Is it better to....

[CrazedGunman502]

Lets say you want to go to a planet. You're at the launch pad. Everything is in position. It's time to launch. But wait, you think... Should I get an orbit of 75km around Kerbin.. Or should I burn up the whole time? If there's something even more fuel efficient, PLEASE let me know.

Please help, thanks.

[SolarLiner]

I guess if IRL interplanetary mission does a Parking orbit before leaving to other planets is because of less dV budget required. It's a guess, but else I think that they should go for a direct ascent launch.

[700NitroXpress]

It depends on what your craft can handle, what mission it's outfitted for, and how quickly it can escape Kerban. If it doesn't require refueling to get to the destination then just go for it.

[Kasuha]

It's better to go to orbit first. Burning straight up to reach heights of Mun and beyond takes more fuel than establishing an orbit and then raising the apoapsis to the same height. And in orbit you can plan maneuvers much more precisely than from launchpad.

It is also better to use good thread names. Such ones which will tell the reader what is the problem.

Edited October 28, 2013 by Kasuha

[JoCRaM]

IRL they run tests in the parking orbit to make sure everything is OK after the stress of launch, and to recalculate orbits based on the actual figures (computeres weren't what they are now, and they couldn't just say the maths is to hard, lets just do patched conics instead)

[AlamoVampire]

Up to you basically. But from an efficiency pov, a LKO at 75km nets biggest oberth effect which is less dV you need in end

[Alistone]

Up to you basically. But from an efficiency pov, a LKO at 75km nets biggest oberth effect which is less dV you need in end

Going straight up:

Pros:

Don't have to worry about gravity turns or steering losses to establish orbit.

Cons:

Gravity drag constantly reducing rocket efficiency. (Orbital thrusting toward the horizon for an ejection burn avoids gravity drag whereas straight up has gravity drag the entire way).

No maneuver node at launch to help plan the course.

Less time low in the gravity well which usually reduces your obert effect.

Bottom line is it works out to just about even; the higher your T/W ratio the more burning you can do going straight up for great oberth effect. If you have a very low T/W ratio you REALLY are better off with doing an orbit first. Generally the orbit first is easier as it allows you to plan an efficient ejection burn during the appropriate window and this is a window where any launch course deviations can be easily corrected.

EDIT: The main penalty for going straight up is gravity drag. The fact that gravity will always pull you down even as you are trying to thrust straight up. Once you are in orbit and speeding toward the horizon this pull of gravity just increases your apoapsis on the far side; it doesn't actually make you lose energy. If you go straight up; it is always slowing you down. This effect is more obvious if you have a low T/W ratio which is actually pretty common given the low thrust hish ISP engines normally used for interplanetary travel.

Edited October 28, 2013 by Alistone

[CrazedGunman502]

Going straight up:

Pros:

Don't have to worry about gravity turns or steering losses to establish orbit.

Cons:

Gravity drag constantly reducing rocket efficiency. (Orbital thrusting toward the horizon for an ejection burn avoids gravity drag whereas straight up has gravity drag the entire way).

No maneuver node at launch to help plan the course.

Less time low in the gravity well which usually reduces your obert effect.

Bottom line is it works out to just about even; the higher your T/W ratio the more burning you can do going straight up for great oberth effect. If you have a very low T/W ratio you REALLY are better off with doing an orbit first. Generally the orbit first is easier as it allows you to plan an efficient ejection burn during the appropriate window and this is a window where any launch course deviations can be easily corrected.

Nailed it, thank you.