Power requirements

[ffutures]

I think someone told me that (ignoring travel times) it actually takes less energy to travel from Earth to Mars than between Jupiter's moons. Can anyone point me at a source for this? And for energy requirements for other interplanetary journeys? Preferably something that I can use without breaking copyright?

Comments

[dsample.livejournal.com]

Take a look at http://en.wikipedia.org/wiki/Hohmann_transfer_orbit

The equations for calculating the energy requirements of orbital maneuvers aren't that complicated. You should have most of the numbers you need to plug into the equations handy. (The one number you might not have is the "standard gravitational parameter" for the sun or Jupiter, which you can find at http://en.wikipedia.org/wiki/Standard_gravitational_parameter)

Aerobraking

[ponderoid.livejournal.com]

I read somewhere the reason that a Mars trip takes less energy than many other trips is because you can do aerobraking at the end. It was some time ago; I can't cite a source, sorry.

[nelc.livejournal.com]

Isn't the energy requirement directly related to the delta-V?

[dsample.livejournal.com]

Yes, but what's the delta-v required to move from Earth to Mars, or from Ganymede to Io?

[nelc.livejournal.com]

The difference in their orbital velocities, to a first approximation?

[ffutures.livejournal.com]

There's also the energy needed to get out of their gravity wells, and move in or out of the gravity well of the Sun / Jupiter.

[dsample.livejournal.com]

It doesn't work that way. Mars has a lower orbital velocity than Earth, but to get from Earth to Mars you have to accelerate to put yourself into an elliptical orbit to take you out to Mars, and then accelerate again to match your orbit to Mars's.

You have to go faster, in order to go slower.

[nelc.livejournal.com]

I just recalled that Jerry Pournelle covered this in a chapter in A Step Farther Out, with a series of tables covering travel between planets and the Jovian moons. Unfortunately, I can't find my copy right now, but IIRC I think he said that the dV requirements for travel between Jovian satellites was a lot kinder in dV requirements than between planets. Especially if you include landing and lift-off requirements, since the planets tend to be bigger than the Galilean moons.

[nelc.livejournal.com]

Absolute value of delta-V?

[ffutures.livejournal.com]

OK, thanks, I may have a copy somewhere. I must be mis-remembering the Jupiter thing.

[ffutures.livejournal.com]

I really only need approximations, so that I can have e.g. a table showing how much fuel is needed for various interplanetary journeys, with earth - mars as 1, earth to jupiter as 4 times as much (or whatever), and so forth.

[nelc.livejournal.com]

Let's see if I can format this properly:






Diagonals are orbital velocities of the planets in km/s; other values are the deltaV's required to go from orbit to orbit. So Earth-to-Mars is 5.7 km/s, greater than any Jovian satellite transfer except Io-to-Ganymede and Io-to-Callisto. And as I said, relative sizes mean that surface-to-surface will add more to Earth-Mars requirements than any given pair of Gallilean satellites. (But it's late and I've forgotten how to work out exactly how much right now...)

[nelc.livejournal.com]

So, I guess if you define a fuel unit as 3 km/s, it takes roughly 1 fuel unit to from one moon to the next one, two units to move two orbits, and three to move three orbits in the Jovian system.

Meanwhile, Earth-to-Mars takes two units, Earth-to-Jupiter takes 6 units, and Mars-to-Jupiter takes 4 units.

(That's provided I haven't made a simple error somewhere... Where's that book, I swear it was right here just a couple of years ago....)

[ffutures.livejournal.com]

Thanks - I've got my own copy somewhere, so don't worry too much, it'll turn up.