Should to an interplanetary transfer burn from the Luanch Pad?

[davidpsummers]

OK, I recently recovered Bill Kerman from being lost in solar orbit for 10 years. He had been stranded when I discovered it took less fuel to escape Kerbin than orbit it. (And of course, at the time I had nothing like the resources I would need to get him back L-).

But we all (?) use parking orbits on our interplanetary missions. Why, if they take more fuel. I had assume it made the transfer burn easier. But it is easy enough to correct for rotation (or figure out how much it affects you escape direction) and launch straight into interplanetary space. Heck, all your stages also either fall back or leave the system, cluttering orbit less.

So, can anyone tell me why we just don't to interplanetary escapes from the launch pad?

[jfull]

What do you mean "It takes less fuel to escape Kerbin than orbit it"?

I can assure you it does not.

[No one]

Personally, I often escape without orbiting but that's because I use massive asparagus monstrosities which are difficult to turn. Also I don't usually use launch windows, I just get into Kerbolar orbit then play with maneuver nodes until I find an intercept.

And are you sure it's easier to escape than orbit? When you're escaping from the ground you're using a radial burn, which is far less efficient than the prograde burn you'd use to escape from orbit.

[SkyRender]

Here's the problem with going straight up to escape Kerbin: gravity. The whole trip up, you're fighting gravity, and as a result you spend a massive amount of fuel pushing away from the planet's pull. When you perform a lateral burn to escape Kerbin instead, you do a minimal amount of fighting the planet's gravity since you're just extending your horizontal velocity for the most part. Of course, the longer your burn takes and the further you get from perfectly horizontal in your burn, the more of your fuel you start surrendering to fight gravity once more. Now strictly speaking you don't have to stay in the orbit you eventually reach when trying to make an interplanetary burn, but unless you do some serious maths, odds are good that you won't arrive above the atmosphere at a point where burning laterally is going to get you where you want to go.

Short version: we go into orbit first because that way we don't fight gravity and we can get a much more precise launch out into interplanetary space.

[Superfluous J]

It feels completely counter-intuitive (and it is) but reaching orbit around Kerbin first and then burning to your destination is more fuel efficient than simply burning straight up.

If you don't believe me (I didn't believe it either) do a test. Build a ship that you can launch straight up to, say, Mun and get into orbit around it. Do that straight up burn and get into Mun orbit. Note how much fuel you have left.

Then do another launch with the same ship. This time get into Kerbin orbit first and once there, do another burn to reach Mun and get into a similar orbit. Assuming you did this part even remotely efficiently, you should notice you saved about 20% or so on fuel.

Of course, nothing is stopping you from just doing the burn straight up method. And if you don't mind spending more fuel by all means do so

[EdFred]

It feels completely counter-intuitive (and it is) but reaching orbit around Kerbin first and then burning to your destination is more fuel efficient than simply burning straight up.

If you don't believe me (I didn't believe it either) do a test. Build a ship that you can launch straight up to, say, Mun and get into orbit around it. Do that straight up burn and get into Mun orbit. Note how much fuel you have left.

Then do another launch with the same ship. This time get into Kerbin orbit first and once there, do another burn to reach Mun and get into a similar orbit. Assuming you did this part even remotely efficiently, you should notice you saved about 20% or so on fuel.

Of course, nothing is stopping you from just doing the burn straight up method. And if you don't mind spending more fuel by all means do so

But you don't have to burn "straight up" to get to Mun, or leave Kerbin.

If done correctly, it should take no more dV to get to Mun (or anywhere) than it does to orbit and then transfer burn.

[Guest Fyre Flare]

Personally, I often escape without orbiting but that's because I use massive asparagus monstrosities which are difficult to turn.

I always do that.

[rkman]

He had been stranded when I discovered it took less fuel to escape Kerbin than orbit it.

It might take less fuel - if the vessel used to do 'direct escape' has a lower mass than the vessel used to do to a traditional ascent to parking orbit, and then escape.

