Estimating Ideal Delta V Beyond Map

[Kchinger]

So I've been playing a few weeks (after playing years ago), figuring things out. How do you know how much delta V a mission profile will take, not just to reach a body or land based on the delta V map? 

For example, I have a craft on the way to the Mun. Payload is 2 relay satellites to go in Munar polar orbits, and then the main craft will head to an equatorial Munar orbit to meet an MPL and satisfy tourists before heading back.

Is the ideal to release the satellites at the AP of my TMI halfway and do their plane change there? How do you estimate the necessary delta V for that maneuver? Or should I get all three craft into a highly elliptical orbit around the Mun and then release the satellites to burn to polar? How do I efficiently get them in 180 degree out of phase orbits with each other?

I feel like I have the basics of estimating delta V and planning simple maneuvers, but I'm struggling to take that next step and be efficient. And I'm a cheapskate so *moar boosters!* isn't as fun for me. 

[paul23]

Landing & getting into orbit is difficult, since a lot depends on the exact trajectory. (Gravity loss & aerodynamic loss, and typically larger gravity loss means lower aerodynamic loss and vise versa).

 

But to get to a different orbit it's quite easy, if we consider a change of orbit two perfect hohmann transfers, calculation isn't hard considering the vis-viva equation, as shown by wikipedia:

 

https://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Calculation

 

This considers an instantaneous delta-v change, at apoapsis/periapsis. So always consider a bit more (like 5%) to account for less efficient burn.

 

To land on a surface you can "guess" the delta v by first considering an orbit at an altitude of 0: so r2 = radius of object. Then calculate the orbital velocity at that altitude  v=sqrt( mu / (2 * radius)). And then consider we need to get to "0" sideways velocity, thus that velocity needs to be added to the delta-v to get to a 0 altitude orbit. (Depending however on the local surface velocity...

 

This is all taking no inclination into account. But since inclination change is always perpendicular to the velocity, it can be calculated separately and added to the total delta-v.

 

https://en.wikipedia.org/wiki/Orbital_inclination_change

 

[AHHans]

19 hours ago, Kchinger said:

For example, I have a craft on the way to the Mun. Payload is 2 relay satellites to go in Munar polar orbits, and then the main craft will head to an equatorial Munar orbit to meet an MPL and satisfy tourists before heading back.

Doing a plane change while already in orbit around the Mun is expensive. The cheapest way to get into a polar orbit around the Mun is to intercept the Mun's SOI not at the equator, but closer to the poles so that you are already in the plane that you want to be. That means if you want to get into a specific polar orbit, then you should time your transfer so that you intercept the Mun's SOI in the correct plane.

So if there is not a good reason to launch all three of them together (e.g. because you only want to use one good, high Isp engine) then I would launch (do the TMI burn) the craft that goes into an equatorial orbit and the satellites for polar orbits separately. Otherwise you could do the the TMI into one orbit (well, plane) detatch the others and correct the intercept of the Mun's SOI to get to the other orbit / plane.

19 hours ago, Kchinger said:

How do you estimate the necessary delta V for that maneuver?

A) You do more math than I'm willing to do here. B) you set up a pair of test maneuver nodes (e.g. one to get to an equatorial orbit and after that a second node to get from there to a polar orbit) and see how much dV you need for the second node. Fiddle around with both(!) to optimize for your application. (Don't forget to vary the time when you do the second burn. I guess halfway to the Mun will be the most efficient, but I don't know.)

19 hours ago, Kchinger said:

Or should I get all three craft into a highly elliptical orbit around the Mun and then release the satellites to burn to polar?

No.

19 hours ago, Kchinger said:

How do I efficiently get them in 180 degree out of phase orbits with each other?

So they should go into the same plane, but 180 degrees out of phase, and stay on opposite sides of the Mun "forever"? Then I'd suggest to capture them into an elliptical orbit, circularize one of them, and then circularize the other when it is 180 degrees out of phase with the other. If you make sure that the orbital period (e.g. as displayed in the maneuver display since 1.7) for both is as identical as you can make it, then they will stay opposite sides of the Mun even if their orbits are not 100% identical.

[mk1980]

you can use this online calculator to do the maths for a so called "resonant orbit":

https://meyerweb.com/eric/ksp/resonant-orbits/

if you want multiple satellites on the same orbit separated by specific phase angle you'd launch them as one vessel into a resonant orbit and then detach and circularize the individual sats at the PE. 

simply put you basically launch them into a polar orbit of (for example) roughly 50km PE and 200km AP and then circularize the first satellite on the first pass through the PE (ie. retro burn to get its AP down to 50km) and then circulatize the 2nd satellite on the 2nd pass. the other satellite should be (close to) the opposite side at this point in time.

 

you can launch the whole thing in one go, even combined with the tourist bus vessel that goes into an equatorial orbit. you just do the munar transit burn with the combined vessel and then decouple the satellite carrier vehicle with the 2 sats and use a tiny correction burn while you're still near kerbin to change the equatorial flyby into a polar flyby for the satellites.