Jump to content
  • 0

Hello, I'm having trouble getting to a low Kerbol orbit, can anybody help?


Ant47
 Share

Question

So, I'm trying to get to a low-Kerbol orbit since I have a contract asking me to get a bunch of scientific measurements from there and also because if a process all of that science in a mobile lab, I can probably upgrade everything I can on my R&D facility level. My current problem is that I run out of fuel on my last stage when my peristalsis is around 3-4 million meters. Whenever I add more fuel, I just use more getting to orbit around Kerbol since my craft is already like 500 tons. I have more then enough funds and leveled-up facilities to build things, but I started playing KSP around a month ago, so I don't know what my best options are for this. 

Just to add: In all of my attempts I just burnt prograde until I was on an escape trajectory from Kerbin, then warped to my Kerbol orbit apoapsis, and when I was there, I would burn retrograde as long as I could.

Edited by Ant47
Realized something:
Link to comment
Share on other sites

9 answers to this question

Recommended Posts

  • 0
9 hours ago, Ant47 said:

Whenever I add more fuel, I just use more getting to orbit around Kerbol since my craft is already like 500 tons.

It sounds like rocket construction is an area in which you can improve further. You'd be surprised just how much proper construction matters. A launch vehicle of five hundred tons in mass should get you really, really far if you built it well. I bet I could get a direct Hohmann transfer to a low solar periapsis in less mass than that, no need for a bi-elliptic one.

If you're encountering a case where adding more fuel to your rocket is no longer improving its dV, you are most likely adding that fuel to an already overloaded stage, thereby causing other stages to get choked by the added mass. The way rockets work is that everything you add on top affects everything that exists below. This may sound daunting - that's like, an unlimited number of variables to take into account!

Good news though - it's much easier than it sounds. Because the way the math works out, there's only one single number that matters for each individual stage, and you can in fact eyeball it most of the time and it'll be fine, so long as you avoid going really far off track. Doing the math just helps you learn how to eyeball it. I've written about it here before. Apologies, it's kind of a wall of text. I have a regrettable tendency towards those. Trust me though, it'll be worth your time :)

 

9 hours ago, Ant47 said:

Just to add: In all of my attempts I just burnt prograde until I was on an escape trajectory from Kerbin, then warped to my Kerbol orbit apoapsis, and when I was there, I would burn retrograde as long as I could.

Note that this is very inefficient, due to something called the Oberth effect. It's a particularly arcane bit of orbital mechanics, and you had no way of knowing this beyond figuring it out through trial and error, so don't be hard on yourself over it :) This is what the forum is for.

The way it works, in simplified terms, is that spending fuel is more efficient the deeper down you are in a gravity well. It's not exactly that, but that's what it boils down to in practice in KSP.

What you have been doing is spending a small amount of fuel to get out of Kerbin's gravity well, then spending a lot of fuel to aim at your actual destination while you are not anywhere near a gravity well. Thus you spent more fuel than you needed to. What you should have been doing instead is spending all your fuel deep in Kerbin's gravity well - in other words, making the full transfer burn in low Kerbin orbit. Counterintuitively, the same amount of fuel - indeed, the same amount of dV - would give you a noticeably deeper Sun periapsis then.

Or, well, you can follow Zheetan's suggestion and do a bi-elliptic transfer. But even then, you'd ideally do the full transfer burn to your high solar apoapsis directly from low Kerbin orbit. Escaping Kerbin before even plotting your transfer is always the least efficient method.

But how do you get from this tiny circle around Kerbin to your destination in solar orbit? Well, imagine Kerbin wasn't there. Imagine if it was just your spacecraft in a solar orbit that happens to be identical to Kerbin's solar orbit. What would you do to lower your periapsis towards the sun? Burn retrograde to your solar orbit, of course. And if you wanted to get out to Eeloo? Burn prograde, of course.

So that's what you do, except that you must do it while circling Kerbin. You are in a low orbit around the planet, but your goal is still to make a burn in the correct direction relative to Kerbin's solar orbit. For example, if you want to go closer to the sun, you want to make a burn on Kerbin's sunlit side, so you are ejected from Kerbin's SOI "backwards". In other words, you make a prograde burn (relative to your low Kerbin orbit) that ejects you retrograde (relative to Kerbin's solar orbit). And going out to Eeloo is the same, just on Kerbin's night side. You make a prograde burn (relative to your low Kerbin orbit) that ejects you prograde (relative to Kerbin's solar orbit). In this way, the burn to escape Kerbin's sphere of influence is at the same time a burn in the right direction relative to solar orbit, so you can just keep burning in the same direction to lower your solar periapsis (or raise your solar apoapsis) despite still being just a few kilometers above Kerbin's atmosphere.

Of course, depending on the amount of dV you must spend and the acceleration your engines provide, making a very large burn in low Kerbin orbit can be very tricky. Because the orbit is so strongly curved, and you traverse a significant portion of the circle in just a few minutes, burns you make there grow less and less efficient the longer they get. If you burn at the maneuver node, you'll burn off-prograde for most of the time, whereas if you follow the prograde marker, you'll not burn in the right direction for most of the time. Rule of the thumb is that if your burn is longer than six minutes (three before the node and three after), you're starting to get inefficient.

You can get around this by splitting the burn. For example, make one burn that raises your Kerbin apoapsis to around where the orbit of the Mun is. (Take care not to actually encounter the Mun, you don't want that here.) Loop around once, and as you come back towards your Kerbin periapsis, make a new maneuver node there and start a new burn in the same direction as the first one - basically continuing what you started.

Once you have an escape trajectory out of Kerbin's SOI, you can keep burning until you have achieved your desired solar orbit destination.

Link to comment
Share on other sites

  • 0

Hi, @Ant47. And welcome to the fun and frustration that is KSP. 