But for one and the same vessel direct escape is not more efficient in terms of delta-v.

[Sirrobert]

But you don't have to burn "straight up" to get to Mun, or leave Kerbin.

If done correctly, it should take no more dV to get to Mun (or anywhere) than it does to orbit and then transfer burn.

Sure, if you time it EXACTLY right, and go directly from circulazation burn to transfer burn.

But that is exactly the same as making orbit first, and than transfering

[blizzy78]

So, can anyone tell me why we just don't to interplanetary escapes from the launch pad?

Transfer windows. Once in orbit, you have all the time you want, and you can't fall back down. But if you want to skip parking orbits and launch directly into your transfer trajectory, you need to do it at the right time, which can be difficult.

[Caelib]

As stated by others, there is no technical difference in Delta-V when comparing with or without a circularization burn first. But I do agree with you that it would be really nice if the maneuver node system would allow for a launch planning ... but I can imagine how insanely difficult it would be to design the interface and take into account the atmospheric drag and how that would completely trivialize learning to reach orbit

[dryer_lint]

I tried to plan a transfer from Kerbin orbit once, but I apparently don't have the patience to do it. I had to move the maneuver node around for like 15 minutes until I finally got it. Now I just burn straight up, escape into solar orbit, and plan my maneuver from there. Plenty of time, and it's easier (in my opinion).

It also might have something to do with what No One (nice username) said. I can't resist strapping on a ton of liquid fuel boosters and things, so my rockets are also pretty hard to turn

[Caelib]

I tried to plan a transfer from Kerbin orbit once, but I apparently don't have the patience to do it. I had to move the maneuver node around for like 15 minutes until I finally got it. Now I just burn straight up, escape into solar orbit, and plan my maneuver from there. Plenty of time, and it's easier (in my opinion).

I used to do that, but it is very fuel-inefficient because you lose the "powered slingshot" from the planet in your burn. What I did was get the Kerbal Engineer mod, which will show the current phase-angle to the planet of your choice ... when you combine this with this awesome, easy to use Transfer calculator ... your life will become a lot easier

Edited June 24, 2014 by Caelib

[NecroBones]

If going into orbit first is taking more fuel, then you're either not efficiently getting into orbit, or you're not efficiently doing your escape burn from orbit. As others have pointed out well, it's cheaper, energy-wise, to orbit first, and then do prograde burn into an escape trajectory.

Any time you're thrusting straight away from the planet, you're losing energy to gravity. Lateral (horizontal) thrust is mostly energy you get to keep. If you start your "gravity turn" at or slightly above 10km altitude, and gradually arc over until you're thrusting horizontally by the time you exit the atmosphere (at roughly 70km altitude), you should get a reasonably efficient launch to orbit. Generally, following the prograde marker on the navball is a good plan. Then, for the escape burn, you should only be thrusting prograde, to take advantage of the "Oberth Effect".

[Victus]

Since I learnt how to dock I just put my main fuel tank in a 200-300m orbit and then add the parts I need. It may take longer but it's very efficient.

[NecroBones]

I tried to plan a transfer from Kerbin orbit once, but I apparently don't have the patience to do it. I had to move the maneuver node around for like 15 minutes until I finally got it. Now I just burn straight up, escape into solar orbit, and plan my maneuver from there. Plenty of time, and it's easier (in my opinion).

That's actually painful for me to read, because of the lost fuel.

You don't need to get a perfect escape to an encounter all at once. You can still do a course-correction after leaving Kerbin's SOI, but still do an efficient escape burn. Getting to orbit is pretty easy. From there, you just need to burn your escape from either the sunny side, or the night side, depending on whether you're going to the inner planets or outer planets, respectively.

If you're not doing prograde burns and thus using the Oberth Effect to get the gravity assist, you're just wasting fuel. Gravity can either work for you, or against you.

[dryer_lint]

Eh... I tried using the transfer calculator, it took me forever to get the maneuver node right... are you guys sure about this?

[Caelib]

Eh... I tried using the transfer calculator, it took me forever to get the maneuver node right... are you guys sure about this?

