Precomputed Low-TWR Interplanetary Transfer Burns

[Red Iron Crown]

That's a matter of personal preference to a large degree. I like >0.5 TWR for general purpose spacecraft, and consider 0.2 to be my practical limit (less than that and it starts getting fairly tedious). However, I know some other players have had success with TWRs as low as 0.05.

If you're using the precomputed burns or calculator in the OP, my rule of thumb is that the preliminary burns should not be longer than 1/6th of an orbit (e.g. about 5 mins in LKO). If your TWR is very low it might be better to use a higher parking orbit for the transfer as the orbital period will be longer, giving more time near periapsis to complete the burns.

[Biggen]

I know this is an older thread but since its a tutorial I figure its still fair game for questions.

When you say to raise the AP in each burn, these burns are happening at the original maneuver node each time, right? You arent arbitrarily burning at some point in the orbit to create an AP anywhere are you?

So if I have a node set for Moho that requires a 25 minute burn am I supposed to start the 1st (and subsequent) burns at the original maneuver node? Or am I supposed to create a new maneuver node at my PE after each burn?

I incorporate the antinormal in my ejection burn for Moho and I know you say this method doesnt take that into account but Id still like to try it. Normally I manually pilot for about 3/4 of the ejection burn since its such a long burn. Woukd like to try another method.

[Red Iron Crown]

1 hour ago, Biggen said:

I know this is an older thread but since its a tutorial I figure its still fair game for questions.

When you say to raise the AP in each burn, these burns are happening at the original maneuver node each time, right? You arent arbitrarily burning at some point in the orbit to create an AP anywhere are you?

So if I have a node set for Moho that requires a 25 minute burn am I supposed to start the 1st (and subsequent) burns at the original maneuver node? Or am I supposed to create a new maneuver node at my PE after each burn?

I incorporate the antinormal in my ejection burn for Moho and I know you say this method doesnt take that into account but Id still like to try it. Normally I manually pilot for about 3/4 of the ejection burn since its such a long burn. Woukd like to try another method.

It's not quite that simple. The idea behind these precomputed burns is to figure out how much ahead of time you need to start doing periapsis kicks to make the final one occur at the time when a single burn or node would get you the desired transfer trajectory. If you have created a node on your current orbit that gives you that trajectory, it's already too late to start kicking unfortunately.

Ideally you know when your perfect burn would occur and at what ejection angle (usually by using a transfer window calculator). Then you can set your first burn ahead of that ideal time and angle by the amounts calculated in the OP. After that, set a new node at periapsis for each subsequent burn, and the final one should include the remainder of the ejection.

I don't have good advice on adding the antinormal part, that's not something I've messed around with, sorry.

As for piloting during the burns, I do so manually and keep pointed between the node target and prograde, watching periapsis altitude to try to keep it constant. Despite being more burns I find it a bit less tedious than one long burn, as it's broken up into more manageable chunks.

Good luck with Moho, I'd call it the most challenging interplanetary transfer in the game.

[Biggen]

For my example, lets ingore radial burns. For Moho one always leaves on its AN node and ignores launch windows. So if im doing the method listed in the OP, I'd need to have my first Pe kick 11 days before reaching that point, right? Then all subsequent Pe kicks take place at the Pe I created at the 1st burn?

The problem is that I dont know "when" Kerbin hits the AN node other than looking at it in map mode. I guess I could eyeball the phase angle pretty easily (there is probably a calculator somewhere that could figure this out for me). Similarly, I should be able figure out an ejection angle once I figure out the proper phase angle. Once I have these two pieces of info I should be able to create a maneuver node but set it so that its 11 days into the future when the angles are correct. That would get me my first Pe burn I believe.

Complicated stuff this rocket science!

Ive made Moho a few times now. Just trying to get it more fine tuned, Building a small outpost out there with plans to a Mohole exploration from a rover (around the top anyway).

[Red Iron Crown]

9 hours ago, Biggen said:

For my example, lets ingore radial burns. For Moho one always leaves on its AN node and ignores launch windows. So if im doing the method listed in the OP, I'd need to have my first Pe kick 11 days before reaching that point, right? Then all subsequent Pe kicks take place at the Pe I created at the 1st burn?

Correct.

9 hours ago, Biggen said:

The problem is that I dont know "when" Kerbin hits the AN node other than looking at it in map mode. I guess I could eyeball the phase angle pretty easily (there is probably a calculator somewhere that could figure this out for me).

The offset to the ejection angle and phase angles are the same (9.3 degrees in the case of the three burn strategy in the OP). So you can start kicking when Kerbin is about that far away from the AN with respect to Moho.

9 hours ago, Biggen said:

Similarly, I should be able figure out an ejection angle once I figure out the proper phase angle. Once I have these two pieces of info I should be able to create a maneuver node but set it so that its 11 days into the future when the angles are correct. That would get me my first Pe burn I believe.

Sounds like a good plan.

9 hours ago, Biggen said:

Complicated stuff this rocket science!

But that's the fun!

9 hours ago, Biggen said:

Ive made Moho a few times now. Just trying to get it more fine tuned, Building a small outpost out there with plans to a Mohole exploration from a rover (around the top anyway).

You've done better than I have. I've only done three Moho missions in years of playing: One was supposed to be a probe landing but it failed when the ion orbital stage burned right through the encounter without capturing, turning it into a flyby. The next was a crewed landing that got stranded on the surface when I spent too much dV getting there in the first place. Third was a rescue mission for the second, which worked more or less according to plan. So I'm one for three.

[Biggen]

Thanks Red Iron for that information!

I think for my Moho transfers, I may just end up raising my Ap enough to do the burn in one full go.  That way I can incorporate my anti-normal burn at the same time as the prograde burn.  With the method in the OP, I'm not sure how to do it properly with the normals.

But I will definitely use this method for going to other planets where you don't need to combine burns like this!

[RedPandaz]

On 2/10/2017 at 7:40 AM, Biggen said:

Thanks Red Iron for that information!

I think for my Moho transfers, I may just end up raising my Ap enough to do the burn in one full go.  That way I can incorporate my anti-normal burn at the same time as the prograde burn.  With the method in the OP, I'm not sure how to do it properly with the normals.

But I will definitely use this method for going to other planets where you don't need to combine burns like this!

How'd it go?