Space Time

[Wintersniper]

I stayed out in space for a good hour and half, transferred up to 1500 km up and then safely returning back to Kearth.

Anyone can shot a rocket at escape velocity, the challenge I find is returning them home after going deep into space. (Good practice whenever the moon comes around. )

the trick there is just get something with a high thrust to mass ratio, and use it to decelerate and then accelerate in the appropriate direction. I personally like to use an ion engine with several fuel tanks for it, because then I often have enough fuel to change course several times before returning to the planet. It however does take forever that way

[foamyesque]

the trick there is just get something with a high thrust to mass ratio, and use it to decelerate and then accelerate in the appropriate direction. I personally like to use an ion engine with several fuel tanks for it, because then I often have enough fuel to change course several times before returning to the planet. It however does take forever that way

That's bad advice. What you want is something with the highest possible Ve (or Isp, same thing), not high TWRs. In orbit, as long as your TWR is > 0, you can do whatever you please. That's why the ion engine works so well after you've achieved orbit, but is utter junk for anything before that.

[defiler86]

the trick there is just get something with a high thrust to mass ratio, and use it to decelerate and then accelerate in the appropriate direction. I personally like to use an ion engine with several fuel tanks for it, because then I often have enough fuel to change course several times before returning to the planet. It however does take forever that way

I do worry a lot about mass/thrust ratio... especially a beginning stages. Often attach 3 engines with a tri-coupler to a single stack of tanks. It's really gets me going, and all my designs reach 10k in the first stage... if it doesn't make that, the rocket is scrapped.

I haven't played with ion engines or anything, usually stick to vanilla parts. But it's always fun to see Kearth get smaller and smaller and then returning them. 8)

[rdfox]

It won't stop. It hit escape speed It will reach 0 m/s at an infinite distance :

Depends on if he fired it straight up or flew downrange. If he fired it straight up, then yes, it will eventually come back home. (Or would, if not for the bug where, when you get far enough away, rounding error causes gravity to cease to affect the vehicle.) Escape velocity has to be *downrange*, parallel to the planet's surface, so that you're flying forward fast enough that the ground falls away from you faster than you fall towards it. (Oversimplified, but close enough.)

You can't escape by just thrusting straight up. No matter how fast you go directly away, in a two-body system (like KSP currently is) with no rounding error glitch, you *will* eventually come back. It may theoretically take longer than the lifespan of the universe, but gravity will win in the end. You have to be in a parabolic/hyperbolic orbit to escape gravity, because it will always keep pulling on you, no matter how far away you are, so you need to move *perpendicular* to it, which it can't slow down.

[DoctorEvo]

Dude, what?

If you hit escape velocity and stop thrusting, you're never coming back. Period. Direction is irrelevant.

[rdfox]

Except that escape velocity is perpendicular to the gravity vector, remember? If you're travelling straight up away from the body you're trying to escape from, it will eventually bring you back.

[DoctorEvo]

Nnnnnnnoooooooooooooooooooooo...........

Sorry, man, but you're wrong. I'm certain of this. I explicitly remember being taught that escape 'velocity' is a misnomer, but gets used anyways out of habit. Kinda like how kinetic energy is generally expressed in terms of velocity, even though it doesn't matter one bit what your direction is.

[rdfox]

Really? I'd always been told that if you were flying radially outward, gravity would eventually win out... *checks wikipedia*

...aw, hell.

OK, I *think* (although now, I'm what the geologists joked NASA stood for in the Apollo days--Never Absolutely Sure of Anything) that the delta-vee required to escape is *lower* in a parabolic/hyperbolic orbit rather than a radial departure, with a lower escape speed, too. But now my brain is starting to hurt because, as Weird Al told us, everything I know is wrong! 8)

[DoctorEvo]

OK, I *think* (although now, I'm what the geologists joked NASA stood for in the Apollo days--Never Absolutely Sure of Anything) that the delta-vee required to escape is *lower* in a parabolic/hyperbolic orbit rather than a radial departure, with a lower escape speed, too. But now my brain is starting to hurt because, as Weird Al told us, everything I know is wrong! 8)

The tangental parabolic escape does require less delta-V due to the Oberth effect, but it requires a higher peak speed.

