Quick and dirty guide to targetted, unpowered re-entries

[khyron42]

Some people care about where they land (apologies to Werner von Braun & Tom Lehrer) and want to do so without using excessive fuel. In preparation for future missions where crew/part recovery will matter, I\'ve done some test flights to determine how to land roughly where you want using just a capsule and chute, plus either an RCS or a throttle-capable engine to deorbit you.

With meticulous planning and an orbit calculator that included the calculus for atmopheric drag, you could do this perfectly. You also probably wouldn\'t have fun with it. Instead, you can reliably land NEAR a target area by following these steps:

1. First, get yourself into a roughly 100 km circular orbit that will pass over your target area. In this case, I\'ll use KSP as the example. This would work for any orbit, but the timing of the deorbit burn would be different and this is the one I figured out the timing for.

2. Switch to map view, hit tab to switch targets to Kerbin, and center your landing target in the view.

3. Zoom out until you can see your whole orbit easily. Then, right click and drag straight down on the map until you\'re looking at your orbit from above. Your landing target should be at the 6 o\'clock position if the planet were a clock.

4. Warp time until your ship reaches about the 10:00 position (120 degrees before it would pass over your target.) Then, repeat step 2 to correct for the planet\'s rotation, and get your ship lined up for a deorbit burn.

5. Ideally at the 9:30 (105 degrees) point, slow your ship down to have a periapsis of just 100 feet or so. The periapsis should be about at the 3:30 position. If you\'re not planning a powered landing, jettison anything you like here but don\'t activate the parachutes yet. Then you can warp time again for 5 minutes until your ship enters the atmosphere. In this example, I made the deorbit burn slightly too early - around 110 degrees/9:40 instead.

6. Just let atmospheric drag do its thing for 5 more minutes of the flight. Pop the chute when you get below 3000 feet or if you\'re going to overshoot your target.

4. Your ship should be within sight of your target when you pop the chute/light the engines for landing. Like I said, I was slightly too early. Still - I was within sight. You can see just how much I was off in the final orbit view.

Your Kerbal\'s will only take about 2.5 G\'s of acceleration this way - way less than they probably took during launch.

You could use this for pretty much any orbit if you line up to be at about 100 km altitude when you\'re at that 105 degrees before target location in the orbit; I used a circular example just to simplify things.

The Really, Really Short Version: if you put yourself into a 100 km x 100 m orbit, you will end up landing about 105 degrees around the orbit from the apoapsis due to atmospheric drag.

[duckunlimited2]

Excellent! This would be helpful. Nice work.

[closette]

Best recipe I\'ve seen so far for this! It would be great if there were a table of de-orbit angles for different starting altitudes. 160km could be a a good circular orbit to start from (for the user) because you can time-warp faster up there.

(And this all assumes prograde equatorial orbits, where your landing spot is moving away as you descend. Polar orbit spot-landings would be much harder to judge unless you can fly over a spot and try to get down as fast as you can!).

I\'ve no idea how to land back at KSC from a Mun return trajectory. If I am returning from the Mun I don\'t have enough RCS delta-v to circularize my orbit first, although aerobraking helps.

[PD]

2 words: Thank You: You did the experiment I never quite got around to!!

PD

[khyron42]

The method I used was: 1. treat the Apoapsis on test flights as 0 degrees and see how far around the globe the eventual landing was. That gives initial angle to try. 2. Try it aiming for a target location, see how badly you miss, and change the angle as needed to compensate.

Basically, 3 landings needed to figure out out: initial landing with no target just to get the rough degrees around the circle, attempt targetted landing and revise the deorbit burn degrees to compensate for error, second attempt to verify the revision.

If people figure it out for other altitudes, post it here and I\'ll update the short version of the guide at least

[Red Iron Crown]

Nice, I\'ve been looking for a rule of thumb for deorbiting but haven\'t had the time to do the experimentation you have. Great job and thanks!