Efficient Kerbin Escape

[Francesco]

I'm either missing something from this discussion, or I'm seeing some seriously fallacious information being shared here.

NO WAY launching straight up from Kerbin is the most efficient way.

NO WAY doing a Hohmann transfer exactly at dusk or dawn is the most efficient way.

There is a reason why we learn about ejection angles - it's not just entertainment.

[SRV Ron]

I'm either missing something from this discussion, or I'm seeing some seriously fallacious information being shared here.

NO WAY launching straight up from Kerbin is the most efficient way.

NO WAY doing a Hohmann transfer exactly at dusk or dawn is the most efficient way.

There is a reason why we learn about ejection angles - it's not just entertainment.

Notice the photos just posted. Launching straight up uses more fuel.

Hohmann transfer is done so that the escape runs parallel to Kerban's orbit. If done when the intercept window to Duna or Eve is present, the longer burn expands or contracts the solar orbit in relation to the prograde or retrograde of Kerban, you will get the most efficient intercept. Otherwise, you will have to loiter in orbit until that window is available.

[Francesco]

Notice the photos just posted. Launching straight up uses more fuel.

That's what I'm saying.

Anyway, even better than measuring the final weight of the ship, you can get a plugin that records expended delta-v (Protractor or Mechjeb just to name a couple.)

[kalor]

Regarding the lack of a gravity turn, here is the wiki link to the explanation of it, and why: http://wiki.kerbalspaceprogram.com/wiki/Gravity_turn

If you burn straight up, you are essentially losing (at least, havent done the math or whatever, but it kinda makes sense...) 1G of potential velocity, due to fighting gravity directly. If you burn at an angle, you are not climbing as fast, but, as an orbit needs lateral velocity as well as vertical, you are still gaining more velocity from the manuever, in the long run, than you are by continuing straight vertical, then turning at apoapsis to make your lateral acceleration.

If you time your launch window properly, you could ignite, burn to gravity turn, gain altitude and velocity, enter your ejection angle, burn for ejection, and be on your way without ever needing to adjust periapses. At least, I think you could... I've always made at least one orbit before ejection, but I'm not planning that far ahead.

[SRV Ron]

Even NASA never did a direct launch to the moon. They launched to orbit, then did the Lunar insertion burn when they reached the insertion window. However, they had to deal with orbital inclination where in Kerbal, we launch at its equator and can do a gravity turn to a direct intercept bypassing getting into a circular orbit.

Edited October 24, 2013 by SRV Ron

[kalor]

Even NASA never did a direct launch to the moon. They launched to orbit, then did the Lunar insertion burn when they reached the insertion window. However, they had to deal with orbital inclination where in Kerbal, we launch at its equator and can do a gravity turn to a direct intercept bypassing getting into a circular orbit.

True. KSC is pretty close to the equator, but the moon has an inlination anyways. However, even if they could have, they wouldnt have. The trip through the atmosphere on a pure vertical launch is significantly more inefficient, and, as we should know, every pound counts

(Also, NASA can't just "add moar boosters!").

[Decent Weasel]

Okay, so just testing that I understand the concept of a straight-up launch being more wasteful than a burn from orbit -

You have to burn a certain amount of fuel to get off the planet, so we'll assume a start condition 80km above Kerbin.

If you burn from 80km straight up to exiting SOI, you are doing work against Kerbin's gravity field the whole way, meaning at all times you need to be doing work to counter 1G and raising your apo has to occur on top of that.

if you burn from an 80km orbit, you have to spend some fuel to accelerate into orbit, but once there you are orbiting and not having to fight against the work done by gravity. All fuel burned from orbit goes directly into raising the apoapsis.

The fuel spent reaching an orbit 80km up is vastly less than the fuel spent fighting gravity directly from 80km to SOI exit.

Am I picturing this right? I mean, I don't debate it, just want to make sure I'm picturing it right.

[kalor]

Okay, so just testing that I understand the concept of a straight-up launch being more wasteful than a burn from orbit -

You have to burn a certain amount of fuel to get off the planet, so we'll assume a start condition 80km above Kerbin.

