Best Way Into Non-Atmospheric Orbit?

[Right]

The question is: What is the most economical way to take a craft into low orbit, from the surface, of a celestial body without an atmosphere (like the moon).

I'm assuming its to thrust directly up until Apoapsis reaches desired altitude, and then circularize. (EDIT: Turns out this is backwards!)

Edited February 12, 2014 by Right

[lyndonguitar]

In my experience it's generally the same. I do complete horizontal then circularize but most of the time I get lazy and don't change the mechjeb ascend options, and I don't see any big difference

[Ascension Islander]

A simple gravity turn with a very abbreviated vertical portion. Basically just get off the ground and quickly but smoothly turn it horizontal, making sure to avoid terrain.

[wasmic]

Yes, the most efficient is to thrust upwards for a few seconds, and then turn horizontal immediately, and thrust until you have an orbit. Then keep thrusting horizontal until your apo reaches the desired altitude, and then circularize at apoapsis.

[Streetwind]

Turn towards the horizon in the direction of rotation of the celestial body, as soon as you can, as hard as you can without falling back down or hitting a mountainside. 5 meters above the ground is just as much an orbit as 50 kilometers, as long as there are no obstructions.

Ascending vertically means spending fuel on countering gravity. But go sideways fast enough, and gravity goes away for free.

EDIT: also, a tighter orbit doesn't require you to accelerate sideways as much, i.e. you need less fuel to launch and circularize right above the surface as opposed to dozens of kilometers out. And the Oberth effect is stronger the closer you are to the celestial body, as well.

Exceptions may apply. if your intent is to escape the celestial body, and the ideal escape vector is straight above you in the sky, then launch straight up. A good example of this is you are returning to Kerbin from the Mun, and your landing site is directly retrograde of the Mun's orbit.

Edited February 11, 2014 by Streetwind

[maccollo]

Go sideways, go sideways, go sideways, and only add so much vertical velocity so that you don't hit any mountains on the way to orbit. This is the cheapest way to do it regardless of your TWR.

[Bobe]

I'm assuming its to thrust directly up until Apoapsis reaches desired altitude, and then circularize.

That is the complete opposite of the most economical way to reach orbit.

On a body without an atmosphere, like some have already said, you want to apply thrust at as low an inclination to the surface as possible as early as possible. Depending on the mass of your craft, there is no point going straight up initially, that method is used on planets with atmospheres to minimize the amount of drag.

When ascending, keep an eye on your ship's pitch relative to its thrust vector. After lifting off, pitch down until the ship's vector starts to drift away. As your velocity increases, you can continue to decrease the pitch as long as you are travelling in the direction you are pointing the ship in, but obviously not below 0 degrees.

[DrMonte]

Would you like to try Mun Extreme EVA challenge?

Burning straight up is never good option unless you're in deep shcrater. Burn as much horizontal as possible, gain orbit, set your appoapsis, at appoapsis change periapsis.

[RSwordsman]

Do your turn almost immediately, as others are saying. You want to be going only as high as necessary to avoid crashing into a hillside; no more.

[Right]

Hmmm, makes sense! Thank you all.

[Stargate525]

Exceptions may apply. if your intent is to escape the celestial body, and the ideal escape vector is straight above you in the sky, then launch straight up. A good example of this is you are returning to Kerbin from the Mun, and your landing site is directly retrograde of the Mun's orbit.

I haven't done extensive maths on this, but personal experiences tell me even that's not a good enough reason. I've found I'm usually better off going around the mun and slingshotting out rather than fighting the gravity well the whole way up.

The extra benefit of this is, if you're low enough on fuel to make this a real issue of anything other than speed, is that the horizontal approach allows you to abort to orbit and await a rescue. If you go straight up, and you're low, you either crash back down into the Mun, or (only slightly better) wind up in a high kerbin orbit that WILL get recaptured by the Mun and impact, or get flung off into deep space.

[Streetwind]

I doubt that circularizing an orbit at more than 600 m/s before you even start burning for escape would be cheaper than a direct ascent in this case... of course, I am open to be proven wrong

How would one go about to test this with little effort? Is there a way to cheat a lander to the correct spot on the Mun (or at least nearby in space)?

Edited February 12, 2014 by Streetwind

[SSSPutnik]

You're fighting gravity the whole way out anyway so...

[Kasuha]

Most effective way:

Step 1: avoid terrain. Burn at sufficient pitch to just clear any ground that might get in your way, put everything else to horizontal speed

Step 2: apoapsis surfing. Push your apoapsis right ahead of you by slight above prograde pitch. Best done in map view once you're sure you're clear of terrain

On bodies with noticeable rotation, take off in the direction where the rotation helps you the most.

[KerbMav]

Step 2: apoapsis surfing. Push your apoapsis right ahead of you by slight above prograde pitch. Best done in map view once you're sure you're clear of terrain

In this regard I keep wondering: Should I burn in bursts, pushing my AP out in front of me, then trottle to zero, surf, burst, repeat - or should I throttle up and down just enough to keep my AP ahead of me? Is there an efficiency loss when not burning at full throttle?

I generally like to burn being close to my AP to use as much dV as possible to raise my PE and not change my AP.

Also: Where should I burn the most? Is there a difference in gaining orbital velocity when still infront/behind of the Mun and burning while my orbit around Mun is parellel to its orbit around Kerbin?

