KSP and light speed

[Der Anfang]

Well, not full blown light speed. But I am curious: Is it possible in stock ksp (without cheats, obviously) to reach, say... 1% light speed? Within a reasonable part count, even?

Well, okay, if not a "reasonable" part count, if indeed we're really going beyond that, I am wondering what a typical 1% light speed spacecraft would actually look like. Maybe you would have to assemble it in orbit, as well?

[Randazzo]

Scott Manley made a video about this very topic:

https://www.youtube.com/watch?v=FCXMpWMEc1w

 

[KerikBalm]

1% of lightspeed = 3,000,000 m/s

I seem to recall low orbit around the sun gets you to about 30,000 m/s... which is 0.01% of lightspeed.. so, there's that...

[Spaceception]

Just now, Der Anfang said:

Well, not full blown light speed. But I am curious: Is it possible in stock ksp (without cheats, obviously) to reach, say... 1% light speed? Within a reasonable part count, even?

Well, okay, if not a "reasonable" part count, if indeed we're really going beyond that, I am wondering what a typical 1% light speed spacecraft would actually look like. Maybe you would have to assemble it in orbit, as well?

Just smash into another craft at high speed, and you'll be traveling at 1000x light speed, I mean, you'll break the NavBall/Altimeter, but you'll be going fast

[Der Anfang]

Just now, Spaceception said:

Just smash into another craft at high speed, and you'll be traveling at 1000x light speed, I mean, you'll break the NavBall/Altimeter, but you'll be going fast

Well, that's kinda funny you mention that. I suppose that's one way to do it in KSP. I'm talking about on more "legitimate" terms, though.

[Anquietas314]

10 minutes ago, KerikBalm said:

I seem to recall low orbit around the sun gets you to about 30,000 m/s... which is 0.01% of lightspeed.. so, there's that...

Assuming you don't bother to lower your apoapsis, if you dive from Kerbin with a periapsis around 100 Mm (Moho's orbit is around 4-6000 Mm for reference) you'd get to more like 90 km/s - but you'll probably explode from overheating before you get that low unless you design the craft to handle it. 60-70 km/s is perfectly doable though, and then you've got whatever delta-V you can get there as well

[zarakon]

No, you're talking about 3 million m/s.  Even if you could pack 30km/s into each stage with ion engines, you'd need 100 stages like that.  Each stage would need to at least double the size of the craft, so even if your final stage was somehow 0.1 tonnes, the first stage would have to be measured in yottatonnes - larger than Kerbin.

[KerikBalm]

well, im testing out some craft right now, and at 200,000 km, its getting very very hot, but I can make a craft that is temperature stable. Orbital velocity is 50 km/sec. Raising my apoapsis from there to Kerbin orbit takes 20 km/sec, so if I did the reverse, and lowered my perapsis to 200,000 km, I'd surely survive the heating, and the velocity at perapsis would be roughly 70,000 m/s, or 0.0233% the speed of light.

19,700 m/s from there to go to a kerbin intercept, 20,800 m/s to escape kerbol... oberth is strong that deep in a gravity well

[luizopiloto]

It's possible to go superluminal... just needs a lot of Dv...

[ProtoJeb21]

52 minutes ago, Spaceception said:

Just smash into another craft at high speed, and you'll be traveling at 1000x light speed, I mean, you'll break the NavBall/Altimeter, but you'll be going fast

Pfft, only 1000 times? Danny2462 shot a Kerbal into the sun at 195000 times the spped of light!

[bewing]

By "legitimate", I assume you mean not driving a ship through the crack in Kerbin's crust to the center, either?

With just orbital mechanics, I think the "best" way to do it would be to use Jool for a gravity slingshot straight down Kerbol's throat. Approach Jool retrograde at the highest speed you can manage, make a very low pass -- carefully nullifying your Kerbol orbital velocity. Then do as KerikBalm is doing -- fall to 300,000 km or so, and burn all the ions you've got. I suspect you could get to around 150 km/s, but the Jool encounter would take some time to set up.

[Waxing_Kibbous]

here is a fun video, not light speed but pretty fast:

 

[cantab]

Without mods, cheats, or glitches, it's just not happening. Period. You're looking at 3 *thousand* km/s of delta-V. Using the ion engine with its 4200 s ISP, to get a 0.04 ton OKTO2 to 1% of the speed of light would take a ship massing 2 x 10³⁰ tonnes. That's a hundred thousand times the mass of Kerbol. And that's if your engines and fuel tanks were all massless.

[Snark]

1 minute ago, Waxing_Kibbous said:

here is a fun video, not light speed but pretty fast

Oh good, now all you have to do is to manage 30 times faster than that. 

