Less efficient?

[Jeebs24]

From take off at Kerbin to orbital injection to the moons or other planets is it more more efficient to orbit Kerbin first (90 degrees) or just shoot straight up when KSC is properly aligned with trajectory you want to go. For example what I have been doing to get to Duna and Jool is I have been waiting for when the planets are in the correct position but also when KSC is in the correct position (right before sunrise) and just shoot straight up. So far it's been working and feel like I have saved time not worrying about getting into orbit.

[The Destroyer]

Orbiting is more effeciant.

How?

Look at mechjeb. in ascent autopilot, hit stats. you will see the line: Gravity speed (or whatever) gravity, when rocket is pointing straight up, waists its Delta-V, almost 2000 Dv is lost because of the main ascent part. thats why we do gravity turns, to minimize the effect of gravity, therefor wasting less deltav. heck, the current route you are taking might be consuming 5000 D-v!

[draeath]

Let the planet's own rotation work for you. That's why we orbit first. They spin pretty fast...

[Markus Reese]

Also orbiting gives you chance for error. It gets quite annoying trying to do the timing etc for a straight launch capture. With an orbit you can tweak and goof as you wish.

[Jeebs24]

Also orbiting gives you chance for error. It gets quite annoying trying to do the timing etc for a straight launch capture. With an orbit you can tweak and goof as you wish.

I've actually had no problem with timing of straight launch captures and it is quite easy. That's the reason I kept doing it but I wasn't quite sure of it's efficiency and I guess everyone has just confirmed it. The question now is to do a full orbit before orbital injection or just do a gravity turn and shoot out?

Well it should easy enough to test and quantify the efficiency.

[EndlessWaves]

I'm fairly sure establishing an orbit is less efficient for your burn method. If you're burning straight up to solar prograde then why would adding extra sideways velocity be a good thing?

What is more efficient is burning at along the curve of the planet in the direction the planet is rotating at some place on the opposite side to the direction you want to go so you end up with a curving trajectory - the exact place for the optimal burn depends on the final orbit size you're trying to achieve. This method will pass through an orbit before reaching an escape trajectory so it makes no difference whether you pause there or not.

[phoenix_ca]

Velocity that's "sideways" now will be "forward" later. Such is or ital mechanics.

[LoganNZL]

It saves you fuel, and speeds you up. I'm not good at doing all the lining up the orbits, and aiming for the right time to accelerate. I usually just end up missing it and get them lost on deep space. I like the sound of not orbiting. As long as you don't plat on doing a return trip; I never have, never will.

[Vanamonde]

Orbit is merely a velocity you pass through as you're accelerating, though it's a particular velocity that has the handy property of keeping you going in a circle. It's neither more or less efficient to shut the engines off and leave yourself at that speed for a while before proceeding, unless you're factoring in an Oberth effect from doing more of your burn low in the gravity well, but I'm not sure how much of an effect that would have for the kind of flight that you're doing.

[Ziff]

Surprisingly, there is almost absolutely no difference between the two methods. I built a 41tn test craft for the purposes of research. I did a total of 4 launches. 2 launches were straight up until I hit escape velocity, 2 were to circular 75km orbits and then boosted to escape velocity. I used MechJeb in both instances to ensure that the vehicle correctly powered down during atmo so as not to waste fuel.

Avg fuel remaining after hitting escape trajectory:

Vertical Method: 558.8L

Orbital Method: 557.2L

The conclusion I draw from this is that whatever fuel is saved by performing a gravity turn and using the rotational speed of the planet is lost by circularizing the orbit.

Reversely, whatever fuel that is saved by not circularizing an orbit is lost due fighting against gravity in a vertical launch.

Edited October 2, 2012 by Ziff
Added Info

[Peppe]

Did some simple tests with a rocket capable of 100km orbit with a little fuel left over.

With the left over fuel i could make the orbit 100km x 360km.

Same ship with a gravity turn and no circularizing i could get ~650km.

Same ship straight up i could get 1.025 million meters.

So if you can time your launch from the pad for an intercept then it seem skipping the gravity turn and orbit stage will save you some delta-v.

Doing same thing with a craft capable of getting to the mun though i found the gravity turn saved fuel. So it might depend on the rocket....