The way I learned it is to make sure your suborbital trajectory gets you to a minimum of 72,000 meters, orient your ship to there the navball displays half ground (brown) and half sky (blue). Then do a full burn, and while doing your burn, click <M> for the map view and continue the burn until you see your orbit line around Kerbin. 

I usually begin doing an insertion roll (change in rocket direction) between 10,000 and 25,000 meters, and reduce main throttle as my speed increases to conserve fuel. That way, I have more fuel to adjust my orbit. 

As long as :prograde: and :retrograde: are above 72,000 meters, and regardless of how ugly that orbit is, you're in orbit!

Link to comment
Share on other sites

  • 0

I think you misunderstood a bit. I can easily get into low orbit around KERBIN, but I mean getting into low orbit around KERBOL (the sun) that's difficult.

Link to comment
Share on other sites

  • 0
5 hours ago, Ant47 said:

I think you misunderstood a bit. I can easily get into low orbit around KERBIN, but I mean getting into low orbit around KERBOL (the sun) that's difficult.

Yes, yes it is.

5 hours ago, Ant47 said:

I don't know what my best options are for this. 

Near space over the sun is far enough away, in relative terms, that a typical Hohmann transfer orbit is no longer the most efficient way to do it, and you're better off using a bi-elliptic transfer, instead.  That's similar to what you've been doing, but your problem right now is that you're burning at an apoapsis that is far too close to the sun to be efficient.   You'll want to boost all the way out to Eeloo at the least.  Farther is better and, counterintuitively, more efficient, but don't go so far as an escape orbit.  You'll need to keep in mind the contract deadline, too.  Once you're at your supra-Eeloo apoapsis, burn retrograde to bring your periapsis down from Kerbin to the sun as you have been.  This has the additional advantage of making your periapsis transit faster, so perhaps you won't burn to death (or at least get by only slightly crispy about the edges).  Remember that you're not near the sun for the purposes of science experiments until you're under one million metres in altitude, so I hope you brought thermal protection.

Thankfully, you can take the measurements while flying by--you don't need to circularise.

The other things that you can do are all the standard advice about designing your vessel properly for the mission, so picking the right engine (for a 500-tonne vessel, it won't be ion engines or Nervs), taking only what you need and nothing else, setting up clever staging, and so forth.

 

On an unrelated note, welcome to the forum!  As newer players go, you're certainly ambitious, but that's not a bad thing.  Good luck, and if you have any other questions, then please feel free to ask.

Edited by Zhetaan
Link to comment
Share on other sites

  • 0
  1. Make your probe as small and more importantly as light as possible. It might seem like you should cram as much fuel as possible on there, but the delta-V gains on that stage are often marginal and the extra weight will reduce the delta-V of your other stages and could end up reducing your total delta-V. You should also keep an eye on your TWR as a low TWR increases burn duration which can lead to cosine losses (due to burning in a direction that isn’t directly prograde/retrograde) that waste delta-V.
  2. After a lot of experimenting, I’ve found that if you want a payload mass below 7 tons (probe plus propulsion) or the cheapest mission possible, it’s better to use the Terrier than the NERV as you get more delta-V that way; however above 7 tons the NERV’s vastly higher ISP finally outweighs its much higher mass and you’ll get more delta-V from it than the Terrier. Both have the same thrust, the Terrier has a gimbal (so you need less powerful reaction wheels to keep it pointing the right way)  but the NERV has an alternator (can generate power when burning) but the NERV is much more expensive (10k funds versus under 400 for the Terrier) and is much further along the tech tree. 
  3. Work smart, not hard- burning retrograde at a very high apoapsis (somewhere beyond Jool’s orbit at least) is a lot more effective at reducing your periapsis because the orbital velocity out there is a lot less- removing 100m/s when you’re going at 1000m/s makes a lot more difference than when you’re going 10,000m/s. You could also try using a Jool gravity assist to lower your periapsis, getting some ‘free’ trajectory adjustment at the cost of requiring a pretty precise flyby to eject you solar retrograde from Jool’s SOI.

edit: it turns out you can crash into the sun very easily with about 4km/s of delta-V in LKO- burn prograde until your apoapsis is at Jool's orbit, then when you reach apoapsis point retrograde and burn until your periapsis hits the sun- you can easily reverse your orbit and go solar retrograde this way.

Edited by jimmymcgoochie
Link to comment
Share on other sites

  • 0

Between Kerbin and the sun is a big old helper. Next time you try this wait until you have what would be a transit window to Eve, but instead of trying to enter orbit there, play with plotting intercepts that will slow your ship and drop it closer to the sun. These slingshots are not easy, however. Real space programs use them not for convenience, but to reduce the size and cost of space probes. 

Link to comment
Share on other sites

  • 0
On 7/17/2021 at 5:05 PM, Ant47 said:

So, I'm trying to get to a low-Kerbol orbit since I have a contract asking me to get a bunch of scientific measurements from there and also because if a process all of that science in a mobile lab, I can probably upgrade everything I can on my R&D facility level. My current problem is that I run out of fuel on my last stage when my peristalsis is around 3-4 million meters. Whenever I add more fuel, I just use more getting to orbit around Kerbol since my craft is already like 500 tons. I have more then enough funds and leveled-up facilities to build things, but I started playing KSP around a month ago, so I don't know what my best options are for this. 

Just to add: In all of my attempts I just burnt prograde until I was on an escape trajectory from Kerbin, then warped to my Kerbol orbit apoapsis, and when I was there, I would burn retrograde as long as I could.

Generally speaking, when running into dV problems the answer isn't moar boosters/struts/fuel, but rather moar stages. That and gravity assists can go a long way.

HTHs,

-Slashy

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...