Here, somebody illustrated the effect in KSP:

[salsathegeek]

My record was 12+hours, but I forgot which of my rockets it was with. (I think it may have been one of the Ork series?) I may try again later.

[Xivios]

A 9 hour flight out to 5000+Km into a fractional orbit returning to land here.

[Frederf]

Really? I'd always been told that if you were flying radially outward, gravity would eventually win out... *checks wikipedia*

...aw, hell.

OK, I *think* (although now, I'm what the geologists joked NASA stood for in the Apollo days--Never Absolutely Sure of Anything) that the delta-vee required to escape is *lower* in a parabolic/hyperbolic orbit rather than a radial departure, with a lower escape speed, too. But now my brain is starting to hurt because, as Weird Al told us, everything I know is wrong! 8)

It all makes sense when you think in terms of energy. In an eccentric orbit you have a fixed total energy consisting of kinetic (T) and potential (U). As you reach peak altitude you have the maximum U (well, least negative) and the minimum T. As you fall back down again into the gravity well you lose U and increase T. To have escape energy your T+U must be greater than the gravitational well's value at infinite distance (usually assigned as 0). If you're in a high orbit you have a lot of U so you only need a little more T to have T+U over the threshold. If you're at the bottom of the well (large negative U) you need a lot of T such that T+U is enough.

The Oberth effect is simple, if an increase in speed costs the same amount of rocket fuel no matter if you're going from 1m/s to 2m/s or from 1000m/s to 1001m/s, then you want to maximize how much T that burn gives you. Since kinetic energy is the classic Newtonian T= 1/2 mv^2, your increase in T is better the bigger v is. Your speed is highest at the bottom of the gravity well because you speed up as you fall.

If you compare T for 1m/s with 2m/s you see that you've increased T by 1.5m. If you compare 1000m/s with 1001m/s you've increased T by 1000.5m. In fact the calculus of it works out that doubling your speed at burn doubles how much kinetic energy you receive. Going from 500m/s to 501m/s in an increase of 500.5m in T.

[defiler86]

A 9 hour flight out to 5000+Km into a fractional orbit returning to land here.

Congrats man. That's amazing.

Mind showing the rocket you done it with?

[icefire]

Longest mission I ever LANDED was around 9 hours 30 minutes. I just recorded me launching and orbiting once, then went to work. (HDD space would have been filled if I recorded the entire mission, sorry) when I came back I started recording again and landed. Botched the reentry though and didn't land close to the pad. But at least it was on the same continent.

[Catfish9200]

After reaching 20556976M in about 90 minutes I had to stop because I was getting extremely bored. (I was still traveling at about 3000 m/s into space and was decelerating with about 0.1 m/s each second.

[Spiz102]

Testing a Ion Drive engine I obtained orbit and burned for several hours to appear at some ludicrous distance and doing 22,000m/s. I then flipped over and began decelerating and left it over night to discover taht, due to a huge lie-in I had managed to slow to a stop, accelerate back towards Kerbin and end up at a similar distance in the other direction, so I braked it to a stop a second time and managed to thrust up to about 15kps towards the planet and cut out thrust, and then left it for a few more hours correcting lateral movement so I managed to impac thte atmosphere at 15kps.... was an interesting reentry at about 36 hours mission time.

Unfortunately I forget to screencap and my parachute had accidentally armed several hours before so it ripped off on entry and I smashed into the deck having slowed to about 130m/s thanks to the old brick wall atmosphere.

[Feragorn]

I had four stacked fuel tanks and a very efficient motor (ion drive or nuclear or something), and I could almost fly one quarter to one half of the circumference of Kearth.