Flight Question Optimal ascent vertical launch

[crisk73]

Hi, I'm trying to minimize ISS interception times in a Falcon9 scenario launch.
If I follow the rule of the half orbit time in LaunchMFD, I may end up with a perfectly aligned plane but pretty much distant from the target. Instead if I launch close to the ISS transit above the site I may end up quite close to the target but with a bad plane alignment due to the off-phase launch. Both cases require a lot of fuel burn to be quickly solved (I don't want to wait forever to sync orbits eventually).
So can I somehow adjust plane alignment in the first stage of the launch so that I can end up close and aligned at the same time? Thanks.

 

[Ripley]

I remember watching an old demo video with Launch MFD were the ship ended very very close to the ISS. Try to search in the scenario folder, or maybe it was a playback. It was called "direct ascent... "

Edit: it was on Youtube

 

[francisdrake]

The optimum launch scenario would be if the ISS passes right overhead just after the groundtrack intersected with the launch site. You launch at groundtrack intersection and the ISS catches up from behind while you are accelerating to orbit. This could be done by tweaking the scenario file.

But this scenario is not very realistic. It is unlikely that all orbital parameters are coinciding.

For low dV requirements you would always launch when the groundtrack passes over the launch site and fly a few orbits to catch up with the space station.

If you have more dV available, you could consider a 'dogleg' approach. In this case the launch happens at a less-than-optimal time, when the ISS is closer to the launch site.
The launch direction is a few degrees off the orbital plane, towards the direction of the groundtrack.

During launch you switch one MFD to 'Align Plane'. Once the angular deviation approaches 0° you deactivate the autopilot and sweep the launcher slightly to the side, to keep the angle at 0°.

Obviously switching off the autopilot requires to be able to finish orbital insertion manually. With this method you can reach the ISS in 2 orbits, which should be an acceptable time span.

 

[Thorsten]

If I follow the rule of the half orbit time in LaunchMFD, I may end up with a perfectly aligned plane but pretty much distant from the target. Instead if I launch close to the ISS transit above the site I may end up quite close to the target but with a bad plane alignment due to the off-phase launch

You've discovered the existence of a planar and of a phasing launch window.

The planar window occurs twice daily when Earth rotates you into the right spot to insert into a given plane (and by-dog-legging you can widen it to perhaps 10 minuts or so), the phasing window occurs more often.

In practice, a mission launches when both coincide - which may mean you really have to wait a lot, you can't launch on any given day and get a reasonable rendezvous with ISS, that's only possible on some days.

 

[crisk73]

@Ripley many thanks for the vid I'll have a look at it.
@francisdrake thank you, that kind of tweaking to the autopilot is what I was looking for, I'll have to get the feeling of how many degrees off the plane to direct the AP after launch. I believe it must be proportional to the offset of the groundtrack relative to the site.

 

[Thorsten]

Earth rotates ~360 degrees in 24 hours, so each minute you wait changes the longitude of the ascending node by 0.25 degrees.

Realistic propellant reserves won't give you more than 1-2 degrees or so via dogleg, so you can readily see how that expands you window by just a few minutes.