Is it more economical to burn retrograde or radially to lower periapsis?

[THX1138]

The goal is to land in a place on the Mun (how I got into this position isn't really important :P) and I need to get my orbit to around 8-10 Km before I start. Is it more economical to burn radially (before being in orbit) to get my periapsis closer or should I wait until I am in orbit and then burn retrograde on the other side?

[Warzouz]

It depends what you want to do and where you are on your orbit.

Usually you go for the retrograde if you're on a near circular orbit. You wait for AP, and burn retrograde until your PE is where you want. You AP should not change much.

Now if you are on a hyperbolic orbit and want to set your PE at the right altitude for aerobraking, you should go for radial burn the farther you can from your target body (even outside SOI).

From you screenshot, I think it's too late for a cheap radial burn : you'll loose much more than you'll gain from a lower PE circularization. Just burn retro at PE, then adjust your altitude by another retrograde burn when you'll be on a elliptic orbit.

OTOH, if you're not on efficiency (have plenty of fuel), just burn radial to change PE and then retrograde at PE to circularize, it'll be quicker

BTW : you could also do a direct landing. Set your PE near the surface, set a node at PE, pull it until it flips. the start landing at the count down = burn time. It's a hyperbolic suicide burn . I did it once or twice. Fun but beware of cliffs !

Edited July 12, 2016 by Warzouz

[Streetwind]

Yeah, it's a bit of an odd spot. At the edge of the SOI, radial is definitely a lot cheaper, and you are still on an escape trajectory, which favors the radial burn. But you're also coming up pretty close to the Mun already.

It's possible that at this spot, there isn't much of a difference between the two options.

But you can try to estimate it. Make a maneuver node, use a radial vector to lower your periapsis to 10 km. Then make a second node at that periapsis and circularize. Add up the dV of both nodes, then remove them. Next, made a node at your flyby periapsis and use a retrograde vector to lower your periapsis to 10 km. Again, make a second node to circularize. Add up the dV.

One of the two is probably going to be less, and that's your answer.

[StarStryder]

The thing with radial burns is that they don't change your orbital energy much.  They just re-direct it.  This is great if you want to use a gravity assist to boost out to higher orbit using the oberth effect.  Burning retrograde to bring your PE in is actually removing orbital energy so hurts your boost.

In your case, you want to get into orbit which means that you need to reduce your orbital energy so a retrograde burn helps and then your final PE burn is also more effective.  Win-win.