Entry: Skylab to Mars IV Thursday, August 05, 2004



I found since last entry that my estimates for dv around Mars are really ultra-sensitive to Mars' orbital position. Hence the propellant mass quoted is potentially off the mark. When Mars is at aphelion the dv is about ~ 1 km/s lower than if Mars was at perihelion. Early September 1986 would find Mars at aphelion, but where would Venus be for the sling-by from the novel? Working on it currently. Transfer time is ~ 384 days, meaning a launch in August 1985.

BTW The mission flight-plan from Voyage is an Opposition Class Hohmann transfer. Usually the dip past Venus is on the way back from Mars to reduce the re-entry velocity at Earth. But it works the other way too, it seems.

And I am wondering about the propellant choice - UDMH/N2O4 is very space friendly, but there are other options for fuels and oxidisers out there. Liquid oxygen is the very definition of cryogenic, but keeping it liquified doesn't take a lot of equipment or power, especially if the tank was wrapped in reflective insulation, for example. A mix of RP-1 and LOX has a decent amount of kick, an Isp ~ 353s, but LOX and UDMH can get ~ 363s, which is cool. Definitely better than the N2O4/UDMH mix of Apollo. The mix is a lot denser (0.97 vs 0.28 gm/cc) than the LH2/LOX used in the Saturn IV-B, so if we assume the tanks are lighter, but the cooling system makes up the difference, then the Interplanetary Maneuver Stage (IMS - another TLA) can mass ~ 13.5 tons like the Saturn IV-B + IU combination.

Propellants that have been tried in real rockets can all be found here...

Propellant Index

Interesting mix is LH2/LF2 - hydrogen and fluorine - which would have a highly toxic exhaust, hydrofluoric acid! Yikes! Glad the rocket makers settled on LH2/LOX for high performance - only exhaust is steam.

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