alterbaron

Wow, this tool is very impressive! I took a peek at your code and coffeescript looks really nice. I might have to try it in my next project.

I updated the chart to cover the more useful range.

Update: New & Improved Chart! Hi all, Ever wanted to land at KSC reliably from low orbit? Then this is the chart for you! NOTE: The x-axis is in units of 100,000 meters! You must be more or less in a circular orbit before using the chart. Look up your orbital altitude on the x-axis. The corresponding y-coordinate gives you your "landing phase angle". Basically, you want to burn so that your periapsis is at as close as possible to 0 meters, and at the given angle AHEAD of your target. It's actually quite simple -- check out the example in the photo album. Example Photo Album Awesome Protip: If you have a station in a fixed orbit, you could mark the relevant angles on a circle of paper, and use it again and again. That's really the way to go. Let me know what you think!

Here's some mathematical details, for those who are curious: Let r = (rx,ry) be the vector from the center of the planet to our ship, and v = (vx,vy) be the velocity vector of the ship. Then the force of drag is given by the following equation: Kp = 1.2230948554874*0.008 FD = -0.5 Kp P0 exp((R0-norm®)/H0)norm(v) d m A v Where Kp is the conversion factor between pressure and density given on the wiki, P0 is the pressure at sea level (1atm for Kerbin), H0 is the atmosphere scale factor (5000m for Kerbin), R0 is the radius of the planet (needed since the vector r extends from the center of the planet), d is the coefficient of drag, m is the ship's mass, and A is the "cross-sectional area" of the ship, which is a constant in-game. The velocity v is a vector, don't forget! Note that in the real world, there is no factor of "m" in the above equation. The force of gravity on the ship is simply: FG = - m(mu/norm®^3)r Where mu is the gravitational parameter of the planet (3.53 x 10^12 for Kerbin). Note that r is a vector! Then the total force on the ship is just F = FD + FG. (A vector sum). Note that norm® means the vector (cartesian) length of r. These equations are used to simulate the trajectory of a ship as it passes through a planet's atmosphere. Here's a quick method that works o.k.: Say the ship has a current position r=(rx,ry) and a velocity (vx,vy). We calculate a net force of F=(Fx,Fy) using the above equations at the current position and velocity. Our acceleration is, by newton's law, a=F/m Call our current time t. Consider a small time step dt. The velocity at time t+dt is approx. v+a*dt. Likewise, the position is approx. r+v*dt. Using this new approximate position and velocity, we can calculate F anew. Do this, rinse, repeat, and you're pretty much done. If you hold on to all your old values of "r" as you go, you have the path that the ship followed through the atmosphere. You also have its velocity at every point in time. Neat, huh?

Well, you never know what people will do . . .

That's true, but wouldn't you feel more like a rocket scientist using a chart?

You don't need to know much calculus to do these sort of calculations numerically (which is what I did here.) But you do need some comfort with forces in physics. Basically, draw the free body diagram for the craft. You'll have a force due to gravity, and a force due to air drag. You take the net force from the free body diagram and set F=ma, or a = F/m. Once you have the acceleration as a function of position and velocity, you can "plug the formula into a computer program to have it numerically solve for the path that the object will take." I say this loosely, because the book-keeping can get rather tedious. But it's simple at heart. This number crunching is based on calculus and can be summarized as "step forward in time, re-calculate forces, update position and velocity, and repeat." Everything else is just a bit of tricky geometry.

Hi all, Here's a little something I made for fun this afternoon: it's a Jool aerocapture chart. Big Version This chart gives the periapsis distance for aerocapture into a variety of different orbits after entering Jool's SOI. To use it, exit time acceleration once in Jool's SOI. Find the curve corresponding to the orbit you want after aerocapture. Find where that curve intersects your orbital velocity (measured on the navball). The y-axis reading then gives the periapsis distance that should aerocapture you into that orbit. This chart should be enough to get you reasonably close (delta-v wise) to any target orbit. For calculating other aerobraking maneuvers, I'd suggest using my aerobraking calculator: http://alterbaron.github.io/ksp_aerocalc (The plot was obtained using the same approach used there.) (NB: This chart assumes that your ship does not have lift surfaces. These might mess up your results if you have any. Use right at SOI entry for best results.) Let me know what you think!

Good catch! Added to to-do list.