Edited October 2, 2012 by Peppe

[Vanamonde]

But since circularizing the orbit is done with a prograde burn, isn't that fuel you'd need to burn anyway, in the direction you'd need to burn it (prograde) anyway? As long as you're not braking to achieve it, I can't see how circularizing would be a loss.

[Kosmo-not]

Inserting into a low Kerbin orbit before doing a transfer burn to another planet is always more efficient.

[Ziff]

But since circularizing the orbit is done with a prograde burn, isn't that fuel you'd need to burn anyway, in the direction you'd need to burn it (prograde) anyway? As long as you're not braking to achieve it, I can't see how circularizing would be a loss.

I refer to it as a 'loss' compared to a vertical escape where you are not circularizing your orbit.

Inserting into a low Kerbin orbit before doing a transfer burn to another planet is always more efficient.

I agree that it should be that way, mathematically speaking, but from the tests I ran it doesn't really appear to be. Try the same experiment that I did and let me know how it turns out.

Edited October 3, 2012 by Ziff

[Vanamonde]

'loss' compared to a vertical escape

In the sense that part of your acceleration vector is not contributing to vertical ascent? I see.

[Kosmo-not]

Okay, I just ran a test for you.

Same ship used in both trials. One did a vertical ascent to 70Mm, while the other went into a 80km orbit around Kerbin first. Both of them circularized at 70Mm to ensure the energy state of both craft were the same at the end.

The craft that went into a 80km orbit first ended with 45.0L of fuel. The one that did a vertical ascent ended with 2.0L of fuel.

I hope this answers your question for you.

[togfox]

I'd like to know if a circularisation burn after gravity turn before shooting for Duna is better than just completing a well timed gravity turn and then going straight to Duna without establishing a Kerbin orbit. With patched conics that would not be difficult to do.

This means only one circularisation (at Duna) and not two.

[Jeebs24]

I finally did some tests using MechJebs for a somewhat controlled test.

Results

Test 1: Shooting Straight Up

- Launched right before sunrise on KSC so I'm going along the orbit of Kerbin.

- Final apoapsis was ~19,470km

Test 2: Gravity Turn

- Launched during midnight in KSC did a little gravity turn and left Kerbin SOI

- Final apoapsis was ~21,000km

Test 3: Full Orbit at 75km

- A little bit past the light sight of Kerbin, pointed rocket at prograde and left SOI

- Final apoapsis was ~24,800km

Test 4: Full Orbit at 80km

- Similar to Test 3 but did a gravity turn 12km trying to get as much horizontal speed before apoapsis reached 80km.

- Final apoapsis was ~27,600km

Maybe I didn't throttle at the right time during Test 2 and Test 3 but nevertheless throttling straight up is far less efficient!

I am doing some tests on different orbit altitudes at the moment to see if it makes a difference.

Edited October 4, 2012 by Jeebs24
Corrections an added Test 4.

[Kosmo-not]

I'd like to know if a circularisation burn after gravity turn before shooting for Duna is better than just completing a well timed gravity turn and then going straight to Duna without establishing a Kerbin orbit. With patched conics that would not be difficult to do.

This means only one circularisation (at Duna) and not two.

If you're talking about getting to the point where you have the option to establish Kerbin orbit but keep burning (your velocity vector is horizontal), they're the same.

Watch the first part of my video where it talks about gravity. It might help.

http://www.youtube.com/watch?v=MBZPH0OOZdY&feature=plcp

Edited October 3, 2012 by Kosmo-not

[Jeebs24]

I just would like to add to my original tests. For Test 3 I did a gravity turn at around 12km and circularized at 80km. Ended with sun apoapsis of 27,600km. It's shooting straight up and get 19,400km vs 27,600km using orbit.

[fishbord]

O_O uhh uhh

[antbin]

it should be that way, mathematically speaking, but from the tests I ran it doesn't really appear to be

MechJeb might be to blame - its ascent autopilot wastes fuel when circularizing (notice how it pitches up to 'surf' the apoapsis)

[Kosmo-not]

MechJeb might be to blame - its ascent autopilot wastes fuel when circularizing (notice how it pitches up to 'surf' the apoapsis)

A couple m/s is not much of a waste.

The math is correct. I think his interpretation of his test results is wrong.