Should I try to push my AP out to be between Mun and Kerbin and throttle up full when reaching it to get a low Kerbin PE as cheap as possible?

[Kasuha]

I'd say, burn all the time, but keep the apoapsis right ahead by pitch. Lower your pitch if it gets too far, increase it if it gets too near. It also means you're keeping your vertical speed near to zero. It is always possible, just with increasing speed (and decreasing eccentricity) it becomes increasingly sensitive to pitch changes.

When your eccentricity is small enough it doesn't really matter if you continue burning or coast to Ap to continue.

I should probably add that this is (AFAIK) cheapest way to get to "whatever" orbit. If you're aiming for orbit at certain altitude the optimum approach is not very simple or straightforward but this approach is close to optimum, too; establish low orbit this way, then perform Hohmann transfer to required altitude.

[Skorpychan]

Generally, I burn straight up until my apoapsis is clear of any nearby terrain, then start going horizontal as I reach it, until my periapsis pops out of the surface. Then I go for an escape orbit, or nudge apoapsis and periapsis up until I'm where I want to be.

[KerikBalm]

I doubt that circularizing an orbit at more than 600 m/s before you even start burning for escape would be cheaper than a direct ascent in this case... of course, I am open to be proven wrong

You realize you get those 600 m/s back when you eject from the mun's orbit.

When you fight gravity, that delta-V is *LOST*

This is most obvious if you sit there, hovering over the mun, your tank could have 2km/s in it, but hover long enough, and you end up with *nothing*.

fight gravity for half that time, and you've lost 1km/s

When you thrust perpendicular to gravity, that delta V is stored, and eventually counts towards your escape velocity when you do your burn. *assuming you time your ejection burn right*

"You're fighting gravity the whole way out anyway so...

No you aren't there is a big difference between thrusting opposite gravity, and thrusting perpendicular to it, as I mentioned above.

Of course the lower gravity is, the less it matters.

On Kerbin, you are losing nearly 10 m/s every second you thrust in opposition to gravity.... on Gilly, well, who cares?

[maccollo]

Exceptions may apply. if your intent is to escape the celestial body, and the ideal escape vector is straight above you in the sky, then launch straight up. A good example of this is you are returning to Kerbin from the Mun, and your landing site is directly retrograde of the Mun's orbit.

This is not true at all.

In order to escape with the highest possible velocity you must beat the maximum potential energy with as much kinetic energy as possible. Because of the oberth effect this means you must expend as much delta V as you can as deep as you can in the gravity well. You can't do that if you're burning vertically.

I'm not good with maths, so I can't prove it that ways. So instead I devised a KSP challenge.

Bilwin Kerman has landed on the moon. Your challenge is to get him back to Kirbin.

He has landed where the retrograde vector of the mun is straight up. Convenient

Here's the quicksave. Dump it into a save folder, load it, and try to get Bilwin home.

https://dl.dropboxusercontent.com/u/22015656/munReturnChallenge.rar

[Streetwind]

Excellent! Now I can confirm for myself if I was wrong.

(...once I get home...)

Edited February 12, 2014 by Streetwind

[Sabreur]

You're fighting gravity the whole way out anyway so...

That's true, but you can limit how hard you have to fight it. I don't have the calculations in front of me, but I'm pretty sure the most efficient way is to first reach orbit, then to use a free-return trajectory to get home.

[Kasuha]

Here's the quicksave. Dump it into a save folder, load it, and try to get Bilwin home.

https://dl.dropboxusercontent.com/u/22015656/munReturnChallenge.rar

I tried it and I must say - pretty cruel

Surprisingly, most effective way to escape a planet does not involve neither pure vertical burn, nor circularizing. Direct ascent keeps trajectory intersecting the originating body all the time but not through the center, rather at certain defined distance. It's not practical on Mun because it doesn't rotate relatively to Kerbin and for optimal escape you need to be at certain angle.

It's also not very practical for manual spaceship piloting, like in KSP.

[maccollo]

I tried it and I must say - pretty cruel

Surprisingly, most effective way to escape a planet does not involve neither pure vertical burn, nor circularizing. Direct ascent keeps trajectory intersecting the originating body all the time but not through the center, rather at certain defined distance. It's not practical on Mun because it doesn't rotate relatively to Kerbin and for optimal escape you need to be at certain angle.

It's also not very practical for manual spaceship piloting, like in KSP.

Well, I think it demonstrates that burning straight up is certainly not an efficient way of escaping a body =P

You can do that all day long and you will never reach Kerbin.

However, if you get into a low orbit of about 10 km you can reach Kerbin with maybe 40 m/s left.

I should have noted that the challenge isn't impossible, and I beat it multiple times before uploading it.

Just in case someone who has a bit less experience has problems beating it and might think I'm pulling their leg.

Edited February 12, 2014 by maccollo

[Kasuha]

You can do that all day long and you will never reach Kirbin.

Just a small note: the name of the planet is Kerbin, not Kirbin

And I wondered if you did that intentionally or if it just came out that way by chance.

I should have noted that the challenge isn't impossible, and I beat it multiple times before uploading it.

Me too, but only just. Some 1.8 units left on better of my two attempts.

[maccollo]

Just a small note: the name of the planet is Kerbin, not Kirbin

...

Derp...