Interplanetary travel from what orbit?

[Tokay Gris]

I think I read somewhere that staring from a low orbit gives you more velocity to start with. I am pretty sure it was a thread here, but I can't find it again.

My thoughts are as follows:

It might well be more efficient to start from a low orbit, since it also costs delta-V to get to a higher orbit...

But around Kerbin, I can easily refuel. Actually, I already have two ships in orbit around Kerbin that can refuel my Duna attempt.

(One was the tug that I build wrong, the other was actually for refuelling, but wasn't needed)

But if I need fuel once I am around Duna, it takes forever to get it there.

So, what to do? Start from low? Or get higher up and refuel?

The calculator gives way less delta-V for a higher orbit....

EDIT: Here is the vessel I am attempting it with:

Edited May 18, 2013 by Tokay Gris

[Rabada]

I'm planing on starting any interplanetary voyages from 500 km orbit so I can use max time warping while waiting for planetary alignment.

[HoY]

From personal experience, starting from a 90 km orbit doing a 30min 2000dv burn does Not work. In order to get a nice 50/50 burn your periaps will actually dip down to 60km, ruining the burn.

This was a TWR issue admittedly (0.15) and wouldn't have been a problem with as little as double that.

[CalculusWarrior]

I think I read somewhere that staring from a low orbit gives you more velocity to start with. I am pretty sure it was a thread here, but I can't find it again.

That, my friend is called the Oberth Effect, and you'll hear its praises sung across the forums, in multiple threads. The effect essentially states that the faster your rocket is moving (i.e. the lower your orbit is) the less delta-v you'll need to expend to get the same result. Circularizing an orbit at a high altitude gives an example of this; your ship requires less delta-v to raise your apoapsis initially than to bring up your periapsis to the required height.

The calculator gives way less delta-V for a higher orbit....

The reason the calculator is doing that is because it ignores the delta-v requirement to actually place your ship into the higher orbit.

So if you try boosting your ship into Duna orbit from LKO, then you'll use all the delta-v required to make the trip, made more efficient due to the Oberth effect. To illustrate this, let's make up numbers: let's say it costs 1000 m/s to boost from LKO. If you do that, you've expended 1 km/s of your fuel supply.

However, if you boost up into a higher orbit, let's say the delta-v required is 600 m/s, plus the 500 m/s required to boost up into that orbit, which gives us a total of 1.1 km/s burned out of your fuel.

Normally, I would suggest burning from a lower orbit, so that you can conserve fuel.

BUT:

You can refuel at the higher orbit. Thus, you can essentially replace the 500 m/s you lost getting up there, and then the trip to Duna will only require 600 m/s, so you'll arrive at Duna with only 600 m/s burned out of your fuel tanks, or a 400 m/s gain over if you had just burned from LKO.

Disclaimer: Any numbers in this post are entirely made up, do not take them as real values or even rough estimations. Any resemblance to actual numbers used for transferring to Duna is entirely coincidental.

So, in conclusion (or TL;DR): Yes, while it may be more efficient to burn at the lower altitude, by refueling at the higher one, you've added your delta-v cost back into your rocket's tanks, which leaves you with more delta-v to play around with.

Hope this helps!

Edited May 18, 2013 by CalculusWarrior

[tavert]

The fuel didn't magically appear in a high orbit, it cost a bunch of energy to get it up there. The difference in delta-V for interplanetary transfers from high orbit vs low orbit is noticeable, but not enormous. The general rule is if you're going from Kerbin's surface to a parking orbit to an interplanetary transfer without any docking then that parking orbit should be as low as possible for best efficiency.

If you already have fuel or a station or something in a high orbit, go ahead and dock there. Just keep in mind there's no reason currently in KSP for putting a station up high. In reality the atmosphere doesn't end abruptly, so higher orbits for permanent stations makes sense. You can mimic this for role-play reasons in KSP if you want to, but remember every time you send payloads up there, you're paying an extra cost for having it higher.

In a different similar thread someone mentioned the concern of phasing orbits to rendezvous with a station, which is a good point. If your station is high then you can wait in a low phasing orbit to catch up to the station. Or, no matter where your station is, you could go into a higher elliptical orbit to let the station catch up to the docking craft. It's a choice of which craft you want to spend more fuel with: your likely heavy refueling craft, or your likely almost-empty craft that need to dock for refueling.

If you have a low TWR as HoY mentioned, you can try the periapsis kick method (splitting the burn into multiple passes) so you don't drop too low by starting long burns early.

[HoY]

If you have a low TWR as HoY mentioned, you can try the periapsis kick method (splitting the burn into multiple passes) so you don't drop too low by starting long burns early.

I actually considered doing this myself, but as I have 5 similarly sized craft all to send to Jool in the same window I didn't want to time warp my orbits if I could help it. They all have more than enough deltaV even with their payloads to reach, unload, and return from Jool without refueling even spending the extra to get to a higher orbit to go in one burn.

[Anglave]

I think I read somewhere that staring from a low orbit gives you more velocity to start with. I am pretty sure it was a thread here, but I can't find it again.

