Orbital insertion economics?

[strider3]

Is it more Delta V "friendly" to have an orbital insertion periapsis at high altitude, and then reduce to desired orbit or to just go for the desired periapsis from the initial ejection burn (or mid-course correction burn)? As an example:

I am heading to Moho from Kerbin, my "working" orbit will be 50 km. Is it more efficient to arrive at 500 km and create an orbit at that height then burn down to my 50 km orbit after...or just shoot for a 50 km periapsis from the git-go and burn to circularize?

Edited June 23, 2020 by strider3

[RoninFrog]

It's better to get a lower Pe right off, that way you can blast through a lot of the SOI at escape velocity and it won't have time to accelerate you as much.

P.S.  I hope you mean "Kerbin to Moho" because if you try to get a 50km Kerbin orbit, you will get a nasty surprise instead...

Edited June 23, 2020 by RoninFrog

[strider3]

19 minutes ago, RoninFrog said:

It's better to get a lower Pe right off, that way you can blast through a lot of the SOI at escape velocity and it won't have time to accelerate you as much.

P.S.  I hope you mean "Kerbin to Moho" because if you try to get a 50km Kerbin orbit, you will get a nasty surprise instead...

Yep...Moho to Kerbin would be...bad! Thanks!!

[king of nowhere]

if you make your orbit higher than it needs to be and then circularize it, you are wasting fuel twice: once to raise your orbit, and once to lower it back. small corrections for a few tens of m/s are normal, but if you need more than that, you're being inefficient.

the gravity turn is by far the most efficient manuever to enter orbit: it's basically an hohmann transfer orbit from the ground to the orbit, and hohmann transfer orbits are the cheapest (or close enough; not sure if there is some complex manuever that would save a bit, but if real rocket scientist use hohmann transfer all the time, there's a good reason. i didn't believe it mattered that much, but after trying i discovered that on kerbin i could save approximately 1 km/s of deltaV by making a gravity turn rather than going straight up and then circolarize. the same goes for any other planet, except where there is no atmosphere it's even more convenient.

you can try for yourself: make a quicksave, get to orbit in the two different ways and write down how much deltaV you expended. you'll be surprised.

[strider3]

5 minutes ago, king of nowhere said:

if you make your orbit higher than it needs to be and then circularize it, you are wasting fuel twice: once to raise your orbit, and once to lower it back. small corrections for a few tens of m/s are normal, but if you need more than that, you're being inefficient.

the gravity turn is by far the most efficient manuever to enter orbit: it's basically an hohmann transfer orbit from the ground to the orbit, and hohmann transfer orbits are the cheapest (or close enough; not sure if there is some complex manuever that would save a bit, but if real rocket scientist use hohmann transfer all the time, there's a good reason. i didn't believe it mattered that much, but after trying i discovered that on kerbin i could save approximately 1 km/s of deltaV by making a gravity turn rather than going straight up and then circolarize. the same goes for any other planet, except where there is no atmosphere it's even more convenient.

you can try for yourself: make a quicksave, get to orbit in the two different ways and write down how much deltaV you expended. you'll be surprised.

I am speaking more of going from Kerbin orbit to another body. I can leave Kerbin for Moho (for instance) and make my maneuver node to arrive at Moho at 50 km or 500 km. If I want to "work" Moho landings from 50 km, I was curious if I should arrive at Moho at 50 km periapses (my required working orbit) or a higher periapses...and burn down to 50 km from a Moho orbit of 500 km.

[Regor]

1 hour ago, strider3 said:

I am speaking more of going from Kerbin orbit to another body. I can leave Kerbin for Moho (for instance) and make my maneuver node to arrive at Moho at 50 km or 500 km. If I want to "work" Moho landings from 50 km, I was curious if I should arrive at Moho at 50 km periapses (my required working orbit) or a higher periapses...and burn down to 50 km from a Moho orbit of 500 km.

arrive at 50km. Look at oberth effect.

[James M]

The lower your periapsis when you arrive at your destination, the less fuel you'll need to circularize as your kinetic energy is at it's highest due to the planets' gravity increasing your ships velocity. 

https://www.reddit.com/r/KerbalAcademy/comments/1jhne0/clearing_up_misconceptions_about_the_oberth_effect/

This post really delves into WHY that works and if you just take that little bit of extra time really reading and doing the calculations, it makes way more sense and I gotta say. It's really cool! 

[strider3]

12 hours ago, James M said:

The lower your periapsis when you arrive at your destination, the less fuel you'll need to circularize as your kinetic energy is at it's highest due to the planets' gravity increasing your ships velocity. 

https://www.reddit.com/r/KerbalAcademy/comments/1jhne0/clearing_up_misconceptions_about_the_oberth_effect/

This post really delves into WHY that works and if you just take that little bit of extra time really reading and doing the calculations, it makes way more sense and I gotta say. It's really cool! 

