Parker Solar Probe (trip to the sun)

[Art Blade]

[fragger]

Very interesting clip, Art I never considered the launch aspect of the mission, but that makes sense.

Understanding orbital mechanics requires one to adopt ways of thinking that run counter to what may seem "logical" or intuitive. Say you're in an Earth-orbiting spacecraft and there is another spacecraft ahead that is sharing your orbit and moving at the same speed as you. To catch up to it, "logic" would dictate that you use your engine to accelerate forwards. But that won't w0#k, because you're travelling in a circle, not a straight line, so when you thrust forward, you'll spiral outwards into a higher orbit, so that in relation to Earth, you will paradoxically lose some orbital velocity. And since you will then be traversing a larger circle than you were before, you'll actually end up above the other ship and falling behind it. To catch up, you need to decrease speed. Earth's gravity will then pull you into a lower orbit, and consequently a smaller circle (and after the initial retro-thrust, you will pick up velocity as you fall into the lower orbit) so that you will catch up to the target ship and pass below it. Then you thrust forward to push you back up into the previous orbit, and if all goes well, you will rise up to meet the target ship.

Orbital docking and maneuvering is a delicate yet complex mathematical ballet, where "speeding up" means "slowing down", and vice-versa. In terms of physics and geometry it's logical, but in terms of our instinctual Earth-bound senses, it seems contradictory.

[Art Blade]

cheers

yep, and well said.

[Dweller_Benthos]

Yeah I did all that in Kerbal Space Program, and getting to the inner planet in the game or a close orbit to the sun took a lot more fuel than just going out to the outer planets. Another way of looking at it, as Fragger mentioned, the closer you are to the gravity source you're orbiting, the faster you orbit. Earth is orbiting the sun at 30 km per second, or 18.5 miles per second. Mercury, the innermost planet, is orbiting the sun at 47 km per second or 29.5 miles per second. In order to get just to Mercury, a spacecraft would have to increase it's orbital velocity by 17km per second. That is a lot of fuel and thrust, or you do what they did, and make a bunch of loops around Venus for the gravity assist. That was also possible in Kerbal Space Program, but was immensely tricky to get it right just once, and that program only calculates one source of gravity at a time, and essentially ignores all the rest of the gravity sources in the system. Doing it for real a bunch of times? Dang.

[Art Blade]

I admire those people who can think up such a trip AND make it w0#k.

[mandru]

Nice find Art. 

...you're travelling in a circle, not a straight line, so when you thrust forward, you'll spiral outwards into a higher orbit, so that in relation to Earth, you will paradoxically lose some orbital velocity. And since you will then be traversing a larger circle than you were before, you'll actually end up above the other ship and falling behind it...

I recall reading a Sci-Fi novel many years ago (sorry I've forgotten the title and author) where this effect was described.

The outcome of using thrust to gain on a ship you were chasing in a similar orbit was "See you later accelerator." 

[Art Blade]

 

[fragger]