Best place for an interplanetary launch facility?

[jlcarneiro]

I explained it in the post above yours (may have been ninja'd)

Thanks, but unfortunately I did not get ninja'd, I actually didn't understand your explanation.

Although now I DO understand ninja'd!

[xtoro]

Ok, lets put it this way because this works in my head...

Gravity Assist: Entering AND exiting a body's SOI without ever orbiting it in order to modify your trajectory and reach a different body. The closer your PE is to the body as you flyby, the more the body will affect your trajectory/speed.

Oberth Effect: IN ORBIT around a body and using its gravity to aid in your acceleration with the goal of raising your AP for cheap, or escaping that body's SOI. The bigger the difference between your AP and PE, the more you will benefit from this effect

Edited July 17, 2015 by xtoro

[jlcarneiro]

The "dumbed down" explanation would be that Oberth maneuver is a powered Gravity Assist, and vice versa.

Thanks for the dumbed down explanation! Maybe it should start a series analog to the "for dummies" one...

Say, you want to get a capture - enter orbit of Eve. If you only skirt its SOI, and start braking, you'll spend a lot of fuel to get the periapsis near the planet's atmosphere. But if you enter at such an angle that you're almost skirting its atmosphere, then burn retrograde at the periapsis, you'll get a capture (an orbit) in a fraction of the delta-V; as you approached it, its gravity was pulling you in and sped you up considerably, making your burn much more efficient - using Eve gravity for changing your velocity relative to Eve. (If you wanted to try capture using gravity assist, you'd have to use Gilly for braking. But Gilly being so small and light wouldn't be much of assistance - the heavier the body the stronger the effect, simply because you're "falling" faster.)

Let me see if I got this right: the Oberth effect independs of the direction I accelerate to? I mean, I assume that, in your example, I'm aproaching Eve counterclockwise (to achieve a prograde orbit), right? But, if I burn retrograde, I'm burning in the opposite direction of the movement, aren't I? I thought Oberth effect increased my speed if I burned in the same direction of the movement, potentializing my speed, isn't it?

Mun yes, Minmus no. Mun is close to Kerbin enough that you need quite a bit of delta-V to drop your periapsis to LKO. Note how the farther the orbit the slower the orbital speed and the less you need to brake to get the periapsis all the way down. So, getting from Minmus to LKO periapsis is very cheap. And again, orbital speed of Mun is high enough that Oberth effect for departure is not insignificant.

I didn't quite get this one either: at the beginning I thought you were saying it was worth to go from Mun directly to Duna. But, at the end, you say exactly the opposite, that Mun's speed is high enough to Oberth effect become insignificant...

[Yasmy]

Mun yes, Minmus no. Mun is close to Kerbin enough that you need quite a bit of delta-V to drop your periapsis to LKO. Note how the farther the orbit the slower the orbital speed and the less you need to brake to get the periapsis all the way down. So, getting from Minmus to LKO periapsis is very cheap. And again, orbital speed of Mun is high enough that Oberth effect for departure is not insignificant.

I think we are going to have to agree to agree. If you look at the numbers I posted earlier, they tell the same story you are telling now.

I did have 3 modifers to warn people that I wasn't talking about every moon to every planet: Occasionally, sometimes and for instance, and I also provided the raw data so people could draw their own conclusions.

Edited July 18, 2015 by Yasmy

[Sharpy]

Let me see if I got this right: the Oberth effect independs of the direction I accelerate to? I mean, I assume that, in your example, I'm aproaching Eve counterclockwise (to achieve a prograde orbit), right? But, if I burn retrograde, I'm burning in the opposite direction of the movement, aren't I? I thought Oberth effect increased my speed if I burned in the same direction of the movement, potentializing my speed, isn't it?

Oberth effect is only about the speed (although still speed relative to whatever body is pulling you). So it doesn't matter if you enter a prograde, retrograde or polar orbit. Your burn affects your speed relative to that body. Both prograde and retrograde burns at the periapsis, and regardless on what direction that periapsis goes, are affected. (Radial/Antiradial burns aren't really affected but at that location they are helpful in adjusting the escape trajectory to your needs, and Normal/Antinormal burns - uh, these seem to suffer a severe penalty.)

Oberth effect is a result of potential energy being converted into kinetic energy in the fuel you carry.

An intuitive way to imagine it. You're in your car, on a road going slightly uphill - the hill is such that if your car runs 10m/s, it will stop over a distance of about 50m, and it takes 10s to stop.

Situation 1: You're stopped. You floor it, for a second, switch to neutral gear. This maneuver accelerated your car by 10m/s. You covered 50m before stopping.