That's a great idea, I'll definitely need to add something like that!

Thanks, that's a really good point!I'll have to add a section on dealing with in-game units.

Jool Rise on Laythe Ascent

Wonderful! What a riot! Score for metaphor: SOI Entry: 1000pts Atmospheric Entry: 500pts Jool Landing: 5000pts Total: 6500pts Those Kerbals sure are made of tough stuff! Is he able to get up and walk around, or is he out of commission? EDIT: I wonder if this technique would make for a viable interplanetary transport network? You could just EVA Kerbals, aerobrake them around a target body, and rendezvous with a station (or have a ship go pick them up)!

The challenge is to send a Kerbal to another planet in only their spacesuit (no craft). You may use a craft to get the Kerbal into orbit (of course), but the interplanetary transfer must be done in EVA. You can either try to land the Kerbal directly (in EVA), or rendezvous him with a lander to land. Proof this can be done. (Duna, not my attempt) (Reddit Thread) My Attempt. (Eve) Points System Entry to SOI of other planet: 1000pts Atmospheric entry: 500pts EVA Landing: 1000pts (Eve), 5000pts (Duna), 5000pts (Jool), 8000pts (anywhere else) Lander Rendezvous + Landing: 1000pts (Eve or Duna), 2000pts (anywhere else) Rules Kerbal must exit Kerbin SOI and enter target SOI without a ship. There cannot be a ship accompanying the Kerbal through interplanetary space. (No ship within a few million meters at least!) Kerbal must be in EVA when Kerbin escape velocity is reached. Additional Rule bending is encouraged. If you can find a hilarious way to technically satisfy the rules and win, go for it! EDIT: Updated scoring to give more weight to Duna landings, as well as moons like Laythe.

I'm working on a guide to the physics behind KSP! This is for those people who want to get really hardcore with their mission planning, want to develop calculators / programs / mods, or who just want to learn some physics. Here's a link to the guide so far: Comprehensive Physics Guide (WIP) This guide (still incomplete) will hopefully be able to take someone with at least basic proficiency in physics and math to a level where the physics of orbital mechanics don't represent a barrier to doing calculations or developing tools for KSP. The guide will be focused on using physics equations as building blocks towards solving practical problems. I feel that this sort of approach can help eliminate much of the confusion that inevitably seems to pop up when dealing with physics. There's still quite a bit to add. I'm planning on adding a section on the vis-viva equation (which was pretty much used and motivated in the section on energy anyway) as well as a few sections on elliptical orbits, eccentricity, .etc By the end of the guide (once it's finished), I hope to derive and explain the formulas for planetary transfer angles, Hohmann transfers, .etc Also, more diagrams are needed at the moment. Please let me know what you think!

http://alterbaron.github.io/ksp_aerocalc/ New Updates! Fixed a bug that would screw up results for retrograde encounters with Jool. Added circularization delta-V prediction. Updates (Aug. 5th) Now more accurate! The calculator can now make use of orbit direction information to calculate a much more accurate result. Unit support! Enter your orbit data in km or Mm for convenience. (Thanks to retoo on GitHub for this feature.) Minor formatting changes. Source Code https://github.com/alterbaron/ksp_aerocalc Photo Album http://imgur.com/a/tB3H9 Quick Guide To use the calculator, first drop your periapsis somewhere just above the atmosphere of the target body. This helps the calculator find a better manouever. Then, enter your orbit data into the form, and the apoapsis you want to have. It will then calculate a constant-velocity manoeuver to get you there. Note: I used to suggest dropping your periapsis all the way inside of the target body's atmosphere. This is not a good idea, as the calculator may have trouble finding a solution if you set it too low. By "constant-velocity", what I mean is that your orbital velocity should not change (much) after the manoeuver. Instead, only the direction of your velocity will be different. To achieve this, the direction in which to thrust will be located exactly between the prograde and retrograde markers on your navball. On the calculator, this is given as an angle from the top of the navball. Don't worry if the orbital velocity is a bit off. What's much more important is to get your periapsis to the right altitude. Once that's set, just coast through the atmosphere. Note that sometimes, due to some buggy behaviour with orbits in the game, the periapsis will jump around as you approach the target body. If this happens to you, just re-run the calculator with the new orbit data to make a correction. If you ever get the message "No Solution!" using the calculator,try raising your periapsis height.