Any good explanations of radial burns

[davidpsummers]

I almost never use radial burns. I think that is because I don't fully appreciate what they are good for. Does anyone know of any good explanations or tutorials on what they do and are good for?

Edited July 14, 2014 by davidpsummers

[allmhuran]

The most common use of radial burns is probably when you are approaching a planet and need to shift your periapsis, either to be lower (so you can aerobrake) or to the other side of the target (so you orbit in the direction you want after slowing down).

Imagine you are in an equatorial orbit around Kerbin and burn hard prograde to escape into solar orbit. You then start planning to return directly to Kerbin.

Since you left equatorially, we know that when we get our next encounter we will again pass by the equator.

When you get that encounter, you notice that you are catching up to Kerbin from "behind" (we burned prograde when we left), your periapsis is, say, 1000km, and it's on the sunny side of Kerbin. That's no good... if our flyby is on the sunny side and we circularize, then we will be in a west to east orbit when we're done! And of course, 1000km is much too high to aerobrake.

In this scenario, you would perform a radial out burn. You can either do it in solar orbit, or wait until you enter the SOI of Kerbin and do a radial in burn.

Aside: While you are still a long way from your eventual periapsis, the burn will be cheap. The closer you get, the more fuel you will need to spend, because you need to adjust your flyby angle more. Imagine shooting an arrow at a tree that is 100m away. If a slight breeze is blowing when you release the arrow, it will be a long way off target when it gets to the tree. But if it has already flown 95m and is on target, it will take a very strong wind to make it miss the tree. Notably, the same is not true of normal corrections, but that's a topic for another time.

Burning radial out will first move your trajectory down into Kerbin's atmosphere on the sunny side, and then right through the planet, and finally over to the dark side of Kerbin. A radial in burn, on the other hand, would push your periapsis even further away and still on the sunny side.

More generally, a radial burn rotates and elongates your orbit. Imagine you are gently holding a bicycle tire vertically, so that the base of the tire is sitting on a flat road. Now using your foot, press down on the inside of the tire slightly ahead of where it is touching the ground. The tyre will rotate and now the part under your foot will be touching the ground. This is a bit like a radial out burn.

Edited July 15, 2014 by allmhuran

[xcorps]

You can use Mechjeb and the SMART ASS module combined with the maneuver planning modules to give you an idea of what radial burns are used for. A transfer from one body to another then making a new peri usually involves a radial burn. You can also get a good feeling for how to burn norm for inclinations.

[qazsedcft]

In short, radial burns rotate your orbital plane around the normal axis. The wiki has some pictures that explain this: http://wiki.kerbalspaceprogram.com/wiki/Maneuver_node

[EtherDragon]

Mathematically, a perfectly executed Radial Burn causes you apoapsis and periapsis to shift while maintaining the same orbital period. It is particularly useful when you are half-way between the apoapsis and periapsis, and want to adjust them both. An example, as mentioned above, is right after SoI change to a minor body, you will be on a hyperbolic trajectory (escaping after fly-by) - this using a radial burn can move your periapsis closer to the surface with much less Delta-V than a retrograde burn.

I use radial burns in every mission where I have my sphere of influence changes - as in from Kerbin to Mun or Minmas, or from the Sun to Duna - each radial burn is done immediately after SoI change for maximum effect.

[LethalDose]

The can also be useful for adjusting intercepts/encounters. I'll use them in combination with burning pro-/retrograde to get earlier planetary encounters. I've also been using them lately to intercept captured asteroids in highly elliptical orbits near their apoapses, but it takes a bit of imagination and skill.

basically, they're useful in certain situations that take some practice and experience to understand or see. They're almost certainly used less frequently than pro-/retrograde and anti-/normal burns

[Oafman]

I use radial burns to correct my interplanetary trajectories when I've failed to launch in the transfer window

[Red Iron Crown]

They adjust eccentricity without changing orbital energy. Not incredibly useful in most situations. The best use I've found is fine tuning encounters and approaches, as others have mentioned.

[Vigelius]

As above: I use them for tweaking intercepts, usually a way before an SOI change where the burns can be tiny. Radial to fiddle with periapsis, normal/antinormal to change my eventual inclination.

[MircoMars]

I use radial burns to push my AP and PE to where I want them to be. some examples:

- align AP or PE with closest approach, so I have a "cleaner" rendevouz burn

- get AP or PE to my position or desired height in front of me, useful for circular orbits with exact periods (CommSats & Stations)

- finetune PE for flyby or capture maneuvers

- correctionburn after "1st-Stage gone wild" or "brute force ascent" maneuvers.

[EdFred]

I also use a radial component when circularizing an orbit, and keeping the PE where it currently is.

[Xavven]

You can also use a tiny bit of radial while burning retrograde to slow your ship down before reentry while keeping your periapsis where you want it. For example, if you're coming back from Jool and enter Kerbin's SOI already going 2000 m/s, you can bet your reentry is going to be hot! If you have Deadly Re-entry installed, your ship will be destroyed if you don't slow down first.

[metaphor]

Any time you want to change a point in an orbit that's less than 1/4 of the way around the orbit ahead of you, a radial in/out burn is better than a prograde/retrograde burn. For example, if you're in solar orbit and you have a planetary encounter you want to tweak that's less than 1/4 of an orbit ahead of you. Also when you want to move your periapsis up/down when on an incoming hyperbolic orbit.

[MonkeyFuzz]

I'll make my radial burns prior to a SOI change to nudge the periapse to where I want it to be, usually at the minimum safe altitude above a body so I can make my orbital insertion burn requiring the least power possible thanks to the Oberth Effect.