The "straight up" flight path

[SlyReaper]

For an interplanetary flight, is there any disadvantage to just waiting until Kerbin rotates to the right position and then just burning straight upwards? As opposed to getting to Kerbin orbit first. So for example, if I want to go to Duna, I'd wait until the Kerbal Space Centre was on the day/night terminator and then launch straight upwards and just keep burning until I got my apohelion to the orbit of Duna.

So you don't get the Oberth advantage from the in-orbit burn, but you don't expend the fuel to circularise a low Kerbin orbit, and you spend slightly less time in the atmosphere due to the lack of gravity turn.

Does any of this make sense?

[astropapi1]

Been there, done that, didn't work.

You simply use too much fuel compared to the traditional method.

[celem]

Well yes. Its what I do when returning from Mun/Minmus. Land on the equator under the moon's retrograde path and launch straight up for free return trajectory.

Off Kerbin its a bit trickier, then planet will spin some lateral into your launch anyway as the air you are moving through in the early stages is spinning with the world.

It makes sense, and probably works after a fashion. I wouldnt like to suggest what it might weigh out at on a dV-cost basis.

[sal_vager]

I've tried direct ascents to the Mun verses orbiting first, the direct ascent always used more fuel.

This wasn't by burning straight up though, but at about 45 degrees for most of the flight.

[Stargate525]

You lose a free few hundred dV of rotational motion. You're also fighting gravity for a longer time. The most efficient way is to wait until the planet's about 45 decrees off, then burning a traditional gravity turn launch and just never flattening out.

[Kerbart]

Been there, done that, didn't work.

You simply use too much fuel compared to the traditional method.

The trick would be to align with Mun for a slingshot to get a more Hohmann transfer like trajectory, I guess. Extra points for then using a slingshot around Minmus for an inclination change. This would be a nice "look ma no hands" challenge to get to Duna.

[EtherDragon]

As pointed out there are some issues with the "strait up" method of going anywhere.

You fight gravity all the way until you leave Kerbin's sphere of influence, using precious propellant to overcome its effects. The best way to conserve propellant with respect to gravity is to establish a stable orbit as early as possible (i.e. an orbit that is 70km x 70km) and then escape from there.

It actually takes considerably less Delta-V to circularize an orbit that already has an Ap above 70km than the amount of Dv "saved" with strait up ascent.

The most efficient way to reach another planet is Direct Ascent. In this kind of ascent you time your launch from the surface so that you have one long continuous burn (with stages) that does all of the following:

Launching from the pad, Reaching space (apoapsis above 70km), and trans-Duna injection.

For the sake of trying it, I did build a rocket that was capable of a strait up ascent - all the way out of the Kerbol solar system entirely! So it is certainly possible to get to Duna this way, just terribly inefficient.

[Taki117]

As far as burning straight up to escape versus using a parking orbit, with a straight up burn you get one shot every so Often (in the case of Apollo once a month) whereas with a parking orbit the time between launch windows is much shorter, more akin to once an orbital period (at 80km about once every half hour) this means if you miss one transfer window it is not that long until the next. (Never mind the dV savings and whatnot)

[Murph]

Direct ascent by going purely vertical isn't an efficient way of doing things, highly likely less efficient than gravity turn, circular low parking orbit, then transfer.

What does work, and saves fuel, is direct ascent by doing a normal proper gravity turn (i.e. progressive turn, not the single sharp 45º at 10k nonsense that floats around, but gradual turn starting below 10k and finishing at horizontal somewhere above 50k), then just skip the circularisation, and burn to transfer or escape before you reach apoapsis. That method requires a carefully timed launch to allow you to basically just keep burning into transfer or escape without ever raising periapsis out of the planet.

As far as I'm aware, it's the single most efficient way of doing a launch, saving the circularisation dV, but is difficult to pull it off in terms of the launch timing. If you don't get the timing right, you lose any advantage over a low parking orbit in corrections required later.

