Chapter 8. Interplanetary Travel on Budget, part 2

This is the last chapter of Duna trilogy, in other chapters I described the mission profile and modules, and planned the interplanetary travel.

Burning

Burns should be done timely, which is quite obvious:

_images/burn-timing.png

If you burn no so fatally wrong, but a bit later, some of the impulse will be canceled out by gravity, and you’ll need more corrections. (Once I made a burn from Laythe to Kerbin about 1 minute later than needed and had to spend 300 m/s of delta-v in addition to 1700 m/s already spent.)

Maneuver nodes in KSP assume instant acceleration, but burns do last long, about 2-5 minutes (5 minutes are approximately 1/6th of the whole orbit), there are gravity losses and imprecise resulting burn.

_images/long-burn.png

In other words, think about orbital energy. If you burn prograde, you add energy, or subtract if burning retrograde. Burning in radial direction (up into the sky or strictly down) adds nothing to the energy but just distorts the orbit. If you do a prograde burn quickly, it’s energy is completely added to orbital energy. In a long burn, you lose some energy into orbit distortion.

While MechJeb plug-in splits burns in halfs: one half before node T, other half after it. But since the ship is heavier on the first half, less 𝛥v is applied then. So I use to split them as 2/3 to 1/3, in other words start at 2/3 of burn time before T. Even if you scratch the atmosphere this way –get as low as 60km– you loose just 1..5 m/s, which is less than 1%.

Usually, because of these small inaccuracies, planet encounter is not achieved in the end of maneuver, so I set up another maneuver node – 5 minutes ahead – and adjust the trajectory again. Or you can just do corrections with RCS visually: in the map view, switch (with TAB key) to the planet and use translation controls in whatever direction is the best.

_images/tn_pic-chap8-screenshot401.jpg

Focus on Duna (TAB key) and see the maneuver node in the back

_images/tn_pic-chap8-screenshot402.jpg

Adjusting the flyby

_images/tn_pic-chap8-screenshot403.jpg

Executing the maneuver: note the purple (real) path has appeared

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Very low periapsis achieved, but it’s still very inclined

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Inclination is being adjusted

Travel and Approach

How do we encounter the planet and get into prograde orbit, the same direction as the satellites? It may seem puzzling, but is quite simple.

Loot at the trajectories and orbital speeds:

_images/encounter.png
note:

How did I calculate this? Orbital velocity is

v=\sqrt{\mu\left({2\over{r}}-{1\over{a}}\right)}

Where μ is standard gravitational parameter of the parent body (Kerbol), r is distance from it, and a is semi-major axis or the average of periapsis and apoapsis.

We encounter Duna with a relative speed of 934 m/s. And now depending on the path, we fly by it on one or another side.

_images/flyby.png

That’s all we need to remember. Set up aerobraking with this calculator to circularize or to intercept Ike after that.

When I travel, I decouple the away stage before encountering Duna and adjust the ship’s periapsis at 30 km.

_images/tn_pic-chap8-drop-i-stage.jpg

This stage is dropped on the way to Duna.

_images/tn_pic-chap8-screenshot426.jpg

I adjust the main ship’s path with Aerobraking Calculator

_images/tn_pic-chap8-poodle-duna-crash.jpg

It hits the planet, without leaving any space debris. A head-on atmospheric reentry is quite spectacular!

_images/tn_pic-chap8-screenshot440.jpg

The rest of the ship aerobrakes

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After aerobraking I get on almost encounter trajectory with Ike and set up circularizing for the lander.

Aerobraking

This is quite simple. After encountering Duna, pause, open the calculator and input all the values. Make sure the planet is chosen correctly.

_images/tn_pic-chap8-aerocalc.png

As you are getting closer, fold all your solar panels and enjoy watching aerobraking. When you are about to leave the atmosphere, adjust apoapsis. That’s pretty much it.

Return

Again, plan it in the calculator and warp to the departure day. Then set up a node and start playing, with the same rules as for departure. Coming down to the inner planets is easier, because those rotate faster, and adjusting the approach is easier: changing angle is a lot cheaper, and you can adjust the encounter to any steepness you need.

When you come back, plan an encounter in prograde orbit and when inside the Kerbin SOI, plan aerobraking to reach apoapsis of 65-70 km. It will make a very smooth reentry without high Gs. I managed to get peak acceleration as low as 3G. This may be useful if you play with Deadly Reentry plugin.

The capsule will brake, then start going upwards, but then apoapsis will lower, and eventually you’ll land.

Watch the apoapsis gradually fall down to within Kerbin atmosphere.

_images/tn_pic-chap8-screenshot162.jpg

Aerobraking at 32 km: already circularized

_images/tn_pic-chap8-screenshot163.jpg

Apoapsis at 175 km, smooth braking (0.9g)

_images/tn_pic-chap8-screenshot164.jpg

Drag reduces a bit as we get higher, but the apoapsis is already within the atmosphere.

_images/tn_pic-chap8-screenshot165.jpg
_images/tn_pic-chap8-screenshot166.jpg

Peak braking: 2.9g in the end of reentry.

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Welcome home!

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See also

One Giant Leap
a photo gallery of Apollo program (with comments in Russian)