[cubinator]

1 hour ago, Der Anfang said:

Well, that's kinda funny you mention that. I suppose that's one way to do it in KSP. I'm talking about on more "legitimate" terms, though.

Another way you can do it is by timewarping really fast straight down into a planet. You'll probably end up flying out the other end at potentially relativistic speeds, depending on how close you manage to get to the exact center where acceleration is infinite.

Now for the real answer:

Let's say you want to make a vehicle with an ion engine that has a delta-V of .01c, or 2 997 925 m/s. We'll say it has limitless energy supply for simplicity. Some things we need to know:
-The Dawn ion engine has an Isp of 4200 in vacuum.
-Xenon's density is 0.1 kg per unit.
-Each fuel tank weighs 0.12 t full and 0.05 t dry.
I'm pretty sure the weight of the engine and probe core will be negligible, so I'll just ignore those in the dry mass calculations. Now I'm going to use this information to calculate the mass of the ship. Here goes nothing:

deltaV = 9.82 * I_sp * ln(M_full / M_dry)

2997925 = 41244 * ln(2.4x)

2.4 is the full/dry mass ratio.

72.6875 = ln(2.4x)

Where did all the big numbers go?

e^72.6875 = 2.4x

Oh, there they are.
Divide it by 2.4, aaand:

x ~ 15 402 362 440 000 000 000 000 000 000 000 t

15.4 nonillion tons.

The Sun's mass is only about 1.989 octillion tons. Our spaceship is 7 743.8 times more massive than the sun, and almost 30 times as massive as the most massive known star. It will collapse into a black hole long before it accelerates to it's full potential because it doesn't have fusion in it's core.

DISCLAIMER: I'm not sure how good that math was, especially with the ln(2.4x) thing. I had problems figuring out what to put inside the ln(). If anyone finds my numbers to be incorrect, please correct me. I'm pretty sure they're ok though.

 

 

Edited March 24, 2016 by cubinator

[theend3r]

Sure, I've reached multiples of c in most of the versions of KSP due to timewarping/exploding on launchpad/phantom forces/kraken drives/other bugs or quirks.

Edited March 24, 2016 by theend3r

[bewing]

... aaaand those calculations are exactly why you don't use the engines to generate the speed. You have to use as many gravity slingshots as you can muster. Voyager is not zooming out of our system at super-high speeds because of an amazing set of engines.

[Skorpychan]

Not with just thrusting. That runs into a similar problem to reaching lightspeed in itself.

To get the required DeltaV, you need more fuel. But that slows you down because of the mass of tankage and fuel, so you need more engines and thus more fuel and so forth. I don't think it's possible to carry enough fuel to get that fast.

[TyrannoFan]

There is no way you could legitimately reach lightspeed in stock KSP because of diminishing return. The larger you make your rocket, the less DeltaV you gain by making it any larger. You would need a gigantic (and I mean bloody huge) craft that would not be stable, would take years to accelerate to lightspeed and would not run on (almost) any computer.

However, you can glitch the game out and reach light-speed. In fact, even faster (source):

Spoiler

 

At the speeds achieved here, you would cross our Milky Way galaxy in ~6 seconds.

[LordKael]

As shown by the math done above, it isn't possible to reach lightspeed KSP, and to reach 1% of light speed is equally impossible, if one uses only fuel. My personal record is just shy of 100kms per second, using cheats and a dozen aerospikes at about 150km from Kerbol.

[Renegrade]

38 minutes ago, cubinator said:

deltaV = 9.82 * I_sp * ln(M_full / M_dry)

Actually, that annoying 9.82 g₀ error was fixed some time ago... it's properly 9.80665 now AFAIK.

[cubinator]

1 minute ago, Renegrade said:

Actually, that annoying 9.82 g₀ error was fixed some time ago... it's properly 9.80665 now AFAIK.

But is it still 9.82 in KSP?

[EladDv]

If Speeds close to C is ever reached consistently in KSP (i presume via Kraken Drives) I'll work on a mod to simulate(partly) space and time dilations (at least locally or path lengths) and inertia(making it impossible to reach to C if you have mass)

[Renegrade]

Just now, cubinator said:

But is it still 9.82 in KSP?

Nope - that's that I was talking about -- in KSP.

[cubinator]

Just now, Renegrade said:

Nope - that's that I was talking about -- in KSP.

Cool. Then, with this value, the mass of the spacecraft would change to...

17 004 397 580 000 000 000 000 000 000 000 t.

17 nonillion tons instead of 15.4 nonillion tons. Not that it really matters at that scale, but at least it's accurate.