[Kasuha]

As far as I'm aware, it's the single most efficient way of doing a launch, saving the circularisation dV, but is difficult to pull it off in terms of the launch timing. If you don't get the timing right, you lose any advantage over a low parking orbit in corrections required later.

The more I think about it, the more I am concerned that what you describe is not the most efficient maneuver. Launch itself is very inefficient as you are sitting near the apoapsis and pushing that apoapsis up and ahead of yourself while you'd need significantly less energy to push that apoapsis by the same amount from periapsis if it was not so deep below the ground. Gravity turn is mostly about getting the periapsis above the ground and atmosphere and when you're almost there, you throw that off and keep rising apoapsis at apoapsis?

I strongly believe direct launch means you do gravity turn, then without interrupting your burn you circularize and continue burning from periapsis of your trajectory. It may mean the periapsis from which you are rising is even lower than you'd need it to be to have stable orbit, further improving Oberth effect.

Or do you have any better reliable reference?

[Smidge204]

Note: "Direct burn" is not the same as "Straight up."

You lose a free few hundred dV of rotational motion.

You actually don't, because that velocity doesn't magically disappear. Try it yourself: Switch to "Orbital" velocity before you launch and watch your prograde vector/velocity reading as you ascend.

However, you don't capitalize on it as much as you could if you go straight up.

You're also fighting gravity for a longer time.

Work against gravity is path independent. What's NOT independent, however, is the amount of work you do otherwise accelerating and decelerating your ship. At the end of the day, either you have sufficient kinetic energy to carry you beyond SOI before your vertical velocity stalls, or you don't.

In theory, a direct injection trajectory (which is not the same as "straight up") is more efficient. On the other hand, a parking orbit gives you lots of advantages that more than make up for the loss of efficiency. If you time it *just right* a direct burn will be more efficient, but that timing is extremely difficult so it's rarely bothered with.

This does not apply for putting yourself into orbit the "dumb" way; e.g. burning straight up then burning towards the horizon. The main reason that's less efficient is you spend a lot more energy getting a high enough Ap that you don't fall back down in the time it takes to get your required horizontal velocity.

"The most energy efficient orbit, that is one that requires the least amount of propellant, is a direct low inclination orbit." ("low inclination" meaning close to the equator, in this case)

The 'Cost' of stopping in orbit on the way to an escape trajectory.

=Smidge=

[technion]

When testing my Eve ascent vehicle, I successfully launched from Kerbin, and flew straight up until I made it to Jool.

So it CAN be done. Given what a horribly overpowered rocket this was for Kerbin, that doesn't mean you want to do it.

[Nao]

Very informative post Smidge , but i'll nitpick at one thing:

This does not apply for putting yourself into orbit the "dumb" way; e.g. burning straight up then burning towards the horizon. The main reason that's less efficient is you spend a lot more energy getting a high enough Ap that you don't fall back down in the time it takes to get your required horizontal velocity.

During straight up burn, you constantly burn "prograde" so you technically don't loose energy going up the way normal launch to orbit does as this is your escape direction anyways. No horizontal velocity needed.

While the method is indeed "dumb" if we have very High TWR rocket the difference between direct ascent and straight up is very small. There was a thread like this many months ago. During testing the least dV to SOI escape (at some speed) was something in the middle of direct ascent and the dumb one.

Also I think the dumb way doesn't loose any more energy due to gravity than a normal launch. As you say yourself "Work against gravity is path independent.". I'm not well versed in orbital mechanics to show it in math (and there isn't any real life calculations to draw from) but i believe the difference in performance between dumb and direct path is more to because of the Oberth effect due to having much less time to accelerate when going straight up.

[tavert]

Once you've hit escape speed, it doesn't really matter what direction you're going. The path dependence only influences the portion of the trajectory when you're burning at finite thrust. For very high TWR this portion is vanishingly small and you can approximate maneuvers as impulsive.

