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BaconofWar

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    BaconofWar reacted to FreedomATX in Orbital Mechanics 101: Avoiding lithobraking.   
    litho- a combining form meaning “stone,” used in the formation of compound words - Dictionary.com
     
    With the relative thrust to weight ratios in the game, getting to orbit and other planets is fairly easy, but I keep seeing pictures of burned up and crashed ships.  I think this boils down to people making a few simple mistakes when navigating between objects.  So, I wanted to cover the basics of how to navigate around in a space-like environment with gravity wells.  I won't touch the math, so you'll have to figure out the details elsewhere.  Other than that, the goal here is to help people navigate, and do so with better fuel efficiency.
     
    First things first, orbits and transfer orbits:
     
    When you first get out of atmo, you should be pressing x a couple times to show the line that usually shows an approximation of where you'll hit the ground.  If it's red, you're not in orbit.  You'll need to burn up and away from the planet, usually about 30 degrees, until you're accelerating away from it.  If your ship is light and has a lot of thrust, you might need to cut this early, and thrust again pro-grade(That is, along your current direction of travel) once you're fairly high up.  If not, you might need to slowly angle down to continue thrusting prograde long enough to achieve orbit.  You'll know you've achieved it when your line turns blue, and comes all the way around to the back of your ship.  Congratulations, you're in an orbit!  Odds are, it will have fairly low eccentricity (it will only be slightly egg shaped.)
     
    But is it a good orbit for where you're going?  Lets say you want to get to Alioth from Sanctuary, and you're in a pretty heavy ship that takes a while to get going.  Your best bet is to set your orbit up for an approximate Hohmann transfer. (https://en.wikipedia.org/wiki/Hohmann_transfer_orbit).  First things first, you need to clean up your orbital inclination.  In the image below, the blue orbit is not aligned with our target planet.  The ideal orbit will be one that is on the same plane as our target, and will require minimal fuel and time to boost out of to get to our target, which is the red orbit. (Attachment orbit1)
     

     
    To make the change from the blue to red, we need to adjust our inclination.  This is done by changing are direction 90 degrees up or down along our current path, and firing our engines ~1/4 of an orbit before the area of inclination we want to adjust.  Your orbit will pivot along an axis that extends from your ship, through the planet, to the other side of your orbit, adjusting the downrange portion 1/4 an orbit away up towards the direction you are thrusting.  (attachment orbit2)  If you're flying fully manually, you will have to adjust your angle as the orbit adjusts in order to stay ~90 degrees off of prograde.  You can avoid this problem almost entirely by starting off in the correct direction. Fly east.
     

     
    The reason you need to do this is because it's cheaper, fuel wise, to adjust the orbital inclination and do a transfer burn out of it.  DU ships often can just burn through the problem, but it has the same problem as real-world: you go through a lot more fuel trying to fight your current trajectory than you would by making an efficient adjustment.

    Note: You should be burning approx where your current orbit crosses the plane of your desired orbit.  See diagram.  It's fairly intuitive once you get the concept, but it doesn't make a lot of sense in the "I accelerate fast this way, I should move this way" sense.
     
    Once your orbit is lined up, you need to prepare for your transfer burn.  Your ideal transfer burn will start at a position that is dependent on your total mass, thrust, and target.  Small ships have it pretty easy, and can use a trick that helps many people get to the Mun in KSP.  Since your thrust to weight ratio (twr) is likely pretty high, you can start accelerating once your target is just over the horizon, and reasonably expect your trajectory to bend out towards the planet fairly quickly.  Burn prograde (along your blue line), and watch as it extends out towards your target down range.  It will eventually turn purple, meaning you've broken away from orbit, and you will continue along the purple path toward your destination.  (attachment orbit3)

    A ship with a lower twr will want to burn sooner, at approx the opposite side of the planet from their target.  See above link about Hohmann transfer, since that's more detailed.
     

     
    Arriving at your target:
     
    Once you're flying towards your target, you shouldn't be aimed directly at the ground.  The safest approach is to slide into an orbit around the planet and then begin lowering your periapsis (lowest point of orbit).  You do this by angling your approach past the planet, so that it appears you're going to miss it, and then starting to brake at an appropriate distance out; for my small ship, this actually starts at about 4su out if I'm trying to shed down from 15km/s down to orbital speed.  If timed well, you'll enter an orbit around the planet instead of shooting past it.  Once there are better scripts out there, orbits should be more  predictable, but for now you can usually hit a reasonably decent orbit by feel.  If you're having trouble with inserting into a good orbit, aim further out from the planet, and start braking sooner. Ride the gravity well in and brake on the low side to bring the top of your orbit down.
     

     
    The bigger your ship, or the less braking power it has, the sooner you should be slowing down.  If you're 5 su out going 29,999, you're not gonna stop in time.  If you overshoot, don't just brake.  You can burn at a weird, off angle, that is retrograde, and thrusting in the same direction as the curve in the purple line.  This will slow you down as well as change your heading, giving you the ability to come back around into an orbit, rather than simply coming to a dead stop and having to rebuild speed to avoid an awkward re-entry.  Your goal is to retain *some* velocity, but to direct it tangential to the surface of the planet.
     
    Once you're in orbit, set your ground target, and again adjust your inclination to bring your orbit over your target.  Remember, you want to be a quarter orbit off of your target to be able to bring your orbit over it, either approaching or past.  Once that's done, it's usually easiest to wait until you're about a quarter orbit or less away from your target and start braking.  Most ships I've seen will be able to brake down to an atmospheric entry vector in that amount of time, and bring themselves in at a reasonable speed.  If your orbit is too high, you might come in at an awkwardly steep angle.  Depending on how good your braking is, you might even come in too fast.  If you're too low, you'll probably come in way too fast, not being able to shed enough speed before kissing the atmosphere.  One of the most confounding things about orbits is that a lower orbit is much faster over the ground.  Since brakes are so effective, and don't consume propellant, I strongly recommend coming in from a higher orbit.  Trial and balls of fire here until better tools are available.  You should also, ideally, be entering with the rotation of the planet (west to east), but I'm not sure if that really applies with DU.

    Adjusting an orbit:

    The rules here are pretty simple:  If you want to adjust your orbits height, you burn on the far side from the side you want to adjust.  Burn pro-grade to raise, and retro-grade to lower.  Inclination, we've already covered.  You can also burn 'straight up' away from the planet to walk an orbit out on the close side and in on the far side, which will translate the entire orbit in the direction you're burning without changing the shape of it too much.  Same goes for burning in, towards the ground.  That's useful for cleaning up a mostly circular orbit, although it's not great on fuel usage.  I'm not sure if there are any in-game tools for adjusting apo/peri, but I can say it's entirely doable with pen/paper/time and knowledge of your weight/thrust.
     
    (Paint 3D is hard to use, okay?)


    Notes:

    DU isn't nearly as difficult to hit a target as a game like KSP.  You can eyeball the crap out of things and still get where you're going, just by using more fuel and thrusting through the problem.  Things like Hohmann transfers are less useful given the acceleration of ships, but the concepts mostly still apply.  I don't expect people to really need this information for their first ship built on a Dynamic Core S with 4 Space Engine Ms attached, but I'm hoping it helps people who have ships that require rockets to get out of atmo, and take a long time to get up/down to speed.  There's also absolute mountains of information skimmed over here, especially concerning orbital inclination adjustment, and quite a bit that I don't know about how DU works.

    Hopefully this helps someone get around easier.  There are probably errors in here, and bad explanations, so certainly call them out I'll be happy to update.
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