Orbit or Direct Escape Trajectory?

[Mythalinear]

Newb question here. If I want to travel to Duna or Eve is it better to start from a stabilized orbit of Kerbin or is it better to just take off and keep the nose up until you reach escape trajectory?

[Claw]

This is one of those posts that always generates a lot of discussion. It really revolves around two points, most of which relies on what you consider "better."

From one perspective, it's easier because you don't have to do much maneuvering or bother establishing that pesky orbit. However, I'd say that it's generally easier to learn how to establish an orbit, which buys you time to get your maneuvers set up and your plan ready before executing.

Other people consider "better" a matter of what is the most efficient, which is usually measured in Delta-V expended. Rather than answer this one directly, I'll let the masses swarm about direct ascent vs. orbital rendezvous.

Welcome to the forums!

Cheers,

~Claw

[legoclone09]

Claw, no argument has happened yet. *sniff*

[Perry Apsis]

Claw, no argument has happened yet. *sniff*

Has so.

[Snark]

Perry Apsis said:

Has so.

Hello, is this the right room for an argument?

(sorry, couldn't resist)

Short answer is:

• going to LKO (low Kerbin orbit) first is much easier from the standpoint of piloting and mission planning; direct ascent requires a lot of skill to get the precise trajectory you want.
• as far as dV requirements are concerned: as long as your LKO is pretty low, in practice there's very little difference in the dV requirements for the two. Very high TWR craft may do a bit better on direct ascent, very low TWR will do much better with LKO-first.

Therefore, my advice would be "go to LKO first." It's a lot easier in terms of skill, and you won't really do much better in dV if you try direct ascent. My impression is that that's what most KSP players do. Tools such as http://ksp.olex.biz/ are very handy for planning your interplanetary ejection burn, and also give you an easy way to play around to see, for example, "how much difference does it make if my Kerbin orbit is this altitude or that one".

[SAI Peregrinus]

The way I do it (interplanetary missions) is launch to LKO, then to low Minmus orbit where I dock with my fuel station and fill up the rocket, then go interplanetary from there. It makes the initial launch lighter and the fuel comes from the Minmus mining outpost, which also saves Spesos in career.

[Wolf Baginski]

One minor advantage is that a parking orbit makes the actual launch time less critical, so Wernher can have a pleasant breakfast before having to shout at the incompetents who built the terrible rocket you're launching. A good breakfast sets him up for the whole day  he'll always shout at you, but indigestion makes it worse.

[Streetwind]

You need to define "direct escape" in order to get a useful answer.

If you wait until your Duna ejection vector is right overhead the launchpad, and burn straight up all the way, then you're wasting a significant amount of fuel.

If you wait until your Duna ejection vector is on eastern horizon, and perform a standard gravity turn, then you have almost no difference in fuel use compared to going into orbit because your launch profile practically ensures that you orbit.

[Mythalinear]

The way I do it (interplanetary missions) is launch to LKO, then to low Minmus orbit where I dock with my fuel station and fill up the rocket, then go interplanetary from there. It makes the initial launch lighter and the fuel comes from the Minmus mining outpost, which also saves Spesos in career.

Whoah! Why have I not thought of this?! How did you build your fuel station and what is the best way to keep it stocked?

[Mythalinear]

Thank you, I have learned much from your answers.

[Montag]

I typically go for an 85 km orbit. I have my boost stages with docking ports so that they can be topped off or residual fuel can be siphoned out.

[Tatonf]

I tried the two different methods from the Mun and found out that the two technics are equivalent in terms on delta-V (which makes sense).

However on Kerbin a vertical ascent requires much more delta-V. I deduced that atmospheric frictions were responsible of that.

As said above you can do a "direct ascent" with a gravity turn but that just mean you're making an orbit without stopping the engines for your escape.

For me it's orbit.

[vixr]

I lean toward orbit and docking with a station...move Kerbs around, fill the tanks, steal snacks and plan the big burn...

[m00nchile]

I tried the two different methods from the Mun and found out that the two technics are equivalent in terms on delta-V (which makes sense).

However on Kerbin a vertical ascent requires much more delta-V. I deduced that atmospheric frictions were responsible of that.

As said above you can do a "direct ascent" with a gravity turn but that just mean you're making an orbit without stopping the engines for your escape.

For me it's orbit.

Orbit will always require less delta V than just burning straight up. Doing this test on the Mun makes this less apparent, because gravity losses are small. Try the same on Tylo, and you'll get different results, even though there's no atmo.

[gogozerg]

The way I do it (interplanetary missions) is launch to LKO, then to low Minmus orbit where I dock with my fuel station and fill up the rocket, then go interplanetary from there. It makes the initial launch lighter and the fuel comes from the Minmus mining outpost, which also saves Spesos in career.

You can do much better if you want to optimize the refueling of the ships leaving Kerbin: Bring a refuel ship from your Minmus fuel/ore production to lower Kerbin orbit. Oberth and Kerbin's gravitational well are our best friends.

Once your supply chain is operational, ore is free, it's just a matter of time and hassle. Travel almost twice as far for the same price.

Edited July 15, 2015 by gogozerg

[Superfluous J]

You can do much better if you want to optimize the refueling of the ships leaving Kerbin: Bring a refuel ship from your Minmus fuel/ore production to lower Kerbin orbit. Oberth and Kerbin's gravitational well are our best friends.

Once your supply chain is operational, ore is free, it's just a matter of time and hassle. Travel almost twice as far for the same price.

The more I'm thinking about it the more sense this idea makes over the "traditional" (for me) Minmus refueling station. First off, you don't have to deal with going to Minmus and back with your interplanetary ships, just the fuel tug. Second, it more closely breaks the dV expenditure in half (~3300 to orbit and <~3300 to get most anywhere else; as opposed to ~3300 to orbit and ~950 to Minmus and << ~4250 to wherever you're going).

