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Brikoleur

Technique for controlled aerocapture

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Aerocapture is a great way to save massive amounts of fuel on interplanetary missions. Its main downside is that it's not possible to plan very well, as your final trajectory depends on your craft, your planetary intersect, your periapsis, and what you do during the capture. Usually aerocapture involves trial and error with a couple of quicksaves/quickloads.

Below I describe a technique for more controllable aerocaptures. It is applicable to Duna, Laythe, and Kerbin*; Eve and Jool would require exceptionally robust designs with creative use of heat shields at least.

Designing the craft

To execute this technique, your craft will need some plane-like characteristics, even if it is not an actual spaceplane. Specifically, it needs to be slippery along one axis and draggy along another axis, you need to be able to control its attitude while it is in the atmosphere, and you need to be able to survive the thermal load of the atmospheric entry (easy for Duna, a significant challenge for Kerbin, depending on where you're coming from). There are various ways to do it for a rocket-like craft -- winglets with control surfaces, big empty fuel tanks and reaction wheels/RCS -- but designing these is left to the reader. Your craft should not rely on heat shields as primary thermal protection as this significantly limits your options in executing the technique.

Below we're assuming your craft has a plane-like geometry: pointy and slippery when pointed prograde, wide and draggy when pointed radial out.

The technique

  1. Do preliminary research to determine a rough aerocapture altitude for your target, e.g. 15 - 20 km for Duna. Set your Pe based on that: if your craft is designed for this technique, you should be going toward the higher end of the range as it will be highly draggy when pointed radial out.
  2. Enter the atmosphere SAS set to orbit/radial out, i.e. 90 degrees. 
  3. Switch immediately to map view and watch your trajectory.
  4. When your flyby turns into an orbit, set SAS to standard mode and nose down to follow prograde.
  5. Fine-tune your orbit by controlling pitch: with a plane-like craft the Ap will fall very slowly if you're tracking prograde, and very quickly if you're at radial out.
  6. Don't forget to burn prograde at Ap to get your Pe out of the atmosphere again.

With plane-like aerodynamics you can get almost as precise control over your Ap as you would by controlled retrograde burns in a propulsive capture. 

*From relatively low-energy transfers. Mun, Minmus, and Duna are not too difficult, Eve should be doable.

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