How to slow down vessel after orbit?

[OhOhOhOhOh!!]

Hello guys. Im new here  . I've been playing KSP for about 22 hours now and It was amazing and I love it so much. So I pick this one contract and its called World's First contract. I've done a lot of World's First contract such as launching the first rocket,Orbiting kerbin and escape the atmosphere. But this one contract that don't understand.Which is the "Return to Kerbin from Orbit". Click "Note" and it says " Slow Down a vessel in orbit and return it to the surface to achieve this goal" . How? What kind of ship should I design? Thank you  

[Geonovast]

6 minutes ago, OhOhOhOhOh!! said:

Hello guys. Im new here  . I've been playing KSP for about 22 hours now and It was amazing and I love it so much. So I pick this one contract and its called World's First contract. I've done a lot of World's First contract such as launching the first rocket,Orbiting kerbin and escape the atmosphere. But this one contract that don't understand.Which is the "Return to Kerbin from Orbit". Click "Note" and it says " Slow Down a vessel in orbit and return it to the surface to achieve this goal" . How? What kind of ship should I design? Thank you  

You need a ship that will still have a little bit of fuel left after getting into orbit.  To de-orbit, point your ship retrograde, and burn the engines until your periapsis is ~30-35km.

Oh, and don't forget your parachutes.  Heat shields are a good idea too, although not always needed.

Example:

Here's a small space station I have going in my science save.  Once I'm done sciencing, the pod will detach, burn retrograde until I have the periapsis where I want, then it will decouple the tank and engine.  I've got two drogue chutes and two normal chutes for landing.

[bewing]

Welcome to the forums! To get to orbit (as you have already done) you burn prograde until your Pe is above the atmosphere -- because burning prograde makes you go faster.

As geonovast said: to reenter, you need to burn retrograde to make you "slow down" a little, until your Pe is back in the atmosphere. After that, the atmosphere will slow you down the rest of the way.

Actually surviving the process of reentering can be easy or tricky, depending on how high your Ap was.

 

[OhOhOhOhOh!!]

Burning retrograde? Oh, I never tried that before. Ok, thank you sir. Will see if I can do this.Thank you again

[Snark]

7 hours ago, OhOhOhOhOh!! said:

Burning retrograde? Oh, I never tried that before.

The nice thing is... that's exactly what real-life spaceships do when they want to come back down from orbit.

So setting up that burn is nicely realistic, in that regard. 

Glad you got it all sorted out!

[Geschosskopf]

On 1/29/2018 at 3:02 AM, OhOhOhOhOh!! said:

Burning retrograde? Oh, I never tried that before. Ok, thank you sir. Will see if I can do this.Thank you again

Welcome aboard!

Yeah, orbital maneuvers are a bit counter-intuitive at first, but they'll soon become second nature.  A couple of key things that help with understanding them are:

1.  Prograde/Retrograde burns change the opposite side of your orbit

Suppose you're in a circular orbit around a planet.  If you burn either prograde or retrograde, you stretch your circular orbit into an ellipse, and this change takes place on the opposite side of the circle from where your ship currently is.  If you burn prograde, you stretch the opposite side out so it will be higher than the original circle.  If you burn retrograde, you pull the opposite side down so it will be lower than the original circle.  However, side where the ship is at the time of the burn stays in the same place.

This is why you burn retrograde to de-orbit.  What you're doing is lowering the opposite side of your orbit to a point where your path will either intersect the ground or be low enough in the atmosphere that drag will do the rest.  Note that until you hit the atmosphere, the point where you did the retrograde burn will remain at the original orbital altitude.  But then atmospheric drag slows the ship some more, which acts like another retrograde burn, so that lowers what is now the opposite side of your orbit (where you were when you started).

2.  "Slowing Down" makes you go faster, and vice versa

The instructions about "slowing down a vessel in orbit" is a bit misleading.  The lower the altitude of an orbit, the faster the ship is moving around the central object.  IOW, suppose your ship is in a 200km orbit.  You burn retrograde, "slowing down", and lower the opposite side of your orbit to 100km.  When you get there, you burn retrograde again to bring down the original side, so you end up in a circular orbit at 100km.  You have now "slowed down" twice with 2 retrograde burns, but your ship will now have a higher velocity in the 100km orbit than it had in the 200km orbit.  OTOH, if you burn prograde twice, "speeding up", so that you end up in a higher orbit than 200km, your ship will have a lower velocity.

Strange, I know, but that's how it works.