The walk of shame

People on Venus hoping to climb out of that gravity-well are pretty much stuck there until Hohmann. Venereans abroad, by contrast, always have an early option back to THEIR home - the fry by. From Earth to Venus as of 3 January 4:37 PM MST I couldn't find a relevant chart on the Internet, so I had to back-of-the-envelope this... in crayon.

I am reverting this post and reposting it, today. Because we can do better... much better.

For background, Bob Zubrin discussed Type II inbound as ideally no better or worse than the classic. It just launches at a different day of the year, at a different angle; and takes (much) longer to get to its inner-planet destination. Usually it also takes up more delta-V to start out, but you might be able to shave that off at the destination depending on the angle you want down there.

Our planets are not ideal. Mars - for one - every now and again happens to get out of whack with Earth due to that ridiculous Martian ellipse, running up delta-V and trip-time, both. In a case like that, instead of running the Type I trip right to Earth, some Mars/Earth Type II windows open where you can run it by Venus on the Mars/Venus Type I. Venus' aerobrake delta's your vessel's V to get your package to Earth on the Venus/Earth Hohmann. One Type I and the outgoing Hohmann, together, improve that "II". Cheaper, if slower.

[JANUARY'S CRAYON: An average Earth-Mars half-synod (ignoring eccentricity) is 390 days. The Type I Hohmann trip there chews up 259 of those. This leaves 131 days before opposition-day. But Zubrin claimed a 30 day stay. Instead of half, I'm guessing typical fry-by windows to be ~289 days of the ~780 day synod - a proportion of 0.37. Or about 133 degrees on the circle.]

For Venus, no body runs interior to its orbit fit for aerobraking, except the Sun (LOL). We are just trying to get there - our journeys thither shall ne'er dip beneath her circle. For that one issue is Earth's ellipse, but that's not so bad. The real issue is the relative Earth / Venus inclination because both happen to be solar-system outliers, on opposite sides.

So today, Don Mitchell sets me right... sort of. He discusses two Type IIs and miscues on at least the latter. Former's not looking good either.

Mitchell's first "Type II" was at synod D in the A-E metonic, after 27 March 1972 in the chart. That Type I was a very quick one; but also 6.1 MJ/kg = kJ/g. (Or, since E/m = v²/2... delta-V was 3.5 km/s.) Venera 8 took that route. Mitchell alleges a cheaper Type II trajectory left on April 4, took 170 days and cost 4.1 megajoules per kilogram. He doesn't say which mission that was. Not. Even. Wrong.

Mitchell cites later Pioneer 13 in 1978, the "Multiprobe". This went on a C year, whose Type I window was 15 August. Pioneer 13 went up 8 August; separated into components mid November and arrived 9 December. Mitchell has that as the Type II - this one is at least... wrong. But a better wrong! Mitchell should have discussed Pioneer 12, which as Colin-Hall 1977 doi 10.1007/BF02186467 pointed out launched 20 May estimating transit time of 186-197 days. This Venus Orbiter duly got to orbit-insertion 4 December.

Then... there's Magellan. This launched from the shuttle 5 May 1989. That was an E synod whose Type I Galileo swung by Venus that October. Magellan took two orbits. Its first orbit went to 9 March 1990 arriving back where it was and where Earth wasn't anymore. Magellan inserted Venus 10 August. I don't even know what you call this. Type IV?

From Pioneer 12, let's go with that 190 day Type II usually opens maybe three months before the classic Type I Hohmann. From Mitchell - and take this with much NaCl - Type II saves delta-V only on the D and E synods. Type II always takes much longer than the later Type I to arrive around the same time. If Oppositional, this longer trip goes out of communication with Earth. Tho' it gets to communicate with the SEL3 anticthon. Tho' I am unsure that Pioneer 12 was Oppositional anymore.

I should point out that Type II has a transfer-angle over 180. That makes it good for orbiters... like Pioneer 12. Clearly Magellan was built for similar.