Kerbin Ejections

[HammyTheHamster]

So I know this is something that has been discussed a few times but I've not come across a thread here, Reddit or elsewhere that seems to ask the right question on this topic that I'm looking for.

What I'm wondering about is the most efficient way to perform an escape burn from Kerbin with an interplanetary craft with possible use of a refueling station on Mun or Minmus. I'm aware that you don't ever strictly need a refueling base but they are fun to set up (though not always to use) and I feel that with massive colony-style ships assembled in orbit they do make some sense rather than launching full tanks all the time.

 

However I'm not sure that there's any real value to them for exit burns given that I have my interplanetary craft fully fueled in either LKO (fuel from launches) or orbit of a moon (fuel from ISRU), obviously the Oberth effect can be used from a fueling station but given the 800-900m/s burn required to lower periapsis (plus plane change if using Minmus) to Xkm would the Oberth effect still be worth it compared to just starting in a circular orbit at Xkm with full tanks?

My guess is that the answer is 'No' for the closer planets like Duna - for which the ideal solution would probably be to burn directly from the fuel station. but what about the further planets like Jool?

 

I still have options for using ISRU within Kerbin obviously but I don't want to go to the trouble of creating a fuel outpost on Minmus (for example) if it isn't actually going to be beneficial.

[Cpt Kerbalkrunch]

Welcome to the forums.

I used to do a lot of refueling at Minmus. For the outer planets, it works extremely well, and is worth it if you don't mind the real-time you'll spend. It won't cost nearly as much as you think to lower your Pe. After fueling your ship, wait until Minmus is just about to pass between Kerbin and the sun (for outer planets, of course). Burn retrograde to Minmus' orbit to lower your Kerbin Pe between 70km and 75km. If I remember right, it costs about 150m/s.

Set your maneuver node and plot your encounter. When you reach Pe, you are now travelling much faster, so that when you accelerate, you'll see how quickly your Ap rises. If you think about it just from there, you're ship fully-fueled in LKO has a low circular orbit (at a "standstill", so to speak), compared to your ship that dropped down from Minmus. It already has a Ap equal to Minmus' orbit, and is moving much faster, and it's only cost you less than 200m/s. You will see the savings in the ability of your ships, but you have to decide if you want to spend real-time doing the refueling or not.

[HammyTheHamster]

Ah OK, I wasn't taking into account that it was quite so cheap to lower PE from Minmus.

Thanks for that, I'm OK with the real-time cost so long as I'm 'doing something' rather than just time-warping.

[FullMetalMachinist]

@HammyTheHamster the concept you're looking for is called a gate orbit. If you're interested in an explanation of the math, there is this thread:

The other thing to worry about if you want to refuel at one of the moons is timing. Say you eject from Mun and now have an elliptical orbit with your AP pointing to where the Mun was when you left. When you get to PE to finish your burn, you need that angle to match the ejection angle that you need for your transfer burn, and you need to reach that PE to do your burn at the right time for the transfer window. Not a trivial problem to figure out. 

The other option is to have a refueling station in Kerbin orbit that you can move to the different gate orbit altitudes. Then when you're going to Jool, put the station in that gate orbit. Then dock your interplanetary ship and refuel it. Now you're in a circular orbit at the correct gate orbit altitude, and can plan and execute your ejection burn like normal. 

[Cpt Kerbalkrunch]

39 minutes ago, FullMetalMachinist said:

@HammyTheHamster the concept you're looking for is called a gate orbit. If you're interested in an explanation of the math, there is this thread:

The other thing to worry about if you want to refuel at one of the moons is timing. Say you eject from Mun and now have an elliptical orbit with your AP pointing to where the Mun was when you left. When you get to PE to finish your burn, you need that angle to match the ejection angle that you need for your transfer burn, and you need to reach that PE to do your burn at the right time for the transfer window. Not a trivial problem to figure out. 

The other option is to have a refueling station in Kerbin orbit that you can move to the different gate orbit altitudes. Then when you're going to Jool, put the station in that gate orbit. Then dock your interplanetary ship and refuel it. Now you're in a circular orbit at the correct gate orbit altitude, and can plan and execute your ejection burn like normal. 

In this thread they're speaking of circular orbits, but what I was suggesting, and I think the OP was referring to, is the drop down from Minmus. At that point, your orbit is highly elliptical rather than circular. In that case, it's all about energy. When you accelerate at Pe, you're getting more benefit than you would at a 300km circular orbit. It's going to be a short burn to the outer planets. And to me, Minmus is the most effective place to mine and refuel because of the low gravity.

When you say the timing is tough, you're exactly right. You need Minmus to be between Kerbin and the sun (or just before actually, because of the time it takes to drop down), and it pretty much never is. You have a choice to go early or late. I prefer to go late. Jool is such an easy target, you can't miss. At the speed you're moving and distance you're travelling, very slight adjustments have a large impact. Moving the maneuver node itself, even a few minutes one way or another, has an even larger impact. It's something you have to play around with to test the possibilities. It works just about as well for Eeloo. Again, it's a long distance, and a decent target, so small adjustments make a big difference.

I wouldn't recommend it for Dres, though. I never tried it this way, and probably never will. I don't use mods, so I'm guessing it wouldn't end well.

Edited May 10, 2017 by Cpt Kerbalkrunch

[Spricigo]

Notice that maths 5used in the mentioned thread can still be applied.  But we need some adjustments,  we need to disregard the burn to circularize at periapsis (we will not do it)  and also consider that the orbital velocity will depend on orbital heigh and semimajor axis (refer to vis-a-vis equation). 

There is also the problems of timing and non-instantaneous change in velocity to consider. But seems that the Oberth effect is the principal difference. 

[Red Iron Crown]

If your aim is to minimize the fuel lifted from Kerbin the best play is to use the payload's fuel to help with the ascent to LKO, then bring a tanker down from Minmus to refuel it there and carry on with the mission. 

[Spricigo]

1 hour ago, Red Iron Crown said:

If your aim is to minimize the fuel lifted from Kerbin the best play is to use the payload's fuel to help with the ascent to LKO, then bring a tanker down from Minmus to refuel it there and carry on with the mission. 

I suppose it results in less overall fuel used (although not sure) also it's less fuel paid with Funds. However refuelling at Minmus allows for a smaller interplanetary ship what I feel it's the most advantageous result we can get. 

But,  as always,  it's a matter of choice and compromises. 

[bewing]

I like taking everything to minmus, refueling there, and then using a spacetug to push the interplanetary vessel into a Kerbin orbit with a very low Pe. And yes, you have to time it properly in advance so that Minmus is in the right place WRT Kerbin for your ejection angle and the timing of the Pe. It's not that hard -- because you already have to know when your window will be.

[Cpt Kerbalkrunch]

I wouldn't tell anyone in this game that my way was better, I would just say what's worked better for me. I've done a ton of refueling (early on because I had bad launches and maneuvers, now when I want to save money with a smaller launch vehicle), and it just seemed that the Minmus system worked the best. The same vessel was actually able to get farther.

It's certainly not perfect, though. The timing to catch your windows takes some practice (though I consider this a good thing; trying to set up encounters taught me quite a bit about orbital maneuvers: I accidentally discovered the free capture at Jool trying for a good encounter on the drop down from Minmus), and you usually need to enable crossfeed on all your decouplers so you can get to Minmus with your onboard fuel, as @Red Iron Crown stated above about getting to LKO. You definitely don't wanna stage away your engines that you're gonna need to get to your destination.

[Kryxal]

I'd suggest putting a satellite in orbit and plotting your escape burn with that if you're going to leave from Minmus.  This lets you plan a drop to hit the same periapsis, where you'll make your transfer burn.  You might need to leave early and adjust your orbit for timing purposes, though.

[steuben]

hmm.... will have to try this for my Eeloo-90 project. Any rough estimates on what it will save on total DV costs?

[Spricigo]

2 hours ago, steuben said:

hmm.... will have to try this for my Eeloo-90 project. Any rough estimates on what it will save on total DV costs?

I'm not sure if that is exactly what you are planing to do but assuming it is, and based :

1.launching from kerbin: 3,4km/s

2.transfer to minmus: 930m/s

3.Minmus orbit: 160m/s

4.Refuel: -4490m/s

5.Transfer to kerbin: 160m/s

6.transfer to Eeloo: 200-300m/s (???I'm just guessing there)

Actually its not difficult to find by yourself how much the transfer burn will cost. (if you have access to KSP, which unfortunately I don’t have right now) Put a small craft a 75kmx75km orbit around Kerbin , raise the apoapsis to the orbit of Minmus, plan the transfer at the periapsis. Off course Yu need to make all the usual considerations about timing, ejection angle, cosine loss.

[Cpt Kerbalkrunch]

5 hours ago, steuben said:

hmm.... will have to try this for my Eeloo-90 project. Any rough estimates on what it will save on total DV costs?

Not sure about exact numbers, but I think I know a decent way to test it if you want. Just take whatever your ship is going to be after launch (as close as you can, anyway), cheat it out to LKO, and make your burn. Then revert flight, cheat it out to Minmus and do the drop-down. It certainly won't be exact (a lot of variables with the Minmus drop-down and subsequent transfer), but it should give you at least a rough estimate.

Haven't done it in a while myself. Think I might test it out to check the numbers as well.

 

At work, so I couldn't send this right away. Got ninja'd by @Spricigo. He's a math guy, so I'd go with his numbers.

Edited May 11, 2017 by Cpt Kerbalkrunch

[Spricigo]

Actually the deltaV map is the math guy.  While the the transfer to Eeloo it's just made up.  (was to lazy to even try a educated guess) 

[Red Iron Crown]

22 hours ago, Spricigo said:

I suppose it results in less overall fuel used (although not sure) also it's less fuel paid with Funds. However refuelling at Minmus allows for a smaller interplanetary ship what I feel it's the most advantageous result we can get. 

But,  as always,  it's a matter of choice and compromises. 

Still better to eject from LKO. When I was using Minmus ISRU to minimize lifted fuel I'd send the tanker-tug down to LKO, dock with the IP ship, burn Ap up to Minmus altitude, top up the IP ship, then undock the tanker and let the IP ship continue the ejection. Less fuel needed in the IP ship than with a Minmus ejection, plus I could reuse the high vacuum efficiency engines on the tanker-tug repeatedly.

[Spricigo]

25 minutes ago, Red Iron Crown said:

Snip

OK.  Now I notice you are really a step ahead.  

Previously my understanding of your method was just "refuelling in LKO, make transfer burn".  

You really top the tanks in the last opportunity.  

[Cpt Kerbalkrunch]

I get'cha now. That sounds pretty efficient, though I don't think I'd call that "burning from LKO". More like "burning from LKO with a reusable booster". 

I'm curious about the real-time. Does it take you longer than a normal refuel? Anything that that takes less real-time, I consider a plus.

[Red Iron Crown]

1 hour ago, Cpt Kerbalkrunch said:

I get'cha now. That sounds pretty efficient, though I don't think I'd call that "burning from LKO". More like "burning from LKO with a reusable booster". 

A two stage burn is still a burn.

1 hour ago, Cpt Kerbalkrunch said:

I'm curious about the real-time. Does it take you longer than a normal refuel? Anything that that takes less real-time, I consider a plus.

I wasn't really paying attention to that, to be honest. The number of trips is similar and it's the same number of dockings. Maybe a little less for the LKO strategy as you don't have to wait for Minmus to be in a favorable position, or mess around with nodes to make sure it is. Just gas and go in low orbit.

That said, I do recall some late evenings running that system but that is just KSP and not specific to any one strategy.