Situation 2: You're moving 100m/s, in neutral gear, then throw the drive gear in, floor it for a second, then back to neutral. You're moving 110m/s now after burning exactly the same amount of fuel as before. Of course you'll cover much more than 10m, because you've started with 100m/s. I don't know how much distance you'll cover thanks to that 100m/s, and I won't tell you exactly how much you've travelled extra due to your acceleration, but it's vastly more than 50m. How? Your deceleration from 110 to 100m/s took the same ten seconds as deceleration from 10 to 0 before. Except while before you ran with average speed of 5m/s over 10s time, now your average speed is 105m/s over the 10s. And then there's the tail of 100m/s exactly the same as if you weren't accelerating.

Now add to that that you're approaching a crest of the hill, the road getting more flat the farther you go.

And now imagine you're accelerating at a bottom of a valley between two hills, and you gained the 100m/s by going downhill from the first hill on neutral.

I didn't quite get this one either: at the beginning I thought you were saying it was worth to go from Mun directly to Duna. But, at the end, you say exactly the opposite, that Mun's speed is high enough to Oberth effect become insignificant...

And again, orbital speed of Mun is high enough that Oberth effect for departure is not insignificant.

Edited July 18, 2015 by Sharpy

[jlcarneiro]

Oberth effect is only about the speed (although still speed relative to whatever body is pulling you). So it doesn't matter if you enter a prograde, retrograde or polar orbit. Your burn affects your speed relative to that body. Both prograde and retrograde burns at the periapsis, and regardless on what direction that periapsis goes, are affected. (Radial/Antiradial burns aren't really affected but at that location they are helpful in adjusting the escape trajectory to your needs, and Normal/Antinormal burns - uh, these seem to suffer a severe penalty.)

Oberth effect is a result of potential energy being converted into kinetic energy in the fuel you carry.

An intuitive way to imagine it. You're in your car, on a road going slightly uphill - the hill is such that if your car runs 10m/s, it will stop over a distance of about 50m, and it takes 10s to stop.

Situation 1: You're stopped. You floor it, for a second, switch to neutral gear. This maneuver accelerated your car by 10m/s. You covered 50m before stopping.

Situation 2: You're moving 100m/s, in neutral gear, then throw the drive gear in, floor it for a second, then back to neutral. You're moving 110m/s now after burning exactly the same amount of fuel as before. Of course you'll cover much more than 10m, because you've started with 100m/s. I don't know how much distance you'll cover thanks to that 100m/s, and I won't tell you exactly how much you've travelled extra due to your acceleration, but it's vastly more than 50m. How? Your deceleration from 110 to 100m/s took the same ten seconds as deceleration from 10 to 0 before. Except while before you ran with average speed of 5m/s over 10s time, now your average speed is 105m/s over the 10s. And then there's the tail of 100m/s exactly the same as if you weren't accelerating.

Now add to that that you're approaching a crest of the hill, the road getting more flat the farther you go.

And now imagine you're accelerating at a bottom of a valley between two hills, and you gained the 100m/s by going downhill from the first hill on neutral.

Thank you, Sharpy!

Sorry to keep bothering you, but I still have some doubts:

• I got both situations you described. But, just to check, that disregard for the direction applies only in space and not to the car example, right? For example, let's take the 2nd situation: if I'm moving at 100m/s and throw the REVERSE gear in, flooring it for one second, it will have a lesser effect, right?
  
• I think I got what you meant on your other post, when you said "Mun's speed is high enough to Oberth effect become insignificant": you meant Oberth effect on LKO, right?
  

[leptoon]

Thank you for all the input! I didn't expect this thread to gain so much attention, still reading all the replies.

I originally planned to launch from Minmus but I think I'll probably use Minmus as a refueling depot and base of operations for interplanetary travel, with a station in MKO to serve as the launch facility. While I may have to haul rocket parts and fuel to the station, it's essential that I try to minimize resource consumption and the most important resource is time. With TACLS, I can't afford to wait years for launch windows from Minmus.

Edited July 18, 2015 by leptoon

[Yasmy]

Thank you, Sharpy!

For example, let's take the 2nd situation: if I'm moving at 100m/s and throw the REVERSE gear in, flooring it for one second, it will have a lesser effect, right?