I think I'm going to modify my mining operation today.

[maccollo]

I tried the two different methods from the Mun and found out that the two technics are equivalent in terms on delta-V (which makes sense).

However on Kerbin a vertical ascent requires much more delta-V. I deduced that atmospheric frictions were responsible of that.

As said above you can do a "direct ascent" with a gravity turn but that just mean you're making an orbit without stopping the engines for your escape.

For me it's orbit.

Most people design lunar asent vehicles with extremely high thrust to weight ratio. As a result the difference is negigable. If you try to redo the experiment with a lower TWR you will notice that there is a difference, and vertical ascent will loose.

[spudman2]

then to low Minmus orbit where I dock with my fuel station and fill up the rocket, then go interplanetary from there.

What if Minmus is on the wrong side of its' orbit?

[gogozerg]

What if Minmus is on the wrong side of its' orbit?

Another good reason to go for LKO refueling.

[Superfluous J]

What if Minmus is on the wrong side of its' orbit?

Minmus' orbit is about 50 (Kerbal) days long. If you plan ahead you can get up there and be ready to eject from Minmus when it's in the right spot, between 0 and 50 days from when you would prefer to launch. Then just orbit Kerbin a few times if necessary to get the right ejection.

Or, like gogozerg said, use the simpler "LKO refueling" approach which - as I said - I have become a fan of for several reasons including it's far simpler.

[GoSlash27]

Most people design lunar asent vehicles with extremely high thrust to weight ratio. As a result the difference is negigable. If you try to redo the experiment with a lower TWR you will notice that there is a difference, and vertical ascent will loose.

I remember being embroiled in a massive flamewar over this one.

Thankfully, it eventually gave way to some excellent science and we were able to prove definitively that a vertical ascent to escape velocity is never as efficient as a prograde burn, although in some cases the difference can be quite marginal.

arkie87 created a very powerful mathematical model of the prograde burn to orbit that we were later able to use to establish mass-optimal thrust to weight ratios for ascent stages using all engines on all of the airless bodies.

Best,

-Slashy

[Tatonf]

Most people design lunar asent vehicles with extremely high thrust to weight ratio. As a result the difference is negigable. If you try to redo the experiment with a lower TWR you will notice that there is a difference, and vertical ascent will loose.

I did that on purpose because if I have low TWR I would burn further away from the Mun and lose some Oberth effect. To make direct ascent equivalent to orbit you have to burn everything before reaching the altitude of the orbit.

Isn't the delta-V gain with the orbit that you all talk about just come from the Oberth effect ?

[Lelitu]

I did that on purpose because if I have low TWR I would burn further away from the Mun and lose some Oberth effect. To make direct ascent equivalent to orbit you have to burn everything before reaching the altitude of the orbit.

Isn't the delta-V gain with the orbit that you all talk about just come from the Oberth effect ?

Actually, most of the delta-v gain comes from not fighting gravity as directly.

It's all trigonometry

With a TWR of 2, going directly up, you provide twice as much thrust as needed to balance weight

so you only accelerate at 1g. Half your fuel is directly lost to gravity.

At 45deg from vertical, you've got a twr of 2 along the hypotenuse.

this works out to an acceleration of sqrt(2) horizontally, and (sqrt(2) - 1) vertically

you gain altitude much more slowly, but lose much less energy to fighting gravity.

Since orbit is very little to do with altitude, and almost all about horizontal velocity relative to the surface

this works out more efficient.

As TWR goes to inifinty, the losses to gravity become proportionally less and less important.

Edit to add: For getting into orbit, altitude is required for exactly two things. Avoiding mountains, and avoiding atmosphere.

On a perfectly spherical world with no atmosphere, you could enter orbit at any nonzero ground seperation.

of course, there are no perfectly spherical worlds, so a few KM above highest point is usually a good idea.

Edited July 15, 2015 by Lelitu

[Weywot8]

The effort that goes into KSP! Is the complex mathematical model still around? I'm curious cause I completely ignore the "1.2 TWR at launch on Kerbin" and just ram vertically as hard as possible to 50-80m/s (drag <4.5ish m/s) under terminal velocity to quickly get past the first 20-25km of atmosphere. Seems to save me a lot of delta-V over the typical gravity turn advice floating around on the forums. It's mostly prograde after that though. Maybe modding the model for atmosphere could help figure it out.

Before 1.04, I did a hybrid and sent everything to the Mun. Kerbin ship launched with extra empty tanks would refuel with Minimus/Mun fuel at the edge of Mun's SOI. With a small timed burn, I'd drop back down to Kerbin correctly inclined, regaining 800m/s deltaV and do the interplanetary burn in atmosphere (60km'ish) near periapsis. Got tired of all the screen-staring and correction maneuvers with ion engines & LN-V's. Shorter burns are much more accurate so those engines are for everything after.

Edited July 16, 2015 by Weywot8

[Superfluous J]

I'm curious cause I completely ignore the "1.2 TWR at launch on Kerbin" and just ram vertically as hard as possible to 50-80m/s (drag <4.5ish m/s) under terminal velocity to quickly get past the first 20-25km of atmosphere. Seems to save me a lot of delta-V over the typical gravity turn advice floating around on the forums.

Optimizing for dV is like optimizing for "distance on one tank of fuel" that car companies sometimes do in their commercials. It sounds better until you realize that they just gave their gas guzzler a bigger fuel tank.

This insufficient car<->rocket analogy was brought to you by Rockomax. Rockomax: If it ain't broke, it hasn't been field tested enough!