Flight Question Two burns to go to Mars?

[EternalFrustration]

Hey everyone, quick question for you:
Is it viable to make two burns on a trip from Earth to, say, Mars? Do one burn that puts your apoapsis out around the Moon's orbit. Then when you come back around to periapsis, do another burn that actually puts you on course to Mars. Any advantage or disadvantage to this, in comparison to doing one large burn?
Thanks for your input,
P

 

[garyw]

Most trips to Mars are more than two burns.

You've got the burn to finalise your Earth orbit then a few hours checking out the probe, the burn to Mars and several course corrections.

What I tend to do is a large burn to Mars then half way into the trip the first mid-course correction, then another MCC at 75% into the journey and maybe one or two more burns to target a specific location or orbit.

The further away you are from Mars the more economic in terms of prop the burns are but the closer you are the more refined the burns can be.

 

[Bonanza123d]

Hey everyone, quick question for you:
Is it viable to make two burns on a trip from Earth to, say, Mars? Do one burn that puts your apoapsis out around the Moon's orbit. Then when you come back around to periapsis, do another burn that actually puts you on course to Mars. Any advantage or disadvantage to this, in comparison to doing one large burn?
Thanks for your input,
P

There is 0 gain in efficiency for what you are trying to accomplish. You would still need the proper dV for escape velocity, there would be from my experience no savings.

 

[Izack]

This sort of plan is a requirement for many missions using vessels lacking the Deltaglider's acceleration. Even if your vessel is capable of this, it could be much, much more efficient to break the manoeuvre into several burns. Recall that escape burns are most effective at periapsis. A longer burn requires more waste burning far away from that crucial point. On the other hand, it is more difficult to plan efficiently. The long-period intermediate orbits and the time they use up may interfere with your mission plan, if the launch window is particularly crucial.

 

[boogabooga]

What Izack said.

If your apoapsis is at the moon's orbit altitude, then you were almost to escape velocity anyway, so I don't see the point.

Otherwise, something very similar to what you describe was tried by the real life Phobos-Grunt probe in 2011. It didn't work, but not because the technique was wrong.

 

[EternalFrustration]

What Izack said.

If your apoapsis is at the moon's orbit altitude, then you were almost to escape velocity anyway, so I don't see the point.

Otherwise, something very similar to what you describe was tried by the real life Phobos-Grunt probe in 2011. It didn't work, but not because the technique was wrong.

So is there a better apoapsis altitude? Or should I just drop the whole idea and stick to just one burn?

 

[boogabooga]

Of course. An infinitely small distance higher than your current apoapsis altitude. This way, all burns will be impulsive and you will have no losses due to non-zero burn duration. Of course, you will need an infinite number of burns.

Seriously, it depends on the acceleration capability of your spacecraft. If you can do better than about 1G, probably don't bother.

You should be thinking in terms of how much velocity to add at periapsis rather than a fixed apoapsis height. For example, if you know the total Delta-V to be added for your escape trajectory, try cutting it in half and apply that much in the first pass. Whatever apoapsis height is, that's what it is. Apply the rest in the second pass. Get it?

Take a look at this:
http://en.wikipedia.org/wiki/Phobos_grunt#2011_launch