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I ran Google AI again, using the 2031 departure trajectory. It says hyperbolic approach velocity is 4.77 km/s.
"for a spacecraft departing earth orbit on February 23 2031 toward mars with 176 day transit what is approach velocity"
Then what is atmospheric entry velocity? 6.86 to 6.89 km/s depending on specific altitude defined for entry interface.
"assuming hyperbolic approach velocity of 4.77km/s to Mars, and aerocapture with apoapsis of 80km above surface, what is atmospheric entry velocity"
So the specific dates and trajectory reduce atmospheric entry velocity a little. And using 4.90 km/s is slightly higher exit velocity. So using Google the higher entry velocity we need to shed 1.99 km/s. That's a little better.
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Rob:
I updated the analysis and the spreadsheet to be more realistic and defendable. It's available from the drop box by means of links here on the forums, in the GW postings thread, if nowhere else. Tom put it up there.
The problem with running other people's software while trying to do orbit mission planning is not knowing what the results really mean, and/or not knowing garbage from good stuff. Computers "happily" process bad data in garbage, and it looks the same as when they process good data into good stuff.
I suggest that you use whatever 2-body orbit "thingie" you prefer to define the trajectory of your ship with respect to the sun. Ship, sun: 2-body. The textbook formulas give you that. The speed and direction of the Earth and Mars with respect to the sun are well known. Planet, sun: 2-body. Textbook formulas.
The problem is the 3-body effect when the ship is near Earth with the sun acting on both, and near Mars with the sun acting on both. ship, planet, sun, 3-body: there are NO textbook formulas for that, only finite-element computer codes can do it "right". And the amateur using somebody else's code often gets into trouble. I know I do, and did.
What I outlined in post 22 above was a magnitude-only close approximation that gets you ship speed (but NOT direction) near the Earth at departure, and near Mars at arrival. It's arithmetic for Hohmann transfers (because the ship and planet velocity vectors are parallel where the orbits touch tangentially). It's vector math for faster trajectories, because the ship and planet velocity vectors are NOT parallel where the orbits cross!
But you can easily get the speed of the ship on its 2-body orbit at the planet distance from the sun, with a textbook formula! I put that into the orbits spreadsheet in the drop box linked from the forums. It may or may not be in the NASA online orbits calculator. The angle between them is something you must figure from the local slopes or angles of the ship trajectory and the planet orbit, and then difference. My spreadsheet produces the plot data in a table that lets you figure that out.
GW
Last edited by GW Johnson (Today 09:07:29)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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