment of delta-v is . Certainly this only holds for infinitessimal burns, since if the delta-v is much greater than the escape velocity, not much of it will be lost (so ) even if that ratio is huge.
This is the linearity that I mentioned: the ratio is , not its square root, and it only applies for small burns. If there are no objections or refinements, I'll restore my edit. -- Tardis ( talk) 14:50, 10 April 2009 (UTC)
"It can be easily shown that the impulse is multiplied by a factor of:
Plugging in 50 km/s escape velocity and 5 km/s burn we get a multiplier of 3.3."
When I plug in those values, I get
which is 4.6 not 3.3. Am I missing something? Art LaPella ( talk) 15:43, 7 April 2010 (UTC)
In a discussion on the Talk:Conservation of energy page, I made the point that the time rate of change of the kinetic energy of the rocket engine is not simply Fv where F is the thrust and v is the engine velocity. A second term which constitutes an energy loss due to the mass of propellant being lost to the exhaust must be included. The correct statement is:
where is the rate at which propellant mass is converted to exhaust mass. As noted by 95.134.115.203, the total power P and thrust F produced by the engine can be assumed constant, so, assuming no heating effects, if Po=Fv is the rate at which the engine increases its kinetic energy, and Px is the rate of energy increase of the exhaust train, then P=Fv+Px or Px=P-Fv and at a high enough velocity, the rate of change of energy of the exhaust train is negative. Thats impossible.
I would edit the article myself, but I am not familiar enough with it to trace the implications of this throughout the article. PAR ( talk) 21:52, 9 April 2010 (UTC)
In astronautics, the Oberth effect is an effect where the use of a rocket engine at high speed generates much more useful energy than one at low speed.
This does not seem to be a good description of the Oberth effect. Ordinary Person ( talk) 00:29, 14 April 2010 (UTC)
We need to do a bit of work on the parabolic example - the example given is not strictly a parabolic example, but rather an example which starts from a parabolic orbit (orbital speed at periapsis equals Vesc) and transitions to a hyperbolic orbit (orbital speed at periapsis after acceleration now exceeds Vesc). Let me dig out my old copy of Stearns' "Navigation and Guidance in Space" and see what he has to say about it; then I'll take a shot at rewriting. Skål - Williamborg ( Bill) 22:34, 22 May 2010 (UTC)
Oberth was born in Nagyszeben (Hermannstadt), which was the part of Hungary that time, not Romania. http://en.wikipedia.org/wiki/Hermann_Oberth I've corrected it. GergelyGergely ( talk) 21:44, 28 November 2010 (UTC)
In the article the 50 km/s is given as escape velocity of Jupiter.
parabolic flyby of Jupiter with a periapsis velocity of 50 km/s Plugging in 50 km/s escape velocity
But in Wikipedia's Escape velocity, Jovian escape velocity is given as 59.5 km/s. We should correct the wrong number. ` a5b ( talk) 04:42, 23 December 2012 (UTC)
I'm not a physicist. I had trouble understanding the Oberth effect article and where this effect comes from. I figured it out after writing down conservation of momentum and conservation of energy at rocket/propellent's frame of reference. There is a transfer of kinetic energy from the propellent to the rocket, not from the planet to the rocket. Actually, if I understand correctly, Oberth effect has nothing to do with being in a gravity field of a planet, except that's where it's practically used. It's a generic effect independent of being near a planet or gravity source.
Mention of gravity slingshot and Oberth effect being used around planets (again, this is only because there is a temporary speed increase) is really confusing and has nothing to do with the Oberth effect itself.
I will write a more detailed explanation when I have time.