Also worth noting that since KSP SoI's are finite (except the sun), technically the lowest-energy orbit that will escape a planet or moon is the one that goes straight up with an apoapsis just barely outside the SoI (but still an elliptical orbit) and a periapsis near the center of the body. However since most escape burns want to get all the way into a hyperbolic orbit, the orbital energy you need to reach is pretty much the same whether your periapsis is at the center of the body or just above its surface.

[Kasuha]

Also I think the dumb way doesn't loose any more energy due to gravity than a normal launch. As you say yourself "Work against gravity is path independent.".

I think talking about "fighting gravity" is misleading. You always fight gravity as you move across gravity potential.

During launch you rise your apoapsis while you're at apoapsis. That's very inefficient maneuver as the best place to do it from is the periapsis. And that's the launch about - the meaning of the maneuver is to rise your periapsis as soon as possible (given atmospheric and terrain conditions) as high as you are so you can start rising your apoapsis from the periapsis.

With direct launch, you don't rise your periapsis, you just keep rising the apoapsis from the place where you are. Which, as your apoapsis grows higher, becomes more and more close to the periapsis and the difference slowly starts being negligible. Unfortunately it only happens when your apoapsis is already that high that you only need negligible acceleration to make it an escape trajectory.

[csiler2]

As far as burning straight up to escape versus using a parking orbit, with a straight up burn you get one shot every so Often (in the case of Apollo once a month) whereas with a parking orbit the time between launch windows is much shorter, more akin to once an orbital period (at 80km about once every half hour) this means if you miss one transfer window it is not that long until the next. (Never mind the dV savings and whatnot)

Apollo performed neither direct ascent or straight up burns. The mission launch windows were approximately a month apart to optimize the Saturn's flight path due to the moon's oscillations across the earth's ecliptic. Vehicle performance requirements increase as the moon moves away from the ecliptic, decreasing the Saturn's propellant safety margins. Launch window spacing also took light requirements for lunar flight operations and vehicle recycling in case of missed launch windows due to scrubs.

Apollo used one to two earth parking orbits for vehicle checks before TLI.

We don't need to worry about such considerations since most of the planets and moons are perfectly aligned with their parent body's ecliptic.

Here's a pretty cool paper from NASA on Apollo Launch Windows.

[Max Grant]

Direct ascent by going purely vertical isn't an efficient way of doing things, highly likely less efficient than gravity turn, circular low parking orbit, then transfer.

What does work, and saves fuel, is direct ascent by doing a normal proper gravity turn (i.e. progressive turn, not the single sharp 45º at 10k nonsense that floats around, but gradual turn starting below 10k and finishing at horizontal somewhere above 50k), then just skip the circularisation, and burn to transfer or escape before you reach apoapsis. That method requires a carefully timed launch to allow you to basically just keep burning into transfer or escape without ever raising periapsis out of the planet.

As far as I'm aware, it's the single most efficient way of doing a launch, saving the circularisation dV, but is difficult to pull it off in terms of the launch timing. If you don't get the timing right, you lose any advantage over a low parking orbit in corrections required later.

I pretty much always do direct ascent . . . not vertical, but as you describe. The reason is, that's how it is really done for most launches post-Shuttle. I watched like a 20 minute digital recreation of the Curiosity launch and noted that it went up, turned towards Mars, and burned until it was going to hit it. They didn't mess around with circularization.

I actually use Kerbal alarm clock to plan intercepts now (though it is not as useful as I'd like) but the good ol' slap a straightedge on the map view also works. I lob my craft into the sky on the night side if I'm going out away from the Sun, on the day side if I'm going in towards it, and point my ship at the target planet at about 70km, and then I do my burn. It works almost every time. I rarely plan a maneuver node for the main burn anymore since I'm getting to where I've memorized the best place to start.

Now for manned missions with multiple parts I don't do it that way, obviously, but for probes and single-vessel trips I never, ever, ever mess around in LKO before I get on my way.

[SnappingTurtle]

I watched like a 20 minute digital recreation of the Curiosity launch

Link to that?