As long as your angle is correct, don't worry too much about the maneuver node being perfect -- as long as it's relatively close. Even NASA has to make adjustment burns for planetary transfers -- remember that the further away you are from your target, the more efficient and PRECISE your adjustments will need to be. Here's how I do it:

1) Escape burn at ~phase angle

2) time accelerate to leave Kerbin SOI

3) setup maneuver node on Ascending or Descending node

4) after burn in step #3 (usually about 1/4 of the total orbital distance), create a new maneuver node to fine-tune approach to target

The most important part is step #1, but it shouldn't be that hard to create your maneuver node for escape and get an intercept if you're at the correct angle.

Edited June 24, 2014 by Caelib

[dryer_lint]

Well, actually, I tried a Duna mission today, and the transfer worked.

Unfortunately, during re-entry, the parachutes ripped the ship to pieces.

Maybe I should just stop doing interplanetary stuff before all my Kerbals are killed.

[Boris_T_Roach]

Well, actually, I tried a Duna mission today, and the transfer worked.

Unfortunately, during re-entry, the parachutes ripped the ship to pieces.

Maybe I should just stop doing interplanetary stuff before all my Kerbals are killed.

Naww the good thing about this game is that you never run out of willing victi.... errr happy crews ready for an adventure in space.

Boris

....another of the parking orbit and boost out crowd (if only because it allows me to build my interplanetary craft in LKO)

[boolybooly]

The OP is right, its cheaper to escape by burning straight up than it is to make orbit first if you test it experimentally.

Nullius in verba.

[davidpsummers]

I have to say, I lot of people have a different view of the cost of orbit than I do.

If you go straight up to any spot over Kerbin, the difference between where you are at and an orbit is the additional lateral delta-V that it takes to establish an orbit.

And in fact all orbits are "falling back down". If you have _any_ random lateral velocity (even a remnant of the velocity you got from planetary rotation) you are in a orbit, you just haven't spent the additional delta-V to make sure it doesn't intersect with the surface.

I'm also not sure that launching directly into transfer is more complicated. You just have to launch at sunrise/sunset and account for rotational velocity.

[rkman]

Now I just burn straight up, escape into solar orbit, and plan my maneuver from there. Plenty of time, and it's easier (in my opinion).

You can do that and be more efficient by not burning straight up but sideways (to the east, starting at about 10km alt), and you'd be more efficient still if you time it so that the escape trajectory of the ship is more or less parallel to- and in the same general direction as Kerbin's orbit around the sun. That's if going to an outer planet, for the inner planets, escape in the opposite direction).

Fuel efficiency might start to matter once we have budgets in the game, not so much for the cost of the fuel but for the cost of the ship.

An inefficiently designed and inefficiently flown ship can easily be twice as big/heavy/costly as an efficient one.

Nullius in verba.

In this case especially yours, i guess.

Edited June 24, 2014 by rkman

[SolarLiner]

The OP is right, its cheaper to escape by burning straight up than it is to make orbit first if you test it experimentally.

Yet the experiments made by dozens of users says the opposite.

AND, if that was more efficient to go straight up, why would no rockets in real life do this instead of using a parking orbit?

The answer is: it is NOT more efficient. IRL as in KSP.

When you go straight up, the basic formula is speed=thrust - gravity - atmospheric drag - planet rotation ( - planet orbital velocity if going for the inner planets)

When you use a parking orbit, the basic formula is speed = thust - atmospheric drag + planet rotation - gravity then speed = thrust + lateral velocity ( + gravity if using Oberth effect)

There is only so much forces helping you when you use a parking orbit that it feels useless to even attempt going straight up.

For my part, I tend to go too lightweight on my rockets that I often dont have enough dV budget, but at least if they need to be maneuverable, I can easily make them maneuverable.

Don't adapt the challenges to your capacities, adapt your capacities to your challenge.

[Lundmunchkins]

I used to do this, then I found it easier to just go into Orbit and do it from there.