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Why can't we use physics warp in space?


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If using Windows, hold down the Alt-key (or similar modifier in Mac/Linux) while pressing greater-than (>) to advance physics warp in space. It will still only go up to 4x. The slash (/) key will cancel it.

Be prepared for the possibility that orbital calculations could lose some precision during a burn at physics warp.

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23 minutes ago, HvP said:

If using Windows, hold down the Alt-key (or similar modifier in Mac/Linux) while pressing greater-than (>) to advance physics warp in space. It will still only go up to 4x. The slash (/) key will cancel it.

Be prepared for the possibility that orbital calculations could lose some precision during a burn at physics warp.

They lost precision pretty badly without!

My apoapsis was almost 10x what I asked for (but I was aiming for just inside the SOI anyway) and my periapsis was 100km below ground.

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15 minutes ago, Loren Pechtel said:

They lost precision pretty badly without!

My apoapsis was almost 10x what I asked for (but I was aiming for just inside the SOI anyway) and my periapsis was 100km below ground.

That's surely a result of having such a long burn time. The orbital prediction assumes a total and instantaneous change in velocity at the exact point of the maneuver node, which is impossible. The best you can do is put the midpoint of your burn right at the maneuver node - half before and half after. Well, technically you can shift it a little earlier to account for your weight decreasing as you lose fuel, but it's trivial. If you have a highly curved trajectory or your target is moving fast enough in relation to you then a long burn time will necessarily be prone to error because all the angles will have changed over the course of your maneuver.

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40 minutes ago, HvP said:

That's surely a result of having such a long burn time. The orbital prediction assumes a total and instantaneous change in velocity at the exact point of the maneuver node, which is impossible. The best you can do is put the midpoint of your burn right at the maneuver node - half before and half after. Well, technically you can shift it a little earlier to account for your weight decreasing as you lose fuel, but it's trivial. If you have a highly curved trajectory or your target is moving fast enough in relation to you then a long burn time will necessarily be prone to error because all the angles will have changed over the course of your maneuver.

I didn't realize it assumed a burn all in one spot but I've long known that the longer the burn the worse the result.  That's by far the longest burn I've ever done and also by far the worst result.  I've never had the error aim me into the planet before!

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1 hour ago, HvP said:

The best you can do is put the midpoint of your burn right at the maneuver node - half before and half after.

That, and also you don't have to follow the maneuver marker precisely.

For example, let's say I'm in LKO and I need to do a heavy burn to eject on an interplanetary trajectory, but I've got a low TWR ship and the burn is longer than about 2-3 minutes (that's the longest I like to do in LKO, otherwise those aiming errors add up).

At the point the burn is supposed to start, the maneuver node marker is well below (i.e. farther from the zenith than) the orbital prograde marker, which is bad.  Any time you're thrusting other than perfectly prograde, you're piling up cosine losses, not to mention there's the accuracy issue.

So what I'll do is, I'll go ahead and aim considerably higher than the maneuver node marker-- say, raise the elevation of my nose about halfway between prograde and maneuver.  Then start the burn.  As the burn continues, gradually the prograde marker will lower.  I just keep pointing my nose about halfway between prograde and maneuver until they coincide, then I hold-maneuver for the rest of the burn... but I deliberately stop the burn a few hundred m/s short of completion, then delete the maneuver node, drop another node a few minutes ahead, and re-adjust that to my target.

Works great, doesn't waste much dV, puts me right on target.

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20 minutes ago, Snark said:

For example, let's say I'm in LKO and I need to do a heavy burn to eject on an interplanetary trajectory, but I've got a low TWR ship and the burn is longer than about 2-3 minutes (that's the longest I like to do in LKO, otherwise those aiming errors add up).

I usually go for an even simpler method - I depart from much higher up, typically 2000-3000km.   This works for capture burns too, capture high then transfer low.

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16 minutes ago, DerekL1963 said:

I usually go for an even simpler method - I depart from much higher up, typically 2000-3000km.   This works for capture burns too, capture high then transfer low.

Well, sure, that works, but then you lose all the Oberth benefit.

It's a matter of taste, I suppose.  For my own gameplay, I'll take "somewhat inconvenient navigation and futzing with maneuver node" over "lose a whole bunch of dV" any day.  :wink:

I'll also split up the burn to help mitigate this, by boosting at Pe on a few successive orbits.  Can only do just so much, of course, because on the final pass that takes you past escape velocity, there are no further passes and you have to do the entire rest of the burn in one go.  But it helps.

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2 minutes ago, Whisky Tango Foxtrot said:

You could start high up, burn retrograde to lower your PE to just above Kerbin's atmosphere, then burn again at PE for the transfer. That way you'll have already gained a lot of speed from your "fall" to Kerbin so you won't need to expend as much dV in your burn. The timing can be difficult, though.

That would waste a lot of dV in first raising and then lowering your orbit.

Most efficient technique is to make a short burn prograde at the correct moment to raise your Ap. Next orbit you'll do the same: at Pe a small burn prograde. Over consecutive orbits you'll increase your velocity.
This is however pretty difficult to plan since the last few orbits within your current SOI could take  multiple days.

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