Hoffman Transfers and Oberth Effect

[MrAnonymous]

Can anyone explain what is Hoffman Transfer and the Oberth Effect. I don't really understand what they mean or what they do.

Edited April 22, 2014 by MrAnonymous

[EdFred]

The first question is, how much understanding do you have on what makes an orbit work?

The second question is, do you know what happens to your orbit when you make prograde/retrograde burns?

[Red Iron Crown]

Hohmann transfer is a low-energy method of moving from one orbit to another, by burning into an intermediary orbit that is tangential to both initial and final orbits. Generally speaking, this is the most commonly used method of getting from one celestial body to another.

The Oberth Effect is the observation that delta-V spent at higher speed changes the specific orbital energy by a greater amount. Essentially, if you want to change your orbit, do your burn at the greatest speed possible, usually by doing so at a lower altitude.

Edited April 21, 2014 by Red Iron Crown
Miscopied link, oops.

[mhoram]

I finally understood of the Oberth effect from the answers in this thread:

http://forum.kerbalspaceprogram.com/threads/60250?p=814201#post814201

Edited September 26, 2015 by mhoram
typo

[MrAnonymous]

The first question is, how much understanding do you have on what makes an orbit work?

The second question is, do you know what happens to your orbit when you make prograde/retrograde burns?

I know what happens when I do those, I just don't know the concept of Hoffman transfers and the Oberth Effect.

[MrAnonymous]

Hohmann transfer is a low-energy method of moving from one orbit to another, by burning into an intermediary orbit that is tangential to both initial and final orbits. Generally speaking, this is the most commonly used method of getting from one celestial body to another.

The Oberth Effect is the observation that delta-V spent at higher speed changes the specific orbital energy by a greater amount. Essentially, if you want to change your orbit, do your burn at the greatest speed possible, usually by doing so at a lower altitude.

Oh awesome!!! Thanks a lot it really helped. I just wanted to understand the concept.

[Dkmdlb]

It's not HOFFMAN.

It's HOHMANN.

[thereaverofdarkness]

The Oberth Effect is from magic pixie dust that floats around celestial bodies. The more gravity they have, the more pixie dust there is, and the further you are away from them, the less you have around you.

The power of your engines is how much thrust they have plus how fast they are burning pixie dust.

I understand nearly everything relating to space travel and astrophysics, but this is the only way I can comprehend the Oberth Effect.

[MrAnonymous]

It's not HOFFMAN.

It's HOHMANN.

Woops, see I'm a noob at the acronyms. Thanks a lot haha.

[Capt. Hunt]

Hohmann's not an acronym, it's named after the scientist who invented the maneuver, Walter Hohmann.

[Rastaman]

The question is, how much dv does the Oberth effect provide? In stock KSP it seems largely insignificant. You maybe have 100 m/s difference between an 80 km orbit around Kerbin and a 1000 km orbit, a difference you rarely even notice. It is massively outclassed by aerobraking for instance, which you can use in most planetary systems.

[Kasuha]

The question is, how much dv does the Oberth effect provide? In stock KSP it seems largely insignificant. You maybe have 100 m/s difference between an 80 km orbit around Kerbin and a 1000 km orbit, a difference you rarely even notice. It is massively outclassed by aerobraking for instance, which you can use in most planetary systems.

Oberth effect does not provide any dv. Oberth effect only allows you to make your spent dv to have bigger impact. Or smaller if you do it wrong.

It does not really depend on whether you use stock KSP or some mods. And it is definitely not insignificant, although some bodies are better to use it than others. Still, it came as quite a surprise to me how much dv can you save by braking low above Moho compared to braking at high Moho periapsis.

Aerobraking is a different story as that allows you to brake in orbit spending virtually no dv. If you need to brake. And majority of bodies in KSP have no atmosphere.

Edited April 23, 2014 by Kasuha

[EdFred]

Oberth effect basically means if you do a burn at a higher velocity your kinetic energy ends up being higher. Stripped down to the basics, if you are doing a gravity assist at a planet, and you perform a 10m/s burn at a periapsis velocity of 1000m/s vs a periapsis velocity of 100m/s, your energy return is higher when you leave the SOI. How?

KE=0.5mv²

I'm going to leave out a lot of extra math, because I think it gets in the way of demonstrating the principle. We are only concerned with the change we get by making the burn, and for simplifying things, lets say our mass is 2kg. So assuming our 1000m/s periapsis and adding 10m/s results in

dKE = 0.5(2)(1010)² - 0.5(2)(1000)²

dKE = 1020100 - 1000000

dKE = 20100kg*m²/s²

If we do the same thing at 100m/s

dKE = 0.5(2)(110)² - 0.5(2)(100)²

dKE = 12100 - 10000

dKE = 2100kg*m²/s²

We basically got almost 10 times the return by burning at a higher velocity than a lower velocity when leaving the SOI. There's a lot of other math involved in entering and leaving the system, but that's basically what Oberth says. So, when performing gravity assists, get your periapsis velocity as fast as you can - and you do that by making the altitude as low as you can.

Edited April 23, 2014 by EdFred
correction of units

[MarvinKitFox]

Oberth effect, in VERY simple speak.

Rocket burning fuel increases its momentum.

Orbits don't run on momentum, they run on Energy.

Energy gain from momentum change *at higher speed* is greater than energy gain from same momentum change at low speed.

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disclaimer:

Very *very* simple speak. Multiple areas of inaccuracy/inexactness exist. This description is qualitative, not quantitative.