- I agree with your analysis. The article is very confusing. The image I have is the combustion of rocket fuel within a chamber. At low speed, the hot gas emitted will bounce off the wall of the chamber in the direction of the spacecraft and add to the exhaust gases emitted in the other direction but a slower speed. At a high speed, the hot gas emitted is traveling only slightly faster than the spacecraft itself and so won't bounce back but will increase the speed of the spacecraft but then be left at a slightly lower speed but in the same direction as it was emitted. No energy is lost in the change of direction of the exhaust gas in this case. No reversed exhaust gases interact and slow down the gases of combustion just after the instant of the first combustion. This is the common sense solution to the paradox, is it not?
The slingshot effect is where the spacecraft is traveling in the same direction as a planet and will gain some of the lateral movement of the emitted gravitons imv. The closer to the planet, the move gravitons it will encounter and therefore the more additional lateral movement it will gain. I simply can't understand the effect without a straightforward mechanical effect for the gravity force, such as an Archimedes spinning helical structure. 2.123.48.102 ( talk) 15:03, 18 November 2013 (UTC) Alan Lowey
The recently launched Mars Orbiter Mission (MOM) - known informally as Mangalyaan, or Mars-craft, from India would make a superb demonstration of the combined Slingshot and Oberth effect. Instead of flying directly to Mars, the $72m (£45m) probe is scheduled to orbit Earth until the end of the month, building up the necessary velocity to break free from our planet's gravitational pull.
[quote] "The fifth orbit-raising manoeuvre of Mars Orbiter Spacecraft, starting at 01:27am (IST).. with a burn time of 243.5 seconds has been successfully completed. The observed change in apogee is from 1,18,642 km to 1,92,874 km," Isro said.
After the successful completion of these operations, the Mars Orbiter Mission is expected to take on the "crucial event" of the trans-Mars injection around 12.42am on December 1. It will reach the orbit of the red planet by September 24, 2014 after taking on a voyage of over 10 months. [end quote]
2.123.48.102 ( talk) 17:18, 18 November 2013 (UTC) Alan Lowey
Added an analysis based on energy conservation to the 'Descripion' section. The folowing sections in the article 'Parabolic Example' & ' Detailed proof' does appear confusing, with the mention of Gravitational potential. I feel that Gravity has nothing to do with Oberth effect( Satyishv ( talk) 05:16, 8 November 2013 (UTC))
The Oberth Effect has nothing to do with energy transfer from a rocket to the rocket's exhaust; when the rocket travels further from the planet quicker (speed increase at periapsis) it spends less time under the influence of higher gravity and so receives less delta v from gravity on its way out to apoapsis. — Preceding unsigned comment added by 114.77.106.242 ( talk) 06:45, 2 April 2014 (UTC)
The following is a plausible-sounding but confused explanation, which I have removed from the introduction:
"Usable energy" is a vague concept at best, and speaking of kinetic energy generating "more power" is just wrong (a power = energy confusion).
An explanation in terms of work (= force x distance) is clear and correct, and appears in the body of the article. Some of the explanations on this Talk page that may seem different are also correct.
