Talk:Oberth effect

message: 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)
replies: text: "I suggest starting from the core idea that propellant imparts a certain amount of momentum" - I also think the article should mention momentum. The Oberth effect could be understood as a consequence of the fact that momentum is proportional to velocity, but kinetic energy is proportional to the square of velocity. MathewMunro (talk) 07:16, 21 April 2020 (UTC)
thread_title: Unclear wording?

message: 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 v_\infty=\sqrt{2e}=\sqrt{v^2+2u}, where e is the specific total energy and u=-\frac{GM}r is the specific gravitational potential energy.  For an instantaneous change in speed, the return in the form of v_\infty per investment of delta-v is \frac{dv_\infty}{dv}=\frac{2v}{2\sqrt{v^2+2u}}=\frac v{v_\infty}.  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 v_\infty\approx v) even if that ratio is huge.
This is the linearity that I mentioned: the ratio is v/v_\infty, 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)
replies: text: The math looks good to me. In any case, its very easy to construct counterexamples to the square root rule given in the current version, so it definitely needs to go. As far as I can tell, that is the main change your revision would make. Wingedsubmariner (talk) 04:12, 13 April 2009 (UTC)

text: This bothered me both times I read it, and I'm really glad to see your explanation here.  I think it should be somehow referenced in or inserted into the page itself; I had the impulse to edit the paragraph to change final velocity to say the speed goes up as the square root as well.  So it should probably be explained better.66.30.14.175 (talk) 19:02, 5 September 2009 (UTC)

text: I haven't checked it carefully, but there's probably nothing wrong with your maths, but the article is about non infinitesimal burns. The article is about impulsive burns (to which many real burns approximate fairly well). This is very different.- (User) Wolfkeeper (Talk) 20:26, 5 September 2009 (UTC)

text: Note that small burns under strong gravity gives a very big Oberth effect; from the equation in the article a 1m/s burn close to the Earth where the escape velocity is say, 10.5 km/s gives a delta-v at infinity of... 145 m/s; over 100x better delta-v!- (User) Wolfkeeper (Talk) 20:31, 5 September 2009 (UTC)
thread_title:  Math check for my reverted edit 

message: "It can be easily shown that the impulse is multiplied by a factor of:
:\sqrt{1 +\frac{2 V_\text{esc}}{\Delta v}}
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
:\sqrt{1 +\frac{2 \times 50}{5}}
:=\sqrt{1 +\frac{100}{5}}
:=\sqrt{1 + 20}
:=\sqrt{21}
which is 4.6 not 3.3. Am I missing something? Art LaPella (talk) 15:43, 7 April 2010 (UTC)
replies: text: no ;-)- Wolfkeeper 16:12, 7 April 2010 (UTC)
thread_title: 3.3?

message: 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:
dE/dt=Fv-\frac{1}{2}\mu v^2
where \mu 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)
replies: text: The energy gained by the mass m1 of the rocket that isn't going to be released during the burn is still proportional to v; it's absorbing a fraction of the kinetic energy being produced that depends only on where you are in the burn in terms of mass ratios. The fact that you're losing energy in the exhaust doesn't seem to matter.- Wolfkeeper 23:21, 9 April 2010 (UTC)
replies: text: But I'll think about ways to clarify this.- Wolfkeeper 23:21, 9 April 2010 (UTC)
thread_title:  dE/dt is not F v 

message: 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)
replies: text: Do you have a good reference?- Wolfkeeper 00:39, 14 April 2010 (UTC)
thread_title: Introduction

message: 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)
replies: text: If you have a good reference that would be very useful, we're desperately short of refs for this.- Wolfkeeper 16:47, 24 May 2010 (UTC)
replies: text: Still short of good references. The Oberth effect is not mentioned in Stearns' or in about a half dozen other similar texts. So I looked for it in the literature. Oberth shows up, of course, but no mention is made of the "Oberth effect". Have to look further next time I'm down at Powell's Books. Williamborg (Bill) 03:11, 1 June 2010 (UTC)
replies: text: I've found Oberth talking about this kind of stuff, but obviously he doesn't use the term 'Oberth effect'. See:  around page 200 (pdf page 209) he talks about energy, and includes a powered slingshot example.- Wolfkeeper 03:59, 1 June 2010 (UTC)
thread_title:  Parabolic example 

message: 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)
thread_title:  Homeland-issue 

message: 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)
thread_title:  50 km/s vs 59 km/s 

message: 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)
replies: text: Gas bouncing back? Gravitons?? This is very confused on several points. 109.149.217.204 (talk) 14:41, 5 December 2015 (UTC)
thread_title:  Need clear example for lay people 

message: 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)
thread_title:  Proof 

message: 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)
thread_title:  Nothing to do with exhaust 

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thread_title:  External links modified 

message: 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)
thread_title:  Can someone please add a section on very-low-thrust applications such as ion-drive or solar sails? 

message: 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)
replies: text: I've reverted your changes because they're wrong. I suggest you read the references if you want to know why they're wrong. GliderMaven (talk) 16:30, 10 February 2019 (UTC)
thread_title:  Removed explanation "Based on work" 

message: The "Impulsive burn" section (first added on 30 January 2016 by GliderMaven and modified somewhat since) states:
: "Whereas the integration of the above energy equation can be done, numerically or otherwise, short burns of chemical rocket engines close to periapsis or elsewhere are usually mathematically modelled as impulsive burns, where the force of the engine dominates any other forces that might change the vehicle's energy over the burn."
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)
replies: text:  The article has changed sufficiently to render this complaint moot. - dcljr (talk) 03:41, 5 December 2019 (UTC)
thread_title:  Impulsive burn burn 

message: 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)
thread_title:  confusion between effect and maneuver 

message: 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:
* at 1 m/s, the rocket starts with 2x1=2 units of momentum. Adding 1 m/s increases the momentum to 2x2=4 units, for a gain of 2 units of momentum.
* at 10 m/s, the rocket starts with 2x10=20 units of momentum. Adding 1 m/s increases the momentum to 2x11=22 units, for a gain of 2 units of momentum.
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)
replies: text: Seven days have passed without comment so I will remove the section related to momentum. Dolphin (t) 23:48, 23 April 2024 (UTC)
replies: text: Done. See my diff. Dolphin (t) 23:50, 23 April 2024 (UTC)
thread_title:  Explanation in terms of momentum