transfer to the sun

[thelordrahl]

hey guys so i am trying to do a mission like scott manley did in one of his interstellar videos to get science from low sun orbit and he managed to to there in about 19 days when I did the xfer to the orbit mine is about 90 days anyone know what could be going wrong im fairly certain i used approximately the same launch and transfer profile here is the video for reference

[mod edit]Corrected imgur link, fixed YT and added a tag.[/mod edit]

Edited July 16, 2015 by thelordrahl
Corrected imgur link, fixed YT and added a [video] tag.

[Reddeyfish]

the orbit in the video has the periapse at about 800 million. Yours is at 2330 million. That's probably why.

[Chiron0224]

I was thinking the same thing. The lower your peri the steeper your sun dive will be and the quicker you'll get there. With Kerbol the gravity is so high that even a small difference in periapsis can have a huge effect on the length of the mission. So be a comet and get dangerously close.

[Kryxal]

Also, what's your day length? I'm guessing you're using 6-hour days, so that's more like 22.5 days compared to 19.

[Snark]

Actually, it won't make that much of a difference.

Your orbital period depends on the average of your apoapsis and periapsis. Most of your orbit time is spent far from the sun, so as long as your periapsis is pretty low, making it lower will have only a small effect on the time taken.

The length of time for an elliptical (or circular) orbit is proportional to (Rpe + Rap)^1.5. Kerbin's orbital period is 426 days. So, let's figure out "what's the fastest Hohmann transfer down to the sun":

- Apoapsis is at Kerbin

- Periapsis is the lowest possible, for a fast orbit-- let's say zero.

That means that your average orbital radius for that highly-eccentric elliptical path will be 1/2 that of Kerbin's orbit. Therefore, its orbital period will be 0.353 of Kerbin's. You only want half of that (i.e. you're interested in "how long to get to the sun", not "how long for a round trip"), so that makes 0.17677 of a Kerbin year. Kerbin takes 426 Kerbin days to orbit the sun.

Therefore, the time taken to reach the sun in this best-case scenario will be 75 Kerbin days.

So yes, you will get there faster if you lower your periapsis, but not by much-- the absolute best you can hope to do will be to reduce the time from 90 to 75 days.

So, if someone did it in 19 days, then that means something else is going on. I haven't watched the video you mention, but I can think of two possibilities:

1. If he did it in "19 days", was that Kerbin or Earth days? (A Kerbin day is only 6 hours.) The above "75 Kerbin days" best case scenario works out to about 19 earth days, so maybe that's it. In other words, not a lot faster than what you're doing now-- your current 90-Kerbin-day path is only 22.5 Earth days.

2. If he did it in 19 Kerbin days, that would mean not doing a Hohmann transfer: it would mean expending a metric crapload of dV to give him a very fast radial-inward velocity from Kerbin's orbit.

[thelordrahl]

thanks for all the replies and the video was made before patch .23.5 so all the days are in earth days rather than kerbin days

[thelordrahl]

thanks for the replies videos was made before .23.5 so all the time was in earth days rather than kerbal days