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Un-necessary kerbin orbit before space travel?


krblman
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In a lot of videos I see on YouTube of people going to other celestial bodies in KSP (usually the mun, but sometimes I watch videos about Duna) they orbit Kerbin before going any further. Is there any advantage to this? I don't do it personally because it looks like a waste of fuel - and in my science mode save there's no room to waste fuel. I only have around 7km/s of Delta V not including the lander which itself have around 2km/s of Delta V to get home.

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There's not really much advantage to orbiting Kerbin... but there's not really a disadvanatge either. You're going to circularize at some stage after launch, whether you then carry on burning to reach escape velocity or not makes no difference to the amount of dV used - you might as well plot an efficient transfer while you're in orbit of Kerbin. It doesnt waste any fuel.. arguably saves you some with better planning.

also un-necessary hypen is unnecessary :P

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I've not noticed any deltaV loss when stopping the burn to orbit Kerbin. For me, It's far easier to get to orbit and leave the system in a proper transfer window then it is to wait for Kerbins rotation to put the KSC where I need it for a constant burn. 

 

I have heard others claim it saves deltaV, so maybe its just my designs, or the way I fly the ships because I don't see the loss.

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1 hour ago, krblman said:

In a lot of videos I see on YouTube of people going to other celestial bodies in KSP (usually the mun, but sometimes I watch videos about Duna) they orbit Kerbin before going any further. Is there any advantage to this? I don't do it personally because it looks like a waste of fuel - and in my science mode save there's no room to waste fuel. I only have around 7km/s of Delta V not including the lander which itself have around 2km/s of Delta V to get home.

Unless your ship is going extremely fast (so fast it will explode before getting out of the atmosphere without heat shielding), you will always lose fuel by not doing a proper gravity turn and going to a proper orbit first.

It is also virtually impossible to go anywhere but the Mun or Minmus doing it that way.

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There are a few reasons why typically going into orbit before leaving kerbin is better than not. All of them stem from the oberth effect.

When you are in a circular orbit, your vertical velocity is zero, and if you start burning prograde, it takes some time before you start gaining altitude fast, at least when compared to launching straight up. When launching straight up, your prograde burn happens as you gain quite a lot of altitude.  If an orbiting craft and a vertically launched craft do the same prograde burn, the vertical craft will gain less energy. While the vertical craft is moving up, gravity is taking away energy and speed, so as it burns and moves up, it is adding velocity onto a velocity that gravity has taken away from, thus adding less than optimal energy. (KE = 1/2mv^2, so at a higher velocity, a change in velocity adds more energy.) Whereas the orbiting craft loses less energy and velocity due to gravity as it burns since it travels less of a distance upwards. This means that the burn is adding velocity onto a higher velocity/energy, which means that it adds more energy than the same burn that is done on a vertically launched craft.  Higher energy means more velocity when the craft finally escapes kerbin.

Another reason orbiting is better is because it allows for ejection burns to be split into multiple orbits, specifically, this allows the split up burns to be completed at the periapsis, where the craft is moving faster, and more energy can added by a certain change in velocity. For a vertical launch, this is not an option, and the longer the burn in a vertical launch, the less efficient it gets, since more burning is happening at a higher altitude. Also, splitting up the orbit into multiple burns means that a small TWR can be used, which allows for a craft to use less engines, leading to a lower dry mass and thus less required fuel.

Edited by J10R
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There is a Δv savings to be had by doing transfer burns as low as possible, and in some cases this means that one never has a proper orbit around Kerbin. But the difference is negligible if your parking orbits are low, and those same orbits make the transfers easier. There's also the problem that some transfers need very high TWR to minimize Δv expenditure.

How high are your parking orbits, anyway?

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17 minutes ago, UmbralRaptor said:

There is a Δv savings to be had by doing transfer burns as low as possible, and in some cases this means that one never has a proper orbit around Kerbin. But the difference is negligible if your parking orbits are low, and those same orbits make the transfers easier. There's also the problem that some transfers need very high TWR to minimize Δv expenditure.

