Using Retrograde Orbits Instead of Aerocaptures

[Carl Sagan The S Stallion]

Prepare thyself, this is a long post.

As many of us are well aware (and are quite frankly hyped about or dreading) 1.0 will include reentry heat and a new aerodynamics system. One problem a player might encounter is when coming out of interplanetary travel, they can no longer perform an aerobraking maneuver without proper heat shielding and a fair amount of luck on their side. Not complaining; just stating the facts.

Now, my question is, can you use a retrograde orbit in place of aerocapture? I could imagine that fighting against the turn of the body you are orbiting would slow you down (maybe not enough for capture, but at least enough to cut fuel consumption).

If this could happen, theoretically, one could send a Voyager probe to Jool and do a retrograde flyby to approach one of its moons (laythe, tylo, or vall) and do a final (or a series of) retrograde orbit(s). From there, you could turn around by going to a higher orbit and turning around.

This may also have a second use as an alternative to potentially unpredictable aerobraking behavior stemming from the new drag and lift system; at least until an updated aerobraking calculator is created.

Edited February 13, 2015 by Carl Sagan The S Stallion
spelling errors; damn autocorrect

[RainDreamer]

I think you are just talking about gravity assist. Gravity assist maneuvers are not only for gaining speed. If you approach a planet in front of its orbital path (doesn't matter which way it spins), you will slow down as its gravity capture you.

About aerobrake though, I think it might still be possible, just not as crazy as before. So instead of dipping real deep in, we might have to skip around on the upper atmosphere a little bit longer.

[AlexinTokyo]

You cannot use a body's own gravity to capture into orbit around that body.

Entering a body's SoI from in front (in respect of its orbital direction) will slow you down relative to that body's parent when you leave the SoI. The exact amount will depend on the exact trajectory and the mass of the secondary body.

In this way, you can use a pass around the Mun to shed some of your interplanetary velocity on returning to Kerbin (not much, as the Mun is too light), but you can't use Kerbin's gravity to get captured (short, of course, of a direct impact trajectory).

[Superfluous J]

Short answer: No.

Longer answer: Kinda. It doesn't matter what side of Eve you fly by it on, if you don't slow down you'll fly right by. It could be spinning like a top or not spinning at all. It does not matter to your space ship.

However, you CAN at Jool (And to a lesser degree Kerbin and Duna) use the moons to slow yourself down. It's a bit tricky but essentially - for Jool - you want to come in a little too wide so you go around the back of Jool and then pass by Laythe or Tylo* on THEIR outside as well, so they slingshot you toward Jool. This new trajectory will very likely give you a completed orbit (and if you plan it all with maneuver nodes, you can keep tweaking until it DOES) and from there you can do whatever you want to do.

*Vall may work too but I've never tried it. Laythe is closer so it has more effect. Tylo's bigger so it has more effect. Vall is smaller than Tylo but further than Laythe.

[GoSlash27]

Prepare thyself, this is a long post.

As many of us are well aware (and are quite frankly hyped about or dreading) 1.0 will include reentry heat and a new aerodynamics system. One problem a player might encounter is when coming out of interplanetary travel, they can no longer perform an aerobraking maneuver without proper heat shielding and a fair amount of luck on their side. Not complaining; just stating the facts.

Now, my question is, can you use a retrograde orbit in place of aerocapture? I could imagine that fighting against the turn of the body you are orbiting would slow you down (maybe not enough for capture, but at least enough to cut fuel consumption).

If this could happen, theoretically, one could send a Voyager probe to Jool and do a retrograde flyby to approach one of its moons (laythe, tylo, or vall) and do a final (or a series of) retrograde orbit(s). From there, you could turn around by going to a higher orbit and turning around.

This may also have a second use as an alternative to potentially unpredictable aerobraking behavior stemming from the new drag and lift system; at least until an updated aerobraking calculator is created.

Unfortunately your velocity will not be affected by the rotation of the planet, so no.

Best,

-Slashy

[ratchet freak]

retro or prograde makes no actual difference in the orbital mechanics, intercepting bodies in the other grade is a bit tricky but as long as you stay out of the atmosphere there is no difference (and even then it's only a difference in the airspeed during aero capture)

[ToyotaTim]

Prepare thyself, this is a long post.

Now, my question is, can you use a retrograde orbit in place of aerocapture? I could imagine that fighting against the turn of the body you are orbiting would slow you down (maybe not enough for capture, but at least enough to cut fuel consumption).

As the others stated above, gravitational wise it wouldn't indeed matter. But I think the OP tries to explain if you benefit from a "counte clockwise" (against the rotation of the planet) aerobraking maneuver instead of a clockwise, because during aerobraking you move against the rotation of the atmosphere. Pretty much the same principal as a heat exchanger (If you're not familiar with heat exchangers go look it up on wikipedia, and search for "countercurrent and parrallel flows diagram" ). Instead of exchanging heat more efficient, you exchange speed more efficient because you move against each other, and not alongside with it.

I'm not too familiar with KSP physics vs. IRL physics concerning the behaviour and rotation of the atmosphere to answer your question though .