Let's do the math. It's just the change in specific kinetic energy (kinetic energy divided by mass):

Forward: de = 1/2 (v+dv)^2 - 1/2 v^2 = 1/2 (+ 2 v dv + dv^2) = 1/2 (2*100*10 + 10^2) = 1050 m^2/s^2

Reverse: de = 1/2 (v-dv)^2 - 1/2 v^2 = 1/2 (- 2 v dv + dv^2) = 1/2 (-2*100*10 + 10^2) = -950 m^2/s^2

So yes, it is slightly less efficient in reverse.

Next, if we take the derivative, de/dv, we get de/dv = v. (Drop the 1/2 dv term, as usual in calculus when taking derivatives. It is infinitessimal.)

This is the Oberth effect: The change in specific orbital energy with respect to velocity is equal to your current velocity.

That's all there really is to it: kinetic energy is quadratic in velocity, so the derivative is linear in velocity.

Edited July 18, 2015 by Yasmy
minus sign error. doh.

[Sharpy]

Thank you, Sharpy!

Sorry to keep bothering you, but I still have some doubts:

• I got both situations you described. But, just to check, that disregard for the direction applies only in space and not to the car example, right? For example, let's take the 2nd situation: if I'm moving at 100m/s and throw the REVERSE gear in, flooring it for one second, it will have a lesser effect, right?
  
• I think I got what you meant on your other post, when you said "Mun's speed is high enough to Oberth effect become insignificant": you meant Oberth effect on LKO, right?
  

1. You just cut off the fastest 100m/s - 90m/s 10 seconds off the trip. Yes, you shortened the way less than you'd lengthen it in the first case. And that's all Oberth effect for ya - it affects itself too! You're decelerating on a lower speed average than you were accelerating at (95m/s vs 105m/s) and so it's less efficient!

After all, that LKO maneuver which is oh-so-efficient at the 2200m/s of your orbital speed? The 200m/s came "free" with Kerbin rotation. The remaining 2000 you've paid for dearly, burning well over 3000 delta-V on keeping yourself from falling down, fighting atmospheric drag and accelerating yourself to the orbit. Lots and lots of fuel went into gaining that speed in the first place. And the even more efficient Oberth effect when "dropping from Minmus"? How did you get to Minmus in the first place?

It only gets "free" when you start moving between "equal level" bodies, e.g. you first get to Eve for a braking assist + Oberth maneuver that throws you out towards Eve's back - that brings your Sun periapsis much much lower on the cheap, and then you can accelerate again when close to the Sun, for a very fast escape trajectory.

2. Once again: I wrote that Mun speed is high enough for Oberth effect to be not insignificant - a negation of a negation, in language this maneuver is called Litotes. That means the effect IS significant to a degree.

[cantab]

Back to the original question, try the Mun. Compared to Minmus it's an easier starting point for interplanetary trips because it's orbital period is shorter so there's less waiting for it to be at the right angle for departure. For some destinations it's also viable to depart directly from Low Mun Orbit - for Eve and Duna that is the more efficient approach. In general the delta-V requirement to reach another planet from the Mun's surface is about the same as to reach it from Low Kerbin orbit. If you do your refuelling in Mun orbit then that's around 640 m/s less delta-V your interplanetary ships need to carry at any one time.

https://www.reddit.com/r/KerbalAcademy/comments/1qu5jv/deltav_charts/

[Red Iron Crown]

But if you also use your Oberth burn trajectory to modify ejection angle, it's back to being a gravity assist. Given a month long orbital period, chances are you want to play with your ejection angle as well as realize greater Oberth gains.

The two are not mutually exclusive manuvers (as if they weren't confusing enough in usage)

Sorry, but that still isn't a gravity assist. A gravity assist requires entry to and exit from the SoI of the body being used for the assist; when starting from the Mun or Minmus the vessel never enters Kerbin's SoI, as the Mun and Minmus (and thus the vessel) are already inside it.

Definitely agree that the "Oberth Maneuver" and gravity assists are often combined, for some types of adjustments it is advantageous to perform the burn when at periapsis of a flyby.

[mhoram]

I hope these in-game pictures help to distinguish between Oberth effect and Gravity assist:

Utilizing the Oberth effect by burning in low Kerbin Orbit.

A gravity assist on your current orbit (blue) by the help of the Mun (brown) with a resulting orbit (purple).

The gravity assist has the effect that the blue and purple orbits are different without spending any fuel.

Utilizing a gravity assist in an efficient manner means to plan the brown gravity assist in such a way that the purple orbit leads to a certain target.

A powered gravity assist.

The gravity assist by the Mun (brown) is enhanced by an Oberth-effect-utilizing maneuver at Mun-Periapsis to bring you on the dotted path that leads out of Kerbins SOI.

Edited September 26, 2015 by mhoram
typo