Increased Breaking dV Required on Non-Optimal Transfers

[DChurchill]

I recently did a non-optimal transfer to Duna which got me to wondering if I had enough dV on board to accomplish the braking burn. Is there a quick rule of thumb I can use? Something using the angle of the two crossing orbits, maybe?

[regex]

No, you'd pretty much have to figure out your orbital velocity at periapsis.  Using that figure and the velocity required for the desired orbit you can come to the delta-V required achieve it.

As I understand it, at least.

Edited May 10, 2016 by regex

[moogoob]

Non-optimal as in high-speed, like how New Horizons flew out to Pluto?

You WILL need more energy, but as @regex said it's not always easy to eyeball the exact amount. If you have an intercept, try focusing Duna in the map and putting a node to capture at periapsis and note how much it takes to circularize.

[regex]

9 minutes ago, moogoob said:

If you have an intercept, try focusing Duna in the map and putting a node to capture at periapsis and note how much it takes to circularize.

Yeah, that's the easy way, lol.

[Enorats]

Who needs braking burns? Duna has an atmosphere. Just slam into that really hard. It'll stop you. Failing that, Duna has a solid surface. Slamming into that is sure to stop you too!

More seriously though, short of a whole lot of math I'm not sure there is an easy rule of thumb to predict such things. Using a node is the obvious solution, but of course you can't actually do that until you're already on your way. By then you're already out of luck if you don't have enough fuel.

[DChurchill]

28 minutes ago, moogoob said:

Non-optimal as in high-speed, like how New Horizons flew out to Pluto?

You WILL need more energy, but as @regex said it's not always easy to eyeball the exact amount. If you have an intercept, try focusing Duna in the map and putting a node to capture at periapsis and note how much it takes to circularize.

Duh. Didn't think of that. Thanks. Hopefully I won't have to try to aerocapture. I've got something like 940 m/s in the last stage before the payload. I've got another 1,700 there, but I didn't want to have to use it.

Non-optimal as in maybe 10 degrees of difference in orbital paths at intercept. I haven't done an interplanetary transfer since .22. So I'm a little rusty.

[regex]

3 minutes ago, DChurchill said:

Duh. Didn't think of that. Thanks. Hopefully I won't have to try to aerocapture. I've got something like 940 m/s in the last stage before the payload. I've got another 1,700 there, but I didn't want to have to use it.

Non-optimal as in maybe 10 degrees of difference in orbital paths at intercept. I haven't done an interplanetary transfer since .22. So I'm a little rusty.

940m/s should be okay to at least get you into a highly elliptical orbit, unless you're doing some serious brachistochrone stuff...  You might also want to try bouncing off Ike, its gravity well should help.

Edited May 10, 2016 by regex

[TMS]

You may need to adjust how many conic patches are drawn.  Can't remember what the default is.

[regex]

Just now, TMS said:

You may need to adjust how many conic patches are drawn.  Can't remember what the default is.

IIRC it's six now in the default settings.

[MaxL_1023]

The amount of delta-v you need to close an orbit eventually converges to your SOI entrance velocity minus escape velocity, if you are torching it on a hyperbolic transfer. To get the exact value, square your entrance velocity, add the square of the escape velocity, square root that, then subtract the escape velocity, taken at your burn altitude. 

 

Circularizing takes about another 500 m\s for Duna.

[Plusck]

(nvm - off topic

Edited May 10, 2016 by Plusck