You're probably talking about the Oberth Effect. You gain more value from your fuel when your ship is moving faster. This means it's usually most efficient to do your burns at periapsis, since that's when your ship is moving fastest. Be aware though, that it doesn't just mean "closer to a planet is better"

My thoughts are as follows:

It might well be more efficient to start from a low orbit, since it also costs delta-V to get to a higher orbit...

But around Kerbin, I can easily refuel. Actually, I already have two ships in orbit around Kerbin that can refuel my Duna attempt.

But if I need fuel once I am around Duna, it takes forever to get it there.

So, what to do? Start from low? Or get higher up and refuel?

The calculator gives way less delta-V for a higher orbit....

The reason it takes less dV from a higher orbit is because you've already spent a bunch of dV to raise that orbit.

You're driving up a hill. If you've already driven part way up and parked, getting to the top is going to take less fuel than it would to drive up the whole hill at once.

You're also right in that it costs dV to get into a higher orbit, so starting lower makes sense. The best solution is to "do both", sort of. Making a big circular orbit is inefficient. What you want to do is start at a reasonable LKO (say 80-100km) and raise your apoapsis (by burning at periapsis) so that your apo points the direction you want to leave Kerbin going.

Instead of wasting dV making a big circular orbit, you only raise half the orbit, so you wind up with an ellipse. Then you keep raising your apo until you have escape velocity. I'd also like to comment that getting an efficient Hohmann transfer to Duna takes only slightly more dV than getting to Mun. You really don't need to worry about refueling in orbit, and you can make the transfer burn from 90km Kerbin orbit just fine.

I'm planing on starting any interplanetary voyages from 500 km orbit so I can use max time warping while waiting for planetary alignment.

While that's fine, I'd like to point out that you can do maximum warp while on the launch pad. So warp until near your window before launch. Or, if your ship is already in orbit, you can return to the space center and launch a new ship (even just a single capsule) and use it to get maximum warp. (then return to space center and go to tracking center to return to your mission).

From personal experience, starting from a 90 km orbit doing a 30min 2000dv burn does Not work. In order to get a nice 50/50 burn your periaps will actually dip down to 60km, ruining the burn.

This was a TWR issue admittedly (0.15) and wouldn't have been a problem with as little as double that.

I've seen it recommended that in LKO you do no more than about a 4 minute burn at a time. At first I thought this meant I would need a higher TWR. HOWEVER, you can easily do multiple Periapsis 'kicks' to raise your orbit. It's actually much simpler than I'd feared.

Set up your burn node and start your burn about 2 minutes before the node. Burn until 2 minutes after.

Delete the node and make a new one at your periapsis. Warp around and repeat.

Do this as many times as needed to reach Kerbin escape. It took my Laythe mission (with only a single LV-N) only 4 kicks, which really didn't take nearly as long as I'd imagined.)

Edited May 18, 2013 by Anglave

[Rabada]

Would you be interested in uploading your Interplanetary Vessel? It looks like you put a lot of work into designing it! It looks great!

Anyways, I came across a thread here (LINK) that covers this question pretty well. It seems the consensus is constructing it at a minimus of 150 km for two reasons. First, you will be able to use 100x time warp. Secondly, according to people of that thread, if you orbit any lower, you run the risk of hitting the atmosphere during docking if you overshoot your target.

[Tokay Gris]

Sure! Just have to look up the means for that.

Actually, it is two vessels. The "Castor", the actual lander (kind of Apollo-style, landing stage stays and has a "brain" to do research, command capsule lifts off again) and the interplanetary tug "Pollux", equipped with enough (I hope) delta-V to get it there.

The idea is to keep the "Pollux" in orbit and dock afterwards head-to-head to get it back.

"Pollux" is here (I hope):

DELETED

An the "Castor" is here:

DELETED

Hope this works, otherwise I have to get my hoster to work on it.

EDIT: Tell me if this worked or if this is some SPAM-site... Seems to me it is. Mea maxima culpa....

EDIT/EDIT: This is junk.... Sorry 'bout that. Deleting the "links".

Edited May 19, 2013 by Tokay Gris

[Tokay Gris]

I'll try this way:

ftp://[email protected]/Craftfiles/Castor.craft

ftp://[email protected]/Craftfiles/IPT%20"Pollux".craft

Seems to work... (left) RIGHT! (of course) click and save file. This is my provider/server, so no spam. I actually pay for this.... ;-)

EDIT: Oh! And any suggestions are ALWAYS welcome!

Edited May 19, 2013 by Tokay Gris

[Horn Brain]

The forum monster ate all of my hard work and analysis, but I did a big thread about this issue a while back. The gist of it is: to minimize overall fuel expenditure (including getting the fuel for your refueling depot into orbit) you should have your station as low as safely possible. To maximize the amount of fuel left in your tanks when you leave Kerbin, you should refuel as high as possible (this is easiest to do at the Mun or Minmus), then dive down to 70 km at just the right time and give a small burn to escape Kerbin. I can't seem to find the thread in any caches, but if you want to look for it, look for "where should I put my station, I do the math so you don't have to", something like that, and my name and you might find it.

[HoY]

I tend to just attach a drop tank with enough fuel to do that big burn out of kerbins SOI, then drop it as soon as its empty. (docking port tank) that way I leave with full fuel