How high are your parking orbits, anyway?

I personally don't do parking orbits, but the first stage of me getting home from anywhere (minmus, duna, etc) is usually at a periapsis of 600km and then i aerobrake

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1 hour ago, GuessingEveryDay said:

Wow. I usually park somewhere between 70-80 km.

Same. I want to say that there's ~50-100 m/s of savings in terms of getting into orbit at 70.x km vs 100 km. (In spherical cow land it's 50 m/s, but the lower orbit should have lesser gravity losses) Plus somewhat more for the transfer from the Oberth effect.

2 hours ago, krblman said:

I personally don't do parking orbits, but the first stage of me getting home from anywhere (minmus, duna, etc) is usually at a periapsis of 600km and then i aerobrake

If you don't care too much about where you land (or are sufficiently good at planning ahead), there's something to be said for just directly reentering from the Mün/Minmus. Duna's a bit more fraught, but for initial capture, the lowest possible periapsis still helps.

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Can't recall actually if they save you d/v (I would say so), but direct burns were always a thing.

The easier one you can experiment is to bring Kerbs in Kerbol SOI. Just burn straight up at 6 in the morning (at dawn) until you reach the right speed to leave the SOI, cash your xp when done and burn back home. Piece of cake. Easy rocket.

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If everything were coplanar, there would be next to no difference between first going to a parking orbit, and just burning straight from the pad to Kerbin ejection, so long as they both followed a gravity turn. Direct ascent might be a hair more efficient in some cases due to not needing any coast phase, but you will not lose much dV by first inserting into a low parking orbit.

Both would be substantially more efficient than anything involving going straight up.

However, interplanetary transfers to inclined targets are usually going to favor going to a parking orbit first, for the simple reason that the point at which you want to launch into Kerbin orbit is dependent on the inclination of the ejection orbit, whereas the actual ejection burn is dependent on the  location of the periapsis of the ejection orbit.

If, for example, you want to meet Minmus 90 degrees after its descending node, you'd want to launch +/- 6 degrees north/south when KSC is underneath one of Minmus's equatorial nodes, and make your Minmus transfer burn roughly 90 degrees before its descending node (i.e. 180 degrees prior to where you want to meet Minmus).

So, you meet Minmus +90 degrees after the descending node, transfer -90 degrees before the descending node, and ascend to parking orbit either at the descending node, or -180 degrees before the descending node (at the ascending node), which means a parking orbit.

EDIT: One way to think about that minor advantage to co-planar direct ascents is that you still have to achieve circular Kerbin orbit prior to your transfer, but you can do so while still in the atmosphere: you can effectively make your transfer burn from a 50 km Kerbin orbit rather than a 75 km Kerbin orbit, as you don't need your "parking" orbit to be stable.

Edited by Starman4308
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@Starman4308: anything you write is theoretically "perfect". But piloting is art and not science. Cosine losses may happen. Easy piloting to achieve and exploit "game achievements" like XPs should be taken into account.

Nevertheless, science is science.

 

 

I like your view of "not stable orbit". That is art.

Edited by Signo
Art
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15 hours ago, UmbralRaptor said:

 

Same. I want to say that there's ~50-100 m/s of savings in terms of getting into orbit at 70.x km vs 100 km. (In spherical cow land it's 50 m/s, but the lower orbit should have lesser gravity losses) Plus somewhat more for the transfer from the Oberth effect.

If you don't care too much about where you land (or are sufficiently good at planning ahead), there's something to be said for just directly reentering from the Mün/Minmus. Duna's a bit more fraught, but for initial capture, the lowest possible periapsis still helps.

I do "park" when I go to duna (atleast not when i wanna go to ike) and set my periaps around 30km to save a lot of fuel, since Duna's atmosphere goes up to 50km its gonna aerobrake

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