If you burn from 80km straight up to exiting SOI, you are doing work against Kerbin's gravity field the whole way, meaning at all times you need to be doing work to counter 1G and raising your apo has to occur on top of that.

if you burn from an 80km orbit, you have to spend some fuel to accelerate into orbit, but once there you are orbiting and not having to fight against the work done by gravity. All fuel burned from orbit goes directly into raising the apoapsis.

The fuel spent reaching an orbit 80km up is vastly less than the fuel spent fighting gravity directly from 80km to SOI exit.

Am I picturing this right? I mean, I don't debate it, just want to make sure I'm picturing it right.

Not quite. Once you are in orbit, everything you are doing is affecting orbit, or working towards the ejection. Think of it like this. If you go straight up from the ground TO 80K, you are losing 1G of delta-v per second, causing a slower increase in velocity. Once you reach orbit, you will have to burn more to get the proper orbital velocity to maintain 80K.

If, however, you burn at an angle (gravity turn), you are still ( in essence) losing 1G per second on the vertical, but you are instead building up velocity in the horizontal. Once you reach your 80K, you then use less fuel to circularize, or to continue along your ejection angle, or... whatever you are doing (unless it is a ballistic arc, but thats another matter).

[Superfluous J]

Okay, so just testing that I understand the concept of a straight-up launch being more wasteful than a burn from orbit -

You have to burn a certain amount of fuel to get off the planet, so we'll assume a start condition 80km above Kerbin.

If you burn from 80km straight up to exiting SOI, you are doing work against Kerbin's gravity field the whole way, meaning at all times you need to be doing work to counter 1G and raising your apo has to occur on top of that.

if you burn from an 80km orbit, you have to spend some fuel to accelerate into orbit, but once there you are orbiting and not having to fight against the work done by gravity. All fuel burned from orbit goes directly into raising the apoapsis.

The fuel spent reaching an orbit 80km up is vastly less than the fuel spent fighting gravity directly from 80km to SOI exit.

Am I picturing this right? I mean, I don't debate it, just want to make sure I'm picturing it right.

I don't believe you are. Once in your 80km orbit, you don't burn UP. You burn FORWARD (or more accurately backward ) to increase your SPEED. In space, it's all about speed. To escape Kerbin's sphere of influence from 80km up, you need to be moving a certain speed. I think it's around 3000 m/s or a little more. It quite literally DOES NOT MATTER which direction you are going*. If you are going at escape velocity, you will escape.

Burning against gravity, you have to burn more to reach escape velocity because some of that fuel is increasing your gravitational potential energy, and (more importantly) you're not adding to your already impressive orbital velocity. If you're going 2500m/s and you burn 500m/s straight forward, you're going 3000m/s. If you burn straight up you're only going (about) 2550m/s. That's 450 m/s you just wasted!

*Well, it DOES matter which direction you are going in one case: If your path will intersect with the planet, or Mun or Minmus. But other than that it's true. It does not matter. At all.

[SRV Ron]

It works because once you establish an orbit, your speed keeps you from falling into the planet. (Actually, you are always falling to the planet but because of your horizontal speed, the surface of the planet has curved away from your ship. So, your ship falls into a stable orbit.) Because the pull of gravity is balanced by your speed in orbit, any additional thrust will expand that orbit with none being expended to fight gravity. Your ship will then find a new balance in a higher orbit, or, if far enough away from the planet, escape its gravity well altogether. Therefore, the fuel savings once in orbit VS the brute force straight up launch.

It,s not a lot depending on design, but it can make a difference between completing an objective or coming up short on fuel close to it.

Using my test rocket, it took 16.3 tons of launch vehicle to send one ton to Kerbel escape going straight up but only 14.6 tons of launch vehicle to send the same ton to Kerbal escape via going into orbit first to use the Hohmann transfer effect. Actual results, of course, will be dependent on your efficiently of your launch vehicle. Asparagus stage Liquid fueled will be far more efficient then SRBs pancake designs where this Novapunch Mod design placed a orange fuel tank into Mun orbit.

Edited October 24, 2013 by SRV Ron

[JebidiahsBigSister]

Oddible : Wh did that graphic? It's fantastic!

[blizzy78]

I'd like to point out that there is a mistake in this graphic, or at least an ambiguity: In the middle graphic, it says to keep as low an inclination as possible, "close to 90 degrees." That would be 90° heading, but 0° inclination.