Re: It's actually for an rpg setting but what the heck

jdnicoll@xxxxxxxxx (James Nicoll) wrote:

In article <0lmuh5948plnk6oab0lko82nrrh7i5av5a@xxxxxxx>,
Eric Ammadon <no@xxxxxxxxxxxx> wrote:
Ben Crowell <crowell09@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

James Nicoll wrote:
In article <r86sh519f8b0dgu8jq4h06k0guk8rqrtc8@xxxxxxx>,
Eric Ammadon <no@xxxxxxxxxxxx> wrote:
David Friedman <ddfr@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

In article <hfkk6h$j47$1@xxxxxxxxxxxxxxxxx>,
jdnicoll@xxxxxxxxx (James Nicoll) wrote:

In article <008ef78a$0$8092$c3e8da3@xxxxxxxxxxxxxxxxx>,
Ben Crowell <crowell09@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
James Nicoll wrote:
Getting back from Venus is a lot more expensive than returning
from Mars, obviously.
Why?
Because Venus has a higher escape velocity.
It's also deeper in the sun's gravity well.
Details, details... <g>

Assuming no aerobraking, for a ship orbiting Venus to go from Venus to
Earth in a Hohmann orbit you need two burns:

Perihelion DV 2.7067 km/sec
Aphelion DV 2.4955 km/sec
Total DV 5.2022 km/sec

Same deal, Earth to Venus

Perihelion DV 2.7067 km/sec
Aphelion DV 2.4955 km/sec
Total DV 5.2022 km/sec


The only difference is the order in which those burns occur.

With aerobraking at the destination, Venus > Earth costs 2.7 km/s, while
Earth > costs 2.5 km/s.

Actually, and since Venus's escape velocity is 10.5 km/s and Earth's is
11.2 km/s, even with areobraking, what you gain from braking at the
aphelion may be lost by having to haul yourself off off a slighly
more massive planet. Earth does have the advantage that it may be
possible to build an orbital elevator; this would not be possible
on Venus.

Your physics makes sense to me, and it makes it sound like the total
amount of fuel needed in both cases is about the same.
The only thing is, I suspect that the cost of a liter of fuel is
going to be a lot higher at Venus than at Earth.

It doesn't make sense to me, he's quoted some naked speeds with no
consideration for anything else. Even the time when a ship leaves
Venus or Mars for a trip to Earth makes a difference, and presumably
he'd be going in the proper direction at the proper time and using the
rotation of Venus rather than fighting it, but it's all unstated
handwavium with a few numbers thrown in to confuse the gullible.

Let's see if I can do this quickly and despite hte fact my coffee has
not kicked in today:

I specified a Hohmann orbit, the usual sort of minimum energy orbit for
this kind of thing, which defined when the ship is leaving (and I posted
a link to launch windows earlier in the thread).

In the Earth > Venus case, you want to turn an almost circular orbit at 1
AU into one whose aphelion is 1 AU and whose perihelion is about .7 AU.
Once you are at 0.7 AU, you will be zipping along fast enough to return
to 1 AU is nothing is done, so you need to slow down a bit so you are
in a 0.7 AU circular orbit.

In the Venus > Earth case, you want to turn an almost circular orbit at
0.7 AU AU into one whose aphelion is 1 AU and whose perihelion is about
.7 AU. You need to speed up a bit to get into that oribt. Once you are
up at 1 AU, your craft will have slowed enough that let to its own it
will fall back down to 0.7 AU so you have to add a bit of velocity to
get into a circular orbit at 1 AU.

The delta vee to go from a circular orbit at 1 AU to a 1 AU > 0.7 AU orbit
is exactly the same as the delta vee needed to go from a 1 AU > 0.7 AU orbit
to a a circular orbit at 1 AU: it's the same operation, reversed. Similarly,
going from a 0.7 AU circular orbit to a 0.7 AU > 1 AU orbit costs the same
delta vee as slowing from a 0.7 AU > 1 AU orbit to a 0.7 AU circular orbit.

Even the orbital elevator comment seems goofy unless there are at
least some reasonable clues about how to solve the associated material
and electrical problems.

This has to do with rotation rates: a space elevator reaches from the
surface of the planet up past a synchronous orbit. On Earth, the
geostationary orbit is at a somewhat reasonable distance: 35,790 kilometers
(but you want ot hang a counter-weight out past the synchronous point
so in practice it would be somewhat longer).

Venus' day is 243 times as long as Earth's day and the sychronous
orbit is therefore farther out: 1.5 million km, if I have not
mucked up my numbers (and again, in practice it would have to be
longer).

I'm glad you got that all worked out, James. <g>

--
arggh, is it priate day again?
.

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