Direction of burn for planetary orbit insertion

[ijv]

I'm wondering what is the best direction to point your burn vector when you want to insert into orbit around a planet from an interplanetary transfer orbit.

As I understand it, for an impulsive burn (or close to it), you would wait till you hit periapsis relative to your target planet, point retrograde and burn away.

But for a more realistic case where the time taken for the burn is non-neglible relative to your fly-by trajectory, whats the best direction to point your engine?. Do you just use the autopilot to keep lined up with your current retrograde direction or do you have to pick a fixed direction for your burn or is it something else entirely?


Thanks,
ijv

 

[Andy44]

Using the autopilot to point anti-track is usually good enough. For something precise, you need numerical tools which we currently don't have in Orbiter. You can also guesstimate, by calculating the time your burn will take, and eyeballing the angle using the orbit HUD.

 

[Tommy]

You can also start with a retrograde burn, then hit killrot when you reach Pe.

 

[Quick_Nick]

I'm wondering what is the best direction to point your burn vector when you want to insert into orbit around a planet from an interplanetary transfer orbit.

As I understand it, for an impulsive burn (or close to it), you would wait till you hit periapsis relative to your target planet, point retrograde and burn away.

But for a more realistic case where the time taken for the burn is non-neglible relative to your fly-by trajectory, whats the best direction to point your engine?. Do you just use the autopilot to keep lined up with your current retrograde direction or do you have to pick a fixed direction for your burn or is it something else entirely?


Thanks,
ijv

If you time your burn so that the periapsis is in the middle of your burn (ex: start burn 100 seconds before PeA, finish burn 100 seconds after PeA) then you can maintain retrograde throughout the whole burn IF you maintain the same acceleration. However, a spacecraft loses mass during a burn which causes an increase in acceleration. You have to compensate for this which means the PeA is no longer completely in the middle of the burn. Some MFDs such as [ame="http://www.orbithangar.com/searchid.php?ID=3035"]BurnTimeMFD[/ame] can calculate this for you and tell you when to burn.

Note: BurnTimeMFD sometimes 'reads' the wrong acceleration if you don't use your engines at least once (for any amount of time) before the calculation.

Alternatively you could do what I sometimes do and mentally calculate the length of the burn based on your engine's current acceleration (acceleration is shown during a burn in the Orbiter glass cockpit next to the fuel levels) and the desired Delta-Velocity and then timing the burn so that the PeA is in the middle and adjusting thrust to maintain the same acceleration throughout the burn.

 

[Andy44]

Note: BurnTimeMFD sometimes 'reads' the wrong acceleration if you don't use your engines at least once (for any amount of time) before the calculation.

Really? Maybe this explains some anomalous burns I've done recently.

 

[Quick_Nick]

Really? Maybe this explains some anomalous burns I've done recently.

I've had it happen with the DGIV. (I've very rarely used the MFD actually) It read one thrust level, then once I actually used the engines for a split-second, it read another value. Also, it'd be a little difficult to get it to caculate for turbo thrust.

 

[ijv]

If you time your burn so that the periapsis is in the middle of your burn (ex: start burn 100 seconds before PeA, finish burn 100 seconds after PeA) then you can maintain retrograde throughout the whole burn IF you maintain the same acceleration. However, a spacecraft loses mass during a burn which causes an increase in acceleration. You have to compensate for this which means the PeA is no longer completely in the middle of the burn. Some MFDs such as BurnTimeMFD can calculate this for you and tell you when to burn.

I take it that the ideal is to have 1/2 your Delta-V before PeA and 1/2 after?
I've been working off the rule of thumb that you do 1/3 of your burn before periapsis as recommended in the TransX manual

ijv


-----Post Added-----

You can also start with a retrograde burn, then hit killrot when you reach Pe.

This might be what I'm looking for. I've been experimenting with really fast transists in a DG-IV (50+km/s Delta-V burns!). When I just track retorgrade, the periapsis of the capture orbit drops below the planets surface.

 

[Quick_Nick]

I take it that the ideal is to have 1/2 your Delta-V before PeA and 1/2 after?

That's probably a much better way of putting it. :lol: