GURPS Space, GURPS Spaceships, Newtonian Flight, and Escape Velocities

NastyGuns · 06-22-2013, 04:35 PM

I'm having some trouble wrapping things correctly in my brain-housing group while attempting to determine the escape velocities from planets and stars. I'm not having trouble with the formula's themselves per se, it's more in determining if I'm converting between information from Space and Spaceships correctly. Here is the situation:

In Space, planets are described in units of Earth Diameters and Earth Masses. And stars are described in Solar Masses and a radius given as some fraction of an AU.

In Spaceships under Newtonian Flight (p. 37), the formulas provided for planets use Earth Masses and Earth Radii. Stars use Solar Masses and Solar Radii.

How do I properly convert between the two?

Planets and Escape Velocity:
One would think all I have to do here is double the number of Earth Diameters to get the proper number of Earth Radii. But that just isn't working for me math wise. If I double 1 Earth Diameter to get the proper 2 Earth Radii, then the equation doesn't work. Or should I leave this equation as is, and just use the Earth Diameter instead of Earth Radii? The equation given is:

EV=6.96 mps * square root of (Me/Re)
Where EV = Escape Velocity; Me = Earth Masses, and Re = Earth Radii

Stars and Escape Velocity:
According to Wikipedia the Sun has a solar radius of 0.0046491 AU. So to make the equation work for stars, I just need to divide the Solar Radii produced in Space by that. So that a correct equation would become:

EV=383 mps * square root of (Ms/(Rs/0.0046491)
Where EV = Escape Velocity, Ms = Solar Masses, and Rs = Solar radii in AU.

Thanks for the help.

vierasmarius · 06-22-2013, 04:41 PM

Quote:

Originally Posted by NastyGuns

How do I properly convert between the two?

Planets and Escape Velocity:
One would think all I have to do here is double the number of Earth Diameters to get the proper number of Earth Radii. But that just isn't working for me math wise. If I double 1 Earth Diameter to get the proper 2 Earth Radii, then the equation doesn't work. Or should I leave this equation as is, and just use the Earth Diameter instead of Earth Radii?

What you're measuring here is a planet's relative linear dimension compared to Earth. A planet that has (for example) a radius x4 that of Earth's radius also has a diameter x4 that of Earth's diameter. You don't need to double anything.

NastyGuns · 06-23-2013, 03:51 AM

Thank you for the reply. I was thinking to just use the first equation as is, since the radii number is just a representation of some unit. Your explaination makes perfect sense, I missed the connection.

Thanks again.

06-22-2013, 04:35 PM	#1
NastyGuns Join Date: Oct 2006	GURPS Space, GURPS Spaceships, Newtonian Flight, and Escape Velocities I'm having some trouble wrapping things correctly in my brain-housing group while attempting to determine the escape velocities from planets and stars. I'm not having trouble with the formula's themselves per se, it's more in determining if I'm converting between information from Space and Spaceships correctly. Here is the situation: In Space, planets are described in units of Earth Diameters and Earth Masses. And stars are described in Solar Masses and a radius given as some fraction of an AU. In Spaceships under Newtonian Flight (p. 37), the formulas provided for planets use Earth Masses and Earth Radii. Stars use Solar Masses and Solar Radii. How do I properly convert between the two? Planets and Escape Velocity: One would think all I have to do here is double the number of Earth Diameters to get the proper number of Earth Radii. But that just isn't working for me math wise. If I double 1 Earth Diameter to get the proper 2 Earth Radii, then the equation doesn't work. Or should I leave this equation as is, and just use the Earth Diameter instead of Earth Radii? The equation given is: EV=6.96 mps * square root of (Me/Re) Where EV = Escape Velocity; Me = Earth Masses, and Re = Earth Radii Stars and Escape Velocity: According to Wikipedia the Sun has a solar radius of 0.0046491 AU. So to make the equation work for stars, I just need to divide the Solar Radii produced in Space by that. So that a correct equation would become: EV=383 mps * square root of (Ms/(Rs/0.0046491) Where EV = Escape Velocity, Ms = Solar Masses, and Rs = Solar radii in AU. Thanks for the help.

06-23-2013, 03:51 AM	#3
NastyGuns Join Date: Oct 2006	Re: GURPS Space, GURPS Spaceships, Newtonian Flight, and Escape Velocities Thank you for the reply. I was thinking to just use the first equation as is, since the radii number is just a representation of some unit. Your explaination makes perfect sense, I missed the connection. Thanks again.