That post was very helpful! Thanks James!

[Curveball Anders]

14 hours ago, James M said:

The lower your periapsis when you arrive at your destination, the less fuel you'll need to circularize as your kinetic energy is at it's highest due to the planets' gravity increasing your ships velocity. 

https://www.reddit.com/r/KerbalAcademy/comments/1jhne0/clearing_up_misconceptions_about_the_oberth_effect/

This post really delves into WHY that works and if you just take that little bit of extra time really reading and doing the calculations, it makes way more sense and I gotta say. It's really cool! 

There is just one thing to keep in mind.

Make sure to adjust your inclination to desired angle as far out/up as possible.

Since the higher your orbital speed (or energy) the more expensive to adjust you inclination.

[king of nowhere]

15 hours ago, James M said:

The lower your periapsis when you arrive at your destination, the less fuel you'll need to circularize as your kinetic energy is at it's highest due to the planets' gravity increasing your ships velocity. 

https://www.reddit.com/r/KerbalAcademy/comments/1jhne0/clearing_up_misconceptions_about_the_oberth_effect/

This post really delves into WHY that works and if you just take that little bit of extra time really reading and doing the calculations, it makes way more sense and I gotta say. It's really cool! 

otoh, what always left me puzzled about that explanation is where the extra kinetic energy comes from. you are burning fuel, converting chemical energy into kinetic energy. the chemical energy of the rocket is always constant, but the ship is getting more energy, where is the extra energy coming from? i've never seen it explained well.

but thinking about it, i just realized the source is the exhaust kinetic energy. see, the fuel also has a kinetic energy, as part of the rocket. the faster the rocket goes, the higher the kinetic energy of the fuel. when you expel fuel, the fuel changes kinetic energy, and if you're going faster, the fuel will change its kinetic energy more.

 

regarding insertion orbit and circularization, another way of seeing it is that if you enter with a high periapsis, then you have to lower your apoapsis from escape to stay in orbit, then you are in a circular high orbit, and you have to lower the orbit at both ends. if you enter with a low periapsis, you have to lower apoapsis from escape to stay in orbit, which has the same cost as it would on the other case. but now you are in an elliptical orbit, so you only have to lower your apoapsis. so you save one manuever. and that without even counting oberth effect. so it means, by entering low you save fuel twice.

 

i never went to moho yet, but i've done a lot of mun insertions; without oberth effect, it is around 200 m/s to drop the other side of the orbit to low mun. with oberth effect, it's 150. and then i'm already in a low circular orbit, while if i enter at high periapsis, i am now in an elliptic orbit and have to run a new manuever to lower my apoapsis. which, depending on how high i was, would be another 50 to 100 m/s. so, by inserting low i can save up to 50% fuel for the insertion manuever

[James M]

7 hours ago, king of nowhere said:

so you save one manuever. and that without even counting oberth effect. so it means, by entering low you save fuel twice.

I still struggle tbh to wrap my head around all the maths but I think in that situation you'd be correct in saying you'd save one maneuver but as such only technically saving fuel once. With a higher periapsis insertion, you'd actually be doing three things. Insertion, transfer orbit (To get your actual orbit where you need it), then to final circulation. With a low insertion however you'd still (basically) save fuel twice because of the oberth effect. I say this because both burns (Insertion and circulation) would be performed at periapsis, where kinetic energy of the craft is at at it's highest.

Sorry if that seemed confusing. Got a lot on my mind right now >.>

[Curveball Anders]

14 hours ago, king of nowhere said:

what always left me puzzled about that explanation is where the extra kinetic energy comes from. you are burning fuel, converting chemical energy into kinetic energy. the chemical energy of the rocket is always constant, but the ship is getting more energy, where is the extra energy coming from? i've never seen it explained well.

The paradox section of the Wikipedia article explains some, but I still don't fully get it.

But I can confirm that "It works in KSP"

[Zhetaan]

On 6/23/2020 at 7:00 PM, strider3 said:

Is it more efficient to arrive at 500 km and create an orbit at that height then burn down to my 50 km orbit after...or just shoot for a 50 km periapsis from the git-go and burn to circularize?

Assuming a reasonably optimised transfer to Moho from Kerbin, going directly to 50 km from Kerbin-Moho insertion saves approximately 195 - 200 m/s over going to 500 km--and that's before the additional burn to adjust the 500 km orbit down to 50 km, which costs an additional 262 m/s.