(Appeal to relevant authority: I completed a graduate-level degree in the Aero-Astro engineering department at MIT.) 109.149.217.204 ( talk) 14:38, 5 December 2015 (UTC)
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Almost the entire article is devoted to high-thrust brief burn duration applications at local periapsis. I don't believe that Oberth intended to describe the effect to be solely occurring at the point of maximum efficiency, but in general with no upward limit to the speed of the vehicle, i.e. it continues up to approaching the speed of light with ever greater effect. In fact, ion-propulsion would be of little significance if it was not applied at least at orbital speeds (noting that with nearly circular orbits for most satellites apogee and perigee are almost identical) already in use today for station-keeping and attitude control. For deep space missions, after the vehicle has been placed in orbit and given a relatively high initial velocity (with chemical rockets), ion-propulsion will occur continuously for most of the duration of the voyage (allowing that its power source is not solar and therefore diminishing in power with distance from the sun). It should be clear that without the Oberth effect (regardless of local gravity), ion propulsion would not be particularly useful. Similarly, for solar sails the very low thrusts obtained (on order of 7 micro-Newtons per meter squared at Earth orbit with practical mirrors) would contribute little energy were it not for an initial high velocity. In fact, since light sails involve no propellant, the concept of "expending propellant in a gravity well" is completely irrelevant. Light sails have been seriously discussed, using current technology, for low-relativistic speed trips to the nearest stars being pushed by lasers. It should be noted that one of the clearest demonstrations that the Oberth effect does NOT violate conservation of energy, is in that the energy lost from the reflected light (from the Doppler effect) EXACTLY balances against the energy gained from the reflector+vehicle kinetic energy gain (assuming a perfect reflector). — Preceding unsigned comment added by Pmarshal ( talk • contribs) 07:40, 4 September 2016 (UTC)
I removed the unsourced section describing this in terms of work because it is highly misleading to lay people and contains highly non-rigorous explanations (for anyone else). Its incredibly unintuitive to talk about work as force over a distance for an object that is already moving (and accelerating none-the-less). Its far more intuitive to talk about this maneuver in terms of leaving the fuel in a lower orbit, so I emphasized that information (which was already in the article). I'm sure an understandable explanation using the concept of work could be written, but that section wasn't it. The contrast between a nailed-down rocket vs a rocket in free space is a completely misguided comparison and confuses the concept rather than illuminating anything. Also, fuel simply does not transfer more energy to a rocket if the rocket is moving faster. That's not how physics works. This maneuver is entirely dependent on the gravity. The speed of the ship is a coincidental detail of the fact that the ship is at the lowest part of the gravity well. 2620:0:1000:1C12:E0B6:7C93:5059:DD09 ( talk) 23:23, 8 November 2018 (UTC)
The " Impulsive burn" section ( first added on 30 January 2016 by GliderMaven and modified somewhat since) states:
When I first read this, it struck me as ungrammatical, as if a key phrase had been omitted after "otherwise" and/or two different sentences had accidentally been mashed together. Then I realized that it's simply being somewhat elliptical: the expectation of a contrast set up by the word "Whereas" is not being "paid off" (as it were) at the end, unless the reader mentally provides the phrase, "in which case integration is unnecessary" (this might seem obvious to those already familiar with the article, but I had merely skimmed over the text up to this point without carefully reading everything that came before). When I did read back through the previous paragraphs I realized that this entire section was more or less duplicating the information in the previous section, " Explanation in terms of work", in the paragraph beginning with "However, integrating this is often unnecessary if the burn duration is short." So, should the "Impulsive burn" section simply be deleted, or does any of it warrant "merging" into another part of the article? (Not sure why I had to turn this into a "story"… :) - dcljr ( talk) 16:19, 26 November 2019 (UTC)
The article makes it extremely hard to understand that the effect is totally independent of the gravity part of the maneuver. I would even say that I understood it less after reading the article. My immediate reaction is that a split would be in order. Amphioxi ( talk) 19:30, 24 October 2021 (UTC)
The article presently claims that the Oberth effect can be explained in terms of momentum. However, the brief explanation merely makes some basic statements about rockets and fails to offer any explanation of Oberth. Two inline citations are provided to support the text about momentum; both citations refer to NASA websites but neither contain any mention of Oberth, either explicitly or by implication.
The section devoted to work and kinetic energy gives a useful numerical example to illustrate the Oberth effect. Readers are entitled to use the same example to explore Oberth in terms of momentum. If they do so they will see the following result:
Considering a 2 kg rocket:
These readers are then entitled to conclude that considerations of momentum do not show any effect related to the initial speed of the rocket. This is not at all surprising - change in momentum is called Impulse and is equal to the nett force multiplied by the time of application of that force; both time and force are independent of the choice of reference frame. This is in contrast to work and energy; work is force multiplied by displacement, and displacement is highly dependent on the choice of reference frame.
Unless someone can promptly improve the section related to momentum it should be removed. Dolphin ( t) 22:19, 16 April 2024 (UTC)