Is there an advantage to a parking orbit?

[rflrob]

It seems like most of my 0.16 designs have been nerfed somehow in 0.17 (I've seen references to a fuel glitch... I wonder if I was unintentionally exploiting that?), but when I get to the point of doing inter-planetary missions, is there an advantage to establishing a parking orbit first, then leaving LKO, or would it be more efficient to simply take off at Kerbol-rise or Kerbol-set (depending on whether I want to go farther in or farther out in the system) and rocket straight upwards? My intuition is that way there's no steering losses, and slightly less drag losses, but I haven't sat down and done the delta-V calculations. Come to think of it, I haven't done any delta-V calculations... seems somewhat un-Jebbish.

[maltesh]

When you head straight upward, you take maximum gravity losses, because the full power of the gravity of at your altitude is opposing your thrust. When you're close to the surface, it's like subtracting 1 from your thrust-to-weight ratio.

If you're at the point in a circular orbit that's optimum for your burn to send you in the direction you want to go, you will be thrusting parallel to the surface of the planet, so gravity isn't dragging on your direction of thrust. Which lets you get to the proper ejection speed quicker, with less fuel spent.

And if you do a proper gravity turn to get to your parking orbit, you can reduce the costs to getting to the parking altitude iver burning straight up.

Basically, every single real-world space launch does something similar.

[Bioman222]

Also, A LKO gives you much more margin for error.

[MaverickSawyer]

Plus, if something goes wonky, you can "abort" and return to the KSC, and rapidly try again.

[Temstar]

Parking orbit opens up the launch window. In a one shot method you have a launch window of maybe about 1 hour each kerbin day. Using a parking orbit you get one sigment of your orbit as launch window every orbit.

In real life using a parking orbit have weight penalty since the departure stage needs to have on orbit re-ignition capability and electrical/hydraulic power to stay "alive" while waiting in orbit. Even then the effect on launch window is so useful that Saturn V was designed for it.

[Kiershar]

Is there big gravity losses past 70km of Kerbin?

Consider a very heavy ship possibly suited for atmospheric landing and return. To put that much tonnage into orbit you need to use multiple high trust engines with bad impulse ratios. If you do a direct ascent, once past the 70km from Kerbin you don't need intense thrust to circularize in a relatively small delay. This opens up the possibility to use a single low thrust nuclear engine that very efficiently consume fuel for great delta-v returns.

Is it possible that for very heavy ships, it is more efficient to spend the tonnage on a single nuke engine designed for direct ascent instead of spending it on enough thrust to circularize in time?

[Accelerando]

In my experience, unless you've already launched with enough high-thrust delta-V to make circular orbit in the first place, low thrust can be a very inefficient way to achieve direct ascent because the long required burn for a single LV-N to place a medium-heavy size ship into stable orbit (even for just one half-size large fuel tank + large command pod) will not be complete before you reach apoapsis.

[Kiershar]

Direct ascent means not circularizing.

[TheHengeProphet]

To oversimplify, think of it using this question: Is it harder to shotput a ball at 30 m/s, or is it harder to use a sling to propel a ball at 30 m/s? Essentially, it uses much less energy to orbit up to speed than it does to brute-force your way out of SOI.

[opticalshadow]

not to mention, for other things, such as meeting up with ships and stations for docking, having a parking orbit will make life tremendously easier.

[Punk]

To oversimplify, think of it using this question: Is it harder to shotput a ball at 30 m/s, or is it harder to use a sling to propel a ball at 30 m/s? Essentially, it uses much less energy to orbit up to speed than it does to brute-force your way out of SOI.

When you are in a stable orbit with engines off, the body you are orbiting is constantly pulling you around it. This means that gravity is working to accelerate you around it without having to use any fuel. If you time it right gravity adds to your acceleration. The hammer throw image is a good one. Its always a better idea to make a gravity turn, whether you pause your acceleration and stay in orbit to fly a lap of Kerbin or not is unimportant. Someone said in a similar post that orbiting is just a velocity you pass through to get to escape velocity. Shooting straight up means that gravity is always working in the opposite direction to your vector, instead of swinging you around and into space it pulls you back down.

[Accelerando]

Direct ascent means not circularizing.

I know that. What I'm saying, if you would read a little further into my post, is that low thrust is not a good idea to get anywhere beyond Kerbin orbit if you didn't already expend sufficient high-thrust delta-V to make it there in the first place. Going beyond Kerbin requires more delta-V than achieving (at least low) circular orbit, as you know.

[Bluesteel]

I found the quickest route to getting an interplanetary stage on its way, is a 4-5 stage SRB Launcher. Pad to Kerbin escape velocity in 2 1/2 mins

[Jeebs24]

I asked the same question rflob. Doing an LKO first does make a big difference. See my test results at http://kerbalspaceprogram.com/forum/showthread.php/22691-Less-efficient?p=286224&viewfull=1#post286224.