Orbital physics.

[cziken20]

I wanted to ask- Kerbin gravity is 1 g, Earth is 1 g, so why they have other orbital speeds? they should have the same, because speed doesn't changed byt the radius of that body, only by gravity, and distance from body- isn't it? Or maybe even i have wrong infos and the orbital speeds are the same?

[Jenteb07]

Orbital speed also depends on the mass of your parent body

[Ralathon]

I wanted to ask- Kerbin gravity is 1 g, Earth is 1 g, so why they have other orbital speeds? they should have the same, because speed doesn't changed byt the radius of that body, only by gravity, and distance from body- isn't it? Or maybe even i have wrong infos and the orbital speeds are the same?

An orbit is nothing more than falling and missing the ground. How fast you need to fall depends on the curvature of the body you're orbitting, which depends on radius. So yea, radius is very important for orbital velocity.

Also, don't confuse surface gravity with mass. Kerbin has the same surface gravity as earth, but it is 10 times as dense and 10 times as small. If you do the maths you come to the conclusion that earth is 100 times as heavy as Kerbin.

[cziken20]

So it means that Earth really needs more speed in orbit? But... why Scott Manley on one of his films calcuated that on Earth you need 4300 m's dV for orbit, and the orbital speed is 7300 m/s, my mind simply don't get it.

[Agamemnon]

that on Earth you need 4300 m's

what you need on earth starts somewhere around 9000 m/s... ksc or baikonur needs more because of location (it also depends on launcher, trajectory, etc)

[peadar1987]

v^2=GM/r. Kerbin has both a lower mass and radius than Earth, but its mass is smaller by a far greater factor, even though the density is higher (mass scales with the cube of radius, so this is usually the case. Reduce the radius by half, and you reduce the mass by a factor of 8, reducing the necessary orbital velocity by a factor of four).

I'm guessing Scott Manlet has made a mistake (sorry for the sacrilege!). Wiki says the delta-v requirement to LEO is closer to 10 km/s: http://en.wikipedia.org/wiki/Delta-v_budget

[Ralathon]

So it means that Earth really needs more speed in orbit?

Yes, the ISS moves at about 7.6km/s while a similar space station in Kerbin orbit only needs about 2.1 km/s

But... why Scott Manley on one of his films calcuated that on Earth you need 4300 m's dV for orbit

You're probably confusing the dV you need for liftoff on Earth with the dV you need on Kerbin. On earth you need about 9.3km/s to get into orbit (the difference between this and orbital velocity is due to gravity losses and atmospheric drag during ascent). On Kerbin you need about 4.5km/s to get into orbit, you can get away with 4.3 if you use a good trajectory.

and the orbital speed is 7300 m/s, my mind simply don't get it.

Recap:

Orbital velocity in low earth orbit: 7.6km/s

dV needed to get into a low orbit on earth: 9.3km/s

Orbital velocity in low Kerbin orbit: 2.1km/s

dV needed to get into a low orbit on Kerbin: 4.5km/s

[TheShadow1138]

Orbital velocity is, as has been stated already, dependent entirely on the mass of the parent body and the distance of the object from the center of that body. If I remember correctly, this is one manner in which a celestial body's mass may be determined. For instance, if we determine the orbital parameters of say, Io orbiting Jupiter, we can sue that information and the vis-viva equation (v^2 = GM*(2/r - 1/a), where "v" is the orbital velocity, "G" is the Universal gravitational constant, "M" is the mass of the parent body, "r" is the distance between the centers of the two bodies, and "a" is the semi-major axis) to determine the mass of Jupiter.

So, If you want to perform a little bit of science on your own, you can launch a vessel to orbit of Kerbin, or any other body and note your velocity and altitude at a certain time, add the parent body's radius to your altitude (this gives your "r" for the vis-viva equation), use your periapsis and apoapsis to determine your semi-major axis, a=radius+(Ap+Pe)/2 (where Ap and Pe are altitudes relative tot he surface), and use the vis-viva equation to calculate the mass of the body you are orbiting. I believe that in the Tracking Station you can get both the radius and mass of each body to use for your calculation and confirm your result.

[cziken20]

But i also want to ask for other scales- engine sin KSP are about 10 times less powerful, aren't they?

[Ralathon]

But i also want to ask for other scales- engine sin KSP are about 10 times less powerful, aren't they?

No they aren't. Power is okay and ISP is pretty close to real life values as well. The biggest difference is the weight. Engines and fuel tanks in KSP tend to be a lot heaver than real life with poorer fuel to weight ratios.

Most empty tanks in KSP weigh 1/9th of the full weight (11%). In real life this number is closer to 4% or so. The engine with the highest Thrust to Weight ratio in KSP (48-7S) has a TWR of 30ish. In real life most engines have TWR's around 100.

[Zuni]

So it means that Earth really needs more speed in orbit? But... why Scott Manley on one of his films calcuated that on Earth you need 4300 m's dV for orbit, and the orbital speed is 7300 m/s, my mind simply don't get it.

I think it's 4300 m/s for KERBIN orbit.

[cziken20]

No they aren't. Power is okay and ISP is pretty close to real life values as well. The biggest difference is the weight. Engines and fuel tanks in KSP tend to be a lot heaver than real life with poorer fuel to weight ratios.

Most empty tanks in KSP weigh 1/9th of the full weight (11%). In real life this number is closer to 4% or so. The engine with the highest Thrust to Weight ratio in KSP (48-7S) has a TWR of 30ish. In real life most engines have TWR's around 100.

I see... i was thinking about it for a bit of time, and kerbal rockets also seem a little to small for me. Are they?

[Ralathon]

I see... i was thinking about it for a bit of time, and kerbal rockets also seem a little to small for me. Are they?

They are compared to real life rockets. But kerbals are also smaller creatures so it makes some semblance of sense. For example, the diameter of Saturn V's first stage is 4 times bigger the diameter of a big orange tank in KSP. The gemini capsule, one of the first and smallest manned capsules was almost as big as the Mk 3 capsule in KSP.

[fireflower]

But i also want to ask for other scales- engine sin KSP are about 10 times less powerful, aren't they?

KSP fudges the spatial dimensions, true. ISP, however, is measured in seconds, and KSP doesn't fudge the time dimension. From looking at the ISP range of kerbal engines, they're roughly equivalent to a space program that only uses RP1/LOX. That isn't really about the engines but about the chemistry of the bipropellant. A great philosopher once said, "respect the chemistry." Human space programs have access to cryogenic fuels like LH2 that has a base ISP over 450 seconds. That isn't "ten times more powerful" than a KSP engine that gets 330 seconds of specific-impulse, especially when you consider the density issues of LH2 compared to RP1. As a rule of thumb, at lower altitudes, the higher density of RP1 trumps the higher efficiency of LH2. A tripropellant system was proposed for the US space shuttle's main engines that ran on RP1/LOX at low altitudes then LH2/LOX at high altitudes. Now and again people ask about a LH2/F/Li tripropellant because it has the highest ISP in chemical rocketry (550 seconds or so), but that's an engineering nightmare and an environmental catastrophe, and once again, they're not "ten times as powerful." If you're interested in human-scale rocketry, there's a realism modpack you could install. Because LEO ÃŽâ€v is no less than twice the LKO ÃŽâ€v it tends to be a humbling experience.