[Lukaszenko]

Getting captured with proper aerodynamics and heat is more difficult than current stock, but it's also more rewarding and nowhere near impossible, unless you're going really really fast. Just takes a bit more planning and maybe some spare fuel to slow you down (for when you ARE going really really fast ).

Also keep in mind that in real life, an aerocapture maneuver has never been performed.

[smjjames]

Wait, I've never heard anything about the new version having reentry heating?

As for the aerocapture, yeah it hasn't been performed because the only craft that would be in a position to actually do those are fragile probes.

[DrD]

As the others stated above, gravitational wise it wouldn't indeed matter. But I think the OP tries to explain if you benefit from a "counte clockwise" (against the rotation of the planet) aerobraking maneuver instead of a clockwise, because during aerobraking you move against the rotation of the atmosphere. Pretty much the same principal as a heat exchanger (If you're not familiar with heat exchangers go look it up on wikipedia, and search for "countercurrent and parrallel flows diagram" ). Instead of exchanging heat more efficient, you exchange speed more efficient because you move against each other, and not alongside with it.

I'm not too familiar with KSP physics vs. IRL physics concerning the behaviour and rotation of the atmosphere to answer your question though .

I'm not sure that's true. The heat arises from the compression of the atmosphere, not from friction as is commonly believed. It's the same reson the atmosphere slows you down. There's no way to separate the heating effect from the braking effect, I think.

[Rakaydos]

Getting captured with proper aerodynamics and heat is more difficult than current stock, but it's also more rewarding and nowhere near impossible, unless you're going really really fast. Just takes a bit more planning and maybe some spare fuel to slow you down (for when you ARE going really really fast ).

Also keep in mind that in real life, an aerocapture maneuver has never been performed.

Depends if you count one of the mars probes doing interplanetary transfer-to-surface no-orbit aerobreaking/capture.

[ToyotaTim]

I'm not sure that's true. The heat arises from the compression of the atmosphere, not from friction as is commonly believed. It's the same reson the atmosphere slows you down. There's no way to separate the heating effect from the braking effect, I think.

You are correct, you can't separate those effects. Maybe my head exchanger example wasn't a very good comparison as I first thought it would be .

[sdj64]

Short answer: No.

Longer answer: Kinda. It doesn't matter what side of Eve you fly by it on, if you don't slow down you'll fly right by. It could be spinning like a top or not spinning at all. It does not matter to your space ship.

However, you CAN at Jool (And to a lesser degree Kerbin and Duna) use the moons to slow yourself down. It's a bit tricky but essentially - for Jool - you want to come in a little too wide so you go around the back of Jool and then pass by Laythe or Tylo* on THEIR outside as well, so they slingshot you toward Jool. This new trajectory will very likely give you a completed orbit (and if you plan it all with maneuver nodes, you can keep tweaking until it DOES) and from there you can do whatever you want to do.

*Vall may work too but I've never tried it. Laythe is closer so it has more effect. Tylo's bigger so it has more effect. Vall is smaller than Tylo but further than Laythe.

You're right. You can also aerobrake at Laythe with a max G load of less than 1 G from a transfer from Kerbin if you time it on the outside of Laythe's orbit. That should be safe enough even if you don't have lots of heat shielding.

For the OP's question, retrograde orbits allow you to aerobrake at a higher altitude but also higher relative velocity to the surface, that actually results in more heating since your transit through the atmosphere has to burn the same amount of speed in a shorter time and distance.

[KerikBalm]

As the others stated above, gravitational wise it wouldn't indeed matter. But I think the OP tries to explain if you benefit from a "counte clockwise" (against the rotation of the planet) aerobraking maneuver instead of a clockwise, because during aerobraking you move against the rotation of the atmosphere. Pretty much the same principal as a heat exchanger (If you're not familiar with heat exchangers go look it up on wikipedia, and search for "countercurrent and parrallel flows diagram" ). Instead of exchanging heat more efficient, you exchange speed more efficient because you move against each other, and not alongside with it.

I'm not too familiar with KSP physics vs. IRL physics concerning the behaviour and rotation of the atmosphere to answer your question though .

Basically, your velocity relative to the air will be higher... this means more reentry heating for the same altitude. It also means more braking force for the same altitude.

It doesn't really change anything except raising the minimum altitude you need to avoid burning up durin aerocapture.

If we were talking about an atmosphere that was really thin (like duna >8km), where the minimum altitude is set by the ground... it can help.

I know I've done some hgh relative velocity aerocaptures at duna where my ship was scraping the tops of the mountains as it flew by with a PE of 8 km (highest mountains are 8.26km ) and barely able to do an aerocapture.

Except the problem with duna is that its rotation is so slow you won't get much help- better off with an ike gravity assist

[Torquemadus]

If entry heating is modelled correctly, players will have to decide whether or not to bring an aeroshield to protect their spacecraft, or brake into orbit using only rocket thrust. Depending on how these are implemented, there might be limits to the size of spacecraft that can be hidden behind stock aeroshields. Aeroshields would look especially cool if they were designed to be foldable to fit inside payload fairings.

I would also suspect that many spaceplane parts will be treated as having heat tiles, while rocket fuel tanks will not. This would give spaceplane fuselages more dry mass than rocket fuel tanks due to the weight of their thermal protection.